Marine Stainless Steel Corrosion

A Stainless Time Bomb

Like most sailors I use a small magnet to test every piece of “stainless steel” marine hardware before I buy it.

While this is a good first line of defence, unfortunately it’s nowhere near enough these days. Even the most respected brands now source their marine hardware from all over the world and there is no way we ordinary sailors can be certain of the quality of the steel used in its manufacture.

We’re also in the unfortunate position of possibly having to bet our life on the quality of the stainless steel on out boats. We rely on our stainless chainplates, turnbuckles, bolts, shackles and other fittings to keep our masts up and we rely on other bolts and fittings to keep our keels on.

The term “stainless steel” covers over 500 different steel alloys of which only 2 are recommended for marine use. These are 304 stainless and 316 stainless. Both alloys consist mainly of iron, chromium and nickel, but the 316 stainless contains 2% molybdenum which increases its corrosion resistance somewhat.

If we could be absolutely certain that all our critical hardware was properly made of 316 stainless steel we could rest easy – couldn’t we?

Afraid not.

There’s an unseen and largely unknown time bomb on board. It’s called ….

crevice corrosion

For marine grade stainless steel to resist corrosion it needs oxygen. Wherever you have stainless steel sitting in stagnant water (i.e. your bilge) or enclosed in wet material without oxygen you’ll get crevice corrosion.

Salt water penetrates the minute crevices in the material caused by forming or welding the piece of hardware or in the crevices formed between the threads of a turnbuckle or bolt and nut. Because there is no oxygen present, the corrosion begins and it will probably not be discovered until the piece fails.

And depending where that piece of hardware is, the failure could be catastrophic.

hose clamps

One of the most critical areas for crevice corrosion is in stainless steel hose clamps. Even the British manufacturer of stainless hose clamps warns that these clamps are not failure proof and should be properly inspected at least once a year. They corrode around the screw threads.

keel bolts

Another major area of concern is stainless keel bolts. If bilge water can get down between the keel bolt and the keelson you have the perfect recipe for crevice corrosion.

When you haul your boat out for a hull clean, after the hull has been cleaned off and allowed to dry, check the hull-keel joint. Any water weeping out of this joint is a warning sign that there is water around the keel bolts and probable crevice corrosion.

turnbuckles

The lower ends of the deck turnbuckles are the most likely to corrode, specifically around the threads. All turnbuckles should be dismantled, inspected, cleaned and lubricated at least once a year.

spreader tips

Turnbuckles at spreader tips because of discontinuous shrouds are a prime site for crevice corrosion. They should be disassembled and checked each year.

maintenance and inspection

The best way to keep crevice corrosion at bay is to thoroughly inspect all the critical stainless fittings at least once a year.

Check your stainless fittings using a magnifying glass to check for hairline cracks. There are crack test dye kits available from most chandleries.

Keep all your stainless clean and polished, so that any corrosion stain is easy to see. Any brown stains along hairline cracks are cause for concern.

… and check your hose clamps NOW!