Wet Corrosion

• Wet corrosion

  • aqueous or electrochemical corrosion
  • liquid present

  liquid: electrolyte (conductive)

  gas: below dew point

  solid: with electrolyte inside solid
• Electrochemical Principles

  1. Corrosion thermodynamics
  2. Electrochemical (partial reactions)
  3. Electrode potential
  4. Polarisation & Passivity
  5. Pourbaix diagrams
• Effects of alloying elements in steel:

  Cr- bcc-

  • 11-12 wt% = min
  • 18-20%= standard max
  • 25-30%= protectivity increase, mech prop decreases

  Ni- Fcc

  • mech prop increases
  • repassivation increases
• Metal Matrix Comoposites (MMC)

  Composites are materials made 2 or more dissimilar materials to achieve properties that cannot get from individual component

  MMC= metal matrix + reinforced metallic or ceramic phase- often stronger + stiffer

  Controlled by:

  1. Individual component in environment
  2. Galvanic interaction between components

  Selective attack of matrix may take place
• Polymers--> non metallic organic mat

  • rubbers
  • Elastomers
  • weaker than allow
  • Tm decreases
  • cheap production price
  • some (teflon) corrosion resistant

  Degradation: swell, discolouration

  
  

  Degradation forms:

  1. UV degradation
  2. Thermal degradation
  3. Chemical degradation
• Ceramics:

  • good chemical stabilty
  • corrosion and HT resistance
  • BUT mech behaviour problems
• Corrosion prevention:

  1. Material Selection
  2. Environmental Control
  3. Design
  4. other protective methods
• Environmental Alteration:

  general trend - exceptions

  1. Lowering temp

  • special condition: T increase, corrosion rate decreases
  • O2 removal when boiling H2O
  • certain temp can promote formation of passive film

  2. Decreasing Velocity

  • Certain V to promote passivation,corrosion decreases
  • SS have better resistance in slow-moving fluid than stagnant
• Inhibitors:

  Classification:

  1.Absorption type

  2. Hydro-evaluation poisons

  3.Scavengers

  4.Oxidiser

  
  

  Limitations:

  • diff environ=diff inhibitor
  • Need constant renewal
  • Some are toxic
• Cathodic Protection:

  1. Using external power supply
  2. Using a sacrificial anode
• Light metals (lighter than steel) : Al,Mg & Ti