Effect of zinc ion on the microstructure and electrochemical behavior of phytic acid based conversion coatings on Q235 steels

Phytic acid conversion coating and Zn 2   + doped phytic acid conversion coating were fabricated on Q235 steels by immersing the steel samples in corresponding coating bath for 600   s. The changes of coating formation process and adsorption action of phytic acid on steel surface due to addition of Zn 2   + in coating bath were investigated. Scanning electron microscopy images exhibited that phytic acid conversion coating was porous network structure consisting of needle-like crystals, but Zn 2   + doped phytic acid conversion coating was one uniform layer except some fine cracks. Energy dispersive spectroscopic, X-ray diffraction and X-ray photoelectron spectroscopic characterizations demonstrated that the roles of Zn 2   + during the coating formation process could conclude to two sides: (i) Zn 2   + could promote the adsorption reaction of phytic acid to form one thicker phytic acid conversion coating on steel surface; (ii) a part of Zn 2   + could form ZnO and aggregation of the complex of phytic acid, Zn and Fe and doped to the formed conversion coatings. And the electrochemical measurements results showed that the corrosion protection efficiency of Zn 2   + doped phytic acid conversion coating was higher than that of phytic acid conversion coating.