Experimental Study On Corrosion Process Of Reinforced Concrete In Sulphate-chlorine Environment

Concrete structures in service environments such as ocean and seashore saline soil for long are not only corroded by chloride ion,but also corroded by sulfate ion,which leads to durability reduction of concrete structures and decrease of service life.However,in the environment of chloride ion and sulfate ion,the corrosion law of reinforced concrete structure is quite complicated.Study on the corrosion mechanism of reinforced concrete caused by coupling effect of chloride and sulfate has great significance on durability evaluation and service life prediction of concrete structures in ocean.In this paper,by designing experiments and using electrochemical test methods,the depassivation process of steel bars in chloride environment was studied,the effect of sulfate on corrosion of steel bars in chloride environment and the transmission of sulfate ion and chloride ion in cement mortar were analyzed,the main research and conclusions are listed as follows:(1)Tests on properties of passive film on the surface of steel bars in simulated pore solution of concrete were carried on,using electrochemical test methods,the change of corrosion potential of steel bar electrodes and electrochemical impedance spectroscopy with solution alkalinity and content of chloride and sulfate ion were analyzed,the formation and depassivation process of passive film on the surface of steel bars were obtained,threshold equation of chloride ion on the surface of steel bars related to alkalinity of simulated solutions and chloride ion concentration were established.The results show that:the time of stable passive film on the surface of steel bars decreases with the increase of solution alkalinity,however.the chloride concentration of depassivation increases with solution alkalinity,and sulfate ion has inhibitory effect on chloride corrosion of steel bars to a certain extent.(2)Replacing concrete with cement paste as the protective layer of steel bars,steel specimens with cement paste protective layer and electrochemical test instruments were designed and made,study on electrochemical impedance spectroscopy test of chloride and sulfate in corrosion solutions was carried on,effect of solution,ratio of water to cement and thickness of protective layer on electrochemical parameters of cement paste steel bars and their degree of corrosion.The results reflect that:in the early stage of corrosion,sulfate slows down the corrosion process of steel bars in cement paste of chloride solution;but in the late stage of corrosion,sulfate accelerates the corrosion process of steel bars in cement paste.Besides,the degree of corrosion of steel increases with the ratio of water to cement of cement paste;however,the degree of corrosion of steel decreases with the increase of the thickness of cement paste protective layer.(3)Using cement paste as the protective layer of steel bars,electrochemical tests were carried on in single chloride solution and different concentration of chloride-sulfate coupling solution,the concentration distribution of sulfate and chloride ion in different depth of cement mortar protective layer was analyzed.The results show that:the concentration of sulfate and chloride ion decrease with the increase of corrosion depth;furthermore,sulfate has inhibitory effect on transmission of chloride ion in cement paste to a certain extent,and the greater the concentration of sulfate ion,the more obvious the inhibitory effect.