46059

Formulation of Dilute Chromate Seal?  

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I am having trouble locating makeup information on a dilute chromate seal tank. I have the DI water and the Cr flake, but I know there is something else involved in the makeup. If anyone could supply the recipe for a 841 gal tank, I would much appreciate it. Thank you for your time and attention.

Derek McCasland
Chemical Analyst - Camden, Arkansas

1. A "seal" for a phosphate coating should be recommended by your phosphate coating supplier.

2. A dichromate seal for conventional Type II anodizing on aluminum would be 100 gm/Liter (13.333 av 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.

3. 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

4. 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.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como

Derek,
Is this a 5% dichromate seal for anodized aluminum, a dilute chromic acid sealing rinse for chromate, black oxide or phosphate, an anti-tarnish rinse for electroless nickel or brass, a post-galvanizing rinse or...? Recipes for 5 wt% dichromate anodize seal, per 100 gallons:

The 5% dichromate anodize seal requires 43.85 pounds of sodium dichromate dihydrate per 100 gallons of solution (balance DI water), adjusted to pH 5.5-6.0. For small pH adjustments, 1 Normal sodium hydroxide solution (preferably reagent grade, as used in labs) is convenient.
Note: An equal weight of potassium dichromate (anhydrous) can be used in lieu of sodium dichromate dihydrate. Also note, MIL-A-8625F gives the pH range as 5.0-6.0; I suggest 5.5-6.0 is better.

To prepare a 5% dichromate solution using chromic acid flake: Dissolve 29.43 pounds of chromic acid in DI water, raise the pH to 5.5 using high quality NaOH* (about 11 pounds) with lots of stirring (about a 50 °F temperature rise overall using solid NaOH, but hot spots on tank bottoms can burn plastic tank liners). Dilute to 100 gallons with DI water and adjust pH to ~5.75.

*Warning: Up to 1 wt% NaCl may be present in the cheapest, diaphragm grade 50% NaOH solutions or pellets thereof. Use membrane, rayon or purified diaphragm grade sodium hydroxide.

For chromic acid sealing rinses after phosphating or black oxide, see MIL-HDBK-205A PHOSPHATIZING AND BLACK OXIDE COATING OF FERROUS METALS.

Note: Using hexavalent chromium requires appropriate protective equipment (such as NIOSH-approved respirators) and EPA permits. If this is a first-time use for your company, hire an EHS specialist to help with EPA,OSHA and local government compliance. Preferably (as in legal), do this before bringing chemicals on-site.

Ken Vlach - Goleta, California

Seal on what and after what?

James Watts
- Navarre, Florida

For Mr Probert's answer

"4. 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."

Can I lower the pH of the solution using acetic acid like #'s 1, 2, & 3?

Thanks

Dimitri Stath
- Whitby Ontario Canada

No acetic. If the pH is high, then you started with high pH water; lower the pH with chromic acid., Bob Probert

Robert H Probert Robert H Probert Technical Services Garner, North Carolina