304 Chromic Acid Anodizing

Chrome Plating and Anodizing Operations

304 CHROMIC ACID ANODIZING TABLE OF CONTENTS 304 CHROMIC ACID ANODIZING 304.1 EQUIPMENT 304.2 SURFACE PREPARATION 304.3 ANODIZING 304.4 POST-ANODIZING

304 CHROMIC ACID ANODIZING Anodizing is used primarily for aluminum and its alloys to improve resistance to corrosion, provide electrical insulation, and enhance the ease of coloring. The process derives its name from the fact that the workpiece forms the anode of an electrical circuit in an electrolytic process that forms a thin oxide film on the metal surface. There are several types of anodizing processes providing a wide variety of plating applications for products ranging from giftware and novelties through automotive trim and bumper systems. Some more demanding applications include products for architectural exteriors for wear-resistant and abrasive conditions such as airplane landing gears. Anodized aluminum can also be used to duplicate semiprecious and precious metals. Gold, silver, copper, and brass imitations are regularly fabricated. Matte finishes can also be produced by etching the aluminum surface, giving the "pewter" look. Figure 304-1 shows some anodized workpieces. Most types of anodizing use the sulfuric acid process. However, where parts may be subjected to considerable stress (e.g., aircraft applications), the potential retention of corrosive sulfuric acid in crevices and recesses of anodized parts could cause problems. Chromic acid, on the other hand, is a recognized corrosion inhibitor, so its retention in crevices and joints does not present a problem. This makes chromic acid the preferred (and specified) anodizing process for aircraft parts, as well as for other assemblies with overlapping joints, recesses, or crevices. Another advantage of chromic acid anodic coatings is their very high resistance to salt spray corrosion as compared to sulfuric acid process coatings of the same thickness.

Cal/EPA – January 2006

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Figure 304-1 Example of Anodizing Workpieces

The chromic acid anodizing process consists of the following steps: 1. Surface Preparation 2. Anodizing, and 3. Post-anodizing operations

304.1 EQUIPMENT General process equipment includes tanks, cathodes, racking system, and power equipment. Equipment selections are dependent on the type of anodizing to be performed. Typical selection of construction materials for tanks includes polypropylene, mild steel, and stainless steel. An external heat exchanger is used to control the temperature of the bath and some form of agitation is needed in the bath to prevent localized high temperatures. Different types of cathodes include aluminum, lead, carbon, and stainless steel. Aluminum cathodes are commonly used because of the better conductivity of aluminum resulting in lower process energy requirements. Common rack materials are aluminum and titanium. Titanium racks are recommended because they do not have to be stripped and have good current carrying capabilities. Aluminum racks are less costly, but they anodize along with the work. Lastly, for chromic acid anodizing, a DC-power source capable of producing up to 40 V and sufficient amperages should be suitable. 6, 8


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304.2 SURFACE PREPARATION The following steps are often used to clean aluminum before anodizing: 1. Soak Clean, 2. Etching, 3. Deoxidizing/Desmutting, and 4. Bright Dipping and Electrobrightening The cleaning steps used for a particular aluminum substrate depend upon the amount of smut and the composition of the aluminum. The aluminum substrate is rinsed between each pretreatment step to prevent contamination of subsequence process baths.

304.2.1 Soak Clean Various cleaners are used to dislodge machining oils, greases, and other surface contaminants from the aluminum surface. These are often alkaline-based or acid-based proprietary cleaners available for surface cleaning. The soak clean step consists of immersing the metal in the cleaning solution, which is mildly agitated with air.

304.2.2 Etching Etching is the removal of some of the aluminum surface from a part using chemical solutions. The purpose for etching aluminum include: 1. To impart a matte finish to the material 2. To remove surface contaminants 3. To hide surface imperfections (scratches, die lines, etc.) 4. To produce an overall uniform finish. Chemical etching is accomplished using both alkaline and acid solutions. The most frequently used etching solution is sodium hydroxide. The degree of etching desired and the composition of the aluminum determine the concentration and temperature of the etch solution, and duration of the etch.

304.2.3 Deoxidizing/Desmutting Deoxidizers/desmutters are used to remove "smut" of residual metallic alloying materials left on the aluminum surface after etching. Many alloys, during their heat treatment steps, will form heat treat oxides. If these oxides are not removed prior to etching or bright dipping, a differential etch pattern can develop, which will cause rejection of the parts. A deoxidizer is designed to remove oxides, but will also remove smut. A desmutter, however, will not remove oxides.


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304.2.4 Bright Dipping and Electrobrightening A chemical or electrobrightening treatment is required where an extremely high luster is desired. These are mainly proprietary solutions. Specifics on the characteristics and use of them are available from chemical suppliers.

304.3 ANODIZING OPERATING PARAMETERS The anodizing process is effective over a wide range of voltages, temperatures, and anodizing times. In general, high voltages tend to produce bright transparent films, and lower voltages tend to produce opaque films. The amount of current applied varies depending on the size of the aluminum part(s). Typical current densities range from 1,550 to 7,750 A/m 2 (144 to 720 A/ft 2). Raising the bath temperature increases current density to produce thicker films in a given time period. Temperatures up to 49 °C (120 °F) typically are used to produce films that are to be colored by dyeing. The anodizing process time usually ranges from 30 to 60 minutes. Typical operating parameters for chromic acid anodizing baths are summarized in Table 304.1. The anodizing process is illustrated in Figure 304-2 . Click on Play in Figure 304-2 to view the process of anodizing.

Table 304-1. Typical Operating Parameters for Chromic Acid Anodizing

(EPA, 1995)


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Figure 304-1 Anodizing Process

When the current is applied, chromic acid acts as a catalyst and breaks down in the bath resulting in the liberation of oxygen and hydrogen. The oxygen is evolved at the surface of the aluminum workpiece where it reacts with the substrate to form an aluminum oxide layer. At the same time, chromic and dichromic acids contained in the bath react with the aluminum oxide film in a dissolving action. This action results in the formation of very fine pores, enhancing the continuation of current flow to the metal surface. Competition between the oxide film growth and oxide dissolution regulates the anodic film properties. As the film thickens, the growth rate decreases until it is equal to the oxide dissolution rate, at which point the film thickness reaches a limiting value dependent on the current density. About half of the oxidized aluminum is retained as anodic film, and the remainder goes into solution to form alumina-chromic acid compounds. The relationship between the cathode and the shape or relative location of the anode is not critical in chromic acid anodizing.

304.3 POST-ANODIZING The anodized parts undergo several post-anodizing steps. These steps include sealing and air drying. Sealing hydrates the aluminum oxide and fills in the pores in the aluminum surface. The sealing process consists of immersing the anodized parts in a solution of boiling water or other solution such as nickel acetate which hydrates the aluminum oxide. The aluminum is then allowed to air dry.


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References 1. California Air Resources Board, 2000, Toxics Enforcement Manual, Part B: NESHAP/MACT Guideline Documents, Volume 2, Chapter I, Chrome Electroplating and Anodizing Operations , Compliance Assistance Program, Sacramento, CA, November 2000, p. 200-2 - 200-11 2. California Air Resources Board, 1988, Control of Emissions from Chrome Electroplating and Anodizing Operations: Submitted for CAPCOA Air Pollution Control Engineering Symposium, December 1988 , Sacramento, CA, Available online at: http://www.arb.ca.gov/toxics/chrome/background.htm, p. 14-55 3. California Air Resources Board, 1988, Technical Support Document to Proposed Airborne Toxic Control Measure for Emissions of Hexavalent Chromium from Chrome Plating and Acid Anodizing Operations: January 4, 1988 , Sacramento, CA, Available online at: http://www.arb.ca.gov/toxics/chrome/background.htm, p. 1-35 4. U.S. EPA, 1993, Chromium Emissions from Chromium Electroplating and Chromic Acid Anodizing Operations-Background Information for Proposed Standards , Volume 1, Document No. EPA 453/R93-030a, p. 3-1 - 3-59 5. Electrochemistry Encyclopedia (http://electrochem.cwru.edu/ed/encycl/), 2002, Article by Robert S. Alwitt, Boundary Technologies, Inc. Northbrook, IL, Anodizing , December 2002, Accessed on September 15, 2005, http://electrochem.cwru.edu/ed/encycl/art-a02-anodizing.htm 6. PF Online an online component of Products Finishing magazine, 2003, Article by Gary Kriesch, Walgren Co., Grand Rapids, MI, Building a World-Class Anodizing Line , Accessed on January 5, 2005, http://www.pfonline.com/articles/pfd0316.html 7. PF Online an online component of Products Finishing magazine, 2001, Answer by Larry Chesterfield, Aluminum Anodizing, Consulting, Indianapolis, IN, Aluminum Anodizing Clinic - Smut and Desmutting , Accessed on January 5, 2005, http://www.pfonline.com/articles/clinics/0201cl_alum2.html 8. Metal Finishing 2000 Guidebook and Directory Issue , Vol. 98, No. 1, Technical paper by: Charles A. Grubbs, Houghton Metal finishing, Alpharetta, GA., Anodizing of Aluminum , p.480-496


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304 Chromic Acid Anodizing

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