Second coating on Hard Anodized Aluminum - sealing helium

[bogiedreamer]

We have a helium/LOX valve that is leaking even with heavy spring PTFE seals. The valve finish is hard anodized aluminum and even with a 4 Ra finish, leakage is higher than desirable.

Can we apply some type of second coating to reduce this leakage? Something to fill up the deep pores in the anodized aluminum?

Many thanks,

 

[israelkk]

Anodize coatings such as Type II and Type III contain micro cracks. This has not been a problem with heavier gases such Air. I am not sure the problem is the anodize coating but if I have the option I use stainless steel instead of Aluminum for Helium gas valve.

 

[IRstuff]

How sure are you that the anodize isn't porous? Anodization usually results in a porous surfaces

TTFN
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[bogiedreamer]

Israelkk, our customer insists on using Aluminum for weight saving purpose. The sulfuric anodization is sealed per MIL-STD-8625. Even with a 4 Ra, 30 Rz, and 10 Rp finish, do you think the microcracks is still relevant?

IRstuff, I don't know. The only think we could do now is to measure the surface finish and they are around 4 Ra, 30 Rz, and 10 Rp which is very good.

 

[IRstuff]

The surface finish tells you nothing about the porosity. We once had leaky pinch-off tubes that were brazed into place. Even though they were seemingly well sealed, there were still sufficient tortuous paths that allowed a nitrogen/argon mix to leak.

TTFN
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[israelkk]

bogiedreamer

According to MIL-A-8625 hard anodize is type III and it is normally NOT sealed.

Here is a quote from MIL-A-8625F

3.8.2 Type III. Type III coatings shall not be sealed where the main
function of application is to obtain the maximum degree of abrasion or wear
resistance. Where Type III coatings are used for exterior non-maintained
applications requiring corrosion resistance but permitting reduced abrasion
resistance, the contract or purchase order shall specify that sealing is
required. Sealing for such Type III coatings shall be accomplished by
Immersion tn a medium, such as boiling deionized water, In a hot aqueous 5
percent sodium dichromate solution, in a hot aqueous solution containing
nickel or cobalt acetate or other suitable chemical solutions (see 6.2). When
Type III coatings are provided unsealed, parts shall be thoroughly rinsed in
cold, clean water and dried after anodizing.

This post appears in other forms too. Please do not double or triple post!

Only Type II (sulfuric anodize) and type I (chromic anodize) require sealing to produce hard surface. What type of aluminum alloy are you using.