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.

                                    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"