Best method for repair corrosion areas on aluminum hull?

[mlt2819]

Hi,

I have a 28ft sailboat (40 years old) and it has a few sections with pitting type corrosion (I'm assuming it's not electrolysis?). It's my first boat and I've been reading as much as I can about how to repair it, but I'm keen to hear from others here about their first-hand experience/advice on the best way to repair these areas. Overall, the boat seems to be in good condition and it was surveyed too (but clearly some of these areas were not looked at closely enough...) Attached below are three shots of the main problem areas. If you have any input on how to tackle this, it would be super helpful! Thanks.

Below [IMG_3601.JPG]: These pits are 1/25 - 3/32" wide (in general) and there are other areas aside from this. Should I sand the pits away completely (leaving behind a concave area of bare aluminum) or should I use an aluminum oxide tipped Dremel bit and sand only inside the pit itself?

http://www.cruisersforum.com/forums/...1&d=1578077316

Below [IMG_4459.JPG]: This area is the worst-looking of all. This area was underneath where the 2 x 12V batteries are stored. The bottom plate - running the length of the boat - is roughly a 1/4" thick and everything else is 3/16". When I removed the batteries, I found the area had water (I didn't check if it was salt or freshwater) and worst of all, the previous owner had dropped a Stanley knife. I have no idea how long it was sitting in that electrolytic broth :| I'm guessing that I need to sand the whole panel to clean aluminum or could I tackle each corroded blotch one by one?

http://www.cruisersforum.com/forums/...1&d=1578077709

Below [IMG_4604.JPG]: This pit is 1/16" deep (on 3/16" plate) and is the deepest one I've come across. No doubt it was caused by the uninsulated steel screw (horrible!). I've since removed the screw and splashed it with a bit of Alumabrite to re-passivate it. I assume I need to completely sand out this entire area so it's down to clean plate and totally smooth? At the minute, I'd rather avoid having to drill this area out and have someone fill weld it/patch it, mainly because in the next few years I plan to haul it out, strip it down to bare aluminum and repaint the boat (this was done 8-9 years ago by the previous owner) - I'm working on the assumption that when it's bare, I'll probably find other spots that need doing and I would rather have it all done properly in one go.

http://www.cruisersforum.com/forums/...1&d=1578077851

Once I'm done with treating these areas on the inside, I plan to leave it unpainted, but I will give it a protective coating with Alumetron (or something similar. Does that sound like an OK course of action?

If you've read this far, thanks very much!

[john61ct]

I would not be removing structural material, what, just to make it look smoother?

[tkeithlu]

Why sand it? As john61ct says, why remove structural material? You're just thinning the stuff that keeps the water out. Clean it and treat it to prevent further corrosion, and get your anodes up to doing their job. Then the question is whether any pits are deep enough that the aluminum needs reinforcement. If it does, you're in for cutting it out and welding in a replacement, or at the minimum welding over the weak spot with a patch. That part will not be fun, at least not when you pay the welder.

[mlt2819]

Originally Posted by john61ct

I would not be removing structural material, what, just to make it look smoother?

Hi, no, not to make it look smoother. It would be to ensure the corrosion is totally removed. I was under the impression that if I don't get rid of it and bring it back down to clean metal then it will continue to corrode.

[tkeithlu]

Try a wire brush on your Dremel tool. Should clean any corrosion out of the holes.

[john61ct]

Aluminum's chemistry is "self-healing" oxidation forms an anti-corrosive barrier, not like rust and steel.

Coating is another way to go, but obviously expensive.

I believe that level of pitting doesn't happen with other grades / purity quality of alloy?

[a64pilot]

There are chemical treatments that improve the resistance to corrosion of aluminum, like alodine for instance and or zinc chromate paint.
Maybe not for a hull, but definitely where the batteries used to be, of course from now on the batteries need to be in boxes.
I would first alodine, then paint with zinc chromate primer, then Epoxy polyamide.
http://www.ppgaerospace.com/Products...de-Primer.aspx
All this is what would be done if it were an aircraft, but why not a boat?

I didn't look at your pictures but assume it’s not so bad it needs to be cut out and replaced

[Q Xopa]

Originally Posted by john61ct

I would not be removing structural material, what, just to make it look smoother?

Of course the 'ideal' is to not remove 'structual' material.

But the reality is if its corroded pitted, dented, eroded etc then it is no longer contributing to the structual integurity of that structure.

So removing the damage/ pits/ corrosion as completely as practical, completey if possible, is preferable. This does mean 'smoothing' out to a nice looking finish. Any irregularities in a surface concentrate stresses and can be the start of a crack etc, referred to as a 'stress raiser'.

Obviously this is reducing the component dimensions and therefore original strength but it is actually stronger than with previous damage or corrosion. Generally called a 'blended repair' or simply 'blending'.

Being an aircraft engineer we do this a lot.

What you have written sounds pretty good

If the repaired section is too small, then replacement of that area, splicing in another section etc, or complete component is required.

After this the aluminium is generally treated with a 'conversion' coating, 'dioxidine' one trade name, but generally a muratic acid solution.

Then a surface treatment is applied, Alodine1200, was the product most commonly used in Aviation.

Then a primer, usually 2 pack yellow chromate PPG, or similar.

This is along the lines of what you have written. Practically speaking you dont need to go to these extents. But if you do something along these lines you are way ahead of what many boaters do, ie nothing because they have little to any knowledge of this stuff.

Corrosion is insidious. If you keep ontop of it, treating it regularly its far more manageable.

When it gets left, it is out of control in no time. When it looks bad, it usually much worse as you start to pull things apart and reveal what's hidden underneath.

[Reefmagnet]

It's the "corrosion" on aluminium that protects the underlying metal. You might want to remove the excess but you certainly don't want to grind off any clean oxide unless your prepping for IMMEDIATE paint and you never want to grind off any raw metal without a very good reason to do so.

[Fore and Aft]

How about some spot welding on those really big pits? I have seen that done on a 100 foot aluminium yacht I surveyed last year and was really impressed with how it turned out.
Cheers

[Q Xopa]

Originally Posted by Reefmagnet

It's the "corrosion" on aluminium that protects the underlying metal. You might want to remove the excess but you certainly don't want to grind off any clean oxide unless your prepping for IMMEDIATE paint and you never want to grind off any raw metal without a very good reason to do so.

Yes as you say 'corrosion' is a somewhat loosely used term.

Pure Aluminium 'oxidizes' quite quickly which is as you say a protective surface, reducing further corrosion- Agreed.

However, it is not completely impervious. Once corrosion gets to be pitted then it's time to start treating it before it gets completely into the intergranular layers. At this point there is no repairing to be done, only replacement.

Also agreed that if you are going to surface coat, paint etc it has to happen quickly before new oxide forms on the surface. We are talking seconds or minutes max. Paint doesn't stick to oxidized aluminum. This is why painting Aluminium is not so straight forward.

[tkeithlu]

Hey folks, don't forget the difference between aluminum and aluminum alloy when it comes to corrosion. Aluminum produces Al2 O3 aluminum oxide, which is self sealing. Aluminum alloy corrodes, even as filliform (the layers you see in a badly corroded bit).

Pure aluminum is too soft to be a good structural material, while aluminum alloy (I can't remember what is used to alloy) is strong. Therefore, the aluminum plate used in aircraft construction, and I assume boats, is aluminum alloy plated with a layer of pure aluminum. Once the plating is breached, the alloy corrodes without being self sealing.

[john61ct]

Obviously no one is talking pure aluminum, magnesium manganese and chromium are added

6000 series alloys like 6082, 6005A, 6061 are very different from 5083, 5383 and 5086

and the **quality** of the actual material delivered to the builder can vary widely

Originally Posted by tkeithlu

aluminum plate used in aircraft construction, and I assume boats, is aluminum alloy plated with a layer of pure aluminum.

I never heard of that, would not assume used for boat hulls.

[warrior 90]

Originally Posted by Q Xopa

Once corrosion gets to be pitted then it's time to start treating it before it gets completely into the intergranular layers. At this point there is no repairing to be done, only replacement.

Question ... I am not an expert for the case, just trying to explore options and limits.

Considering following all the surface pre treatments and treatments for protection after repair recommended by You.... is the following repair method with aluminum brazing at lower temp an option or can it be considered at least a temporary fix to slow down the process once corrosion has started to affect intergranular layers ?

Since this method will not reach the required melting point temp for aluminum this is only a bond.

Of course not for structural problems or treatment of high pressure tanks.

https://www.youtube.com/watch?v=k21JDVdGalY

[thinwater]

This sort of damamge is very common in both steel and metal tanks.

* If the pits are widely scattered, weld the pits and grind smooth if needed (there is nothing wrong with leaving the bead.

* IF there is structural damage, that's more complicated, but I think you are just talking a few pits.

If it were me, and I inspect tanks for a living, I would clean the area well, wire brush the spots I'm interested in, and draw a large circle around each with welder's chalk. I would then hire a welder for a few hours to weld all the pits.


Good as new. I'd feel good about that.


(Inspector tip. If you want to quantify how deep they are, get one of these. Easimating the depth ofpit visually is much more difficult than it seems. This does it in a jiffy.)
https://www.amazon.com/Welding-Gauge...qid=1578168088