Influence of HVOF Parameters on Corrosion Resistance and Microstructure of Thermal Sprayed NiWCrBSi Coatings

摘要

Preliminary studies have been conducted in order to characterize plain carbon steel samples, which have been HVOF thermal sprayed by using commercial NiWCrBSi powder. In this work, the influence of fuel/oxygen ratio and spraying distance on corrosion resistance and morphology of NiWCrBSi commercial alloy were evaluated. The equivalent ratio, φ, was varied between 0.8 and 1.2 and the spraying distance was also varied between 380 and 570 mm. The corrosion resistance was evaluated according to ASTM G-5 standard using a 3.5% NaCl solution. Coatings characterization was performed by means of optical and scanning electron microscopy (EPMA) before and after corrosion testing. Results regarding porosity, microstructure and corrosion resistance, and their relationship with the spraying parameters are reported. The results indicate that the equivalent ratio and spaying distance affects the corrosion resistance and properties of coatings, and the lowest corrosion current density was obtained for an equivalence ratio of 1.2 and a spraying distance of 380 mm. However, it was shown that the as-deposited HVOF coatings present values of corrosion current densities, I_(corr), which are of the same order and sometimes higher than the value corresponding to the uncoated substrate, indicating that these coatings are not suitable for corrosion protection in aqueous media.