Study into mechanical and electrochemical properties of coating deposits and welded-coated components using the HVOF (High Velocity Oxy-Fuel) process.

Abstract

The present study examines the metallurgical, mechanical and corrosion properties of High Velocity Oxy-Fuel (HVOF) thermal spray coatings of Inconel 625 powders on plain and welded surfaces of mild carbon and stainless steel (304). The research work carried out focused on coating adherence to base substrate, coating integrity and mechanical behavior of coating at the weld-substrate interface when subjected to tensile and fatigue loads. The solid bar and sheet specimens were tested and prepared in accordance with the test standards "Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings". The specimens were tensile and fatigue tested pre and post exposure to brine. SEM and EDS analysis were carried out to examine the surface morphology and microstructure of the coatings. The specimens that were subjected to a corrosion environment for an extended period of three weeks failed at a lower tensile load than those that were not exposed to corrosive environment. Shear deformation of the adjacent zones at the coating-substrate interface resulted in the total failure of the coating. However, in some cases, the coating was almost free from voids or pores and was impervious to electrolytic or any other contaminants. This result was observed from tests performed on different sets of specimens to identify brine exposure effects on coating adherence to the substrate. Foreign materials were observed beneath the coating due to improper surface preparation during the specimen preparation prior to coating. From tensile tests results it was observed that the bond strength of the coating reduced due to corrosion effect which caused degradation of the coating adhesion to the specimen's surface. This is probably due to the formation of oxides on carbon steel surfaces during coating. Due to expected progressive oxidation, coating adherence was continuously degraded with lowering bond strength. Fatigue results revealed that an excess of 15 thousand cycles without cracking was demonstrated by the stainless steel coated specimens assuring the fatigue strength. The fatigue life resistance of the stainless steel coated specimens exceeded .that of the coated carbon steel specimens. Moreover, the imperfection at the interface resulted during coating could cause early initiation of cracks in the coating, therefore, thus lowering the fatigue strength