stainless steel and corrosion

[ford]

I am currently replacing rotten frames and I find that most of the damage is situated where the ss water tanks were in the bilge.Any connection ?

[Peter Sibley]

Greetings Ford,how clean was it under there,much in the way of accumulated mud,dust and old vegetable peelings?Any leaks in the s/s tanks ?

[Scott Rosen]

Most likely the tank installation didn't allow for proper ventillation and/or proper runoff of water, condensation, etc. You ought to address that when you have the tanks out of the boat and are making the repairs.

[PugetSound]

What the others have said regarding ventilation is true, but I assume you already know that. The most critical factor in preventing rot (dry or wet rot) is ventilation. However . . . . . .

Are you sure that the deterioration of the frames really is rot? There may well be a connection between the cres tanks and the frame deterioration if the tanks were sitting directly on the frames. The same process of galvanic corrosion (which is an electrical process caused by the different potentials between metals -or between the different constituents of an alloy) which affects metals can also cause deterioration in wood (called electrolysis). The actual damage in the wood is the result of acids formed by the galvanic reaction.

If there is NO corrosion on the outside of the cres tanks, then I would rule out electrolysis. The problem is that deterioration of cres isn't always all that apparent. Have the tanks checked in critical areas (such as around the heat affected zone of the welds) using PT/NDT procedures to see if there is any crevice corrosion going on. If so, then that may be your culprit. Otherwise, if it really does look like rot, then it probably is rot.

[ford]

pugetsound, thanks for your reply, the rot I have is actually from galvanic corrosion, not the dry rot variety, it was simply that the greatest area of damage is where the water tanks were positioned in the bilge, I have at this point replaced about 60% of the frames in this area with more to go. The more the nails were eaten away the faster the salt water [electrolyte] dampened the frames and planks via the endgrain and the faster the whole process seemed to take, 250 milliamps running thru' the boat, is it a coincidence that the area of tanks was worst hit?

[Jim Swarthout]

Gentlemen,

This problem is an obvious case of the over abundant use of zincs and noble potentials! It is common for many boat owners to assume that if a little zinc is good; more must be better! In order to maintain peace within the noble house the quantity of zinc material must be based upon the various participants. I will not go into a long diatribe about the nobility of metals, but will simply comment that PugetSound was correct in his analysis of your problem.

[ford]

jim, interesting reply, except there was no zinc on the boat, but yes a mix of metals below the waterline, ie; stainless steel keelbolts, bugger.