Shocking finish: Medical Manufacturing

Aluminum parts that are anodized using electrical energy or chromate conversion coated can offer wear and corrosion resistance, among other benefits.

While most shop workers know how aluminum parts are machined, they might be less familiar with what happens to the parts afterward. Anodizing and chromate conversion are two common processes for finishing aluminum parts. Both processes can provide corrosion protection, wear resistance and surface preparation for painting.

These processes are applied to aluminum consumer, commercial and industrial products. Parts range from satellite components that need protection from the harsh environment of space to bicycle and boating components.

Courtesy of Aluminum Anodizers Council

Aluminum parts anodized in a chemical bath and dyed red.

Anodizing and chromate conversion are specialized processes that are almost always done by a finishing company. “Very few machine shops or fabricators, even large ones, do it in-house,” said David Kraft, vice president of sales for Master Metal Finishing, Paterson, N.J. “Not only is there a big capital investment, there are environmental issues with handling the chemicals.”

Anodizing services are performed on a contract basis or piecemeal. Cost considerations include part size, the process used and any add-ons, such as part masking, special packaging or special inspection. “To anodize a part could cost pennies or hundreds of dollars,” Kraft said. “We have big, intricate 8 ‘-long parts that we mask and ‘hardcoat’ anodize and those can run $300 or more each. We also do some small, high-volume parts that cost maybe 12 cents each.”

Standard turnaround for most anodizers is 3 to 5 days, but expediting is common. “Our process is the end of the line so a lot of times we make up the time that might have gotten lost in the machining process,” said Steve Goodsett, corporate product manager, Pioneer Metal Finishing, which has seven locations including one in Green Bay, Wis. “Some parts have a 2-day turnaround—run within 24 hours and shipped back in the next 24 hours.”

Parts are packed in various ways, including bulk, layered on a skid or individually in containers made just for that part.

Anodizing Details

According to Dr. Jude Mary Runge, president of Comprehensive Metallurgical Consulting, Lombard, Ill., and a member of the Aluminum Anodizers Council, Wauconda, Ill., it is hard to say what percentage of aluminum parts are treated. “But anodizing is the most common finish for aluminum parts,” she said. “It is also the most reliable corrosion- and wear-resistant finish. As a metallurgically integrated finish, it doesn’t peel or delaminate as other coatings can.”

Anodic coatings are most commonly applied to protect aluminum alloys, although other nonferrous metals, such as magnesium and titanium, can be anodized. Ferrous metals cannot be anodized.

“Because of the chemistries and electricity used for anodizing, you need alloys that do not contain iron,” said Jason Ouimette, production manager, Poly-Metal Finishing Inc., Springfield, Mass. “The anodizing process actually dissolves those irons into the bath solution, so there has to be a nonferrous substrate for the anodic coating to adhere to.”

Courtesy of Walgren Co.

Workers loading and unloading hardcoat anodized spool valves for transmissions.

The first step is to rack each part individually. The part needs to remain in the same position throughout the entire process. The anodic coating is nonconductive, so if the part moves slightly, the coating could move on to an area that is already anodized and will not conduct the electrical current.

“One of the largest overheads for an anodizer is labor,” Goodsett said. “You have to clip each part separately. And it is not always a given on how you are going to do that. Wherever you make contact to hold the part, there is going to be a small bare spot. Racking requirements have to be agreed upon in advance.”

Anodizing tanks are approximately 10 ‘ long × 5 ‘ deep × 4 ‘ wide, with some larger and some smaller. While only one or two large parts can be anodized at a time, thousands of small parts can be done at one time. “We run up to 12,000 parts in a load,” Kraft said.

Next, parts are immersed in an alkaline and/or acid bath to remove oils, fluids and dirt. After each chemical process, the parts are rinsed with fresh water.

The parts then go into an etch tank. A matte finish is created with hot solutions of sodium hydroxide. “The chemical etch bath actually attacks that material,” said Michael Pecjak, vice president of operations, Anodizing Specialists Inc., Mentor, Ohio. “It removes a thin layer of aluminum, and any oxides that may have started to occur naturally are broken down. This gives a clean, active surface to be anodized.”

An option before anodizing is bright dip—a brightening process that creates a glossy look on the part. The process takes place in a bath of a phosphoric and nitric acid mixture that chemically smoothes the aluminum’s surface.

Anodizing Conversion

Anodizing is an electrochemical conversion process where the surface of the aluminum part is converted to aluminum oxide by passing an electrical current through an acid bath in which the part is immersed. The anodic coating thickness and surface characteristics are tightly controlled, according to Pecjak.

“The aluminum naturally tries to oxidize, but we do it in more of a controlled atmosphere,” he said. “That produces a good, even coating. When it oxidizes on its own, it is uneven.”

Depending on the coating thickness, the anodizing process itself can take 10 to 60 minutes, with 30 minutes being typical. The longer the part is left in the bath, the thicker the coating, up to a point.

There are three main types of anodizing: chromic, sulfuric and hard. Type I is a chromic-acid anodize. The coatings are from 0.00002 ” to 0.0003 ” (0.5µm to 7.6µm) thick.

“Chromic anodizing is a great maskent prior to hardcoat or other types of anodize you want to apply to the part,” Ouimette said. “We do chromic anodizing all over and then machine the areas that need a hardcoat anodize. The hardcoat only anodizes bare aluminum. It won’t anodize the other areas that have chromic anodize on them.”

Types II and III are sulfuric acid-based and utilize the same chemistries. Ouimette said: “By changing the temperature of those chemistries, we can produce two completely different anodic coatings. Type II is typically run at 68° to 72° F. Utilizing the same anodic solutions, running at 28° to 32° F produces a Type III hardcoat anodize. It changes the pore structure with that lower temperature.” Also, more electrical current is used to produce the hardcoat anodize.

With Type II, about two-thirds of the anodic coating thickness penetrates into the base material, and the remaining third builds on top of the base material. The coating is 30 percent thicker than the aluminum it replaces. (The same is true for Type I but it is so thin, it is almost a nonissue.)

Type II anodizing produces a coating thickness from 0.00007 ” to 0.001 ” (1.8µm to 25.4µm). (Thickness includes the buildup and penetration amount.)

For Type III, 50 percent of the anodic coating thickness penetrates into the base material, and 50 percent builds on top. The coating thickness is 0.0010 ” to 0.0025 ” (25µm to 63.5μm).

The finish produced with hardcoat anodizing provides wear resistance equivalent to other materials with a hardness up to 68 HRC, according to Goodsett.

However, hardcoat anodizing is generally about 30 to 50 percent more expensive than Type II, so if maximum wear resistance isn’t required, Type III is not generally used. “With Type III, you must apply twice as much electricity as with Type II,” Goodsett said. “So you need a larger DC rectifier machine to provide the electricity as well as a much larger chilling system to keep the anodic solution at the required low temperature.” Besides the larger equipment investment required for Type III, it also requires significantly more energy.

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