FORMS of
CORROSION
1.General Corrosion
2.Localized Corrosion
 Pitting
 Crevice
 Filiform
FORMS OF
CORROSION
3.Galvanic Corrosion
4.Environmental Cracking
 Stress corrosion cracking
 Hydrogen-induced cracking and sulfide stress cracking
 Liquid metal embrittlement
 Corrosion fatigue
5. De-alloying
Damage mechanism: A mechanical, chemical, physical or
other process that results in asset degradation
DM
• Some damage mechanisms can occur before an asset is ever put into service
• Some damage mechanisms occur more or less gradually while in service; other damage mechanisms are sudden and episodic
Example Damage Mechanisms
Uniform corrosion can be slowed or
stopped in 5 basic ways
• 1. Slow down or stop the movement of
electrons
 Coat the surface with non-conducting medium
(paint, lacquer or oil)
 Reduce the conductivity of the solution in contact
with the metal, or keep dry
 Wash away conductive pollutants regularly
 Apply a current to the material (cathodic protection)
2. Slow down or stop oxygen from reaching the
surface (by coating)
3. Prevent the metal from giving up electrons
 Use a more corrosion-resistant metal higher in
the electrochemical series
 Use a sacrificial coating that gives up its
electrons more easily than the metal being
protected
4. Apply CP
5. Use inhibitor
 Select metal that forms an oxide that is
protective and stops the reaction
 Control and consideration of environmental and
thermal factors is also essential
Localized Corrosion
• Pitting Corrosion, Crevice Corrosion,
Filiform Corrosion
Definition : Pitting Corrosion
• higher rate of localized attack than surrounding areas
• Attacked areas-Pits narrow and deep, or shallow and
broad
• Penetration rates can be rapid
Pitting Corrosion
• Pitting corrosion takes place when the passiavated oxide film on
stainless steel (or alloys) is removed at localized areas.
• The exposed metal (called depassivated area) forms the anode and
relatively large area covered by passivated film forms the cathode.
• This leads to very high current concentration at a very small anode area
and it results in pits in small localised area.
Pitting is initiated by:
a. Localized chemical or mechanical damage to the protective oxide
film
 water chemistry cause breakdown of a passive film, decrease pH value, low
dissolved oxygen concentrations (which tend to render a protective oxide film less
stable) and high concentrations of chloride.
b. Poor application of a protective coating
c. The presence of non-uniformities in the metal structure of the
component, e.g. nonmetallic inclusions.
Pitting is initiated
by
• Theoretically, a local cell that
leads to the initiation of a pit can
be caused by an abnormal
anodic site surrounded by
normal surface which acts as a
cathode
• Pitting is more dangerous than
the uniform corrosion as it may
go undetected under a coating,
scale etc and the end result is a
through hole or leakage etc.
Pitting Mechanisms
• Local corrosion cell
• Pitting happen due to unfavorable anode/cathode area ratio , especially
in conductive, bulk environments
• Oxygen depleted inside pit
• Potential difference between pit interior and exterior
• Stages- initiation
propagation
termination
re-initiation
Pitting corrosion can be prevented through
• 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.
Crevice Corrosion
• Localized attack where access of surrounding environment is restricted
• One of the area is speeded up. Eg: occurs when oxygen cannot penetrate a
crevice and differential aeration cell is set up. Corrosion occurs rapidly in the
area with less oxygen.
• The potential for crevice corrosion can be reduced by:
 Avoiding sharp corners and designing out stagnant areas
 Use of sealants
 Use of welds instead of bolts or rivets
 Selection of resistant materials
Crevice Corrosion
• Localized attack where access of surrounding
environment is restricted
• One of the area is speeded up. Eg: occurs when
oxygen cannot penetrate a crevice and differential
aeration cell is set up. Corrosion occurs rapidly in
the area with less oxygen.
• The potential for crevice corrosion can be reduced
by:
 Avoiding sharp corners and designing out
stagnant areas
 Use of sealants
 Use of welds instead of bolts or rivet
 Selection of resistant materials
• Metal-to-metal
Crevice Corrosion
• Metal-to-nonmetal
• Deposits, debris, or corrosion products
• Disbonded coatings
Crevice Corrosion
• High Cl- concentration inside crevice
• Metal ion hydrolysis in crevice
• pH of crevice electrolyte becomes very acidic
• Passive film breakdown in crevice
• Crevice area become active potential
• Outside, surrounding area become passive potential
• O2 concentration cell is the main driving force
• Mechanism: inside crevice, O2 depleted; outside crevice, O2 high
• O2 diffusion into crevice difficult, but Cl- migration into crevice much easier
Crevice Corrosion
• Small anodic vs large cathodic area ratio under immersion conditions
• Accelerated localized attack inside crevices
• Tighter crevices worse than loose crevices
• Increasing turbulence or flow rate-crevice corrosion worse because O2
reduction rate (cathodic reaction) outside crevice increased
• Mechanism similar to pitting corrosion
Filiform corrosion
• Definition : Filiform Corrosion
• Filamentary corrosion on metal
surface beneath coatings
• Also known as under-film
corrosion
Filiform corrosion
• Occurs on salt-contaminated metal surfaces
• Salt attracts moisture through coating film in
humid conditions
Galvanic corrosion
• Corrosion of one metal accelerated
due to electrical contact with
another metal in an electrolyte.
• Results from a difference in
oxidation potentials of metallic ions
between metals. The greater the
difference in oxidation potential, the
greater the galvanic corrosion.
Galvanic corrosion
• The less noble metal will corrode (i.e.
will act as the anode) and the more
noble metal will not corrode (acts as
cathode).
• Perhaps the best known of all corrosion
types is galvanic corrosion, which
occurs at the contact point of two
metals or alloys with different electrode
potentials.
• Severe corrosion if anode area (area
eaten away) is smaller than the
cathode area. Example: dry cell battery
Galvanic Corrosion
Important considerations
• Anode-to-cathode area ratio very important
• Polarization behavior very important
• Potential difference less important
• Exposure environment – water/soil, atmospheric
• Electrolyte resistivity
• Small-anode/large-cathode-very undesirable
• Corrosion concentrated on small anode
Design for Galvanic
Corrosion
• Material Selection: Do not connect dissimilar
metals. Or if you can’t avoid it:
• Try to electrically isolate one from the other
(rubber gasket).
• Make the anode large and the cathode small
• Bad situation: Steel siding with aluminum fasteners
• Better: Aluminum siding with steel fasteners
• Eliminate electrolyte
• Galvanic of anodic protection
Selective Attack
This occurs in alloys such as brass, when one
component or phase is more susceptible to attack than
another and corrodes preferentially, leaving a porous
material that crumbles. It is best avoided by selection of
a resistant material, but other means can be effective
such as:
 Coating the material
 Reducing the aggressiveness of the environment
 Use of cathodic protection
Dealloying:
• When one element in an alloy is anodic to the
other element.
• Example: Removal of zinc from brass (called
dezincification) leaves spongy, weak brass.
• Brass alloy of zinc and copper and zinc is anodic
to copper (see galvanic series).
Dealloying:
The alloy may not appear
damaged
May be no dimensional
variations
Material generally
becomes weak – hidden to
inspection
Dealloying
 Two common types:
 Dezincification – preferential removal of zinc in brass
 Try to limit Zinc to 15% or less and add 1% tin.
 Cathodic protection
 Graphitization – preferential removal of Fe in Cast
Iron leaving graphite (C).
Corrosion Fatigue
• Metal mechanically degrades faster than expected under the combined action of
cycling loading and corrosion.
• Happen due to joint action of corrosion and cyclic stress, the metal fractures and
causes corrosion fatigue.
• crack development is due to quick fluctuating stresses that are below the tensile
strength.
• It can be avoided by;
 Use of inhibitors and coatings to stall time before corrosion fatigue cracking starts
 controlling and bringing down the pressure and vibration fluctuations
 use of high performance alloys which are resistant to corrosion fatigue
Corrosion Fatigue
• Above certain threshold, microscopic
cracks initiate
• Cracking usually begins at stress risers
(sharp corners, grooves, cross-section
changes, stencil marks, corrosion pits)
• Cracks can initiate at stresses well below
yield strength
• Cyclic stresses but no corrosive
environment
Fretting Corrosion
• Caused by relative motion between 2 surfaces in
contact by a stick-lip action resulting in
breakdown of protective films or welding at the
contact areas, allowing other corrosion
mechanisms to operate.
• It can be avoided by;
 Designing out vibrations
 Lubrication of metal surfaces
 Increasing the load between the surfaces to
stop the motion
 Surface treatments to reduce wear and
increase the friction coefficient
Stress Corrosion Cracking (SCC)
• Combined action of a static tensile stress and corrosion forms
cracks and eventually leads to catastrophic failure of the
component.
• Some cases the mechanism starts with intergranular corrosion
• It can be avoided by;
 Reducing the overall stress level and designing out stress
concentrations
 Selection of suitable material not susceptible to the
environment
 Designing to minimize thermal and residual stresses
 Use suitable protective coating
Stress Corrosion Cracking
(SCC)
• Spontaneous corrosion induced cracking of a
material under static (or residual) tensile stress.
• Problem w/ parts that have residual stress –
stamping double whammy – residual stress at
bends = SCC + stress concentration.
• AKA environmentally assisted cracking (EAC),
other forms:
 Hydrogen embrittlement
 Caustic embrittlement
 Liquid metal corrosion
Factors:
 Must consider metal and environment. What to watch for:
 Stainless steels at elevated temperature in chloride solutions.
 Steels in caustic solutions
 Aluminum in chloride solutions
 Requirements for SCC:
1.Susceptible alloy
2.Corrosive environment
3.High tensile stress or residual stress
• Material selection for a given environment
Design for Stress
Corrosion
Cracking
• Reduce applied or residual stress - Stress relieve to
eliminate residual stress (i.e. stress relieve after heat
treat).
 Introduce residual compressive stress in the service.
 Use corrosion alloy inhibitors.
 Apply protective coatings.
Erosion
Q&A
• Name 5 methods to control corrosion
• Identify Forms of Corrosion
The water pipes leading into your house are made
of lead, while the rest of the plumbing in
your house is iron. To eliminate the possibility of
lead poisoning, you call a plumber to replace the
lead pipes. He quotes you a very low price if he
can use up his existing supply of copper pipe to
do the job.
a) Do you accept his proposal?
b) What is your suggestion?
Pb
Cu
Fe