Saturday, July 12, 2014

Rust Never Sleeps*

Since about Memorial Day weekend, my time has been split between getting the new shop going and tending to a problem with my aluminum boat.  The boat is stored in the garage, and so it lives an easy life.  Not exposed to rain, or other environmental problems; when we take it out, we wash it down, run the outboard for 10 minutes worth of freshwater flush, followed by renewing oil or grease layers and generally doing our best to keep it in top shape.  During the work putting in the shop, we rolled a toolbox out of its normal position to behind the boat, and there it stayed for a couple of months. When I moved that toolbox back, I was stunned to find this hole in the back of the boat, on the left in this picture.  The visible metal on the right is just a scuffed area where the buckle of the strap (just visible on the right edge) rubbed against it one day. 
You're looking forward from behind the boat near the right (starboard) side.  This is the transom, the vertical wall that's the back of the boat; the outboard pushes on this and so it has to be strong.  It undoubtedly gets the strongest forces the boat experiences.  

There's no mark of impact; no evidence anything hit the boat; if anything, the exposed metal looks dissolved.  Since this is just inches away from that strap on the right (used in strapping the boat to the trailer for towing), and is large (about 3/4" long) I absolutely would have noticed it if it had been there when we last had the boat out.  No, this formed while the boat was sitting in the air conditioned garage. 

So my free time in the last month or so has been spent trying to understand what caused it, and if it's fixable.  Some boat mechanics I've been able to talk with say it's galvanic corrosion caused by saltwater getting trapped in there, between the plywood core and the thin aluminum skin.  (I do have a picture or two that appears to show salt crystals on the wood).  The dissimilar materials set up a corrosion process.  Experts tell me the only way to fix this is to tear the boat down to bare metal and replace all the wood.  Rebuild it.  There is some chance that this is electrolysis; that some current flows in the boat's metal hull from a damaged wire or something, and this current is what caused the corrosion.  Last weekend, I put a kill switch in line with the battery, so that the battery will be completely disconnected when we're not on the water.  That will help a lot if it's electrolysis, but have no effect if it's galvanic corrosion simply from some saltwater getting trapped in there. 

There are other holes in the boat.  These are smaller, on the order of 1/8" or less diameter.  There are some on the left (port) side of the transom and along the on bow both sides.  Only a couple on each side.  Funny thing is they appear to be on the level of the bottom of the plywood deck of the boat.  The experts I spoke with say that when a pinhole appears in the aluminum skin, the other side of the skin looks like a large corroded crater, many times larger in diameter.  

Today, I sanded the holes down and especially along the edges of the bigger ones, using polymer/silicone wheels with SiC abrasive embedded in them.  Once the edges of all of the holes were clean, I degreased everything with denatured alcohol, and then sealed them all with epoxy putty.  We'll have to wait until tomorrow to see how well it bonded and some time (probably on the water) to see how well it holds up.

My gut feeling is that it isn't electrolysis, it's just galvanic corrosion.  That means the boat is scrap - or soon will be.  Holes will continue to appear, and enlarge.  We can repeat today's work over and over, chasing new holes whenever they form, but eventually the boat corrodes away.  The shame is that this is only a 9 year old boat - I've only had it 2 1/2.   

* with apologies to Neil Young: it's not rust, it's aluminum oxide.  The other word for aluminum oxide is sapphire, but Sapphire Never Sleeps just doesn't have the right connotation.


9 comments:

  1. Get some bondo and patch it and then keep the bondo with the boat in case it develops more holes. I know this won't appeal to the engineer in you but the repairs will work and probably outlast you.

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  2. That's pretty much what I just did. Marine-Tex epoxy is pretty much "marine Bondo".

    My inner science geek looks at the boat and puzzles over "why are there holes here but not there?" I know to some degree that's unknowable, but there's a very peculiar pattern to where the holes are and it just makes no sense.


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  3. Sapphire never Slumbers!

    73,
    III
    gb

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  4. From what I've read and heard from people I've talked to, there are times when contaminants get "rolled" into the aluminum sheet as it's being manufactured. They don't usually get caught until years later when they start to pop out of the surface.

    I've seen similar things happen when people wire brushed Aluminum before painting it. Small amounts of steel from the wire brush get embedded into the surface, and you have little galvanic cells just waiting for some moisture to get in there.

    Running an aluminum boat in salt water is maintenance intensive, and as you've found out, even the best maintenance sometimes isn't enough.

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  5. It also depends what alloy the aluminum is. Some, like 2024 will do better against corrosion. Some like 7075 T3 and harder will corrode by just looking at it. The heat treating causes the alloy to stratify out of solution and you get inter granular corrosion.

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  6. Here's something for the inner geek:
    https://en.wikipedia.org/wiki/Galvanic_anode

    I'm trying to remember more about it, but I think motorcycles with aluminum engine blocks have an issue with contaminants as well ... magnesium? salt? don't remember. I've heard Internet Horror Stories about the engine block being eaten, but ... it's the internet, so who knows.

    73, Jim

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  7. Looks like you’re up for more repairs in the next days, eh? Good thing you were able to check the condition of your boat before it was totally damaged by the corrosion. I hope you’ll be able to find a solution for all those damages and fix the boat as soon as possible. And I hope you get to use your boat more often after the repairs. Cheers!

    Brent Vandenbroek @ Central MM

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  8. Some Cessna's from the 70's(?) suffered from a kind of inter-granular corrosion IIRC. Causes like incorrect prep for Alodyne plus saltwater and such.

    The fix was full panel replacement.

    itor

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  9. Groan. I've heard of IG corrosion. Regardless of cause, the cure is the same: take it apart to bare metal and rebuild it, replacing as much aluminum as necessary.

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