The process of anodizing, or controlled oxidation, of aluminum and aluminum alloys is more than seven decades old. The primary intent of anodizing aluminum and aluminum-alloy parts is to protect the highly reactive surface against corrosion in aqueous environments, such as humid air and sea water. Because the anodic coating can be produced in a variety of colors, painted anodized parts are used in architectural applications. Furthermore, because the anodization process produces a hard ceramic coating, many times harder than that of the substrate from which it is formed, anodic coatings are also used to protect aluminum parts from abrasion, especially sand abrasion.
Traditional anodizing is an electrochemical oxidation process. The part to be anodized is connected to the positive terminal of a DC power source and a nonreactive metal, such as stainless steel, is connected to the negative terminal. The aluminum part, which is the anode, and the stainless steel cathode are immersed in an electrolytic bath and a DC voltage is applied across them. The potential difference is of the order of 20-100 V, and the current densities are 1-10 A/dm2. The electrolytic baths comprise aqueous solutions of chromic acid, orthophosphoric acid, sulfuric acid, oxalic acid, or combinations thereof. Because the electrolytic baths have appreciable resistivity, and because the anodization process itself is exothermic, the temperature of the electrolytic bath increases greatly during anodizing. Since the anodizing process is quite sensitive to temperature, the bath temperature is controlled rather closely by a heat exchanger or refrigeration equipment.
Today's advanced anodizing technologies include several proprietary hard-anodizing processes that employ a wide range of electrolyte compositions and operating conditions, and a limited number of aluminum alloy compositions. The type and thickness of coating obtained greatly depend on these three factors. The military specification MIL-A-8625F, for example, lists at least six types and two classes of electrolytically formed anodic coatings on aluminum and aluminum alloys for nonarchitectural applications.
Despite many decades of experience and the expensive equipment employed by the traditional anodizing plants, the acid-bath-based DC anodizing process has severe limitations:
- By the very nature of the low-voltage DC power employed, the anodic coating is quite porous: often the volume percent of pores is as much as 50 percent.
- The electrolytic baths are made up of extremely low-pH acidic electrolytes, and thus the process does not meet many of today's environmental regulations.
- The expensive equipment, such as the electric power supplies and heat exchanger, makes the process capital-intensive.
- The traditional process, for reasons not fully understood, cannot be used for anodizing aluminum alloys containing high concentrations of Cu and Si. Thus many aerospace and automotive parts cannot be satisfactorily anodized, if at all.
- The present process, while appropriate for a limited range of the wrought-aluminum alloys, cannot be used for anodizing other reactive metals, such as Ti, Zr, Mg, etc., and intermetallic compounds and metal-matrix composites. Thus, most of the promising aluminum-based advanced alloys and composites cannot be protected by the traditional anodizing process.
- Above all, the hardness of even the so-called hard anodic coatings is far below the hardness of alpha-alumina, the principal component of the anodic coating. Accordingly the full strength potential of the anodic layer cannot be realized by the traditional process. Indeed, the other potentially beneficial properties of aluminum oxide, such as the high thermal and electrical resistivities and the high dielectric breakdown strength, are not even addressed.
This state of affairs is primarily due to the porosity of the coating produced by the traditional acid-based electrolytic processes at low power levels, and to a certain extent the poor bonding between the aluminum-alloy substrate and the anodic layer.