What sort of stainless steel do I use for my boat?

Stainless steel is a generic term used for a steel alloy that is highly resistant to corrosion. The word “alloy” means it is iron mixed with other metals, which in this case is more than 10.5 per cent or more of a metal called chromium. It’s the same stuff that is plated in a thin layer on top of steel parts to create the shiny finish known as chrome.

Stainless steel is very good at resisting corrosion brought about by oxidation, the process that creates rust on normal steel.

Getting all scientific, the chromium oxide mixed in with the steel creates a thin protective barrier all over the surface of the stainless steel. 

Stainless steel is also self-healing, meaning that if the stainless steel surface is nicked or scratched, the chromium oxide barrier re-forms itself to offer corrosion protection that’s as good as when the bit of steel was new.

One of the other benefits of stainless steel is its strength. If you’ve ever replaced a boat trailer’s bearings, you’ll notice that the bearings and the housings they come in are made from stainless steel.

The downside to all that strength is that when stainless steel fails, it is often quite sudden and unexpected. Unlike normal steel, which will stretch and twist, stainless steel fails – and here’s a word we don’t use often – catastrophically, meaning that one minute it is working fine, and the next it’s in several pieces.

That makes it very important to inspect stainless steel that’s under a lot of stress – say, attached to a mast stay or holding an outboard engine out of the water – at least once a year looking for signs of fatigue such as cracking.

Because it is resistant to rust, stays nice and shiny, and needs little attention over its lifespan, stainless steel is pretty much maintenance-free.

The main places you will see stainless steel on a boat are on the railings, any metal handrails, pushpits and pulpits, ladders, deck fittings, snapper racks and anchor rodes.

The best kind of stainless steel to use on boats is type 316, which has a higher resistance to corrosion compared with more common grades of stainless.

You can tell if a component is a grade you need by looking for a “316” stamped somewhere on the part.

Anything that doesn’t have that stamp is likely to be type 304, which has less corrosion resistance than 316 to chlorides. Why is that important? Well, saltwater contains salt, also known as sodium chloride. Being able to resist chlorine in saltwater therefore has its benefits.

You’ll know if you’ve mistakenly bought 304 stainless steel because once exposed to seawater and salt air, it will start rusting as the chlorine from the salt begins to eat into it. You can clean the rust off, but you’ll notice that over time the surface of the stainless steel will become pitted as the surface is eaten away by corrosion.

If you see rust on a stainless steel component, replace it rather than clean it, because if the corrosion weakens the stainless steel, it will let go with a bang rather than a whimper.

About the only downside to 316 stainless steel is that you do pay more for it than the cheaper 304 grade. That’s because 316 stainless steel’s higher resistance to chlorine corrosion comes from more metals mixed in with the iron, altering its chemical make-up.

However, the extra expense of 316 stainless steel over 304 is easily compensated by the extra durability you will get from it.

Always carefully inspect stainless steel parts before you buy them to ensure they have no manufacturing flaws that are likely to result in an unexpected failure.

Be careful, too, about working with stainless steel. Stainless steel plate is surprisingly difficult to bend, and even more difficult to drill – always use sharp bits and drill at very low speeds using a pilot hole for larger bit sizes.