Frequently Asked Questions
about the Anodizing Process
Anodizing is a process for finishing aluminum alloys which employs electrolytic oxidation of the aliuminum surface to produce a protective oxide coating. The anodic coating consists of hydrated aluminum oxide and is resistant to corrosion and abrasion. Conventional coatings are 0.1 to 1.0 mil think and are essentially transparent, although they may be colored.
Because anodic coatings are transparent, ant alteration or improvement of the surface prior to anodizing will carry through the anodizing process. Accordingly, most anodizing process lines are set up with chemical pre-treatments in place and the term “anodizing” has come to include those auxiliary steps.
Anodizing is accomplished by immersing the work in a series of solutions where various operations are performed. The solutions are held in open-top tanks and the work passes through each in turn. The work usually consists of discrete items which are placed on racks and carried through the process in batches. Overhead cranes move the racks of work from one tank to the next. The work is rinsed thoroughly after each operation in order to avoid contamination and interference with the next solution.
The cleaning process is generally accomplished by soaking the work for several minutes in a water-based solution containing acids or alkalines. Alkaline cleaners employ mild alkalies such as sodium phosphates, carbonates, and borates; along with wetters. These products work by emulsifying and saponifying shop and forming oils and are inhibited so that they do not etch the metal. They are used on work that has not yet been buffed.
Pre-treatment is used for decorative purposes to improve the appearance of a surface prior to the anodizing step. The most common pre-treatments are Etch and Bright Dip. Etch is accomplished in a solution of caustic soda (sodium hydroxide) along with additives to improve the quality of the finish. The etch imparts a satin, matte finish by dissolving aluminum from the surface in a microscopically irregular pattern causing reflective light to be scattered. After Etch it is a common practice to dip the metal in an acidic desmut/deoxidizer solution to remove any residue of alloying agents or oxides.
Anodizing is the step which produces the actual coating. It is accomplished in an electrolytic cell using sulfuric acid as the electrolyte. The work is made the positive electrode, or anode (hence anodizing). When direct current (DC) is passed through the acidic solution, water is decomposed liberating oxygen at the surface of the work. The oxygen combines with the aluminum to form the coating which is a transparent and microscopically porous layer of aluminum oxide. The thickness of the coating is a function of the current density (amps/sq ft) and the length of time it is applied. The durability of the coating is, or course, a function of it’s thickness.
Whether it is to anodized or not, all aluminum should be stored to prevent metal-to-metal contact when moisture is present and unsightly “water etch” will result. Minor “water etch” can be removed, but severe cases can not.
Lubricants used in forming and fabrication can also be corrosive to aluminum and cause undesirable etching. Be sure to use lubricants approved for aluminum.
Choose the right alloy. Some alloys and tempers respond better than others to pre-treatment and anodizing. Variations in temper, composition, etc., can lead to differences in color after anodizing. Consult us or your metal supplier for more information. Always use the same alloy and temper throughout any one job. \If you are welding an assembly, be sure to use the proper alloy welding wire. Consult your supplier.
If practical, perform all fabrication work (cutting, welding, bending, grinding, buffing, etc. before anodizing. you will not want to disturb the anodized coating after it is formed. The one exception to this rule may be the drilling and tapping of holes or other operations where exact dimensions are critical. Expect to lose a little metal from all surfaces during anodizing.
Also remember that the coating has a ceramic-like quality in that it cannot be stretched or compressed much. Severe bending of an anodized article will cause the coating in the convex to surface to open up and on shatter or “craze” on the concave surface. Tubes that will be bent after anodizing should receive minimal coating. Severe heating will also cause the coating to craze.
Avoid the use of paints, varnishes, adhesive tapes, etc to avoid residues. We use water-based cleaners which are effective on fab oils and buffing compounds but not on materials that require solvents.
Avoid delays in getting your job to us. Aluminum is an active metal and when unprotected, is subject to damage from fumes, mists, even oily fingerprints.
Yes…to a limited extent, especially if it gets and “etch” pre-treatment. Much minor “noise” in the surface: small nicks and scratches, some die lines, etc. should be diminished, if not removed entirely, in the etch bath.
In most cases, the standard racking method is called 2” Each End for parts over 66”. For parts under 66”, we use a method called End to End and rack each part vertically. Anything other than these are considered Special Racking which usually means racking the unexposed area of the part to hide the rack marks. This can require custom racks be made and can decrease the parts per rack which will increase the racking price. Special racking requirements should be indicated at the time of quoting to provide the most accurate price.
We offer champagne bronze, medium bronze, dark bronze, and black. Keep in mind that with anodize coloring, color intensity (depth, shade) varies with coloring bath chemistry and temperature, coating thickness, coating porosity, immersion time, and several other variables. Therefore, the color from one batch (rack) to the next will seldom be the exact same intensity. A slight variation in anodize color is normal and should be expected. An acceptable color range should be established for each job.
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