Chemically driven coating systems can be simpler in terms of application apparatus but are more difficult to design in theory and also to practice. Schuman, in Handbook of Environmental Degradation of Materials (Second Edition), 2012 17.5.2 Chromate-Based Conversion CoatingĬurrent-driven processes are a mainstay of hard anodize coating systems but suffer limitations in apparatus and electrical requirements. 97 In general, all these conversion coatings need to be used in combination with an overlay coating for efficient corrosion protection, especially in aggressive environments. 96 Effort was also made to produce conversion coatings based on zinc manganese phosphating electrolytes on magnesium alloys. 93,94 As these conversion coatings impart good adhesion properties, a few attempts have been made to produce chrome-free coatings in electrolytes based on stannates 95 and RE salts. 90 Alternate chrome-free conversion coatings based on alkali potassium permanganate solutions (MAGPASS-COAT®) and from solutions containing vanadates, molybdates and tungstates have also been attempted. New alternatives to the chromating process based on phosphate permanganate or fluoride zirconate have been proposed. 90 Nevertheless, the use of chromates is limited by the new environmental regulations in Europe since 2003, owing to growing health concerns associated with hexavalent chromium. The adhesion to organic coatings is also greatly improved by this treatment. In addition to corrosion protection, these films offer an inhibitive effect inherent to the chromate film. The treatment can be used for the prevention of damage by corrosion of magnesium alloy components during storage and transport. Dietzel, in Shreir's Corrosion, 2010 3.09.6.1.1 Chemical conversion coatingsĬhromating is a known surface treatment for aluminum, zinc, and magnesium for years and this treatment develops a very thin layer (to about 1 μm maximum).