Best anodize method for SW corrosion resistance

[circlespot]

We are using 6061-T6 plates in a seawater environment (usually not in contact with seawater though covered in salt from incidental contact and sometimes sitting in a pool of stagnant seawater for long periods of time)Our current anodize spec is "HARD ANODIZE PER MIL-A-8625F TYPE III, CLASS 1,.002 NOMINAL, NICKEL ACETATE SEAL."

This thread (thread307-28313) suggests using a dichromate seal for maximum corrosion resistance.

I am having trouble finding reference material on the best anodize methods for seawater resistance, any pointers to such information would be greatly appreciated.

I have Also investigated using 'marine grade' alloys, specifically 5086. I cannot find any information on the relative corrosion resistance of this alloy vs. 6061 or if and/or which anodize to specify for the alloy. I would want to have some reference data rather than just the 'marine grade' pedigree before switching to it.

Non-aluminum alloys would be cost prohibitive, and the current part is mostly performing satisfactorily but I would like to know that we are giving it the best shot at surviving through proper material and anodize specification.

Thanks in advance

 

[MikeHalloran]

I'll tell you what little I think I know.

5086 is widely used for marine structure, e.g. the boat itself. In that service, it's usually coated with some seriously expensive marine paint. I'm not sure if anodizes well, or at all.

As I understand anodizing, it leaves the aluminum covered with aluminum oxide, which is always micro-cracked, hence the need for a sealer.

Your part is in what is actually a pretty hostile environment. I've seen generic 'anodized aluminum', about which I have no other details, corrode pretty quickly in more benign conditions.

Freshwater boat builders use clear anodized aluminum on exterior parts.

Saltwater boat builders mostly don't. Any aluminum exterior parts are powdercoated, usually white, sometimes clear, sometimes argent. You might want to take a look at powdercoating; it's incredibly tough stuff.



Mike Halloran
Pembroke Pines, FL, USA