Dangers of Crevice Corrosion

11 Jan

Most sailors are familiar with the phenomenon of crevice corrosion in chainplates and rigging hardware and we all know it is good practice to make a thorough visual inspection of this gear at least once a year. That said, many of us are also on a budget and pinching pennies can sometimes lead to questionable decisions about the working life of hardware. That’s precisely the problem with crevice corrosion. It is difficult to detect and warning signs are often fairly minor; a severely compromised fitting may exhibit just a touch of rust staining or a very minor surface pitting and it is tempting to think that the damage cannot be much worse than it looks. Let me tell you firsthand that THIS IS NOT THE CASE! In my short sailing career I have done a lot of fairly tough sailing on old boats and I have a seen a handful of stainless steel failures due to crevice corrosion. In every incident there was telltale visual evidence of corrosion, however small. Unfortunately I learned this lesson again over the weekend where an accident in my workshop can serve as a fine illustration of the dangers of ignoring crevice corrosion.

Here’s another shot of the block at the top of this post:

Clearly it’s in very rough shape. However, if we ignore the cracks from where it broke it didn’t look so bad when we decided to use it:

Not great, for sure, but not terrible. There is definitely some visible rust staining, enough that I would not have put this back into service on a boat. However, I’m always pinching pennies and a block and tackle is a very useful thing to have around a shop so I put it into a drawer rather than the trash. It failed when we tried to use it to lift this table:

The table looks beefy and it is, for a coffee table, but it’s still only just over 100lbs of load on a tackle that was designed for the mainsheet of a 27′ bluewater sailboat. I didn’t give it a second thought. It turns out the block was just strong enough to get the table about 12′ into the air but not strong enough to hold it for long. When it went with a bang the table came crashing down and my shopmate, who was 8′ in the air on a ladder guiding it into place, came down neatly on top the whole mess. Fortunately he took a lot of the force out by holding onto the line as the table went down and he saved this beautiful hand-carved antique from shattering into a hundred pieces. Unfortunately, he got a hell of a rope burn. Still, we’re lucky that neither of us was seriously injured in the accident.

The takeaway is obvious- if you have a piece of stainless hardware that shows even the slightest rust staining or pitting consign it to the scrap heap immediately! Don’t even save it for ‘light’ use, it’s just not worth the risk. You might end up with a nasty rope burn like this, or much worse.

If you’re interested, here’s a bit of the science behind this. The mechanism of corrosion  is fairly simple, and has to do with the properties that make ‘stainless’ steel resistant to corrosion in the first place. Stainless steel gets its corrosion protection from a very thin layer of chromium oxide which naturally forms on the surface when oxygen (generally from the air) is present. If this oxide is scratched or buffed away it will simply reform as long as oxygen is still present. However, in any situation where there is no oxygen to  renew the protective layer stainless will be quite susceptible to corrosion. This is why stainless should never be used for crucial fittings below the waterline. Unfortunately, stainless doesn’t need to be completely submerged to be in an anaerobic (without oxygen) environment. This failed block exhibits one of the most common modes of corrosion for stainless- it looked just a little rusty from the front but when the strap sheared off and we were able to see the backside it is obvious that this is where most of the corrosion was occurring. It’s likely that small amounts of seawater displaced the air between the stainless strap and the plastic body of the block and caused corrosion which was rampant but hidden.
Worse, once corrosion starts in stainless it rapidly penetrates the metal in highly destructive ways. Unlike regular steel which rusts over the whole of its surface corrosion in stainless tends to start at one or more points and then move inwards through the piece. This is where the term crevice corrosion comes from- a small amount of initial corrosion creates a microscopic crevice in the material which becomes an oxygen-free breeding ground for further corrosion. As corrosion travels deeper into the material it greatly weakens it, eventually leading to a fracture like the one in the strap on our block. It also makes the piece far more vulnerable to stress cracking from repeated motion. Another classic example is the all-too-common failure of in-deck chainplates at the point just below where they enter the deck. Often seawater gets into this joint and sits against the chainplate creating crevice corrosion where it cannot be seen. Any movement due to pumping of a loose rig or incorrect angles in the original installation exacerbates the issue by causing stress fractures and eventually the chainplate can shear off, sometimes without exhibiting any symptoms of corrosion abovedecks. Just another reason why all stainless chainplates should be pulled and examined every 10-20 years!

This article was syndicated from Safe At Harbour But Meant For The Sea: DIY Sailing with Paul Calder

Comments

  1. Herb Clark

    Excellent advice, well written, and very timely as we get ready to enjoy spring sailing. Nothing like a careful inspection of all the stainless before heading out.
    Cheers to the New Year!

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