Iron Phosphate Coating
Iron phosphate conversion coatings are produced on a ferrous metal surface by contacting the metal surface with an acidic phosphate-containing solution which reacts with the ferrous metal surface. Iron present in the metal substrate undergoing treatment chemically reacts with the phosphating solution and redeposits on the surface to form a low weight, tightly adherent amorphous iron phosphate coating which provides highly effective corrosion resistance and serves as an excellent base for subsequent painting.
The economic and practical desirability of producing phosphate coatings at lower temperatures has been well recognized, and the iron phosphating field has been active with attempts to produce suitable low temperature phosphating compositions and processes.
Iron phosphate is the thinnest of phosphate films. In the application process, an iron oxide base is developed, followed by a flat or amorphous metal phosphate topcoat. The treated metal surface will typically have a gray to blue iridescent or blue-gold iridescent color, depending on the coating weight and the base metal.
In an iron phosphate solution, the metal surface is etched, releasing
some iron into the bath. When metal ions are etched from the part
surface, the surface becomes positively charged. The metal ions in
the bath are converted to iron phosphate, negatively charged. A pH
rise occurs at the interface of the solution and the part, causing the
iron phosphate ions to deposit an amorphous coating on the metal
surface.
Iron phosphate coatings can be applied by hand wiping, with a handheld spray wand, immersion, or a spray washer. The number and type of process stages is directly dependent on finished part requirements. A cleaner/coater combination followed by a rinse is the typical minimum chemical cleaning and phosphating process used. The addition of stages in the process can provide enhanced performance.
