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Why a pH control in chromic anodize?

I have seen requirements for the control of pH in a chromic acid anodize bath. I have never used pH as a control parameter. Why is pH important to the anodize bath, and if the pH is high, how do you reduce it without exceeding your requirements for chromic acid concentration?

Ed Durham
Shop Employee - Columbus, Georgia, USA
January 7, 2009

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First of two simultaneous responses -- January 13, 2009

I checked several reference books and did not see a requirement to check pH. It is possible that an OEM requires it for some reason. I the mil spec and ASM do not require it (I do not have) then I highly doubt its need.
If pH is high and your chrome content allows it, you could add more chromic acid. You can NOT add sulfuric or nitric acid and I do not think that you can use hydrochloric acid either.
Colorimetric indicators can not be used and electronic methods are poor as the pH is low. Also, the probes do not like chrome solutions, so great care must be taken with their use.

James Watts
- Navarre, Florida

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Second of two simultaneous responses --

Some of the spec writers have seen the handbook chart showing the relation between %/wt, oz/gal, and pH. Humor them, it will not hurt, unless you are heavily contaminated with sulphate and/or chloride, you will find that you can hold their pH low and still have the oz/gal in range. One of my clients just got hit with that requirement and I have found that if I have a clean solution, about 4 or 5 %/wt, the pH is around 0.62 to about 0.82. But by dragging the type II and III solution dripping over the type I, the sulphate limit is 500 mg/L and that also skews the pH.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

January 13, 2009

Good luck trying to accurately measure pH below 1. It takes some special equipment and procedures.

Leo Herringon
consultant - Grand Rapids, MI, USA
February 3, 2009

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February 4, 2009

According to some references, including the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] (edited by Larry Durney) 4th edition Page 407; pH will act as a measure of free chromic acid or "effective chromic acid". The pH is suggested as 0.6. Other references give a range of 0.5-0.7. This should be measured with a meter, adjusted for temperature and the proper buffer (1.0). Depending on the type of meter, electrode selection will vary. This is sensible since methods to determine chrome content can pick up some trivalent chrome compounds and other by products such as aluminum dichromate and chromium dichromate

Gene Packman
process supplier - Great Neck, New York

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February 6, 2009

For optimum pH electrode use, you should use a calibration solution(buffer)with a pH of "0", since you need to bracket the desired pH to be tested. Any decent reference manual will show that most electrodes tail off quite badly at about pH of 0.1, so even that is not good. Since pH is logarithmic, a tiny bit off will make a huge difference in the amount of chrome.
As several have stated before, contaminates affect the true amount of chromic acid. Chrome is very hard on electrodes, so the choice of a proper electrode is going to be the high end of probe prices and it will take a lab grade meter to be able to effectively handle the low pH.

Controlling the solution by pH is a poor option compared to analytical testing.
If that is what it takes to keep the customer happy, then spend the bucks on the equipment. It always has other uses also, so it is not a total loss.

James Watts
- Navarre, Florida