Corrosion
Basics
Lecture 1
1
What is Corrosion
• With a few exceptions, most of metals are unstable and
they react with surrounding environment.
• As the surrounding environment gives these metals,
opportunity to combine chemically and return to their
original stable level as they were found in nature ( e.g.
oxide, hydroxide, or sulphide etc.)
• Corrosion is a natural process which converts refined
active metal into a more chemically stable material.
• It is the gradual destruction of materials by chemical or
electrochemical reaction.
What is Corrosion
• The most common well known example is Iron rusting. Where it
forms oxides / hydroxide by reacting with oxygen and water.
• This reaction damages iron surface . The product is distinctive
red/ orange colour.
• Other type of corrosions take place due to corrosive gases or
liquid available in environment or dissolved in water like Co2,
H2S, NH3, organic acids etc.
• In absolute terms no metal is corrosion resistant. These may be
having an insignificant reaction, undetectable with eye.
• Stainless steel, Monel , Inconel also can get corroded in
specific environments.
3
Consequences of Corrosion
1. Corrosion degrades properties of materials, like strength,
appearance, permeability to liquids etc.
2. Degradation makes it weak due to a loss of material and
reduction in metal strength e.g. Hydrogen embrittlement,
Sulphide stress cracking, Chloride attacks etc.
3. This may result in plant safety , loss of life, financial losses etc.
Some examples :
– Reduced value of product due to surface marks or mixture
with rust contamination.
– Leakage from vessels and pipes making plant unsafe.
– Loss of property of component e.g. roller bearing, heat
transfer across corroded tubes.
– Damage to valves, pumps, boiler or pressure vessel.
4
Consequences of
Corrosion
Few examples:
1. A tube to tube sheet joint may
leak and damage the exchanger
performance.
2. A tank or pressure vessel may
leak.
3. Distillation trays or tower
internals may fail, and impact
column performance.
Finding out Corrosion rate and corrosion
allowance
• Corrosion rates can be expressed in a variety of ways
such as :
– Percent weight loss,
– Milligrams per square centimetre per day
– Milligrams per square inch per hour.
• However above values do not express corrosion
resistance of any particular metal.
• An engineer’s out looks is always the equipment or
plant design life.
• The most desirable way of expressing corrosion rates is
mils per year (mpy) .
6
Finding out Corrosion rate and corrosion
allowance
• Corrosion rates are established by placing sample
coupon in the environment for a fixed time and checking
loss of weight with respect to time.
• Accordingly the additional thickness is added to
component based on the design life of item.
• This additional thickness which takes care of future
reduction of thickness is called Corrosion Allowance.
Applied to all process wet areas.
• Normally in process industry the designed life of an
equipment is taken as 20 years.
7
Finding out Corrosion rate and corrosion
allowance
• Periodic inspection and thickness measurements of
equipment is carried out to decide balance life.
• Normally for process equipment made of CS a corrosion
allowance of 3mm is a common practice.
• It may go as high as 4.5 or 6mm for CS.
• Some times a corrosion allowance is also specified on
stainless or other corrosion resistant steels (1 or 2 mm)
• Trays and tower internals of CS are normally given ½ the CA
provided for vessels. Applied to both side wetted surface.
• Normally no corrosion allowance is specified on exchanger
tubes.
8
Classification of Corrosion
Corrosion can be classified in different ways, such as
I. Chemical and electrochemical
II. High temperature corrosion
III. Wet corrosion
IV. Dry corrosion.
Dry corrosion occurs in the absence of aqueous
environment, usually in the presence of corrosive
gases / vapours mainly at high temperatures.
9
Acid theory of corrosion
• Corrosion is a complex electrochemical phenomenon.
• It starts at a particular spot on the surface of metal.
• The electrons released at this anodic spot move through
the metal and combine with OH to form Hydroxides.
• Rusting of iron makes hydrated oxide, Fe(OH)3, FeO(OH),
Fe2O3.H2O. The process needs presence of water, oxygen
and an electrolyte.
• Water is normally acidic which is believed to be available
from carbonic acid (H2CO3) formed due to dissolved CO2 in
water in moist air condition of atmosphere.
Chemical formula for rust
• The chemical formula for rust is Fe2O3 and is commonly known
as ferric oxide or iron oxide.
• The final product is below
4Fe + 3O2 + 6H2O → 4Fe(OH)3.
• The rusting process requires both oxygen and water. The
process is accelerated by presence of acids, strains in the iron
or rust itself.
• The loose porous rust Fe(OH)3 slowly transforms into
Fe2O3.H2O , which is the familiar red-brown stuff
• Air with RH over 50% provides the necessary amount of water
good to initiate corrosion. RH above 80% corrosion is worse.
11
Surface Pitting Corrosion
Pitting corrosion
• Pitting is among the most common and damaging forms of
corrosion in passivated alloys.
• In the worst case, almost all of the surface will remain protected,
but tiny local flaws degrade the oxide film.
• Corrosion at these points is amplified, and forms a small pit.
• While the corrosion pits only nucleate under fairly extreme
circumstances, they can continue to grow even when conditions
return to normal.
• Since the interior of a pit is naturally deprived of oxygen and locally
the pH decreases to very low values and the corrosion rate
increases.
13
Pitting corrosion
• A thin film with a small hole on the surface can hide a thumb sized
pit below. This corrosion is often difficult to detect due to the fact
that it is usually relatively small in size at the surface.
• Pitting results when a small hole, or cavity, forms in the metal,
usually as a result of depassivation of a small area.
• This area becomes anodic, while part of the remaining metal
becomes cathodic, producing a localized galvanic reaction.
• The deterioration of this small area penetrates the metal and can
lead to failure.
14
Pitting corrosion
▪ When corrosion starts on a metal
surface at certain small spot becomes
an initiation point.
▪ The pit becomes deeper so its bottom
has low oxygen which makes it more
anodic.
▪ The out side pit surface area has higher
oxygen concentration, hence become
cathodic.
▪ Pitting is more severe in sea water. It
can take place even in Stainless Steel. A pitted flange face
Dry corrosion due to gases
1. Dry corrosion or oxidation occurs when oxygen in
the air reacts with metal without the presence of a
water or liquid.
2. Dry corrosion is sensitive to high temperature.
3. Dry corrosion is classified into three types:
a. Oxidation,
b. Molten-salt corrosion
c. Hydrogen attack.
Dry corrosion due to gases
1. Gases responsible for dry corrosion are:
• H2
• SO2
• CO2
• Cl2
• etc.
2. Here corrosive effect depends mainly on the chemical affinity
between the metal and the gas involved.
3. Degree of attack depends on type of protective or non
protective films formed on the surface.
4. If volume of corrosion film formed is strong and non-porous,
it does not allow further gas penetration.
e.g. Ag + Cl2 → 2AgCl (protective film)
Dry corrosion due to gases
5. If lower thickness of protective layer forms
pores/cracks and allow the penetration of corrosive
gases ,
6. It leads to further corrosion of the underlying metal
Example :
H2 gas at high temperature reacts with carbon at
boundary on the layer of Iron structure converting
it to Methane.
This is called Hydrogen attack, where metal develops
cracks and blistering.
Passivation of surface
19
Passivity
1. Passivity occurs when an oxide layer forms a
continuous film on a metal surface which prevents
further oxidation (corrosion).
2. Metals which build layer of metal oxide on surface,
exhibit passivity.
3. The metal oxide acts as a barrier by separating the
metal surface from its environment and prevents
corrosion.
20
Passivity
4. In order to provide passivity, this oxide layer must be
both stable and firm.
5. Products of corrosion must be strong and insoluble in
the environment.
6. Metals like Zirconium, Chromium, Aluminium and
stainless steel form oxide films when exposed to the
atmosphere or to water.
7. The film is so thin that it's invisible to the naked eye but
very effective in giving these metals passivity and thus
corrosion resistance.
21
Why Stainless steel does not
corrode
22
Why Stainless steel does not corrode
• Stainless steel contains a minimum Cr content of 10.5%. The
chromium reacts with the oxygen in the air and forms a
protective layer that makes stainless steel highly resistant to
corrosion and rust.
• Even with these impressive features, stainless steel can also
rust .
• Some types of stainless steel are more prone to corrosion than
others, depending on the chromium content.
• The higher the chromium content, the less likely the metal will
rust.
• But, over time if not maintained correctly or in high Chloride
environment it can also corrode.
Other type of Corrosion (next lectures)
1. Crevice corrosion
2. Hydrogen attack
3. Inter-granular corrosion
4. Chloride attack
5. Sulphide Stress corrosion
cracking
6. Sulphur attack
7. Mercury attack
8. Molten salt corrosion
9. Caustic corrosion
10. Microbes corrosion
24
Thank you
25
