Over zinc'd?

[bobk]

The diver who just cleaned the bottom speculated the running gear is over zinc'd. The 48 MY has long shafts and has three zinc's per shaft, one each on the rudders and trim tabs. The latter are probably a size larger than necessary. Anyway when he was scraping the crud off an eight month old 5/16" stainless bolt that attaches a stainless swim platform support to the stainless transom bracket, the bolt broke. The post mortem shows a very large pit into the center of the bolt next to the bolt head. Can this be a result of too much zinc? If not, what could cause the pitting?

Bob

 

[Brian Degulis]

It's not the zincs it's CREVISE COROSION very common with stainless there is a lot of information about it's caused by a lack of oxegon usually in places that cannot be seen without removing the bolt.

Brian

 

[sammidog]

I suspect Brian is dead on here.

 

[bobk]

I htought of that too, but this is a bolt that attaches a piece SS pipe to a U shaped fitting. No Caulk etc. So there ought to be lots of water/oxygen about.

Bob

 

[Genesis]

Naw, its not overzinced. Its very, very hard to overzinc a fiberglass boat and do damage. You CAN overzinc a WOOD boat and its NOT GOOD if you do!

Almost certainly crevice corrosion and is very common with stainless. One of the reasons you need to check that sort of thing regularly.