Best Plating Option for AL 6061 3

[ogray]

What is considered to be the most corrosion resistant plating finish for Al? Here is my problem. I have an Aluminum housing that, according to spec, must be "hard coated per MIL-A-8625, TYPE III, class 2, dyed black". This housing must also pass a 30 day salt fog requirement per MIL-STD-810. The first unit I have plated failed miserably during the 30 day salt fog period.

Can anyone recommend a similar or comparable process to hard coat that will stand up to the requirements of this test?

Thanks in advance

 

[BrianR]

This is not a direct reply to your question, but on reading MIL-A-8625F there are a number of options re sealing the finished product and this affects it's corrosion resistance. This spec seems to be for abrasion resistance rather than corrosion protection. A copy of the spec can be had at

http://astimage.daps.dla.mil/

 

[IRstuff]

Slight disagreement on MIL-A-8625F.

Paragraphs 3.7.1.2 and 4.5.3 show that Type III Class 2 coatings should be capable of surviving 14 days of salt spray with little more than minor pitting. I don't have ASTM B 117, so can't really compare the salt spray vs salt fog of 810F.

If the coating failed miserably, then perhaps the coating was sufficiently conforming to 8625 ??

TTFN

 

[roberth]

Have you considered ceramic coatings? I would suggest a thermal spray coating of alumina or zirconia. Please feel free to contact me with any questions.

Regards,

Robert Hall
Phone: 716-826-9103

 

[kenvlach]

Hi,
I'm in the anodizing business and have comments on the above.
ogray,
For best corrosion protection, anodize should be sealed using a dichromate solution. This is not the default, it must be specified on purchase order. Also, dichromate sealing affects dye colors (and vice versa, nobody wants dye in their dichromate seal), and is normally only specified for 'clear' Class 1 anodize. If a customer wants both Class 2 (dyed) and dichromate sealing, we do what is called 'duplex sealing:' seal in normal nickel acetate for 1/2 normal time, rinse, then finish sealing in dichromate. As for your parts failing miserably, I'm not surprised, Type III anodize typically has some 'crazing' in the layer, which makes dichromate sealing even more imperative. For your case, there are clear supplementary sealants which would allow the color to show & extend the corrosion resistance.

IRstuff,
"Paragraphs 3.7.1.2 and 4.5.3 show that Type III Class 2 coatings should be capable of surviving 14 days of salt spray" isn't true. The corrosion requirements of MIL-A-8625F do not apply for Type III coatings.

We have done hard anodizing w. dichromate sealing on undersea sonar parts for Navy contractors who used an epoxy topcoat. The epoxy was there in part due to rough handling on ships. The parts competed successfully with titanium parts.
Also, hard anodized aluminum competed successfully with stainless steel for handrails for marinas. If the anodizing is done right, with supplemental protection on a case-by-case basis, it can be used in corrosive environments. Maintenance may be required, same for stainless steel. Hard anodize, with dichromate sealing, plus a suitable powder coating, can withstand almost any environment as long as the temperature isn't too high.
Thanks,
Ken