Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Larry Zitko, ChromeTech, Inc.
May, 2003

Specific Gravity Monitoring

Q. I am not very knowlegdable about the plating industry and I have some simple questions--Is there a type of platng process where it is important to do a continuous monitoring of the liquid density (specific gravity) during the process? What types of processes would benefit from this monitoring?

A. You have brought up an interesting topic, and one that I have dealt with for nearly 20 years. As one of our services, ChromeTech routinely analyzes customer chrome plating baths. Plating facilities send us samples of their baths, typically on a monthly basis, in order to:

• quantify active major chemical components (chromic acid, sulfate, fluoride, etc.)
• quantify some specific and detrimental impurities (trivalent chromium, chloride, etc.)
• provide a general assessment of overall bath impurities (typically comprised of many species of dissolved metallic cations like copper, aluminum, iron, etc.)

Although it is commonplace to use atomic adsorption spectroscopy (AA) or other means to quantify metals individually, I have not found this data to be any more valuable than to know the "overall" amount of dissolved impurities in the hard chromium plating baths. My reasoning is that most of these dissolved metals have the same adverse effects on the performance of the plating bath and the deposit properties.

In other words, one could try and decide which metals are present, quantify them individually, then add them up. Or, one could use some simpler and less expensive means to estimate the amount of overall bath impurity. Your question may have relevance to the latter methodology.

In our lab, we measure the density of each plating bath sample using accurate Baume hydrometers. We also quantify, by titration, the amount of chromic acid present. We also know the density of a brand new bath, and can attribute virtually all of its density to the amount of chromium trioxide that has been dissolved in water. In other words, there exists a strong relationship between the density of the bath, as expressed in degrees of Baume, and the chromic acid concentration. This relationship is often shown in tabular form in Baume charts from various sources.

However, as a bath ages, the density of the bath increases from the presence of dissolved impurities other than the chromic acid content. It is a relatively straightforward process to mathematically "factor out" the chromic acid concentration, and simply attribute the additional bath density to unknown dissolved impurities in the bath. I have successfully used this technique for decades to derive a "Contamination Index" for each chrome plating bath analyzed in our lab. Furthermore, this value for this index can help to explain process and deposit issues, and also serve as an important factor when considering the life expectancy for the bath. Unless chromic acid purification technology is utilized, the Contamination Index for a bath typically rises continuously over the years until the bath is so contaminated that it is replaced with a new bath.

Now, we can extrapolate on this thinking to introduce a concept that applies directly to your question. It would be possible to use two companion technologies to automatically monitor the overall contamination of a hard chrome plating bath in real time, or at least at scheduled intervals:

• One analytical system to measure chromic acid concentration.
• A second analytical system to measure bath density

Data from these systems would then be fed to a software tool to calculate, display and log the Contamination Index, and even annunciate when the index value reaches unacceptable levels.