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Frequently Asked Questions

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Frequently Asked Questions

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.

General Questions

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.

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.

Because anodizing is such a versatile process there are thousands of different applications. Just to name a few, these include:

  • Architectural products like windows and doors
  • Appliances
  • Automotive
  • Lighting
  • Food preparation equipment
  • Furniture
  • Sporting goods
  • Marine

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.

Review the "What is Anodizing" section on this website. Contact AAC if you require additional information. AAC also provides workshops and seminars for anodizers.

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.

Yes, both hardcoat and conventional anodized products are often used for aluminum cookware.

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").

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.

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.

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.

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.

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.

For safety reasons, the Council does not advocate "at home" anodizing, and cannot provide the requested information.

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 interall@tin.it, 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.

Steel and stainless steel can't be anodized; the process baths used to anodize aluminum would attack and dissolve steel parts.

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.

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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Types & Coloring Questions

To our knowledge, there is no commercial process for zinc anodizing. While it is theoretically possible that a thin anodic oxide layer could be produced on zinc, it likely would be too thin to impart any of the beneficial surface properties we see with an aluminum oxide. No other metal has the anodizing characteristics of aluminum.

Please define Class I and Class II Anodic Coatings

Class I and Class II anodic coatings are designations created by the Aluminum Association for the purpose of codifying the specification of anodized aluminum. The American Architectural Manufacturers Association (AAMA) publication, AAMA 911-92, Voluntary Standards for Anodized Architectural Aluminum, includes the following definitions from The Aluminum Association publication #45, Designation System for Aluminum Finishes:

Class I. High performance anodic finishes used in exterior applications receiving periodic maintenance such as curtain walls. Minimum coating thickness of 0.7 mil (18 microns).

Class II. Commercial anodic coatings used in interior applications or exterior applications receiving regularly scheduled cleaning and maintenance such as store fronts. Minimum coating thickness of 0.4 mil (10 microns).

The above publication goes on to call out the designations of other more specific coatings and methods of pretreatment of the aluminum prior to anodizing.

Coating thickness can be measured by an "eddy current," nondestructive test instrument as per ASTM B 244-79, or by cutting a cross-section of the anodized aluminum, mounting it in a slide, polishing the edge, and reading the coating thickness directly with a microscope as per ASTM B 487-85 (1990).

Class I and Class II coatings should not be confused with Type I, Type II and Type III anodic coatings as described in the authoritative anodizing standard, MIL-A-8625. Type I anodize refers to chromic acid anodizing. Type II is nominal "clear" sulfuric acid anodizing. Type III is "hardcoat" using sulfuric acid or mixed chemistry electrolytes.

Various publications on both aluminum and anodizing from the above organizations may be obtained from:

The Aluminum Association, Inc.
900 19th St., N.W.
Suite 300
Washington, D.C. 20006
202-862-5100
Fax 202-862-5164
www.aluminum.org

AAMA
1827 Walden Office Square
Schaumburg, IL 60173-4628
847-303-5664
Fax 847-303-5774
www.aamanet.org

There are several organic dyes that can be used to achieve a black color; these dyes are available to anodizers from several suppliers. Chemical suppliers can offer advice regarding the specific application. Dye selection depends upon alloy and application. What works well for exterior may not yield good results for interior applications.

Another means is an electrolytic coloring, a post-anodic coating procedure where AC current is applied to the anodized part in an electrolyte consisting primarily of tin sulfate, sulfuric acid, and various stabilizing agents. This may not produce the "deepest" blacks, however. Choice of alloy, coloring media, and coating thickness may affect the depth or intensity of color.

The surface pretreatment is also important ... the eye will perceive a "darker" black on a smooth (buffed) surface than on a heavily etched surface.

There are many different types of anodizing. When we talk about anodizing, we generally are talking about sulfuric acid anodizing that produces a porous oxide layer. There are other processes where other electrolytes are used such as chromic acid, phosphoric acid and sulfuric-boric acid. Using these different electrolytes produces oxide layers with different properties than those produced with sulfuric acid. In addition, process conditions such as temperatures can be changed to produce very hard oxide coatings.

There are four ways to color aluminum:

  1. Dye: The freshly anodized part is immersed in a liquid solution that contains dissolved dye. The porous anodic coating absorbs the dye. The intensity of color is related to the thickness of the anodic film, the dye concentration, immersion time and temperature, among other things.
  2. Electrolytic Coloring (a.k.a. "two-step"): After anodizing, the metal is immersed in a bath containing an inorganic metal salt. Current is applied which deposits the metal salt in the base of the pores. The resulting color is dependent on the metal used and the processing conditions (the range of colors can be expanded by overdyeing the organic dyes). Commonly used metals include tin, cobalt, nickel, and copper.
  3. Integral Coloring: This so-called one-step process combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades while more abrasion resistant than conventional anodizing.
  4. Interference Coloring: An additional coloring procedure, recently introduced, involves modification of the pore structure produced in sulfuric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces light-fast colors ranging from blue, green and yellow to red. The colors are caused by optical interference effects, rather than by light scattering as with the basic electrolytic coloring process.

Generally yes. It is important to remember that color is not defined by simply "light" and "dark." Like the controls on a conventional TV set, the hue and chromaticity will have an impact on appearance as well as light-to-dark. Color is at least a three dimensional thing, and cannot be adequately represented on a one dimensional light-to-dark axis. It may be impossible for the color of a flat sheet to perfectly match an extrusion anodized at the same time because the color of the metal may be fundamentally different. Range samples are good for adjusting expectations and for representing the amount of color variation that one can expect. However, if representing the exact color is important, then one must make samples from the exact lot of material you expect to use. When striving for uniformity, it is similar to buying fabric or carpet because you want all the material to come from a single lot.

The answer is relative. Anodizers have made incredible improvements in controlling the color of anodized aluminum. The color consistency is comparable to metallic paints.

Bright dipping is a process for increasing the specularity or brightness of aluminum by leveling the microscopic roughness or "peaks and valleys" on the surface of the aluminum. The process can not do macropolishing or smoothing of scratches or pits. This limitation underscores the need for careful handling prior to bright dipping.

Most commercially available bright dip baths consist mainly of phosphoric and nitric acid. Additives are introduced to reduce nitrogen oxide fumes and to enhance the brightening ability of the bath.

This can be a vague term, but usually hardcoat refers to a very thick and hard anodic coating. This kind of anodizing is accomplished with a bath similar to the standard sulfuric process, but with the temperature reduced to about 0°C to slow the dissolution rate. A higher voltage is applied to enable the coating to continue to build after the insulation value of the coating starts slowing down the coating formation.

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Specifications & Alloy Questions

Mechanical properties of aluminum, such as ultimate strength and yield strength, along with percent elongation, reportedly have been tested before and after anodizing without exhibiting any difference in the strength of the aluminum. Aluminum is a very soft metal and if not protected can be abraded away. The anodic oxide is much harder that the aluminum and the anodize process is used on parts like pistons so that the parts will last much longer.

Alloy 380 is a high-silicon alloy (about 8 percent). The solubility of silicon in aluminum is about 1.5 percent. When the casting cools, silicon precipitates out and forms these gray mottled areas. Silicon does not anodize, so the adhesion around these areas is likely to be poor. To minimize this problem, the surface finisher should pretreat the casting in a nitric acid/ammonium bifluoride solution under well-ventilated conditions. Doing so will effectively dissolve much of the surface silicon and thereby enrich the surface with anodizable aluminum.

The best test for lightfastness is to expose production color anodized panels outdoors, facing south, positioned on a rack that is tilted 45 degrees, in an area that is not shaded. Usually after a month or so the exposed panels can be compared to sections of the same anodized panels that were retained indoors to get an idea of how well they will perform.

Alternatively, a bench-top accelerated exposure device (approximate cost $10,000) or a full-size carbon-arc or xenon-arc light exposure apparatus (approximate cost $40,000) will yield information in a shorter period of time. The latter provides lightfastness data that can be referenced to military specification MIL-8625F, which calls out 200 hours' exposure. It has been found that 200 hours in this unit correlates to about 80 days' exposure to natural light and weathering, based on an annual worldwide average. Weathering services are also available (e.g., Atlas Weathering Services, 800/255-3738).

Information on light-fastness can usually be obtained from the dye supplier. Customers' needs and product applications should be discussed with the dye supplier before tests are run. Light-fastness of an anodized part depends on many factors--not just the choice of dye. Equally important are the porosity and thickness of the anodic film, the degree of saturation, as well as the type and quality of the seal.

There are a number of different items that can be specified when ordering anodized aluminum. What you specify depends upon your product requirements. Some of the most common items that are specified are as follows:

  • Thickness or weight per area of the anodized coating
  • Color of the anodized coating
  • Gloss of the finish
  • Quality of the seal of the anodized coating
  • Corrosion resistance measured by salt spray testing
  • Aluminum alloy used and its temper

Most aluminum alloys will build aluminum oxide in an anodizing tank, so the answer to this question depends on the anodizing process and the desired result. Copper containing 2000 series are generally the most difficult to anodize and 5000 or 6000 series are the easiest.

Castings are challenging to anodize because they are often porous. The alloy preferred for anodizing castings is 518. C443 is also good, but it is not inherently corrosion resistant. These are also the alloys preferred for painting since paint pretreatment will attack a poor casting similar to anodizing chemicals.

You can specify a range of coating thickness from 0.00001" to 0.005" based on what your product is to be used for and how you want it to look. In general, thicker coatings are used for products to be used outside or in corrosive environments, and thinner coatings are used for parts to be used in interior applications. When anodized products are to be used out of doors, anodized film thickness is usually specified at 0.000400" minimum or 0.000700" minimum. Parts used in automotive applications are usually specified at 0.000300". Parts used in interior applications are usually specified between 0.000100" and 0.000350". There are some applications where coating thickness of 0.00001" is used. Hardcoat anodizing, commonly used in high abrasion applications, may range from .001" to .005" in thickness.

Example

Inches 0.001"
Mils 1.0 mil
Microns 25.4 µ

0.001" is read as one thousandth of an inch
0.0001 is read as one ten thousandth of an inch
1.0 mil is also read as one thousandth of an inch
0.1 mil is one ten thousandth of an inch
25.4 µ is read as 25.4 microns

Equivalents:
0.001" = 1.0 mil = 25.4 µ

For a Class I coating of 0.7 mil the equivalents would be:
0.7 mil = 0.0007" = 17.78 microns

 

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Quality Issue Questions

Many factors influence the appearance of anodized aluminum architectural components. Many architects and designers find that to be one of the attractive features of anodized aluminum: their buildings assume a slightly different character depending upon the weather, the time of day, the season of the year, or the angle of observation.

Differences in appearance will also be influenced by the alloy, anodic film thickness, surface texture of the aluminum from the mill supplier (mill finish surface roughness, longitudinal versus transverse surface roughness), and the process of the anodizer (clean and anodize, etch and anodize, bright dip and anodize, and combinations). Critical to all this is the angle of observation, especially as it relates to the primary source of illumination.

If the product is colored, another level of complexity is added, since the shade and hue must also be controlled. Furthermore, if the material is being used as a light reflector, very specific photometric specifications are usually required.

Anodized aluminum is very suitable for applications involving exposure to sea water and is routinely used for parts such as masts. Corrosion resistance is good; however, the parts should be kept clean. The buildup of dirty surface deposits can provide sites for corrosion, particularly if there is associated acidity. Coloring using dyes may be a problem because many dyes are susceptible to the effects of sunlight and may fade or change color.

Generally speaking, organic dyes are well suited to indoor applications and have enjoyed such use for over 50 years. Their light-fastness, however, depends on the dye chosen, the amount of dye that is actually contained by the coating, and the conditions of seal.

Two-step electrolytic coloring uses metallic salt solutions to produce bronze or black colors that are sufficiently light-fast for exterior, as well as interior, applications. Colors produced using organic dyes are often susceptible to change due to the effects of ultraviolet (UV) rays from sunlight. However, they should be satisfactory in indoor applications where they are not exposed to sunlight.

Some organic dyed anodized aluminum can show significant color change within one year of outdoor exposure. If we assume that is 10 hours exposure to sunlight per day, then indoor finishes may be able to withstand one hour of sunlight per day for 10 years.

The purple hue is an iridescent color--the result of a very thin film on the surface of the black anodized part. Other iridescent colors can be produced, and can appear even on clear anodized parts, but are most evident on black anodizing. An analogy is a black asphalt road after rain. A droplet of oil causes iridescent rings that are quite apparent on black asphalt but can hardly be seen on a light-colored concrete road.

There are a number of possible causes of the film. One is sealing smut. Hot water sealing blocks the pores of the anodic coating, thus improving its weathering resistance, reducing its adsorptive properties, and sealing in any pigment. However, some sealing product forms on the outer surface of the anodic coating. This is sealing smut. It consists of very small, needle-like crystals of aluminum hydroxide. They act as a thin film that can generate iridescent colors. Anti-smut additives for sealing baths are available, which minimize the problem. The formation of smut can be favored by high pH. The bath chemistry should be controlled as recommended by the supplier of the sealing additive. Alternatively, the smut can be wiped off. Before the invention of anti-smut additives, people used to wipe down parts with lanolin in white spirit. This temporarily masks the smut as much as it removes it. Manual removal is time-consuming and can be hard work.

Anodizing under conditions that are too aggressive can lead to iridescent effects. In particular, if the bath temperature is too high, the surface region of the porous anodic coating (the part of the coating that was first formed during anodizing) can be dissolved in the acid solution to the extent that it is very much more porous. This is called a "soft" coating. In general, anodizing at over 75°F for more than 45 minutes can produce a soft coating. A simple abrasion test to detect a soft coating is described in British Standard 6161: Part 18: 1991.

Also, purple iridescence has been seen with certain types of black dyed parts if the film thickness is too low or the dye bath is inadequately controlled. The advice of the supplier of the dye should be followed.

This discoloration could be weathering bloom, chalking, or chemical attack.

Weathering bloom may occur when sealing is inadequate, and the surface of the anodized coating has become etched during weathering. Acidic pollutants in the environment promote this effect.

Chalking may occur when the anodizing quality is not high enough. For instance, anodizing at too high a temperature can result in "soft" coating, which, when exposed to the atmosphere reacts a bit like mud, cracking and swelling with variations in humidity. Sometimes this leads to the development of iridescent colors, particularly on dark bronze or black anodized parts. Eventually, the mud-cracked layer spalls off, and then the degradation of the underlying layers commences in a similar manner.

It may be possible to restore the original appearance by an abrasive cleaning using a pumice powder or a fine grade of synthetic abrasive (e.g., "Scotchbrite"). Coarser abrasives such as sandpaper or steel mesh should be avoided, along with acid or alkaline cleaners, any of which would damage the anodized coating.

It is worth noting that the degradation of anodized coatings can be reduced substantially by regular washing with water containing a suitable wetting agent or a mild soap solution followed by thorough rinsing.

Chemical attack occurs when acid or alkaline materials come in contact with the anodized finish. The most common occurrences are when mortar or muriatic acid (used by masonry trades) are allowed to dwell for a time on a window or other anodized aluminum building component. Once the finish is visually affected, irreversible damage has occurred and the discolored item must be re-anodized or replaced.

There are many different tests used to evaluate anodized finishes. Some of the most commonly used tests are as follows:

American Society for Testing and Materials (ASTM)

  • B136-84 (2008)e1 – Standard Method for Measurement of Stain Resistance of Anodic Coatings on Aluminum
  • B137-95 (2004) – Standard Test Method for Measurement of Coating Mass Per Unit Area on Anodically Coated Aluminum
  • B244-09 – Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments
  • B680-80 (2004) – Standard Test Method for Seal Quality of Anodic Coatings on Aluminum by Acid Dissolution
  • B117-07a – Standard Practice for Operating Salt Spray (Fog) Apparatus

These are just a few examples; many other standard test methods may be used to test anodized aluminum.

Electrical contact must be made to each part that is anodized. The more electrical current required, the bigger the electrical contact must be. The size of the contact therefore depends on the anodizing process and the size of the part being anodized.

The usual rule of thumb is that if you can feel a scratch by rubbing your fingernail across the surface, you will be able to see the scratch after anodizing. It is always helpful for the finisher to understand the application. It is also good for the finisher and client to agree on a viewing distance. If a part is to be viewed from 10 feet away, like a window or roofing component, then the inspection may be relatively insensitive to scratches. However, if the part is to be viewed from 24" or closer, then even a scratch which you cannot feel may be unacceptable.

Parts can be welded prior to anodizing. The use of 5356 welded rod is strongly recommended, though some discoloration will still occur. 4043 is the worst choice because it will turn a smutty black when anodized. Grinding away the weld before anodizing will result in decreased mechanical integrity and will not solve the appearance variation problem.

It is not a good idea to weld after anodizing. Because most welding process require electrical conductivity the anodic coating must be ground away where the weld will be applied. This normally results in an unsightly mess around the welding area.

Welds can discolor for a couple of different reasons. First, the metallurgy of the welding wire is different than that of the alloy being welded. Since the finish produced in the anodizing process is somewhat dependent on the metallurgy, the metallurgical difference will show up as a shade difference. Secondly, during the welding process a significant amount of heat is built up around the weld. This heat buildup actually changes the temper of the aluminum immediately surrounding the weld bead. Since a temper change is really a metallurgical change, again this shows up after anodizing as a color difference. These areas are commonly called halos or ghosts.

There are a few things that can be done to minimize the color differences. First, excellent results may be produced by using welding wire alloy 5356. This alloy reportedly produces the best color or shade match when used to weld 6xxx series alloys. The second problem, concerning halos or ghosts, is a little more difficult to solve. Try using as little heat as possible to accomplish the job. This can be something of an art and is dependent on the individual doing the work. Another possibility is to put the aluminum that is being welded in contact with a chill block that will draw the heat away from the working area. One other note: Some people grind the weld bead smooth and mechanically finish the weld area in an attempt to avoid the color difference. This practice, however, will not help hide the discoloration.

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Cleaning Anodized Aluminum Questions

Factors such as concentration of the detergent solution, duration of exposure, and temperature will influence the results. If the anodized aluminum is cleaned at room temperature and promptly rinsed with clean water, then there should be no problem. If cleaned at an elevated temperature or with prolonged exposure without rinsing, then the cleaning solution would start to attack the anodic oxide and etch the metal. Mild soap is generally preferable to detergent for routine maintenance cleaning.

Cleaning anodized aluminum is easy with the right technique. Because anodizing is so hard, you want to use an abrasive cleaning technique with a gentle soap. Do not use harsh acidic or alkaline cleaners because they may destroy the finish. Use solvents with care as they may stain the finish. Regardless of the technique, be sure to try a test area first. One recommended technique is to use an abrasive cleaning sponge with mild dish washing liquid. Always try a test small area first to prevent a widespread problem. (For more detailed advice, obtain a copy of Care of Aluminum from The Aluminum Association. )

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