Corrosion investigation of nitrided chromium coating layer on nickel in non-aqueous electrolytes

Abstract

Nitrided chromium layers with different thickness (0.5, 1 and 3 μm) are prepared from electroplated chromium on nickel substrates. The CrN phase that forms in the coating layer provides better corrosion resistance than chromium, a nitride-free layer. Anodic polarization tests in a non-aqueous electrolyte consisting of propylene carbonate and 1 M tetraethylammonium tetrafluoroborate clearly demonstrate that the nitridation of chromium improves the corrosion resistance as can be confirmed by the reduction of corrosion currents. There is a strong inverse correlation between the corrosion currents and the nitriding time, which further supports the positive effect of chromium nitridation on corrosion resistance. By contrast, no definite relationship between the corrosion currents and the CrN layer thickness is observed mainly because the amount and distribution of pre-existing cracks formed during the chromium electroplating prior to the chromium nitridation do not seem to depend on the CrN layer thickness. Sheet resistance measurement by four-point probe before and after the immersion in the electrolyte shows a negligible change in resistance, indicative of the high stability of CrN coating layers as bipolar plate materials under the operating conditions of non-aqueous redox flow batteries.