MIL-A-8625 Type II vs Type III

[CanosSSCS]

Hello all,

I have a part that will be exposed to "plant" water with all kinds of unknow metals. The part is made of Al 6063-T6. My main goal is to protect the part against corrosion. I've been doing some digging around and I am unclear as to what would be a better anodization for the intended operation, Type II or Type III. I see that type III produces a thicker and harder coat, which helps with wear resistance, but might not necessarily be as effective against corrosion as type II with sealing. Also, when it comes to sealing, what would be an "optimal" solution? I see nickel acetate, and nickel fluoride are commonly used. I appreciate any insight in this regard.

best,

Carlos

 

[Ti_Metallurgist]

Taking a stab at this but others can likely provide better response.

What temperatures and solutions do you expect the part to see? If just "dirty" water at ambient conditions, either option is probably OK.

Typically Type III hardcoats are used for wear resistance in the un-sealed condition. Leaving anodic surfaces un-sealed will lend itself more to corrosion than with sealing, of course, even a hot water seal will help. The type II sulfuric acid anodize, with a sodium dichromate or duplex (sodium dichromate + nickel acetate) will likely be your best bet to keep corrosion away.