Ask Sail: Seeing Green

One of the stainless steel chainplates on my Olson 911S is tinted green both above and below deck. The fasteners that bolt the plate to the internal bulkhead do not show the same condition.
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Mono D'Angelo of Grosse Ile, Michigan, asks:

One of the stainless steel chainplates on my Olson 911S is tinted green both above and below deck. The fasteners that bolt the plate to the internal bulkhead do not show the same condition. From time to time, the green substance must be mixing with water, as I find small drips coming from one of the through-deck bolts inside the boat. There is no water discoloration on the wooden bulkhead belowdecks, and a tap test does not suggest any weak spots or rot. I sail on the Great Lakes, so salt is not a factor. I don’t know what is causing this condition, but I’m concerned about the integrity of the chainplate. 

Don Casey replies:

DonCasey

The condition you describe is unusual, so it may be that the tinting you see is not really green. One male in twelve is red/green colorblind, so first have a woman confirm the color for you. If the discoloration really is green, there are three possibilities I can think of.

The most likely explanation is that the source of the tinting is some copper-based component. Perhaps the attached toggle or turnbuckle is bronze and is leaching copper oxide down the chainplate. Perhaps the chainplate is fastened with bronze bolts or a brass washer or nut. Could the chainplate itself be plated bronze rather than stainless steel? The second possibility is acid attack, which in some circumstances can result in a greenish discoloration of stainless steel. The likely source would be a two-part teak cleaner or muriatic acid. A third possible source would be organic matter, perhaps mold living in wet deck core, but I do not see how this would cause discoloration above the deck.

In any case, if you have water trickling down the chainplate it may be wet and oxygen-deprived where it passes through the deck. This could lead to destructive corrosion. I would extract the chainplate and examine it for corrosion. You should also check the deck for core exposure and damage, and examine all the components to see what metals are involved.

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