1. 8 Forms of Corrosion: Uniform Pitting
Crevice Corrosion or Concentration Cell
Galvanic or Two-Metal
Stress Corrosion Cracking
Intergranular
Dealloying
Erosion Corrosion

2. Uniform Corrosion Most common – i.e. steel exposed to environment.
Uniform in nature – leaves scale or deposit over entire exposed area – this is called rust which is really iron-oxide – Fe(OH)3 or Fe2O3
Fairly predictable and therefore the effects can be minimized!
i.e. corrosion proportional to current, proportional to time (corrosion rate)
< 2 mils/yr – necessary for food containment
20 mils/yr = conservative estimate for general atmospheric corrosion.
Really general form of galvanic corrosion – i.e. anode and cathode random and in same material!
Prevented by
Removing electrolyte (i.e. lower relative humidity below 30%)
Choose material that doesn’t rust in a particular environment – look at potential-pH diagram!
Add design “allowance” for rust

3. Uniform (or general) corrosion of steel in water:

4. Uniform Corrosion Corrosion penetration rate (mils/yr):
Constant depending on desired units
Weight loss after exposure time t
Exposure time
density
Exposed area

5. Uniform Corrosion: Corrosion rate in terms of current:
r = rate in terms of mol/m2-s
i = current per unit surface area of material corroding
N = # of electrons associated with ionization of metal ion
F = constant = 96,500 C/mol

6. A rate of less than 2 MPY is necessary for food containers
A rate of less than 20 MPY for many industrial applications

7. EXAMPLE 1: MIG Welding tank METAL: Carbon Steel
ENVIRONMENT: Industrial en
FORM OF CORROSION: General
METHOD TO CONTROL! Surface is painted for protection

8. EXAMPLE 3: Machine Shop Table METAL: Carbon Steel
ENVIRONMENT: Industrial en
FORM OF CORROSION: General
METHOD TO CONTROL! Surface is painted for protection. Aggressive environment (molds dragged across
surface) led to scrapping off of paint. Note corrosion where paint is scraped off in line.

9. EXAMPLE 4: Dumbbell METAL: Cast Iron
ENVIRONMENT: Indoor (exercise room)
FORM OF CORROSION: General
METHOD TO CONTROL! Surface is painted for protection. Note, portion of dumbbell where paint was
abraded off due to handling shows significant corrosion while areas that are better protected from abrasion
retained paint and therefore show little corrosion.

10. EXAMPLE 5: House Drain and Drain Cap 1 year old cap 30 year old cap
METAL: Cast Iron
ENVIRONMENT: Residential basement – water exposure
FORM OF CORROSION: General
METHOD TO CONTROL! Surface is painted for protection. Note the 1 year old cap shows significant
corrosion already!

11. 60 YEAR OLD OIL PUMP

12. Kinzua Viaduct – see photos!! (1882/1900)

13. Crevice or Concentration Cell
Local attack (corrosion) in crevice due to change in chemistry of electrolyte making it more aggressive – i.e. stagnant fluid = lower oxygen concentration = decrease in pH.
Can be between metal surfaces or non-metal surfaces in contact with metal.
Very destructive since highly localized!
How design around?
Leak proof weld
Better gasket design
Avoid stagnant water

14. Crevice or Concentration Cell
Good example – crevices and recesses or under deposits of dirt or corrosion products where the solution is stagnet.
Crevice must be wide enough to allow solution to penetrate yet narrow enough for stagnancy (i.e. few thousandths of an inch).

15. Crevice or Concentration Cell
Depending on the environment developed in the crevice and the nature of the metal, the crevice corrosion can take a form of:
pitting (i.e., formation of pits),
filiform corrosion (this type of crevice corrosion that may occur on an aluminium surface underneath an organic coating),
intergrannular attack, or
stress corrosion cracking.

16. KEY – In crevice there are high concentrations of H+ and Cl- ions which are especially corrosive!

17. Crevice corrosion between pipe and I-beam:
Rubber pads just accelerated the attack – why???

18. Track Fastener - Taipei
EXAMPLE 2:
Track Fastener - Taipei
METAL: Ductile Cast Iron per ASTM D412
ENVIRONMENT: Corrosive Salt Water (Salt Spray)
FORM OF CORROSION: Crevice Corrosion + General
METHOD TO CONTROL! Surface is degreased, sand blasted and phosphatized for corrosion protection
Surface is then painted with Chemlock elastomer primer and bonding adhesive.
Note: Salt water trapped between elastomer and steel led to crevice corrosion which led to underbond
corrosion. The adhesive to metal bond then failed causing the elastomer to delaminate. Resulted in return
of several million dollar worth of product + replacement costs (labor and components).

19. Pitting Corrosion:
Extremely localized corrosion that leads to the creation of small holes in the metal surfaces
The driving power again is the lack of oxygen around a small area. This area becomes anodic while the area with excess of oxygen becomes cathodic.
More of a problem in stagnant solutions.
Very destructive since highly localized.
Prevention?
Material selection
Avoid stagnant flow

20. Pitting Corrosion:
Similar in chemistry to crevice corrosion except it happens in pits.
Occurs in “pits” of metal surfaces where again, electrolyte is aggressive (stagnant).
More of a problem in stagnant solutions.
Very destructive since highly localized – may go undetected until failure occurs.
Gravity causes pit to grow downward – corrosion rate can increase with time

21. Pitting Corrosion:
A pit can be initiated by a localized surface defect, scratch or slight variation in composition.
Stainless steels are especially susceptable to this form of corrosion.
Prevention?
Material selection
Avoid stagnant flow
Alloy SS with about 2% molybdenum.

22. 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.
Refer to Galvanic Series (Figure 13-1)
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.
*Meet requirements of a corrosion cell!! See slide 4

23. Galvanic Series:

24. Note, positions of ss and al**
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)
Lead Tin Solder
Cast iron
Low-carbon steel (mild steel)
Manganese Uranium
Aluminum Alloys
Cadmium
Aluminum Zinc
Beryllium
Magnesium
PASSIVE – will not corrode – act as cathode. These elements are least likely to give up electrons!
Note, positions of ss and al**
ACTIVE – will corrode – act as anode. These elements most likely to give up electrons!

25. Big Cathode, Small Anode = Big Trouble

26. 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

27. 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

28. Steel bolt (less noble) is isolated from copper plates.
See handout! – Read Payer video HO

36. Comparison of EMF series and galvanic series
EMF Series
Galvanic Series
absolute
 relative
quantitative
qualitative
pure metals only
 metals & alloys
half-cell potential 
corrosion potential
standard conditions
 any specified conditions
based on thermodynamic analysis
 based on thermodynamic analysis
used for theoretical calculations
 used for practical applications

37. Stress Corrosion Cracking:
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.
Other forms:
Hydrogen embrittlement
Caustic embrittlement
Liquid metal corrosion

38. 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
3 Requirements for SCC:
Susceptible alloy
Corrosive environment
High tensile stress or residual stress

42. Design for Stress Corrosion Cracking:
Material selection for a given environment (Table 13-2).
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.

43. Stress Corrosion Cracking:
See handout, review HO hydron!

44. 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.

45. 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.

46. 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!!

47. 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.
Mostly in the material chemistry.

48. Intergranular Attack:

49. Example 2: Intergranular Attack:
Exfoliation of high strength Aluminum alloys.
Corrosion that preferentially attacts the elongated grains of rolled aluminum.
Corroded grains usually near surface
Grain swells due to increase in volume which causes drastic separation to occur in a pealing fashion.

50. 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).

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

52. 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).

53. Erosion: Forms of Erosion: Liquid Impingement Liquid erosion
Slurry Erosion
Cavitation
Read definitions on page 523.

57. EXAMPLES of UNIFORM CORROSION

62. Examples of Galvanic Corrosion

76. How to avoid (or control) Corrosion?
Material Selection! Remember – environment key. Look at potential pH diagrams!!!
Eliminate any one of the 4 req’ments for corrosion!
Galvanic - Avoid using dissimilar metals.
Or close together as possible
Or electrically isolate one from the other
Or MAKE ANODE BIG!!!

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

78. How to avoid (or control) Corrosion?
Material Selection! Remember – environment key. Look at potential pH diagrams!!!
Eliminate any one of the 4 req’ments for corrosion!
Galvanic - Avoid using dissimilar metals.
Or close together as possible
Or electrically isolate one from the other
Or MAKE ANODE BIG!!!

79. 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.

80. 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
Impressed current, etc..

81. Corrosion Control:
Anodic Protection – Zinc coating of steel. KNOW HOW THIS WORKS!!

82. DESIGN for Corrosion

83. DESIGN for Corrosion
Bracket easier to replace than pipe!