| Reinforcing steel corrosion as a major disease for civil engineering has not been overcame for long time.Developing alloy corrosion-resistant steels with high corrosion resistance but attractive economy in replacing plain carbon steel,seems to be the dominant solution to prevent reinforcing steel corrosion persistently,showing good prospects.In recent years,Research Institute of Jiangsu Shasteel Iron and Steel prepared out a new alloy corrosion-resistant steel,Cr10Mo1,by means of adjusting alloying elements composition and optimizing hot working technologies,which has a corrosion-resistance(measured by the critical chloride threshold level)higher than 10 times that of plain carbon steel,and could bring about potential high durability(75～100 years)of concrete structures.It is of great significance to investigate corrosion behaviour of the alloy corrosion-resistant steel,evaluate its corrosion resistance and clarify its anticorrosive mechanisms,in order to provide scientific theoretical basis and technical support for its popularization and application in civil engineering.This thesis comprehensively using electrochemical test and microscopic analysis methods,studied corrosion behaviour of Cr10Mo1 alloy corrosion-resistant steel(marked as "CR")and plain carbon steel(marked as "PC",as a comparison)in simulated concrete pore solutions with different pH values(13.3,12.0,10.5 and 9.0)at different stages of steel corrosion that is from passivation stage,passivity maintaining and breaking stage to corrosion propagation stage in the perspective of steel reinforcement corrosion lifecycle in concrete.The major contents including:passive behaviour of the steels in environments of different pH values without chloride and with various chloride concentrations(0.2 M,0.4 M,0.6 M,0.8 M,1.0 M),the attacking behaviour of chloride at different concentrations(0 M,0.2 M,0.6 M,1.0 M,2.0 M)to the passive films on the steels fully passivated in solutions of different pH values and corrosion propagation behaviour of the steels under high chloride(4.0 M)corroding in environments of different pH values.The thesis led a comprehensive method for corrosion resistance evaluation of alloy corrosion-resistant steels,replenished the anticorrosive mechanisms of them,and could provide guidance for systemic research on corrosion of alloy corrosion-resistant steels.The main conclusions and innovations in this research are shown in the following four parts:(1)The passivation behaviour of CR steel affected by different pH values and Cl-concentrations was explored,based on the characterization and analysis of the structural composition and electrochemical performance of the passive film formed on the steels.In solutions without presence of chloride,as the pH decreasing,CR steel has a significant improving passivity,which is the opposite case for PC steel.The passive film formed on CR steel bears a bilayer structure:the outer layer mainly contains Fe3O4,Fe2O3 and FeOOH/Fe(OH)3,while the inner layer is enriched in Cr2O3 and CrOOH/Cr(OH)3.The passive film formed on PC steel consists of Fe3O4,Fe2O3 and FeOOH/Fe(OH)3.When the pH drops,Cr oxides have a further enrichment in the passive film of CR steel and the film becomes thicker,thus the passivity of CR steel increases.While Fe-oxides in the film layer of PC steel are difficult to form,the film growth is arduous and the film thickness reduces,causing a decreasing passivity of PC steel.In solutions with presence of chloride,the steels show degraded passivation as chloride increasing.When the Cl-concentration not more than 0.2 M,CR steel can passivate at all pH values,and the passivity shows enhancement as pH dropping all the same.However,with chloride above 0.6 M,CR steel still has good passivity at high pH(13.3,12.0)but hardly passivates even occurring pits at low pH(10.5,9.0),suggesting that CR steel has decreasing passivity with pH in presence of high chloride,which is contrary to the trend when in presence of low chloride.Therefore,the weakening effect of chloride on passivation of CR steel depends on pH level of the solution:the lower the pH,the greater the weakening.In solutions of pH 13.3,increasing chloride also hinders the passivation of PC steel,and the steel cannot be passivated when chloride more than 0.4 M.To sum up,compared to PC steel,CR steel shows passive-like characteristics.(2)The reasons for the good stability and high resistance of passive film formed on CR steel were announced by dissecting the structural composition and electronic structure evolution of the film layer under C厂 attacking and investigation of metastable pitting and passive film rehabilitation of CR steel in presence of Cl-.The critical chloride threshold level to depassivate CR steel in solutions of pH 13.3,12.0,10.5 and 9.0 is 3.8 M,2.6 M,0.52 M and 0.36 M,respectively.For PC steel in solution of pH 13.3,the depassivating chloride level is 0.23 M.So,it can be stated that CR steel has a critical chloride threshold level more than 10 times(conservative estimation)that of PC steel.When the passive film on steel encounters with chloride,Cl-occupies anion vacancy of the metal oxides in the surface area of the film,and destroys the electronic structure of the oxides,resulting in the dissociation of metal cations in the oxides lattice and then the gradual dissolution of the oxides phase in the film surface,correspondingly,the whole film thickness reduces.In lower pH environments,the electronic structure of protective Fe/Cr oxides in passive film of CR steel is readily damaged,leading to the accelerated destruction of the film.Therefore,high pH is still beneficial to the stability of passive film on CR steel when exposed to chloride.The weakest regions of passive film on steel are most vulnerable to breakage and preferentially form pitting nucleation under Cl-attack.During the pitting metastable growth,passive Cr oxides can be formed in these regions of CR steel,repairing the locally damaged passive film when in high pH(13.3～12.0)environments with Cl-even at 2.0 M.Therefore,submitted to chloride,passive film of CR steel has much stronger self-repairing capability than that of PC steel.(3)The inhibition mechanism for CR steel corrosion propagation is expounded,after examining the structural composition and hampering action on corrosion mass transport of rust layer on CR steel.After stable pitting forming,corrosion rates of the steels show a decreasing trend as the time goes on,and finally tends to be basically stable.The accumulating rust layer produced by the corrosion reaction can somewhat hinder corrosion propagation of the steels.Corrosion rate of CR steel rises notably with the pH dropping in the same immersion time,indicating that the densification progress of rust layer on CR steel slows down in lower pH environments.In solution of pH 13.3,the corrosion rate of CR steel is significantly lower than that of PC steel,signifying the rust layer on CR steel is much more obstructive.This is because the rust layer of PC steel is loose and contains many run-through holes and cracks,facilitating the invasion of corrosive substances from the outside environments.For CR steel,although the outer rust layer is also porous,the inner rust layer is relatively compact and tightly adheres to the metal matrix,which could prevent corrosive chloride penetrating into and eroding the matrix.There is an obvious enrichment of Cr element in the inner rust layer of CR steel,which promotes more fine grain a-Fe2O3 formation,and thus the inner rust layer becomes very stable and dense.(4)From the corrosion behaviour characteristics of CR steel in passivation stage,passivity maintaining and breaking stage and corrosion propagation stage,the anticorrosive mechanisms of CR steel are summarized.The anticorrosive mechanisms of CR steel can be stated as:Cr element in the metal matrix takes part in the passivation processes of the steel,and the formed passive film presents abilayer structure with the inner layer enriched in Cr species and the outer layer mainly containing Fe species.Even if in very low pH or high chloride environments,passive Cr oxides can still form,making the CR steel to be passivated.So CR steel behaves passive-like performance.For Cr oxides in passive film of CR steel,the lattice point defects are mainly cation vacancy,which can inhibit the adsorption and penetration of Cl-anions into the film.In addition,the electronic structure of Cr oxides is difficult to be damaged by Cl-action.These result that the passive film on CR steel is much higher resistant to Cl-attack than that of PC steel.When the weakest regions of passive film on CR steel breaks and produces pitting nucleation under high chloride attacking,reparative passive layers containing Cr oxides can be formed afresh in these pit nucleating regions,and continue to protect the metal,if the solutions have considerably high pH values(13.3～12.0).Cr element in the metal matrix participates in the corrosion processes of CR steel,contributing to the formation of the stable and adherent inner rust layer with Cr enrichment,which as a barrier hinders the transport of corrosion substances,and so the corrosion propagation of the steel is inhibited. |
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