And Related Processes
Anodizing usually refers to the electrochemical oxidation of a metal, most commonly aluminum. The part is made anodic (hence the term anodized) in various electrolytes. Some of these include sulfuric, chromic and oxalic acids. Kaehr uses sulfuric/oxalic combination electrolytes. Soft anodize refers to coatings produced in electrolytes that operate at close to ambient temperature. The coatings tend to be more porous than hardcoat anodize, which is produced in electrolytes at much lower temperatures. The hard anodic coatings are also much denser and are thicker, and have high dielectric values. The more porous coatings can be dyed a wide variety of colors, and then sealed to "close" the pores. Since coating is an integral part of the base metal, anodic coatings are very durable and not subject to chipping such as paint might be under harsh service conditions. Technical personnel are always available at Kaehr to answer questions concerning these anodic coatings. The following are the specific types of anodizing that Kaehr Corporation can provide.
Anodizing usually refers to the electrochemical oxidation of a metal in a suitable electrolyte. The item to be coated becomes the anode in the solution, hence the terms anodize. Here only the anodization of the aluminum is discussed. Although there are several chemicals that can be used to create the electrolyte, by far the most common one worldwide uses sulfuric acids in various concentrations. When conditions such as temperature and current density are controlled properly, the surface "grows" aluminum oxide. The buildup for this type of anodizing ranges usually from 0.0002/0.0005 inch, although the coating thickness itself would be double this or 0.004/0.0010 inch. Sometimes the anodic coating is described as one half buildup and one half penetration, although technically the anodic coating does not penetrate but rather builds from the underlying aluminum surface. The film formed is porous (microscopically) and can be dyed a variety of colors or left in the natural condition, For both, the anodic coating must be sealed to "close" the pores to prevent the dye from leaching or other material from staining the surface. One property of the aluminum oxide coating is that it is an electrical insulator so if electrical grounding is desireable then those areas much be masked and a conductive coating such as chromate applied.
The same general theory described for clear and color anodizing applies to hardcoat anodize. However, since the electrolytes are operated at lower temperatures, the chemical attack on the aluminum oxide coating being formed, is less and consequently denser. Harder and thicker coatings can be obtained. Buildups of 0.001/0.0015 inch per surface are common resulting in film thicknesses of 0.002/0.003 inch. These properties result in much higher electrical dielectric values than conventional coatings. The alloying elements in the aluminum tend to make theses coatings naturally much darker than the softer coatings (typically a dark gray for 6061 alloy) so they are not normally used for decorative coatings, but for the other properties listed earlier. Because the coating tends to get darker as the thickness increases, dying is usually impractical for lighter colors, black being by far the most common. To improve chemical resistance or sliding wear characteristics, the coating is sometimes impregnated with fluorocarbons such as Teflon. This does not significantly affect the thickness of the anodic film.
Chromate conversion coatings are used to give metal surfaces, in this case aluminum, some protection from corrosion at a reasonable cost. These coatings were first deveolped around seventy-five years ago to improve paint adhesion and are still used today for the same purpose. The process differes from anodizing in that it is not an electo-chemical one, but strictly a chemical reaction. One similarity for both, though, is that the aluminum substrate is an integral part of the coating. As uncoated aluminum has a tendancy to naturally oxidize and therefore form an electrically insulating surface, chromates have found extensive use in the electronics industry for their relatively conductive properties. The coating is used many times in conjunction with anodized or painted parts to provide good electrcal grounding areas, while at the same time providing excellent corrosion protection. Chromates with higher hexavalent chromium content will tend to yellow to light tan, while those with higher trivalent chromium content tend to be clear. Because of the thinness and low abrasion resistance of chromate coatings, they are seldom used for decorative applications unless coated with an organic type material.