Southern Metal Finishing

Re: Troubleshooting Hexavalent Chromium Baths

[color="Blue"]This article was published in the Nov/Dec 05 issue of Southern Metal Finishing. If you would like register to receive our free newsletter and review our online archives please visit[/color]

By: Shane Moore of Scovill Fasteners, Clarksville, GA

Many defects and difficulties often arise when plating hexavalent chromium.  Many of these defects can often be prevented with routine maintenance of the plating solution.

First I will begin with the basics.  The concentrations of the chromic acid, sulfate, trivalent chrome and secondary catalyst (if applicable) must be analyzed frequently.  After the analysis is complete check to ensure that they (including the chromic acid/sulfate ratio) are within the recommended operating parameters of the technical data sheets.  If they are not in compliance, make the necessary adjustments.

Next, a chromium solution will often build chlorides, usually from poor rinse water and drag in from a nickel tank rinse (containing Nickel Chloride) a hydrochloric acid activate rinse, or a woods nickel strike rinse.  To remove chloride contamination from a chromium bath, dummy plate at 100-ASF with an anode/ cathode ratio of 1:1.  Make sure your ventilation is adequate as the chlorides will be driven out of the plating solution as the chloride gas.  Analyze the plating solution for the chloride concentration after a few hours of this electrolysis.

Another contaminant to a chromium bath is an excessive amount of trivalent chromium.  The recommended procedure for reducing the trivalent concentration  is by dummy plating with a large anode to a small cathode ratio (approximately 10:1).  This re-oxidizes the trivalent chromium back into hexavalent chromium as a result of the oxygen being generated at the anode surface.

Both of the above conditions (trivalent chromium increase and chloride contamination) can be dramatically reduced with the frequent use of a porous pot.  Ask you local equipment distributor (CPC) for more information on this equipment.

Now a word about anodes.  Typically the anodes should be made of lead, alloyed with either 6% tin, or 6% antimony.   Some anodes contain an alloy of all three elements, but the former is the most common.  The anode surface should be checked periodically for yellow scale build up.  This yellow chromate film normally forms during inactive periods. Eventually, this build up will severely decrease the plating speed of the bath and will need to be cleaned.

A few other preventative maintenance procedures that I will mention include checking to ensure you are operating at the correct temperature and current density, observing the meters on the rectifier to make sure you have no interruptions of current, periodic cleaning of all contacts such as anodes and cathodes as well as the hooks.  It is always a good idea to dummy plate for 5 minutes at mid - high current before the beginning of each shift.

Article Contributed By: Shane Moore (Scovill Fasteners)