Chain Plate Crevice Corrosion – Part 1

I just happened to be examining our forestay chainplate the other day.  Not for any particular reason.  I  was just there and decided to give it a good look.

I’m glad I did.  What I saw was not reassuring.  Right at the top of the weld where it is attached to the bow roller was a very small crack.  Not a good sign.  Not a good sign at all.

There is no shortage of posts and articles decrying the use of stainless, especially 304, in marine applications such as chainplates.  And yet, many times, those same people denouncing the use of the stainless go right back to it to replace their bad or failed piece.  And quite often it’s for the very same reason that the boat builder used it:  cost.  Stainless, even 316,  is relatively inexpensive compared to its more suitable counterparts.

Granted, this particular piece of stainless has probably lasted for 20+ years.  But, it’s the fact that its failure is so secretive and deceptive.  A piece of stainless can look absolutely perfect, but beneath the surface, horrible things are going on.  Stainless seems to corrode from the inside out.  Here’s a failed chainplate that the article says was inspected and deemed to be good before it failed catastrophically.  What’s even worse here is that it appears someone decided to take two pieces of stainless plate and weld them together to make a single thicker plate.

I decided to try to file down our crack just to see how deep it went.  If I was able to remove about a 64th or so and the crack was gone, then it was just a surface flaw.  I doubted it, but it was worth a shot.  Though, my confidence in it would be forever diminished.  The crack was still there.  I understand why it’s there.  Welding stainless consumes the chromium which makes stainless, well, stainless.  This crack is right at the top of the weld where it also happens to be not smooth and polished.  This means that there was a micro-crevice for salt water to be captured and then start corroding the metal.

So, the only option here is to replace it.  I looked at the prices on bronze versus 316 stainless.  It’s about double.  This chain plate is 1/4″ thick and 1.5″ wide.  I decided to go ahead and replace it with C954 aluminum bronze plate that is 3/8″ thick and 2″ wide.  I am going to cut out the forestay chainplate from the bow roller and then reinforce the bow roller separately from the chain plate.  I am also doing the chainplate for the backstay.  This is going to require me to find a local welder or machine shop that can put a bend in each plate.  Shouldn’t be too hard.  The cost will be the major factor.

I’m also going with bronze bolts. The current stainless bolts have quite a bit of corrosion around them.  As the bolts are external and subjected to salt water directly, no point in going back with stainless.

I will say this is not inexpensive.  The 2 plates cost $65 each and the bolts are a total of $75.  To replace this all with 316 stainless would be about half that price.  But, when I look at the fact that these items are holding the mast up, I come to feel that this is completely cheap.  A mast is WAY more expensive than this.  And the beauty of this is that I won’t have to worry about it ever again.