1.0 Introduction The practice 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 highly reactive surface against corrosion in aqueous environments, such as humid air and sea water. Because anodic coating can be produced in a variety of colors, painted anodized parts are used in architectural applications. Furthermore, because anodization process produces a hard ceramic coating, many times harder than that of substrate from which it is formed, anodic coatings are also used to protect aluminum parts from abrasion, especially sand abrasion.
2.0 Traditional Anodizing Traditional anodizing is an electrochemical oxidation process. The part to be anodized is connected to positive terminal of a Direct Current (DC) power source and a nonreactive metal, such as stainless steel, is connected to negative terminal. The aluminum part, or anode, and stainless steel cathode are immersed in an electrolytic bath and a DC voltage is applied across them. The potential difference is of order of 20 -100 V and 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 electrolytic baths have appreciable resistivity and because anodization process itself is exothermic temperature of electrolytic bath increases greatly during anodizing.
Since anodizing process is quite sensitive to temperature, bath temperature is controlled rather closely by heat exchanger or refrigeration equipment. Today's advanced anodizing technologies include several proprietary hard anodizing processes that employ a wide range of electrolyte compositions, operating conditions and a limited aluminum alloy compositions.
The type and thickness of coating obtained greatly depends on composition of electrolytic bath, operating conditions and alloy compositions. 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 non architectural applications.
Despite many decades of experience and expensive equipment employed by traditional anodizing plants, acid bath based DC anodizing process has severe limitations.
· By very nature of low voltage DC power employed, anodic coating is quite porous. Often volume percent of pores is as much as 50%. · Because of low current densities employed, it takes many hours to produce a coating of a few tens of micrometers thick. · The electrolytic baths comprise extremely low pH acidic electrolytes and thus process does not meet many of today's environmental regulations. The expensive equipment, such as electric power supplies and heat exchanger, makes process capital intensive. · The traditional process, for reasons not quite apparent, 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 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 promising aluminum based advanced alloys and composites cannot be protected by traditional anodizing process. · Above all, hardness of even so called hard anodic coatings is far below hardness of alpha alumina, principal component of anodic coating. Accordingly, full strength potential of anodic layer cannot be realized by traditional process. · Indeed, other potentially beneficial properties of aluminum oxide, such as high thermal and electrical resistivities and high dielectric breakdown strength are not even addressed.
This state of affairs is primarily due to porosity of coating produced by traditional acid based electrolytic processes at low power levels, and to certain extent poor bonding between aluminum alloy substrate and anodic layer.
3.0 The Microplasmic Process In recent years, Microplasmic Corporation, a start up R&D company of Peabody, MA, U.S.A. has developed a unique anodizing technology, called Microplasmic Process for all types of aluminum alloys. It is an electrochemical micro arc oxidation process for which a US patent is pending. A controlled high voltage AC power is applied to aluminum part submerged in an electrolytic bath of proprietary composition. Due to high voltage and high current, intense plasma is created by micro arcing at specimen surface and this plasma in turn oxidizes surface of aluminum specimen. Thus process is called Microplasmic Process. The oxide film is produced by subsurface oxidation and considerably thicker coatings can be produced.