Electrochemical Corrosion Performance of TiN, TiCN and TiBN Multilayer Coatings on Hardmetal Substrates

Three types of gradient plasma-assisted chemical vapour deposition (PACVD) coatings were produced on WC-Co hardmetal substrates: a TiN coating, a gradient TiCN coating with alternating TiN/TiCN layers and a multilayer TiBN system of TiN/TiB2 layers. Their corrosion behaviour in a chloride environment was compared using direct current and alternating current electrochemical techniques. Potentiodynamic polarization, linear polarization and electrochemical impedance spectroscopy were carried out in 3.5 wt.% NaCl at temperature 20 ± 2 °C in a three-electrode cell with a saturated calomel electrode (SCE) reference. After 1000 s open circuit stabilization, TiN coating showed superior corrosion resistance with Ecorr = 15 mV vs. SCE, versus TiCN (Ecorr = −281 mV) and TiBN (Ecorr = −304 mV). Linear polarization resistance/Tafel analysis showed significantly higher polarization resistance of TiN (Rp = 1559 kΩ∙cm2) than TiCN (195.4 kΩ∙cm2) and TiBN (243.6 kΩ∙cm2), with the lowest corrosion current density, jcorr = 10.97 nA∙cm−2 and corrosion rate vcorr = 117.2 × 10−6 mm∙y−1. TiCN showed the highest jcorr (360.8 nA∙cm−2) and vcorr (3.32 × 10−3 mm∙y−1). Electrochemical impedance spectroscopy fitting with a R(QR) circuit confirmed this, with the highest charge transfer resistance at the substrate–electrolyte interface (Rct) for TiN (8.198 × 104 Ω∙cm2), lower for TiBN (7.929 × 104 Ω∙cm2) and lowest for TiCN (1.435 × 104 Ω∙cm2), indicating TiN as the best barrier and TiCN as the most permeable.