Study On Passivation And Depassivation Of Cr10Mo1 Corrosion-resistant Steel Bars In Marine Concrete

Chloride-induced corrosion of steel bars is the main cause for performance deterioration of marine concrete,which poses severe threats to the security and service life of concrete structures.The development of new-type alloy corrosion-resistant steel bars to replace plain carbon steel bars is a promising way to solve the problem with broad application prospect.In this study,the passivation and depassivation behavior of Cr10Mo1 corrosion-resistant steel under different p H,temperature and dissolved oxygen content was systematacially investigated using various electrochemical test methods and microscopic analysis methods;the corrosion resistance performances of the corrosionresistant steel bars were also scientifically evaluated.The time-dependent behavior of steel bars in simulated pore solutions of marine concrete and in cracked concrete under real marine environment was studied and the prediction model of service life for marine concrete was established.Based on the prediction model,the service life of a cross-sea bridge was predicted.The main conclusion of this paper are as follows.(1)The formation rate of passive film for steel bars accelerates with increasing p H value,temperature and dissolved oxygen content of simulated pore solutions,accompanied by more stable film structure and much enhanced corrosion resistance of steel bar;the passivation time decreases with increasing p H value,temperature and dissolved oxygen content,and these factors have less influence on corrosion-resistant steel bar.Compared with plain carbon steel bar,the passive film structure of corrosionresistant steel bar is more compact and the Chromium oxide in passive film still maintains its passivation status even at a low p H value and decreased dissolved oxygen content.This indicates that the Chromium oxide provides steel bar with more corrosion resistance capacity.(2)The critical chloride ion concentration when steel depassivating increases with increasing p H value while decreases with increasing temperature and dissolved oxygen content.The critical chloride ion concentration of corrosion-resistant steel bar in simulated pore solution is 10 times higher than that of plain carbon steel bar.When under the conditions of relatively high p H value,low temperature and low dissolved oxygen content,the corrosive pitting point can repassivate even at high chloride ion concentration(2.0M),achieving repairing of passive film.(3)In simulated pore solutions of concrete with chloride ions,the speed of chloride ions to penetrate passive film increases with decreasing p H value,increasing temperature and dissolved oxygen content,leading to a more difficult self-repairing process of passive film and thus degraded corrosion resistance.At the early stage of corrosion,the steel bars were corroded at a fast rate,while after a certain time the corrosion rate gradually decelerates to a stable level,as a result of the blocking effect of the accumulated rust layer.At the later stage of corrosion,the corrosion rate of corrosion-resistant steel bar is lower than that of plain carbon steel bar in all the different corrosive environment.(4)The steel bar corrosion of cracked concrete in real marine environment is influenced by crack width of concrete,crack feature and marine corrosion zone,and these factors have greater influence on corrosion-resistant steel bars than plain carbon steel bars.With a crack width of concrete less than 0.2 mm,the corrosion-resistant steel bars maintain its passivation state even after a corrosion time of 5 years.The corrosion degree of steel bars in real marine environment is ordered as: multi-cracks>single crack,longitudinal crack>transverse crack,splash zone/tidal zone>underwater zone;An exponential evolution equation was obtained between corrosion current density of steel bars and corrosion time for both cracked concrete in real marine environment and indoor simulated pore solution of concrete.The acceleration coefficient of steel bar corrosion in simulated pore solution compared with that of concrete in real marine environment is55.71.(5)Based on DuraCrete model and taking into consideration of temperature,oxygen content,initial chloride concentration in concrete and acceleration coefficient,a two-stage service life prediction model including both the initiation period and propagation period of corrosion.On the basis of measured concrete cover thickness,diffusion coefficient of chloride ion,design parameters,etc.of the service life of piers in the cross-sea bridge of Qinglian railway is predicted to be 195 years.