Marine grade stainless

Marine grade stainless steel is a misunderstood description. There is no industry recognized definition for a marine grade stainless steel though it is routinely used by many end-users. In a marine environment it must be made clear as to whether the stainless steel is submerged in seawater or is simply near enough to the seashore such that it can be attacked by the chloride present in seawater by splashing or carried by onshore breeze. Chloride ions can cause localized corrosive attack (pitting and crevice corrosion) of susceptible stainless steels.[1]

When the stainless steel will be submerged, a PREN > 40 is typically specified as the minimum for resistance to seawater. Stainless steels, such as super austenitic stainless steels (for example UNS S31254 or N08367), or super duplex stainless steels (for example UNS S32760 or S32750) meet this requirement.[2][3]

Near the seashore 316L is typically considered the minimum grade of for use in such a marine environment.[4]

Concentrations of chloride in seawater can vary, and splash zones can cause concentrations to increase dramatically by evaporation, thus the corrosive severity of marine environments can vary. The resistance of any stainless steel near the seashore will also be dependent on whether the stainless surface can be rinsed by rainfall, which will reduce the tendency for surface chlorides to concentrate by evaporation. Thus, the underside of overhangs will be more susceptible to corrosion due to lack of rinsing, also desert locations near the seashore will typically be more corrosive than seashores in locations with high rainfall.

Visible evidence of corrosive attack in a marine environment is known as “tea staining”.[5]