MULTILAYER COATINGS FOR ANTI-CORROSION APPLICATIONS

Due to low mass densities, magnesium alloy materials are utilized to reduce vehicular weights. However, when a magnesium alloy part is fastened with a carbon steel bolt, galvanic corrosion will occur. To stop or slow down galvanic current flow, an isolation material may be inserted between the magnesium alloy part and the steel bolt. In a previous study, an aluminum oxide thin film (100 - 300 nm) or a silicon nitride thin film (20 - 850 nm) was coated on a steel substrate as a barrier layer. In this study, besides the single layer coatings, double layer and triple layer coatings are fabricated with various combinations of the silicon nitride thin film and oxide films. After a multilayer coating is applied onto a 1050 carbon steel disc, the galvanic current between the coated steel disc and AM60B magnesium alloy is obtained. Using the lowest current value as the criterion, a candidate multilayer stacking design is then selected for the coating on the carbon steel bolts. The coated bolts are subsequently evaluated by a salt spray testing method. So far, the best layer stacking has three layers in the following sequence: silicon nitride base layer, aluminum oxide and UV curable oxide topcoat.