Corrosion behavior of hybrid coatings: Electroless Ni–Cu–P and sputtered TiN

Protective coatings for mild steel (MS) are prepared by combining two techniques: electroless (autocatalytic) deposition of a Ni–Cu–P layer and reactive DC sputtering of TiN/Ti atop. These hybrid coatings are designated to provide corrosion protection together with wear resistance and hardness. The corrosion behavior of the system Ni–Cu–P/MS is compared with the conventional Ni–P/MS and its superiority is demonstrated by the weight loss method. This justified the choice of the ternary coating as intermediate amorphous layer, aiming to improve the impermeability of the inherently porous TiN/Ti. The structure of the hybrid TiN/Ti/Ni–Cu–P/MS is studied by TEM of thinned cross-sections prepared by the focused ion beam (FIB) technique. Electrochemical characterization (via potentiodynamic polarization curves) of TiN/Ti/MS and hybrid TiN/Ti/Ni–Cu–P/MS in two model media: 5% NaCl and 0.5 M H2SO4 shows the outperformance of the hybrid coatings. SEM–EDS study of the local corrosion areas is conducted on FIB prepared sections. The role of internal stress in corrosion destruction process is discussed. The importance of bias voltage applied to the substrate during sputtering is considered. The advantage and prospects are outlined of MS finishing with hybrid TiN/Ti/Ni–Cu–P coating to ensure reliable corrosion protection joint with the excellent mechanical properties of TiN.