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Discussion Starter · #1 · (Edited)
About a year ago, I noticed that my forestay chain plate had crevice corrosion. I took it to a boatyard, they cut off the old chain plate, fabricated a new chain plate which they attached to the old piece (which is not structural, and had no corrosion) that sits on the deck (don't know what its called). I now notice that rust stains are appearing behind the chain plate (see photos). I bedded the chain plate in Boatlife caulk if that makes any difference. It appears like it may be coming from the area where the new chain plate and old piece were joined. Any ideas?
Sadly, do to the structural nature of the piece, I went out of my way to use one of the more pricey and reputable (I thought) yards in the area. They apparently cut the original chain plate off without establishing a jig. The bolt holes were off by 5/8 of in inch, and it took them two tries to get the fit right! So much for getting what you pay for! Anyway, is this rust stain a problem?
Edit: See post #12 for the solution.
 

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Chainplate failure

Ah yes, Chain plates. Our boat is now in a yard getting all of its chain plates replaced due to corrosion from sail water intrusion. I few ideas: How old is the standing rigging? Was the welding done with 316 stainless? Was all slag removed and the surfaces buffed clean? Could the rust be coming down from your forestay or fittings? If any of these questions are a concern, you should repair the item (items), very soon, as a total rig failure will cost you more than the boat is worth. Trust me, we found out the hard way.
 

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That's not good. Chances are very good that the rust is forming at the welded portion, since welds are generally far more susceptible to corrosion-especially if the stainless steel wasn't 316L.

One real problem could be if the two parts that you welded together, the old part that isn't structural and the new part that is, are of different alloys. If one is 304 and the other 316, then you've got a problem, since one is more noble than the other.

Another common problem is when they use the wrong alloy for welding rods in doing such a repair.

Since you don't say what alloy was used in the repair or what alloy was used on the boat, it is hard to say what is going on. But any chainplate that is bleeding rust to that degree is not going to be reliable IMHO.
 

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I've just finished reading a document on stainless passivation becuase we are building systems with an extensive amount of stainless piping that will be subject to corrosive chemicals.

What may be happening here is that the welding has not been properly cleaned and passivated. I had a similar problem on some rails I welded myself on a boat. I used used all the right materials but still had extensive rusting. I was finaly able to stop it by extensively grinding away the heat affected layer.

I understand that they make a pickling paste for removal of the oxidized region around welds, followed by a passivating gel that restores the passive layer taht keeps stainless from corroding. I beleive you can find this at www.mcmastercarr.com.
 

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Gary-

Part of the problem is that in many cases, the weld itself is going to cause problems, especially if the alloys involved were the low-carbon versions. Also, it depends a lot on what the two pieces were made of as well as what welding rod/wire was used to weld them together.
 

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Discussion Starter · #6 ·
Great! Several people recommended machine shops to do the job, but I insisted on using a marine shop to avoid issues just like this! I don't know what alloy was used because I didn't think I needed to, given the job was done by one of the largest boat yards in the area! Silly me! Well, I'll send the shop an email with the same pictures, this should be amusing! About as much fun as when they initially called me and said they had lost their "reference points" on the piece after cutting it in half! I knew I was in trouble then! I used a ton of caulk to bed the piece. I wonder how that much water is getting under there?
 

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Discussion Starter · #7 · (Edited)
Contacted the shop. They say they only use 316 Stainless, have no way of knowing what the old piece (which they welded to) is made of (surprising and disappointing). They theorize that I may not have bedded the piece sufficiently, and may have allowed a void for corrosion to form (of course). I contend that even if that is the case (doubtful, I used a TON of caulk), it seems like a lot of rust from stainless is a very short time. They said that we really can't know whats going on until the piece is removed (of course), but don't think there is a structural issue as the entire chainplate is new. If they didn't know what they were welding to, I think they probably should have mentioned this could happen when they encouraged me to reuse the old piece. Pulling that piece again has all the appeal of doing my own Root Canal! I'm so glad I didn't cut corners, payed top dollar, and used a reputable marine shop! Grrrrrrr!
 

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This is a really good reason not to re-use old, unknown stainless. :)
Contacted the shop. They say they only use 316 Stainless, have no way of knowing what the old piece (which they welded to) is made of (surprising and disappointing). They theorize that I may not have bedded the piece sufficiently, and may have allowed a void for corrosion to form (of course). I contend that even if that is the case (doubtful, I used a TON of caulk), it seems like a lot of rust from stainless is a very short time. They said that we really can't know whats going on until the piece is removed (of course), but don't think there is a structural issue as the entire chainplate is new. If they didn't know what they were welding to, I think they probably should have mentioned this could happen when they encouraged me to reuse the old piece. Pulling that piece again has all the appeal of doing my own Root Canal! I'm so glad I didn't cut corners, payed top dollar, and used a reputable marine shop! Grrrrrrr!
 

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Down here in the sub-tropics, stainless steel just takes a little longer to rust than any other type of exposed metal.
 

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Mixing different grades of stainless and welding them is a good way to accelerate the process though.
Down here in the sub-tropics, stainless steel just takes a little longer to rust than any other type of exposed metal.
 

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Discussion Starter · #12 · (Edited)
I took the chain stay to a (non marine) machine shop I have confidence in. The machinist inspected the weld and found it to be suitable. He said he would have had no problem joining the new 316 chain plate with the unknown grade of stainless as long as the weld was done properly. We took the chain roller assembly apart (as the marine shop would have done to do the job originally). We found a lot of rust between the roller mechanism where they are bolted together. The machinist told me that, because the stainless surface between the parts was rough (as opposed to polished like the exposed stainless) the rust was especially able to get a "tooth". Given the fact that the rust didn't exist before the repair, we assume the marine shop failed to protect the surfaces when they reassembled the parts. Salt water was able to get between the surfaces and presto, stained stainless! We cleaned and coated them with Lanocote before reassembling (see photo). I still find it odd that the rust would have run down and accumulated where it did, but there was almost no rust anywhere else. The really sad thing was that I could have taken the roller mechanism apart without taking the chain plate off the boat (no fun at all, especially when I did it a year ago!) Rust coming from roller mechanism never occurred to me, given the location of the rust stain. Hope it solves the problem!
Edit: 1/22/13 (four years later) It has!
 

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