Mysterious Corrosion pattern on aluminum hull

[pcaker]

On my 18 ft Ocean Sport, about 20 years old, the fully zinc protected aluminum hull is showing a strange pattern of corrosion on bottom. At rear right and rear left, and centre 3/4 forward, rectangular areas of corrosion have appeared, forcing off the multi layer bottom paint applied 6 years ago, just before I bought it. Each area is about 1-1.5 sq.ft and neatly rectangular. At all the stainless fittings I have installed on the deck, with liberal silicone, not a trace of corrosion shows, generally the 3/16" hull shows little corrosion. There is no "higher" metal anywhere near the affected areas, as far as I can see.
My wife has suggested a plausible explanation. She thinks corrosion may have originated when bare hull boat was placed on 3 steel stands, which would have been placed exactly where the corrosion is. If a salt water wet boat was left on unprotected steel stands for a period, corrosion might well have started. Then if boatyard had failed to wash & scrub those areas before applying the bottom paint - hard to believe - the corrosion could have got locked in under the paint, until breaking out recently. I do not have a better explanation, does anyone else? I have just sandbasted (glass beads) the areas back to bare metal, and thankfully there seem to be very few pits, except on the keel angle at the forward corrosion.

 

[zurk]

no idea about your issue but i recommend applying 8 thin coats of interprotect 2000e now that you have bare metal. you can then antifoul on top of that. it should solve the issue permanently and they can dump it on steel stands all day long after the coat has bonded.

 

[tubber]

Is it possible the boat stands were carpeted and used previously on boats with actual copper bottom paint?
Or the boat was not epoxied/primed properly beneath the stands, but then they painted those spots without the same pre-treatment as the rest of the bottom.

 

[pcaker]

Thanks for that idea

no idea about your issue but i recommend applying 8 thin coats of interprotect 2000e now that you have bare metal. you can then antifoul on top of that. it should solve the issue permanently and they can dump it on steel stands all day long after the coat has bonded.

Boat is trailered home and washed down with fresh water after every use, so this level of protection should not be required. I should have mentioned this, but post was getting rather long. There is no significant corrosion anywhere else on the hull, it is the 3 neat areas of the corrosion that is the mystery.

 

[spring fever]

Thanks for that idea

Boat is trailered home and washed down with fresh water after every use, so this level of protection should not be required. I should have mentioned this, but post was getting rather long. There is no significant corrosion anywhere else on the hull, it is the 3 neat areas of the corrosion that is the mystery.

Got a picture??

 

[Rayvon]

Sounds to me like who ever did the bottom paint had it on a center keel block and stands for the port and stbd. stern corners. they did the interprotect epoxy and conversion primer on everything they could except where the block and stands were.When they finished bottom painting over the epoxy and conversion primer,they didn't follow through and do the same where the block and stern stands were.Probably just slapped some bottom paint on the bare metal.Pretty common probably,because there's 2 coats min. of interprotect and the conversion primer and then at least 2 coats of bottom paint which all takes time.

 

[Rayvon]

On your cleaned /blasted area's can you see an area that clearly has a different coating under the bottom paint around the corroded area's?

 

[BC Coast Pilot]

You said you are using zinc anodes, should you not be using Aluminum anodes? The most likely answer to your corrosion is that they didn't barrier the areas where the stands were and you're seeing the result of years of direct interaction betwèen the aluminum hull and copper based paint

 

[pcaker]

Got a picture??

Corroded area is speckly area top right on photo. View is looking up at right rear of hull bottom. Uncorroded smooth area to left is a transom extension I added 3 years ago, so boundary between 2 areas is bottom edge of old transom.

 

[pcaker]

Sounds to me like who ever did the bottom paint had it on a center keel block and stands for the port and stbd. stern corners. they did the interprotect epoxy and conversion primer on everything they could except where the block and stands were.When they finished bottom painting over the epoxy and conversion primer,they didn't follow through and do the same where the block and stern stands were.Probably just slapped some bottom paint on the bare metal.Pretty common probably,because there's 2 coats min. of interprotect and the conversion primer and then at least 2 coats of bottom paint which all takes time.

Yes, I think this very plausible, thanks.

 

[Rayvon]

Yes, I think this very plausible, thanks.

I think so because I've never seen boat stands with just steel on top,usually plywood or rubber.Even the navy ships sit on crush blocks of wood.

 

[Brando]

It doesn’t look like you have any barrier coat on there.. once the bottom paint gets flaky your just trapping saltwater up against the hull. You say your trailering only so either strip the paint off, barrier coat then re bottom paint or strip it and leave it as is.

 

[pcaker]

It doesn’t look like you have any barrier coat on there.. once the bottom paint gets flaky your just trapping saltwater up against the hull. You say your trailering only so either strip the paint off, barrier coat then re bottom paint or strip it and leave it as is.

Thanks for recommendation

 

[pcaker]

Thanks for recommendation

The irony is that bottom paint was only applied by vendor, at claimed $6,000 cost, because a prospective buyer before me made it a condition of sale, then pulled out after job was done! For me the bottom paint has caused more problems than benefits !

 

[pcaker]

Sounds to me like who ever did the bottom paint had it on a center keel block and stands for the port and stbd. stern corners. they did the interprotect epoxy and conversion primer on everything they could except where the block and stands were.When they finished bottom painting over the epoxy and conversion primer,they didn't follow through and do the same where the block and stern stands were.Probably just slapped some bottom paint on the bare metal.Pretty common probably,because there's 2 coats min. of interprotect and the conversion primer and then at least 2 coats of bottom paint which all takes time.

Yes, I agree with you - its now looking 99% certain that something like you describe happened. But for $6,000, the boatyard should surely have done a better job.

 

[Alex_c]

Sounds like a giant pain. When I got my boat, I was unsure of the condition of the bottom paint/primer, so I had it blasted off. I never re-applied since the boat lives on its trailer. Have had zero issues since - maybe just blasting and then sanding off the hull might be the way to go

 

[pcaker]

Sounds like a giant pain. When I got my boat, I was unsure of the condition of the bottom paint/primer, so I had it blasted off. I never re-applied since the boat lives on its trailer. Have had zero issues since - maybe just blasting and then sanding off the hull might be the way to go

Yes, that's what I would like to do, What size is your boat and what did you pay to get bottom coat removed?

 

[Alex_c]

Yes, that's what I would like to do, What size is your boat and what did you pay to get bottom coat removed?

I have a 16’ Lifetimer. I got it done at Tri-City blasting in Poco, and since I was on a budget at the time, I told them to go as far as $200 would get it. They definitely went above and beyond, but I still had to remove some of the epoxy primer myself. I pulled the boat off the trailer in my driveway, and sanded the rest off with a 120 grit sanding pad on an angle grinder, being careful not to eat into the metal. I then finished by sanding with an orbital sander to buff out any scratches. I would expect to pay a lot more than I did before n 2016 to have the whole boat done.

 

[BC Coast Pilot]

Just wondering if you've questioned the material your anodes are made of?
If you're using zinc on an aluminum hull you may have effectively made a battery between the zinc/aluminum/seawater

 

[ClintR]

Just wondering if you've questioned the material your anodes are made of?
If you're using zinc on an aluminum hull you may have effectively made a battery between the zinc/aluminum/seawater

Same for the zincs on my outboard? Should I be changing them over?