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topic 1541

Help on chlorides in Chromic Anodize.


I am looking for a lab procedure to test for chlorides in a Type I chromic anodize bath. I have tried a tradition conversion with silver nitrate. The content I received was approx. 8g/L. After further review, I can hypothesize that some chromium nitrate was formed. I am currently testing that hypothesis. I am looking at further digestion with HNO3 to dissolve the chromic nitrate and leave the silver chloride behind. Any further help or experience would be appreciated.

Ira Donovan, M.S.F.
Kansas City, Missouri


The following should work:

1. Take a 50 ml sample, and dilute with DI to 250 mls.

2. Add 20 mls 42 degree baumé Nitric Acid and boil.

3. Add 5 mls Silver Nitrate and allow ppt. to fall out.

4. Wash filtered ppt.with 2% Nitric Acid, and dry.

5. Weigh as Silver Chloride, calculate as follows:

grams of Silver Chloride X 8.2 = g/l of Sodium Chloride.

It is important to note, that as in all gravimetric procedures quantitative techniques are critical to results found..

This is a variation of the basic procedure found in The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick, R. Pinner, P.G. Sheasby.

Ward Barcafer, CEF
aerospace - Wichita, Kansas

To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)

March 12, 2011

how to reduce chloride content in chromic acid anodizing bath?

(we have to maintain the chloride < 0.20 g/l but we observed 2 g/l )

Priya G
plating shop employee - Chennai, India

March 16, 2011


Two things you need to do, firstly find the source of chloride contamination (2 g/l is massively high and will be detrimental to the anodising process) and secondly, rather than trying to play around with processes to reduce the chloride content just discard the bath and make up a fresh solution (ensure you use distilled or demineralised water to make up the solution).

Brian Terry
Aerospace - Yeovil, Somerset, UK

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