With magnesium alloys, corrosion is an ever present reality. For optimum corrosion protection, a chemical conversion coating or anodizing treatment is required prior to the application of an organic paint system. These treatments roughen and chemically modify the surface for maximum paint adhesion. A typical anodize treatment (Dow 17) is shown in Fig. 3.7. In this treatment, the part is first alkaline cleaned and then anodized in a solution of NH4HF2, Na2Cr2O7 ■ 2H2O, and H3PO4 heated to 160-180° F using either an AC or DC current (5-50A/ft2). This method produces a two-layer coating; the first layer is a thin light-green coating (0.2 mil) done at lower voltages that is followed by a thicker dark-green coating (1.2 mil) done at higher voltages. The thicker coating enhances
corrosion protection and forms an excellent base for paint but can be susceptible to spalling under impact, deformation, or flexing.
Porous casting surfaces are normally filled with a penetrating resin prior to paint application. Primers usually contain zinc chromate or titanium dioxide pigments for improved corrosion resistance. Both air drying and baked-on paints are used, with the baked-on paints being harder and more resistant to solvents. Depending on the application, vinyl alkyds provide resistance to alkali, acrylics for resistance to salt spray, alkyd enamels for exterior durability, and epoxies for abrasion resistance. Vinyls can withstand temperatures up to about 300° F. Higher temperature finishes include modified vinyls, epoxies, modified epoxies, epoxy-silicones, and silicones. It is important to maintain the integrity of the paint system when the part is placed in-service, as the chemical conversion and anodized surfaces will readily corrode if exposed to the atmosphere.
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