Fixing pitting aluminum

[fish18]

Well unfortunatley my 20 yr old boat has some pitting, since starting this project of repainting my boat the process has revealed the pits.
I have sanded and wire wheeled the boat down to bare aluminum where the paint had deteriorated and discovered the pits. My intentions are to apply self etch primer to the bare spots then apply a metal primer to the intire boat then apply the paint. Will these steps prevent the pits from getting worse and stop the aluminum from pitting again? Thank you

 

[Quackrstackr]

If your pitting was under the paint, putting more paint over the top of it is more than likely not going to stop it.

Check all of your electrical connections and make sure nothing is grounding on your boat.

 

[Mike Redmond]

:?: Make shure you are wearing your life jacket,,and check if the water is warm///Mike

 

[fish18]

First I don't have any electrical in the boat that would have caused this. Second the bad spots have been stripped of paint and are down to bare aluminum which revealed the pitting to begin with.

My best guess would be the pits were formed by some type of reaction from the wood that had been used for the decks throughout the years.


Old paint stripped off to bare aluminum, so what do I use to stop the pitting BEFORE putting on the new primers and paint? OR now that the aluminum is no longer subjected to any type of foriegn matter and cleaned to bare metal will the pits stop getting worse?

 

[Quackrstackr]

You did not give any particulars about your pitting. Pitting underneath paint like that is normally caused by electrolysis. If the paint is original paint (and you did not specify) and is still intact, the pitting should not be from wood previously used on the boat... but it could be if there was newer pressure treated wood in contact with bare aluminum at some point and has since been painted over.

To my knowledge, there is nothing that you can apply to aluminum to reverse the effects of pitting.. which is essentially what you are asking.

Remove the problem (which you think you have) and clean well, and the pitting stops.

If the boat ever sat in the water for extended periods of time in a marina type setting, it doesn't necessarily have to have had electronics on it to still be a victim of electrolysis.

https://www.yandina.com/electrolysis.htm

 

[fish18]

OK the boat still has the original paint. Some of the paint is flaking and peeling so since I have to paint the new aluminum I put on the boat I decided to paint the whole boat. So I sanded and removed all flaking paint and discovered the pitting. All pitting was found inside the boat mainly on the bottom but some of the worst pitting is on the front deck portion of the boat.
Today I found a NEW spot looked like a bird terd on the outside of boat no where near any other corrosion. After sanding it to bare metal I find it is not only pitted but has eatin a hole thru. It is above the water line.

I have used the boat in saltwater but it has alway's been launched and loaded the same day. I have had wiring in the boat along time ago. I have had 3 wooded decks and floors on boat all of which were treated lumber which were in direct contact
with the original paint all screwed together with what I had at the time. Right now No wiring NO wood, in boat, only wiring is all part of the trailer, Galvanized aluminum not experiencing this pitting problem.

So it sounds as if the boat has undergone some type of chemical reaction or ELECTROLYSIS, Since the boat has been exposed to all the things that would corrode and pit the aluminum over a period of 20 yrs my only choice is to sand it down to bare metal, etch prime it, primer it, then paint.
I really don't know what else to do. Hey thanks I am truly at a loss here as I have searched long and hard for a cure on the internet, No such luck, sand ,clean, repaint with no guarantee it will stop further corrosion.

 

[GrumMan]

This may be a loooong post LOL. I'm among other thing's a structures tech. I cringe at some of the boat mod pics and see huge areas of corrosion that are just painted over. Gonna have problem's later. In order for corrosion to form in or on a metal, 3 thing's must be met;
There must be an electrical potential difference within the metal, there must be a conductive path between the areas of potential, and there must be some form of electrolite wether liquid or gas covering the areas of potential. Your boat may never see an electrical component, wire or battery and still corrode just as readily due to natural atmoshere exposure. It is a natural process that is almost impossible to prevent, we can only control it. The wood did not cause your corrosion, it's not a good conductor, my guess is it held moistre to the metal, which is a great conductor!
By your description, I guess pitting is the type of corrosion you have. Several other types will cause what look's like pitting, but require different treatment's. If the corrosion is not around lap joint's or rivet's, I'd say you are correct with pitting, so that's what I'm going with. Not sure if this site times out on post's, so I'll do this in a series of post's. I am disabled, can't sit long, and type slow.

 

[GrumMan]

First, all traces of corrosion must be removed down to every little piece of powder. Very important or you have not removed the potential for it to continue. You can do this mechanicaly, or chemicaly. Mechanicaly sound's like what you are doing, so I'll go with that. You can use a scotchbrite pad with any house hold cleaner that does not contain chloride. You can use aluminium wool, or an aluminium brush. More drastic measures if it's real bad would be a rotary file or power grinder using rubber wheel's impregnated with aluminium oxcide to grind out every trace of corrosion damage. Try not to go too deep, as you don't want a boat made out of aluminium foil LOL. DO NOT use a steel wire brush or steel wool! Tiny bit's of steel can break off and imbed in the aluminium and will cause even more severe corrosion. If you have a bead blaster, you can also hit it with glass bead's smaller than 500 mesh. After abrasive brushing or blasting, you can examine it with a 5-10 magnifying glass to make sure you got it all. Sand the area out smooth using 280 grit, then 400 grit. Finally, clean with solvent or an emulsion cleaner, dry, then treat the surface with a corrosion inhibiting conversion coating. Alodine would be the best choice.

 

[GrumMan]

Next is protective coating. Alodine is a protective coating. It makes an oxide film over the aluminium to prevent air or moisture from touching it.This help's it resist further corrosion. If you've coated it correctly, you should see a yellowish brown even coating over the treated area. Any break's in the fim show's that you still have some sort of grease or oil residue under it. Work it and try again. Funny thing about alodine. It causes corrosion! You are putting a layer of corrosion over the corrosion you just cleaned LOL. Without some boring chemistry talk, same principle, but not controlled, is why you can go to a junk yard, wack the frame of a car that's been sitting for 50 year's and still see shiny metal under the rust. Gently rough up the alodine with 400 grit. Then prime. I'd recommend zinc chromate primer as the best. Reason being it is an inhibiting type of primer with a slightly porous film so water can penatrate it (I know, sound's defeatist LOL) which cause some of the chromate ion's to be released and held on the surface of the metal. This ionized surface prevent's electrolitic action and inhibit's the formation of corrosion. Yellow or the dark green work the same. Don't let the color slow your choice. Same result.

 

[GrumMan]

Now the easy part. Apply the zinc chromate. First spray a light tack coat. Allow that to set up then apply the full coat. Make sure there are no fingerprint's or oily residue of any kind and paint the color of your choice. I recomment an acrilic laquer or polyurethane enamel.

Hope this give's you the information you were looking for fish, and a little extra info. If your interested as to the why's, want to get into molecular chemistry, or I've misinterpreted the type of corrosion your dealing with (if it is around lap joint's and rivet's) let me know and I'll bore ya ta death some more LOL.

 

[fish18]

THANK YOU!!!! Great responce to my problem. Couple questions.
Will Alodine harm my existing paint which is not corroding? Are there any alternatives to Alodine treatment?( Proly Not ).

 

[rome8899]

WOW! That's some heavy, and useful info. GrumMan, thanks. And just a reminder to fish18. The chemical in the new treated wood eats aluminum. Scott