Chemmat 305 - Corrosion

• Dry corrosion is oxidation of high temperature corrosion with the absence of liquid
• Oxidation reactions are controlled by reaction thermodynamics, reaction kinetics and oxide scale protective ability
• Reaction thermodynamics is what decides whether the reaction will occur
• Reaction kinetics is the transport processes/reaction steps
• Experimental methods are for kinetics (oxidation rates) and Characterisation of oxidation products
• Kinetics (oxidation rates) are experimented by mass gain/change by:

  1. Measuring before and after oxidation method.

  2. Use thermogravimetric method to know continuous rate use thermo-balance with record system

  This all corresponds to the amount of O2 taken
• Characterisation of products ( knowing if good or bad oxidation):

  1. Visual: Colour, oxide thickness, smoothness, porosity and broken scale.
  2. Optical Microscopy: cross section to check thickness of cut, oxide scaler, porosity/crack, microstructure and contact with surface layer
  3. SEM, TEM ( high resolution) and AFM
  4. Crystal oxide structure: XRD, TEM (make film that shows us the electron attraction pattern/crystal structure) - crystallised/ amorphous - can make without crystal.
• Thermodynamics of oxidation:

  G=H-TS

  G<0 spontaneous

  G>0 does not take place

  G=0 equilibrium
• Ellingham Diagram is G plotted with T to decide the dissociation of oxygen partial pressure
• Equilibrium constant

  k= products/ reactants
• G= - R T ln K
• G= R T ln PO2
• PO2 is where the metal and oxide coexist/ dissociation pressure of oxide.

  If Partial pressure> PO2, oxidation occurs/ is stable

  If Partial pressure< PO2, oxidation does not occur/ is unstable/ oxide will decompose
• Prevention of Oxidation:

  1. Make PO2 (Partial pressure of O2) very low
  2. Using a reduction atmosphere (H2)

  O2 partial pressure used as reference point to compare the relative reactivity of metals in the same oxygen atmosphere
• Oxidation Kinetics:

  1. Supply of O2 to reaction surface
  2. Absorption of O2 into the oxide scale
  3. Reaction between metal & O2 into lattice
  4. Transport of reactants through the oxide layer - OXIDATION RATE IS CONTROLLED BY SLOWEST STEP WHICH IS THIS STEP
  5. Diffusion of selective metal ions in alloy
• Oxidation rate laws:

  1. Linear: Y=KL t + C1 Y=oxide thickness or mass gain unit area. KL=linear rate constant
  2. Parabolic Y^2= Kp t + C2 Kp= Parabolic rate constant
• Ellingham diagram plotted 1 mole of oxygen to compare the same O2 partial pressures by using O2 partial pressure as the reference point. O2 partial pressures are the driving force for different reactions.
• Effect of temperature on oxidation rate

  D=Do exp(-Q/RT)

  Kp=Ko exp (-Q/RT)

  Kp= parabolic rate constant. Ko= a constant. Q= Activation energy for diffusion
• K=Ko exp (-Q/RT)

  lnK=lnKo- (Q-RT)

  lnK ~ 1/T linear

  Slope used to find Q. (-Q/R)
• Oxidation of common metals :

  Below 570

  TOP Fe2O3 1%

  TOP Fe3O4 4%

  MIDDLE FeO 95%

  Fe
• Non- stoichiometric is faster than stoichiometric
• FeO is p-type semiconductor

  High mobility of Fe2+ and high E-

  High growth rate

  porous structure
• Oxidation of Alloys:

  -different G- selective oxidation

  -different diffusion rate- may accelerate selective oxidation

  -complex oxide may form (spinel)

  -Solid solubility between oxides [(Cr-Fe)2O3]

  -O2 may diffuse into alloy resulting in either internal( damage material) or external (good protective layer)
• Protectivity:

  1.Pilling Bedworth Ratio (Rp-b)

  2.Stress Generation & Relief
• Pilling Bedworth Ratio (Rp-b)

  Rp-b = (oxide volume/metal volume)

  Rp-b < 1 Non protective- oxide cannot cover metal completely

  Rp-b >>1 Non protective- high stress- likely oxide scale- breakdown

  Rp-b ~ 1-2 Protective Eg. Al, Si, Cr
• Stress Generation and Relief

  a)Growth STRESS

  1. Pb ratio- volume difference of metal/oxide- thickness increase, stress increase
  2. Crystal mismatch- Epitaxal relations between metal and oxide
  3. Composition change
  4. Formation of new oxide
  5. Influence of specimen geometry( more stress in corners)
• Stress Generation and Relief

  b) THERMAL STRESS

  different expansion- not always one temperature

  Linear coefficient of expansion of metal NOT=oxide

  Mme/Mo >1 tensile (+)

  Mme/Mo <1 compressive- good cause close together and not easy to breakdown

  Mme/Mo >>1 tensile (++++)
• Relieving Stress:

  1. Plastic deformation of scale ( Steady Oxidation)
  2. Plastic deformation of metal (Steady Oxidation)
  3. Fracture of scale (Breakdown Oxidation) NOT FOR RELIEF
  4. Loss of scale adhesion to metal (Breakdown Oxidation). NOT FOR RELIEF If breakdown is severe, useful life is before these REE- Reactive element effect - Y, Ce can be used to improve resistance
• Materials used at High Temp

  Application

  1.Energy production and use, power stations, turbines

  2.Furnace parts
• Temp for common metals

  -C and low alloy steels up to 500C

  -Fe based alloy - S.S- Cr2O3 formers up to 800C

  Ferrite (bcc) - relatively cheap

  Martensite (bcc) - good mech prep

  Austenite (fcc) - good comb of mech +chem

  -Ni based alloys - up to 1000C

  -Ni of Fe based alloys with Al - up to 1200C

  Above 1200C use ceramics
• Effect of Al % on resistance

  -Fe/Ni alloys with Al

  1. Compact and dense
  2. Low diffusion rate(growth rate decreases)
  3. More chemically stable than Cr2O3 (composition does not change)
  4. Volatile Cr2O3 forms >1000C

  Problem: Al2O3 breaks easily (brittle)

  high Al%= mechanical properties decrease
• Why most oxidation reactions are faster at higher temperature:

  Oxidation reactions are thermally activated reactions. Higher thermal energy can accelerate mass transfer (diffusion) and other reaction forces
• Oxides are more stable at lower temperature as their formation of free energy is more negative
• Easier to reach passivity when ic is lower

  More stable when passive state is wider

  lower Ip therefore lower corrosion rate