Anodizing...
"Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. Anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors."
"Aluminum alloys are anodized to increase corrosion resistance, to increase surface hardness, and to allow dyeing (coloring), improved lubrication, or improved adhesion. Anodized coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminum. As a result, the coating will crack from thermal stress if exposed to temperatures above 80 °C. The coating can crack, but it will not peel. The melting point of aluminum oxide is 2050 °C, much higher than pure aluminum's 658 °C. (This can make welding more difficult.)"
The Process...
"The anodized aluminum layer is grown by passing a direct current through an electrolytic solution, with the aluminum object serving as the anode (the positive electrode). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminum anode, creating a build-up of aluminum oxide. Alternating current and pulsed current is also possible but rarely used. The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V. Higher voltages are typically required for thicker coatings formed in sulfuric and organic acid. The anodizing current varies with the area of aluminum being anodized, and typically ranges from 0.3 to 3 amperes of current per square decimeter (20 to 200 mA/in²)."
Sulfuric Acid Anodizing...
"Sulfuric acid is the most widely used solution to produce anodized coating. Coatings of moderate thickness 1.8 μm to 25 μm (0.00007" to 0.001") are known as Type II, as named by MIL-A-8625, while coatings thicker than 25 μm (0.001") are known as Type III, hardcoat, or engineered anodizing. Very thin coatings similar to those produced by chromic anodizing are known as Type IIB. Thick coatings require more process control, and are produced in a refrigerated tank near the freezing point of water with higher voltages than the thinner coatings. Hard anodizing can be made between 25 and 150 μm (0.001" to 0.006") thick. Anodizing thickness increases wear resistance, corrosion resistance, ability to retain lubricants, and electrical and thermal insulation. Standards for thin sulfuric anodizing are given by MIL-A-8625 Types II and IIB, AMS 2471 (undyed), and AMS 2472 (dyed). Standards for thick sulfuric anodizing are given by MIL-A-8625 Type III, AMS 2469, BS 5599, BS EN 2536 and the obsolete AMS 2468 and DEF STAN 03-26/1"
See Wikipedia, Anodizing, http://en.wikipedia.org/wiki/Anodized_aluminum (As of Dec. 3, 2008, 16:30 EST)
"Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. Anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors."
"Aluminum alloys are anodized to increase corrosion resistance, to increase surface hardness, and to allow dyeing (coloring), improved lubrication, or improved adhesion. Anodized coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminum. As a result, the coating will crack from thermal stress if exposed to temperatures above 80 °C. The coating can crack, but it will not peel. The melting point of aluminum oxide is 2050 °C, much higher than pure aluminum's 658 °C. (This can make welding more difficult.)"
The Process...
"The anodized aluminum layer is grown by passing a direct current through an electrolytic solution, with the aluminum object serving as the anode (the positive electrode). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminum anode, creating a build-up of aluminum oxide. Alternating current and pulsed current is also possible but rarely used. The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V. Higher voltages are typically required for thicker coatings formed in sulfuric and organic acid. The anodizing current varies with the area of aluminum being anodized, and typically ranges from 0.3 to 3 amperes of current per square decimeter (20 to 200 mA/in²)."
Sulfuric Acid Anodizing...
"Sulfuric acid is the most widely used solution to produce anodized coating. Coatings of moderate thickness 1.8 μm to 25 μm (0.00007" to 0.001") are known as Type II, as named by MIL-A-8625, while coatings thicker than 25 μm (0.001") are known as Type III, hardcoat, or engineered anodizing. Very thin coatings similar to those produced by chromic anodizing are known as Type IIB. Thick coatings require more process control, and are produced in a refrigerated tank near the freezing point of water with higher voltages than the thinner coatings. Hard anodizing can be made between 25 and 150 μm (0.001" to 0.006") thick. Anodizing thickness increases wear resistance, corrosion resistance, ability to retain lubricants, and electrical and thermal insulation. Standards for thin sulfuric anodizing are given by MIL-A-8625 Types II and IIB, AMS 2471 (undyed), and AMS 2472 (dyed). Standards for thick sulfuric anodizing are given by MIL-A-8625 Type III, AMS 2469, BS 5599, BS EN 2536 and the obsolete AMS 2468 and DEF STAN 03-26/1"
See Wikipedia, Anodizing, http://en.wikipedia.org/wiki/Anodized_aluminum (As of Dec. 3, 2008, 16:30 EST)