Stainless Steel A2 vs A4 or 304 vs 316
At Boat Fittings the vast majority of our stainless steel fasteners (nuts, bolts and washers) as well as our shackles, cleats hinges and deck fittings are 316 (sometimes called A4) and more generally known as Marine Grade Stainless Steel. In our experience no commonly used stainless steel is absolutely 100% guaranteed not to suffer from any staining (ie light surface rust) when subjected to marine (ie salt water) conditions for an extended period of time.
That said, there are two main classifications of stainless steel commonly available and in use. These are called A2 and A4. A2 is also known as 304, whereas A4 is also known as 316. When buying stainless steel for external marine use (on the outside of our boat) we should choose A4 (316) if we want it to keep it’s shiny stain-free appearance over the seasons. If we use A2 stainless parts on the outside of our boat it will only be a matter of time before some degree of surface rust (staining) becomes visible like in the photograph below.
For internal fittings and fixtures (ie on the inside of the boat) as long as they are not likely to be submerged in areas of the bilges, A2 fixings and fittings are less likely to be an issue. This is the reason that a few of the hinges that we sell at boatfittings.co.uk are A2 grade and these would be generally fine for inside lockers, chart table lids etc. We would not recommend them for external marine use unless a degree of staining is acceptable. This is more often a cosmetic issue rather than a structural or functional problem.
For some reasons in the manufacturing supply chain it is more common for cast components (eg cleats, fairleads and the thicker types of hinges) to be made from 316 stainless, where as parts made from thinner-wall stainless steel which are bent or pressed into their final shape are more commonly available in 304 (ie not marine-grade).