Assessment of the corrosion rates and mechanisms of WC-Ni nano-structured HVOF coatings in H_2SO_4 and NaCI solutions

摘要

This paper reports an investigation of the effects of high temperature surface heat treatment on the crossion performance for ST-37-based WC-Ni HVOF-sprayed MMC coatings.The nanostructured specimen was produced from sprayed samples by heat-treating at 1100°C in a vacuum chamber. Examination of the heat treated surface was conducted in terms of microstructure morphology, and phase analysis. Corrosion resistance using potentiodynamic polarization in 3.5% NaCI and 5% H_2SO_4 solution and open-circuit potential (EOC) measurements and electrochemical impedance spectroscopy (EIS) of the various coatings before and after heat treatment were evaluated before polarization tests. Corrosion products on the coatings were conducted in terms of morphology, chemical composition by scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). Atomic Absorption Spectroscopy was also used for measuring solved coatings elements (corrosion Products) in the solutions after electro-chemical tests. During the building up of the WC-Ni coating using the (HVOF) spraying process, new phases such as W2C, Ni2W4C and amorphous phase was existed in the as sprayed coating. Heat treatment of this coating at high temperature also resulted in partially dissolution of WC particles and formation of new crystalline nano-size inter-metallic NiW phase. The results indicated that significant improvement of corrosion can be achieved after heat treatment as a result of the elimination of discrete splat-structure, micro crevice and porosity. In addition, the formation of nano-structure of the WC phase was considered to be beneficial for the corrosion performance. On the other hand, all of the WC-IONi coatings have shown better corrosion resistance in the NaCI than H_2SO_4 Solutions.