Seawater Resistance of Stainless Steels
Stainless steels are susceptible to crevice or pitting attack in chloride bearing waters. Their behavior has been studied by a number of investigators. There is considerable variation in the percentage of apparently identical sites where attack occurs, when it occurs. It is useful to describe results in terms of the percentage of apparently identical sites where attack occurs at a given chloride concentration. Very tight crevices increase the likelihood of attack. Rough surfaces, sheared edges, scratches and similar imperfections also tend to increase the incidence of attack. Crevice or pitting attack also occurs under deposits and under biofouling growths attached to the metal surface.
Relative resistance can be described by the chloride concentration below which there is little likelihood of crevice attack occurring. The ability of chlorides to concentrate in some crevices means that occasional attack may occur at lower concentrations than shown in the following table. Nevertheless, the table provides useful guidelines.
Guidelines for relative resistance of stainless steels to crevice attack in natural waters
Alloy Chloride concentration below which crevice corrosion is rare
400 series: crevice attack occurs in fresh waters
304SS: 100-200 ppm
316SS: 1000 ppm
The 4 1/2% Mo and duplex stainless steels are more resistant than type 316, but suffer varying degrees of crevice attack in brackish water and seawater.
The 6% Mo stainless steels have excellent resistance to crevice attack in seawater.
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