| Corrosion of rebar is key factor to degrade the durability of reinforced concrete.The corroding and passivating behavior of black rebar with mill scale is definitely different from that of the polished one.Therefore,intensive research on black rebar with mill scale is urgently needed and much helpful to approach the truth of rebar corrosion in concrete than focusing on the polished one,as well as to design durable reinforced concrete.The microstructures,transporting,and influence on the corrosion susceptibility of black rebar of mill scale of mill scale are investigated using scanning electron microscopy(SEM),Mott-Schottky plots,and other methods.The results show that:(1)the mill scale could retard the matrix to dissolve at the very initial stage of immersion,but play a negative role in later stage,owing to its inherent microdefects;(2)mill scale demonstrates as an n-type semi-conductor;(3)mill scale could form a galvanic couple with the matrix,and the caused galvanic corrosion leads the thereafter corrosion of rebar.The passive characteristics of black rebar with mill scale in saturated Ca(OH)2 solution is studied by dynamic or linear polarization together with open circuit potential measurement.The results show that:(1)black rebar with mill scale seems able to be passivated phenomenally;(2)the open circuit potential and linear polarization resistance of black rebar increases with the immersed duration,and the prepassivated one shows much more stable;(3)when considering the Mott-Schottky plots actually reflect the charge transfer in electrode reactions at the metal/solution interface,four different processes within 0.6~0.8V/SCE range are determined for black rebar in saturated Ca(OH)2 solution.The redox processes of black rebar with mill scale are studied by cyclic voltammetry.The results show that:(1)there is no significant difference during anodic oxidation between the black rebar and the polished one;(2)cathodic reduction of black rebar is the same as the polished one if the highest anodic potential does not enter the passivation range,otherwise the Fe(II)reduction process disappears.The oxidation products on the surface of black rebar with mill scale in different conditions are studied using Raman spectrum.The results real that:(1)the mill scale is composed of inner original Fe3O4 and outside γ-Fe OOH after the black rebar is immersed in saturated Ca(OH)2 solution for 140d;(2)the outside mill scale turns to α-Fe2O3 while the inner remains in Fe3O4 after the black rebar is anodically polarized for 120h;(3)no new phase is found at the mill scale/matrix interface in both cases mentioned above;so it can be concluded that,(1)the phenomenal stability of black rebar under long-term immersion or anodic polarization is both owing to the oxidation of Fe(II)to Fe(III)within the mill scale,and(2)the expected passive film as formed on polished rebar surface is difficult to generate at the matrix/scale interface in black rebar. |
