Effect of surface treatments on the thermal emissivity of AISI 304L stainless steel

Abstract

Abstract The low thermal emissivity of AISI 304L stainless steel is one of the critical design requirements for controlling radiative heat transfer among the components of cryogenic and superconducting systems used in nuclear fusion machines such as the ITER tokamak. This study aims to determine the lowest emissivity of SS304L at room temperature using electromagnetic wave theory and practically achieve this by enhancing the surface quality. This study investigates the effects of mechanical polishing and electropolishing on the surface roughness and emissivity of SS304L. The electromagnetic wave theory yields an emissivity of ~ 0.098 at ambient temperature when using the electrical conductivity of SS304L, which is 72 µm ohm-cm. The SS304L sample plates were mechanically polished and then electropolished to reduce emissivity up to the theoretical value. Emissivity was measured at room temperature using a portable emissometer following the ASTM C1371 standard. The surface roughness (Ra) and emissivity of the as-received SS304L are 4.24 ± 0.45 µm and 0.35 ± 0.01, respectively. After mechanical polishing followed by electropolishing, these values were reduced to 0.06 ± 0.01 µm and 0.11 ± 0.01, respectively. It is observed that the experimental emissivity of the optically smooth electropolished SS304L at room temperature is marginally higher than the theoretical value owing to the existence of certain surface irregularities, residual stress, and the development of a passive oxide layer on the outermost surface. The measured emissivity is consistent with the theoretically predicted value derived from the electromagnetic wave theory.