perhaps the most practical from a corrosion resistance and cost standpoint. It is composed of

slightly more than 18 percent iron and not more than 0.08 percent carbon (Driscoll, 1986).

Chromium and nickel give the Type 304 alloy resistance to corrosion; the low carbon content

improves weldability. Type 316 stainless steel is compositionally similar to Type 304 with

one exception    Type 316 has a 2 to 3 percent molybdenum content and a higher nickel

content that replaces the equivalent percentage of iron. This compositional difference

provides Type 316 stainless steel with an improved resistance to sulfur containing compounds

and sulfuric acid solutions (Barcelona et al., 1983). Type 316 generally performs better than

Type 304 under reducing conditions.

For either formulation of stainless steel, exposure to corrosive conditions may result in

corrosion and the subsequent contamination of samples by metals such as chromium or

nickel. According to Barcelona et al. (1983), Type 316 stainless steel is less susceptible to

pitting or pinhole corrosion caused by organic acids or halide solutions. However,

Laboratory studies by Hewitt (1989) and Parker et al. (1990) showed that rusting began

within 1 to 2 days for pieces of both Type 304 and Type 316 casings exposed to well water

with high dissolved oxygen. Recent work by Barcelona and Helfrich (1986, 1988) and

Barcelona et al. (1988a) suggests that biological activity may alter geochemistry near stainless

steel wells. Iron bacteria, which oxidize ferrous iron to ferric iron, can cause encrustation of

any type of casing material, including PVC or PTFE, if the water contains ferrous iron

(Lloyde and Heathcote, 1985). Encrustation can lead to failure of the screen due to blockage

(Lloyde and Heathcote, 1985). Under anaerobic conditions, sulfate reducing bacteria can

actively cause corrosion of stainless steel (Lloyde and Heathcote, 1985).

The following advantages and disadvantages of stainless steel are highlighted by Aller

et al. (1989) and by Nielsen and Schalla (1991):

Advantages of stainless steel well casing and screen materials:

High strength in wide range of temperatures;

Readily available;

High open area screens available;

Suitable for driven wells;

Not degraded by organic solvents;

Low potential for sorption of organic compounds; and

Best material for monitoring trace level organics.

November 1992

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