Stainless steels are susceptible to crevice 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 stationary crevices increase
 the likelihood of attack.  Rough surfaces, sheared edges, scratches and
 similar imperfections also tend to increase the incidence of attack.
 Crevice attack also occurs under deposits and under biofouling growths
 attached to the 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.

                                    TABLE
           Guidelines for relative resistance of stainless steels
                     to crevice attack in natural waters

Stainless steel     Chloride concentration below which
                    crevice attack is rare

430 & series 400    Crevice attack occurs in fresh waters

304                 100-200 ppm

316                 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.

For further information:  NiDI 11 003 "Guidelines for Selection of Stainless
  Steels for Marine Environments, Natural Waters and Brines"
