Is it OK to spray epoxy primer over anodized cast aluminum?

[starz]

What is the best method of protecting cast aluminum from corrosion when it will be attached to stainless steel and dumped into sea water where it would reside for 10 years without any access to it?

Ok, what i am talking about is not part of a boat, but rather a washing machine.

I'm posting here because folks at the washer forums haven't come up with a solution yet. I've seen a few posts here talking about similar problems so i figured you guys might have some experience dealing with these materials in such environments.

The aluminum part is called a spider-arm. It's attached to a stainless steel drum. The drum is of course where you put your clothes. This setup is (afaik) always on a front load washer. The thing is, the spider always corrodes and breaks - mine did in just 3 years. It's inevitable. Lots of theories as to why that happens...galvanic corrosion, water PH levels, too little detergent, too much detergent, using cold water, keeping the washer door closed...and so on.
Those who say its NOT galvanic corrosion argue that if it was, the spider would have corroded at the points where it meets the stainless steel drum (the 3 ends), and at the shaft. Although most of the corroded spiders i've seen corrode a few inches from the shaft, i'm yet to see most of the corrosion happening at the spider-arm end joints...or immediately at the shaft. So they are of the opinion that the reason it is corroding is because of soap and high PH water.

In any case, i am getting ready to put in a new spider-arm and i would like to treat/coat it (and/or the stainless steel directly under it) so that i could get at least 10 years out of it.

I am thinking of first etching it with phosphoric acid, then anodizing it, and finally spraying it with a 2 part epoxy primer. What do you guys think?

 

[starz]

anyone?

 

[VicS]

What is the best method of protecting cast aluminum from corrosion when it will be attached to stainless steel and dumped into sea water where it would reside for 10 years without any access to it?

Ok, what i am talking about is not part of a boat, but rather a washing machine.

I dont understand why you are dumping your washing machine in sea water and leaving it there for 10 years ........... Never mind the aluminium spider the electrics will be shot well before 10 years have passed.


As far as epoxy coating the aluminum spider is concerned read the coating manufacturers product data sheet and follow the recommended procedure .

E.g. for International's Interprotect epoxy primer see http://www.yachtpaint.com/MPYACMDatasheets/Interprotect+eng+A4+Y+20161123.pdf

A quick look into it seems to suggest degreasing and sanding with 24 -120 grit aluminium compatible paper

 

[noelex]

The spider arm will suffer very rapid corrosion in the application you describe, so a 3 year life is not unreasonable.

To prolong the life it needs to be electrically isolated from the stainless steel and the central bearing. While some isolation from plastic spacers and chemical treatments could be achieved, the long term isolation would be limited.

Aluminium is very corrosion resistant in the right application, but this is not the right application.

I would replace the spider arm with a stainless steel fabrication.

 

[Pasarell]

I don't understand the use for this either but would be interesting to know. I'm guessing it's not as a washing machine!
I guess there are some insulation pieces between the Al spider arm and the SS drum otherwise galvanic corrosion would take place, but then it would also in a normal washing machine on land.
If the Al arm is isolated from other metals at all points do you have any electrical connection that could be causing electrolysis? Is the drum still being spun? If so, maybe the stresses on the arm underwater are causing a fracture in the Al casting. The forces could well be very different from what it is designed for.
If you want to coat then miss out the galvanising. Degrease, etch prime and epoxy system. Key to the epoxy system will be getting enough on. Too little and you are just wasting your time

 

[vyv_cox]

The problem with applying epoxy to aluminium is that it can be difficult to achieve a bond between the two. Ideally you would use an etch primer but AFAIK there are no epoxy versions of these. If you apply a conventional etch primer you will not be able to apply epoxy over the top. As said, coarse abrasion is an answer but I suspect that the bond will not be perfect and water will creep in.

Galvanic corrosion is not an issue here. If it was the aluminium would be far more corroded at points close to the stainless steel, which it is not. The problem here is general corrosion.

 

[lw395]

I've had reasonable results epoxy coating aluminium alloy parts.
wire brush to remove loose corrosion.
Maybe wet sand, but at least scrub with water and detergent to degrease.
Rinse in hot water.
Heat it up to dry thoroughly.
Brush on epoxy (e.g WEST or SP) while it's still warm, say 60 celcius.

 

[PaulRainbow]

I'd try Hammerite special metals primer and a couple of coats of smooth Hammerite.

Or, buy a better washing machine.

 

[howard easton]

What is the best method of protecting cast aluminum from corrosion when it will be attached to stainless steel and dumped into sea water where it would reside for 10 years without any access to it?

Ok, what i am talking about is not part of a boat, but rather a washing machine.


I'm posting here because folks at the washer forums haven't come up with a solution yet. I've seen a few posts here talking about similar problems so i figured you guys might have some experience dealing with these materials in such environments.

The aluminum part is called a spider-arm. It's attached to a stainless steel drum. The drum is of course where you put your clothes. This setup is (afaik) always on a front load washer. The thing is, the spider always corrodes and breaks - mine did in just 3 years. It's inevitable. Lots of theories as to why that happens...galvanic corrosion, water PH levels, too little detergent, too much detergent, using cold water, keeping the washer door closed...and so on.
Those who say its NOT galvanic corrosion argue that if it was, the spider would have corroded at the points where it meets the stainless steel drum (the 3 ends), and at the shaft. Although most of the corroded spiders i've seen corrode a few inches from the shaft, i'm yet to see most of the corrosion happening at the spider-arm end joints...or immediately at the shaft. So they are of the opinion that the reason it is corroding is because of soap and high PH water.

In any case, i am getting ready to put in a new spider-arm and i would like to treat/coat it (and/or the stainless steel directly under it) so that i could get at least 10 years out of it.

I am thinking of first etching it with phosphoric acid, then anodizing it, and finally spraying it with a 2 part epoxy primer. What do you guys think?

I once owned a Prout cat with an ally outdrive leg and a black anodised and fading ally mast. The mast irritated me so I removed the anodising with a hot concentrated caustic soda solution ( and lots of protective gear) before coating with an etch primer from that dutch paint outfit whose name currently escapes me
and then spraying with epoxy primer and poly two coat top coat. That treatment worked well with no issues in the 7 years I kept the boat though it was such a ballsache to do that I would buy a new mast if faced with the same issue again.

As for the Silette outdrive leg, I tried the same approach. It was better than any other paint but it still didnt last more than 3 years use in sea water. The problem was either the screw holes which left bare ally edges or scratches or dings or even fish eyes in the epoxy coat. Once salt water got to even the smallest patch of bare ally, it corroded and travelled along under the paint

I dont know whether this is practical for you but it seems to me that the issue is adhesion of the paint to an oxidised surface. What might work is to paint a clean surface and whilst the paint is still wet, give the surface a good abrade with a wire brush. This should generate a rough surface of bare unoxidised ally under the paint film and maybe better adhesion.

 

[starz]

I dont understand why you are dumping your washing machine in sea water and leaving it there for 10 years ........... Never mind the aluminium spider the electrics will be shot well before 10 years have passed.

I am sorry for not being clearer. I didnt mean i would, literally, be dumping my washer water and leaving it there for 10 years. I was just portraying that the part i am posting about would be subjected to a similar environment - you know, with water and soap.

The spider arm will suffer very rapid corrosion in the application you describe, so a 3 year life is not unreasonable.

The application i meant to describe is actually the normal application of a washer.

To prolong the life it needs to be electrically isolated from the stainless steel and the central bearing. While some isolation from plastic spacers and chemical treatments could be achieved, the long term isolation would be limited.

Aluminium is very corrosion resistant in the right application, but this is not the right application.

I would replace the spider arm with a stainless steel fabrication.

Who does fabrications as such?

...Or, buy a better washing machine.

Haha...they're all the same from the inside.

 

[Tranona]

I am sorry for not being clearer. I didnt mean i would, literally, be dumping my washer water and leaving it there for 10 years. I was just portraying that the part i am posting about would be subjected to a similar environment - you know, with water and soap.


The application i meant to describe is actually the normal application of a washer.


Who does fabrications as such?


Haha...they're all the same from the inside.

Not sure why you have this problem. I am on only my fourth washing machine of this type in close on 40 years (latest is 2 years old) and have never experienced this problem. 1 was a Hoover and the others all Hotpoint so among the cheapest. Previous 3 all failed because of the drum bearings breaking up and averaged over 12 years each.

 

[lw395]

Not sure why you have this problem. I am on only my fourth washing machine of this type in close on 40 years (latest is 2 years old) and have never experienced this problem. 1 was a Hoover and the others all Hotpoint so among the cheapest. Previous 3 all failed because of the drum bearings breaking up and averaged over 12 years each.

My guess would be some sort of water softening?
Or is it the practice of raising the pH of the water to stop it dissolving lead pipework?

 

[Iminei]

Buy a Miele. Seriously.

(Edit: That was the advice given to me by a washing machine repair man 12 years ago when we bought our current machine)

 

[ghostlymoron]

Trouble with Miele washers is the eyewatering initial cost. A lot to pay for a ten year warranty.
We replaced our Zanussi last year after 20 yes, the main bearing was the problem not the spider.