Oxidation And Reduction Of Mill Scale Of Rebar In Simulated Concrete Pore Solution

Rebar corrosion is a main factor threatening the durability of reinforced concrete.The electrochemical behavior of rebars with mill scale?balck rebars?is quite different from those without that.Therefore,the oxidation and reduction of mill scale in simulated concrete pore solution is crucial to understand the mechanisms about the corrosion and passivation of steel rebar.The microstructure,composition and charge transfer characteristics of mill scale are investigated by means of SEM,XRD,Raman spectroscopy and Mott-Schottky plot.The results show that the mill scale is composed of Fe O,Fe₃O₄and a little?-Fe₂O₃ from inside to outside with a flaky surface of microdefects,and it performs as an n-type semiconductor in the saturated Ca?OH?₂ solution which is commonly used as a simulated concrete pore solution.The oxidation and reduction of mill scale under polarization in saturated Ca?OH?₂ solution are explored using cyclic voltammetry and Mott-Schottky curves,as well as Raman spectroscopy.The results show that the electrode processes of black rebars are similar to those without mill scale and the electrode reactions on the black rebars are mainly affected by the oxidation and reduction of mill scale.Both rebars with and without mill scale exhibit an obvious n-type charge tranfer characteristics within-0.6V?-0.2V/SCE and 0.2V?0.5V/SCE potential ranges.The oxidation and reduction of mill scale are controled by the redox of Fe????Fe???occuring in the outer surface layer.When anodicaly polarized at 0.3V/SCE,the Fe₃O₄ in the surface layer of mill scale is inward oxidized to?-Fe₂O₃,and when cathdicaly polarized at-1.1V/SCE,the?-Fe₂O₃disappears.None newly formed phase is found at the matrix/mill interface at the conditions mentioned above.The oxidation and reduction behavior of mill scale under natural immersion conditions in saturated Ca?OH?₂ solution are detected by electrochemical methods and surface analysis.The results show that the Fe₃O₄ in the surface layer of mill scale also could be oxidized to form a trivalent iron compound.Galvanic couple can form between the mill scale and matrix when both simutenously expose to the solution,which leads to the oxidation of the matrix.The donor densities of rebars with and without mill scale shift positively with the increasing dissolved oxygen concentration in the range of 0.3ppm?9ppm,indecating the iron intersitials are the major donor carriers in the charge transfers.The oxidation and reduction of mill scale on rebars in saturated Ca?OH?₂solutions can be finaly summarized to the redox of Fe????Fe???of Fe₃O₄ in the surface layer of mill scale,which can be employed to interpret the“passivation”and corrosion mechanism of black rebars.