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.