Magnesium Pretreatment

Q. I am the manufacturing engineer in a plant making components for electronic equipment. My responsibility is the paint line. Are there any books or literature available for proper surface pretreatment of magnesium prior to painting? S.W.

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A. Pretreating magnesium is really easy. It is pretreated by cleaning, deoxidizing and priming prior to painting. Since magnesium is more alkali resistant than aluminum, it can be degreased using strong alkaline cleaners. Deoxidizing can be done by acid pickling with chromic acid or by blast cleaning. However, blasting must be done carefully so that oily residues and corrosion-causing contaminants are not driven into the magnesium surface. All surfaces should be degreased before blast cleaning.

Because the details of these pretreatments are beyond the scope of Painting Clinic, I will only list the chemical pretreatments for magnesium. More detailed information will be provided by pretreatment chemical suppliers. The pretreatments for magnesium are: Type I Chrome-Pickle Treatment, Type II Sealed Chrome-Pickle treatment, Type III Dichromate Treatment, Type IV Galvanic anodizing Treatment, the HAE process, Acid Chromate Anodic Process. Many of these pretreatments involve multiple stages, which is probably the reason people say magnesium may be difficult to pretreat. If your volume of magnesium products is low, you could consider out-shopping the pretreatment to a custom coater or electroplating shop.

Primers for magnesium generally contain zinc chromate pigment. Certain zinc chromate pigmented primers are actually pretreatments and should be applied directly to bare, cleaned magnesium and not be applied over other pretreatments. After priming, most paint finish systems can be used for painting magnesium.