Oxidation behavior of Ni-based coatings deposited by PTA on gray cast iron

Abstract

The aim of this investigation was to study the effect of PTA current (100 and 128 A) on the oxidation behavior of nickel-based hardfacing coatings deposited on gray cast iron. The oxidation behavior of coatings held at 800 and 900°C for 2h in air was studied by thermogravimetry (TGA). The surface, as well as the cross‐section, of the coatings was characterized by scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD). TGA results indicate that the coating produced with lower arc current exhibits more effective oxidation resistance than that produced with higher current. This behavior could be correlated with the dilution promoted by the PTA process, which changes the chemical composition of coatings. As a consequence, different kinds of oxide scales were detected in each coating after isothermal oxidation. In the specimen produced with a lower arc current and lower dilution a protective layer of Si–O is formed, while in the specimen produced with a higher current two layers could be identified: an external one of Fe2O3, with small features of Fe3O4 and NiFe2O4 spinel rich phases, and an internal one of Fe3O4 with small amount of a dark phase evenly distributed rich in silicon, nickel and iron. The isothermal oxidation curve at 900°C of the coating with higher dilution showed two stages: at an early stage, the weight increase over time is almost linear whereas, in a second stage, a parabolic law could be fitted to the experimental data. The other specimens followed only a parabolic law.