How to protect cast aluminum from corrosion? 1

[Radkeyz]

The aluminum part i want to protect is part of a washing machine. It's called a spider-arm, and it's attached to a stainless steel drum. The drum is of course where you put your clothes. It spins at speeds of 1200rpm. The load (clothes) can be unevenly distributed in the drum, therefore it could be put through high stress. This setup is (afaik) only on front load washers.

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, not using bleach...and so on.

Those who say it's 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 up 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. What would be the best method?

Greatly appreciate any help i can get!

Click here for more pics (Google image)

 

[Compositepro]

About all you could do is paint it. Looks like general corrosion from wash water containing bleach. Aluminum casting is not going to last long in that environment. I would call it a bad design, or planned obsolescence.

 

[Radkeyz]

So painting it would be better than priming it with epoxy? Or anodizing it? Or coating with coal tar epoxy? Or zinc chromate?

 

[Compositepro]

Those are all just basically different types of paint, except for anodizing. Anodizing would help a little, and would help paint adhere better.

 

[Radkeyz]

OK. Can you suggest a paint?

 

[romke]

the real reason for the spider to rot away in a relatively short time nowadays is the fact that over the years we have changed the washing process in such a way that we have created a environment in which certain type of bacteria thrive. we use less phosphates and more silicates in the detergents for environmental reasons, use less bleach because dyes used in modern fabrics tend to fade much sooner and we wash at lower temperatures to decrease energy use. that creates a environment where certain bacteria that are living in worn clothing thrive in: no bleach and low temperature (< 60 deg C) and a relatively soft metal where they can feed on.

so far the industry has not come up with a suitable solution, a water soluble bactericide that is effective in killing the bacteria in the environment of a washing machine while being ecologically acceptable has not yet been found.

you could call the use of aluminium a design flaw. however, you need a material that is both relatively light and strong - modern plastics so far seem to be unable to fulfil those requirements.

coating the spider with a coating that makes the aluminium out of reach for the bacteria might be a solution. apart from paints etc as suggested i think powder coating the specific part might be a solution. with that you probably can get a denser coating then when applying it with a brush.

the speed at which the spider rots away is highly dependent on local water quality - and water quality may vary quite a bit, even in the same city.

 

[Radkeyz]

Very insightful romke! So i guess now its between 2 part epoxy primer or powder coating. Love to hear your opinion.

 

[romke]

both should work. but i guess that a powder coating would be better, because it's surface is likely more "slippery" and possibly less diffuse.

 

[racookpe1978]

Powder coatings do best when there is absolutely no erosion or scratching/wear ANYWHERE on the part so the plastic coating is 100% retained everywhere for the lifetime of the part. (You will need to look at your assembly process and the screws/bolts involved to make sure "nothing" can cut or rub through the thick plastic layer now coating the Al. Also, that coating itself make mean some other parts and surfaces need to be larger (or smaller!) clearance to assemble and install the now-thicker aluminum spider with coarser, more rounded edges and used-to-be dimensional surfaces.

 

[Radkeyz]

racookpe1978.

The AI arm bolts to the stainless steel. Would tightening down the bolts break the powder coating?

 

[racookpe1978]

Smoothly tightening the bolts would not break down the somewhat-flexible powder coating. A clamping force through the plastic would probably be OK. But a skidding or irregular twisting force or slamming force will cut the plastic. In a hardware store, look at the powder-coated handles on tools, or the thin powder-coated decorative "steel" fence and railing parts.

Your problem would come in when the modestly thick powder coating plastic layer (applied as a sprayed-on powder then melted in a furnace) would fill-in drilled bolt holes, or cover a surface where the arms attach so the stack height of the assembly is too big.

 

[Radkeyz]

racookpe1978,

Your problem would come in when the modestly thick powder coating plastic layer (applied as a sprayed-on powder then melted in a furnace) would fill-in drilled bolt holes, or cover a surface where the arms attach so the stack height of the assembly is too big.

You mean clearance might be an issue?

 

[chicopee]

I would like to know if the manufacturer had anything to say about your problem.

 

[dgallup]

First, this post belongs someplace else, it has nothing to do with tribology.

Second, there is no such thing as aluminum eating bacteria! That set my BS meter off immediately. There are bacteria that excrete acids that will corrode aluminum.

Third, this part looks like it could easily be replaced with a stainless steel stamping. With a little development it probably could match the weight, strength and cost of the aluminum part.

If there is really much of a market for such a thing, the OP should take the entrepreneurial initiative and start the ball rolling. I expect the market is actually very small, only places with the wrong water conditions.

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