Nano-mechano-electrochemistry of passive metal surfaces

In situ nanoindentation tests were performed to evaluate the mechanical properties of the iron (100) and (110) surfaces passivated potentiostatically for 1 h in pH 8.4 borate solution after or without immersion in 5 x 10(-2) M K2Cr2O7 solution for 24 h. It was found from the measured load-depth curves at a maximum load of 400 muN that the hardness (3.2-3.3 GPa) of the passivated iron (110) surfaces was larger by 10% than that (2.9-3.1 GPa) of the passivated iron (100) surfaces. Moreover, the hardness of the passivated iron (100) and (110) surfaces increased slightly with increasing formation potential of passive film, which was ascribed to the presence of passive film. The dichromate treatment increased the hardness of the iron (100) and (110) surfaces to some extent. The loading discontinuity appeared frequently at a load of about 90 muN in the load-depth curves for the passivated iron (110) surfaces as compared with the passivated iron (100) surfaces. The loading discontinuity in the load-depth curves for the iron 110) surface, however, disappeared completely due to the dichromate treatment. (C) 2001 Elsevier Science Ltd. All rights reserved.