Wet Corrosion

    Subdecks (7)

    Cards (75)

    • 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