Anodizing of aluminum and aluminum alloys is accomplished by immersing articles to be anodized in an electrolyte, connecting the articles to one terminal of a power supply, this terminal being positive during an entire anodizing cycle or portion thereof, and connecting the cathode in the electrolyte to the other terminal of the power supply. The characteristics or properties of the oxide film produced on the surface of aluminum and aluminum alloy articles can change dramatically depending upon the composition of the electrolyte, its temperature, the waveform of the applied voltage, and the program under which the voltage is varied. In the present specification, the term "aluminum" is used to include the alloys of that metal unless the text indicates otherwise.
A very thin and nonporous oxide film is formed on an aluminum article when a water solution of a weak acid, which does not dissolve the oxide film, is used for anodizing. Weak acids include boric acid, citric acid, etc. Thickness of the oxide film in this case, is generally less than one micron and the dielectric properties of this thin coating improve with the increased purity of the aluminum being coated.
Porous and much thicker oxide films are obtained when aluminum articles are anodized in water solutions of strong acids, which do partially dissolve the oxide film simultaneously with its formation. Such strong acids include sulfuric acid, chromic acid, oxalic acid etc. In this case, the thickness of anodized coatings may be from several microns to hundreds of microns. The properties of these thick coatings are strongly dependent on the temperature of the electrolyte. At room temperature (about 70.degree. F. or 20.degree. C.) a rather soft oxide film is produced, with a thickness ordinarily in the range of about 8-10 microns. A low DC voltage of about 15-18 volts is used in this case for anodizing. Anodizing, under the conditions just described, is called "conventional anodizing" and is widely used when the appearance or corrosion resistant properties of the oxide surface are of primary importance rather than the mechanical properties of the oxide film.
About three decades ago it was first discovered that very hard oxide films with a sapphire hardness may be obtained if the temperature of the electrolyte is about 32.degree. F. or less. In the years following, this discovery has been implemented and successfully employed in what has come to be known as "hard anodizing processes." While different in details, all hard anodizing processes have certain common features.
A much higher voltage than that of conventional anodizing is employed in a hard anodizing process, because the input resistance of the immersed system has an increased resistance at low temperatures which requires more voltage to achieve a given current level in the system. Usually such high voltage cannot be initially applied to the articles being anodized. The initial voltage is usually no more than about 10-20 volts since at higher voltages a deteriorated oxide coating is produced or the aluminum article can start "burning", which is the catastrophic dissolving of the aluminum. The final voltage may reach nearly 100 volts at the ends of a hard anodizing cycle, the specific final voltage depending on the particular alloy, its temper, and the film thickness. Thus, in a hard anodizing process. The voltage is gradually raised from an initial value to a final value to produce the intended oxide coating without burning of the articles. It is very probable that anodizing with a gradually increased voltage does not enable the provision of an oxide film with a homogeneous structure. As indicated in the article by Keller, Hunter and Robinson in the Journal of the Electrochemical Society, Volume 100, 1953, pages 411-419, the oxide film formed in strong electrolytes has a cell structure, each cell being hexagonal with a pore in its center, the pore being perpendicular to the aluminum surface. The distance between pores of adjacent cells is proportional to the applied anodizing voltage. As a result, the oxide film formed by a non-constant voltage will have a non-uniform structure gradually changing as the voltage and the thickness of the oxide film increases. Therefore, the properties of this oxide film are believed to be inferior to those of films having a homogeneous structure.
In addition to the higher anodizing voltage, a hard anodizing process employs a rather high concentration of a strong acid in the electrolyte to provide an electrolyte having reasonable "universality". By the "universality" of the electrolyte, it is meant that any alloy irrespective of its composition or temper can be hard anodized with the same acid concentration. Some alloys, however, especially those with a high copper content, would not be hard anodized as readily as other alloys if both the acid concentration and the temperature of the electrolyte are lowered to a certain degree. A universal electrolyte preferably includes a concentration of sulfuric acid of about 300 grams per liter or more and at temperatures about 32.degree. F. Such high acid concentration can prevent the formation of oxide films with more than 50-60 microns thickness on some alloys.
An especially effective technique for providing a universal hard anodizing electrolyte is the addition to the electrolyte of an organic extract sold under the trademark "SANFRAN" and produced by the Sanford Process Corporation. This additive is an acidic aqueous extract obtained by boiling a mixture of brown coal, lignite, or peat in water, and the process for obtaining such extract is described in U.S. Pat. No. 2,743,221. The hard anodizing process using the Sandord acidic aqueous extract is now widely employed in the United States and in foreign countries and has become known as the Sanford Process. This process is further described in U.S. Pat. Nos. 2,897,125; 2,905,600; 2,977,294; and 3,020,219.
It would be of great practical benefit to provide a hard anodizing process in which the amount of electrical power required for the process is reduced in order to reduce the cost of consumed electrical energy. The cost factor is especially important by reason of the greatly increased cost of electrical energy during recent years and the expectation of still further increases of energy cost in the future. During a hard anodizing process, about half of the electrical energy is consumed by the electrochemical process of forming the oxide film itself, as governed by Faraday's Law, while the other half of the electrical energy is consumed by the refrigeration system used to control the temperature of the electrolyte. A reduction in the amount of electrical energy consumed by the hard anodizing process can be achieved if the anodizing voltage is reduced, without sacrificing either the speed of anodizing or the quality of the anodic oxide film. Further reduction of consumed electrical energy can be provided if the temperature of the electrolyte is increased without diminishment of anodizing speed or resulting quality of the oxide film.
A disadvantage of an electrolyte of high acid concentration is the increased cost of waste water treatment which is required to meet modern antipollution standards. It is therefore extremely desirable to reduce the acid concentration required for providing an oxide film on aluminum alloys and to reduce the cost of waste water treatment without sacrificing the ability to anodize different alloys in the same electrolyte.