The Anodizing Answer to Engineering Challenges

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Anodizing

By Mark Shortt
Editorial Director, Design-2-Part Magazine

Strong resistance to corrosion, abrasion, and wear along with various colors and aesthetic designs are obtainable through anodizing, a process that "grows" an oxide finish from the base metal.

Painting, powder coating, anodizing, and plating are some of the techniques that protect raw aluminum from the corrosion that occurs from exposure to atmospheric conditions. Among them, anodizing provides some of the best protection and the widest variety of surface treatment options, including the use of multiple colors.

Anodizing is an electrochemical oxidation process that imparts corrosion resistance and a decorative finish to aluminum and other metal parts immersed in an acid bath. The process converts the surface of the workpiece to a hard and durable, porous oxide layer, which, because of its porosity, can be impregnated with dyes to obtain various colors. Used predominantly on aluminum, the versatile surface finishing technique is also being used now on other reactive metals, such as titanium, magnesium, and zinc.

Today, anodizing service providers are developing and offering new options ranging from multi-coloration to more efficient and cost-effective methods for providing basic anodizing services, according to Michael Deming, president and general manager of Deming Industries Inc., Coeur d'Alene, Idaho.

"Multi-color anodizing presents one of the greatest opportunities for manufacturers to consider the use of anodizing in their product," says Deming. While giving customers control over the cosmetics of their product, the process provides shorter lead times and a finish that is typically more durable and cost-effective than many alternatives, he asserts.

The emergence and widespread use of electrolyte additives have enabled job shops to increase the speed of anodizing, and thus develop coatings in a much shorter period of time while also reducing costs. Electrolyte additives have also paved the way for the greater availability of hard coat anodizing, Deming says. At the same time, improvements to process control equipment are raising the bar, industry-wide, for anodizing quality.

Meanwhile, improved dye pigments are allowing anodizers to obtain cosmetically acceptable finishes with thinner coatings, at significantly lower cost. As a result, anodizers now have the ability to provide customers with more affordable coatings, Deming says.

Advanced technologies currently being used in the anodizing industry include higher-grade rectifiers (with high-amperage capabilities), cathodes, and tanks. Today's process technology includes specialized processing lines for batch, continuous coil, continuous parts, and basket anodizing. Other advanced technologies currently in use are new sealers, buffers, wetting agents, and automated and semi-automated rinsing designs.

Products that make use of anodized aluminum include building exteriors, such as curtain walls and roofing systems; home appliances, such as refrigerators, microwave equipment, and coffee brewers; and automobile components, such as trim parts, wheel covers, name plates, and control panels. Other specific uses include storm doors and window frames; coolers, pans, and grills; and clocks, telephones, and electronic products.

Anodized parts do not chip, peel, or flake because the anodic coating is integral to the base metal, as opposed to being deposited on the surface. Another positive attribute of the process is its non-hazardous nature: anodizing is a natural oxide process that produces no harmful or dangerous by-products.

"No finish on aluminum has a more natural look," says Mark Thomas, vice president of sales and marketing for Light Metals Coloring, Southington, Connecticut. "It's a natural, beautiful look."

However, users should realize that most anodizing dyes are organic and will eventually fade if subjected to direct sunlight 24 hours a day, says Cynthia Piazza, sales manager, Anodizing Industries, Inc., Los Angeles, California. Also, anodizing is not recommended for marine uses, which are better served with an electroless nickel finish, she explains.

Unlike applied coatings, anodizing is a reactive finish that integrates with the underlying aluminum to achieve "total bonding and unmatched adhesion," according to the website of the Aluminum Anodizers Council (AAC), an international trade association based in Wauconda, Illinois. Because the volume of oxide produced is thicker than the metal that it replaces, the resulting oxide can increase part dimensions by varying percentages under normal conditions.

Thomas says that failure to consider beforehand the dimensional changes that result from anodizing is a relatively common mistake in the design engineering of components. All finishing service providers should be contacted at the beginning of a project to prevent costly adaptations at the end, he advises.

Batch and Continuous Coil Methods

Batch anodizing and continuous coil anodizing are methods that differ by the physical types and shapes of the metals to be processed. In batch anodizing, parts are racked and then immersed in a series of treatment tanks. Continuous coil anodizing, on the other hand, consists of continuously unwinding pre-rolled coils through a series of cleaning, etching, and anodizing tanks. The coils are then rewound for shipment and fabrication.

Batch anodizing is used for processing bent metal parts, castings, and extrusions. It is also used to anodize machined aluminum parts, flashlight bodies, and cosmetic cases. The continuous coil method is used for processing high-volume sheet, foil, and lighting fixtures, as well as reflectors, louvers, and spacer bars for insulated glass.

Both methods comprise five carefully controlled, calibrated stages that occur in the following sequence: cleaning, pre-treatment, anodizing, coloring, and sealing.

In both methods, the process begins with the use of alkaline or acid cleaners to remove grease and surface dirt from the part. Cleaning is followed by a pre-treatment stage in which the surface of the part can be either etched or brightened. Etching uses hot solutions of sodium hydroxide to remove minor surface imperfections and create an appealing matte surface finish; a dull finish can be achieved by removing a thin layer of aluminum. Brightening is accomplished with a concentrated mixture of phosphoric and nitric acids, which chemically smooth the surface of the aluminum to create a near-mirror finish.

In the anodizing stage, the aluminum (or other metal) part is immersed in an acid electrolyte bath and functions as the anode (the tank is the cathode). Passing an electrical current through the bath builds an anodic oxide that combines with the metal. Oxide thickness and surface characteristics can be tightly controlled to meet end product specifications.

The sealing process closes the pores in the anodic film, resulting in a surface that resists staining, abrasion, crazing, and color degradation. Employed in the final stages of the anodizing process, some of the newer sealers can be run at lower temperatures, helping to reduce overall costs, Thomas explains.

Types of Anodizing

The authoritative anodizing standard, MIL-A-8625, details Type I, Type II, and Type III anodic coatings. Type I refers to chromic acid anodizing; Type II, to nominal "clear" sulfuric acid anodizing. Type III is hardcoat anodizing in which sulfuric acid or mixed-chemistry electrolytes are used, according to information published on the website of the AAC (www.anodizing.org).

Performed in a dilute sulfuric acid solution at room temperature, Type II anodizing produces thin finishes less than 1 mil (0.001 inch) thick. The process is regularly used for architectural coatings.

Type III anodizing, performed at lower temperatures, gives a thicker finish that is extremely hard and highly resistant to wear and corrosion. Like Type II, the process is usually performed in a dilute sulfuric acid solution, but at a temperature of approximately 32F. Hard-anodized components are favored for engineering applications that require a highly wear-resistant surface, such as pistons, cylinders, and hydraulic gears. Often left unsealed, the coatings may be impregnated with waxes or silicone fluids in order to obtain specific surface properties, according to the website of the AAC definitions.

Coloring Techniques

Breakthroughs in coloring techniques have made a wider range of colors available for use with anodizing. Deming Industries offers customers a choice of 13 different colors, including clear, black, green, gold, brown, blue, turquoise, purple, and orange- and burgundy-tinted reds.

"In our opinion, the most significant advanced technology in the anodizing industry today is in the area of multi-color anodizing," insists Deming. "In our case, we have developed the ability to provide customers with complete control over the cosmetics of their product while obtaining better quality, service, and lead times, and, in most cases, cost advantages over previously used options."

Companies come to Deming to request custom multicolor finishes of all types. And whether they call for development of leaf patterns for camouflage, or five-color logos and graphic layouts, all of the jobs present challenges, Deming says.

"The main challenges in providing multicolor services are in designing a process that will control all color locations, be affordable and capable of high volume, and still maintain the high quality of the coating," he explains. "All challenges are met through two basic concepts we follow. First, a complete process is engineered for each customer's application to ensure that customer-specific requirements are met. And second, all of our processes are based on a coating that is designed to meet or exceed military specification for Type II anodizing, so as to ensure the consistent high-level performance of all our coatings."

Anodizers can achieve coloring by any of four different techniques: electrolytic coloring (also known as the two-step method), integral coloring, organic dyeing, or interference coloring. In electrolytic coloring, performed after anodizing, the metal is immersed in a bath that contains an inorganic metal salt. Current is then applied, depositing the metal salt in the base of the pores. The resulting color depends on the metal used and the processing conditions. According to the AAC, the process offers "color versatility and the most technically advanced coloring quality."

Integral coloring combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades. The method provides greater resistance to abrasion than conventional anodizing. It is also the most expensive process, according to the AAC, because it requires significantly more electrical power.

Interference coloring is a recently introduced coloring procedure that involves modification of the pore structure produced in sulfuric acid, according to the AAC. Metal deposition at the base of the pores, where the pores are enlarged, produces light-fast colors such as blue, green, yellow, or red. The procedure is unlike the basic electrolytic process in that optical-interference effects, rather than light scattering, cause the formation of the colors. However, the procedure has not been successful in the U.S., according to an AAC spokesperson.

Freshly anodized parts can also be immersed in a liquid solution containing a dissolved dye, which is absorbed by the porous anodic coating. Organic dyes can be used to produce a wide variety of vibrant, high-intensity colors that provide excellent weather fastness and light fastness, according to the AAC.

More About the Job Shops

Deming Industries, Inc. provides custom anodized coatings to a variety of companies that manufacture parts and products for the high-tech electronics, aerospace, robotics, medical equipment, and sporting goods markets. Its clients' end products include genetic engineering equipment, locomotive controls, industrial test equipment, archery products, and paintball guns, among others. In addition to solid color anodizing, the company provides custom camouflage anodizing, anodized graphics, and DDS processing for non-reflective surfaces.

One of the largest independent aluminum anodizing job shops in the U.S., Light Metals Coloring anodizes parts for customers in industries ranging from food preparation equipment to aerospace. The company prides itself on being diverse enough to accommodate a wide range of customer needs, from 0.0001 coating thickness to specific voltage breakdown minimums. Orders come direct from OEMs and other job shops, such as stampers, screw machine houses, extruders, and sheet metal fabricators. LMC processes some 100,000 pieces per month, running three shifts per day to handle jobs ranging from "16-foot-long parts to bulk anodizing in a basket," according to Mark Thomas, vice president, sales and marketing.

Anodizing Industries, Inc. processes a variety of items, ranging from pliers, television components, and shower doors, to aerospace, military, and high-performance auto parts. One of its jobs involves anodizing hydraulic fittings that are used on Formula One race cars. The company also anodizes medical parts, writing instruments, brake calipers, and hockey and lacrosse sticks. The firm offers both Type II and Type III anodizing, along with high-resolution image transfer printing on irregular objects and silk screening.

Specialty anodizing services offered by All Metals Processing Inc. (AMP), Stanton, Calif., include Type I chromic anodizing to aerospace and military specifications, and boric sulfuric acid anodizing to most aerospace specifications. The company, which recently completed the addition of a 12,000-sq-ft, state of the art anodizing facility, also specializes in cadmium, nickel, and silver plating, phosphate coating, and powder coating. The firm is now branching out across the nation to new markets.

The Aluminum Anodizers Council, headquartered in Wauconda, Illinois, is a trade association of firms engaged in aluminum anodizing. For its members, the Council publishes Technical Bulletins, such as the Aluminum Alloy Reference for Anodizing, and Comparison of Colored Aluminum Anodic Finishes, which cover a range of anodizing subjects. It has also established a Web Forum, a threaded discussion forum that facilitates the exchange of information on aluminum anodizing. For more information from the Aluminum Anodizers Council, visit the organization's website at www.anodizing.org.

JobShop.com thanks the Aluminum Anodizers Council for making information from its website available for use in this article.

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