Chrome plating chemistry lesson

A. Hello Cheah. We appreciate your interest.

Chromium is an element and 'chrome' is a slang word for it. Chrome plating is the electrolytic deposition of chromium from an aqueous solution containing (usually) Chromic Acid (H2Cr04) and a "catalyst", usually sulfuric acid.

The exact steps in the deposition process (i.e., all the intermediate species of ions and compounds) have been the subject of large and deep research projects that are beyond my understanding or that of most high school students. But if we look at just the beginning and the end, rather than trying to understand every intermediate state, the situation is that we start with chrome ions in solution in a Cr+6 state as chromic acid, and we end up with Cr+0 metal -- and the driving force towards this is a DC current source which supplies 6 electrons to the workpiece for each atom of chrome to be deposited on it. However, chrome plating is exceptionally inefficient (much less efficient than most other plating processes). It's usually only about 12-15 percent efficient, which means that only 12-15 percent of the electricity supplied is actually applied towards reducing hexavalent chromium ions to chromium metal. The other 85-88 percent is 'wasted' in separating water molecules into hydrogen and oxygen.

You may be interested in our FAQ, "Introduction to Chrome Plating". Further, while your text book may not have much information on chrome plating, your library might. I have 3 books just about chrome plating.
and most plating books will have a chapter on chrome plating. Best of luck.

A. Chrome plating generates a huge amount of heat, so probably half of the wasted electricity goes into heat rather than gas. Some chrome plating processes add a "catalyst" which very probably is not a true catalyst. I think that it has a higher efficiency by raising the hydrogen and the oxygen overvoltage potential which means that less gas is formed. Big trade secret as to how and why it works!

A. Hi James. Yes, it is possible to do chrome plating from a trivalent rather than hexavalent bath, and it is widely practiced for decorative nickel-chrome plating. But plating from a trivalent chromium bath is not easy either, introduces its own problems, and requires special proprietary chemistry, special anodes, and in some approaches, membranes to separate the anode ions from the cathode ions.

If you use the search engine to look for "trivalent chrome plating", you will find a bunch of threads on the subject.

Trivalent chromium and hexavalent chromium are also both used in processes for chromate conversion coating/passivation of aluminum and zinc plated components so, to reduce confusion as you review the threads, skip those that don't relate directly to chrome plating. Good luck.

Regards,