We are hard anodizing alloy 3003 aluminum cookware and have a color inconsistency. We are trying to achieve a black color, if possible, and sometimes do. We are anodizing in a 150 g/liter sulfuric acid bath at 25 amps per sq ft at 58 volts with a small ramp time. Is there a way to push this to the darker side? R.G.
Going darker may not mitigate the color variance that you see. Color differences in this process could be caused by any number of conditions. The most prominent of these are metallurgical inconsistencies and coating thickness variance.
Metallurgical conditions are sometimes difficult to pin down, but if you are willing to experiment, try etching a load, or a few pieces, in caustic soda and see if the "smut" produced by the etching process is consistent in color. If not, chances are it’s a metal problem of some sort. This could be mill batch to batch differences in chemical composition, temper or even in ingot casting technique. Take this up with your metal supplier. It is also possible that your forming process is changing the temper of the metal by work hardening. This could vary from piece to piece and could affect the color achieved in anodizing.
Anodizing variables can be dealt with first hand in your shop. Make sure the rack contact with each pan is consistent, tight and large enough to carry the current. Check part spacing on the rack and part positioning on the load. Parts too high or too low on the rack or those on the ends of the load may vary in color from the rest. Some parts may be too close together. Check the coating thickness of each part on each of several loads to see if it is uniform. (I assume you process only like part numbers on the same load). Make sure all of the bus bar connections between the tank and the power source are clean and tight. Check air agitation to see that it is uniform throughout the tank. Is the bath cooling system capable of holding temperature within ± 2 F and is the bath temperature uniform from one end of the tank to the other and from top to bottom? Check electrical joint connections on the load bars, too.
In processing, there are certain conditions that will foster darker color. Lower bath temperature can produce darker color for the same current density. At a given current density and lower temperature, voltage will increase. Voltage produces the color. Higher voltage results in darker color. Lowering the sulfuric acid concentration will also result in higher voltages for the same current density. Decreasing both bath temperature and concentration could make it somewhat easier to get a darker color. (Watch that the combination doesn’t result in burned parts. Air agitation becomes increasingly important under these conditions to carry away the localized heating caused by increased voltage). With some diligence in instituting these procedures you will be able to tell whether the underlying cause of color inconsistency is due to metal or the process.