Surface layer microstructure evolution during interdiffusion annealing of chromium electroplated iron substrates

Abstract

Objective of the present study was the understanding of the interdiffusion behaviour of chromium in deposited layers on a pure iron substrate. For this purpose, the layer substrate systems were vacuum‐annealed for 6 h, 12 h and 24 h at temperatures between 970 °C and 1090 °C, respectively. After the interdiffusion treatment, samples were analyzed by means of scanning electron microscopy (SEM) in combination with energy‐dispersive X‐ray spectroscopy line analysis (EDX) and electron backscatter diffraction (EBSD). The results revealed that interdiffusion causes multimodal microstructure evolution. The applied chromium layer remained fine‐grained due to the formation of nano‐sized chromium oxides on grain boundaries during vacuum annealing. At the interface to the substrate some of those fine grains grew into the interdiffusion zone and caused stress relief during interdiffusion. Details of the microstructural changes, like a concentration step formation on account of a Cr concentration‐dependent shift of the fcc to bcc phase boundary and the formation of a pore network in the interface area, were discussed using the results of the microstructure analysis as well as phase diagrams and thus contributing to the development of interdiffusion coated plain carbon steels.