Understanding the treatment process
By Mike Mavrigian
The use of aluminum components during an engine build is common from a standpoint of weight savings and appearance. This can include, but not limited to aluminum pulleys, engine mounting plates, -AN plumbing hardware, pistons, aluminum main caps, rocker arms, brackets, valve covers, water pumps, balancers, timing covers, timing pointers, carb spacers, or any of a myriad of aluminum components. Aluminum requires a surface treatment of some type in order to prevent corrosion and to enhance appearance, which may include paint, powder coating or anodizing (clear or a color).
Anodizing offers far more than appearance enhancement. Hard anodizing for applications such as pistons and main caps for instance, provide increased Rockwell hardness and durability.
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Depending on the specific parts involved, these may be available already anodized (clear or in a color), or you may wish to create a custom color scheme to coordinate with the color of the block, engine bay or body.
It’s important to understand that anodizing is not a coating, and has nothing in common with coatings such as paint, powdercoat or plating. Anodizing involves an electrochemical process (electrolytic oxidation) that alters the surface of an alloy component, with the primary benefit involving surface protection, and a degree of wear-resistant surface hardening. Anodizing creates a protective oxide layer on the surface of the aluminum piece. The addition of a dye (where needed) provides the desired color, which is absorbed into the oxidized surface. Aluminum materials may be anodized clear or in any of a variety of colors, including “clear.” Keep in mind that the primary purpose of anodizing is to protect the aluminum parent material from corrosion, and to provide a harder, more durable surface.
During the process, aluminum oxide is generated during anodizing at/near the surface, creating aluminum hydrate, which provides the additional surface hardness. Since the anodized surface is also fairly porous, it readily accepts a color dye where desired. Varying levels of hardness are possible, depending on the component’s application.
The treatment is referred to as anodizing because the part being treated forms the anode of an electrical circuit. The process alters the microscopic texture of the alloy surface and the crystalline structure of the aluminum surface.
Hard coat anodizing is available, with emphasis on superior wear resistance. The primary purpose of hard anodizing is to provide a high level of added strength and increased resistance against wear.
THE PROCESS
Bare aluminum pieces are first cleaned/etched in a tank using an electrically conductive solution. Anodizing is accomplished in a tank that features an anode (positive electrode) and an inert cathode (negative) that conducts electricity, in a bath of electrolyte (sulfuric or chromic acid). After etching, the pieces are rinsed and subjected to a neutralizing solution and rinsed again. The anode layer is then bonded to the part. Once the thickness of the base layer is achieved, the pieces are rinsed and color may be applied using metallic salts or other compounds.
The process, from start to finish, includes a cleaning in a caustic soda, followed by a rinse. Next is a “de-smut” in nitric acid, which removes the slimey “smut” created in the cleaning process. Next is immersion in an anodizing tank using a diluted nitric acid bath, followed by another rinse. If a color is desired, next is immersion in a dye, followed by a rinse, finalized in sealing process involving a nickel acetate solution. It’s critical that each rinse is performed using a de-ionized water batch, using ultra-pure water that contains no minerals.
As compared to “normal” anodizing, the Type III hard anodizing process penetrates the aluminum material at a depth of about 0.001-inch, and provides an added hard layer about 0.001-inch above the surface. Again, this is applicable to aluminum pieces that require added wear resistance such as pistons. This level of hardness achieves a Rockwell of C6-C70, handling up to about 932°F (500°C). Hard anodizing also increases surface lubricity.
Typically, the anodizing process applies to aluminum alloys only. Stainless steel cannot be anodized, as the chromium in stainless steel prevents the surface to be etched, preventing the oxide from forming and etching into the material. Anodizing steel or iron materials is not beneficial, as steel does not contain the aluminum necessary to create a protective layer. Basically, the acids used will merely result in rust which will flake off. While the process can be applied to aluminum castings, results may be sketchy due to irregularities or impurities in cast materials, as a dull gray or non-uniformity color may result. While aluminum castings can certainly be anodized, the resulting appearance may or may not be as desired. More predictable appearance is provided with aluminum billet/machined/forged materials.
The appeal of the anodizing process involves avoiding surface corrosion, to apply a color to the surface, and depending on the level of anodizing, to increase surface hardness. The final anodized part’s surface will readily allow the surface finish to be revealed. Clear anodizing serves to provide the required corrosion resistance without altering the natural color of the alloy, providing a somewhat satin sheen.
If you want high gloss (on clear or color dyed pieces), the part must be polished prior to anodizing, or by applying a “bright dip” treatment following anodizing. Bright dip is an electrochemical process that smooths microscopic roughness and provides a glossy finish. Bear in mind that not all anodizing shops offer bright dip, as the solution required for the electrochemical process creates vapors that can contaminate other nearby solution tanks, requiring added handling and maintenance (one shop that I’m aware of that does offer bright dip is McNichols Anodizing & Polishing, but other may also offer this, so check around if needed).
Pre-polishing the parts prior to anodizing is likely the preferred option if you have difficulty finding a bright dip service. If you want a satin or semigloss finish, the part can be buffed with a light abrasive (such as Scotchbrite) prior to anodizing. If you want a matte finish, don’t perform any polishing or buffing.
If you are concerned about obtaining a specific color and/or finish, consider sending the shop a few sample pieces to have anodized before sending your prized items. If you send a paint sample of the color you desire, keep in mind that the anodizing shop may or may not be able to exactly match the paint sample, but in many cases, they should be able to obtain results that will be close enough to be satisfactory. For custom colors, be prepared to potentially pay a bit extra.
BENEFITS OF ANODIZING
• Increases material hardness and wear resistance
• Provides superior adhesion for paints, primers or adhesives (if you plan to paint over the piece for some reason)
• Will accept color dyes
• Helps to prevent galling
• Reduces electric current passage
ANODIZING TIPS
Common automotive applications for clear or color-dyed anodizing include aluminum pulleys, carburetor spacers, air cleaner lids, brackets, –AN hose and fittings, spark plug wire dividers, miscellaneous spacers, boost pressure regulators and wastegates, etc. As long as the component is made of a cast or machined billet aluminum alloy, it can be anodized. As noted earlier, castings may be anodized, but appearance results will vary depending on the makeup of the casting (specific alloy, casting porosity, etc.).
Another potential issue to be aware of is material uniformity. If you plan to have pieces color anodized, for best results all components should be made from the same material stock. Variances in the alloy mix can (and usually do) result in color variances and/or slight blemishes. For example, if you fabricate a handful of items from a variety of aluminum stock (sheet, round or square tubing, etc.), differences in the alloy formula can easily result in varying shades of the desired color (for example, slightly lighter or darker blue). If you grab miscellaneous pieces of aluminum from your raw stock inventory or from a local new or scrap metal supplier without paying attention to the alloy content and/or grade of aluminum, even when the batch of items is anodized at the same time using the same color dye, you may end up with slightly mis-matched colors. Be aware of this potential concern. If dissimilar alloy materials will be in close proximity to each other when installed, be aware that the colors may not perfectly match. This is not the fault of the anodizing shop.
You may have noticed when buying color-anodized –AN hardware that even from the same manufacturer, blues or reds may not be consistent, but are usually close enough to satisfy your expectations.
-AN aluminum hose ends, fittings and threaded plugs are commonly available off-the-shelf in various color-dyed anodizing (blue, red, black or clear being the most commonly available finishes). Depending on the anodizing shop’s color selection, some shops may be limited to the most common colors (red, blue, black) or a virtually unlimited range of colors (purple, gold, copper, green, etc.). If during the course of a custom build you wish to have certain pieces anodized in a custom color of your choice, there are two ways to accomplish this: it is possible to order a batch of hose ends, fittings and plugs in raw form (fully machined but not anodized), allowing you to turn these over to a custom anodizing shop for hardness and color treatment.
The other option is to deal with pieces that have already been anodized. A custom anodizing shop can strip the color and re-anodize in the color of your choice. However, be aware that the result may be a slightly duller/matt/flat finish due to the fact that the pieces had already been oxidized previously, and because the existing color needs to be stripped off.
Regardless of whether you’re dealing with a first-time color or a re-color, if you prefer a glossy appearance, the pieces will need to first be polished or treated to a “bright dip” following the desired color application. The bright dip option is not available from all anodizing shops, so if you desired a gloss finish, find out if your shop of choice can offer this.
Some anodizing shops are willing to try to actually attempt to color-match to a specific color. Be aware, though, that trying to color-match a specific color sample isn’t as easy as it sounds. The primary variable is the aluminum alloy itself and how it tends to absorb the dye. If you desire a specific custom color, the shop may ask you to provide a color sample. If the shop is willing to try matching the color, they may be able to anodize a small batch for your review. I realize that I’m repeating myself, but again, bear in mind that all of your aluminum parts must be made of the same grade of the alloy. If fabricating parts, it’s best to make everything from the same batch of aluminum stock, if you expect the color of all parts to be consistent. Even with that said, don’t be surprised if colors don’t match perfectly. But using the same raw stock will usually result in matching color.
Also, be aware that aluminum castings can present a challenge in terms of achieving the desired color and surface finish, such as cast aluminum valve covers as an example. It is possible, due to the casting process, to end up with color deviation and potential color blotching. If cast aluminum valve covers are to be anodized, it’s best to first polish the surfaces, which will increase the chance of achieving acceptable color and color consistency. If you wish to apply a color to cast valve covers, you’re better off by painting or powder coating.
Note: other materials that can be anodized include titanium and magnesium.
EXAMPLES OF ANODIZING SOURCES
• Fort Wayne Anodizing: Fort Wayne, IN (260) 424-1030
• Luke Engineering: Rittman, OH (330) 925-3344
• McNichols Polishing & Anodizing: Redford, MI (313) 538-3470
• Met-Fin Company: North Wales, PA (215) 699-3505
Listed above are a few anodizing shops that I’ve used in the past with great results. Refer to your local phone directory, or perform an internet search for anodizing shops in your region.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2024EPQ3/#p=50