Study On Natural Passivation And Depassivaation Behavior Of Stainless Steel In Simulated Concrete Pore Solution

The durability of steel reinforced concrete in a chloride environment has always been a key scientific problem that has attracted widespread attention from scholars at home and abroad.Corrosion of the reinforcing steel is generally considered to be the main factor causing the decrease in durability of reinforced concrete structures.Therefore,in-depth study and discussion on the corrosion behavior and mechanism of steel rebar in concrete structures have important scientific significance and engineering application value for further taking better anti-corrosion measures and reasonably predicting the remaining service life of concrete structures.In practical engineering applications,the reinforced concrete structure usually needs to go through two stages:the curing stage and the application stage.Correspondingly,the surface state of the steel rebars and their environment are completely different,so their corrosion behaviors are also significantly different.However,a perusal of literature has shown that the vast majority of the current studies on the corrosion behavior of steel rebars in simulated concrete pore solution have confused these two stages,either ignoring the passive state of the steel rebars,or inconsistent with the environment of the curing stage to which the steel rebars exposed;or using the anodic oxide film instead of the natural passivation film to study the corrosion resistance of steel rebars.The obtained results are often far from the practical field performance,and it is difficult to guide specific practice.Therefore,in this thesis,the passivation behavior of stainless steel(SS)in simulated curing stage concrete pore solution,including ordinary carbon steel,and the depassivation process of naturally formed passive film on SSs in simulated service stage concrete pore solution were systematically studied using different corrosion electrochemical methods,combined with the surface observation and analysis techniques.The influence and mechanism of the SSs microstructures,alloying elements and environmental factors on the passivation and depassivation behavior of SSs were analyzed and explored.For the environment where the steel rebars are exposed during the concrete curing stage,it is obvious that the natural passivation process is more consistent with the actual growth of the passive film on the steel rebar surface in the field.The research results have shown that in the high alkalinity simulated concrete pore solution,with the prolonging of passivation time,a natural passive film with the gradient chemical composition and the layered structure was formed on the surface of the SSs.Its chemical composition and corrosion resistance were closely related to the alloy elements contained in the substrate and the passivation time.The outer layer of the natural passivation film formed on HRB400 steel was mainly Fe3+oxides,and the inner layer was mainly composed of Fe2+oxides.The outer layer of the natural passive film of 430 ferrite steel and 304 austenitic steel constituted of Fe and Cr oxides and hydroxides,enriched in Fe3+ compounds,while the inner layer was mainly composed of Fe2+and Cr3+oxides,enriched in Fe2+species.During the natural passivation process of duplex stainless steel(DSS)2304 and 2205,alloying element Mo directly participated in the growth process of passive film,and affected the composition distribution and thickness of the passive film by modifying the content of other oxides in the passive film.At the initial stage of natural passivation,the corrosion resistance of DSS 2205 with high Mo content was weaker than that of DSS 2304 steel with low Mo content since the partial substitution effect of Mo oxide on Cr oxide in the passive film was dominant.With the increase of passivation time,the synergistic effect of Mo-Ni alloy elements gradually dominates the passivation process,thereby increasing the content of Cr oxides within passive film formed on the surface of 2205 steel and effectively promoting the improvement of its corrosion resistance.Aiming at the environment of the steel rebar surface during the concrete service stage.the effects of concrete carbonation(pH value of simulated concrete pore solution),Cl-concentration and temperature on the depassivation process of natural passive film formed on SSs surfaces were studied.The results proved that with the decrease of simulated solution pH,Fe3O4 in the passivation film formed on the steel surface gradually decomposed into porous FeOOH and Fe(OH)3,so that the ratio of Fe2+/Fe3+within the passive film decreased,while the content of Cr oxides slightly increased.For the depassivation process of natural passive film,the competitive adsorption balance on the passive film surface between OH-with the inhibition effect on depassivation and Cl-with the promotion action determined the degradation process of the passive film.In the solution with high pH and low Cl-concentration,the inhibition effect of OH-on the depassivation process was predominant,the natural passivation film was in a stable state,with stronger self-repairing ability and improved pitting corrosion resistance.With the decrease of solution pH value and the increase of Cl-concentration,the promotion activity of Cl-played a dominant role in the depassivation process,resulting in that the passive film was unstable and began to degrade.Temperature had significant influence on the passivation behavior of the steels tested and the semiconductor properties of the natural passivation film formed on their surfaces.As the temperature of the solution increased,the concentration of dissolved oxygen in the solution reduced and the oxidation rate of the oxide in the passivation film increased,resulting in a gradual increase in the resistance value of the passivation film of HRB400 steel,and its stability and corrosion resistance were improved.For stainless steel,with the increase of solution temperature,the resistance of the passivation film first increased and then decreased.This phenomenon might be related to the changes of oxidation of the components and their state within the passive film formed on SSs surfaces.In an alkaline solution containing Cl-,with the increase of temperature,the activity of Cl-increased,and the ability and rate of penetrating the passive film was accelerated,leading to the decrease in the stability of the passive film and the acceleration in the degradation of passive film,and ultimately the corrosion resistance of the prepassivated steels was reduced.