final report.edit

SURFACE TREATMENT OF AEROSPACE COMPONENTS 2014
DSCE, CHEMICAL ENGINEERING DEPARTMENT Page 1
ACKNOWLEDGEMENT
First and foremost we wish to express our gratitude towards Dr. Ravishankar R., Head of
Department of Chemical Engineering, for all the help extended to us at every stage of the
project.
We are indebted to Mr. Sunil H., our guide and Assistant Professor in the Department of
Chemical Engineering, for his valuable guidance, encouragement and suggestions through
the course of the project. Without his guidance this project would not have been completed.
We are also grateful to Mrs. Vidhya Karthikeyan, SCI - Engr ‘SD’ and Mr R Sundara
Rajan , Manger (STF), LPSC, ISRO for their valuable guidance and such a wonderful
opportunity to be a part of such an esteemed institution.
Finally, we would like to thanks our parents for their unending support to us in all
endeavours that we pursue.

TABLE OF CONTENTS
I. List of Figures
Figure No. Description
2.1 Electrochemical Attack
2.2 Cost of corrosion in the US
2.3 Change in thickness of metal
2.4
Effect of voltage and temperature on unit barrier
thickness
2.5
Relationship between voltage, current density and
temperature
2.6
Relationship between concentration of electrolyte and
unit barrier layer thickness
2.7 Effect of bath temperature on porosity
2.8
Relationship between bath voltage and current density to
treatment time
2.9
Effect on anodizing time on film growth and dimension
of work-piece
2.10
Coating ratios for various alloys
2.11
Automated ultrasonic cleaning system
2.12
Conversion of existing normal tank to ultrasonic tank
3.1
Schematic diagram of anodization tank
3.2
Dyed specimen

3.3 Schematic diagram for vapour degreasing
3.4 Cavitation
3.5 Schematic diagram for ultrasonic solvent cleaner
4.1 Thickness VS time
4.2 Temperature VS gain in weight

II. List of Tables
Table No. Description
2.1 Galvanic series of metals
2.2
Specifications of anodization in aerospace
series
2.3 Oxide films produced by various treatments
2.4 Coating of oxide films
2.5
Effect of operating conditions on properties
of coating bath
2.6 Anodic oxide coating composition
2.7 Initial rating of CASS
2.8
Selection of cleaning agents for ultrasonic
solvent cleaning
3.1 Process sheet for pickling- passivation
4.1
Relationship of coating thickness with
anodizing time
4.2
Relationship between weight gain and
temperature
4.3 Initial rating of CASS
4.4 Intensity of Stain

III.
Abstract 09
Chapter 1
Introduction
Introduction 11
Chapter 2
Literature Survey
2.1 Corrosion 13
2.2 Mechanism of Corrosion 15
2.2.1 Electrochemical Attack 16
2.2.2 Basic Process 19
2.3 Chemistry of Corrosion 19
2.4 Factors that Control Corrosion 20
2.5 Forms of Corrosion 22
2.5.1 Surface Corrosion 22
2.5.2 Dissimilar metal Corrosion 22
2.5.3 Inter Granular Corrosion 23
2.5.4 Stress Corrosion 23
2.5.5 Fretting Corrosion 24
2.6 Consequences of Corrosion 24
2.7 Anodization 26
2.7.1 History 27
2.7.2 Anodized Aluminium 27
2.7.3 Specifications 29
2.7.4 Mechanism of Anodization Process 30
2.7.5 Barrier Layer 32
2.7.5.1 Thickness 32
2.7.5.2 Effect of Operating Conditions On Barrier Layer 34
2.7.6 Porous Layer 37
2.7.6.1 Porosity 37
2.7.6.2 Mechanism of Porous Film Growth 39
2.7.7 Coating Ratio 40
2.7.8 Anodic Oxide Coating Composition 44
2.7.9 Comparison of AC & DC Anodization 46
2.8 Properties & Tests of Anodic Oxide Coating 47
2.8.1 Apparent Density 47

2.8.2 Coating Thickness 48
2.8.3 Porosity 52
2.8.4 Adhesion 54
2.8.5 Sealing Efficiency 55
2.8.6 Corrosion 56
2.9 Types of Anodization 59
2.9.1 Chromic Acid Anodizing (type I) 59
2.9.2 Sulphuric Acid Anodizing (type II & III) 59
2.9.3 Organic Acid Anodizing 60
2.10 Dyes and Colours 60
2.11 Sealing 61
2.12 Mechanical Considerations 62
2.13 Laboratory Testing 62
2.14 Environmental Impacts 62
2.15 Ultrasonic Solvent Cleaning 64
2.15.1 System Design 67
2.16 Pickling-Passivation 71
2.16.1 Pickling 71
2.16.2 Passivation 71
2.16.3 Test for Detemining Effectiveness of Passivation 73
2.17 Chemical Cleaning 75
2.17.1 Types of Chemical Cleaners 75
2.17.2 Common Cleaning Agents Used 76
2.18 Vapour Degreasing 77
2.19 Objective 78
Chapter 3
Materials and Methods
3.1 Materials 80
3.2 Pre-treatment of Aluminium for Anodization 80
3.2.1 Mechanical Cleaning 81
3.2.2 Ultrasonic Cleaning 81
3.2.3 Acetone Rinsing 82

3.2.4 Drying 82
3.2.5 Alkali Cleaning 82
3.2.6 Rinsing in Water 82
3.2.7 Acid Cleaning 82
3.2.8 Rinsing with DM Water 83
3.2.9 Process Flow sheet for Cleaning Cycle 84
3.3 Anodization Process 85
3.2.1 Construction of Anodization Tank 85
3.2.2 Process Description 86
3.4 Post-treatment of Anodized Aluminium 87
3.4.1 Dyeing 87
3.4.2 Sealing 88
3.4.3 Process Flowchart 90
3.5 Pickling-passivation Process Sheet 91
3.5.1 Process flowchart for pickling-passivation 93
3.5.2 Solvent Cleaning 94
3.5.3 Alkali Cleaning 94
3.5.5 Pickling 94
3.5.7 Passivation 94
3.5.9 Rinsing in DM Water 95
3.5.10 Drying 95
3.6 Flowchart for chemical cleaning of satellite tankages 97
3.7 Vapour Degreasing Technical Specifications 97
3.7.1 Vapour Degreasing System Description 98
3.8 Ultrasonic Solvent Cleaning System Description 100
3.8.1 Ultrasonic solvent Cleaning Technical Specification 101

Chapter 4
Results and Discussion
4.1 Test for Thickness 104
4.2 Test of gain in Weight with Temperature 105
4.3 Test for Porosity 106
4.4 Corrosion Test (CASS test) 107
4.5 Test for Sealing 108
Chapter 5
Conclusion
Conclusion 112
Chapter 6
References
Bibliography 114

ABSTRACT
Anodization is the common designation for Anodic oxidation of the certain metals to form
stable oxide film on their surface. But prior to anodization or other coating on metal
surfaces, certain other surface treatments are employed there include pickling-passivation,
vapour degreasing, ultrasonic solvent cleaning andor chemical cleaning. All these methods
in some way or the other render corrosion and abrasion resistance to the metal surfaces.
Aerospace industries employed such surface treatment methods mainly for providing
resistance to the metal components against corrosion. The metals used predominantly
include Aluminium, Magnesium, and Stainless Steel of various grades and Titanium.
The liquid propulsion system center, ISRO, specializes in treating surfaces of components
parts of satellite launch vehicles. Being offered to carry out project work at this highly
prestigious institution is a matter of great pride. The project carried out was challenging as
high specification and accuracy had to be maintained while performing the experimental
work.
Tests were carried out to realize which treatment methods could be employed to which
specific metal for using aerospace industry and whether requirements like thickness,
hardness, porosity, corrosion resistance etc, were up to the required specifications.

Chapter 1
Introduction

INTRODUCTION
Most metals exist in nature in combined form as their oxides, carbonates, hydroxyl,
chlorides, and silicates. During extraction these are reduced to their metallic state from their
ores and extraction considerable amount of energy required. Consequently, isolated pure
metals can be regarded as I excited state than corresponding ores and they have natural
tendency to revert back to the combined state. Hence when metals are put into use in various
forms they are exposed to the environment such as dry gases, liquids etc. thus destruction of
metals start at the surface. This type of metal destruction may be due to direct chemical
corrosion by the environment or by electrochemical attack.
Any process of deteoriation of metal, through an unwanted chemical or electrochemical
attack, starting from its surface is termed as corrosion.
The process of corrosion is slow and occurs only at surface of metals, but losses incurred
due to corrosion are enormous. In general, the life and strength of structure is reduced very
much due to corrosion is 1/5th of the total world production. It is very difficult to assess the
exact losses incurred due to corrosion. Various methods have been developed to protect
metals and to prevent corrosion. But even today there is no method used that can assure
100% protection. The most common methods employed are painting, electroplating,
anodization, galvanizing etc.
This report gives the details about various methods of cleaning towards corrosion protection
in aerospace industry.

Chapter 2
Literature Survey

2.1 CORROSION
Corrosion is an irreversible interfacial reaction of a material (metal, ceramic and polymer)
with its environment which results in its consumption or dissolution into the material of a
component of the environment. Often, but not necessarily, corrosion results in effects
detrimental to the usage of the material considered. Exclusively physical or mechanical
processes such as melting and evaporation, abrasion or mechanical fracture are not included
in the term corrosion.
Corrosion is primarily associated with metallic materials but all material types are
susceptible to degradation. Degradation of polymeric insulating coatings on wiring has been
a concern in aging aircraft. Even ceramics can undergo degradation by selective dissolution.
The fundamental cause or driving force for all corrosion is the lowering of a system’s Gibbs
energy. The production of almost all metals involves adding energy to the system. As a
result of this uphill thermodynamic struggle, the metal has a strong driving force to return to
its native, low energy oxide state. This return to the native oxide state is what we call
corrosion and even though it is inevitable, substantial barriers (corrosion control methods)
can be used to slow its progress toward the equilibrium state and it is this rate of the
approach to equilibrium that is often of interest. This rate is controlled not only by the nature
of the metal surface, but also by the nature of the environment as well as the evolution of
both.
Most corrosion processes involve at least two electrochemical reactions. A corroding surface
can be thought of as a short-circuited battery; the dissolution reaction at the anode supplies
electrons for the reduction reaction at the cathode. A short circuit is the electrical connection
made by a conductor between the two physical sites, which are often separated by very
small distances. Electrode potential difference between the reinforcing bars and electrolyte is
the driving force for the charge transfer to occur. Their electrode potentials will change with
the corrosion reaction rate until a stable or equilibrium state (Ecorr) is achieved. At this
potential the anodic (ia) and cathodic (ic) current densities are opposite and equal and to the
state (Icorr) achieved. It is graphical represented as a polarization curve (shown in fig 2.1).
Deviation from the steady-state condition can be expressed by the electrode polarization
potential, also known, as over-potential (a or c) where,

a= E – Ecorr
c= Ecorr – E
Where,
A=potential at anode
C=potential at cathode
Icorr =polarization at state achieved
Ecorr=polarization at equilibrium state
The study of corrosion processes involves the use of many of the same tools that are used by
electrochemists studying batteries, fuel cells, and physical and analytical electrochemistry.
The application of mixed potential theory to corrosion was originally presented by Wagner
and Traud and discussed later in the Journal of the Electrochemical Society by Petrocelli. In
1957, Stern and Geary theoretically analyzed the shape of polarization curves providing the
basis for the primary experimental technique (electrochemical polarization) used in
electrochemical studies of corrosion.
The formation of surface oxide films is critical in mitigating the rate of metal dissolution, so
person study corrosion have much in common with those studying dielectrics for other
purposes. It is these thin (< 10 nm) native oxide films that make the technological use of
metallic materials possible by serving as barriers to dissolution. Traditionally, corrosion is
classified into eight categories based on the morphology of the attack, as well as the type of
environment to which the material is exposed. Uniform or general corrosion is the most
prevalent type of corrosion but fortunately, it is predictable and can be controlled by various
methods such as painting the surface or applying a layer of a sacrificial metal like zinc to
steel. This sacrificial corrosion of the zinc surface layer to protect the underlying steel is
actually a form of galvanic or bimetallic corrosion. In this case, like in a battery, we are
using corrosion to our advantage. The surfaces of some metals (like aluminium, stainless
steel, and titanium) are protected from uniform corrosion by an extremely thin oxide films
that forms naturally. Many practical applications of materials depend on the presence of this

protective oxide. We would not be able to use planes made from aluminium if it were not for
this thin protective film. Unfortunately, this film can breakdown locally, resulting in forms
of corrosion like pitting of aluminium plates, crevice corrosion of stainless steel fasteners, or
stress corrosion cracking of pipes in nuclear reactors.
In light of the thermodynamic basis for corrosion it is not surprising that costs associated
with corrosion are high. Several studies over the past 30 years have shown that the annual
direct cost of corrosion to an industrial economy is approximately 3.1% of the country’s
Gross National Product (GNP). In the US, this amount rises to over $276 billion per year.
From Fig. 1, the highest segments of the cost of corrosion are associated with utilities,
transportation, and infrastructure. The Department of Defence alone has corrosion costs of
$20 billion. Because of the significant economic, safety, and historical impact of corrosion
on society and because corrosion of metals is an electrochemical process, it is also not
surprising that the Corrosion Division is one of the oldest divisions within ECS and was
established in 1942, but corrosion has been an important topic in the Society since 1903.
Reviews of the early literature and history of the Division were prepared by Uhlig and
Uhlig’s Corrosion Handbook is a good overall source of corrosion information for
consultation purpose.
2.2 MECHANISM OF CORROSION
Modern corrosion science was set off in the early twentieth century with the local cell model
proposed by Evans and the corrosion potential model proved by Wagner and Traud. The two
models have joined into the modern electrochemical theory of corrosion. They describe
metallic corrosion as a coupled electrochemical reaction consisting of anodic metal
oxidation and cathodic oxidant reduction. The electrochemical theory is applicable not only
to wet corrosion of metals at normal temperature but also to dry oxidation of metals at high
temperature. Metallic materials corrode in a variety of gaseous and aqueous environments.
Here we restrict ourselves to the most common corrosion of metals in aqueous solution and
in wet air in the atmosphere. In general, metallic corrosion produces in its initial stage
soluble metal ions in water, and then, the metal ions develop into solid corrosion precipitates
such as metal oxide and hydroxide .We will discuss the whole process of metallic corrosion
from the basic electrochemical standpoint.

2.2.1 ELECTROCHEMICAL ATTACK
An electrochemical attack may be likened chemically to the electrolytic reaction that takes
place in electroplating, anodizing, or in a dry cell battery. The reaction in this corrosive
attack requires a medium, usually water, which is capable of conducting a tiny current of
electricity. When a metal comes in contact with a corrosive agent and is also connected by a
liquid or gaseous path through which electrons may flow, corrosion begins as the metal
decays by oxidation.
During the attack, the quantity of corrosive agent is reduced and in turn it completely reacts
with the metal, becoming neutralized. Different areas of the same metal surface have
varying levels of electrical potential and, if connected by a conductor, such as salt water,
will set up a series of corrosion cells and corrosion will commence. All metals and alloys are
electrically active and have a specific electrical potential in a given chemical environment.
This potential is commonly referred to as the metal’s “nobility.” The less noble a metal is,
the more easily it can be corroded. The metals chosen for use in aircraft structures are a
studied compromise with strength, weight, corrosion resistance, workability, and cost
Fig 2.1
Electrochemical attack

balanced against the structure’s needs. The constituents in an alloy also have specific
electrical potentials that are generally different from each other. Exposure of the alloy
surface to a conductive, corrosive medium causes the more active metal to become anodic
and the less active metal to become cathodic, thereby establishing conditions for corrosion.
These are called local cells. The greater the difference in electrical potential between the two
metals, the greater will be the severity of a corrosive attack, if the proper conditions are
allowed to develop. The conditions for these corrosion reactions are the presence of a
conductive fluid and metals having a difference in potential. If, by regular cleaning and
surface refinishing, the medium is removed and the minute electrical circuit eliminated,
corrosion cannot occur. This is the basis for effective corrosion control. The electrochemical
attack is responsible for most forms of corrosion on aircraft structure and component parts.

+ Corroded End (anodic, or least
noble)
Magnesium
Magnesium alloy
Zinc
Aluminium (1100)
Cadmium
Aluminium 2024-T4
Steel or Iron
Cast Iron
Chromium-Iron (active)
Ni-Resist Cast Iron
Type 304 Stainless steel (active)
Type 316 Stainless steel (active)
Lead-Tin solder
Lead
Tin
Nickel (active)
Inconel nickel-chromium alloy (active)
Hastelloy Alloy C (active)
Brass
Copper
Bronze
Copper-nickel alloy
Monel nickel-copper alloy
Silver Solder
Nickel (passive)
Inconel nickel-chromium alloy (passive)
Chromium-Iron (passive)
Type 304 Stainless steel (passive)
Type 316 Stainless steel (passive)
Hastelloy Alloy C (passive)
Silver
Titanium
Graphite
Gold
Platinum
– Protected End (cathodic, or most
noble)
Table 2.1
Galvanic series if metal

2.2.2 BASIC PROCESSES
The basic process of metallic corrosion in aqueous solution consists of the anodic
dissolution of metals and the cathodic reduction of oxidants present in the solution:
MM→ M2+
aq + 2e−
M anodic oxidation ------(2)
Where M anodic oxidation.
2Oxaq + 2e−
M→ 2Red (e−
redox) aq cathodic oxidation -------(3)
2.3 CHEMISTRY OF CORROSION
Common structural metals are obtained from their ores or naturally-occurring compounds by
the expenditure of large amounts of energy. These metals can therefore be regarded as being
in a meta-stable state and will tend to lose their energy by reverting to compounds more or
less similar to their original states. Since most metallic compounds, and especially corrosion
products, have little mechanical strength a severely corroded piece of metal is quite useless
for its original purpose. Virtually all corrosion reactions are electrochemical in nature, at
anodic sites on the surface the iron goes into solution as ferrous ions, this constituting the
anodic reaction. As iron atoms undergo oxidation to ions they release electrons whose
negative charge would quickly build up in the metal and prevent further anodic reaction, or
corrosion. Thus this dissolution will only continue if the electrons released can pass to a site
on the metal surface where a cathodic reaction is possible. At a cathodic site the electrons
react with some reducible component of the electrolyte and are themselves removed from
the metal. The rates of the anodic and cathodic reactions must be equivalent according to
Faraday’s Laws, being determined by the total flow of electrons from anodes to cathodes
which is called the “corrosion current”, Icorr. Since the corrosion current must also flow
through the electrolyte by ionic conduction the conductivity of the electrolyte will influence
the way in which corrosion cells operate. The corroding piece of metal is described as a

“mixed electrode” since simultaneous anodic and cathodic reactions are proceeding on its
surface. The mixed electrode is a complete electrochemical cell on one metal surface.
Such electrochemical reactions are most common in acids and in the pH range 6.5 – 8.5 the
most important reaction is oxygen reduction 2b. In this latter case corrosion is usually
accompanied by the formation of solid corrosion debris from the reaction between the
anodic and cathodic products. If solid corrosion products are produced directly on the
surface as the first result of anodic oxidation these may provide a highly protective surface
film which retards further corrosion, the surface is then said to be “passive”.
2.4 FACTORS THAT CONTROL THE CORROSION
RATE
Certain factors can tend to accelerate the action of a corrosion cell. These include:
1. Establishment of well-defined locations on the surface for the anodic and cathodic reactions.
This concentrates the damage on small areas where it may have more serious effects, this
being described as “local cell action”. Such effects can occur when metals of differing
electrochemical properties are placed in contact, giving a “galvanic couple”. Galvanic
effects may be predicted by means of a study of the Galvanic Series which is a list of metals
and alloys placed in order of their potentials in the corrosive environment, such as sea water.
Metals having a more positive (noble) potential will tend to extract electrons from a metal
which is in a more negative (base) position in the series and hence accelerate its corrosion
when in contact with it. The Galvanic Series should not be confused with the
Electrochemical Series, which lists the potentials only of pure metals in equilibrium with
standard solutions of their ions. Galvanic effects can occur on metallic surfaces which
contain more than one phase, so that “local cells” are set up on the heterogeneous surface.
Localised corrosion cells can also be set up on surfaces where the metal is in a varying
condition of stress, where rust, dirt or crevices cause differential access of air, where
temperature variations occur, or where fluid flow is not uniform.
Stimulation of the anodic or cathodic reaction. Aggressive ions such as chloride tend to
prevent the formation of protective oxide films on the metal surface and thus increase
corrosion. Sodium chloride is encountered in marine conditions and is spread on roads in

winter for de-icing. Quite small concentrations of sulphur dioxide released into the
atmosphere by the combustion of fuels can dissolve in the invisibly thin surface film of
moisture which is usually present on metallic surfaces when the relative humidity is over 60-
70%. The acidic electrolyte that is formed under these conditions seems to be capable of
stimulating both the anodic and the cathodic reactions.
In practical terms it is not usually possible to eliminate completely all corrosion damage to
metals used for the construction of industrial plant. The rate at which attack is of prime
importance is usually expressed in one of two ways:
(1) Weight loss per unit area per unit time, usually mdd (milligrams per square
decimetre per day)
(2) A rate of penetration, i.e. the thickness of metal lost.
If suitable water treatment with corrosion inhibitors is used a life of at least twenty years
might be expected. This, of course, is ignoring the fact that at some time before the metal
corrodes away the tubing may have thinned to a point where its required mechanical
strength is not attained. When designing equipment for a certain service life engineers often
add a “corrosion allowance” to the metal thickness, permitting a certain amount of thinning
before serious weakening occurs. In a cooling water system the factors influencing the rate
of attack are:
a) The condition of the metal surface Corrosion debris and other deposits - corrosion
under the deposits, with a possibility of pitting (severe attack in small spots)
b) The nature of the environment
pH - in the range of 4-10 corrosion rate is fairly independent of pH, but it increases rapidly
when the pH falls below 4. Oxygen content - increase in oxygen concentration usually gives
an increase in corrosion rate.

Flow rate - increased water flow increased oxygen access to the surface and removes
protective surface films, so usually increases corrosion, but can sometimes improve access
for corrosion inhibiting reactants.
Water type - very important, in general low corrosion rates are found with scale-forming
(hard) waters. Aggressive ions which accelerate corrosion are Cl-, SO4
2-
but quite complex
interactions may occur between the various dissolved species in natural waters.
2.5 FORMS OF CORROSION
There are many forms of corrosion. The form of corrosion depends on the metal involved,
its size and shape, its specific function, atmospheric conditions, and the corrosion producing
agents present. Those described in this section are the more common forms found on
airframe structures.
2.5.1 Surface Corrosion
Surface corrosion appears as a general roughening, etching, or pitting of the surface of a
metal, frequently accompanied by a powdery deposit of corrosion products. Surface
corrosion may be caused by either direct chemical or electrochemical attack. Sometimes
corrosion will spread under the surface coating and cannot be recognized by either the
roughening of the surface or the powdery deposit. Instead, closer inspection will reveal the
paint or plating is lifted off the surface in small blisters which result from the pressure of the
underlying accumulation of corrosion products.
Filiform corrosion gives the appearance of a series of small worms under the paint surface.
It is often seen on surfaces that have been improperly chemically treated prior to painting.
2.5.2 Dissimilar Metal Corrosion
Extensive pitting damage may result from contact between dissimilar metal parts in the
presence of a conductor. While surface corrosion may or may not be taking place, a galvanic
action, not unlike electroplating, occurs at the points or areas of contact where the insulation
between the surfaces has broken down or been omitted. This electrochemical attack can be

very serious because in many instances the action is taking place out of sight, and the only
way to detect it prior to structural failure is by disassembly and inspection.
The contamination of a metal’s surface by mechanical means can also induce dissimilar
metal corrosion. The improper use of steel cleaning products, such as steel wool or a steel
wire brush on aluminium or magnesium, can force small pieces of steel into the metal being
cleaned, which will then further corrode and ruin the adjoining surface. Carefully monitor
the use of non-woven abrasive pads, so that pads used on one type of metal are not used
again on a different metal surface .
2.5.3 Inter-granular Corrosion
This type of corrosion is an attack along the grain boundaries of an alloy and commonly
results from a lack of uniformity in the alloy structure. Aluminium alloys and some stainless
steels are particularly susceptible to this form of electrochemical attack. The lack of
uniformity is caused by changes that occur in the alloy during heating and cooling during the
material’s manufacturing process. Inter-granular corrosion may exist without visible surface
evidence. Very severe inter-granular corrosion may sometimes cause the surface of a metal
to “exfoliate.” This is a lifting or flaking of the metal at the surface due to delamination of
the grain boundaries caused by the pressure of corrosion residual product build-up. This type
of corrosion is difficult to detect in its initial stage. Extruded components such as spars can
be subject to this type of corrosion. Ultrasonic and eddy current inspection methods are
being used with a great deal of success.
2.5.4 Stress Corrosion
Stress corrosion occurs as the result of the combined effect of sustained tensile stresses and a
corrosive environment acting on the metal. Stress corrosion cracking is found in most metal
systems; however, it is particularly characteristic of aluminium, copper, certain stainless
steels, and high strength alloy steels (over 240,000 psi). It usually occurs along lines of cold
working and may be trans-granular or inter-granular in nature.

2.5.5 Fretting Corrosion
Fretting corrosion is a particularly damaging form of corrosive attack that occurs when two
mating surfaces, normally at rest with respect to one another, are subject to slight relative
motion. It is characterized by pitting of the surfaces and the generation of considerable
quantities of finely divided debris. Since the restricted movements of the two surfaces
prevent the debris from escaping very easily, an extremely localized abrasion occurs. The
presence of water vapour greatly increases this type of deterioration. If the contact areas are
small and sharp, deep grooves may be worn in the rubbing surface.
2.6 CONSEQUENCES OF CORROSION
The consequences of corrosion are many and varied and the effects of these on the safe,
reliable and efficient operation of equipment or structures are often more serious than the
simple loss of a mass of metal. Failures of various kinds and the need for expensive
replacements may occur even though the amount of metal destroyed is quite small. Some of
the major harmful effects of corrosion can be summarised as follows:
1. Reduction of metal thickness leading to loss of mechanical strength and structural failure or
breakdown. When the metal is lost in localised zones so as to give a crack like structure,
very considerable weakening may result from quite a small amount of metal loss.
2. Hazards or injuries to people arising from structural failure or breakdown (e.g. bridges, cars,
aircraft).
3. Loss of time in availability of profile-making industrial equipment.
4. Reduced value of goods due to deterioration of appearance.
5. Contamination of fluids in vessels and pipes (e.g. beer goes cloudy when small quantities of
heavy metals are released by corrosion).
6. Perforation of vessels and pipes allowing escape of their contents and possible harm to the
surroundings. For example a leaky domestic radiator can cause expensive damage to carpets

and decorations, while corrosive sea water may enter the boilers of a power station if the
condenser tubes perforate.
7. Loss of technically important surface properties of a metallic component. These could
include frictional and bearing properties, ease of fluid flow over a pipe surface, electrical
conductivity of contacts, surface reflectivity or heat transfer across a surface.
8. Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion products.
9. Added complexity and expense of equipment which needs to be designed to withstand a
certain amount of corrosion, and to allow corroded components to be conveniently replaced.
In light of the thermodynamic basis for corrosion it is not surprising that costs associated
with corrosion are high.
Several studies over the past 30 years have shown that the annual direct cost of corrosion to
an industrial economy is approximately 3.1% of the country’s Gross National Product
(GNP). In the United States, this amounts to over $276 billion per year. It is revealed that
the highest segments of the cost of corrosion are associated with utilities, transportation, and
infrastructure. The Department of Defence alone has corrosion costs of $20 billion [6]
.
Fig 2.2
Cost of corrosion in the US

2.7 ANODIZATION
Anodizing is an electrolytic passivation process used to increase the thickness of the
natural oxide layer on the surface of metal parts. The process is called "anodizing" because
the part to be treated forms the anode electrode of an electrical circuit. Anodizing
increases corrosion resistance and wear-resistance, and provides better adhesion for paint
primers and glues. Anodic films can also be used for a number of cosmetic effects, either
with thick porous coatings that can absorb dyes or with thin transparent coatings that
add interference effects to reflected light.
Anodizing is also used to prevent galling of threaded components and to make dielectric
films for electrolytic capacitors. Anodic films are most commonly applied to
protect aluminium alloys, although processes also exist
for titanium, zinc, magnesium, niobium, zirconium, hafnium, and tantalum. Iron or carbon
steel metal exfoliates when oxidized under neutral or alkaline micro-electrolytic conditions,
the iron oxide (actually "ferric hydroxide" oxy hydrated iron oxide, also known as rust)
forms minute anodic pits and large cathodic surface, these pits concentrate anions such
as sulphate and chloride accelerating the underlying metal to corrode. Carbon flakes or
nodules in iron or steel with high carbon content (high carbon steel, cast iron) may cause an
electrolytic potential and interfere with coating or plating. Ferrous metals are thus
commonly not subjected to anodization.
Anodization changes the microscopic texture of the surface and changes the crystal
structure of the metal near the surface. Thick coatings are normally porous, so a sealing
process is often needed to achieve corrosion resistance. Anodized aluminium surfaces are
harder than aluminium but have low to moderate wear resistance that can be improved with
increasing thickness or by applying suitable sealing substances. Anodic films are not only
much stronger and more adherent than most types of paint and metal plating, but also more
brittle. This makes them less likely to crack and peel from aging and wear, but more
susceptible to cracking from thermal stress.

2.7.1 History
Anodizing was first used on an industrial scale in 1923 to protect Duralumin seaplane parts
from corrosion. This early chromic acid process was called the Bengough-Stuart process and
was documented in British defence specification DEF STAN 03-24/3. It is still used today
despite its legacy requirements for a complicated voltage cycle now known to be
unnecessary. Variations of this process soon evolved, and the first sulphuric acid anodizing
process was patented by Gower and O'Brien in 1927. Sulphuric acid soon became and
remains the most common anodizing electrolyte.
Oxalic acid anodizing was first patented in Japan in 1923 and later widely used in Germany,
particularly for architectural applications. Anodized aluminium extrusion was a popular
architectural material in the 1960s and 1970s, but has since been displaced by cheaper
plastics and powder coating. The phosphoric acid processes are the most recent major
development, so far only used as pre-treatments for adhesives or organic paints. A wide
variety of proprietary and increasingly complex variations of all these anodizing processes
continue to be developed by industry, so the growing trend in military and industrial
standards is to classify by coating properties rather than by process chemistry.
1.7.2 Anodized Aluminium
Aluminium and its alloys are anodized to increase corrosion resistance, to increase surface
hardness, and to allow dyeing (colouring), improved lubrication, or improved adhesion. The
anodic layer is non-conductive. When exposed to air at room temperature, or any other gas
containing oxygen, pure aluminium is capable of self passivation by forming a surface layer
of amorphous aluminium oxide 2 to 3 nm thick, which provides protection for some time but
the thickness of this layer is usually not uniform. Aluminium parts are thus anodized to
greatly increase the thickness of this layer for corrosion resistance. The corrosion resistance
of aluminium and its alloys is significantly decreased by certain alloying elements or
impurities: copper, iron, and silicon, so 2000, 4000, and 6000-series alloys tend to be most
susceptible. Anodizing the parts not only enhances corrosion resistance but also their ability
to retain dye which is not possible in case of untreated metal. Although anodizing only has
moderate wear resistance, the deeper pores can better retain a lubricating film than a smooth
surface would.

Anodized coatings have a much lower thermal conductivity and coefficient of linear
expansion than aluminium. As a result, the coating will crack from thermal stress if exposed
to temperatures above 80 °C. The coating can crack, but it will not peel. The melting point
of aluminium oxide (2050 °C) is much higher than pure aluminium (658 °C). This at times
makes welding more difficult. In typical commercial aluminium anodization processes, the
aluminium oxide is grown down into the surface and out from the surface by equal amounts.
So anodizing will increase the part dimensions on each surface by half of the oxide
thickness. Anodized aluminium surfaces are harder than aluminium but have low to
moderate wear resistance. Attempts are being made to overcome this problem of further
improvement of thickness of anodized layer and sealing.

2.7.3 Specifications
STA
NDA
RD
NOTES
COMME
NTS
BS EN
2101:1991
Chromic acid anodising;
Alloy category 1: Min film thickness 2.5 μm
Alloy category 2A: Min film thickness 1.5 μm
Alloy category 2B Min film thickness 1.0 μm
Sealing; Type A = Unsealed, Type B = Hot
water sealed,
Category 2 alloys shall preferable be
dichromate sealed
AEROSPACE
SERIES
BS EN
2284:1991
Sulphuric acid anodising;
Class A – Unsealed anodising
Class B – Sealed anodising
Thickness class 1 – 12 to 25 μm
Thickness class 2 – 6 to 12 μm
Sealing as specified; Dyed aluminium – hot
water seal,
Undyed aluminium – hot water seal or
dichromate seal.
AEROSPACE
SERIES
BS EN
2536:1995
Hard anodising;
Category 1 alloys < 1 % Cu : 30 μm to 120 μm
film thickness
Category 2 alloys 1% to 5% Cu: 30 μm to 60
μm film thickness.
Note: Restrict thickness on splines & threads
to 25 μm
Sealing is either hot water or dichromate seal
AEROSPACE
SERIES
Table 2.2
Specifications of anodization in aerospace series

2.7.4 MECHANISM OF ANODIZING PROCESS
When a current is passed through an electrolyte in which an aluminium anode is employed,
the negative charged anion migrates to the anode where it is discharged with loss of one or
m
o
r
e
electrons. In aqueous solution, the anion consists parts of oxygen which chemically units
with the aluminium and the result of the reaction depends on a number of factors,
particularly nature of electrolyte , the consequent reaction product which are formed, and the
operation conditions such as current potential, bath temperature, and time of treatment. In
simple terms the following oxidation reactions at the anode can occur:
1. The anode reaction products may be soluble in the electrolyte. In this case metal is dissolved
until the solution is saturated. This reaction takes place in some strong inorganic acids and
bases.
2. The reaction product may be almost insoluble in the electrolyte and from a strongly adherent
and practically non-conducting film on the anode. In this case film growth continues until
0
10
20
30
40
50
60
0 0.5 1 1.5 2 2.5 3 3.5 4
A'
A
B
Fig 2.3
Change in thickness of the metal sheet A, anodized on both sides to the coating
thickness indicated by the curve B, and A’ anodized on one side only, in sulphuric acid
at 15 A/dm2
DC at 20˚C

the resistance of the film prevents the current from reaching the anode. They can be formed
in a number of electrolytes of which borate or tartarate solutions are the most common
examples. Such films, formed at high voltage, find application in the production of the
electrolytic condensers and for protection for very thin aluminium coating, example, those
applied by vacuum deposition.
3. The reaction products may be sparingly soluble in the electrolyte and form a strongly
adherent film that is non-conducting when dry, over the anode.
In this case film growth takes place as above but is accompanied by dissolution of film at the
surface. Pores are thus formed in the coatings that are wide enough to allow continuous
access of the current to the metal. Film growth continues while the electrical resistance
increases. When the rate of film growth has decreased until it is equal to the rate of
dissolution of the film in the electrolyte, the film thickness remains constant.
The maximum film thickness varies with the electrolyte and the operating conditions,
especially the temperature which affects the dissolution velocity. The way in which the film
thickness and thickness of the basic sheet vary with time is shown in the following figure.
The curve A refers to the total increase in thickness of a sheet anodized on both sides to the
coating thickness indicated by curve B, while the curve A’ refers to the dimensional change
of a single surface. The coating reaches its maximum thickness in just almost 2 hours and it
may be seen that upto this point, for every 3 microns of coating formed the metal surface
retreats approximately to 2 microns and the exterior surface advances 1 micron.
These are the conditions of industrial anodization process that are based chiefly on chromic,
sulphuric or oxalic acid.
4. The reaction products may be moderately soluble. Under these conditions electro-polishing
may be possible if a suitable electrolyte is used. Apart from the reactions considered there
are a variety of less important possibilities, for example, where the reaction products may
form loosely adherent, spongy or powdery deposits, as when the anodizing solutions become
contaminated or when anodizing under special operations. A continuous adherent insoluble
film, a few molecules thick, may render the metal passive [7]
.

2.7.5 Barrier Layer
2.7.5.1 Thickness
It was shown as early as 1932 by Steoh and Miyata that the anodic oxide film consists of
two layers, the porous thick outer layer growing on an inner layer which is thin, dense and
dielectrically compact, and usually called the active layer, barrier layer or dielectric layer.
This layer is very thin, i.e, usually between 0.1 and 2.0% of the total film, and its thickness
depends on the composition of the electrolyte and the operating conditions. It has been
established that the barrier layer formed in anodizing is of the nature of the natural oxide
film formed in the atmosphere and that the barrier layer and porous films can also be
distinguished coatings and on electro polished surfaces.
In anodizing, the barrier layer is formed first and its thickness varies directly with the
forming voltage. The barrier layer is non porous and conducts current only due to its
thinness and faults in its skeleton. The outer layer, on the other hand, is micro porous and
built upon a columnar structure. As long as no dissolution occurs in the electrolyte, the
barrier layer is formed in a thickness of 14 A per volt. This is the theoretical maximum
approached only in solutions in which little or no solvent occurs: thus, Holland and
Sutherland obtained film thicknesses of 13 A per volt in 3% ammonium tartarate solution
used in the protection of vacuum coated aluminium mirrors. Capacity measurements of
barrier layers by Ginsberg and Kadan have given values of 14 A per volt for films formed in
barrier layer electrolytes and 11.5 A per volt for barrier layers for porous anodic coatings [8]
.

The following table gives barrier layer and total thickness of oxide films produced by
various treatments.
Treatment Temperature
(˚C)
Barrier
Layer
Thickness
(A˚)
Total thickness
( m)
Structure and
composition of
coating
Dry air 20 10-20 0.001-0.002
Amorphous
Al2O3
Dry air 500 20-40 0.04-0.06
Amorphous
Al2O3 + - Al2O3
Dry
oxygen
20 10-20 0.001-0.002
Amorphous
Al2O3
Dry
oxygen
500 100-160 0.03-0.05
Amorphous
Al2O3 + - Al2O3
Humid air 20 4-10 0.05-0.1
Boehmite +
Hydragillite
Humid air 300 8-10 0.1-0.2 Undetermined
Boiling in
water
100 2-15 0.5-2.0 Boehmite
Autoclavi
ng in
water
150 About 10 1.0-5.0 Boehmite
Chemical
oxidation
7-100 2-8 1.0-5.0
Boehmite +
Solution anion
(e.g.CrO4 ,PO4)
Normal
anodizing
18-25 100-150 5-30
Amorphous
Al2O3 + solution
anion
Hard
anodizing
+6- -3 150-200 150-200
Amorphous
Al2O3 + solution
anion
Barrier
film
anodizing
50-100 300-400 1.0-3.0
Crystalline Al2O3
+ Amorphous
Al2O3 + solution
anion
Chemical
polishing
50-100 About 5 0.01-0.1
Boehmite
+Solution anion
Electro
polishing
in H3PO4 –
butyl
alcohol
50-60 50-100 0.1-0.2
Al2O3 (structure
not determined)
+ solution anion
Table 2.3

2.7.5.2 Effect of Operation Conditions On the Barrier Layer
The way in which anodizing time affects the thickness of the barrier layer has already been
discussed. The way in which other variables affect the unit barrier thickness, i.e, the
thickness per volt of applied potential is shown below.
Electrolyte Type
The unit barrier thickness as shown in the following table which also gives other dimensions
of these coatings, referred to in greater detail below.
Electrolyte
Conc
.
Temperature
(˚C)
Unit barrier
Thickness
(˚A/volt)
Pore
thickness
(˚A/volt)
Wall
diameter
(˚A)
Phosphoric
acid
Oxalic acid
Chromic
acid
Sulphuric
acid
4
2
3
15
25
25
40
10
11.9
11.8
12.5
10.0
11.0
9.7
10.9
8.0
330
170
240
120
Table 2.4
Temperature of Electrolyte
The effect of temperature on the unit barrier thickness at different voltage is shown in the
following graph. It is seen that the effect of voltage is negligible. Increasing temperature
may decrease the unit barrier thickness slightly due to increased rate of dissolution of the
oxide, but under, some conditions the reverse has been observed.

Fig 2.4
Effect of voltage and temperature on unit barrier thickness for coatings on 99.99%
aluminium formed in 15% sulphuric acid
Current Density
The following graph shows the relation between voltage, current density and temperature
during sulphuric acid anodizing. Increase in temperature decreases the minimum voltage at
which the current density rises steeply with the forming voltage. However, the current
density has little effect on unit barrier thickness
8.8
9
9.2
9.4
9.6
9.8
10
10.2
0 10 20 30 40 50 60 70 80
unitbarrierthickness
(angstorm/volt)
Bath Temperature (˚C)
0˚C20˚C40˚C60˚C
70˚C
0
20
40
60
80
100
120
0 5 10 15 20 25 300 deg

centration of Electrolyte
The effect of electrolyte concentration is seen in the following graph. At constant voltage
and temperature and with the use of very low concentrations the unit barrier thickness
approaches a maximum of 14˚A/volt, as at this concentration the solvent action is low.
Increasing the concentration causes a drop in the unit barrier thickness that reaches a
minimum between 35 and 65% (weight %) sulphuric acid. This is followed by a marked
increase upto 90% where it changes sharply to an almost negligible value. The decrease in
unit barrier thickness at higher concentration is by no means related to the rate of
dissolution, nor is it directly related to the degree of dissolution of sulphuric acid as related
to the electrical conductivity, suggesting that some other influences barrier thickness at high
acid concentrations. In other electrolytes, such as chromic, oxalic or phosphoric acid, the
barrier thickness is influenced by the same factors to a very similar extent.
0
2
4
6
8
10
12
14
16
0 20 40 60 80 100 120
Y-Values
Fig 2.6
Relationship between concentration of sulphuric acid and unit barrier thickness of
coating formed on 99.99% aluminium at 20˚C and 15V
Fig 2.5
Voltage, current density and temperature relationship during coating of 99.99%
aluminium in sulphuric acid

2.7.6 Porous Layer
2.7.6.1 Porosity
As well as the thickness, the porosity of the coating varies with the dissolution velocity and
the conditions and rate of film growth, and these depend on the operating conditions and the
type of electrolyte.
As far as the last is concerned, it is probable the pH of the solution is the most important. An
example of this is seen most strikingly when aluminium is anodized in phosphate solutions.
While a phosphoric acid electrolyte gives a thick and extremely porous anodic oxide
coating, a buffered phosphate solution on the other hand gives a non-porous barrier film the
phosphate content of which is an integral part of the film and is proportional to its thickness.
Due to the effect of dissolution, the outer layer of the coating have the greatest porosity.
Examination by electron microscope shows the presence of pores in the striated structure.
Edwards and Keller also found vertical lines 6 x 10-9 inches apart near the metal interface,
which they believe locate the pore centres from which the coating grows.
The pores are very absorptive and it was determined that when a film, formed in sulphuric
acid with a volume of 15 ml/sq m was boiled in a 1% potassium dichromate solution for one
hour, the coating took up 0.48 gm of chromium per sq m, in other words the dichromate
content of the 140 ml of solution or 10 times the volume of the coating, was absorbed and
concentrated in the pore surface as fresh solution continued to diffuse in to the pores. The
effect of anodizing temperature on the absorption capacity of the film, i.e, on its porosity, is
shown in following graph.

The largest pore diameter of coating produced industrially is found in the phosphoric acid
coating that is used as a base for electro deposition. Next come the oxalic acid films
particularly films produced by the AC process. Thus it was found by direct electron
microscope examination that thin DC sulphuric acid films give approximately 800
pores/nm2
(pore diameter 0.015 nm; porosity 13.4%). While DC oxalic acid coating gives 60
pores/ im2
(pore diameter 0.075 nm; porosity 8%, amended to 12% to allow for pore sections
not appearing on the surface due to their direction). No pores have been found on the barrier
layer, and on examining films formed in ammonium borate and disodium phosphate, found
on determinable structure under the electron microscope.
Chromic acid coatings, due to their relatively low solubility are more closely allied to barrier
films and have a smaller pore diameter. The total porosity of coatings formed in chromic,
sulphuric and oxalic acid coatings has been variously estimated from 12-30 %. More
detailed investigations on the mechanism of anodic oxidation show that the number of pores
and their volume are largely dependent on the forming voltages.
0
10
20
30
40
50
60
70
0 5 10 15 20 25 30 35 40
bathvoltage(volts)
temperature (˚C)
Fig 2.7
Effect of bath temperature on water absorption of coating, i.e., on porosity. Film 10
microns thick produced in 20% sulphuric acid at 20 A/sq ft subsequently immersed
in water for 30 mins

While the ratio of effective “metal surface” apparent surface is extremely low. i.e., the
effective current density is much larger than the calculated values. The pore area is
extremely large. True values for the current density of porous film formation cannot easily
be determined.
Values based on the average pore diameters do not take into account the considerable
decrease pore diameter at the lower layers or the electrical relationship through the dielectric
layer or the relationship between current density and voltage during film formation at
constant current density is shown in the following graph.
2.7.6.2 Mechanism of porous film growth
When aluminium is made anodic in the anodizing electrolyte, it will depend on the operating
conditions, i.e., voltage, solubility of the reaction products, concentration, temperature, etc.
There are two possibilities for a reaction in a sulphuric acid electrolyte, in one of which the
O2-
ion and water react directly with the aluminium, while in the other the aluminium
0.1 A/sq dm
0.5 A/sq dm
1 A/ sq dm
2 A/sq dm
5 A/sq dm
10 A/sq dm
0
20
40
60
80
100
120
0 20 40 60 80 100 120 140
bathvoltage(volts)
Time (seconds)
Fig 2.8
Bath voltages and constants current densities in relation to the treatment time in 2 %
oxalic acid, DC at 17-18˚C

sulphate first formed is hydrolysed to the hydrate. Once oxide has been formed on the metal
surface, the anion can no longer make contact with the aluminium.
The oxidation reaction is given empirical by the simple equation:-
2 Al + 3 O → Al2O3 + energy
The above reaction takes place at the metal-oxide interface.
2.7.7 Coating ratio
A useful concept in determining the course of anodic oxidation is the “coating ratio”. This
term represents the weight of coating divided by the weight of aluminium reacting (the last
being the combined weight of metal converted into oxide plus that going into the solution)
200
interuppted dye
adsorbed
normal
0
50
100
150
200
250
0 20 40 60 80 100 120 140
thickness(mil)
1mil=25microns
time (minutes)
Fig 2.9
Effect of anodizing time on film growth and on the dimensions of the part being
anodized

Assuming that the coating is composed of Al2O3 the coating ratio has a theoretical ratio of
1.89. In practice the anodic coatings nearly always contain some of the solution anion. In the
case of sulphuric acid anodizing on anodizing on 99.95% aluminium, approximately 12-
14% SO3 is found in the coating which gives a maximum coating ratio of 2.2.
From the foregoing discussion on the effect of operating variables, it follows that, other
condition being constant, the coating ratio will decrease with time and increased by reducing
the bath temperature and acid concentration or by increasing the current density and voltage.
It can also be increased by addition of a certain amount of oxalic acid.
Decrease of coating ratio is approximately linear with time at constant current density. The
voltage rises as the coating thickness increases and this reflects increasing dissolution of the
coating during its growth associated both with a larger active surface area with progressive
dissolution in the pores and increase in the local temperature due to the higher voltage
required as the coating grows in thickness.
The effect of increasing the current density is to speed up the rate of growth. The effect of
current density will last the whole course of normal anodizing and becomes even more
pronounced in time if the bath temperature has a very pronounced effect on increasing the
2S M18
245 T3
755 T6
99.95 Al
615 T6
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
0 50 100 150 200
coatingratio
metal removed (milligram)
Fig 2.10
Coating ratios for various alloys treated as anodes in 15% H2SO4 at 1.1˚C with a
current density of 2.5A/dm2

coating ratio. Thus at 10˚C, which is a temperature commonly used in hard anodizing, the
effect of anodizing in reducing the coating ratio is negligible at current densities as low as
24 amp/ sq. ft, while at current densities above 48 amps/ sq. ft prolonging the treatment
causes a progressively steep rise in the coating ratio. This rise can be explained as being due
to a decrease in solution rate within the pore channel due to build-up of the solution product.
The excess of dissolution products may be pictured as due to the rapid dissolution at the
high temperatures obtaining at the pore base (estimated at 125 ˚C), which cannot be dealt
with by the diffusion rate further up the pore in cooler solution. For the same reason, it is
noteworthy that increase in the coating ratio with time at low temperatures and high current
densities is inevitably associated with a steeply rising voltage, and in practice, these
conditions therefore present serious disadvantages. Different alloys behave rather differently
in cold sulphuric acid electrolytes. The coating ratio for commercial aluminium tends to rise
with time at relatively low temperatures, due possibly to stronger initial dissolution. Anodic
oxide films that contain heavy metals dissolve rapidly from the surface and it is difficult to
obtain uniform coatings of any thickness. Sometimes the coating ratio is used as a control
method for evaluating the efficiency of the anodizing process and production work.

EFFECT OF OPERATING CONDITIONS ON PROPERTIES OF
COATING BATH
Change in
operating
conditions
Limitng
film
thickness
Hardness
Corrosion
resistance
porosity voltage
Temperature
increase
↓ ↓ → ↑ ↓
Current
density
increase
↑ ↑ → ↓ ↑
Reduction in
time
-- ↑ ↓ ↓ ↑
Decrease in
acid
concentration
↑ ↑ → ↓ ↑
Use of AC ↓ ↓ ↓ ↑ ↓
Increase in
homogeneity
of alloy
structure
↑ ↓ ↓ ↑
Use of less
aggressive
electrolyte
↑ ↑ → ↓ ↑
↑ = increase → = passes through a maximum ↓ = decreases
Table 2.5

2.7.8 ANODIC OXIDE COATING COMPOSITION
The mechanism of anodic oxidation is very complex and is still largely controversial. The
theories developed are numerous and complicated and hence will be dealt with in a very
brief manner. As from the discussion of aluminium oxide films in general, the compositions
of coatings depend largely on the electrolysis employed and differing and different workers
have reached sometimes, contradictory conclusions.
Thus, Bengough and Sutton found 0.4-0.7 chromic acid in the coatings produced by chromic
acid while Pullen and Scot, working with coatings sealed in water found them to consist of
almost entirely anhydrous aluminium oxide Al2O3 with less than 0.1% of chromium. At the
same time the latter authors found the sealed sulphuric acid coatings to have 13% SO3 and
the oxalic acid coatings to contain about 3% of (COOH) 2. The composition coated by Scott
for the sealed sulphuric acid coating is:
Aluminium oxide - 72%
Water - 15%
Sulphur trioxide - 13%
These measurements were made with thick porous coatings detached from the base metal,
by Scott obtained almost identical results some 25 years later using thin coatings and a
different analytical technique. According to Mason the sulphate content of the normal
sulphuric coating is between 13% - 17% is higher at lower temperatures of operation and
increases with current density. Spooner has given the following composition:
Compound Unsealed coating
Water sealed
composition
Al2O3 78.9% 61.7%
Al2O3.H2O 0.5% 17.6%
Al2(SO4)3 20.2% 17.9%
H2O 0.4% 2.8%
Table 2.6

These figures are equivalent to SO3 contents of 14.2 and 12.6% respectively but the H2O
contents are 0.5 and 5.4%. Edwards and Keller have reported water contents of 1.0% and
6.0% in sulphuric acid and anodized coatings. These figures suggest a possibility of a partial
hydration of the coating. Phillips reports water content in coatings produced in oxalic acid
equivalent to the formula 2 Al2O3.H2O.
The use of radioactive tracer methods for estimating sulphate by using sulphuric acid
incorporating some of the isotopes of S35
has enabled the role of sulphate to be examined
with much more precision. Thus Brace and Baker have detected sulphate during the first few
seconds of anodizing at about 15% slowly falling as anodizing proceeds, and it even reaches
1.5% during chromic anodizing. E.Raub and his co-workers have more recently applied the
same techniques to quiet a detailed examination of sulphate incorporation from coatings
prepared in electrolytes of very low sulphate content based on sulphur sulphosalicyclic acid
or maleic acid. They have demonstrated that although the electrolytes contained only 0.35%
and 0.5% of sulphuric acid, this was essential for the proper operation of the process and the
coatings was found to contain from 5-10% of the sulphate ion.
Anhydrous aluminium oxide is very hygroscopic even when heated to red heat, and the
electrolytically produced coating is like a gel in equilibrium with the vapour pressure.
Change in weight of the anodized aluminium rises steeply with humidity above about 70%
relative humidity and on a 20 micron coating the weight may rise to as much as 600mg/sq
cm. As might be expected, water uptake by sealed coatings is markedly lower at the higher
humidities, although at lower humidity there is often little difference.
On heating, anodic oxide films lose ionic conductance, due probably to a decrease in
aluminium ion, until the film is undistinguishable from an amorphous film formed in dry
oxygen. As electric conductance of the film is proportional to its moisture content, this
property has been utilized has been utilized in an instrument designed to determine
humidity.
Most investigators agree that the coating consists mainly of anhydrous aluminium oxide
which is either amorphous or in the ƴ - Al2O3 or ƴ1
- Al2O3 state, though there is
disagreement concerning these forms. ƴ - Al2O3 is intermediate in formation temperature

between the two ƴ - Al2O3 stages while ƴ1
- Al2O3 is the form produced on heating the
monohydrate ƴ - Al2O3.H2O to 650˚C. Ruziewiez observed changes in the
photoluminescence of anodic films when heated at 450˚C and 680˚C.
Franklin found at least 3 types of oxide in films formed in boric acid-borax solution:
1. Anhydrate layer at the outside of the film
2. Irregular patches of crystalline ƴ1
- Al2O3 within the coating.
3. Amorphous oxide as bulk of the film
On the other hand there is now a great deal of support, example for the view that the
proportion of crystalline oxide in the film increases with the film thickness while the effect
of pre-treatment on the structure of anodic oxide coatings on aluminium cannot be ruled out.
Thus there is some evidence that absorbed chloride, present example, on the aluminium foil
that has to be etched before use in capacitators increases the amount of boehmite present
after anodizing in barrier film electrolytes compared with foil that is etched in hydrofluoric
acid. In practice, this effect is removed by rinsing in hot water containing silicon after
hydrochloric acid etching.
2.7.9 Comparison of AC and DC anodization
In order to overcome the problem of liberated oxygen forming a passive layer on the surface
on which anodization is to be done, it is desired to use AC instead of DC.
The use of AC is done not with the change of polarities but in a pulsated mode DC so that
the passivation of the surface is reduced. In industry AC has also been used successively but
anodized surfaces with AC have certain inherent deficiencies.
The film produced by AC is more transparent. The advantage of AC is due to the lower of
film in comparison with DC films obtained at lower current densities.
AC films can be dyed more deeply and more uniformly than DC films. DC anodizing at low
voltages could also obtain most of the advantages of AC processes. But in some cast
materials, more uniform dyeing can be obtained by using AC process due to the very
efficient degreasing action of AC.

No sound film, thicker than about 12 microns, could be obtained with AC and therefore this
process is not suitable when a high degree of resistance to pitting corrosion is required.
The pore numbers of Ac films are slightly higher than for equivalent DC films, butt AC
films have a tendency to hydrate in sealing to a much greater extent.
The natural colours of AC films are yellowish. The depth of the colour is a function of the
film thickness only and not of other anodizing conditions. The colour is greatly diminished
by boiling water sealing and is greatly intensified by copper or ferrous ions in anodizing
bath.
Abrasion and corrosion resistances of AC films are much lower than those of equivalent DC
films. When the comparison is made is made between films thicker than 6 mm at low
temperatures and high current densities, the comparison becomes more favourable to AC
films.
1.8 PROPERTIES AND TESTS OF ANODIC OXIDE
COATINGS
As discussed previously, the properties of the coatings obtained by the various anodizing
processes may vary considerably, and depending on the specific application for which the
work is to be treated, it is often possible by varying the solution, the operating conditions,
the after-treatment, or even the composition of the basic metal or alloy, to obtain
improvement in the properties aimed at. This section comprises of discussing the physical
and chemical properties of anodic coatings along with the testing methods that have been
employed, certain of which might with advantage be incorporated in routine control and
inspection in aerospace practices, where being specific is particularly important.
2.8.1 Apparent density
The apparent density (specific gravity) of anodic oxide coatings may vary within quiet
appreciable limits depending upon the operating conditions and the basic metal. The

variations are due to differences in porosity and in foreign inclusions in the film, hence the
term apparent density is more appropriate and now generally preferred.
The following table shows apparent densities of sulphuric acid anodic oxide coatings with
anodizing time and basic metal composition.
Anodizing
Time
(Mins)
Apparent Density
(gm/ m3
)
99.99% Al Al -3% Mg Al-Mg-Si Al-Cu-Mg
5 3.3 3.4 3.5 2.9
10 3.2 3.3 3.1 2.4
20 3.0 2.6 2.8 1.9
30 3.0 2.5 2.4 1.8
40 2.6 2.5 2.3 1.6
50 2.4 2.3 2.4 -
60 2.2 2.3 2.3 -
Table 2.7
2.8.2 Coating thickness
The thickness of coatings normally produced by the different anodizing processes has been
described earlier. As has been seen, the increase in film thickness is not linear with the
treatment time, but a maximum, or limiting, film thickness may often be reached when
equilibrium is established between the rate of film growth and the rate of dissolution of the
film in the electrolyte. The operating time is often critical. However, in that the metal will
continue to decrease in thickness after the maximum film thickness has been obtained, while
in case of some alloys, example, certain Al-Mg and Al-Mg-Zn alloys, there may even be an
actual decrease in film thickness after the maximum is reached.

In general, the limiting thickness and the rate of film growth increase with rise in current
density and pH of the solution, with vigorous agitation and with greater homogeneity of the
alloy, and decrease with rise in temperature and the presence of heterogenous phases of
alloying constituents that accelerate the dissolution of the film in the electrolyte. The most
effective method of producing thicker coatings is to use low temperatures. In the case of
dielectric non-porous films, which are practically insoluble in solution, such as films
produced in the boric acid electrolyte, the film growth of the barrier type film ceases when
the breakdown voltage is equal to the voltage applied.
In most cases anodizing first increases the dimensions of the work and then reduces them
again after reaching a maximum. In general, the sulphuric acid film itself has a volume
approximately 1.5 times that of the metal from which it is formed but this ratio is slightly
higher in oxalic acid anodizing and hard anodic films may be 2.0 times the volume of the
metal removed during their formation.
In practice, it is often desirable to measure the film thickness periodically, both as a check
on the solution and on the quality of the work, as the thickness of the coating influences its
resistance to corrosion and wear. This is of particular importance where the work is to be
dyed or where close dimensional tolerances have to be obtained together with adequate
protection. It is of course, essential where specification is to be maintained.
DETERMINATION OF COATING THICKNESS
Numerous methods have been suggested, but no technique that is both simple and accurate
has been evolved suitable for all types of techniques that is both simple and accurate has
been evolved suitable for all types of techniques. Methods for determining the thickness of
anodic coatings in routine inspection should preferably not destroy the film and at least
should not affect the basic metal. Some film thickness meters are now accurate and
dependable but the referee methods in cases of dispute inevitable involve destruction of the
coating.
a) Direct Microscopic Measurement
This is an adaptation of the method for determination of thickness of mirrors and can be
only used for transparent films. It is a non-destructive method for measuring coating

thickness. A microscope is employed whose adjustment includes a micrometer device. The
microscope is first focussed on the surface of the coating, then on the coating-metal
interface. The difference measured on the micrometer device gives the optical film thickness
that must then be multiplied by the refractive index, which is 1.59 for unsealed and 1.62 for
hot-water sealed films. The accuracy of this method increases with the thickness of the
coating and the magnification of the microscope (should be upto 1000 dia). Modification of
this technique is to immerse the article in oil which causes a reduction in the effective
refractive index.
The air-film interface may be focussed with greater accuracy by rubbing the surface lightly
with pencil. If the surface is highly reflective, the shadow of the pencil mark is taken as the
metal-coating interface, or the distance between the pencil marking and its mirror image is
determined, being equivalent to twice the optical thickness.
b) Eddy current measurement
The most reliable non-destructive methods for measuring coating thickness are those carried
out with meters based on the eddy current principle which are designed so that the strength
of a high frequency current flowing through the search coil is dependent on the distance of
the coil from a conducting surface.
Out of the earliest commercial instrument was the Isometer, in which the test head is a small
coil energized by a high-frequency oscillator. The associated magnetic field induces eddy
currents in the basic metal. The depth of penetration of the current is inversely proportional
to the square root of the frequency and directly proportional to the conductivity of the basic
metal. Thus, when the conductivity of the coating differs from that of the basic metal the
coating thickness (i.e., the distance between the probe and the basic metal) is linear to the
output of the amplifier, which is measured on a dial. In order to prepare calibration curves
for the specified aluminium alloy, non-metallic foil of known thickness is placed on the
uncoated metal.
Eddy Current instruments have been used for several years, principally as flow detectors, as
alloy sorters or resistivity-measuring devices. The majority of instruments require a zero-
setting on uncoated metal of the same composition or alloy type as the anodized metal to be

measured, and standardizing on coatings of accurately known thickness, which are
sometimes supplied as plastic films.
The commercial instruments vary in accuracy, stability and dependability with the course of
time. Defects of the earlier models were associated with zero drift requiring very frequent
rechecking, a high sensitivity to temperature changes, a response that was not always linear
and probes whose characteristics are not always stable.
c) Micro-section
The traditional method for determining the anodic coating thickness without equivocation is
the preparation of a standard metallurgical micro-section that can be viewed with a high-
power microscope fitted with a calibrated micrometer eye-piece, or which is equipped with a
projection screen on which direct measurements at a known magnification can be made. It is
destructive, very time consuming, allows only a small portion of the surface to the surface to
be examined and requires some skill and experience because of the tendency for the edge of
the coating to bevel or chip. In fact, in inexperienced hands it can be less certain than the
eddy current method.
The British Non-Ferrous Metals Research Association has given the following guidance:
“Sections shall be cut using a fine jeweller’s saw to avoid deformation and blurring of cut
edges. The cut edges of anodic coatings require support to retain a true profile during
polishing and for ease of differentiation between coating and mounting medium. This may
be achieved in a variety of ways, the following method being recommended.”
The anodized surface of the specimen should be tightly wrapped with a single layer of
smooth aluminium foil and folded at one end to retain in place. The wrapped specimen is
mounted using a suitable thermosetting resin. Fine particles of resin powder should be first
packed around the specimen to ensure the complete filling of voids and the pores finely
filled using the coarser resin particles. The anodized specimen should be perpendicular to
the face of the completed mount, a deviation of 100
introducing, however, only an error of
2% in the thickness. After mounting, the specimen must be free from voids between the
section and the mounting medium.

Mounted specimens are ground on emery paper using water or white spirit lubrication and
the minimum pressure applied to avoid bevelling of the surface. Initial grinding should
employ 100 or 180 grade emery to reveal the true specimen profile and remove any
deformed areas. A final polish for 2-3 minutes on a rotating wheel charged with 4-8
micrometer diamond paste particles and white spirit lubrication should suffice to remove
emery scratched for final examinations.
Where very soft aluminium substrates are being prepared emery particles may become
embedded during grinding. This may be minimised by totally immersing emery papers in
lubricant during grinding or by using a copious flow of lubricant. If emery particles do
become embedded they may be removed by applying a short, light hand polish with metal
polish after grinding and before diamond finishing.
The most convenient microscope magnification for viewing the section is 1000 because
1mm on the screen is equal to 1 micron film thickness. The accuracy with which the coating
can be measured is generally about + 0.5m, and the average of several determinations is
taken.
It should be noted in passing that most other method of film thickness determination are
calibrated by means of standards that have, or should have been measured by micro-section.
They cannot therefore be any better in absolute accuracy than the micro-section method, and
determinations of density are subject to proportionate errors that obviously become greater
when applied to thinner films. The most accurate calibrations require the techniques of
interferometry and expensive equipment.
2.8.3 Porosity
There are semantic problems associated with applying the concept of porosity to anodic
oxide coatings because here we are dealing with materials whose nature is inherently
porous, while at the same time superimposing on this secondary concept borrowed from the
terminologies of electrodeposits which is related more to discontinuities in an essential
homogenous medium. These two properties are sometimes distinguished by referring to
them as micro and macro porosity, but this is not altogether ambiguous because the micro

porosity is on a scale far below the reach of any microscope and can only be imperfectly
seen at magnifications approaching 100,000 with an electron microscope, while macro
porosity can be associated with a number of factors, including inter-metallic constituents,
which are of truly microscopic dimensions.
The porosity, which is part and parcel of anodic oxide coatings, formed in acid electrolyte
arises from dissolution of the coating in the electrolyte so that continuous film formation
becomes possible, and the dimensions of these pores are a function of the electrolyte, the
operating conditions and the thickness of the coating. The nature and magnitude of this
porosity is important because it affects the resistance to abrasion, to corrosion, the case with
which a coating may be dyed or otherwise impregnated, and the efficiency with which it can
be sealed. In very thin anodic coatings the pores can be produced to an extremely uniform
size, example, to within + 10%, and much of our knowledge of pore structure has come
from observation of such films, while in practice they have been used for the filtration of
gaseous colloids or colloidal suspensions. It has also been shown that micro porosity can
also be influenced by the texture of the metal surface, decreasing with the smoothness of the
surface, and being less in electro-polished surfaces than in mechanically polished surfaces
after anodizing.
While micro porosity is closely linked with the physical properties of the coating as noted
above, the continuity or macro-scale features are related to corresponding features or faults
in the basic metal, or to extreme operating conditions, and these may be detrimental to
appearance or corrosion resistance.
The quantitative estimation of porosity will vary to an extent with the method of definition
or test because the dimensions involved are such that they will admit some molecules and
not others and so that gaseous absorption or theoretical calculation of the void space may not
correspond with what can be absorbed in the nature of a solid pigment.
Lead Acetate Absorption
A number of methods have been devised which are based on impregnation of the anodic
coating. In one such test, the specimen is anodized, dipped for 10 minutes in distilled water

in order to remove traces of electrolyte, dried at 110˚C for 30 minutes, and weighed.
Subsequently the specimen is immersed in lead acetate solution for 2 hours, again washed in
distilled water, dried and re-weighed. The gain in weight caused by impregnation with lead
acetate is determined and porosity is calculated.
Toluene Absorption (real density)
Immersing coatings in toluene and using Archimedes principle, to measure the density as
opposed to the apparent density, pore volume can be calculated. With unsealed coatings
formed on 99% Al in sulphuric acid, the density was found to be 2.96 which corresponded
to a pore volume of 15.8% while after sealing the real density became 2.65. Using an alloy
containing 4.5% Cu, 1.5% Mg, a pore volume of 47% was found. In an oxalic acid
electrolyte there was very little change in real density with rise in anodizing temperature and
only a slight increase in porosity, which is different from sulphuric acid where the role of
temperature is very important.
Dielectric constant
Making measurements of the apparent density and the apparent dielectric constant and then
assuming values for the real density and the real dielectric constant, which is assumed to be
2.95 and 8.70 respectively, the porosity of coatings formed in sulphuric acid can be
estimated. The porosity of oxide formed is 15% in sulphuric acid at 4 A/sq dm, 30˚C for 30
minutes is 24-26% while the porosity of film formed at 1˚C is 10-14%. When aluminium is
anodized at more than 20˚C a rapid increase in porosity is observed. The results can be
confirmed by measuring the amount of transformer oil that can be absorbed by unsealed
coating.
2.8.4 Adhesion
The adhesion of the oxide coating is normally much better than that of the electrodeposits
but the film tends to be weak vertically to the surface, i.e. , the film is apt to crack
transversely to the direction of rolling. When bent, the coating cracks in parallel lines but
will not strip off as electrodeposits do. Care must be taken, however not to leave anodized

work in the electrolyte when the current is switched off as this tends to loosen the film and
to decrease adhesion. In general, the adhesion of the film increases with increasing
temperature, acidity and the use of DC, as well as low current densities and longer treatment
times. As the coating is an integral part of the surface, no adhesion test is used for anodized
aluminium in normal circumstances.
2.8.5 Sealing Efficiency
In spite of the profound effects of sealing on properties and performance of anodic oxide
coatings, there has been a need for a good infallible test since many years to indicate how
efficiently sealing has been performed. The most frequently employed property has been the
increased resistance to chemical attack and this can be judged visibly by the appearance of
the coating, or by the extent to which it absorbs a dye-stuff. After attack, it can also be
assessed quantitatively by a photometric measurement of the depth of this dyeing, or finally
with more certainity by measuring the loss of weight. Recently impedance measurements
have become popular. The various methods that have been employed as discussed below.
Certain tests have been used in production control to assess the efficiency of sealing. It is
important to note that sealing tests (as well as those which measure electrical breakdown of
the sealed anodic oxide coating) may give misleading results on coatings that have been
stored for some time before testing. The changes that take place in the film are so profound
that after a period of 7 to 8 weeks of storage, it may be impossible to distinguish between
well and badly sealed films by some tests.
Dye Stain Test
These are commonly employed when one needs to know whether sealing has been
performed or omitted. At one time this was all that one needed to know and they were the
sole tests of sealing, but it is now known that failure to absorb dye represents only the initial
stages of the sealing operations and they are therefore regarded as resistance-to-marking
tests that may be specified where a surface is intended for mild indoor service.

The acid violet test is a commonly used test. Two separate drops of a dye solution is made
by dissolving 1gm C.I Acid Violet No. 34 or a corresponding dyestuff in 50ml distilled or
deionised water , are applied at room temperature to the anodized surface and allowed to
stand for 5 minutes. The test piece is then rinsed in running water and the test area swabbed
for 15 seconds with cotton wool in a detergent solution (1gm sodium dioctylsulphosuccinate
in distilled or deionised water, allowed to stand for 12 hours and made upto 100ml). The test
piece is rinsed and dried with filter paper without rubbing. The sample passes the test if no
mark remains.
Acidified Sulphite Test
In the sodium sulphite sealing test the specimen is immersed in a solution containing
10gm/lt anhydrous sodium sulphite adjusted to pH 3.75 with glacial acetic acid and then to
pH 2.5 with 5N sulphuric acid. The solution is kept at 90-98˚C and the specimens are
immersed for 30 minutes.
A numerical rating system is based on visual standards and ranges from 5 for a perfect
specimen with little or no change in appearance and no bloom to 0 for removal of the
coatings.
In practice, a rating of 3, which is accorded to a surface with a light bluish tinge and “light”
bloom, is usually considered acceptable, while a rating of 2, corresponding to a blue-grey
surface with “moderate” bloom is normally considered to be insufficient for acceptance. On
bright-anodized materials, there is little visual difference between ratings 5 and 0 (coating
removed) and a simple test with a flashlight and battery must be performed to check whether
any coating is left.
2.8.6 Corrosion
CASS Test
The letters CASS stands for “copper-accelerated acetic acid-salt spray test” which is
operated at a higher temperature than the acetic acid-salt spray test and includes a proportion

of cupric chloride which is perhaps the best additive for promoting rapid attack of exposed
aluminium.
The solution used for spraying is made by dissolving 50 + 0.02gm of cupric chloride in
water containing less than that 100ppm of total solids or having a conductivity of less than
0.002 s/m, and diluting to 1 litre. Glacial acetic acid is added to adjust the pH to 3.2 + 0.1.
The cabinet in which the test is carried out in frequently made from Perspex but may be
constructed from, or lined with a material resistant to corrosion, and containing supports to
hold the specimens so that the significant surfaces are at an angle of 15-30˚ to the vertical
and facing upwards. The operating temperature inside the cabinet is 50 + 1˚ and the test
solution is sprayed through nozzles at a rate that the spray collected over a horizontal area of
8000 sq. mm during 8 hours averages 1.5 + 0.5 ml/hr, taking care that no liquid falling from
the specimens or parts of the cabinet is collected. Baffles prevent direct impingement of the
spray onto the specimens. The air used to provide the spray is humidified by passing through
saturation tower containing water at a slightly higher temperature than the enclosure so that
the latter is maintained, about 57˚C usually being about the required level.
The test requires 8 hours to complete, after which the specimens are rinsed in clean running
water to remove any deposits of salts, and dried. The frequency of corrosion spots is
determined by counting the number of spots where there is evidence of basic metal
corrosion and a rating is allocated from 0-10 according to the frequency of pitting, where,
where 10 is the best result completely free from corrosion and 0 is the worst. The method of
counting is described by British Standard 3745: 1964. The significant area is divided into
5mm squares and the corrosion frequency is the ratio of squares with corrosion pits
expressed as a percentage of the total number of squares. The size of a corrosion spot is
defined, as the area of penetration and discolouration without penetrations are not counted.
Spots of more than 1 square amounted only once, but cracks are continued in all squares
entered.

The “initial ratings” are then assigned according to the following table.
Initial CASS rating Percentage of squares affected
10 0
9 0-0.5
8 0.5-1.0
7 1.0-1.5
6 1.5-2.0
5 2.0-3.0
4 3.0-6.0
3 6.0-12
2 12-25
1 >25
Table 2.7
For local concentrations of spots the 50 x 50 mm area with the greatest number is chosen,
the number of 5mm squares with spots in it is counted. The initial rating is reduced by 10 for
every 10 squares. Samples are unsuitable if more than 10 spots can be included in any two 5
mm squares, if any spots present are greater in area than 2.5 sq mm, or in the case of cracks
transversing more than one square the area affected in any one square exceeds 2.5 sq mm.
Acetic Acid-Salt Spray Test
This test is carried out in much the same manner as the CASS test and uses very similar
apparatus, but the test solution comprises of 50 + 5gm of sodium chloride in one litre of
demineralised water that is adjusted to pH of 3.2 + 0.1 with glacial acetic acid.
The solution is sprayed into the cabinet with clean air of controlled pressure and humidity,
taking care to see that it does not impinge directly onto the specimens under test, so that the

collected spray rate averaged over a period of atleast 8 hours on 8000 sq mm of horizontal
surface is 1.5 + 0.5 ml/hr measured with atleast 2 collector vessels. The solution must not be
recirculated. The temperature inside the cabinet is maintained at 35 + 2˚C and the specimens
are supported on inert supports as far as possible with the significant surfaces at an angle of
15-30˚ to the vertical.
After an exposure period of 24 hours the specimens are rinsed in cleaning running water and
dried for inspection. The present requirement is that there shall be no pitting to comply with
the test. The acetic acid-salt spray test is quick to reveal defects in the continuity of the
coating but is quiet a mild test for the coating itself, and its use on thicker coatings has been
largely superseded by the CASS test as described above.
2.9 TYPES OF ANODIZATION
2.9.1 Chromic Acid Anodizing (Type I)
The oldest anodizing process uses chromic acid. It is widely known as the Bengough-Stuart
process. In North America it is known as Type I because it is so designated by the MIL-A-
8625 standard, but it is also covered by AMS 2470 and MIL-A-8625 Type IB. In the UK it
is normally specified as Def Stan 03/24 and used in areas that are prone to come into contact
with propellants etc. There are also Boeing and Airbus standards. Chromic acid produces
thinner, 0.5 μm to 18 μm (0.00002" to 0.0007") more opaque films that are softer, ductile,
and to a degree self-healing. They are harder to dye and may be applied as a pre-treatment
before painting. The method of film formation is different from using sulphuric acid in that
the voltage is ramped up through the process cycle.
2.9.2 Sulphuric Acid Anodizing (Type II & III)
Sulphuric acid is the most widely used solution to produce anodized coating. coatings of
moderate thickness 1.8 μm to 25 μm (0.00007" to 0.001") are known as Type II in North
America, as named by MIL-A-8625, while coatings thicker than 25 μm (0.001") are known
as Type III, hard-coat, hard anodizing, or engineered anodizing. Very thin coatings similar
to those produced by chromic anodizing are known as Type IIB. Thick coatings require

more process control, and are produced in a refrigerated tank near the freezing point of
water with higher voltages than the thinner coatings. Hard anodizing can be made between
25 and 150 μm (0.001" to 0.006") thick. Anodizing thickness increases wear resistance,
corrosion resistance, ability to retain lubricants and PTFE coatings, and electrical and
thermal insulation.
2.9.3 Organic Acid Anodizing
Anodizing can produce yellowish integral colours without dyes if it is carried out in weak
acids with high voltages, high current densities, and strong refrigeration. Shades of colour
are restricted to a range which includes pale yellow, gold, deep bronze, brown, grey, and
black. Some advanced variations can produce a white coating with 80% reflectivity. The
shade of colour produced is sensitive to variations in the metallurgy of the underlying alloy
and cannot be reproduced consistently.
Anodization in some organic acids, for example malic acid, can enter a 'runaway' situation,
in which the current drives the acid to attack the aluminium far more aggressively than
normal, resulting in huge pits and scarring. Also, if the current or voltage is driven too high,
'burning' can set in; in this case the supplies act as if nearly shorted and large, uneven and
amorphous black regions develop. Integral colour anodizing is generally done with organic
acids, but the same effect has been produced in laboratory with very dilute sulphuric acid.
Integral colour anodizing was originally performed with oxalic acid, but sulphonated
aromatic compounds containing oxygen, particularly sulphosalicylic acid, have been more
common since the 1960s. Thicknesses up to 50μm can be achieved. Organic acid anodizing
is called Type IC by MIL-A-8625.
2.10 DYES AND COLOURS
Sulphuric acid anodising produces a porous surface which can easily accept dyes, the larger
the pore the better the dye will take. The range of colours is huge, however alloy choice will
limit some of the colours, and hard anodise film tend to be a dark grey colour which will
limit the choice even more. It is worth remembering that the surface finish of the component
before anodising and the amount of etching and de-smutting will have an effect on the
brightness of the coloured film at the end of the process, castings particular can be difficult

and lead to patchy finishes. In general the surface finish of the part will not improve by hard
anodising in fact the surface roughness is more likely to double during processing. Organic
pigments are the most cost effective but do not have the same colourfast stability and
performance in sunlight than that of the more expensive inorganic systems. They also have a
ceiling of around 150oc before the pigment starts to break down. More lightfast systems are
available with inorganic systems which are electrolytically deposited but they are
considerably more expensive to apply.
A third method of colouring is to introduce the colour within the anodic film; here organic
acids are mixed with the electrolyte. This process is limited to use on high volume bespoke
lines where the need to change to a clean or alternative colour is not required.
2.11 SEALING
Any recently anodised aluminium substrates will have a porous surface to the anodic film,
these pores at this stage absorb any colouring or lubricants. The larger the pore size the more
important it is to seal. The time taken to seal a part is roughly equivalent to the time taken to
anodise. If no additional requirements are specified the surface can be left and allowed to
seal naturally over time or can be sealed. Sealing can be accomplished in a number of ways,
the simplest being a hot de-min dip which converts the oxides into its hydrated form, the
resultant swelling fills the surface pores, this method does have a downside in that it reduces
the abrasion resistance slightly.
A second method is cold sealing. Here the pores are filled with a nickel solution which has
the added advantage of significantly reducing energy costs compared to hot sealing and does
not produce a seal smut. The downside being the part must be dry before handling.
The third method of seal is a hot sodium or potassium dichromate seal; this does leave a
slight yellowish colour on the part.
Additional materials can be added to a seal such as PTFE, typically parts are also printed
before sealing to stop any dye bleed out. Processes such as silkscreen, sublimation transfer
or digital printer are frequently used.

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