Chapter 13 Corrosion

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Chapter 13 - Corrosion

• What is Corrosion???
• Forms of Corrosion
• How to design for Corrosion
• Dr. Payer ASM video tapes worth the effort

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METALS WANT TO CORRODE they want to exist as
oxide compounds because oxides contain less
energy and are more stable!!
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What is Corrosion??

• Electrochemical reaction involving an anode and a
  cathode.
• Deterioration of a material because of reaction
  with the environment.
• Combines many elements of engineering and impacts
  ALL engineering disciplines Chemical
  Engineering, Mechanical Engineering, Material
  Engineering, Electrical Engineering and Civil
  Engineering

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What is Corrosion??

• Corrosion involves the interaction (reaction)
  between a metal or alloy and its environment.
  Corrosion is affected by the properties of both
  the metal or alloy and the environment. The
  environmental variables include
• pH (acidity)
• Oxidizing power (potential)
• Temperature (heat transfer)
• Velocity (fluid flow)
• Concentration (solution constituents)

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What is Corrosion??

• Cost? EQUALS 3 5 of GNP/ year or 700/person
  based on 2006 estimate 300 billion US only
  (corrosion of steel the biggie)
• Combination of the material and its environment
  - Examples
• No Problem
• Lead in Water
• Aluminum in atmosphere
• Nickel in hydraulic fluid
• BAD
• Steel in marine environment
• Cu in Ammonia
• SS in chloride (Sea water)
• Lead in wine

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Requirements for Corrosion
Ionic Current Path
ANODE
CATHODE
Where Corrosion Occurs!!!!
Electronic Path
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Anodic partial process (oxidation of iron)
Fe Fe 2 2e- 2H 2e- H2

Fe 2H Fe 2 2e-
Cathodic partial process (reduction process H
reduced)
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• Previous corrosion was Fe in HCL.
• Can also have Fe corrode in water most common
  form of corrosion (i.e. steel left outside).
• The anodic corrosion reaction is the oxidation of
  iron Fe Fe2 2e-
• The cathodic or reduction reaction is the
  reduction of oxygen O2 2H2O 4e- 4OH-

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Relationship between the rate of corrosion,
corrosivity of an environment and corrosion
resistance of a material.
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Methods to Control Corrosion
There are five methods to control corrosion

• material selection
• coatings
• changing the environment
• changing the potential
• design

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• Key Parameters
• pH next slide
• Oxidizing power measure of relative tendency to
  corrode or oxidize a solution of low oxidizing
  power will corrode only those metals at the lower
  end (more active) of an emf series.

Description of Environment in terms of oxidizing
power (E) and pH
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pH - log10H
pOH - log10OH-
Pure water has a pH of 7 Strong acids have lower
pHs which means they have more H ions! Strong
alkalis have low pOHs which means they have
more OH- ions!
The more ions, the more toxic the solution.
But not that simple go to materials Pourbaix
(potential pH diagram)!!
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Recall, Corrosion is the degradation of a metal
by an electro-chemical reaction. One half of
this is the dissociation reaction of a metal M
into a metal ion, Mz , releasing electrons e- M
? Mz ze- where z, an integer of 1, 2, or
3, is the valence of the metal. Acidic
environments, with high H (and thus low pH)
stimulate this reaction thus a metal such as
copper, in sulphuric acid solution, reacts
rapidly
Cu ? Cu2 2e H2SO4 ? 2H
SO42- Some metals are resistant to attack by
some acids because the reaction product, here
CuSO4 , forms a protective surface layer thus
lead-lined containers are used to process
sulfuric acid because lead sulfate is
protective. Most metals are immune to attack by
alkalis because their hydroxide, formed in the
reaction, is protective. There are, however,
exceptions, notably aluminum, that forms
non-protective aluminum hydroxide, Al(OH)3.
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Metals behavior as function of oxidizing power
(E) and pH
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The Right material depends on the environment.
Polarization can have a major effect on metal
stability.
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Often several approaches to control corrosion
Often several system constraints pertain
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Eight forms of corrosion can be identified based
on appearance of the corroded metal. These are

• Uniform
• Galvanic, or two-metal
• Pitting
• Crevice or Concentration Cell
• Intergranular
• Stress corrosion cracking
• Erosion-corrosion
• Dealloying

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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)
• lt 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 doesnt rust in a particular
  environment look at potential-pH diagram!
• Add design allowance for rust

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Uniform (or general) corrosion of steel in water
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Uniform Corrosion

• Corrosion penetration rate (mils/yr)

Constant depending on desired units
Weight loss after exposure time t
Exposure time
density
Exposed area
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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
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A rate of less than 2 MPY is necessary for food
containers A rate of less than 20 MPY for many
industrial applications
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EXAMPLE 1 MIG Welding tank
METAL Carbon Steel ENVIRONMENT Industrial
en FORM OF CORROSION General METHOD TO CONTROL!
Surface is painted for protection
Question The tank sees tension stress due to
internal gas pressure, would this lead to stress
corrosion cracking as well as general??
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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.
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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.
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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!
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60 YEAR OLD OIL PUMP
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Kinzua Viaduct see photos!! (1882/1900)
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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

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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).

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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.
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KEY In crevice there are high concentrations of
H and Cl- ions which are especially corrosive!
WHY? Low oxygen levels (stagnant) means ions have
nothing to react w/ except the metal!!
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Crevice corrosion between pipe and I-beam
Rubber pads just accelerated the attack why???
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EXAMPLE 2 Track Fastener - Taipei
A manufacturers nightmare!!
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).
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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

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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

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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.