Effect of pulse current and coating thickness on the microstructure and FCCI resistance of electroplated chromium on HT9 steel cladding

Abstract

In this study, we examined the interrelations among electroplating parameters, such as pulse current and electroplating duration; structural properties (such as crystalline structure, grain size, micro-cracking, and coating thickness); and physical properties (such as hardness and Fuel and Cladding Chemical Interaction (FCCI) barrier properties) of chromium barriers. Chromium coatings were electroplated onto HT9 steel disks using direct current and pulse current (PC) for different durations to investigate the influence of PC and coating thickness on the resulting microstructure and FCCI resistance. PC and a coating thicker than 20 μm were found to be effective for improving the barrier properties of Cr coatings. Scanning electron microscopy and electron backscatter diffraction analyses revealed sound microstructures without micro-cracking and twofold grain structures including large and small grains near the outer surface and HT9 substrate of the pulse-plated Cr coatings thicker than 20 μm, respectively. These resulted in reduced interdiffusion area and interdiffusion depth in the microstructure of the diffusion-coupled specimens. Heat-treatment of Cr coatings at 650 °C for times ranging from 50 to 1000 h revealed that the grain size increases with increasing heat-treatment time. The measured Vickers hardness of Cr coatings decreased with increasing grain size and resulted in only 42.6% hardness of as-electroplated Cr coatings after 1000 h of heat-treatment.