Corrosion Behavior And Mechanism Of Reinforcing Steel Under Severe Composite Corrosive Medium

Currently the ongoing implementation of"Maritime power strategy"and"The Belt and Road"is taking place,while a growing number of major infrastructure projects and strategic military projects are under construction or being planned in China's coastal areas,islands and reefs areas and western areas,to name a few,South Island Reef construction,Shenzhen-Zhuhai Passage,Shenzhen-Zhuhai Intercity,Bohai Bay Passage,Yarlung Zangbo River Hydropower Station and Sichuan-Tibet Railway.According to statistics,China has been the country with the largest amount of cement production/consumption for many years.In practice,reinforced concrete structures in above-mentioned harsh environment such as maritime,ports,salt-lake/saline soil is not only subjected to loading but also attacked by composite corrosion medium such as chloride,sulfate,magnesium and carbonation from surrounding environment.Premature structure failure induced by acceleration of performance deterioration brings great hidden dangers to people's life and property or even national strategic engineering safety.In this sense,it is of great urgence and significance to investigate the corrosion behavior and mechanism of reinforced concrete in demanding environment and composite corrosion media to explore the evolution of concrete degradation and improve the concrete durability.In present study,based on actual engineering needs in salt-lake/saline soil areas,the corrosion evolution of reinforcing steels under composite corrosion medium of chloride and sulfate in harsh environment was studied using electrochemical methods and advanced microscopic characterization methods.The reliability and stability verification of electrochemical impedance data was discussed as well as the basic principles to select rational equivalent circuits.Dielectric properties and AC conducticity of steel corrosion interface are initially reported from theoretical deduction.Additionally,steel corrosion evolution of reinforced concrete structures and sulphate attack of cementitious materials was in situ,nondestructively monitored by X-CT method.Main innovative achievements of the study are as follows:1.Steel corrosion evolution in composite aggressive mediumIn order to investigate the corrosion evolution of steel bars under the action of single erosive medium and composite erosive media,open circuit potential?Ecorr?,linear polarization resistance?Rp?and corrosion current density?Icorr?of steel under different concentrations of chloride,sulfate and chloride-sulfate combinations in simulated concrete pore solution was monitored.The relationship between Ecorr and Rp was established and corrosion threshold values were obtained.Results showed that:?1?As the pre-passivation time of steel bars in alkaline environment extended,passive film approached compactness,leading to high corrosion resistance.?2?The chloride threshold value in simulated concrete pore solution was estimated to be 0.04mol/L⁰.06mol/L,while the critical concentration for steel corrosion under sulfate attack was 0.02mol/L⁰.03mol/L.Under composite chloride-sulfate condition,the presence of sulfate ions further reduced polarization resistance of steel bars and increased the corrosion rate.?3?A certain statistical relationship between Ecorr and Rp or Icorr was established accordingly and can be used to predict corrosion state and rate of reinforcing steels.2.Electrochemical impedance characterization of corrosion and corrosion mechanismTaking electrochemical impedance spectroscopy?EIS?response of steel corrosion as the entry point,the Kramers-Kronig transform was put forward for verification of EIS data and further the basic selection principles to select rational equivalent circuits for interpretation of EIS were obtained.The EIS method was initially applied to calculate thickness of the passive film/oxide of the steel during corrosion process.Corrosion products induced by chloride or sulfate are analyzed by means of XPS and XRD.The results show that K-K transform carried out by qualitative comparison and quantitative calculation depicted that experimental data accorded with the law of causality,linearity and stability.The equivalent simulation circuit of R?Q?R?QR???demonstrated the best fitting effect on the corrosion of steel in simulated concrete solution.The selection principles can be summarized as qualitative comparison between simulated data and tested data,the chi-square value used to quantitatively characterize the discrepancy between simulated data and tested data,and the physical meaning of circuit parameters evolution.It was inferred that chloride ion or sulfate ion played as catalysts in the corrosion reaction of steels and the corrosion products mainly included?-FeOOH,?-FeOOH,Fe?OH?₃,Fe₂O₃ and FeO.Additionally,compared with results of ToF-SIMS test,the method inducted from EIS demonstrated effective in predicting passive film/oxide thickness of corroded steels.3.Corrosion of reinforced concrete under salt lake brine solutionThe corrosion development and evolution of reinforcement concrete in brine environment of the western salt-lake were studied under full immersion condition and wet-dry cycles,respectively.The experimental factors such as concrete strength,cover thickness,steel type and corrosion inhibitor were considered.The results show that under full immersion condition,the average Rp value of C50 concrete was 3.8 times higher than that of C30concrete,leading to a lower corrosion risk.As the cover thickness increased from 10mm to20mm,the average Rp value has increased by 50%.The introduction of calcium nitrite corrosion inhibitor?2%by weight of cement?led to 22%increase in Rp of reinforcing steels.In case of dry-wet cycles,concrete with the same strength,cover thickness and steel type exhibited higher corrosion risk than those under full immersion condition.This was mainly due to the accelerated ingress of harmlful ions by dry-wet cyclings,leading to low Rp value and high corrosion risk.4.X-CT observation of steel corrosion and sulphate attack of cementitious materialsElectrochemical methods were used to probe into the accelerated corrosion of reinforced concrete with different proportions,cover thickness and steel types.The temporal evolution and spatial distribution of corrosion products was in situ,three-dimensionally and non-destructively obtained by X-CT test.In addition,elements distribution at steel/rust/concrete interface was obtained by SEM-EDS analysis and the thickness of the corrosion product layer was statistically obtained.The results showed that X-CT as a powerful tool can be effectively applied to 3D non-destructive characterization of steel corrosion in concrete.The average thickness of corrosion product layers determined by SEM-EDS analysis was in accordance of results from electrochemical methods and X-CT scanning.Additionally,the X-CT observation of sulfate attack of cementitious materials was explored from theoretical analysis to experimental test and 3D non-destructive visual characterization of sulphate attack for cement paste and mortar was realized.Combined with SEM-EDS test,the damaged depth obtained by X-CT was compared with the sulfate ingress depth.The results showed that X-CT can be used as a powerful tool for three-dimensional,non-destructive and visual characterization of sulphate attack of cement-based materials.Sulfate attacked and damaged depth observed by X-CT was about 47%of the sulphate ingress depth tested by SEM-EDS.