Shocking finish

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.

Finishing Up

Parts emerge from the anodizing bath with a clear finish but the resulting porous coating can be dyed many colors, such as blue, green, red, black or gold.

Because the anodic layer is so thin, Type I anodic coatings are not typically dyed. But Type II coatings are light-colored in their natural state and can be dyed. Type III produces a dark brown or gray natural color, which results in darker hues when dyed.

While the coloring is cosmetic, it can be used to identify different parts and devices. “The medical industry is starting to use color more often to identify surgical tools,” said Dave Meyer, inside sales, Bowers Manufacturing Co., Portage, Mich. “We anodize machined medical parts, such as drill handles.”

The parts can be sealed in a variety of process baths, including nickel acetate, nickel fluoride, sodium dichromate and hot, deionized water after anodizing and dyeing to seal the coating pores, which increases corrosion resistance and dye retention and resistance to UV fading. (A hot-water seal is not used on dyed parts as it will leach out the color.)

The parts can also be left unsealed so the pores stay open. “When you apply paint and primers, it goes into those pores,” Ouimette said. “Once [the paint] cures, it locks into those pores.” 

Although most aluminum alloys can be anodized, the process is more effective for low-copper alloys. “For the most part, we anodize the 6000 series, some 3000 series, 5000 series and 7000 series,” Meyer said. “You can anodize 2000 series, but there are issues with it because of the high copper content, which doesn’t anodize as well so parts can come out smutty or with a film.”

Aluminum alloys have different rates of anodic coating formation, so coating thickness will vary. “They all take different process times,” Kraft said. “If I put 2000 and 6000 together and ran it for the time I normally run 6000 at, the 2000 would become overcoated and powdery. If I ran it for the time I run 2000 at, the 6000 wouldn’t get enough thickness.” 

Courtesy of Servi-Sure Corp.

This step clip rack from Servi-Sure is one of many styles of racks for anodizing parts. The user pushes the part onto the V-shaped clip and the clip springs back to hold the part in place.

Problems can also arise with cast and die-cast aluminum parts. “The aluminum used in die casts, which is injected, has a high level of silicon,” Goodsett said. “Therefore, you can’t get as thick of a coating and it comes out very uneven. Also, the coating comes out dark gray so it is hard to color. Poured castings anodize better, which results in a thicker coating, but there is always the chance of porosity in the part from the casting process.” 

Anodizing alters part or hole dimensions. “When a customer is making a part that requires an anodic coating, they make it to a size that once we apply the proper anodize thickness, it will bring all those critical diameters and bores into their final tolerances,” Ouimette said. “Customers often give us a diameter or bore that they are trying to keep track of to make sure that we hit that final dimension.” That requires checking the dimension before, during and after anodizing.

Other processes used to anodize, such as chemical etching and brightening, can also affect dimensions. “You have to be careful, especially with fine threads,” Meyer said. “You don’t want to etch a part too long because it will affect the threads. And when you anodize, it can reduce the hole diameter.”

Anodic coatings can be removed with a chemical etch but there will be some dimensional loss, which can cause problems, according to Ouimette. “Depending on the coating thickness and how tight the tolerance is, once you strip it, the needed thickness can be more than we can put back on.” 

Environments and Impact

The life of anodic coatings varies greatly based on the amount of environmental exposure. For instance, parts exposed to salt water or subjected to chemical washes or high temperatures can corrode quickly, and sun exposure might cause the color to fade. Coatings not subject to heavy exposure can last for years.

Hardcoat anodized parts are extremely durable. “If you have two parts that are hardcoat anodized and they are wearing on each other, they will virtually last a lifetime,” Pecjak said. “As far as exposure, hardcoat will last longer than a Type II anodize as well just because it is a much thicker coating.” 

Courtesy of Walgren Co.

A rack of spool valves for transmissions is hoisted from one anodizing process tank into another.

Anodizing is an environmentally friendly process because it is a reinforcement of naturally occurring oxidation. “Anodizing is ‘green,’ ” Meyer said. “Anodized parts are recyclable. You can melt them down and not have to worry about the anodic coating. If paint is applied, however, you might have to strip it off before you can recycle the part.”

Anodic byproducts are green as well. “A lot of the chemicals we use can be recycled or reused in different markets, such as in making fertilizer,” Meyer said. “Some of our chemicals are used by municipalities to treat wastewater.”

Another Conversion

Chromate conversion is a chemical-treatment process similar to anodizing but electricity is not used. Also known as chemical film or Alodine (brand name for Henkel Technologies’ conversion), chromate coating leaves virtually no buildup on the part so it does not alter part dimensions or make holes smaller. The coating thickness is 0.00001 " to 0.00003 " (0.3µm to 0.8µm).

“It does not penetrate the aluminum,” Ouimette said. “It provides a great substrate for painting or priming. It also gives you a good substrate for any type of maskents to adhere to. And it gives you light corrosion resistance but not as much as anodizing.”

Also, unlike anodizing, chromate conversion provides an electrically conductive surface so it is widely used in the electronics industry.

Whether using a chromate conversion or anodic coating, it is best to do as much machining as possible beforehand because the increased surface hardness makes machining more difficult after coating. Also, the parts must be coated before they are assembled as assembled parts cannot be chromate coated or anodized due to streaking, bleeding out or solution entrapment issues.

The market for these processes continues to grow as engineers find more uses for “lightweight” aluminum as a substitute for hardened steels or painted steel parts, and as new aluminum alloys continue to be developed with the strength of steel. CTE

About the Author: Susan Woods is a contributing editor for CTE. Contact her at (224) 225-6120 or susanw@jwr.com.

Contributors

Aluminum Anodizers Council
(847) 526-2010
www.anodizing.org

Anodizing Specialists Inc.
(440) 951-0257
www.anodizingspecialists.com

Bowers Manufacturing Co.
(800) 669-9114
www.bowers-mfg.com

Master Metal Finishing
(973) 684-0119
www.mastermetal.com

Pioneer Metal Finishing
(800) 994-7654
www.pioneermetal.com

Poly-Metal Finishing Inc.
(800) 628-8356
www.poly-metal.com