Effect of corrosion layer of steel bar in concrete on time-variant corrosion rate

Abstract

Corrosion current density of steel bar in concrete was measured in active corrosion process under a designed artificially controlled climate environment. The active corrosion process shows the characteristics of the time-variant corrosion rate, and the three phases of the corrosion process are presented. The corrosion rate decreases at first; this is followed by a steady state phase; finally after concrete cover cracking caused by corrosion, an ascending phase of the corrosion rate is observed. The mechanism of the time-variation characteristics is discussed based on the microstructure of the interfacial transition zone (ITZ) between steel bars and concrete at different corrosion levels. The microstructure shows that the porous interfacial transition zone gradually transforms into a dense corrosion layer composed of concrete and corrosion products due to expansion of the corrosion products. The layer is called as corrosion layer for short in this paper. The main reason for the descent of the corrosion rate is that transportation of oxygen and moisture is retarded due to the dense corrosion layer. When the equilibrium between rates of consumption and transportation of oxygen is reached, the corrosion rate tends to be steady. The concrete cover cracking offers new access for transporting oxygen and the corrosion rate speeds up.