Influence of galvanic coupling on the formation of zinc phosphate coating

Abstract

The influence of galvanic coupling of mild steel (MS) with titanium, copper, brass, nickel and stainless steel (SS) on the phosphatability is elucidated. The galvanic couple accelerates metal dissolution, enables quicker consumption of free phosphoric acid and facilitates an earlier attainment of point of incipient precipitation, resulting in higher amount of coating formation. The surface morphology of the coatings exhibit more uniform coating for the mild steel substrates phosphated under coupled conditions. XRD pattern of the zinc phosphate coating formed under coupled condition confirms the presence of phosphophyllite rich coating. The potential-time measurements are also carried out. The study reveals that galvanic coupling of mild steel with metals that are nobler than steel during phosphating proved to be beneficial in accelerating the coating formation.