Surface morphology, microstructure and emissivity of rhenium electrodeposited from molten salts

Abstract

Rhenium (Re) has a high melting point and unique high temperature mechanical properties, making it an optional material that acts as a high emissivity coating on the outer wall of iridium/rhenium combustion chambers, improving their radiant heat dissipation. In this study, the effects of the electrodeposition (ED) process parameters in molten salts on the surface morphology, microstructure, and emissivity of Re coatings were investigated. The emissivity of the coating was related to the oxygen content, preferred orientation and, particularly, its morphology. The emissivity of the Re coatings increases with decreasing average radius or increasing surface density of the coating characteristic microstructures. With increasing cathodic overpotential, corresponding to high CsCl content or high current density or low molten salt temperature, the Re coating tended to change from lateral growth mode to outward growth mode, with preferred growth plane of the coating changing from (200) to (110), and meanwhile the surface morphology of the Re coating changing from stout branch-like to spiked starfish-like, leading to an increase in emissivity. The optimal deposition process parameters for the high emissivity black Re coatings were as follows: temperature ranging from 720 °C to 780 °C, cathode current density of 50 mA/cm2 to 90 mA/cm2, and the CsCl content higher than 60 wt%. The obtained black Re coatings had an emissivity higher than 0.7, with a preferred growth crystal plane of (110), and a typical spiky starfish shaped micromorphology.