Development of Composition Modulated Multilayer (NiTi) Alloy Coatings for Improved Corrosion Protection of Mild Steel

A new class of materials known as composition modulated multilayer (CMM) alloy coatings has created an ever-increasing interest in materials research due to their improved functional properties. In this direction, an effort has been made to improve the poorer corrosion resistance performance of conventional monolayer nickel–titanium (NiTi) alloy coatings (due to inherent induced type of codeposition), their multilayer alloy coatings have been developed. CMM (NiTi) alloy coatings have been fabricated electrolytically on mild steel (MS) from a citrate bath, using the glycerol as additive. Multilayer alloy coatings of varying matrices have been developed by periodic modulation of direct current (DC), in terms of pulse height and pulse duration. Coating configurations in both composition and thickness of alternate layers were optimized for best performance of the alloy coatings against corrosion. Corrosion behaviours were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization methods. Experimental study revealed that the corrosion rate (CR) of CMM coatings decreased successively with increase in number of layers up to certain limit, and then started increased. Experimental findings demonstrated that CMM (NiTi) alloy coating having optimal configuration, represented as (NiTi)_2.0/4.0/120 is approximately five times more corrosion resistant than its monolayer counterpart, deposited from the same bath for same duration. Development of coatings in layered pattern was confirmed by scanning electron microscopy (SEM) analyses. EDX and XRD techniques confirms the composition and phase structure of alloy coatings. The corrosion mechanism responsible for delayed corrosion of multilayer coatings has been explained, and experimental results are discussed.