All military aviation platforms contain parts with magnesium castings. Magnesium is an electrochemically "active" metal which leads to moisture induced corrosion and galvanic corrosion when mating with dissimilar metals. During overhaul phase of magnesium aviation components, protective coatings like primer and topcoat paints are stripped off to allow visual and penetrant inspection of the casting. After passing inspection, magnesium castings are dipped in a chemical conversion coating such as DOW-7 or DOW-19 before being placed back into service. Both of these DOW coatings contain hexavalent chromium (Cr6+). The objective of the Tagnite demonstration at the Corpus Christi Army Depot (CCAD) plating shop is to eliminate the use of hexavalent chromium DOW-7 and DOW-19 tanks that are currently being used. This will be accomplished by masking off the dissimilar metals and applying the anodized coating Tagnite to these legacy magnesium components. The advantages of the Tagnite anodize coating are twofold: 1) The Tagnite electrolyte and anodic coating contain no hazardous materials, and 2) The Tagnite coating has better corrosion resistance than the currently used hexavalent chromium coatings.
Historically, anodic coatings could not be used to coat fully assembled magnesium housings because the anodic process is an oxidation process and would rapidly convert the steel components to rust. Technology Applications Group (TAG) has developed the technology to mask off dissimilar metals such as steel in completed magnesium components in order to apply the Tagnite coating to legacy components without harm to the dissimilar metals.
The Immersion Tagnite coating was found to be compatible with five repair techniques; DEVCON Al liquid, DEVCON Ti Paste, Tungsten Inert Gas Welding, High Velocity Oxy Fuel and Al Cold Spray. The MIL-PRF-23377 primer adhesion and galvanic corrosion response was equal to or better than the performance of the DOW 19 coating. The Brush Tagnite coating performed as well or better than DOW 19 under neutral and SO2 acidified salt fog testing, coastal outdoor exposure, MIL-PRF-23377 primer adhesion and galvanic corrosion testing. Fatigue testing was performed but was inconclusive due to a very limited sample size. The demonstration of the masking technology/Immersion Tagnite on three (3) H-60 magnesium components at the CCAD Plating Shop was successful with no damage to any dissimilar metals. There was no information available on the cost to run the DOW-7/DOW-19 tanks at CCAD so the cost assessment was based on the replacement cost of the H-60 components. The cost to apply the Immersion Tagnite was between 5 – 11% of the replacement cost for the H-60 demonstration parts. Therefore, if the parts lasted at least one additional overhaul cycle there would be a significant cost savings.
The demonstration was very successful and CCAD is moving forward to approve this process for production parts. The primary implementation concern is the 9 – 12 month lag time between the training/demonstration phase and actual production. TAG proposed a chemical stripping method to remove the legacy coatings, however, this effort was not successful. An alternate media blast technology was studied during the demonstration phase and found to be effective in removing legacy coatings.