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Aluminum Alloys Preferred for Anodizing

Higher purity alloys are always preferred for anodizing. Alloying elements such as copper and silicon do not anodize and leave microscopic voids in the aluminum oxide film. (See Anodizing Assemblies with Other Metals?) Since the anodizing process converts only the aluminum to aluminum oxide to form the anodized finish, higher purity aluminum will yield a denser and harder layer of aluminum oxide. High concentrations of some alloying elements will also affect the surface finish and color of the anodized finish and will reduce the effectiveness of the sealing process causing reduced corrosion and wear resistance and decreasing fade resistance in dyed parts.

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Anodizing Wrought Alloys and Extrusions 

The most popular alloys used for anodizing are 5000, 6000 and 7000 series alloys. These alloys will provide consistently excellent quality finishes for hardcoat and conventional anodizing. High purity alloys like 1100 and 3000 series will also form very good finishes. Although 2000 series alloys are popular alloys because of their strength and machining characteristics they are not the best choice for good anodized finishes because of their high copper content.  It has all of the disadvantages noted in the above paragraph. Alpha Metal Finishing Co. regularly provides acceptable anodized finishes for parts made from 2000 series alloys, however a superior anodized finish will be obtained with higher purity alloys.

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Anodizing Castings

Cast aluminum parts in general will not anodize as well because of their tendency toward porosity. Pores do not anodize and contribute to the same type of problem that highly alloyed aluminum parts encounter. Good high-density castings without porosity will anodize with good results. It should be noted that in color anodized castings some difference in color will exist between machined surfaces and the natural as cast surfaces. Some of the better casting alloys are 514, 518, 520, 535, 713, and 771. Again the same principle applies: higher purity aluminum, (less alloys), anodize better, producing better finishes.

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Dimensional Growth During Anodizing

As previously mentioned, anodizing is the process of electrochemically converting the surface of an aluminum part to aluminum oxide. Aluminum oxide occupies about two times the volume as that of raw aluminum. Therefore anodizing will cause parts to grow dimensionally. This factor should be taken into consideration when designing parts that will be anodized.

Typical standard clear and color anodizing creates an aluminum oxide film in the range of .0002 to .0008 inches, (.005 to .020 mm), on each surface. Hard anodizing is typically in the range of .0005 to .003 inches, (.013 to .076 mm), the most common being .002 inches. (.051 mm).

The process of hard anodizing a part to .002 in. film thickness will therefore grow .001 in. on each surface or .002 in. in overall dimension.

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Effects of Sharp Edges  and Corners on Anodizing

During the anodizing process, the aluminum oxide film grows perpendicular to the surface of the part. Because of this there will be less coverage on the sharp corners of parts leaving a thin spot in the anodic coating. That thin spot will then be a site of low dielectric strength and reduced corrosion resistance. In the case of parts being hard anodized, because it is a site of low electrical resistance, an increased amount of current will flow at that spot, frequently causing the part to overheat and produce a discoloration or “burn”. In some cases the temperature becomes so hot that the anodic coating and the underlying aluminum are actually dissolved in the sulfuric acid electrolyte and large amounts of material are removed from the part.

This phenomenon very rarely causes a problem in conventional anodizing. However, in the hardcoat anodizing process where power densities can approach 3000 watts per square foot, sharp corners or points can initiate a “burn” which can destroy portions of the part. Being aware of this potential problem, Alpha Metal Finishing has developed procedures and computer control technology that will minimize this possibility. However it is much better practice to design parts without sharp points or edges.

A different but equally undesirable phenomenon occurs with sharp inside corners. The two surfaces growing perpendicular to each other impinge upon each other at the corner creating a seam. The seam is not contiguous like normal anodizing and can expand and contract with temperature allowing the environment access to the base aluminum at its root. MIL-A-8625 recommends a minimum radius of .030 in. on both inside and outside corners. 

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Anodizing Aluminum Assemblies

Aluminum parts that have been assembled together will anodize successfully. However the designer should be aware that lap joints such as spot welds and joints that are not completely watertight can become traps for cleaning agents and sulfuric acid. In some cases it is impossible to completely rinse all of the sulfuric acid from the joint. If this happens, the joint then becomes a site of increased corrosion potential, and in the case of color anodized parts trapped acid, can cause the color to become bleached out during the process. Alpha Metal Finishing has developed processes and procedures to minimize this problem. However, as above, it is much better practice to consider this potential problem during the design phase of your product and plan to anodize before assembly if at all possible.

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Anodizing Assemblies with Other Metals

Anodizing is similar to electroplating inasmuch as both processes use electrical current and an electrolyte. However, in the anodizing process the part is made the anode, thus the name “anode-izing”. The anode in an electroplating cell provides the supply of metal ions to the plating bath. Any metal that is anodic in the cell is sacrificed and is plated onto the cathode. Under normal anodizing conditions the anodic aluminum part does not dissolve, but oxidizes, forming the anodized coating. However, most other metals such as steel or copper that are attached to the aluminum part will plate off and effectively disappear.

We highly discourage attempts to anodize parts with any non-aluminum inserts or attachments. Although they could conceivably be masked, even a small imperfection in masking will guarantee the disappearance of any non-aluminum parts from the assembly and could damage the part due to the extra heat generated.

The best practice is to plan to assemble bushings and other non-aluminum parts after you anodize them.

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MSDS and FDA approval 

There is no MSDS for anodizing.  It is not hazardous material and would therefore not require one. In fact, it is not even a material. Anodizing is a process used to form aluminum oxide by electrochemically oxidizing the surface of an aluminum part in a sulfuric acid electrolyte.

Likewise there is no FDA "document” on anodizing.  An anodized aluminum surface is composed of aluminum oxide and some aluminum sulfate.  Aluminum sulfate is on the FDA GRAS list section 182.1125.  In addition all of the aluminum compounds listed by the Select Committee on GRAS Substances are considered GRAS with no reservations.  Aluminum oxide is not listed however it is chemically inert and probably the least reactive of all aluminum compounds.

Anodized aluminum is considered “generally recognized as safe” (GRAS) by the FDA but it is not on any of their published lists. However, a search of their website will find numerous anodized food processing and biomedical products that have been approved.  The following is a quote from a letter, dated June 16, 1994 to Anacote Corporation Long Island City, NY "... FDA considers anodized aluminum which is processed and handled in a manner consistent with good manufacturing practices for food contact articles as generally recognized as safe (GRAS)..." The FDA website states; “The use of a substance is GRAS (generally recognized as safe) because of widespread knowledge among the community of qualified experts, not because of a listing or other administrative activity.” http://www.cfsan.fda.gov/~dms/grasguid.html

In the 1940’s and 50’s you could buy butter in anodized dishes and other food products in anodized tumblers. A variety of hardcoat anodized fry pans and other cookware are currently on the market and we have anodized instruments for laparoscopic surgery, parts for endoscopes, heart pumps, and food processing equipment for many years. 

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