2. CORROSION
• Corrosion is the deterioration of a metal as a result of
chemical reactions between it and the surrounding
environment.
• It is an oxidation process. It causes loss of metal.
• The responsible factors for the corrosion of a metal are the
composition of the metal, the environmental chemicals,
temperature and the design.
• Corrosion comes in many different forms and can be
classified by the cause of the chemical deterioration of a
metal.
2
3. Example: Formation of rust on
the surface of iron
Formation of green film on the
surface of copper
3
4. FORMS OF CORROSION
• Uniform or General attack
• Galvanic or Two-metal
• Crevice
• Pitting
• Intergranular
• Selective leaching or Parting
• Erosion corrosion
• Stress corrosion
• Hydrogen damage
4
5. GENERAL ATTACK CORROSION
• It is also known as uniform attack
corrosion, general attack corrosion is
the most common type of corrosion
and is caused by a chemical or
electrochemical reaction that results
in the deterioration of the entire
exposed surface of a metal.
• Ultimately, the metal deteriorates to
the point of failure.
5
6. Mechanism
• The anodic reaction in the corrosion process is always the
oxidation reaction:
M = M+ + e (1)
• In acidic environments, i.e., pH<7, the cathodic process is
mainly the reduction of hydrogen ions:
2H+ + 2e = H2 (2)
• In alkaline or neutral environment, i.e., pH=7 or pH>7, reduction
of dissolved oxygen is the predominant cathodic process that
causes uniform corrosion:
O2 + 2 H2O + 4e = 4OH- (3)
• With uniform distribution of cathodic reactants over the entire
exposed metal surface, reactions (2) and/or (3) take place in a
"uniform" manner and there is no preferential site or location
for cathodic or anodic reaction.
6
7. • General attack corrosion accounts for the greatest amount of
metal destruction by corrosion, but is considered as a safe
form of corrosion, due to the fact that it is predictable,
manageable and often preventable.
• Uniform attack can be prevented by
a. Use thicker materials for corrosion allowance
b. Use paints or metallic coatings such as plating,
galvanizing or anodizing
c. Use Corrosion inhibitors or modifying the environment
d. Cathodic protection (Sacrificial Anode or Impressed
Current -ICCP) and Anodic Protection
• Most of the other forms of corrosion are insidious in nature
and are considerably more difficult.
7
8. GALVANIC CORROSION
• Galvanic corrosion (also called bimetallic corrosion) is
an electrochemical process in which one metal corrodes
preferentially to another when both metals are in electrical
contact, in the presence of an electrolyte.
• This same galvanic reaction is exploited in primary batteries to
generate an electrical voltage.
• A galvanic couple forms between the two metals, where one
metal becomes the anode and the other the cathode.
Three conditions must exist for galvanic corrosion to occurs:
1. Electrochemically dissimilar metals must be present
2. The metals must be in electrical contact
3. The metals must be exposed to an electrolyte
8
9. Mechanism
• Different metals and alloys have different
electrochemical potentials (or corrosion potentials) in the same
electrolyte.
• When the corrosion potentials of various metals and alloys are
measured in a common electrolyte (e.g. natural seawater) and
are listed in an orderly manner (descending or ascending) in a
tabular form, a Galvanic Series is created.
• It should be emphasized that the corrosion potentials must be
measured for all metals and alloys in the same electrolyte
under the same environmental conditions (temperature, pH,
flow rate etc.), otherwise, the potentials are not comparable.
• The potential difference between two dissimilar metals is the
driving force for the destructive attack on the active metal
(anode).
• The conductivity of electrolyte will also affect the degree of
attack. The cathode to anode area ratio is directly proportional
to the acceleration factor. 9
10. Prevention of Galvanic Corrosion
• Select metals/alloys as close together as possible in the
galvanic series.
• Avoid unfavorable area effect of a small anode and large
cathode.
• Insulate dissimilar metals wherever practical.
• Apply coatings with caution. Paint the cathode (or both) and
keep the coatings in good repair on the anode.
• Avoid threaded joints for materials far apart in the galvanic
series.
10
11. CREVICE CORROSION
• Crevice Corrosion refers to the localized attack on a metal
surface at, or immediately adjacent to the gap or crevice
between two joining surfaces. The gap or crevice can be
formed between two metals or a metal and non-metallic
material. Outside the gap or without the gap, both metals are
resistant to corrosion.
• The damage caused by crevice corrosion is normally confined
to one metal at localized area within or close to the joining
surfaces.
11
12. Mechanism
• Crevice corrosion is initiated by a difference in concentration of
some chemical constituents, usually oxygen, which set up an
electrochemical concentration cell.
• Outside of the crevice (the cathode), the oxygen content and
the pH are higher - but chlorides are lower.
• Chlorides concentrate inside the crevice (the anode),
worsening the situation. Ferrous ions form ferric chloride and
attack the stainless steel rapidly.
• The pH and the oxygen content are lower in the crevice than in
the bulk water solution, just as they are inside a pit.
• Once a crevice has formed, the propagation mechanism for
crevice corrosion is the same as for pitting corrosion.
12
14. Prevention of Crevice Corrosion
• Use welded butt joints instead of riveted or bolted joints in
new equipment
• Eliminate crevices in existing lap joints by continuous welding
or soldering
• Avoid creating stagnant conditions and ensure complete
drainage in vessels
• Use solid, non-absorbent gaskets such as Teflon
• Use higher alloys (ASTM G48) for increased resistance to
crevice corrosion
14
15. PITTING CORROSION
• Pitting Corrosion is the localized corrosion of a metal surface
confined to a point or small area, that takes the form of
cavities. Pitting corrosion is one of the most damaging forms of
corrosion.
• Pitting factor is the ratio of the depth of the deepest pit
resulting from corrosion divided by the average penetration as
calculated from weight loss.
• Pitting corrosion is usually found on passive metals and alloys
such aluminium alloys and stainless alloys when the ultra-thin
passive film (oxide film) is chemically or mechanically damaged
and does not immediately re-passivate.
• The resulting pits can become wide and shallow or narrow and
deep which can rapidly perforate the wall thickness of a metal.
15
16. 16
ASTM-G46 has a standard visual chart for rating of pitting corrosion
17. Mechanism
• For a defect-free "perfect" material, pitting corrosion is caused
by the environment (chemistry) that may contain aggressive
chemical species such as chloride. Chloride is particularly
damaging to the passive film (oxide) so pitting can initiate at
oxide breaks.
• The environment may also set up a differential aeration cell (a
water droplet on the surface of a steel, for example) and pitting
can initiate at the anodic site (center of the water droplet).
• For a homogeneous environment, pitting is caused by the
material that may contain inclusions or defects. In most cases,
both the environment and the material contribute to pit
initiation.
• The environment (chemistry) and the material (metallurgy)
factors determine whether an existing pit can be repassivated
or not.. An existing pit can also be repassivated if the material
contains sufficient amount of alloying elements such as Cr, Mo,
Ti, W, N, etc... 17
18. Prevention of pitting corrosion
• Proper selection of materials with known resistance to the
service environment
• Control pH, chloride concentration and temperature
• Cathodic protection and/or Anodic Protection
• Use higher alloys (ASTM G48) for increased resistance to
pitting corrosion
18
19. INTERGRANULAR CORROSION
• Intergranular corrosion is sometimes also called
"intercrystalline corrosion" or "interdendritic corrosion". In the
presence of tensile stress, cracking may occur along grain
boundaries and this type of corrosion is frequently called
"intergranular stress corrosion cracking (IGSCC)" or simply
"intergranular corrosion cracking".
• "Intergranular" or 'intercrystalline" means between grains or
crystals. As the name suggests, this is a form of corrosive
attack that progresses preferentially along interdendritic paths
(the grain boundaries).
• Positive identification of this type of corrosion usually requires
microstructure examination under a microscopy although
sometimes it is visually recognizable as in the case of weld
decay.
19
21. Mechanism
• This type of attack results from local differences in
composition, such as coring commonly encountered in alloy
castings.
• Grain boundary precipitation, notably chromium carbides in
stainless steels, is a well recognized and accepted mechanism
of intergranular corrosion.
• The precipitation of chromium carbides consumed the alloying
element - chromium from a narrow band along the grain
boundary and this makes the zone anodic to the unaffected
grains.
• The chromium depleted zone becomes the preferential path for
corrosion attack or crack propagation if under tensile stress.
• Intermetallics segregation at grain boundaries in aluminum
alloys also causes intergranular corrosion but with a different
name - "exfoliation".
21
22. Prevention of intergranular corrosion
• Use low carbon (e.g. 304L, 316L) grade of stainless steels
• Use stabilized grades alloyed with titanium (for example type
321) or niobium (for example type 347). Titanium and niobium
are strong carbide- formers. They react with the carbon to form
the corresponding carbides thereby preventing chromium
depletion.
• Use post-weld heat treatment.
22
23. DEALLOYING
• Dealloying is the selective corrosion of one or more
components of a solid solution alloy. It is also called parting,
selective leaching or selective attack.
Common dealloying examples are decarburization,
decobaltification, denickelification, dezincification and
graphitic corrosion.
• Graphitic corrosion is the deterioration of gray cast iron in
which the metallic constituents are selectively leached or
converted to corrosion products leaving the graphite intact.
• Graphitic corrosion should not be confused with another term
graphitization, which is used to describe the formation
of graphite in iron or steel, usually from decomposition of iron
carbide at elevated temperatures.
23
25. Mechanism
• Different metals and alloys have different
electrochemical potentials (or corrosion potentials) in the
same electrolyte.
• Modern alloys contain a number of different alloying elements
that exhibit different corrosion potentials.
• The potential difference between the alloying elements is the
driving force for the preferential attack on the more "active"
element in the alloy.
• In the case of dezincification of brass, zinc is preferentially
leached out of the copper-zinc alloy, leaving behind a copper-
rich surface layer that is porous and brittle.
25
26. Prevention of dealloying
• Select metals/alloys that are more resistant to dealloying. For
example, inhibited brass is more resistant to dezincification
than alpha brass, ductile iron is more resistant to graphitic
corrosion than gray cast iron.
• Control the environment to minimize the selective leaching
• Use sacrificial anode cathodic protection or impressed current
cathodic protection
26
27. EROSION CORROSION
• Erosion Corrosion refers to the combined action
involving erosion and corrosion in the presence of a moving
corrosive fluid or a metal component moving through the fluid,
leading to accelerated loss of metal.
27
28. Mechanism
• The mechanical effect of flow or velocity of a fluid combined
with the corrosive action of the fluid causes accelerated loss
of metal.
• The initial stage involves the mechanical removal of a metal's
protective film and then corrosion of bare metal by a flowing
corrosive occurs. The process is cyclic until perforation of the
component occurs.
• Erosion-corrosion is usually found at high flow rates around
tube blockages, tube inlet ends, or in pump impellers. The
photo shows erosion corrosion of a cast aluminum pump
casing due to excessively high flow rate of the coolant.
• Cavitation-corrosion is a special form of erosion-corrosion. It
is caused by water bubbles produced by a high-speed
impeller, which then collapse and cause pits on the metal
surface.
28
29. Prevention of Erosion Corrosion
• Streamline the piping to reduce turbulence
• Control fluid velocity
• Using more resistant materials
• Using corrosion inhibitors or cathodic protection to minimize
erosion corrosion
29
30. STRESS CORROSION
• Stress-corrosion cracking (SCC) is a cracking process that
requires the simultaneous action of a corrodent and
sustained tensile stress. This excludes corrosion-reduced
sections that fail by fast fracture. It also excludes
intercrystalline or transcrystalline corrosion, which
can disintegrate an alloy without applied or residual stress.
SCC may occur in combination with hydrogen embrittlement.
30
31. Mechanism
• Stress corrosion cracking results from the conjoint action of
three components:
(1) a susceptible material
(2) a specific chemical species (environment)
(3) tensile stress
• For example, copper and its alloys are susceptible to ammonia
compounds, mild steels are susceptible to alkalis and
stainless steels are susceptible to chlorides.
• There is no unified mechanism for stress corrosion cracking in
the literature.
31
32. • Various models have been proposed which include the
following:
• Adsorption model: specific chemical species adsorbs on the
crack surface and lowers the fracture stress.
• Film rupture model: stress ruptures the passive film locally
and sets up an active-passive cell. Newly formed passive film
is ruptured again under stress and the cycle continues until
failure.
• Pre-existing active path model: Pre-existing path such as grain
boundaries where intermetallics and compounds are formed.
• Embrittlement model: Hydrogen embrittlement is a major
mechanism of SCC for steels and other alloys such as
titanium. Hydrogen atoms diffuse to the crack tip and embrittle
the metal.
32
33. Prevention of stress corrosion cracking
• Avoid the chemical species that causes SCC.
• Control of hardness and stress level (residual or load).
• Introduce compressive stress by shot-peening for example.
• Use of materials known not to crack in the specified
environment.
• Control operating temperature and/or the electrochemical
potential of the alloy.
33
34. HYDROGEN DAMAGE
• It is a general terms which refers to mechanical damage of a
metal caused by presence of hydrogen.
• Hydrogen damage may be classified as:
1) Hydrogen blistering
2) Hydrogen embrittlement
3) Decarburization
4) Hydrogen induced cracking
34
35. HYDROGEN BLISTERING
• Hydrogen Blistering (HB) refers to the formation of subsurface
planar cavities, called hydrogen blisters, in a metal resulting
from excessive internal hydrogen pressure. Growth of near-
surface blisters in low-strength metals usually results in
surface bulges.
35
36. Mechanism
• Hydrogen ions are reduced to hydrogen atoms that adsorb on
the steel surface. Some of the hydrogen atoms will diffuse
through the steel and accumulate at hydrogen traps, typically
voids around inclusions.
• When hydrogen atoms meet in a trap and combine, they form
hydrogen gas (H2) molecules in the trap.
• The accumulation hydrogen gas inside the extremely small
cavity will lead to the buildup of excessive
internal hydrogen pressure. At certain times, this internal
hydrogen pressure will become sufficient to cause the steel to
blister.
• Blisters occur usually in low strength steels (<80ksi yield
strength) and are formed preferentially along elongated
nonmetallic inclusions or laminations in line pipe steels.
36
37. Prevention of hydrogen blistering:
• Control of impurity of steel.
• Avoid the hydrogen source.
• Baking to remove hydrogen.
37
38. HYDROGEN EMBRITTLEMENT
• Hydrogen embrittlement (HE) is a process resulting in a
decrease of the toughness or ductility of a metal due to the
presence of atomic hydrogen. Hydrogen embrittlement has
been recognized classically as being of two types.
• The first, known as internal hydrogen embrittlement, occurs
when the hydrogen enters molten metal which becomes
supersaturated with hydrogen immediately after solidification.
• The second type, environmental hydrogen embrittlement,
results from hydrogen being absorbed by solid metals. This
can occur during elevated-temperature thermal treatments and
in service during electroplating, contact with maintenance
chemicals, corrosion reactions, cathodic protection, and
operating in high-pressure hydrogen.
38
39. Mechanism
• In the absence of residual stress or external loading,
environmental hydrogen embrittlement is manifested in
various forms, such as blistering, internal cracking, hydride
formation and reduced ductility.
• With a tensile stress or stress-intensity factor exceeding a
specific threshold, the atomic hydrogen interacts with the
metal to induce subcritical crack growth leading to fracture.
• In the absence of a corrosion reaction (polarized cathodically),
the usual term used is hydrogen-assisted cracking (HAC) or
hydrogen stress cracking (HSC).
Prevention of hydrogen embrittlement
• Control of stress level (residual or load) and hardness.
• Avoid the hydrogen source.
• Baking to remove hydrogen.
39
40. DECARBURIZATION
• High-temperature Hydrogen Attack (HTHA) refers to the loss of
strength and ductility of steel by high-temperature reaction of
absorbed hydrogen with carbides in the steel, resulting in
decarburization and internal fissuring.
• High-temperature hydrogen attack is also referred to as hot
hydrogen attack or decarburization. It occurs in carbon and
low-alloyed steels exposed for an extended period to hydrogen
under high pressure and at high temperature.
40
41. Mechanism
• Atomic hydrogen (formed in corrosion processes or by
dissociation of molecular hydrogen in a gas stream at the steel
surface) diffuses into steel. At grain boundaries, crystal
imperfections, inclusions, discontinuities and other defects, the
atomic hydrogen reacts with the dissolved carbon or with
the metal carbides, forming methane:
8H+C+Fe3C =2CH4+3Fe
• Because of the pressure build-up of the methane in the steel, this
results in the formation of intergranular cracks (refer to the
micrograph above), fissures and blisters, often extending to the
surface of the steel. This form of hydrogen damage sometimes
resembles the low-temperature hydrogen blistering. Moreover,
the decarburization process leads to the loss of carbon in the
steel and hence a reduction in tensile strength and an increase in
ductility and creep rate. Interestingly, the reverse process (Eq.1
above), carburization, can also occur in hydrogen-hydrocarbon
mixtures such as that encountered in petroleum-refining
operations
41
42. Prevention of High-temperature hydrogen attack
• Avoid high carbon steels
• Use higher alloyed steels
• Use a safety margin of 30°C when using Nelson curves
42
43. HYDROGEN-INDUCED CRACKING
• Hydrogen-Induced Cracking (HIC) is the stepwise
internal cracks that connect adjacent hydrogen blisters on
different planes in the metal, or to the metal surface. It is also
known as stepwise cracking.
• HIC is especially prevalent in iron alloys because of the
restricted slip capabilities in the predominantly body-centered
cubic (BCC) structure. HIC is generally limited to steels having
a hardness of 22 or greater on the Rockwell C scale.
43
44. Mechanism
• Hydrogen-induced cracking results from atomic hydrogen
being absorbed by solid metals.
• This can occur during elevated-temperature thermal
treatments and in service during electroplating, contact with
maintenance chemicals, wet H2S, corrosion reactions,
cathodic protection and operating in high-pressure hydrogen-
containing environments.
Prevention of hydrogen-induced cracking
• Control of stress level (residual or load) and hardness.
• Avoid the hydrogen source.
• Baking to remove hydrogen
44
