Effect of mixed cations on phosphate coating of steel

Abstract

Effect of variations in the constituents of phosphating baths having Zn2+, Mn2+, Ni2+ and Ca2+ on performance of produced coatings on steel surfaces, have been studied. It is observed that a bath having Zn + Mn + Ni as their phosphates and Ca as nitrate, provides excellent coating on the surface. Addition of urea (U), hexamine (HA), guanidine nitrate (GN) and nickel nitrate (NIN) provides different types of effects. HA doubles the rate of coating formation but produces a coarser and poorer corrosion resistant film. Thermal resistance, spot corrosion resistance, alkali resistance tests and immersion resistance tests have been performed to determine the characteristics of the coatings. Salt spray resistance (B 117) studies indicate that the phosphated surface having passivation treatment of Phosave (a trade name) provides an excellent improvement in controlling of appearance of brown rust on the surface. Electrochemical studies indicate that the produced coating is more resistant in saline environment than sulphurous acidic environments. SEM studies indicate that the addition of GN in phosphating baths has a strong influence in transforming the morphologies of phosphate crystal structure from coarse to finer one.