Phosphate coating on steel surfaces by an electrochemical method

Abstract

Electrochemical coating of phosphates on mild steel and stainless steel surfaces was carried out under galvanostatic conditions. Mild steel samples could be coated with needle-shaped crystals of mixed phosphates of Zn and Fe, when an electrolyte solution containing zinc oxide, orthophosphoric acid and sodium hydroxide was used and a cathodic direct current (DC) or an alternating current (AC) was passed through it. However, when calcium chloride was added to the electrolyte solution, both mild steel and stainless steel samples could be coated with heavy and uniform deposits of phosphates on passing a cathodic DC. It was possible to coat mild steel surfaces by this ‘calcium modified’ process under AC conditions as well, which led to etching of the mild steel surface followed by coating of phosphate. The coating appeared to contain rich deposits of small and cubic crystals containing mixed phosphates of zinc, calcium and iron, in addition to some needle-shaped crystals, when studied under a scanning electron microscope connected with an energy dispersive X-ray analyser. In ‘Salt Spray’ Tests, it was observed that the mild steel samples coated with phosphate had better corrosion resistance characteristics than the blank sample, without phosphate coating.