Material Sciences

1. CHAPTER 5
CORROSION & NON FERROUS
METAL

2. • Why does corrosion occur?
• What metals are most likely to corrode?
• How do temperature and environment affect
corrosion rate?
• How do we suppress corrosion?
CORROSION AND DEGRADATION

3.  Corrosion:--the destructive electrochemical attack of a
material.--Al Capone's ship, Sapona,off the coast of Bimini.
THE COST OF CORROSION

4. INTRODUCTION

5. INTRODUCTION

6. Corrosion?

7. Why Do Metals Corrode?

8. Corrosion can lead to failures in plant infrastructure
and machines which are usually costly to repair, costly
in terms of lost or contaminated product, in terms of
environmental damage, and possibly costly in terms
of human safety.
Why corrosion should be avoid?

9.  Corrosion: the chemical or electrochemical
reaction between a material, usually a metal, and its
environment that produces a deterioration of the
material and its properties.
Corrosion

10. Categories of Corrosion
1) Dry corrosion
2) Wet / Electrochemical corrosion

11. Dry corrosion

12.  Dry corrosion or oxidation occurs when oxygen in the
air reacts with metal without the presence of a liquid.
 Dry corrosion is surely the most visible of all corrosion
processes, e.g. rusty bridges, flag poles, buildings and
outdoor monuments.
Dry corrosion

13.  Typically, dry corrosion is not as detrimental as wet
corrosion, but it is very sensitive to temperature. If
you hold a piece of clean iron in a flame, you will soon
see the formation of an oxide layer!
 The differences in the rate of dry corrosion vary from
metal to metal as a result of the mechanisms
involved.
Dry corrosion

14.  In dry corrosion the oxygen
has to be able to make contact
with the metal surface.
 Initially this is not a problem,
but as soon as corrosion starts
to occur the oxide layer, that
forms on the metal surface,
will limit the amount of oxygen
that can further react with the
metal.
Dry corrosion

15.  Uniform corrosion is characterized by corrosive attack
proceeding evenly over the entire surface area, or a large
fraction of the total area. General thinning takes place until
failure.
 If surface corrosion is permitted to continue, the surface
may become rough and surface corrosion can lead to more
serious types of corrosion.
Uniform Corrosion

16.  The formation of an oxide layer on the surface of a
metal will, in some instances, lead to a reduction in
the rate of corrosion.
 When a metal oxidises and forms an outer layer, this
layer can remain on the surface of the metal and limit
further corrosion by inhibiting the ability of oxygen or
other corrodents to reach the metal surface. This is
known as passivation.
Formation of an Oxide Layer

17.  Passivation of a metal surface
through the formation of an oxide
layer is found in many common
metals and alloys.
 Aluminium naturally forms a
protective oxide layer (or scale)
which slows down further
oxidation and corrosion.
 Stainless steel has chromium
added to it, which forms a very
protective oxide layer that
prevents further corrosion.
Passivation

18.  Not all oxide layers that form
on metals are protective.
 If the oxide does not form a
continuous layer on the
surface of the metal, it will not
be able to reduce the amount
of oxygen reaching the metal
surface.
Cont.

19.  Also known galvanic corrosion.
 It will occur if an “electrochemical cell” is produced.
 An electrochemical cell consists of an Anode, a
Cathode, a Connection, and an Electrolyte.
Wet / Electrochemical corrosion

20.  The anode is the metal that corrodes. It undergoes
oxidation and therefore loses electrons.
 The cathode can be a metal or any other conducting
material. It undergoes reduction and therefore gains
electrons. The reaction that occurs at the cathode is
not necessarily related to the material that it is made
from.
Wet / Electrochemical corrosion

21.  The connection is necessary for the electrons to
travel between the anode and cathode and can be
either physical direct contact or some form of wire.
 An electrolyte must also be present to allow for
migration of ions between the cathode and anode
and participate in the formation of corrosion
products.
Wet / Electrochemical corrosion

22.  Wet corrosion therefore involves an oxidation reaction at
the anode and a reduction reaction at the cathode.
 In the oxidation reaction metals give up electrons to
become positively charged ions.
Reaction at anode:
Wet / Electrochemical corrosion

23.  The electrons generated from the metal are transferred to
another material. This is the reduction reaction and occurs
at the cathode.
Reaction at cathode:
Wet / Electrochemical corrosion

24.  For the electrons that are formed at the anode to
move to the cathode there must be a path that they
can follow. This usually means a physical contact
between the anode and the cathode.
 There must also be a way for the ions produced to
come together so they can react to form the
corrosion products. This usually is provided by a liquid
(water, electrolyte) or moist conductor.
The Electrochemical Cell

25.  The essential components are:
1) Anode
2) Cathode
3) Connection
4) Electrolyte
 All of these components need to be present for corrosion to
occur. If you remove even one of them then there will be no
corrosion. This is the theory behind corrosion prevention.
The Electrochemical Cell

26.  If you have two metals in contact, with an electrolyte
present, how do you determine which metal will
corrode?
 Eg. Zn and Cu
The Electrochemical Cell

28.  A metal higher in the series has a higher corrosion
resistance than one below it in the series.
 From the table we can see that gold (Au) has the
highest corrosion resistance, and that if we were to
join a steel pipe (Fe) with a copper fitting (Cu) then it
is the steel that would corrode because it is below
copper in the series.

32.  Galvanic corrosion can cause unwanted accelerated
corrosion when it is not considered during design or
construction, however it can also be used to advantage.
 When considering which metal will corrode we can look at
the galvanic series. Metals closer to one another generally
do not have a strong effect on each other, but the further
apart two metals are, the stronger the corroding effect on
the one higher in the list.
 Anodes and cathodes arise in many ways. As well as
connection between two different metals, a plain metal
surface can have anodic and cathodic areas. For example:
- Grain boundaries can be anodic with respect to grain
interiors.
- Cold worked regions are anodic to regions not cold
worked.
Cont.

33. 1) Varying stress
2) Varying oxygen concentration
3) Crevice corrosion
4) Intergranular corrosion / Grain
interface
Forms of corrosion

34.  Occurs in similar metals with varying
stress concentration area.
 Anode: high stress
 Cathode: low stress
Varying stress corrosion

35.  Stress corrosion is another form of corrosion that is
important to many fields including civil structures.
 Stress-corrosion occurs when a material exists in a
relatively inert environment but corrodes due to an applied
stress. The stress may be externally applied or residual.
 This form of corrosion is particularly dangerous because it
may not occur under a particular set of conditions until
there is an applied stress. The corrosion is not clearly
visible prior to fracture and can result in catastrophic
failure.
Varying stress corrosion

36.  Many alloys can experience stress corrosion, and the
applied stress may also be due to a residual stress in
the material. An example of a residual stress could be
a stress remaining in a material after forming, or a
stress due to welding.
 Stress corrosion cracking will usually cause the
material to fail in a brittle manner, which can have
grave consequences as there is usually little or no
warning before the failure occurs.
Varying stress corrosion

37.  Stress corrosion is a form of galvanic corrosion,
where stressed areas of the material are anodic to the
unstressed areas of the material.
 Practically the best way to control stress corrosion
cracking is to limit or reduce the stresses a material is
under while it is in a corrosive atmosphere.
Varying stress corrosion

38. Varying stress corrosion

39. Varying oxygen concentration

40.  Is localized corrosion that has the apperearance of
cavities (pits) and occurs on freely exposed surfaces.
 Cathode: large passive area surrounding the pits.
 Anode: area of the pits (since the pit is a cavity, it is
difficult for the solution to leave the pit.)
Pitting

41. Pitting

42.  Is localized corrosion that occurs at mating or closely
fitting surfaces where easy access to the bulk of
corrosive environment is hindered.
 May occur at mating surfaces, assemblies of metals
and nonmetals and deposits on the surfaces of
metals.
Crevice corrosion

43.  Crevice corrosion occurs when two components are joined
close together to form a crevice. Corrosion occurs as the
crevice accumulates water.
 If the crevice is small enough a differential oxygen
concentration in the water can form. When this happens
the base of the crevice becomes anodic to the upper
region.
 Crevice corrosion often occurs under bolts and rivet heads
as well as in shielded areas and under dirt or sand deposits.
Crevice corrosion

44. Crevice corrosion

45. Crevice corrosion

46.  Is a microstructural corrosion that occurs along the grain
boundaries of austenitic and ferritic stainless steels and
chromium containing nickel alloys.
 The heat of fabricating or processing operation can cause
the chromium and carbon in stainless steels and chromium
containing nickel alloys to combine forming, forming
chromium carbide.
 The grain boundaries become depleted in chromium and
susceptible to corrosion in specific environments.
Intergranular corrosion

47. Intergranular corrosion

48.  Intergranular corrosion occurs when the grain boundaries in
a metal form an anode and the interior of the grain acts as a
cathode. In serious cases this can lead to the grains falling
apart.
 This type of corrosion is a particular problem in stainless
steels, however it can also occur in other metals.
Intergranular corrosion

49.  In stainless steels the problem occurs
after the metal is heated to between
425°C and 870°C. During the heating, the
chromium in the stainless steel reacts
with carbon in the steel and forms
particles of chromium carbide at the
grain boundaries. The regions near the
grain boundaries become depleted in
chromium.
Intergranular corrosion

50.  This means that the regions around the grain boundaries are
no longer protected by the chromium passivation, and
therefore corrode intergranularly.
Intergranular corrosion

51.  a) Cathode and anode protection
 b) Material selection
 c) Coatings (metals, organic and non
organic)
 d) Design
Corrosion Prevention Techniques

52.  Introduced by Humphrey Davey (1824).
 Cathodic Protection is an electrochemical means of
corrosion control in which the oxidation reaction in a
galvanic cell is concentrated at the anode and
suppresses corrosion of the cathode in the same cell.
1) Cathodic Protection

53.  Figure shows a simple cathodic protection system. The steel
pipeline is cathodically protected by its connection to a
sacrificial magnesium anode buried in the same soil
electrolyte.
Cathodic Protection

54. Concept : supply more electrons to
the system or suppresses metal
dissolution.
Two methods : external power
source and galvanic coupling.
Cathodic Protection

55.  By impressing a direct current between an inert
anode and the structure to be protected. Since
electrons flow to the structure, it is protected from
becoming the source of electrons (anode). In
impressed current systems, the anode is buried and a
low voltage DC current is impressed between the
anode and the cathode.
Cathodic Protection

56.  Negative terminal to
object.
 Positive terminal to
carbon electrode.
 Electrons will be
supplied to the tank.
 This will prevent
corrosion.
Cathodic Protection
a) By impression electric current from an external
power source

57.  By coupling a given structure (say Fe) with a more
active metal such as zinc or magnesium. This
produces a galvanic cell in which the active metal
works as an anode and provides a flux of electrons to
the structure, which then becomes the cathode. The
cathode is protected and the anode progressively
gets destroyed, and is hence, called a sacrificial
anode.
Cathodic Protection

58. Concept : sacrificial anode
 Example:
Magnesium anode is connected to steel
to prevent corrosion.
Cathodic Protection
a) By appropriate galvanic coupling

59.  Sacrificial anode continuously “consumed” by corrosion
and needs replacement
 Example:
 Zinc: used broadly,e.g. galvanized zinc coating is a common
distributed sacrificial anode for steel and magnesium: used
for underground pipeline protection, i.e. in soil and other
low conductivity environments.
Sacrificial anode method

60. Cathodic Protection

61.  Predict the general corrosion rate and
susceptibility to localized attack of candidate
materials.
 The scope of selection is not limited to metals.
2) Materials Selection

62.  Choose material that is suitable with the
environment. For example stainless steel is
sensitive in nitric acid environment.
 Choose metals closer to one another
according to galvanic series.
 Choose material suitable with the
application.
Material Selection

63.  Protective coatings are a simple way to reduce corrosion, by
limiting the exposure of the metal to a corrosive
environment.
 Paint is a very common protective coating, but tar, pitch,
bitumen and plastics are also used.
3) Protective Coatings

64.  An important consideration for protective coatings is to
ensure the coating is well adhered to the metal, and that it
remains intact or is regularly repaired/recoated.
 A further form of protective coating is to plate a coat of
another metal onto the surface of the metal you wish to
protect. One type of this coating is known as galvanising,
where zinc is plated onto iron or steel. In the case of
galvanising, the zinc acts as an anode and corrodes
preferentially to the iron or steel.
Protective Coatings

65. Coating
Metal coating:
 Example: Plating,
‘cladding’, ‘hot dipping’
dan ‘flame spraying’.
 Zinc, nickel, tin, cadmium,
gold, copper and
platinum are metals used
for plating.

66. Organic and non organic:
 Coating applied to metals to protect
them from environment.
 Example of organic coating: paint,
lacquers, varnish, tar.
 Example of non organic coating: enamel,
ceramic
Coating

67.  The final, and most efficient way to prevent corrosion is to
design your component or process to eliminate or reduce the
possibility of corrosion. Some of the things to avoid are:
 Design principles that are used to avoid corrosion include
prevention of crevices by using continuous welds, avoiding
locations where corrosives can concentrate.
 Joins which require fasteners (e.g. bolts) usually create
crevices. Welding is a better alternative, however, the weld
shape should be monitored to ensure no crevices are present.
 Avoid varying stress area such as sharp corner.
 Avoid using metals further apart with one another in galvanic
series.
4) Design

68.  You can alloy metals to change their corrosion
behaviour. Stainless steel is a very good example of
this. Chromium is added to the steel to enhance its
corrosion resistant properties. This works because
the chromium in the metal forms a passivating layer,
similar to that produced by aluminium.
 You can also add small amounts of another metal to
an alloy, which can shift its position in the galvanic
series making it more cathodic than it was previously.
5) Alloying

69. NON FERROUS METAL

70. ENGINEERING MATERIAL

71.  Nonferrous metals and their alloys do not contain iron
as a principle ingredient.
 These are used in industry because of the following
characteristics:
 a)Ease of fabrication
 b)Resistance to corrosion
 c)Good electrical and thermal conductivity
 d)Light Weight
DEFINITION

72.  1)Aluminum
 2)Copper
 3)Zinc
TYPES OF NON FERROUS METAL

73.  This metal is highly used as it is good conductor of electricity. It is soft, malleable and
ductile material with a reddish brown appearance. Its specific gravity is 8.9 g/cm3 and
melting point is at 1083 °C.
 It is largely used in making electrical cables and wires for electrical machinery and
appliances. Copper alloys are categorized into two categories: copper zinc alloys and
copper tin alloys. Brass is usually used for costume jewellery whereas bronze is used as
bushings.
 The most widely used copper zinc alloy is brass. It has a greater strength than
that of copper but has low thermal and electrical conductivity. Brass is used for tube
manufacturing, plumbing fittings and musical instruments.
 The alloys of copper and tin are termed as bronzes. The useful range of
compositions is 75-95% copper and 5-25% tin. This alloy is comparatively hard, resists
surface wear and can be shaped into wires, rods and sheets easily. Bronze is used for
making gears, air pumps, bushings and condenser bolts.
Copper (CU)

74.  It is a white metal produced by electrical process from alumina oxide which is
prepared from clay material called bauxite. It is a lightweight material having a specific
gravity of 2.7 g/cm3 and a melting point of 660°C. In its pure state, aluminium is weak
and soft but becomes hard and rigid once mixed with other materials (aluminium
alloys). It has good electrical conductivity and good resistance to corrosion. Due to its
light weight, it is used heavily in the aerospace industry. The commonly used
aluminium alloys duralumin and Y-alloy.
 Application:
 • Beverage can
 • Aluminium foil
 • Window cladding
 • Roof frame
 • Stairs
 • Bottle cap
Aluminium (Al)

75.  Zinc is a bluish white metal which is in pure state. It has a specific
gravity of 7.1 g/cm3 and its melting point is 420 °C. It is not very
malleable and ductile at room temperature. It offers high
resistance to atmospheric corrosion. It is used for covering steel
sheets to form galvanized iron due to its high resistance to
atmospheric corrosion. Zinc has its alloys too. The usual alloying
elements for zinc are aluminium and copper. The aluminium
improves mechanical properties and also reduces the tendency
of zinc to dissolve iron. Copper increases the tensile strength,
hardness and ductility. The alloys from zinc are used for washing
machines, oil burners, refrigerators, radios and television sets.
Zinc (Zn)