Stainless in a Marine enviorment

[Electronics Guru]

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Stainless Steel

Because there are so many misconceptions about stainless steel (a misleading term in itself, though not as bad as ‘inox’) it’s probably worth momentarily delving into the technicalities. As well as iron and carbon, stainless steels include a number of alloying elements. Of these the most important is chromium (Cr.). If there is more than 12% in the alloy, a complete layer of chromium oxide surrounds the metal. This layer, the ‘passive’ film, is resistant to most things and will self-repair in the presence of oxygen. Chromium-only stainless steels tend to be brittle, so about half as much nickel (Ni) is added to create a more usable material. 304 stainless (or A2) is one of the more commonly available and includes 18% Cr and 10% Ni. If you have a stainless sink or exhaust pipe it’s likely to be 304 and, as anyone who’s ever tried cleaning a sink or pulpit will know, is somewhat prone to attack from the organic acids generated by food, fingerprints and other pollutants.

The chemical and food industries alleviate these problems by adding a dash of Molybdenum (Mo). Thus 316 stainless (or A4) typically comprises 17% Cr, 11% Ni, 2 % Mo and is widely used to store and transport some very aggressive substances. So, you might think that this is the perfect stuff to use as a fastener in or through wood, and from the sole perspective of chemical attack you’d be right. But we need to reconsider the environment in which the fastener is doing its job. Imagine a bolt, nail or screw fastening a plank to a frame underwater. The head, at or near the surface, will be oxygenated enough to maintain its passive film. The shank, buried deep in the structure, is likely to be starved of oxygen but will be surrounded by various acids and chlorides. In these circumstances, the passive film may break such that the stainless becomes ‘active’. This has two effects: firstly, look back at the galvanic series and you’ll see that the difference between active and passive electropotentials in 304, and to a lesser extent in 316, is enough to cause galvanic corrosion. Like brass, stainless can form its own galvanic couple. Secondly, without the oxide layer, the stainless will corrode about as fast as steel. The upshot is that stainless fasteners below the water-line - irrespective of the grade - may be no better than mild steel. Above the water-line (more oxygen and less electrolyte) such fastenings are fine, but unless you value the extra lustre of 316, there’s little point in paying for it.

While on the subject , I’d like to tackle the nonsense of shot-blasted stainless fittings which seek to ape the appearance of galvanised fittings. The ability of the passive film to self-repair is optimised if the surface of the stainless is highly polished. By forming millions of sharp peaks during shot-blasting, you significantly reduce this ability, which is why such fittings rust. If you want the appearance of galvanised fittings, try galvanised fittings.