Anodizing Non Aluminum Substrates

Magnesium Anodizing. Three methods of anodizing magnesium are widely employed by industry. One uses only internal voltage generated as a result of a galvanic couple, and two use an external power source. The first method, often referred to as galvanic anodize or the Dow 9 process, uses a steel cathode electrically coupled to the magnesium component to be anodized. Dow 9 coatings have no appreciable thickness and impart little added corrosion resistance. However, the resulting coating is dark brown to black, which makes it useful for optical components and for heat sinks in electronic applications. This coating also serves as an excellent paint base.

The other anodizing processes, known as the HAE and Dow 17 processes, use an external power source. Both processes deposit an anodic layer about 50 pm (2 mils) thick, but they differ in that the solution used for Dow 17 coatings is acidic, a combination of ammonium bifluoride, sodium dichromate, and phosphoric acid, whereas the HAE process employs an alkaline bath. Details for both processes may be found by consulting military specification MIL-M-45202.

Titanium Anodizing. While extremely corrosion resistant in itself, titanium and its alloys are often anodized to impart properties other than corrosion resistance. For instance, in wear situations, titanium components are very prone to galling. In order to overcome its tendency to gall, titanium is often anodized in a caustic electrolyte. This application is detailed in the SAE specification AMS 2488.

Decorative colored coatings on titanium can be achieved by anodizing in slightly acidified solutions of phosphoric or sulfuric acid. By controlling the terminal voltage, vivid colors from magenta to cobalt blue can be obtained. Such decorative uses have been widely utilized by the jewelry industry for years, and these coatings are now finding functional use for medical implants and dental instruments.

Zinc Anodizing. Zinc can be anodically treated in a wide range of electrolytes using either alternating or direct current to form decorative, yet protective, coatings. Anodic coatings on zinc and zinc alloys are covered in military specification MIL-A-81801. The zinc to be anodized may be wrought or die cast zinc parts or zinc coatings obtained by electroplating, mechanical deposition, thermal spraying, or galvanizing.

Electrolytes are formulated from such materials as phosphates, silicates, or aluminates to which are added chromates, vanadates, molybdates, and/or tungstates. Solutions are typically heated to 65 °C (150 °F), and anodize times vary from 5 to 10 min. The resulting coatings are 30 to 40 pm (1.2 to 1.6 mils) thick and are either green, gray, or brown, depending on the electrolyte used. For optimum corrosion resistance, anodic zinc coatings should be sealed using a material such as sodium silicate or an organic lacquer or enamel.

Thermal Spray Coatings

Robert C. Tucker, Jr., Praxair Surface Technologies, Inc.

0 0

Post a comment