Reducing Zinc Chromate in Wash Water

Q. I run the paint line at a sheet metal fabrication shop that produces storage cabinets and shelving. Because many of our products are used outdoors or in corrosive atmospheres, we use a zinc chromate primer on all of our painted parts.

Our waste disposal company says our paint sludge from the wash water has too high a chrome level, which in turn results in a higher than normal cost of disposal. What would the effect on corrosion performance of our products be if we reduce the chrome in the primer? G. K.

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A. When you say sludge, you must mean the residue in the water wash spray booth. This is actually collected paint overspray—wasted paint. You can reduce paint overspray, use less paint, use less solvent and lower your disposal costs by using more efficient paint application methods such as air and airless electrostatic spraying, rotating electrostatic spraying and high-volume, low- pressure spraying equipment.

Zinc chromate pigments provide corrosion resistance in paints by reacting chemically with the base metal to inhibit the corrosion reaction. In order to be effective, chromates must be present at a specific level. Reducing the amount of zinc chromate in your paint formulation will therefore reduce the corrosion resistance afforded to the substrate.

A viable alternative would be to use paints containing corrosion inhibiting pigments other than zinc chromate. There are paints formulated using a host of these environmentally friendly pigments. Ask your paint supplier to provide you with samples and data sheets. Choose one that has corrosion resistance equal to that of your present primer.