Can epoxy primer be sprayed on top of anodized cast aluminum?

Discussion in 'Metal Boat Building' started by gskz, Aug 10, 2017.

  1. gskz
    Joined: Aug 2017
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    gskz New Member

    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?
  2. jorgepease
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    jorgepease Senior Member

    I'd 3D print a new one out of Titanium lol )) ... No seriously I have no idea but I would think the Anodizing or maybe even powder coating would be enough. It's never happened to me, maybe you should test your water and treat it if you find it's causing the problem.
  3. Nick.K
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    Nick.K Senior Member

    I don't have much experience of coating aluminium but I have quite a lot to do with seeing coatings come off which makes me think your solution probably won't work. Getting it professionally powder-coated and baked would almost certainly give a more reliable finish than home brewed etching, anodising and coating but I doubt that would last either in the situation you describe. (Anodising and coating probably not compatible in any case). Maybe you could isolate the spider from the drum with say a teflon sheet between and figure a way to isolate the fastenings too? I guess the water level rarely reaches the centre of the drum so that part stays mostly dry? Epoxy is easily sprayed with an airless sprayer.
  4. Ike
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    Ike Senior Member

    It probably is galvanic corrosion. Back in the 80's when I was managing a repair facility for the Coast Guard, on aluminum boats we would put a dielectric material, usually plastic or a rubber substitute, between the stainless steel fittings and the aluminum hull. No corrosion. If you look at a galvanic series table, aluminum is on the anodic (less noble) end and stainless steel is on the cathodic (noble) end. Add water, an electrolyte, and you have a battery. It's a weak battery but a battery nonetheless. Current flows, and the aluminum corrodes. Either separate the two or use some other material as a spider. (a structural grade plastic would probably work) Some engineer forgot his basic chemistry that he learned in engineering school.
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Phenol novolac epoxy resin is used for coatings where higher chemical and heat resistance is required. You might look at that if you proceed with coatings. I doubt powder coating would last indefinitely.
  6. gskz
    Joined: Aug 2017
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    gskz New Member

    Can't find that here. Does it have another name? Or any alternative?
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Look up novolac epoxy. It's not a very common epoxy on the retail level. It's regularly used in concrete floor coatings, intended for harsh environments.,, are a few locations to check out.

    Having just put a new set of bearings in the other half's front load, I'll bet your problem isn't a coating, but a ground wire. There should have been a "bond" wire carried throughout the machine, including the shaft, which is carried through a bearing carrier (race) usually. If this bond is broken or not connected in the system someplace, you'll get a lot of corrosion quickly. Additionally, the seal that is supposed to protect the shaft and bearing(s) typically is the press fit style, which work for a while, but not very long. If you want the new one to last, apply a bead of polyurethane sealant (3m-5200 or similar) to the seal seat before you drive the seal in place. Some will suggest silicone, but I wouldn't, just not nearly as much adhesion as polyurethane.
  8. missinginaction
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    missinginaction Senior Member

    I just can't see any coating staying put under the conditions that those spiders have to work. I'm not a chemist though. PAR, checking the ground is a good idea and one that the average consumer won't understand.

    I replaced a drum/spider assembly on a Whirlpool Duet a couple of years ago. It was covered under a lifetime warranty from Whirlpool. I called and explained the problem. They balked for a moment because I wasn't a "service tech" but when I politely offered to send them a digital photo and described the problem in detail their attitude changed quickly and the assembly arrived a few days later. It wasn't a free repair since when the spider let go it damaged the plastic outer drum but it wasn't that big a deal. I put a new set of bearings in it since I had it apart.

    Our original spider lasted from 2003 to 2016. I believe we got good mileage out of the original because we use detergent according to the manufacturers instructions. Sparingly. I believe that Whirlpool still offers that lifetime warranty on the drum assembly. I'd be reluctant to buy a machine that doesn't warranty their spiders for life. They're just too prone to failure.

  9. gonzo
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    gonzo Senior Member

    That is an aluminum casting riveted to a stainless steel plate and having a stainless steel shaft press fit into it. They made a galvanic battery. Further, there is an electrolyte in the solution of detergent, water, etc. The coating, or lack of is not the main problem. The design is flawed. Sacrificial anodes may help.

  10. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    google aluthane

    exactly what the doctor ordered
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