Iron, exposed to moist air, will react with oxygen in the air to form iron oxide. This oxidation process is called rusting.

iron + oxygen iron(III) oxide. 4Fe(s) + 3O2(g) 2Fe2O3(s)

  • Rusting requires both oxygen and water.
    • Salt, acid and higher temperatures will accelerate oxidation (rusting.)
  • Iron and steel are most commonly protected by painting, plastic coating, and metal plating.
    • Galvanizing (Zinc plating) is another alternative
  • The oxide formed by oxidation does not firmly adhere to the surface of the metal and flakes off easily causing “pitting.” Extensive pitting eventually causes structural weakness and disintegration of the metal.
  • Aluminum, forms a very tough oxide coating (Aluminum oxide Al2O3 ) which bonds to the surface of the metal preventing the surface from further exposure to oxygen and corrosion.
  • The patina we see on brass and bronze is really corrosion but it protects the metal from further deterioration. In some cases it is considered aesthetically pleasing. The oxidation of copper produces a protective coating of copper oxide, which is red, but then thickens to give a familiar green patina and/or the dark brownish color of bronze statuary.

Types of corrosion

  • General corrosion / uniform corrosion. Metal corrodes uniformly all over the surface.
    • Graphitization. Gray iron is a matrix of iron and graphite that acts as a lubricant during machining. A galvanic action can take place between these dissimilar materials that will cause the iron to gradually go into solution, leaving the graphite behind. This can cause the casting to break off into pieces.
    • Selective leaching can occur when di-ionized water circulates through pipes and pumps and extracts those minerals that were removed during the di-ionization process.
  • Local corrosion. Part of the structure corrodes at a considerably higher than average rate. The categories of local corrosion are:
    • Pitting corrosion. The corrosion effect is concentrated on localized areas and leads to pitting.
    • Crevice corrosion proceeds at locations covered by a corrosion product and other deposits (dirt or trash). Crevice corrosion typically occurs in small cavities, gaps, recession, etc.
    • Galvanic corrosion requires two different metals, constituting a corrosion cell. A structure should contain metals that are as close to each other as possible in the galvanic electric series.
    • Intergranular corrosion proceeds along the metal grain boundaries.
    • Interfacial corrosion occurs at water-air interfaces.
    • Cavitation Erosion occurs when passivated metal pieces are removed as the bubbles collapse, leaving the active metal exposed to corrosion. The process is ongoing.
  • Selective corrosion occurs when one element in an alloy dissolves faster than the others.
  • Exfoliation corrosion attacks the exposed material end grain and can work its way parallel to the metal surface, creating products of greater volume than the original material and causing splitting of material layers, leading to a stratified appearance
  • Tuberculation corrosion– the knob like mound byproducts of corrosion that build up on the interior walls of unlined piping  and pumps, restricting liquid flow. A severe problem with the iron pumps used in municipal water systems.

Combined effect of mechanical factors and corrosion.

  • Mechanical wearing as well as static or dynamic stresses often act in combination with corrosion. The main categories for the combined effect of mechanical factors and corrosion are:
    • Stress corrosion occurs when a metal in a corrosive environment is exposed to static or residual stress that results in fracture. Shot peening and heat treating are a couple of the remedies to this type of corrosion
    • Corrosion fatigue is caused by the combined effect of corrosion and varying state of stress.
    • Erosion corrosion is acceleration in the rate of corrosion caused by high velocity of a liquid, or solid impurities carried by a liquid.
    • Cavitation corrosion is erosion caused by the combined effect of corrosion and the pressure caused by the breaking of gas bubbles formed in liquid (cavitation) damaging the passivated coating
    • Fretting corrosion occurs between two metals rubbing against each other under corrosive conditions. The rubbing removes the passivated layer that had formed on the metal parts. Lip or grease seals are another common cause for the removal of this protective layer.