Corrosion resistance of ZrN films on AISI 304 stainless steel substrate

The corrosion resistance of ion-plated Zr, ZrN and ZrN/Zr films on commercial AISI 304 stainless steel has been investigated by electrochemical measurement. The electrolyte, 0.5 M H2SO4 containing 0.05 M KSCN, was used for the potentiodynamic polarization. The potentiodynamic scan was conducted from −800 to 800 mV (SCE) with scan rate ranging from 10 to 600 mV/min. The N/Zr ratios of the ZrN films determined by X-ray photoelectron spectroscopy (XPS) were essentially stoichiometric. The composition depth profiles measured by secondary ion mass spectrometry (SIMS) indicated that the compositions in the ZrN films were uniform from the film surface to the 304 stainless steel substrate. Experimental results showed that the corrosion current density Icorr and passive current density Ip increased with increasing polarization scan rate for the bare AISI 304 stainless steel specimens. Compared with the bare substrate, the Icorr and Ip for the coated specimens decreased at least 1 order of magnitude. The bi-layer ZrN/Zr coating possessed the highest corrosion resistance among the three coated-specimens. Because of the cathodic control of the galvanic corrosion, the corrosion potential of the coating specimens was slightly higher than that of bare metal substrate. The corrosion power Q, i.e. the integrated electric charge per unit area of the specimen during potentiodynamic polarization test, was an effective index to evaluate the corrosion resistance of the coated stainless steel substrate. The pinhole density played a significant role in corrosion resistance of the transition metal nitride coatings. Normalized critical passive current density (NIcrit) was closely related to the exposure area, and a linear relationship between Q and NIcrit was held.