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Surface Technology Part 4 Corrosion

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It is an electro-chemical reaction. It happens in two parts ... Corrode in neutral aqueous solutions, even without oxygen. includes Na, Mg, Be, Al, Ti, and Fe ... – PowerPoint PPT presentation

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Title: Surface Technology Part 4 Corrosion


1
Surface TechnologyPart 4Corrosion
  • Professor Kenneth W Miller
  • Office A108
  • Phone 0841 9348 0324

2
Outline
  • Mechanisms of Corrosions
  • Types
  • Causes

3
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

4
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)

5
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

6
Galvanic Couple
  • Oxidation is a half reaction
  • Reduction is a half reaction
  • They must happen together
  • galvanic couple

7
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

8
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

9
Reactions and Rate
  • Reactions depend on Standard Electrode
    Potential
  • Reaction rate depends on temperature
  • Reaction rate depends on concentration

10
Electrical Potential
  • Standard emf series shows half reactions
  • Two reactions are required
  • oxidation
  • reduction

11
Standard emf Series
  • Idealized reactions with solutions of the metal
    ions
  • Does not address effects of dilution, formation
    of protective layers, or secondary reactions

12
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

13
Potential Fe Cu
14
Potential Fe Cu / Fe - Zn
15
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)

16
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

17
Very Base Metals
  • emf lt -0.4V
  • Corrode in neutral aqueous solutions, even
    without oxygen
  • includes Na, Mg, Be, Al, Ti, and Fe

18
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

19
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

20
Noble Metals
  • emf between gt 0.7V
  • includes Pd, Pt, Au

21
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

22
Uniform or General Surface Corrosion
  • Evenly distributed loss of material over a
    surface
  • Allows corrosion evaluation through material
    thickness

23
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

24
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

25
Galvanic Corrosion
  • Occurs around the junction of dissimilar metals
  • Typical of riveted and bolted joints
  • Corrosion products (reduction) can cause problems
    through volume increase

26
Rust Formation
  • Formation of ferriferous oxide and hydroxide
    corrosion
  • Iron and Steel
  • Most common problem in steel bodies and frames

27
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

28
Fretting Corrosion
  • Combination of a corrosive medium (e.g. salt
    water) and friction
  • Similar to erosion
  • Starts attack at surface asperities

29
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

30
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)

31
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

32
Vibration Corrosion Cracking
  • Stress corrosion with fatigue loads
  • Typically results in transcrystalline fissures
  • Not always visible

33
Controlling Factors
  1. Material
  2. Environment
  3. Stress
  4. Geometry
  5. Temperature
  6. Time
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