Mechanism of Nucleation and Growth of Passive Film on Steel Reinforcing Bar at Different Durations of its Exposure in Concrete Pore Solution at Nanoscale

Abstract

The nanoscale passive film formation on steel reinforcing bars at different stages of their exposure in concrete pore solution is characterized by the application of atomic force microscopy, Raman spectroscopy, electrochemical impedance spectroscopy, electrochemical noise, and cyclic voltametry studies. Based on this, a new description of the sequence of reactions during the formation of passive film on steel reinforcing bars is suggested in this paper. The results generated during this investigation revealed that the surface of the steel reinforcing bars prior to their embedding in a concrete pore solution remain covered with a film of ferric hydroxide (Fe(OH)₃). The passive film on the reinforcing bars is formed by the dissolution of this oxide film followed by a series of chemical reactions. The nanotechnological investigations conducted in this paper indicated that a protective passive film on a reinforcing bar’s surface develops within 24 hours of its exposure to the concrete pore solution. Further exposure helps growth of the film over a period of up to 7 days. Beyond this period of exposure, no significant changes in either the structure or the protective property of the film are recorded. In solid concrete, however, the film formation and its growth is slow compared with the concrete pore solution, and a minimum period of 20 days is needed to form a complete protective film on the reinforcing bars embedded in concrete.