corroding aluminium - help! can you stop it?

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phantomlady

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Hi everyone,
We have the problem of a badly corroded aluminium toerail on our boat - Beneteau First 42. I think I inadvertently set off the chemical reaction a few years ago whilst using a powerful teak cleaner on the decks and allowing it to pool under the lip of the rail rather than being vigilant in my rinsing off :(
My question is this....is it just unsightly or will it eventually just corrode away? Can you reverse it? Was it indeed a reaction that stripped off some sort of coating? Has anyone else had a similar problem? If so, what did you do about it.
Thanks in advance.
 
Hi

Firstly, you have to place the picture of scabby jumping off the boat, it would make a great 'caption required' post. :)

Your ally was probably galvanised (a chemical process that protects the ally) this galvanising is not liked by teak cleaners and will react badly as you have found out.

Not sure if it will get worse over time, how much worse has it got over the two years?

My first guess (blind) would be to rub the corroded area with wet n dry paper (wet) to see if this helps, you will remove the galvanising by doing this but it has gone already by the sound of it and it will look much better and then apply a protective metal wax to seal?

EDIT: Anodising instead of galvanising!!!!!!!!!!
 
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No, you can't galvanise aluminium! It is almost certainly anodised.
I have used oxalic acid on our decks without any effect on the ali toe rails. But there are a couple of small areas of electrolytic corrosion where the stainless stanchions are very close to the toe rail. Are your corrosion areas similarly close to stanchions?
In any case DON'T use wet 'n dry on anodised aluminium! It will corrode!
 
Sorry M.R, but you don't galvanise alloy, galvanising is for steel; you anodise it, which looks like a smooth tough paint coating.

The cleaner seems very unlikely to be relevant to me; salt air & water eventually gets to alloy.

It has to go a long way to be a problem to strength, but can become unsightly; the real cure would be to remove the toerail, strip it as clean as poss then coat it, but that's pretty serious hassle.

Most people ( well, me ) would clean it as well as possible inc light sanding, then treat with a suitable alloy paint.

I see I'm a slow typist !
 
It sounds as though what ever you used may have removed the anodising... the protective oxide layer that is formed by an electrochemical process.

Not a lot you can do to restore it if that is indeed what has happened.

There is a product call Alochrom ( or Alochrome) that can be used to protect non anodised aluminium. I have no idea where you would get it .. I have only heard of it in connection with the aircraft industry. Maybe if you can source some and clean the aluminium back to bright clean metal it will solve the problem.
Maybe Seajet knows more than I do

Any idea what it was that caused the problem?

BTW aluminium is not galvanised ... that's a zinc coating applied to steel . Usually by dipping in molten zinc

I am an even slower typist
 
I presume the teak cleaner (acid) has removed the anodised protection of the aluminium, and this allows residual acid (if any left) or sea water to attack the ali directly.

Classically you need to neutralise the acid (tho' it's a bit late for that) by using e.g. water with baking soda in it, and brushing it underneath and in any cracks.

Short of removing the rail, and recoating the aluminium underneath with perhaps epoxy as protection, a new toe rail seems to be the best long term answer. :(
 
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ali corrision

your toe rail would have been anoidised, if you have stripped this off you will need to remove all the corrosion and depending on budget reprotect, good priming and alloy wheel paint can be a good finish
 
Alochrome supplier

You can get alochrome and other aluminium protection treatments used in aircraft industry from Light Aero Spares (or LAS Aerosopace as they seem to be called now). They can be bought in small quantities and LAS will provide advice over the phone. Their usual customers are owners and operators of small private aircraft so they have time for new and small-purchase customers. Look at;

www.lasaero.com
 
Reminds me of an old story.

A man goes into a clinic and asks if he can have an operation.

Doctor: How can we help you today?

Patient: I'd like to be castrated.

Doctor: Erm... that's a big operation for a 20 year old, are you sure you have thought this through?

Patient: Doc, I am an adult, I have thought it through very well and I am of sound mind and body, it's what I want and my partner said she would prefer it.

Doctor: It's up to you I suppose and yes, we can help as soon as you like.

Waking up from the op, the chap heard moaning noises coming from the bed next to him.

When the nurse came over the man asked if everything was okay with him, as he sounded like he was in a lot of pain.

The nurse explained: Yes he is okay, he is just recovering from being circumcised.

The man sat bolt upright and clicked his fingers.

THAT'S THE WORD!!!! :)
 
Acid an aluminium do not go well together. Once the acid is neutralized marine grade alumium will not corrode in seawater. Anodizing is not needed to prevent this corrosion.
The problem with an aluminium toerail is there is a lot of dissimilar metals (stainless steel stanchions, bolts and blocks) attached to a small mass of aluminium. If you can isolate these components and also prevent crevice corrosion where the aluminum is bonded to the deck, you will get no further problems, but this is difficult to do.

Aluminium toerails are not a great idea on a fiberglass boat, but I do not think you will get much change in the problems you have experienced from the acid, particularly if you can isolate the dissimilar metals, which is often not done in production boats.
 
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There are a number of place to get Alocrom or Alodine from, as well as the places already mentioned, this place has everything you need as well:
http://shop.trimite.com/brush-alocrom-1200-kits

I would suggest that if the corrosion is on the underside of the toerail, then your options would appear to be fairly limited:
1. Lift entire toerail, repair/treat corrosion and replace - not a job to be undertaken lightly.
2. Cut out corroded section and replace with new if possible, de-corroding and re-protecting as required otherwise.
3. Monitor and treat in place as much as possible.

To treat the corrosion properly, you do need to get access to the damaged piece, squirting an alkali or acid into a crack will, unless flushed out totally, lead to further corrosion. In fact you may well accelerate the process.
 
I have corrosion on my toe rail. It appears to be due to the anodising being damaged along the lower edge during hull polishing or painting. It appears to be forming quite deep pits similar to stainless crevice corrosion. The pits tend to hold salt crystals after sailing. I dont think alochrom would be much use as it is very thin and the wrong colour. The best treatment seems to be to carefully mask off the affected area then burnish the affected pits using a dremel type tool or even scraping out with a knife. The pits are then carefully filled with a mixture of epoxy and aluminium powder. (Obtainable from GRP suppliers) It needs as much powder as the epoxy will take. It's important to try to get the fill slightly above the surrounding level as it tends to shrink on curing. It's sometimes necessary to use several layers of masking tape to build up a thick edge. Once it has cured but before it's totally hard the tape can be removed and the fill shaved down carefully using a stanley knife blade. This can be done easily without damage to the surrounding area. The repair can then be carefully burnished by hand taking care not to damage the surrounding anodised surfaces. The epoxy mix will eventually take on the appearance of aluminium with a reasonable colour match to the surrounding silver anodising. It certainly seems to arrest any further corrosion and bonds well to the rough surface. This method also works well on minor mast damage caused by chafing from blocks etc.
 
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