Experimental and computational study of zinc coordinated 1-hydroxyethylidene-1,1-diphosphonic acid self-assembled film on steel surface

Zinc coordinated 1-hydroxyethylidene-1,1-diphosphonic acid (Zn-HEDP) film was fabricated on steel surface by coordination self-assembly method. The surface morphology and chemical composition of Zn-HEDP film were investigated by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), and the structural and valence state of Zn-HEDP film was determined by Fourier transform infrared spectroscopy (FTIR), Raman spectra and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of Zn-HEDP film in 3.5wt% NaCl solution was investigated by using potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS) techniques. The experimental results show that Zn-HEDP film is in a smooth surface with disturbed nanoparticles. The protection efficiency of Zn-HEDP film to steel in 3.5 wt% NaCl solution is firstly increased and then decreased with the increasing of zinc concentration, pH value and assembly time. The optimum preparation conditions for Zn-HEDP film are the zinc concentration of 10 mmol/L, pH value of 5.0 and assembly time of 30 min, under which the protection efficiency of film is reached to 95.79 %. The coordination between zinc ions and HEDP can effectively increase the binding energy with Q235 steel. Zinc ions can improve the compactness and thickness of Zn-HEDP films as bridging agents, which prevent Q235 steel from corrosive solution.