Formulation & use of Dichromate Seal for Anodized Aluminum

A. Hi Dave,

Potassium is a slightly smaller atom than sodium and is a little faster thru a diffusion layer. I seriously doubt if there is any difference from a functional standpoint between the two when used as a seal.

You will have to get a customer "buy off" to be legal to cert it to the Mil Spec. You can ask the controlling agency to make it sodium or potassium. You have a reasonable case. It will take them about a year to process the change. Civil servants are not interested in making work for themselves and have to have lots of industry people to agree before they dare make a decision.

A. Chemically speaking, the two are nearly identical. Both the sodium and potassium donate a single + charge to the bath.

A. MIL-A-8625E, 25 April 88, under 3.8.1.1, states in part, that you can use sodium dichromate, boiling deionized water, or other suitable chemical solutions. Potassium dichromate is suitable in my book, because we called for it in our process specification at Bendix. If your customer calls for it, I would say that it is suitable.

A. The book The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick, R. Pinner, P.G. Sheasby has the info you need.

A. We used to prepare dichromate seal as:
Concentration: Sodium Dichromate 7 oz/Gal Temp.: 195-205 °F DI water

A. A semi educated guess is the black out put is a metal based dye and the green output is an organic based dye that the chromate is reacting with. Switch to a nickel based seal and you should have good results and a black finish with both dyes, plus you have eliminated a source of hex chrome which makes many tree huggers happy.

A. Dichromate Seals are ALWAYS yellow-greenish tint if properly applied. Dichromate Seals leave the best corrosion protection. Dichromate Seals should never be used on dyed work: not just because it changes the color of the dye but also because Dichromate Seals are for maximum corrosion protection and ALL dye slightly lowers corrosion protection. So why would you want to put on Black dye to lower corrosion protection and then discolor it with dichromate which raises corrosion protection? Or just shooting yourself in the foot !

A. Hi, read Mil-a-8625f section 6.11 it will explain duplex seal nickel seal then dichromate. Also 3.5 last sentence.

A. 1. A dichromate seal for conventional Type II anodizing on aluminum would be 100 gm/Liter (13.333 avoir. oz./gal) of either sodium or potassium dichromate, 200-212 °F, pH 5.0-6.0 adjusted with Sodium or Potassium Hydroxide and acetic acid ⇦this on eBay or Amazon [affil links] .

2. A seal for Type I Chromic Acid Anodizing would be Sodium Dichromate 6 to 8 oz/gal, pH 5.0-6.0 with NaOH or acetic acid , 194-212 °F

3. However, if it is Chromic Acid Anodizing for Boeing then use: in 100 gallon: 26 grams of chromic acid and 26 grams of either sodium or magnesium chromate, pH 3.2-3.8, 180-200 °F.

A. Hi HL,

It sounds to me that you are over-treating the parts, leaving them in the seal too long. Simple solution is to reduce the time in the seal.

A. Hi Hammad Abdul. The majority of anodized parts are probably free of Cr VI content. So yes, green dyeing followed by a different type of seal is theoretically possible.

But please don't keep the question abstract. Instead, please try your best to describe your actual situation: Who are you (specifier, anodizer, etc.) ... who is writing the specification ... what is the specification ... what are the parts ... how are they used ... etc. To simply answer the question of what is theoretically possible might mislead you rather than help you :-(

Regards,

Q. Sir, my question about carcinogenicity of Potassium dichromate (Cr VI Source ) is during dying when dipping anodized parts in that dye, continuously heating to boiling point and evaporating Cr VI. Not the question about anodized part after sealing in potassium dichromate in which the dye is enclosed then in pores not open and also reduced to some extent I think to Cr III(non-carcinogenic), if I can explain my question as above for you Sir,

I am student in NUST university working on anodizing project nanoporous structure and also on its dyeing with some some safe chemicals in search; it's a simple question if have some idea about dyeing in safe chemicals please.

Regards

A. Hi again. As previously noted, yes, parts can be dyed with organic dyes, and usually are. I've personally never even heard of this process of dipping anodized parts in boiling Cr VI to dye them. Why are you specifying dipping them in hexavalent chrome instead of green organic dye?

Regards,

A. Hi Hammad,

Sealing anodizing in Cr(VI) solutions is normally carried out to improve corrosion resistance. The resultant yellow/green color is a by-product of the process, dichromate seals are not actually there to impart color.
If you only want a color then use organic dyes, many commercially available dyes can be used, I have even used clothes dyes before successfully (not ideal, but right conditions and it will work).

Q. Sir,
Again the question stays whether the Sealing solutions Potassium or sodium Dichromates are CrVI sources that are carcinogenic or not?

A. Hi again Hammad. Yes, potassium or sodium dichromates are CrVI sources that are carcinogenic -- just as you yourself said in your first posting. A possible source of confusion on this though is that, for practical purposes, government regulations rather than God's knowledge determine what we call "carcinogenic". For example, a couple of decades ago they were called 'possible carcinogens' or 'probable carcinogens' ... the actual facts regarding carcinogenicity didn't change in that period -- knowledge and laws changed.

Regards,

A. Hi Bob,
The way the vast majority of anodizing dyes work, by deposition in the pores of the anodized surface and then permanently set in place during the sealing process- you just aren't going to have success trying to dye OVER a seal. Nor would I try to tint the dichromate seal bath, as it is unallowable under the spec, and I would not trust the seal- adding an azo dye to what will ultimately be the surface layer kind of defeats the purpose of using such a robust seal method to begin with. I am going to assume that you are using dichromate seal because there is a customer callout for it, and you would be failing to provide the coating as called out by messing with the seal bath composition.
A black dye under nickel seal is pretty standard and doesn't shift the color, if the customer is okay with nickel seal instead.

As an aside, there is a lot of mention in this thread of 'powdery' surface finish on dichromate sealed parts, and running at pH 6.6, which is higher than the generally agreed target, but acceptable within the spec, has worked very well for us, both in terms of visual results and salt spray.

A. Hi Debashish. I don't think plastic balls will actually change the boiling point. The boiling point of water should be 94 °C up to an elevation of about 5600 feet (1700 meters), which I believe is higher than Bangalore -- have you checked it with several thermometers? But it does seem that the boiling point of water in Bangalore would indeed be lower that 98 °C.

The chromic acid should raise the boiling point, but probably not by much (I can't easily find a chart for dichromate). Can you give us a specification number so we can get a better understanding of this requirement?

Regards,