Deciphering the role of W content, triple junctions, and heat treatment on the corrosion performance of Ni–W alloy coatings used for automotive applications

Abstract

In this study we focus on understanding the influence of tungsten (W) content, heat treatment and grain size on the corrosion behaviour of Ni–W coatings. Using pulse and pulse reverse electrodeposition techniques, Ni–W coatings with different W contents (3, 7.5, 12, 17, 20 and 25 at.%) were deposited from a standard ammonia-citrate bath. Coatings were heat treated at 700 °C in vacuum for 1 h and then characterized to determine the surface morphology, grain size, and phase transformations. To evaluate the corrosion performance of the coatings, potentiodynamic polarization tests were performed in 0.6 M NaCl solution. Polarization test results indicated that as-deposited Ni–W coatings had better corrosion resistance compared to heat treated coatings. In the as-deposited condition, corrosion resistance increased up to 17 at.% W. However, further addition of W decreased the corrosion resistance due to high volume fraction of triple junctions and intercrystalline regions. On the other hand, heat treated coatings suffered from micro-galvanic corrosion due to the precipitation of Ni4W and NiW phases, and therefore showed higher corrosion rates compared to as-deposited coatings. The changes observed in the corrosion behaviour of as-deposited and heat treated Ni–W coatings were rationalized based on surface composition, mixed potential theory, and triple junction corrosion theory.