Marine grade stainless steel
Stainless steel is naturally corrosion resistant however corrosion can still occur when the metal is exposed to harsh or aggressive environments such as a salt water or marine environment. What gives stainless its resistant properties is the formation of a protective chromium oxide skin on the surface of the metal, protecting the base metal (and importantly the iron present) from exposure to moisture and therefore preventing the formation of iron oxide or rust.
Sodium chloride (or salt) from sea water can attack this protective layer and restrict it from naturally reforming and so leaving the base, iron rich metal exposed and susceptible to pitting or crevice corrosion.
Although there is no industry standard marine grade stainless steel, 316 or 316L are the most common grades used for this application due to the inclusion of molybdenum in their composition. Although molybdenum is found in some other grades of stainless steel, it is the relative high concentration (2.0 – 2.5%) present in 316 that helps to stop the salt water causing pitting or crevice corrosion however, it does mean that these grades are more expensive than some other grades such as 304.