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

by Larry Zitko, ChromeTech, Inc.
June, 2003

Aeration in Chrome Tanks

Q. How much aeration is required for the chrome tanks. We seem to be over aerating and producing more mist than our ventilation system can handle. We are using balls to reduce the surface area of the tanks, however the aeration keeps the balls pushed out of the way exposing as much as 1/3 of the tank surface.

A. All hard chrome plating tanks benefit by some kind of solution agitation. Otherwise, two undesirable events may occur.

1. Temperature can stratify in the tank, with the top region warmer than the bottom region. I have measured as much as 7-8 degrees F. difference in some tanks. This can cause hardness/brightness problems with the chromium deposits on ling parts that are vertically oriented in a deep vertical plating tank.
2. Plating chemicals can form a concentration gradient, rather than having a uniform concentration throughout the bath.

I have successfully used air agitation in most of my plating system designs through the years. But it's important to deliver the right kind of air to the hard chromium plating tank. Here are some suggestions:

• Never use air from the big, oil-lubricated shop air compressor. Delivery pressure is much too high (100+ psi), and the pressure regulators that are commonly used to reduce the pressure to your plating tank can fail in a manner that may blow the chrome solution out of the tank. This presents both a health risk to building occupants and also a significant environmental incident. Moreover, the entrained oil will contaminate the plating bath, leading to higher-than-normal trivalent chromium concentrations, adhesion problems and so forth.
• Instead, consider using mild, oil-free air delivered from a dedicated rotary-vane, oil-less compressor. Size the unit so that the working pressure is slightly higher than the weight of the column of fluid that must be displaced by the air. Deeper tanks need higher pressure than shallower tanks, assuming that the air sparger (see next bullet) is located at the bottom of the tank. As an example of the ballpark pressure range, I have installed many rotary-vane compressors that are rated for only 10 psi.
• Don't introduce the air agitation into the tank at the end of an open pipe. Instead, fabricate a bottom air "sparger", or distribution manifold. 1-1/4" or 1-1/2" schedule 80 PVC or CPVC is a good choice. This horizontal pipe is drilled with a series of very small holes (1/32" - 1/8") that distribute the agitation air over the length of the tank, rather than only in one location. You can put some 1/2" lead wire in the interior of the sparger to keep it from floating.
• Rotary vane compressors can't be fitted with pressure regulators, because they run very hot and fail quickly when the discharge is deadheaded or restricted so much that discharge pressure exceeds the design working pressure. Instead, you can install an adjustable pressure relief valve on the discharge piping, and then adjust the degree of agitation to a slow, rolling action which does not generate a lot of chrome mist at the liquid surface. This wastes some air but works well.
• Mechanical agitation may be used instead of air agitation. The over-the-side type vertical pumps are safest, because if a leak develops, the leaking chromic acid will still be confined to the plating tank. As with the air strategy previously discussed, the discharge port of the chemical pump should be directed to a bottom air sparger. Be sure and consult the pump manufacturer for chemical compatibility issues for the materials of construction for the pump. Again, you want only mild agitation in a chrome plating tank.
• I'm not fond of the floating balls. They seem to get in the way, get trapped in the parts and fixtures, reduce the desirable evaporation of water vapor from the top of the tank, and have a low efficiency at reducing chromium emissions.