Corrosion protective double layer hard coatings

Abstract

A Cr + TiN double layer was coated on an SS-420 substrate using a hollow cathode discharge gun. The compositional analysis of the samples carried out by glow discharge optical emission spectroscopy confirmed the Cr/TiN constituents. X-ray diffraction pattern studies showed that due to the presence of chromium in the TiN matrix, preferential growth of TiN was changed from (200) to (111) reflection plane, leading to formation of a Cr + TiN double layer hard composite. However, at the same time, the hard coating layers exhibited unavoidable defects exposing them to corrosive environments. In this work, the corrosion protective behaviour of the Cr + TiN double layer was examined by potentiodynamic measurements in sodium chloride solution. It was inferred from this study that the increase in corrosion potential from −505 mV to −452 mV corresponding to the bare substrate and Cr + TiN coatings, respectively, led to the formation of more corrosion protective layers. The electrochemical impedance spectra analysis of the SS-420 substrate, TiN and Cr + TiN layers revealed that corrosion resistance increased and the electrochemical impedance of the Cr + TiN layer decreased. This is attributed to the corrosion protective properties of the resultant coating and its reduced permeability to the corrosive environment.