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Test procedure for sodium dichromate solution for galvanizing?

January 7, 2011

I want to know the actual lab test procedure for Dichromate Bath.

What can be checked

1) The Percentage of Chromium in respect of total volume ?

or

2) Percentage of dichromate in respect of total volume ?

and What would be first time preparation ration for water and Dichromate powder/Chromic Acid ?
Very confusing please advice.

Anup Dash
Galvanizer - Haridwar, U.K. , India

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January 13, 2011

Sir:

Sodium Dichromate it the preferred chemical rather than chromic acid (Cr03). In a new batch add acetic acid [on eBay or Amazon] (glacial) and enough HCl to bring the pH down to about 4.5. Readjust pH periodically with con. HCl to maintain pH between about 4 to 4.5. Avoid below pH 3 or above pH 5. pH 3 or lower gives product brown color. pH 5 or above does not work. Zinc +2 ion can be titrated with disodium EDTA with suitable indicator in basic pH buffer. Total dissolved solids can be measured by a conductivity meter. Properly made up a chromate solution of sufficient size for the product can last several years and pass a 40 hour salt-spray test.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA