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1. Corrosion
Lecture 4
Types of Corrosion: Part 1
Dr. Omar Nashed

2. Uniform Corrosion:
The main cases of metals suffering generalized corrosion are:
 Carbon steel when exposed to the atmosphere, or immersed in neutral or
acidic solutions, in carbonated concrete, in soil and sea water
 Aluminium in low and high pH solutions
 Stainless steel in acidic solutions.
 Carbon steel in CO2-containing environments (sweet corrosion)
 Zinc in acidic solutions
 Lead throughout the entire pH range when insoluble corrosion products,
such as carbonates or sulphates, do not form.
Uniform corrosion, also called generalized corrosion, is a type of corrosion that affects the entire exposed surface of
active metals in contact with an aggressive environment because anodic and cathodic zones coincide.

3. Uniform corrosion can be prevented or reduced by
 Proper material and coating
 Inhibitors
 Cathodic protection
• Corrosion rate varies widely from very low values, some μm/y, to tens of mm/y depending on the metal
and the aggressive environment
• Although generalized corrosion causes the greatest amount of corrosion products, it is in general
less insidious than localized corrosion because corrosion rate (i.e., thickness loss rate) is often low
and predictable, with good accuracy, and is easily monitored during operating; nevertheless,
attention should be paid to its consequences.

4. Galvanic Corrosion (two metal corrosion):
Galvanic corrosion, also called bimetallic corrosion, occurs when two
metals immersed in an electrolyte are in electrical contact and have a
different practical nobility
The less noble metal, M, with more negative potential, works as anode
and its corrosion rate is accelerated by the coupling. The noble metal, N,
with more positive potential, behaves as cathode, hence its corrosion rate
decreases up to a halt.
Materials with electronic conductivity can work as cathode, such as
magnetite that forms near welds, magnetite in boilers, and also
graphite and sulphides in industrial processes.

5. Galvanic coupling depends on the following main four factors:
1. Practical Nobility:
The driving voltage set by the galvanic coupling is the difference between the two free corrosion potentials in the
environment, which depends on nature, composition and structure of the metal, presence of oxide films or other
compounds on metal surface, composition, temperature and oxidizing power of the electrolyte. The rank of
practical nobility of metals depends on the environment to which the coupling is exposed: aerated, stagnant or
turbulence conditions.
2. Effect of area:
Another important factor in galvanic corrosion is the area effect or the ratio of cathodic to anodic area. The larger
the cathode compared with the anode, the more oxygen reduction, or other cathodic reaction, can occur and,
hence, the greater the galvanic current.

6. 3. Electrolyte Resistivity
Corrosion rate of a galvanic coupling depends strongly on electrolyte resistivity; assuming the same driving
voltage, corrosion rate decreases as resistivity increases; for instance, in high resistivity electrolytes like fresh
water galvanic effects are often negligible, whilst in high conductivity ones like seawater corrosion rate is at least
two orders of magnitude greater.
4. Geometry of the Domain
Domain geometry is another important factor influencing galvanic corrosion: for example in small diameter tubes,
as it happens on atmospherically exposed surfaces, the extension of affected (working) areas is greatly reduced;

7. Prevention
Prevention of galvanic corrosion is achieved by:
• Avoiding dangerous couplings with a choice of metals close in
scale of practical nobility.
• Separating coupled metals, for example, by insulating flanges
• Taking care that the anodic-to-cathodic area ratio is not
unfavourable (Sa _ Sc)
• Applying paints on both surfaces or only on the cathodic one.
Avoid painting of anodic metal, only.
• Applying cathodic protection

8. Crevice corrosion
The presence of cracks, gaps, screens or deposits on a metal surface can
give rise to a localized corrosion form, called crevice corrosion or
interstitial corrosion and corrosion under deposit. Crevice corrosion is a
concern in many environments for active-passive alloys as stainless
steels, nickel alloys and titanium.
Typically, stainless steels suffer from crevice corrosion in seawater or in
chloride-containing solutions, present in most of industrial plants as in
chemical, petrochemical, pharmaceutical, food processing, as well as in
biomedical, nuclear and civil engineering.

9. Mechanism:
The mechanism of crevice corrosion of stainless steels in chloride-containing solutions follows several stages:
First stage called incubation or oxygen depletion stage, during which oxygen inside the gap is consumed through the
corrosion reactions occurring on passive stainless steel, namely, oxygen reduction and passive film growth, as follows:
𝑂2 + 2𝐻2𝑂 + 4𝑒−
→ 4𝑂𝐻−
𝑥𝑀 + 𝑦𝐻2𝑂 → 𝑀𝑥𝑂𝑦 + 2𝑦𝐻+
+ 2𝑦𝑒−
The second stage starts once oxygen is completely depleted in the crevice. The elimination of oxygen inside the crevice
brings stainless steel in active condition. During this stage, two important processes take place: inside the crevice, metal
ions concentration can exceed 1 M, so hydrolysis reactions take place:
𝑀 → 𝑀𝑥+
+ 𝑥𝑒−
𝑦𝑀𝑥+ + 𝑥𝐻2𝑂 → 𝑀𝑦(𝑂𝐻)𝑥 + 𝑦𝑥𝑒𝐻+
The metal salts hydrolize in water into an insoluble hydroxide and a free acid. The chloride and hydrogen ions accelerate
the metal dissolution rate.

10. Factors
• Nature, composition and structure of metal: In the case of stainless steels, the increase in chromium content and even
more the presence of molybdenum and nitrogen is beneficial in promoting a stable passive film.
• Environmental factors favouring crevice are chloride content, acidity, temperature, potential and bacterial activity: as each
of these factors increase, crevice likelihood increases. For example, incubation time for stainless steel AISI 316 grade is a
few weeks in seawater (pH around 8, chloride content 20 g/L) and increases to several months in industrial waters (same
pH and chloride content below 1 g/L). However, with same chloride content, if pH drops from 8 to 5 or if the solution is
contaminated by bacteria, incubation time decreases by one order of magnitude.
• Deposits that may produce crevice corrosion are sand dirt, corrosion products, and other solids. The deposit acts as a
shield and created a stagnant condition.
• Contact between metal and non-metallic surfaces can cause crevice corrosion as in the case of gasket.
• Crevice size: a crevice must be wide enough to permit liquid entry but sufficiently narrow to maintain a stagnant zone.
The key parameter of crevice corrosion is the critical crevice gap size (or critical interstice size), defined as the minimum
that allows the aggressive environment to enter the interstice but impedes the diffusion of oxygen. Critical gap size is
between 0.1 μm and 0.1 mm, depending on metal composition.

11. Prevention:
 The prevention of crevice corrosion has to be carried out, primarily, in design and construction phases
in order to avoid crevices as cracks, gaps and deposits.
o Use welded butt join instead of riveted or bolted joints
o Close crevices in existing lap joints by continuous welding
o Design vessels for complete drainage avoid stagnant area
 Inspect equipment and remove deposits frequently.
When crevice conditions are inevitable, its prevention follows two ways:
• Selection of resisting material.
• Cathodic protection (in the case of stainless steels, iron anodes are often used).

12. Pitting is a form of severe localized corrosion which produces a deep
penetrating attack, called the pit, with diameter mostly less than a few
millimetres. These pits or holes occurring most often isolated in a number
varying from a few to several hundred per square meter.
• The term pitting mostly used for the typical localized attack occurred on
active-passive metals in oxidizing chloride containing environments.
• Pitting is one of the most destructive and insidious forms of corrosion.
• It causes equipment to fail because of perforation with only a small
percent weight loss of the entire structure.
• It is often difficult to detect pits because of their small size and
because the pits are often covered with corrosion products.
• It is difficult to measure quantitatively and compare the extent of
pitting because of the varying depths and numbers of pits that may
occur under identical conditions.
• Pitting corrosion required months and years to perforate a metal
section, therefore it is difficult to be tested in laboratory.
Pitting
Figure: Typical forms of pitting corrosion

13. Pitting corrosion follows two distinct stages: pit initiation and pit
propagation.
• The initiation stage is the time required for the local breakdown of passive
film, which is produced by the action of specific chemical species present
in the environment, such as chloride ions (Cl−).
• This step might last a few weeks up to several months, depending on
metal and operating conditions.
Pitting propagation is the result of a macrocell mechanism: the anodic area is
inside the pit while the cathodic zone is the external surrounding passive
area, where oxygen reduction is the most common cathodic process.
Penetration rate is high because cathodic to anodic area ratio is as high as
100.
Once a stable pit initiates, a macrocell current starts flowing, as shown in
Figure, between the anode (i.e., where passive film has broken down and
metal dissolves) and the surrounding passive zones acting as cathode. Inside
pit, the solution becomes gradually more aggressive as hydrolysis reaction of
metal ions proceeds, hence acidification increases and pH drops to values
close to 3–4:
𝑀𝑧+ + 𝑧𝐻2𝑂 → 𝑀(𝑂𝐻)𝑥𝑧 + 𝑧𝐻+
Conversely, on cathodic zones outside the pit, pH increases, then passive
Pitting Mechanism

14. Prevention:
Only shallow pits, less than 0.3 mm deep, can be recovered by washing with alkaline, chloride-free solutions
(for example sodium carbonate).
To prevent pitting on susceptible metals, two strategies are followed:
• Selection of resistant metals.
• Application of cathodic protection.

15. Thank you
Q & A