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Frequently Asked Questions
What is anodizing?
A coating of aluminum oxide is grown from the aluminum by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications.
What is the purpose of anodizing?
The purpose of anodizing is to form a layer of aluminum oxide that will protect the aluminum beneath it. The aluminum oxide layer has much higher corrosion and abrasion resistance than aluminum. There are some types of anodizing that produce a porous oxide layer that can be colored with organic dyes or metallic pigments giving the aluminum a decorative and protective finish.
What is anodized aluminum good for?
Because anodizing is such a versatile process there are thousands of different applications. Just to name a few, these include:
It only takes a few hours to process and pack a part. Most anodizers need anywhere from a few days to a few weeks to plan, process and invoice for projects. Lead times of longer than six weeks for anodizing are rare.
Where do I learn more about aluminum anodizing?
Review the "What is Anodizing?" section on this website. Contact AAC if you require additional information. AAC also provides workshops and seminars for anodizers.
Is anodizing environmentally friendly?
Yes. Anodized aluminum is often used on cookware. (However, FDA approval depends on the anodizer and on the process.) In contrast to electroplating process, it is not difficult for anodizers to comply with all federal and state environmental regulations. There are no RCRA heavy metals used with anodizing.
Is any anodizing process acceptable for use in cookware?
Yes, both hardcoat and conventional anodized products are often used for aluminum cookware.
From a fabricator's viewpoint, is it best to do as much machining as possible before anodizing because of the increased surface hardness?
Machining before hardcoat anodizing is much easier and saves considerable wear and tear on the tools. A good rule of thumb is that the hardcoated surface has about the same hardness as nitrided steel ( about 50 + Rockwell "C").
Does anodizing alter the bolt/nut thread pattern and diameter?
Yes. Anodizing is a process that converts aluminum to its oxide. The oxide is thicker than the aluminum that is consumed, which means the dimension of the anodized part changes. The amount of change will depend on the anodizing process conditions (temperature, current density, etc.) and alloy. Under nominal Type II anodizing conditions, the rule of thumb is 2/3 in 1/3 out; for example, a coating that is 0.6 mil thick will have consumed 0.4 mil of aluminum. Under hard coat (Type III) anodizing conditions the ratio changes to ½ in ½ out. Keep in mind that when calculating the shrinkage of a hole, you must double the amounts given because a hole has two sides, for example, the hole diameter reduction for a 0.6 mil Type II coating would be (1/3 of 0.6 mil) x 2 = 0.4 mil. Another example to consider is the hole reduction of a 1.5 mil Type III coating (1/2 of 1.5mil) x 2 = 1.5 mil.
Other processes in an anodizing line (including, for example, chemical etching and brightening) will affect dimensional changes. Different anodizers use slightly different process parameters. With all these variables, it is a good idea for the design engineer to contact the anodizing plant under consideration and ask for their input.
Given that anodizing is an electrochemical process, can areas be masked or holes plugged to prevent coverage of an area? (E.g., to create letters or symbols on a surface or to prevent buildup in machined bores or threads.)
Yes, sections of a part can be masked. Flat areas can sometimes be more difficult to mask; holes and bores can usually be masked without too much difficulty. Lettering may be accomplished more satisfactorily either by casting them into the part or using laser engraving after anodizing.
Is there a simple process for stripping the anodize from a part?
Although it is common practice, and such parts can be reanodized, there are hazards associated with the removal of anodic oxide from a part. Consult a professional anodizer to explore the details.
Can you advise me on trying to touch-up a part that has been scratched and gouged? This is a small printed circuit board that has a metal frame around it. The frame has a hardcoat finish on it, but over the years it has been damaged. Is it possible to touch-up the hardcoat in order to restore it to its original dimensions?
It might be possible to repair the anodized coating on the frame. However, if the basemetal has been affected, it is not possible to eliminate the scratches or gouges except by a mechanical repair of the substrate through sanding, buffing, etc., after removing all the anodic coating.
If the hard anodized frame cannot be easily removed from the circuit board, it is possible that the frame could be salvaged by a technique called brush anodizing. Brush anodizing can be thought of as a portable anodizing operation and has been characterized thusly: "If you can't bring the part to the anodizing tank, bring the tank to the part." This process is often used to touch up hardcoat or other types of anodic finishes in the aircraft, aerospace and other industries. For example, a hardcoated pneumatic or hydraulic cylinder on a aircraft that has been scratched or gouged may be brush anodized to keep the finish continuous over the part in order to prevent corrosion in the scratched area or to restore the hard wear surface. Aircraft landing gear surface finish repair is one important application of brush anodizing, but there are many other applications as well.
In brush anodizing the surface around the area to be repaired is masked off. The repair area is then etched to remove the anodic coating. This is done so that a "feathered" edge, which is very likely to occur at the edges of the scratched or worn repair area, will be eliminated. If the "feathered" edge were allowed to remain, the repaired anodic coating would be thinner in this area. The etch solution is flushed away. A tubular cathode rod with many holes in its wall is placed very close to the area to be anodized. Electrolyte is then passed through the cathode and allowed to flood the area. An apparatus to keep the electrolyte cool is also part of the system. The part being repaired becomes the positive, or anode, side of the electrical circuit. In this way the repair area can be anodized to the same coating thickness as the adjacent area of old anodic coating and the repair has been accomplished.
More information on brush anodizing, and brush anodizing services, can be obtained through Mr. Gary Torgerson at Brush Plating Specialties, 760-727-3656.
What is an alkaline etch?
Etching aluminum in sodium hydroxide is by far the most common pretreatment for aluminum that is to be anodized. It imparts a matte finish to the end product. Alkaline etches are also used to strip off anodic coatings. However, under no circumstances should an etch-only finish be used in a finish product because there is no way to prevent the rapid onset of corrosion.
I play paint ball and would like to anodize my gun. It is aluminum and I read an article that you can anodize at home with battery electrolyte and battery charger. If there is any information you can give me, I would greatly appreciate it.
For safety reasons, the Council does not advocate "at home" anodizing, and cannot provide the requested information.
I am interested in setting up a small anodizing business. Do you have any information available?
There are a couple of good, general reference books pertaining to anodizing, either of which may prove useful.
The Technology of Anodizing Aluminium by Arthur W. Brace is published by Interall Srl. The hardcover Third Edition contains 350 pages and includes numerous photographs and diagrams. Copies are available through the Aluminum Anodizers Council publications order form ($US295 for members; $US350 for nonmembers) or through the publisher at Interall Srl, Via Marinuzzi, 38, 41100 Modena, Italy; telephone +39-059-282390, fax +39-059-280462, e-mail firstname.lastname@example.org, website www.interall.it.
The Surface Treatment and Finishing of Aluminum and its Alloys by P. G. Sheasby and R. Pinner is co-published by Finishing Publications Ltd and ASM International. The sixth edition includes CD-ROM and two-volume set of books totaling 1,387 pages with illustrations and tables. The first volume deals with mechanical surface treatments, electrolytic and chemical polishing, cleaning and etching, conversion coatings and decorative and protective anodizing. The second volume covers architectural applications, hard anodizing, coloring and sealing of anodic oxide, properties and tests of anodic oxide, organic finishing, vitreous enameling and effluents. Appendices address aluminum alloys and finishing specifications. At last report, the set sold for $425.00 to nonmembers of ASM and $375.00 to members. Vol. 1 ISBN #0-904477-21-5; Vol. 2 ISBN #0904477-22-3; CD ROM ISBN #0-904477-23-1. Available from www.asminternational.org or ASM International, Materials Park, OH 44073-0002; telephone 800/336-5152.
Can steel and/or stainless steel nuts and bolts be anodized?
Steel and stainless steel can't be anodized; the process baths used to anodize aluminum would attack and dissolve steel parts.
Has anyone ever tried to anodize an ASTM A463 Type 1 hot dip aluminum coated steel?
Problems will arise if there is any area where the steel under the aluminum coating is exposed to the electrolyte: the current will flow from that area and not anodize the aluminum. Such problems occur where pin-hole areas in the aluminum coating do not cover the steel. If the steel is completely coated, the aluminum coating can be anodized successfully.
I am trying to glue a rubber grommet to clear anodized aluminum. I am using an industrial grade instant adhesive on 6061-T6 alloy. The grommet is acting as a mechanical pivot so there is some amount of force, but not a lot. I am having difficulty getting the adhesive to stick. Do you think it is the adhesive I am using, or does the sealing of the anodic coating reduce the adhesion? Could the anodize coating be coming off?
It's probably not the fault of the adhesive and, no, the anodizing is not coming off. You have most likely touched on the problem in your question. The answer to your problem can be applied across any number of situations involving the adhesion of bonding agents or organic coatings (paints) over anodized aluminum. Anodizing can be an excellent surface for these applications, but the anodizing must be done with this in mind. The solution to your problem involves the method of rinsing and sealing of the anodic oxide after anodizing.
It is quite common to seal anodic coatings on so-called "proprietary" solutions that contain certain wetting agents (surfactants). This is done primarily to help prevent the formation of smut on the surface of the part. Smut detracts from the appearance of the product and makes it look dirty or hazy. If it is known that the anodic coating is to be used as a base for paint, or that adhesives are going to be used (caulking around windows in an architectural application, for example), the anodized parts may be sealed in either near-boiling deionized (DI) water or a dilute solution of commercially available nickel acetate. Sealing with room temperature nickel fluoride is also acceptable in this case. All three of these methods are free of surfactants. It also helps if the parts can be thoroughly rinsed in clean DI water before and after the sealing step. This will give a clean, "non-slippery" surface (no wetting agents) to which paint and most adhesives will bond. (Anodized aluminum that is to be painted is sometimes left unsealed altogether.) It would also be advisable to prime the anodized surface prior to applying the adhesive by wiping with a highly volatile solvent such as methyl ethyl ketone (MEK) or acetone to remove all dirt, fingerprints, and other possible contaminants.
Of course, you will still have to determine, by testing, which adhesive will give the best service for your application.
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Disclaimer: The information and recommendations provided are believed to be accurate and reliable. It should not be assumed, however, that all responsive information has been provided, or that additional information may not be relevant under certain circumstances or conditions. AAC assumes no responsibility or liability for the use or misuse of any information, materials, processes or techniques described, and it makes no warranty, guarantee, or representation whatsoever as to the absolute validity or sufficiency of any information provided. AAC does not "approve" or "endorse" any specific products, methods, or sources of information. This information should not be referenced in any way which would imply such approval or endorsement.