My friend Glen and I were working on Mallard's rig the other night when Glen spotted a hairline crack in one of the stainless shackles. The shackle was holding on, but only just and I was very easily able to pull the two bits apart. I would appear that a small crack had at some time started in the shackle, water had found it's way in and the insidious corrosion started. Luckily we spotted it just in time or something bad might have happened sooner rather than later.
For crevice corrosion to start there has to be certain factors in play. Firstly as it's name implies there has to be a crevice where water can collect, if the crack is too wide then air can get in and crevice corrosion is unlikely. Secondly there has to be moisture to enable the corrosion to occur. We had both with the shackle, it is quite likely that over tightening the shackle at some point in the past has fractured the top of the bow by an imperceptible amount, sea water got in and the corrosion followed shortly after. Sea water is an ideal electrolyte and although crevice corrosion can and does occur with fresh water salt water exacerbates the problems and failure will often be sooner than would be the case with fresh water.
It should be noted that crevice corrosion only occurs under very specific circumstances, namely; there has to be a microscopic crack, this crack then has to fill with moisture which then becomes de oxygenated and finally the part has to go undisturbed allowing the corrosion to take place. All these factors were in place with the shackle it seems. The only cure is a sharp lookout and to be aware of the factors that allow crevice corrosion to take place.
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