Corrosion of Materials Other Than Steel

6 Corrosion of Materials Other Than Steel Cast Iron 

Coating of the exterior of cast iron pipe with bituminous or lower oil coatings has often been used to provide a measure of protection. More recently tape wrapping with self adherent tapes have been found to be a very cost-economical method of providing an efficient barrier between the metal and a corrosive soil Fig. 10.6. CP can then be applied provided the pipe sections are electrically joined. 

Zinc may be used as a reference half-cell in soils. However, the main application of zinc in buried applications is in galvanized steel, for example, in the fabrication of culverts. Performance may be adequate unless soils are poorly aerated, acidic, or highly contaminated with chlorides, sulfides, and other corrosive ions. Well-drained soils with a coarse texture (the sandy type) provide a high degree of aeration. It should also be borne in mind that zinc corrodes rapidly under highly alkaline conditions. Such conditions can arise on the surface of cathodically overprotected structures. The degree of corrosion protection afforded by galvanizing obviously increases with the 

Stainless steels are rarely used in soil applications, as their corrosion performance in soil is generally poor and not better than bare steel. Localized corrosion attack is a particularly serious concern. The presence of chloride ions and concentration cells developed on the surface of these alloys tends to induce localized corrosion damage. 

Since pitting tends to be initiated at relatively high corrosion potential values, higher redox potentials increase the localized corrosion risk. Common grades of stainless steel and even the very highly alloyed versions are certainly not immune to MIC. 

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