casing materials. The strength and rigidity of metallic casing materials are sufficient to

withstand virtually any subsurface condition encountered in a ground water monitoring

situation, but metallic materials may be subject to corrosion during long term exposure in

certain subsurface geochemical environments.

Corrosion is defined as the weakening or destruction of a material by chemical action.

Corrosion of metallic well casings and well intakes can both limit the useful life of the

monitoring well installation and result in ground water sample analytical bias. It is important,

therefore, to select both casing and screen that are made from corrosion resistant materials.

Several well defined forms of corrosive attack on metallic materials have been

observed. In all forms, corrosion proceeds by electrochemical action, and water in contact

with the metal is an essential factor. According to Driscoll (1986), the forms of corrosion

typical in environments where well casing and well intake materials are installed include:

General oxidation or "rusting" of the metallic surface, resulting in uniform

destruction of the surface with occasional perforation in some areas;

Selective corrosion (dezincification) or loss of one element of an alloy, leaving

a structurally weakened material;

Bi metallic corrosion, caused by the creation of a galvanic cell at or near the

juncture of two different metals;

Pitting corrosion, or highly localized corrosion by pitting or perforation, with

little loss of metal outside of these areas; and

Stress corrosion, or corrosion induced in areas where the metal is highly

stressed.

To determine the potential for corrosion of metallic materials, the natural geochemical

conditions should first be determined. The following list of indicators can help recognize

potentially corrosive conditions (modified from Driscoll, 1986):

Low pH    if ground water pH is less than 7.0, water is acidic and corrosive

conditions exist;

High dissolved oxygen content    if dissolved oxygen content exceeds 2

milligrams per liter, corrosive water is indicated;

Presence of hydrogen sulfide (H S)    presence of H S in quantities as low as 1

2

2

milligram per liter can cause severe corrosion;

November 1992

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