Type III anodizing- max corrosion resistance seal

[thlum]

This question is an extension of thread 404-66479. I am working on an aluminum housing for a lighting fixture for the marine environment. It will be used in underwater applications. It is machined from 6061 aluminum, and proposed to be Type III hardcoat anodized (natural, no dye). Is dichromate the best sealing technique for maximizing corrosion resistance? If the anodizer I am currently working with only does boiling deionized and nickel fluoride, which is the better choice? How do these two compare to nickel acetate? I've seen Metalast's reports, but they appear to be specific to Type II anodizing.

 

[israelkk]

Here is the quote from MIL-A-8625F ANNODIC COATINGS FOR ALUMINUM AND ALUMINUM ALLOYS

3.8.2 Type 111. Type III coatings shall not be sealed where the main function of applicatlon is to obtain the maximum degree of abrasion or wear resistance. Where Type III coatings are used for extertor non-maintained applications requlrlng corrosion resistance but permltttng reduced abrasion resistance, the contract or purchase order shall specify that sealing is required. Seallng for such Type III coatings shall be accomplished by Imnersion in a medium, such as bolling deionized water, In a hot aqueous 5
percent sodium dichromate solutlon, 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 annodizing

 

[thlum]

Thanks for the response, Israelkk. But I want to know which sealing method would provide the BEST corrosion protection for a submerged saltwater application-- it would seem that not all sealants are equal..

 

[sreid]

I would call General Magnaplate.

http://www.magnaplate.com/

Look at Magnaplate HCR. But talk to them about your application. In my humble opinion, a little pricy but worth every cent.

 

[TVP]

Of the options you mentioned, sealing with sodium dichromate is the best for marine corrosion resistance. Nickel acetate is better than hot water. Duplex sealing (nickel acetate first, then sodium dichromate) offers the best overall corrosion resistance for anodized aluminum.