Chrome Bath Help: Chrome to Acid Ratio

A. Hi Marco.

The rule of thumb for conventional Sergeant's chrome plating baths is 100:1 ratio of chromic acid to sulfate (the molecular weight of sulfate is so close to sulfuric acid that it doesn't really matter much).

I don't quite understand the numbers you are giving us, but if you are saying the chrome or chromic acid is 27% (270 g/l of chromic acid), that would be reasonably conventional ... and it would call for a target of 2.7 g/l of sulfate (or sulfuric acid. We have several threads about this, so search the site for "chromic sulfuric ratio".

But a general approach in stuff like this is to test stuff in the lab and scale up ... why did you "redo" a 5300 liter tank instead of just making adjustments? Thanks.

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Marco,

Chrome ratio:
There are two types of ratio
1) hard chrome ratio -- chromic acid to sulfuric acid ratio 100:1
2) decorative chrome ratio == chrome ratio should be 125 to 140 with using catalyst.

A. Hey there, well, with 28°Bé you have approx. 345 g/l CrO3 containing in your bath. If you are using a conventional chrome plating bath, than the ratio between sulfuric acid and chromic acid should be max. 1% (3.4 g/l Sulfuric acid). Remember, sulfuric acid is your primary catalyst for your chrome process. On the market some other additives available which increases hardness, appearance, throwing power and efficiency. You have described a grey deposit, so you should check at least your sulfuric acid content via centrifugal method and adjust if necessary. The density of the electrolyte can also be influenced by metallic contaminations. Regards, Dominik Michalek - Mexico City, Mexico Centrifuges on eBay or Amazon (affil links) A. Baumé is a poor estimate of the condition of the chrome bath. For good and consistent results, you need to do a titration for the chrome and analyze the sulphate with a precipitation and centrifuge method which is quick easy and reliable. Kocour makes a hand powered centrifuge that is OK, if you maintain a constant and correct RPM. Baumé measures everything and it all contributes to a higher reading. Chrome, acid, trash and etc. James Watts - Navarre, Florida A. Brian, First I should have thought you would have read the book before starting. Second, I hope you are an industrial user who is knowledgeable in the handling of these hazardous materials and in the disposal thereof. If not STOP NOW. You may be getting yourself ready for a lot of time spent in litigation over waste, storage, use and safety regulations. The correct ratio of chrome to sulphate (not acid, but the sulfuric acid contributes the majority of the sulphate) depends on whether you are running a non proprietary or a proprietary bath. The proprietary bath with have technical data which will tell you what ratio is suited for that chemistry. The Baumé (Be) is giving you an idea of the total dissolved solids in the bath. By using a chart, an estimate of the amount of chromium or chromic acid can be arrived at. One then determines the sulphate by analysis and calculates the ratio and makes additions accordingly. Gene Packman - Great Neck, New York

RECOMMENDED CHROME PLATING BOOKS
for Shops, Specifiers, & Engineers

"Chromium Plating"
by Weiner & Walmsley (1980)
avail from eBay, AbeBooks, or Amazon

"Hard Chromium Plating"
by Robert K. Guffie (1986)
avail from AbeBooks, or Amazon

"Electrodeposition of Chromium from Chromic Acid Solutions" by George Dubpernell (1977)
avail from eBay, AbeBooks, or Amazon

"Hard chromium plating: A Handbook of Modern Practice"
by John David Greenwood (1971)
avail from eBay

"Chrome Plating Simplified" by Clarence H. Peger (1977, looseleaf)
very rarely avail from Amazon
but copies are available in a few libraries)

"A Chromium Plating Bath With The Fluoride Ion"
by Alfred Perlenfein (2013)
avail from eBay, AbeBooks

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? Brian, hello.
Sorry, I got more questions than answers. Apart from chromic acid and sulfuric acid content,
1. What is the operating temperature?
2. What is the current density applied ?
3. What is the plating time?
4. Hard Chrome/Decorative chrome?
5. Substrate ? Chrome applied over Nickel?

A. SK,
They are all interdependent for a quality coating. Personally, I would buy two of the recommended books from amazon or whoever has used books in your country. Read them cover to cover before you spend a single rupee on anything else. There is a massive amount of information and sorting it all out is not simple as every plater has a tweak or two on one of the two or three main process methods.
Chrome plating is as much an art as a science, so consider working for a chrome plater for a year before jumping into the business.

Ed. note: we apologize for causing some confusion; we don't think that SK was asking how to chrome plate, but was asking Brian for his parameters so that he could help with the gray plating problem.

A. Sometimes you'll need more Sulfuric Acid as catalyst. When I started with metal finishing I had the same issue "gray chrome". I tested many ratios and today I rather to use 5:100 instead the most common 1:100.

Hi, Sandry. That is very interesting; I have never heard of sulphate ratios even half that high!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi

It's not dimensionless, you go by mass -- it's grams to grams or ounces to ounces, etc.

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey