Abstract: A process for hard anodizing of aluminum and aluminum alloys by use of a low voltage having a DC component and a superimposed AC component. The electrolyte can have relatively low acid concentration and can be of higher temperature than usually employed for hard anodizing.

Abstract: An improved anodizing process produces on aluminum and on alloys of aluminum a hard oxide film of greater thickness and better abrasion resistance then can be obtained in the conventional Sanford anodizing process which uses an organic substance sold under the trademark "Sanfran" as an additive to the electrolyte. In the improved process, the DC voltage employed in the conventional process is modulated by a sinusoidal AC voltage of standard industrial frequency and the modulation factor is not permitted to exceed 100% to prevent reversal of the current through the oxide film. The DC voltage is AC modulated during the entire term of the process. In an initial period of 1 to 2 minutes, the DC voltage is raised to about 10 volts and then is raised at a rate of about one half volt per minute to a level within the range of 14 to 19 volts. Upon reaching that level, the DC voltage is held constant for a dwell period of at least five minutes.

Abstract: A method and system for anodizing and particularly for hard anodizing of aluminum and aluminum alloy articles in which a wide range of operating voltages can be employed without danger to personnel and without burning of the articles. For continuous (dynamic) processing, a plurality of electrolyte-containing tanks are employed with an associated cooling system for maintaining the electrolyte at a low operating temperature. Each tank, cathode, electrolyte and cooling apparatus are electrically insulated from ground, while each cathode, which can be the tank itself, is connected to a predetermined negative potential of a power source which source comprises one or more separate power supplies. A conductive conveyor system is provided to carry articles through the several tanks and is connected to ground to which is also connected a positive terminal or terminals of the power source.