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;
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.