Why do I get rust stains around my stainless steel fittings
Stainless steel is known in German as ‘Rostfrei’ and in French as ‘Inoxydable’. These are all misnomers as they all imply that stainless steel does not oxidise or corrode. In fact it oxidises almost instantly on exposure to air to form a layer that is microscopically thin. It is the chromium and nickel content in the alloy that oxidises and forms a coating that is transparent and most importantly is impervious to oxygen. This effectively prevents further oxidisation and is known as passivation. The only time you are likely to see the oxide coating is if the metal is heated such as welding where it turns blue.
Stainless steel needs the constant presence of oxygen to remain corrosion resistant because should the passivation layer be damaged it needs oxygen to immediately re-oxidise. Where stainless steel in encased and denied air, particularly if there is high salinity present, anaerobic pitting corrosion will occur. All corrosion is essentially electrochemical, with erosion of electrons from the anodic surface to the cathodic surface within an electrolyte or saltwater.
Pitting corrosion is, as its name implies, deep pitting rather than the uniform surface corrosion found on mild steels. Passivated stainless steel is widely separated from active stainless steel on the galvanic scale. Any damage to the passivation layer becomes anodic. Due to the tiny size of the anodic pit compared to the surrounding metal which is cathodic, galvanic action results in highly concentrated aggressive erosion of the metal. The ferrous component of the corrosion that washes out of the fitting appears as surface rust.
This kind of pitting corrosion is extremely insidious as it can erodes deep into the structure of the metal. It can lead to stress corrosion fracturing which is the propagation of cracks in a corrosive environment. This can cause the sudden failure of metals that are subjected to stress loads. For that reason if rusty water stains emerge from structural steel fittings such as chainplates or keel bolts the fittings should be withdrawn and inspected. They should be replaced if necessary