Metal finishing Q&As since 1989

Mil-A-8625 Sodium Dichromate seal: make up and troubleshooting

Q. I want to install a small 185 gallon Sodium Dichromate seal tank for MIL-A-8625 / MIL-PRF-8625 [on DLA] MIL-A-8625 TY IB & TY II anodized parts. I have found two makeup plans, but I am uncertain as to how to calculate the amount of sodium dichromate to add to DI water. Can anyone show me the calculations?

Plan 1- Shows 56 to 64 g/l of Sodium Dichromate.

Plan 2- Shows 4.0 - 6.0 percent solution by weight of Sodium Dichromate.

Q. I am a lab tech/ intern at a metal finishing company, and we came across a problem with the sodium dichromate seal tank. The pH of the tank is 6.0, which puts it within the range of 5.6-6.0. The sodium dichromate concentration is 6.4 oz/gal which equals to around 47 g/l which is also within range. However, the parts go in charcoal black, and are coming out green (unacceptable). Possible problems?

A. Need more information. What kind of anodizing? Alloy? Why dark gray going in? Is it dye? You always shift the color of the dye with dichromate sealing. A greenish tint is normal on some alloys with regular non-dyed Type II anodizing on common alloys. What thickness? What temperature of anodizing?

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

May 25, 2012

A. I agree with Mr. Probert, we need some more info. I can only assume that this is type II anodize, in which case I imagine they will be a yellowish green color. As Mr. Probert asked, why are the parts so dark?

Scott Merritt
- Eastman, Georgia, USA
July 11, 2012

A. Unless it is 8625 Type III Clear …
Build up on Type III 8625 will begin with a Grey to a dark Grey in a bath of 20-25 °F; the dichromate will look like an olive color

Thanks,

Wendy McGaha
Aerostar Industries, LLC - Huntsville, Alabama USA
July 26, 2018

Sodium dichromate seal tank - pH adjustment

Q. Hi all,

We sulfuric anodise to MIL-A-8625 / MIL-PRF-8625 [on DLA], and are having minor powder issues. From trials it does not appear to be an anodising tank issue, and parts have been stopped at this stage and inspected with no trace of powder (white). It appears after sealing in sodium dichromate with (yellow/green) powder. I have reduced the sodium dichromate content to mid limit (50 g/l), but still powder issues continue ...

I wish to make a minor adjustment to my sodium dichromate seal tank from 5.8-5.9 pH to 5.6-5.7 pH range. What acid is best to use without further side effects? (I have seen acetic acid [on eBay or Amazon] mentioned).

Thanks Ian

A. Chromic Acid

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

November 13, 2018

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A. Along with adjusting your pH, look for the possibility of contamination in your Dichromate Seal. One possible source is stray Nickel from a nearby bath, entering as rinse overspray, dragover, or even condensed mist dripping out of a shared ventilation system (speaking from experience here... luckily it was an easy fix). If you take a sample from your Dichromate Seal bath, pH adjust it UP to about 8-8.5 with Sodium hydroxide, and see particulate settling- you've got a potential problem, and I would have it sent out to be tested for Aluminum and Nickel (if you use a Nickel seal too) levels. Nailing down the actual contaminant will also help you nail down what you can do to prevent recurrence.
Bath age is important as is making up fresh baths with deionized water. I run my seal bath at pH 6.5-6.7 with no powdery residue until the bath gets pretty old and has to be dumped for contamination, and still don't see salt spray issues even at that point. One clue I use that the bath is getting dirty is that the pH starts to drift slightly downward.
You may in fact solve your problem by adjusting your pH as planned, but this is another thing to monitor over time.

Rachel Mackintosh
- Greenfield, Vermont
November 20, 2018