I was looking at an aluminum boat this morning. The sides of the boat and the hull of the boat look to be in excellent condition. No scratches, no marks, slight discoloration. The transom of the boat had many many deep pitting holes through the aluminum. I'm not sure how they got there but it looks like electrolysis to me. The previous owner put about a dozen stainless steel screws through the transom into the wood. What do you think would cause those holes?
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I would say Electrolysis. Could be a bad ground in the electrics using the hull as a ground to water. Was the boat used in salt water
Galvanic corrosion from the two different metals. Stainless causes aluminum to weaken and corrode faster than normal.
All sheep are eventually led to slaughter
I asked that question, he said he never use the boat in saltwater but the previous owner may have.Originally Posted by dartag1829
Electrolysis can occur in freshwater. It's worse in saltwater and on aluminum boats. Make sure all grounds are on one source, preferably the cranking battery.
"Better to remain silent and be thought a fool than to speak out and remove all doubt."
I can't figure out why you would only occur on the transom, and the sides and the bottom would be so clean?
Is there an engine on this boat, lf so what does the anode look like?
Now a larger potential problem could be the transom plywood leeching and pitting the aluminum. Because it is only affecting the transom, this could be the cause.
Last edited by Chautauquanuy; 11-16-2019 at 04:54 PM.
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Could be that the plywood is damp. If it is damp it could be causing the galvanic corrosion even when the boat is not in the water. I would be willing to bet that all those holes for the stainless screws are causing the wood to stay damp right at the point where the stainless screw and aluminum transom meet.
But, if the wood transom was replaced with ACQ pressure treated wood that would be a much bigger issue. The copper in the ACQ will react with the aluminum. From the way the pitting looks I would want to know if the transom wood is ACQ treated.
I questioned the guy selling the boat if he replace the transom because there was a section of the transom cut out to use a short shaft motor. And it looked like the wood was newer, don't get me wrong not brand new but maybe the last 10 years
Is the wood in the transom plywood? That is what I think it looks like in the first picture.
Is there plastic or rubber washers under the screw heads between the screw heads and the transom?
Huntertn is correct
What he said. Dont ask me how I know
Definitely wood in the transom, it is plywood. The stainless steel screws have rubber spacers under them but they are definitely dry rotted away.
Dissimilar metals will cause corrosion.
That would be my guess
Put new aluminum skin on outside of transom and new wood on inside. I bought a boat that had this done and it was a good boat. Don't have it now but wish I did. Well worth redoing.
Yep. Looks to me like the transom was replaced with treated wood. I bought one like that once. Transom was Swiss cheese.
Good to know other people have seen this before, I'm convinced that the guy use pressure treated wood to replace the transom
I see this all the time. It is the fault of the wood transom, but not always due to the same things. In some instances, it's due to galvanic corrosion, in other's it's poultice corrosion.
You're on the right track with the ACQ plywood, but depending on the age of the boat, don't immediately blame the boat owner. Aluminum boat companies have been putting pressure treated plywood in boat transoms for decades. This was more or less okay with the old style pressure treating, but when the newer style pressure treating came into play, the copper content wreaked havoc with any aluminum it was in contact with - dissimilar metals and whatnot. Despite that, pressure treated plywood was still used in boat transoms, even by some companies you would expect to know better. Some companies even use it still today.
However, the transom corrosion isn't just galvanic corrosion from copper, it's prevalent in plenty of boats with untreated wood. In this instance, it's primarily poultice corrosion. Poultice corrosion has to do with water being trapped up against aluminum, without airflow. Aluminum is actually highly subject to corrosion - it corrodes in seconds when exposed to oxygen. However, unlike iron, where the rust keeps flaking off and getting deeper, eventually rusting away to nothing, aluminum oxide actually forms a protective layer on the surface, which seals the metal below off from oxygen. This is what gives aluminum a commercial value, and gives aluminum it's 'corrosion resistance'. Even bright, shiny, brand new sheet aluminum is technically corroded, and this corrosion happens within seconds of cutting through the metal.
In the case of a poultice corrosion cell, water has been trapped up against the metal. The aluminum naturally pulls oxygen out of the surroundings to create aluminum oxide, which in this instance, is pulled from the trapped water. This leaves the water slightly acidic, which then breaks through the protective oxide layer, which causes the aluminum to pull more oxygen, which leaves the water more acidic, and the cycle continues, until your transom looks like this. The thing with poultice corrosion is it isn't just wood that can cause it, but anything that can trap water up against the metal - one of the key contributors is poured in urethane foam - that flotation foam is the kiss of death for many aluminum boats.
The stainless bolts aren't usually that big of an issue. Yes, stainless is a more noble metal than aluminum (hence it becomes the cathode, leaving the aluminum as an anode), but they aren't drastically far apart on the galvanic scale, so it's usually not that big of a concern. Furthermore, the effect is minimized due to the mass of the items in question - a tiny aluminum fastener in the middle of a stainless panel would corrode away in no time, but a small stainless fastener in the middle of an aluminum panel doesn't have much of an effect.
That said, I do usually see the damage more concentrated around the bolts, but I still don't usually contribute it to galvanic corrosion from the bolts. My thoughts are that the bolt can provide a path for water intrusion. Secondly, the bolts are pressing the aluminum tight to the wood, whereas in the areas away from the bolts, the contact between the wood and metal isn't as tight.
If you start seeing corrosion cells all the way through to the outside of the transom, you can pretty much be certain that most of the surrounding areas are corroded 90% of the way through. That transom skin is shot.
Normally, we fix that by simply cutting the transom skin off, and welding a new one one, and replace the wood core with a transom core made from aluminum rectangular tube (this assumes a welded transom corner). A lot of times, the transom wood stops an inch from the sides, and 6 inches or so from the bottom of the boat, so we can often just cut out where the wood touched, and be left with perfectly fine metal around the perimeter. In the case of the boat that you looked at, with a riveted transom to hull seam, I'd probably just drill out the rivets, and bend a new matching transom piece to rivet in place. Alternatively, we'd just cut the back 2" off the boat, to get rid of the rivet seam, and weld a new skin on.
The long and short of it is if you're just looking for a functional boat, pass on that one. It's a worthwhile fix if the cost of the boat is already sunken money, but not worthwhile to add onto the purchase cost of one you're looking at.
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Who is John Galt?
I don't Guess what Caused it, I KNOW, and the Above Poster is CORRECT!
Way Back When a lot of the Aluminum Manufacturer thought that if they used Treated Wood for the Transom it would last longer than the former used Non-Treated Wood did.
HUGE MISTAKE!!
The Chemicals in the Treated Wood reacts with the Aluminum and you see the results. DO NOT BUY THIS BOAT UNLESS YOU PLAN TO REMOVE AND COMPLETELY REPLACE THE TRANSOM!!!
There is no fix. The only option is to remove and replace. G3, who makes a fine boat, was one of the first to go that route. Several of there Bass/Bay boats are now sitting looking Perfect-- except the Transom is ready to fall off.
Most of this happened, if I recall correctly, in Aluminum Boats built in the late 1970's through the early 1990's.
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