Enhanced corrosion resistance of duplex coatings

A series of electrochemical tests have been carried out to investigate the corrosion behaviour of physically vapour deposited TiN, CrN and (TiAl)N coatings on plasma nitrided En40B steel. Surface and subsurface characterisation before and after corrosion testing were performed using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). The experimental results indicate that all three duplex coating systems possess superior corrosion resistance over the individually plasma nitrided or PVD coated En40B steel, highlighting the importance of the iron nitride subsurface in determining the corrosion resistance of duplex coating systems. It is also demonstrated that among these three duplex coating systems, the corrosion resistance increases in the order of TiN/PN, CrN/PN and (TiAl)N/PN. The critical potentials corresponding to the onset of transpassive behaviour for both the CrN/PN and the (TiAl)N/PN duplex coating systems are above the practical potential range (< 1 V vs SCE). The corrosion mechanisms involved are discussed and the roles of the iron nitride sublayer and the coating-substrate interface are emphasised. Finally, based on the present findings and in conjunction with previously reported results, an optimum duplex coating system towards combined improvements in tribological, corrosion and fatigue properties is proposed.