Effects Of Grain Size On The Corrosion Behaviors Of Zinc And Trivalent Chromium Passivation Film On Zinc

Pulse current (PC) and direct current (DC) electrodeposition were used to synthesize nanocrystalline (NC) zinc coatings from citric acid bath. X-ray diffraction was employed to study the grain size of the depositions and examine their preferred orientation. The effects of grain size on the electrochemical corrosion behaviors of zinc and trivalent chromium passivation film on zinc have been studied respectively by electrochemical methods.The effect of grain size on the corrosion behavior of zinc was investigated by using potentiostatic and potentiodynamic polarization methods. Electrochemical corrosion behavior was characterized by pitting potential, induction time and stable pit growth rate which analyzed according to statistical method. The results showed that refining the grain size increased the sensitivity of E_pit refer to potential sweep velocity, changed the type of the pit generation from model B1 (pit generation and pit repassivation occurred at different places) to model B2 (pit generation and pit repassivation occurred at the same place), accelerated the pitting initiation process, inhibited the stable pit growth process.The effect of grain size on the corrosion behavior of trivalent chromium passivation films formed on PC and DC electrodeposited zinc coating were studied by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The results showed that trivalent chromium passivation film could inhibited cathode reaction, restrained corrosion process. Refining the grain size decreased the film's capacitance, increased the module value and the transfer resistance of trivalent chromium passivation film, improved corrosion performance of trivalent chromium passivation film.