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Fake Sodium Dichromate?

2004

Hi all,

Has anybody heard about the EU trying to do away with Sodium Dichromate in their vendetta against chrome? We just received some aluminum slugs in for rework that were originally anodized in France and labeled as a 205 (Sodium Dichromate) - I'm not entirely sure yet what finish is actually on these parts, but it definitely isn't the Type II Anodize with Sodium Dichromate Seal that 205 means to us.

Here is what we know:

1. The parts look like a Type II anodize dyed yellow - almost identically like it in fact.
2. According to the customer that rejected them and sent them to us, leaving the parts in tap water overnight strips the color from the part.
3. The parts completely fail the Nitric Acid seal test - a couple of seconds in nitric and they looked bare.
4. The parts are non-conductive, even after the nitric acid removed the color.

We suspect that the parts were anodized clear and then coated with some type of dye to make them appear to be yellowish (too yellow to match with sodium dichromate parts though) - but apparently not dyed in the same manner as a MIL-A-8625 / MIL-PRF-8625 [on DLA] Type II Class 2 because they completely failed the seal test. It is possible that they were anodized, dyed and just not sealed - I'm trying to test that now soaking a couple of pieces in hot water to do a basic hot water seal on them, then I'll try the nitric again.

Has anybody seen anything similar? Is sodium dichromate going down along with Chromate Conversion Coatings?

Thanks,

Jim Gorsich
Compton, California, USA

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Dichromate (Cr2O7--) and chromate (CrO4--) are really the same thing, only differing in proportion as the pH changes, and both are hexavalent chromium. Although I haven't read the EU directive with respect to dichromate sealing, I think that the short answer is it's history regardless of exactly what words the EU directive contains today. Nickel acetate will be replacing most of it by my guess.

I haven't seen that kind of imitation part or defective part myself, but it's interesting, so thanks for keeping us posted.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

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So, just to keep everyone updated: I tried the hot water seal and that gave some positive results, so I tried a 30 minute seal in nickel acetate and got a significant amount of the dye to survive the nitric acid (I wasn't expecting much better even if the parts were a Type II anodize dyed yellow with no seal - it has been six months since manufacture and I'd be amazed if sealing the parts is as effective with that kind of time delay).

So, my suspicion is still that the parts were type 2 anodized and dyed and the manufacturer is trying to pass them off as sodium dichromate. Presumably, like Ted pointed out, to get rid of the chromate. Anybody heard of anything similar?

Jim Gorsich
Compton, California, USA
2004

To find out if it is anodized or conversion type coat please use eddy current for thicknesses, the figure less than 5 microns lead us to look for chemical conversion, so it will be easy to look among OXIDE COATING ,BOHEMITE, MBV,PHOSPHATE. AND MANY OTHER AFTERWARD.

Hadi Khosravi
plating and anodizing - Tehran, Iran
2004

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Eddy Current does sound like a good way to determine what the finish is exactly, but unfortunately I don't own a meter. At this point we're just going to strip it and re-anodize it, but I was wondering if anyone else had seen this happening and if it was a new trend or just a mistake.

Jim Gorsich
Compton, California, USA
2004

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2004

Jim,

If you can not find any bare spot then forget anodic coating (lack of rack contact). Then precisely you have to look for chemical conversion of alkaline chromate. The coating is performed by ALORK processes. which is similar to MBV. Please study Wernick bible chapter 5. These processes are old but are immersed on aluminum heat exchanger of finned or complicated type. Coating are sealed in dichromate and dried without rinsing. a similar process is called EW.

Hadi Khosravi
plating and anodizing - Tehran, Iran

I had a chemical supplier try to tell me to just seal my anodize in the chromate since its all the same. Needless to say I never dealt with that company. First I don't know if these new non chromes will stick to an anodized finish, but it may be what they tried to do. I thought the yellower finish was done with Potassium Bichromate instead of Sodium though, but if it came off in tap water, it probably was just not hot enough when sealed. All dichromate seals need to be 190-boiling don't they?

Wally Schwetz
- Melrose Park IL USA
2007