types of corrosion

1. CORROSION FAILURES
THIRUNAVUKARASU.H
13MY12

2. Content
• Over view of various types of corrosion .
• Factor influencing corrosion failures .
• Analysis of corrosion failure

3. Definition of Corrosion
• Corrosion is the deterioration of materials by chemical
interaction with their environment.
• unstable due to higher energy comes to lower state.
Classification of corrosion
1. wet corrosion
2. Dry corrosion

4. 8 Forms of Corrosion:
• Uniform
• Pitting
• Crevice Corrosion or Concentration Cell
• Galvanic or Two-Metal
• Stress Corrosion Cracking
• Intergranular
• Dealloying
• Erosion Corrosion

5. Uniform corrosion
• Uniform corrosion is characterized by
corrosive attack proceeding evenly over the
entire surface area, or a large fraction of the
total area.

6. Pitting corrosion
• Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the
creation of small holes in the metal. The driving power for pitting corrosion is the
depassivation of a small area, which becomes anodic while an unknown but potentially vast
area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates
the mass of the metal, with limited diffusion of ions. The mechanism of pitting corrosion is
probably the same as crevice corrosion.

7. • Concentration cell corrosion occurs when two
or more areas of a metal surface are in contact
with different concentrations of the same
solution. There are three general types of
concentration cell corrosion:
• metal ion concentration cells
• oxygen concentration cells, and
• active-passive cells.
Crevice Corrosion or Concentration Cell

8. Galvanic Corrosion:
• Possibility when two dissimilar metals are electrically
connected in an electrolyte
• Results from a difference in oxidation potentials of metallic
ions between two or more metals. The greater the difference
in oxidation potential, the greater the 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.

9. GALVANIC SERIES
Galvanic Series in Seawater (supplements Faraq Table 3.1 , page 65), EIT Review Manual, page 38-2
Tendency to be protected from corrosion, cathodic, more noble end
Mercury
Platinum
Gold
Zirconium Graphite
Titanium
Hastelloy C Monel
Stainless Steel (316-passive)
Stainless Steel (304-passive)
Stainless Steel (400-passive)
Nickel (passive oxide)
Silver
Hastelloy 62Ni, 17Cr
Silver solder
Inconel 61Ni, 17Cr
Aluminum (passive AI203)
70/30 copper-nickel
90/10 copper-nickel
Bronze (copper/tin)
Copper
Brass (copper/zinc)
Alum Bronze Admiralty Brass
Nickel
Naval Brass Tin
Lead-tin
Lead
Hastelloy A
Stainless Steel (active)
316 404 430 410
Lead Tin Solder
Cast iron
Low-carbon steel (mild steel)
Manganese Uranium
Aluminum Alloys
Cadmium
Aluminum Zinc
Beryllium
Magnesium

10. Big Cathode, Small Anode = Big Trouble

11. 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

12. Design for Galvanic Corrosion?
• Galvanic severity depends on:
– NOT
• Not amount of contact
• Not volume
• Not mass
– Amount of separation in the galvanic series
– Relative surface areas of the two. Severe
corrosion if anode area (area eaten away) is
smaller than the cathode area. Example: dry cell
battery

13. Steel bolt (less noble) is
isolated from copper
plates.

14. Stress Corrosion Cracking:
• Spontaneous corrosion induced cracking of a
material under static (or residual) tensile stress.
• Problem with parts that have residual stress –
stamping double whammy – residual stress at bends
= SCC + stress concentration.
• Also known as environmentally assisted cracking
(EAC), other forms:
– Hydrogen embrittlement
– Caustic embrittlement
– Liquid metal corrosion

15. 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.

16. Stress Corrosion Cracking:

17. Intergranular Attack:
• Corrosion which occurs preferentially at grain
boundries.
• Why at grain boundries?
– Higher energy areas which may be more anodic
than the grains.
– The alloy chemistry might make the grain
boundries dissimilar to the grains. The grain can
act as the cathode and material surrounding it the
anode.

18. Intergranular Attack:
• How to recognize it?
– Near surface
– Corrosion only at grain boundries (note if only a
few gb are attacked probably pitting)
– Corrosion normally at uniform depth for all grains.

19. Example 1: Intergranular Attack:
• Sensitization of stainless steels:
– Heating up of austenitic stainless steel (750 to
1600 F) causes chromuim carbide to form in the
grains. Chromuim is therefore depleted near the
grain boundries causing the material in this area
to essentially act like a low-alloy steel which is
anodic to the chromium rich grains.

20. Example: Intergranular Attack:
• Sensitization of stainless steels:
– Heating up of austenitic stainless steel (750 to
1600 F) causes chromuim carbide to form in the
grains. Chromuim is therefore depleted near the
grain boundries causing the material in this area
to essentially act like a low-alloy steel which is
anodic to the chromium rich grains.
– Preferential Intergranular Corrosion will occur
parallel to the grain boundary – eventually grain
boundary will simply fall out!!

21. Design for Intergranular Attack:
• Watch welding of stainless steels (causes
sensitization). Always anneal at 1900 – 2000 F after
welding to redistribute Cr.
• Use low carbon grade stainless to eliminate
sensitization (304L or 316L).
• Add alloy stabilizers like titanium which ties up the
carbon atoms and prevents chromium depletion.

22. Intergranular Attack:

23. 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).

24. Dealloying:
• Danger!
– The alloy may not appear damaged
– May be no dimensional variations
– Material generally becomes weak – hidden to inspection!

25. 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).

26. EROSION-CORROSION
(“Flow-Assisted” or “Flow-Accelerated” Corrosion)
• An increase in rate of attack on a metal
because of relative movement between a corrosive fluid
and the metal surface.
Removal of the metal may be:
– as corrosion product which “removes” the surface because of the high
fluid shear and bares the metal beneath;
– as metal ions, which are swept away by the fluid flow before they can
deposit as corrosion product.

27. EROSION-CORROSION
Remember the distinction between erosion-corrosion and
erosion:
– erosion is the straightforward wearing away by the
mechanical abrasion caused by suspended particles . . . e.g.,
sand-blasting, erosion of turbine blades by droplets . . .
– erosion-corrosion also involves a corrosive environment . . .
the metal undergoes a chemical reaction.

28. EROSION-CORROSION
Erosion-corrosion produces a distinctive surface finish:
grooves, waves, gullies, holes, etc., all oriented with respect to
the fluid flow pattern . . .
Erosion-corrosion of condenser
tube wall.

29. EROSION-CORROSION
Vulnerable equipment includes:
• - pipes (bends, elbows, tees);
• - valves;
• - pumps;
• - blowers;
• - propellers, impellers;
• - stirrers;
• - stirred vessels;
• - flow-measuring orifices, venturies;
• - turbine blades;
• - nozzles;
• - metal-working equipment (scrapers, cutters,
• grinders, mills);
• - spray impingement components;
• - etc.

30. EROSION-CORROSION
Prevention of Erosion-Corrosion
• design (avoid impingement geometries, high
velocity, etc.);
• Corrosion free environment.
• Materials selection (use Cr-containing steels);
• use corrosion-resistant coatings.

31. Methods to Control Corrosion
There are five methods to control corrosion:
 material selection
 coatings
 changing the environment
 changing the potential
 design

32. How to avoid (or control) Corrosion?
• Material Selection! Remember – environment
key. Look at potential pH diagrams!!
• Galvanic - Avoid using dissimilar metals.
– Or close together as possible
– Or MAKE ANODE BIG!!!

33. How to avoid (or control) Corrosion?
• Pitting/Crevice: Watch for stagnate water/
electrolyte.
– Use gaskets
– Use good welding practices
• Intergranular – watch grain size, environment,
temperature, etc.. Careful with Stainless
Steels and AL.

34. How to avoid (or control) Corrosion?
• Consider organic coating (paint, ceramic, chrome,
etc.) – DANGER IF IT GETS SCRACTHED!!
• OR BETTER YET, consider cathodic protection:
– such as zinc (or galvanized) plating on steel
– Mg sacrificial anode on steel boat hull

35. DESIGN for Corrosion

36. Surface Treatment (Coatings)
• Organic paints
• Chromating and phosphating:
– The Process - chromating and phosphating are
surface-coating processes that enhance the
corrosion resistance of metals. Both involve
soaking the component in a heated bath based on
chromic or phosphoric acids. The acid reacts with
the surface, dissolving some of the surface metal
and depositing a thin protective layer of complex
chromium or phosphorous compounds

37. Material Selection:
• Noble material are selected . Which reduces
the corrosion

38. Cathodic Protection (CP)
• Cathodic protection (CP) is a technique to control
the corrosion of a metal surface by making it work as
a cathode of an electrochemical cell. This is achieved
by placing in contact with the metal to be protected
another more easily corroded metal to act as the
anode of the electrochemical cell. Cathodic
protection systems are most commonly used to
protect steel, water or fuel pipelines and storage
tanks, steel pier piles, ships, offshore oil platforms
and onshore oil well casings.

40. Factors influencing corrosion
• Nature of metal.
• Nature of corrosive environment.

41. 1.Nature of Metal:
Purity of metal:
• Impure the metal, more is the rate of corrosion.
• This is because impurities act as “minute
electrochemical cells.”
Physical state of metal:
• Physical state of metal like grain size, stress
affects the corrosion rate.
• Stressed areas undergo more corrosion.

42. Position in Galvanic series:
• More the oxidation potential i.e if metal is
higher in the galvanic series.
• it is more anodic and hence more is the rate
of corrosion.
Nature of Oxide layer:
• Corrosion rate depends upon the nature of
oxide layer whether it is stable, unstable,
porous etc.

43. Passive nature of metal:
• Some metals like Ti, Al, Cr show more
corrosion resistance than expected from their
position in the electrochemical series.
• These are called passive metals.
• This is due to formation of thin protective
layer on metal surface.

44. Nature of corrosive products:
If the corrosion products are soluble or
volatile in nature, then corrosion increases.
Relative area of anode and cathode:
smaller the anodic area, more is the
corrosion rate because oxidation of anode occurs
at faster rate.

45. 2. Nature of corroding environment:
Temperature:
Rate of corrosion increases with increase in
temperature.
Moisture:
More the moisture, more is the rate of corrosion.
pH Value:
Lesser the pH i.e. acidic conditions, more is
corrosion.
Nature of electrolyte:
Presence of salts in the electrolyte increases the
rate of corrosion.

46. Presence of impurities in
atmosphere:
• Impurities increase the rate of
corrosion.
• Corrosion is more in industrial
areas and sea.
• This is because of presence of
gases like CO2, H2S, SO2

47. REFERENCES:
• Herbert H Uhlig and Winston Revie , "Corrosion and Corrosion
Control – An Introduction to Corrosion Science and Engineering",
Third Edition, John Wiley & Sons, 1985.
• Mars G.Fontana, "Corrosion Engineering", Third Edition, McGraw
Hill Inc., 1987.
• Rajnarayan, "Metallic Corrosion and Prevention", Oxford
Publications, 1988.
• Kenneth R Trethewey and John Chamberlain, "Corrosion for Science
and Engineering", Second Edition, Longman Inc., 1996.
• Denny A. Jones, "Principles and Prevention of Corrosion", second
edition, Prentice Hall Inc., 1996.
• ASM Hand book – Vol 13: Corrosion, ASM International, 2001.