Process Fundamentals

The application of a phosphate coating for paint-based application normally comprises five successive operations: cleaning, rinsing, phosphating, rinsing, and chromic acid rinsing. Some of these operations may be omitted or combined, such as cleaning and coating in one operation. Additional operations may be required, depending on the surface condition of parts to be phosphated or on the function of the phosphate coating. Parts exemplifying these exceptions are:

• Heavily scaled parts, which may require pickling before cleaning.

• Parts with extremely heavy coatings of oil or drawing compounds, which may require rough cleaning before the normal cleaning operation.

• Parts that are tempered in a controlled atmosphere before being phosphated, which may not require cleaning and rinsing before phosphating.

• Parts that are phosphated and later oiled for antifriction purposes, which may have the chromic acid rinse omitted, because corrosion resistance is not required. (Some rust-preventive oils negate the need for a chromic rinse while still providing excellent corrosion resistance.)

• Automotive parts when electrodeposition of a primer is involved. (A deionized water rinse is required after the chromic acid rinse.)

Hexavalent chromic acid for a passivating rinse is no longer used in some plants because of strict effluent controls imposed by the Environmental Protection Agency (EPA). Other, less restricted materials, such as phosphoric acid and various proprietary compounds, are being used.

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