MIL-A-8625, TYPE II, CLASS 2 with sealer?

A. Sealers have never been called "clear coat". You should not have to specify "sealer" but you should specify which sealer: Nickel Acetate (the most popular and clear), Cobalt Acetate, Plain Hot Water (no toxicity), Dichromate (certainly not clear). Sealing is hydrating the aluminum oxide. The hydrated molecule is larger than plain aluminum oxide, and this "clogs" the pores and retards corrosion.

A. Unless you are requesting Type 3 Hardcoat anodizing, the sealing process (it is a process, rather than a coat - it affects the characteristics of the existing coating rather than adding new material) is a required part of the MIL-A-8625 specification and no anodizer should be omitting it and you can write your callout as "MIL-A-8625 T2C2 Black". If you are worried, specify the callout as "MIL-A-8625 Type 2 Class 2 dyed black and sealed with Nickel Acetate". This way you'll simultaneously dodge the possibility of your anodizer using sodium dichromate on the parts - which, while allowed, would discolor them some noticeable amount.

A. Unless your customer needs a "dichromate seal" you can say nothing and your anodizer will automatically seal in either nickel acetate or hot water. If you want maximum corrosion protection then say "Nickel Acetate Seal".

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A. Hi Carlos. Yes.

Parts must always be fully sealed or they will fade. This can be done in hot water or otherwise. If properly sealed, how much they fade will depend on how stable the dyes are. Even the best organic dyes will fade in sunlight. Please try your best to describe your own situation. Thanks.

Regards,

A. Hi Burain. I'm glad your material complies with a spec, but AMSQQA200/225/250 includes more than 55 standards, so it's not telling us anything specific. All anodized parts discolor to some extent, because the anodized coating is not glass-clear, although for certain alloys with thin coatings and a non-chromate sealer it may be almost unnoticeable. I think we'd need to know the type of alloy, the anodizing thickness, and the sealing method to be able to hazard a guess as to where a "yellow" color came from. A picture might help too. But certainly if they were chromic acid sealed, they'll be yellow. Thanks.

Regards,

A. Hi. Apparently I misunderstood your problem. It is not that a coating that was intended to be clear is yellowish, but that a yellow coating is discoloring? But so I don't misunderstand a second time, what do you mean by discoloration? Is the yellow color washing away, or is it turning black, or getting spots of some sort?

Have you considered a double seal consisting of nickel acetate followed by potassium dichromate? Bob Probert discusses this in letter 57367.

Regards,

A. I suspect the pH of the dichromate seal was out of range.

A. My experience with anodizing military specs is that they have an end goal (coating properties) for you to meet, and don't specify how you get there. In the case of your question, you could possibly use a Ni acetate seal, as long as your product meets all the end goal requirements (salt spray pass, coating weight, coating thickness, etc.)

The print for the part should specify if the dichromate sealing is required, or any other medium (Ni acetate, DI water, Ni fluoride, etc., etc.). If it doesn't, then I interpret the spec that the coater can process in his medium of choice, as long as the coating properties in the spec are met.

Thank you Mark for the response.

If I understand correctly, if you were to receive a request to use a specific sealer type then you would do so and state it on the cert, otherwise sealing options are open for sealing and referencing not needed.

In our case we requested (as the print specified) dichromate sealer. Should we expect the cert to state this or only if we requested it to be stated? The cert we received only references the Mil spec.

I didn't formally say "Hi" to everyone either. Nice to be here. I've been working around aviation parts for a while...even worked for 7 years at an aircraft component repair shop supervised the plating shop. So I've done a little anodizing but as someone early said we never have all the answers. I certainly don't. :-)

A. If it were me, yes, I would request that the cert stated such, especially if it's called out on the print. That should be an easy thing for your vendor to accommodate, and gives you peace of mind.

That being said, I wouldn't get upset with your vendor for not stating his sealing mechanism this first go-round, as you did not request them to do so. But going fwd, it shouldn't be an issue.

Thanks Marc, I'll pass this info on to our purchasing dept.

A. Hi Adam,

For corrosive environments such as operations in a salt laden environment I would choose dichromate sealing over nickel acetate.

End of the day you are the customer so if you want sodium dichromate seal, and you specify that, anything else is non-compliant so you do not have to accept it. If the supplier you are currently using refuses to meet your requirements it's time to find a new supplier who will! Suppliers don't dictate the product, the customer does!

A. Hi Marjan,

Sounds like sulfuric acid was left trapped in the hole, which has then attacked the aluminum. I see this quite often. It is down to a poor rinsing regime at the anodising company. I suspect if you were to do an elemental analysis you will find aluminum, sulfur and oxygen and not a lot else.

To rescue the parts you can polish out the damage, but it's going to depend on the tolerances you are working to.

A. Hi Marjan,

The size of hole is what is going to cause you a problem. If the anodizing solution gets to the bottom of the hole and there are no air locks then it will anodize all the way down, if you have air locks or pre-treatment solutions become entrapped you get no anodizing. If you have successfully anodized and then go into the dye/seal tanks without successfully removing the sulfuric acid then you will end up etching and the hole will not look anodized. My company, which builds a lot of small parts for the Aerospace industry recognized this problem a number of years ago and so allow the touch up of bare metal surfaces in blind holes with a conversion coating (MIL-DTL-5541 [on DLA] Type 1 Class 1A). This contains hexavalent chromium so may well not be allowed in your application. There are other trivalent chromium conversion coatings that may be acceptable to you, whatever you do you do not want to leave a bare metal surface.

In the meantime you do need to go back to your treatment shop and make sure they understand they are causing you a problem due to poor rinsing practices. You may not always get the hole anodized, but there are no excuses for leaving process solutions trapped in holes that can cause you problems further down the line.

This is very helpful. Greatly appreciate your help.

Thanks,

A. MIL-A-8625F paragraph "6.10.1.1 Holes. In the case of small holes and tapped holes, coating thickness can vary from no film to a full normal coating. Holes, both tapped and not tapped, over 1/4 inch should be anodized." I read this to mean that holes smaller than 1/4 inch do not have to be anodized.
It is not surprising that a small, blind holes retain anodizing electrolyte. Once it wicks down a hole the surface tension can make it nearly impossible to remove with rinsing, no matter how good or long. As the sulfuric acid dries out it forms the white aluminum sulphate crystal which blooms out of the hole. A better practice is to plug these holes.

A. Hi Tom. Guessing can only suggest things for you to look at, not resolve the problem, but two guesses ...
1. The yellow color is from a dichromate seal, a part of the duplex sealing process.
2. The etching process, limited de-smutting, type of alloy, and thickness of the anodizing layer is leaving/creating a yellowish tint. If this is the case, rather than the dichromate, we'd need a lot of data to go further.

Luck and Regards,

A. Lots of possibilities as you can see. Some alloys of aluminum will render a yellowish oxide so check out that option.

A. One of the most neglected areas in anodizing, regardless which part of the line is rinse water. Change it even if you think it is clean. Filtering with activated charcoal canisters also works.

 Hi Ken,

Thank you for your advise, we have overcome the issue with extra DI water spraying when lifting up the parts from anodizing tank.

You are absolutely right, the rinsing water needs to be changed regularly even if we see it clean.

Your advise is highly appreciated.