Mechanoelectrochemical Properties of Passive Iron Surfaces Evaluated by an In Situ Nanoscratching Test

In situ (as kept at 0.25 V vs. SHE in solution) and ex situ (in air) nanoscratching tests were performed to the iron single crystal (110) and (100) surfaces passivated at 0.25 V (SHE) in pH 8.4 borate solution to evaluate the correlated mechanical and electrochemical properties, i.e. , mechanoelectrochemical properties of the passive surfaces. The friction coefficient of the passive iron (100) surface was larger than that of the passive iron (110) surface, irrespective of in situ or ex situ nanoscratching. The friction coefficient obtained with in situ nanoscratching for both the passive iron (110) and (100) surfaces were significantly larger than that obtained with ex situ nanoscratching. The increase in the friction coefficient due to in situ nanoscratching is explained in terms of a series of mechanoelectrochemical reactions taking place at the moving front of the indenter, such as active dissolution from the film rupture sites followed by repassivation.