[SlaterSpeed]

I think you have a few options. in order of preferance

1. Make them out of brass
2. Nickle plate them
3. Type 2 anodize them

Personaly i feel that all 3 of those options offer acceptible protection but it is clear that others have higher standards than i do. If you make things to sell to the public you need to cater for them. Arguing the toss does not sell the caps! The way you will sell the most caps is by producing brass caps. This is the same reason that i have started making copper/brass waterblocks insted of copper/alu. Not because of my needs/oppinions but because i want them to sell and therfore you must choose the option that is most desirable to whoever is buying.

FYI i run an aluminium res with an aluminium cap. The res and cap are not anodised or plated in any way. When i take the cap off there is allways a few drops of condensed coolent on its underside. So far after 6months it has not corroded. Thats why i do not worry too much but it does not mean it cant happen.