Ni20Cr coating on T24 steel pipes by HVOF thermal spray for high temperature protection

Abstract

This investigation deals with the deposition of Ni20Cr coatings on the surface of T24 steel pipes using High Velocity Oxy-Fuel (HVOF) technique to improve the high temperature oxidation performance. The main parameters of the thermal spraying process as tube rotational speed, spraying distance, number of layers, gun speed over the tube surface or H2/O2 relation have been studied and optimized. The best parameters have been selected through the analysis of coating properties, employing six spraying conditions. The optimized parameters provided a homogeneous and fully dense coating with very low oxide content (4 ± 1 vol%), thickness of at least 260 ± 20 μm, micro-hardness around 460 ± 30 HV0.1, and adhesion strength higher than 29 ± 7 MPa. Thermal tests have been used to analyze the oxidation protection effectiveness of the coating at high temperatures. The optimized coating provided useful high temperature protection to T24 steel pipe up to 750 °C, while the uncoated steel only withstands up to 600 °C. After the exposure of the uncoated sample to 650 °C for 360 h, the oxide layer formed on the surface had more than 120 μm of thickness. However, in the coated samples exposed at 650 °C and 750 °C, no degradation of T24 substrate has been found. Thin layers of 1 μm (in the coating exposed at 650 °C) and 5 μm (in the coating exposed at 750 °C) of protective Cr2O3 and NiCr2O4 oxides have been observed on the external surface of the coatings.