Surface Technology Part 4 Corrosion

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Surface TechnologyPart 4Corrosion

• Professor Kenneth W Miller
• Office A108
• Phone [phone]

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Outline

• Mechanisms of Corrosions
• Types
• Causes

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Fundamentals of Corrosion

• It is an electro-chemical reaction
• It happens in two parts
• oxidation or loss of electrons
• reduction or gain of electrons
• Any electrical joint of dissimilar metals
• batteries
• thermocouples

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Oxidation

• Anode donates electrons
• General Reaction form is
• M ? Mn ne-
• Examples
• Fe ? Fe2 2e-
• Al ? Al3 3e-
• This results in a positive ion and free
  electron(s)

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Reduction

• Cathode receives electrons
• General form of reaction is
• Mn e- ? M(n-1)
• Examples
• O2 2 H2O 4e- ? 4 (OH-)
• 2 H 2 e- ? H2
• Loose electrons join with other atoms resulting
  in a neutral atom or less positive ion

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Galvanic Couple

• Oxidation is a half reaction
• Reduction is a half reaction
• They must happen together
• galvanic couple

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Complete Reactions

• Combine one (or more) oxidation with one (or
  more) reduction
• Rust
• 2 Fe O2 2 H2O ? 2 Fe2 4 (OH)-
• ? 2 Fe (OH)2
• then 4 Fe (OH)2 O2 2 H2O ? 4 Fe(OH)3
• Result is an insoluble compound
• Some reactions remain as ions in solution

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Complete Reactions

• A similar reaction is aluminum oxidation to form
  Al2O3, an insoluble compound
• Another reaction lead-acid batteries
• Use lead plates H2O, and H2SO4
• Pb SO4-2 2H ? PbSO4 2e- 2H
• Pb PbO2 2SO42- 4H ? 2PbSO4 2H2O
• PbO2 SO4-2 4H 2e- ? PbSO4 2 H2O

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Reactions and Rate

• Reactions depend on Standard Electrode
  Potential
• Reaction rate depends on temperature
• Reaction rate depends on concentration

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Electrical Potential

• Standard emf series shows half reactions
• Two reactions are required
• oxidation
• reduction

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Standard emf Series

• Idealized reactions with solutions of the metal
  ions
• Does not address effects of dilution, formation
  of protective layers, or secondary reactions

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Reactions and Rates

• Standard Reaction
• V2 is the cathode or reducing material
• V1 is the anode or oxidizing material
• Must be positive, or V1 and V2 are reversed

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Potential Fe Cu
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Potential Fe Cu / Fe - Zn
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Reactions and Rates

• Nernst Equation, addresses temperature and
  concentration
• R Universal gas constant
• R 8.3145 J / mole K
• F Faraday constant
• F 1.6027733 x 10-19 C / electron
• F 96,485 C / (mole of electrons)

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Reactions and Rates
molar concentrations (a)

• Numerator components are anode materials
• Denominator components are cathode materials
• Result still must be positive

• Nernst Equation at 25C

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Very Base Metals

• emf lt -0.4V
• Corrode in neutral aqueous solutions, even
  without oxygen
• includes Na, Mg, Be, Al, Ti, and Fe

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Base Metals

• emf between -0.4V and 0.0 V
• Corrodes in neutral aqueous solutions with oxygen
• Corrodes in acids to produce hydrogen, even
  without oxygen
• includes Cd, Co, Ni, Sn, and Pb

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Semi-Noble Metals

• emf between 0.0 V and 0.7V
• Corrodes in aqueous solutions only with the
  presence of oxygen
• includes Cu, Hg, Ag

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Noble Metals

• emf between gt 0.7V
• includes Pd, Pt, Au

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Types of Corrosion

• Group I
• identifiable by visual inspection
• Uniform, Pitting, Crevice, Galvanic, Rust
• Group II
• identifiable with special inspection tools
• erosion, cavitation, fretting, intergranular
• Group III
• identifiable by microscopic examination
• exfoliation, de-alloying, stress-corrosion
  cracking

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Uniform or General Surface Corrosion

• Evenly distributed loss of material over a
  surface
• Allows corrosion evaluation through material
  thickness

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Pitting Corrosion

• Local corrosion forming holes and pits
• Depth is typically greater than diameter
• Damage is localized and hard to measure
• Damage is difficult to predict and model
• typically requires a statistical model
• May be covered with corrosive products to hide
• Possible serious weakening with little material
  loss

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Crevice Corrosion

• Attacks crevices in material
• gaskets, fastener heads, disbonded coatings,
  clamps, and lap joints
• Localized corrosion sensitive to
  micro-environment
• May cause a localized anode condition at the base
  and cathode at the surface

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Galvanic Corrosion

• Occurs around the junction of dissimilar metals
• Typical of riveted and bolted joints
• Corrosion products (reduction) can cause problems
  through volume increase

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Rust Formation

• Formation of ferriferous oxide and hydroxide
  corrosion
• Iron and Steel
• Most common problem in steel bodies and frames

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Erosion Corrosion

• Corrosion accelerated by relative motion of
  electrolyte
• Not typical of auto bodies except in extreme
  cases in wheel wells
• May be accelerated by cavitation

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Fretting Corrosion

• Combination of a corrosive medium (e.g. salt
  water) and friction
• Similar to erosion
• Starts attack at surface asperities

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Intergranular Corrosion

• Corrosion along grain boundaries
• May be a function of material segregation along
  grain boundaries
• May attack precipitates along grain boundaries
  (Cr in stainless steel)
• Typical problem in welds

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Exfoliation

• A type of intergranular corrosion typical of
  high-strength aluminum alloys
• Starts (usually) at exposed grains, typically on
  a machined surfaces such as holes or edges
• Attacks following grain boundaries
• Volume of corrosion products separates grains
  (leafing)

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Stress Corrosion Cracking

• Combination of a corrosive medium (e.g. salt
  water) and tensile stress
• Stress can be external or internal (residual)
• Not always visible without microscopic evaluation
• May cause transcrystalline or intercrystalline
  fissures

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Vibration Corrosion Cracking

• Stress corrosion with fatigue loads
• Typically results in transcrystalline fissures
• Not always visible

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Controlling Factors

1. Material
2. Environment
3. Stress
4. Geometry
5. Temperature
6. Time