Current Density Gauge for Anodizing

The Current Density Gauge allows the plater to know precisely what the current density is in the tank for optimum anodizing quality.

After mounting the Current Density Gauge on a bus bar, the operator presses the button for an instant digital display of the current density. Monitoring the displayed current density (in ASF) allows the operator to set the correct constant amperage on the rectifier for the duration of the anodize cycle.

WHY ANODIZE BY CURRENT DENSITY?

When the surface area of part isn’t known, many anodizers make a guess at the correct voltage and maintain a constant voltage on the tank. Even if the area is known, its just faster and easier to just read the current density.  With constant-current anodizing, the voltage varies over time to ensure that the part receives an unchanging amount of electrical current as the anodized film grows. Constant current provides a constant rate of film growth, which means the 720 rule can used to calculate the rate of that film growth. (in thickness per minute) Therefore, a selected current density will determine the time required to achieve a specific film thickness. If you have a way to determine the thickness accurately based on time then you don’t need to interrupt the process to take measurements.

Products Finishing article on constant voltage vs. constant current by Larry Chesterfield. Another article discussing constant voltage vs constant current control in Finishing and Coating.

THE 720 RULE

The 720 rule can be used for aluminum parts to accurately determine the time needed to build thickness.

Minutes to anodize = [mils (of coating desired) x 720] / amps per square foot.

So for example, if you want to build 0.001″ thickness and your current density is 15 ASF, then it will require 48 minutes. (720 x 1mil/15ASF=48mins)  Online calculator here.

CONSTANT VOLTAGE PROBLEMS

The problem with constant voltage anodizing is that there are many variables that change the voltage: bath temperature, acid concentration, aluminum concentration, the aluminum alloy, and the voltage will change during the cycle because of the resistance of the oxide layer!  So its hard to know what the correct voltage should be. If the anodizer get it wrong then problems with “mossy oxides” or inability to seal will result. In addition, as the anodic film builds, the resistance increases and the current density will decay as the anodizing process progresses. This produces an anodic oxide that is less abrasion resistant, and increases the time required to form the necessary oxide thickness.