Studies on Corrosion, Mechanical and Wetting Properties of the Thermal Sprayed Coatings on Low Carbon Steel

Abstract

Low-carbon or mild carbon steels are very attractive materials throughout the industrialized world in diverse applications but are susceptible to corrosion. This problem can be mitigated and the service lifetime of the low carbon steel can be increased by the application of protective coatings with good mechanical properties while reducing maintenance costs. In the present work, different corrosion-resistant materials like metal alloy (Inconel-625), ceramic-metal composite 25(NiCr)-75Cr3C2 and ceramic-graphene oxide nanoplatelets(GNP) composite (Al2O3-GNP and ZrO2-GNP) were coated using thermal spray techniques like weld overlay, High-Velocity Oxyfuel (HVOF), and Atmospheric Plasma Spray (APS) techniques respectively. The structural, morphological and compositional studies were carried out by XRD, FTIR, Raman Spectroscopy, XPS, FESEM-EDAX, TEM, and BET characterization techniques. The corrosion studies were conducted using the three-electrode electrochemical system. The stability of the coatings was studied using immersion tests upto 14 days. The mechanical and wetting properties of samples were studied using Vicker’s microhardness tester and contact angle measurements respectively. ANSYS FEA simulation showed that alternate skip weld overlay of SS-309Mo as the buffer layer by GTAW and Inconel-625 as final layer by SMAW process for the 6 mm thick low carbon steel substrate preheated to 100°C, to be the best model with 18 MPa surface residual stress among twelve combinations. Using the conditions of the best model, SS-309Mo and Inconel-625 have been coated on low carbon steel. The weld overlay coated Inconel-625 had nearly the same corrosion resistance as that of bulk Inconel-625 with increased microhardness. 25(NiCr)-75Cr3C2 cermet coating on low carbon steel using HVOF process showed hydrophobic behaviour with improved microhardness and corrosion resistance. The α-Al2O3- (X wt. %GNP) and ZrO2-(X wt. %GNP) (where X= 0, 0.5, 1.0, 1.5 and 2) composite coatings by APS process were successful in the retention of GNPs in the composite. The surface corrosion resistance increased by six orders of magnitude when coated with 2.0 wt.% GNP reinforced α-Al2O3 nanocomposite, in comparison with bare Al2O3 coating. The increase in corrosion resistance is due to the hydrophobic nature of in-situ reduced GNP. In addition, the mechanical properties have improved with the addition of GNP. The corrosion rate of ZrO2-2 wt. % GNP coating is 130 times lesser than that of ZrO2. Further, the mechanical and wetting properties of the coatings showed a similar trend as that of corrosion behaviour.