Tagnite Coating for Magnesium Alloy
Description And Advantages
The Tagnite coating is the most corrosion and abrasion resistant anodic coating available for magnesium alloys. The Tagnite coating is formed in an electrolytic cell and is classified as an anodize coating. The coating was designed as a replacement for the Dow 17 and HAE anodize coatings which were invented in the mid 1940's. These two coatings lack the properties needed in today's demanding aerospace and defense industries. Like typical anodize coatings, the part to be coated is connected to a conductive rack which carries the part throughout the coating process. Once placed in the Tagnite coating tank, the rack and part become the anode and the tank holding the electrolyte serves as the cathode.
The electrolyte used to form the coating is an alkaline solution clear in color, containing no chromium (VI) or other heavy metals and operates below room temperature (40o to 60o F). This contrasts sharply with the Dow 17 electrolyte, a chromic acid based electrolyte containing a significant amount of chromium (VI), operating between 160o and 180o F and is dark orange in color. The HAE electrolyte (dark purple in color) contains permanganate, an oxide form of the manganese metal, and is a heavy metal similar to chromium.
The rectifier used to apply Tagnite was designed specifically for the coating. It employs a unique waveform optimized for the coating process. To ensure maximum corrosion resistance and high dielectric strength, the Tagnite coating is applied at voltages exceeding 300 volts DC. The Dow 17 coating may be applied with AC or DC power and typically does not exceed 100 volts. HAE is applied using AC current and typically does not exceed 125 volts.
Unlike Dow 17 and HAE anodize which primarily build thickness by surface deposition resulting in large pores and a softer coating, Tagnite is formed by the build up of hard, tightly packed magnesium oxide from the base of the coating. The coating consists mostly of hard magnesium oxide with minor surface deposition of hard fused silicates.
The Tagnite coating can be applied as thin as 0.10 mil or as thick as 0.9 mil depending upon the alloy being coated. The white color of the TAGNITE coating does not vary with coating thickness, a clear demonstration that the coating is primarily hard white magnesium oxide.
