Zinc Anodes in Soil

Cathodic protection is a very successful method of protecting structures buried in soil. This protection can be applied to pipelines, storage tanks, and building foundations. 

In certain soils, zinc anodes used in conjunction with coatings have lives of up to 5 years. After this time, the zinc anodes can be replaced and an additional period of protection obtained. Because the electrochemical potential of zinc is relatively low, current flow is moderate and there is no damage to any coating on the pipelines or structure. Since soils vary in their content of water and salts, the electrical resistivity may vary from day to day as well as depending on the type of soil. However, if the resistivity of the soil is greater than about 5 fl cm, the electrochemical potential will be too low to produce an adequate current, and protection will be less. 

To overcome this problem in soils of high resistivity, the space around the structure and the zinc anode is backfilled with a more highly conductive material. Soils containing significantly more dissolved sulfates and chlo- 

A particular hazard to buried structures is the growth of sulfate-reducing bacteria, which turn sulfates in the soil into particularly aggressive compounds. The slightly alkaline environment produced by cathodic protection greatly discourages the growth of these bacteria. 

On buried structures and pipelines that are initially wrapped either with or without the use of cathodic protection, the coating will eventually deteriorate. When it is inconvenient, uneconomical, or impossible to lift and rewrap, continued protection for the structure may be had by installing sufficient zinc anodes. Provided the zinc anodes are replaced when necessary, a relatively straightforward job, the useful life of the structure may be extended indefinitely. 

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