Chemical conversion coatings play an important role as a coating—alone or for its enhanced adhesion properties. Though they do not compare with anodizing, either as a finish or in the range of applications in the aerospace industry.
The oxide film formed by various anodizing processes is mechanically superior and produces a much higher corrosion- and abrasion-resistant layer than the chemical conversion coatings. The various processes all use an electrical current to form the oxide film. The current passes through an electrolyte in which aluminum is the anode, hence, the name “anodizing.” The nature of the electrolyte, the reaction produced and operation parameters determine the structure and properties of the formed oxide film.
This overview will provide a short explanation of the various anodizing processes used in the aerospace industry today.
THE VARIOUS ANODIZING PROCESSES
Many electrolytes have been tested, used and patented during the last century, leaving only a few as important industrial processes. According to the “bible” of anodizing, “The Surface Treatment and Finishing of Aluminum and its Alloys” by Wernick, Pinner and Sheasby, the three most important ones are chromic acid, sulfuric acid or oxalic acid.1 Acids as phosphoric acid and boric sulfuric acid mix are now used in the market for anodizing in the aerospace industry.
Chromic acid anodizing, or CAA, was the first commercial anodizing process patented in 1923 by Bengough and Stuart, followed closely by the first sulfuric acid anodizing (SAA) process patented in 1927.
The oxalic acid was introduced by the Japanese in the middle of the 1950’s. The main interest today is as an additional acid in hard coat anodizing, or HCA, to produce a harder coating faster than that obtained with a pure sulfuric acid electrolyte.
Phosphoric acid anodizing, or PAA, and boric sulfuric acid anodizing, BSAA, were both developed by the Boeing Company, the first one as a structural bonding surface and the other as a replacement for CAA for non-critical fatigue parts. The most commonly used anodizing process is the sulfuric acid anodizing process, but for the aerospace applications this picture looks a little different.
Chromic acid anodizing is mostly used for protection of critical structures with all kinds of joints. The corrosion resistance is excellent relative to the thickness of the coating, which normally lies in the range of 0.08 – 0.2 mil. The oxide film is softer and less porous than those formed by the other processes, and is formed without any significant fatigue loss of the material. The film is easily damaged, and the color is light opaque gray. When this film is sealed in a dichromate seal, a greenish color appears.
The process is voltage controlled with a ramping in the beginning of the process increasing up to 40 volts depending on the type specified. Two types are specified in the military specification MIL-A-8625F, type I and Type IB, whereas the first is conventional coatings produced by a voltage of around 40 volts and Type IB uses a voltage of 20 to 22 volts.
Sulfuric acid anodizing can be divided into two main uses, for Type II coatings and Type III coatings. Type II is primarily used for decorative or protective applications, whereas hard coat oxide films, Type III, are used for engineering applications, i.e., the aerospace industry.
MIL-A-8625F specifies the Type III coatings as those formed by treating aluminum and its alloys electrolytically to produce a uniform anodic coating. This gives a variety in the process operations procedures as long as a heavy, dense coating is produced.
The resultant hard film is very dependent on the aluminum alloy used.2 The first processes used higher current densities and lower temperatures of the electrolyte. These process parameters give some difficulties with higher copper alloys of the 2000 series—some of the favorite alloys for the aerospace industry. Therefore, a lot of work has been done to reduce these difficulties.3,4 Addition of oxalic acid to the sulfuric acid electrolyte has been one of the main modifications. Additionally, variation in electrolyte temperature and the use of different electrical sources and pulse methods have been developed.5,6,7
Phosphoric acid anodizing is basically used for structural adhesive bonding in high-humidity environments. This process is known as the Boeing Process and is carried out at 10-15 V. The formed oxide film has a greater durability under adverse conditions than film formed in chromic acid and sulfuric acid. One of the reasons for the great adhesive property is said to be due to the morphology of the oxide film, which should be a film of pores with whiskers or protrusions on the top surface of the formed film.
The last anodizing process mentioned is the new boric sulfuric acid. This is an alternative to the chromic acid electrolyte, which contains hexavalent chromium. Note: Hexavalent chromium is carcinogen and has to be phased out of metal finishing processes. Therefore, hexavalent chrome-free electrolytes are necessary. The formed oxide film from the boric sulfuric electrolyte has a paint adhesion that is equal, or superior, to the one formed on chromic acid. The process is voltage controlled and is ramped to 15 V. A seal in a hot dilute chromate solution is required to achieve satisfactory corrosion resistance.
The above processes are the basis of the anodizing we do in the aerospace industry today. It should be remembered that operating conditions might vary within a wide range, and that most of the specifications are general guidelines. Therefore, the most important part to remember is to define the performance criteria before choosing the right anodizing process.
- Wernick, S., Pinner, R. and Sheasby, P.G., “The Surface Treatment and Finishing of Aluminum and its Alloys”, 5. Ed., Finishing Publications LTD., Teddington, Middlesex, England, 1987.
- Juhl, A. Deacon, “Hard Anodizing of Aerospace Aluminum Alloys”, Light Metal Age, June 2009.
- Lerner, L., Sanford Process Corporation, “Hard Anodizing of Aerospace Aluminum Alloy”, presented at IMFAIR09, 10-11 June, 2009, Royal Air Force Museum, Cosford, Shropshire UK.
- Schaedel, F., “Improving Anodize Wear and Corrosion Resistance by Combining Modified Electrolyte Chemistry with Advanced Waveform Pulse Ramp Technology”, AAC 17th Anodizing Conference & Exposition, October 28–30, 2008, San Francisco.
- Munk, F., “State of the Art Hardcoat Anodizing Power Supplies”, IHAA, 9th Technical Symposium, Canada, Sept., 2002
- Juhl, A. Deacon, “Pulse Anodizing of Extruded and Cast Aluminium Alloys”, Ph.D. thesis, Inst. of Manufacturing Engineering, The Technical University of Denmark, July, 1999.
- Juhl, A. Deacon, “Why it Makes Sense to Upgrade to Pulse Anodizing”, Metal Finishing, July/August 2009.
Anne Deacon Juhl, Ph.D., is a manager for AluConsult, LCC, San Diego, Calif., and has authored more than a dozen papers on many topics pertaining to the anodizing process. She holds a Ph.D. in pulse anodizing of extruded and cast aluminum alloys from the Technical University of Denmark. Dr. Juhl is also a Sr. Instructor for the International Surface Finishing Academy (ISFA) where she conducts several anodizing workshops the ISFA around the country each year. Dr. Juhl is also a member of the following organizations: International Hard Anodizing Association, Aluminum Anodizers Council, and ASM International. She may be reached at firstname.lastname@example.org.