Ah yes. What exactly IS Stainless Steel
The definition “Stainless Steels”, are those ferrous alloys that contain a minimum of 12% chromium for corrosion resistance.That leaves the definition of SS fairly wide open. The "Austenitic" grades are those alloys which are commonly in use for stainless applications. The austenitic grades are not magnetic. The most common austenitic alloys are iron-chromium-nickel steels and are widely known as the 300 series (what we usually use is 304 ande 316 SS for Marine Applications) The "Martensitic" grades were developed in order to provide a group of stainless alloys that would be corrosion resistant and hardenable by heat treating. The martensitic grades are straight chromium steels containing no nickel. They are magnetic and can be hardened by heat treating. The martensitic grades are mainly used where hardness, strength, and wear resistance are required. (e.g. 410 & 416 series) The "Ferritic" grades have been developed to provide a group of stainless steel to resist corrosion and oxidation, while being highly resistant to stress corrosion cracking. These steels are magnetic but cannot be hardened or strengthened by heat treatment. They can be cold worked and softened by annealing. As a group, they are more corrosive resistant than the martensitic grades, but generally inferior to the austenitic grades. Like martensitic grades, these are straight chromium steels with no nickel. They are used for decorative trim, sinks, and automotive applications, particularly exhaust systems. (e.g. 409 & 430) Here is a link to discussion on this http://www.sppusa.com/reference/white_paper/wp_ss.html So what does this all mean? Well, unless it says 304 or 316 SS then it might be one of the other alloys, but that does not mean its bad it just means that its different. All the different "types" are specifically engineered for a different purpose, for instance one might need to have a greater strength or on the other end of the spectrum a greater need to resist corrosion (e.g. medical grade Stainless Steel). Normally one will note that very corrosion proof SS is typically very brittle (high chromium) and not necessarily ideal in a marine environment where strength is important. From another site on Marine Stainless Steel and its uses "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." http://www.classicmarine.co.uk/Articles/metals.htm One other thing most people will notice is that 304 and 316 Marine Grade SS prices are increasing . I have heard the reason is mostly due to the price of "nickel" which has quadripled in the past few years (316 is up to 14% nickel)
