Corrosion Characterization and Performance Evaluation of Co-Cr alloys by Laser Powder Bed Fusion and Suitability of their use for Pressure Containing Down-hole Wear Components

Tungsten containing Cobalt based alloys (Co-Cr-W-C) possess an attractive combination of corrosion resistance, erosion resistance and heat resistance properties. This property combination coupled with their low magnetic permeability values make them suitable for many flow components in the measurement and logging while drilling (M/LWD) systems in the oil and gas industry. However, flow components manufactured from investment cast Stellite 6 alloy (Co-Cr-W-C) have shown that they are susceptible to cracking under operating conditions due to the brittleness of the material and the complex geometry of these parts. One possible solution was to produce the same part through additive manufacturing (AM) with a Co-Cr-Mo-C system which exhibits significant ductility enhancement compared to cast Stellite 6. The current study evaluated parts fabricated using a laser powder bed fusion additive manufacturing using a Cobalt 28 (Co-Cr-Mo-C) alloy. These AM parts, when put under pressure, led to a loss of pressure/failure of the component because of a network of interconnected pores. Microstructural evolution and defect density of these AM parts are characterized in this study. The role of chemistry in the microstructure has been studied using energy dispersive spectroscopy mapping and porosity quantification is studied using computed tomography scanning. The difference in the wear behaviors of these two families are characterized using low-high angle dry erosion tests. Comparison of their corrosion performance was performed by exposing them to alkaline brines with high concentration of chlorides typically seen in the drilling environment. The corrosion properties were studied using electrochemical methods such as open circuit potential, linear polarization resistance, and cyclic potentiodynamic polarization along with high temperature immersion testing. Optical microscopy techniques were further utilized to characterize the microstructures of the samples pre/post corrosion testing to further assess the role of microstructural defects on corrosion rate of the AM parts compared to investment cast Stellite 6 parts.