Q:
What Are Some of the Important Factors
in Oxidation of Fe Alloys?





A:
  • Rust is non adherent, and therefore, is not a
    protective coating.

  • Rust (Fe2O3/Fe3,O4) forms electro-chemically
    at ambient temperatures in moist air. Higher
    temperatures accelerate the process in some
    instances, e.g., process equipment and power
    generation systems, etc.

  • Differences in electrical potential among small areas
    on the steel surface forming anodes and cathodes
    connected by an electrolyte (moist air, etc.)

  • Impurities in the electrolyte increase conductivity, which
    increases the rate of corrosion. Transfer of electrons
    from the anodic grains to the cathodic adjacent grains at
    the start, then to the rust as it forms.

  • On a steel surface, there are millions of exposed anodes
    and cathodes (grains) with minute current flows among
    them in the presence of an electrolyte (moist air).




  • As the rusting process proceeds, anodes and cathodes
    (grains) are consumed and new ones are created. As
    the metal is consumed in the anodic area, a small electric
    current begins to flow. The iron ions produced in the anode
    combine with the environment and form the loose, flaky iron
    oxide known as rust.

  • As anode areas corrode, new material of different
    composition and structure is exposed. A gradual change
    takes place in the electrical potentials and the anodic
    and cathodic site locations.

  • As the rusting process proceeds, previously uncorroded
    areas are attacked, and a uniform surface corrosion is
    produced. The process continues until the steel is entirely
    consumed.