Corrosion

• Corrosion


  Interactions between a material and its environment leading to a degradation in the material properties
• Three factors that govern corrosion


  • The components of material
  • Protective equipment
  • Environment
• Corrosion of metals


  An unintentional and destructive attack on a metal; it is an electrochemical process that typically starts at the surface
• Three ways metals corrode


  • Dry corrosion - Direct oxidation of metals
  • Wet corrosion - Exposed to air and moisture, Reacts with acid rain
  • Galvanic corrosion - Close contact between two different metals
• Corrosion of ceramics


  Usually entails simple chemical dissolution; however, in contrast to metals have electrochemical processes
• Corrosion of polymers (degradation)


  When exposed to a liquid solvent, it can dissolve or absorb, causing swelling. Heat and electromagnetic radiation, particularly ultraviolet light, can alter the molecular structures of these substances
• Polymer degradation processes


  • Swelling and dissolution
  • Scission - Severance or rupture of molecular chain bonds
  • Weathering - Resultant degradation which may be a combination of several different processes
• Electrochemical nature of aqueous reaction


  Process between chemical and electrical reaction
• Types of electrochemical reactions


  • Galvanic cell
  • Electrolytic cell
• For metallic materials, the corrosion process is normally electrochemical
• Components of an electrochemical cell


  • Anode - Oxidation occurs; metal corrodes
  • Cathode - Reduction occurs
  • Electrolyte - Substance that lies between the electrodes; aqueous environment
  • Electrical connection - Enables the movement of electrons from the anode to cathode
• Anode


  Positively charged electron, attracts electrons or anions, source of positive charge, electron acceptor
• Cathode


  Negatively charged electron, attracts cations or positive charge, source of electrons, electron donor, may accept positive charge
• Molarity


  Number of moles of solute per liter of solution
• Standard half cell


  Used to determine the standard electrode potential (Eo) cell for any voltaic cell and predict whether a specific redox reaction is product-favored
• Electromotive force (EMF)


  Generated by coupling the standard hydrogen electrode to standard half-cells for various metals and ranking them according to measured voltage
• Nernst


  Enables the determination of cell potential under non-standard conditions
• Types of corrosion


  • Atmospheric
  • Galvanic
  • Corrosion accelerated by mechanical stress - Failure of a component, Stress corrosion, Fretting corrosion, Corrosion fatigue, Impingement corrosion
• Corrosion penetration rate


  Also known as corrosion rate, material removal rate as a result of the chemical action and is an important corrosion parameter
• Corrosion rate equation: W = the weight loss after exposure time, t = time, ρ = density, A = exposed specimen area, K = constant (534 in mpy; 87.6 in mm/yr), CPR = expressed in mpy or mm/yr
• Corrosion rate in terms of current equation


  i = current per unit surface area, n = number of electrons, F = 96,500 C/mol, r = rate (mol/m2s)
• Types of polarization


  • Activation - Reaction rate controlled by slowest step
  • Concentration - Reaction rate limited by diffusion in solution
• Forms of corrosion


  • Atmospheric corrosion
  • Erosion corrosion
  • Selective corrosion
  • Uniform corrosion
  • Pitting corrosion
  • Fretting corrosion
  • Stress corrosion
  • Intergranular corrosion
  • Corrosion fatigue
• Factors affecting corrosion


  • Structural design
  • Environment
  • Applied or internal stress
  • Composition and structure
  • Temperature
• Corrosion prevention and protection methods


  • Material selection - Metallic, non-metallic
  • Improvement in material
  • Design of structure
  • Alteration of environment - Temperature, velocity, removing oxygen/oxidizers, changing concentration
  • Cathodic protection
  • Coatings