Galvanic corrosion problems 1

[musashi99]

Galvanic corrosion problems.

I have a cast Aluminum housing (alloy 380). That has been chromate converted, had an anit-gas coat applied and than an epoxy based power coat applied. When the unit is assembled there are several 17-4 ph components in the assemble. While going through the Navy required salt spray and humidity tests we have galvanic corrosion occurring.

I was hoping to combat this problem by hard coat anodizing the aluminum housing and then sealing (if needed) and paint with the powder coat.

A few folks are not sure that the sealer or the powder coat will stick to the hard coat anodize. Any comments are appreciated.

 

[CoryPad]

What is anit-gas?

Alloy 380 does not have great corrosion resistance, any chance to change alloy?

You should use regular anodizing, not hard anodizing, if you want corrosion resistance. You definitely can use a powder topcoat over anodizing. If you are powdercoating, then you don't need the sealer.

 

[MikeMech]

You could try separating the aluminum and 17-4 components with a coat of non conductive paint. I've used this to good effect with stainless screws in aluminum in salt water environments...

 

[tbuelna]

musashi99,

You need to seal and isolate the interface between the 17-4 parts and the 380 housing. If the 17-4 parts are permanently installed (like a pin, bushing or insert) then they should be installed "wet" with a primer. If the part is removable, then the interface should be some material that is galvanically compatible with both parts, and the interface should also be sealed.

Most topcoats don't bond well with bare aluminum. So some sort of surface treatment like chem film or anodize is usually first required on the aluminum. If you're applying a topcoat like paint or powdercoat, then a standard anodize should be sufficient.

Hope that helps.
Terry

 

[musashi99]

anti-gas can best be described as a sealing primer coat.

So hard coat anodize does not afford better corrosion protection than Type II? Remember that I am also trying to create an electrical barrier between the Stainless and the Aluminum. That is why I am considering Type III hard coat.

I am also thinking of putting a layer of Di-Chromate over the top of the anodize. As far as I know the powder should stick well to the Di-Chromate.

The anodize coupled with the Di-Chromate the powder coat should offer improved corrosion resistance while a the same time electrically isolating the two dissimilar metals

Any thoughts are appreciated.