1157 Selective Corrosion

Corrosion, which occurs at preferred sites on a metal surface, may be described as selective corrosion. As metals are rarely uniform in composition or structure and contain grain boundaries as a result of solute segregation during solidification, spatial separation of anodic and cathodic sites may readily occur. Typical examples of selective corrosion include (i) intergranular corrosion and (ii) selective leaching or demetallification.

Intergranular corrosion is caused primarily by a difference in the chemical activity between the atoms within the grain boundary region and the alloy-metal matrix. The concentration of impurities at the grain boundaries and enrichment/depletion of alloying elements in the grain boundary area therefore influence intergranular corrosion. Heat treatment processes that cause preferential migration of elements within the material can result in sensitization of the microstructure, for example, 304 stainless steel with carbon contents above 0.05%, heated in the temperature range 500-800°C suffer chromium carbide precipitation at the grain boundaries. Sensitization may be prevented by quenching quickly through the critical temperature range, using very low carbon content steels (<0.05%) or adding carbide stabilizers such as Ti or Nb (<1%).

Exfoliation of Al alloys is another example of selective grain boundary corrosion being especially dramatic in alloys where there is a strong rolled texture, that is, in wrought alloys, which do not recrystallize during heat treatment. Elongated pancake-shaped grains are susceptible as corrosion attack occurs along paths parallel to the working direction. The corrosion products formed have a volume greater than the original material, causing swelling and eventual mechanical separation of individual layers. Alloys that are susceptible to exfoliation, include, Al-Cu, Al-Zn-Mg, and Al-Mg. However, exfoliation is not encountered in alloys having an equiaxed grain structure.

Dezincification of brasses is a well-known example of selective corrosion. The area around a dezincified zone is mechanically weak and porous and therefore lowers the mechanical properties of the alloy. Susceptibility of the 70Cu/30Zn alloy may be decreased by the addition of 0.05% As (arsenic). Graphitization of cast-iron is another example of selective corrosion, gray cast-iron being most susceptible. In this case graphite flakes are more noble than the other phases in the alloy and act as cathode sites with the matrix acting as the anode. The addition of Ni (13-36%) gives rise to an austenitic matrix and improved resistance to corrosion. White cast-iron contains no graphite and therefore is not susceptible. In the case of spheroidal cast-iron the graphite particles are found as discrete spheroids and not interlinked as in the case graphitic cast-iron; therefore this type of cast-iron does not suffer from graphitization.

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