1.1 - What is Anodizing?

Anodizing is a chemical process that accelerates and controls the formation of an oxide coating on an aluminum substrate or machined part. The process consists of making the aluminum the anode (+) in a suitable acid and water electrolyte where the walls of the chemical container act as a cathode (-). When electrical DC or AC/DC pulse current is applied to the electrolyte, oxygen atoms (-) are aggressively attracted to the anode (aluminum part) to form an aluminum oxide film at the surface. The coating formed is an integral part of the substrate and it rises both above and below the original surface (as illustrated below). Aluminum oxide has a hexagonal structure with an open pore at the center. The closing of this pore with a chemical such as nickel acetate is referred to as Sealing; whereas the filling of this pore with organic dye or metal to reflect light is called Coloring. Aluminum oxide is the 3rd hardest element known to mankind, but its characteristics can be influenced by the choice of the electrolyte as well as temperature.

Sulfuric Acid
MIL-A-8625 Type II
A sulfuric acid electrolyte is the dominant form of anodizing for commercial and industrial products. A common commercial coating would be applied at 72°F for 25 minutes to achieve .0003/5 inch thickness of aluminum oxide, making the substrate electrically non-conductive (all aluminum ladders are anodized as a safety precaution against electrical shock). The coating color is a light opaque gray.

Dyes
The aluminum oxide is an amorphous coating (like a sponge) when first formed so that organic or inorganic colored dyes including black, red, blue, green & gold, etc., can be absorbed immediately after processing. For maximum absorption, the coating thickness is usually in the range of .0007/8 inches.

Two Step
And we don’t mean the Country & Western dance. After anodizing, metal such as tin can also be electrolytically deposited into the pores of the oxide formed to acquire popular color shades ranging from light bronze to brown. This is referred to as architectural coloring and is most often seen on aluminum window frames.

Boric-Sulfuric Acid
MIL-A-8623 Type IC
The boric-sulfuric acid process has primary use in industries where structural aluminum will carry loads that may cause metal fatigue, as in the transportation and aerospace industries. Boric-sulfuric acid electrolyte produces thin, opaque, slightly iridescent coatings that are much thinner than those produced in the sulfuric electrolyte solutions. The boric-sulfuric acid process forms an oxide coating that has low resistance to abrasion, but a high degree of flexibility to act as an excellent base for paint.

Hardcoat
MIL-A-8625 Type III
Hardcoating is often referred to as hard anodizing. It is similar to conventional sulfuric anodizing, but with lower temperatures. It is most often applied near 40°F for 60 minutes, along with higher amperage and voltage. Sometimes an additive such as oxalic acid is added to increase hardness and conserve energy. Hardcoat has a high resistance to abrasion and corrosion, with thicknesses ranging from 0.0005 to 0.002 inches or more. This type of finish is used for lightweight parts requiring strength and high dielectric resistance. Eventually sealed with hot water, nickel acetate, or sodium dichromate, these hard coatings achieve a highly functional yet attractive finish for industrial OEM equipment. The anodized hard surface can be formed on all alloys (except those with more than 5% copper). However, the more pure the alloy, the harder the coating. Thus, the 1,000, 3,000, 6,000 and 7,000 series alloys will form better and harder coatings than the 2,000 series. Engineers and machinists will often choose alloys because of their machining ability and tensile strength, but these are not always the most “hard coat friendly."

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