Study on the Growth and Corrosion Resistance of Manganese Phosphate Coatings on 25Cr2Ni4WA Alloy Steel

The growth process and corrosion resistance of the phosphate coating on 25Cr2Ni4WA alloy steel fabricated by a high temperature manganese phosphating were investigated by XRD, SEM, EDS and electrochemical polarization method, respectively. It is found that the phosphate coating consists of lots of close packed lump crystallites and mainly composed of (Mn,Fe)5H2(PO4)4·4H2O. The coating weight increased rapidly, and then the increase rate slowed down when phosphating process was carried for about 5 min. The corrosion potentials of the phosphated steel shifted positively about 250 mV than the bare steel, indicating that the phosphating can improve the corrosion resistance of the 25Cr2Ni4WA alloy steel.

A Study on Corrosion-Resistant Properties of Unleaded Bi-Containing Automotive Epoxy Primers

The aim of this work is to evaluate the difference in the corrosion resistance between unleaded containing bismuth automotive epoxy primers and leaded one. It was also discussed the effect of zinc phosphate pretreatment. The specimen coated unleaded epoxy primer showed 0.5V higher corrosion potential than that of bare steel. The result of salt spray test did not indicate remarkable difference of corrosion resistance in all specimens above 10 thickness up to 1200 hours. In the cyclic corrosion test, unleaded epoxy primers on phosphated substrate performed good corrosion properties until 800 hours. The lead free primer performed the equivalent corrosion resistance as leaded coating on phosphated steel, but slightly inferior to that of leaded on bare steel. These results show that the pre-treatment of zinc phosphating is effective as well as pigment changing in performing anti-corrosion properties in automotive bodies.