Research On The Corrosion And The Intelligent Protection Of The Reinforcing Steel In Concrete For Marine Engineering

Corrosion of the steel rebar is the major deterioration cause of reinforced concrete structures in marine environments.Chlorides from seawater are responsible for the corrosion of steel embedded in the concrete structures.When chloride concentration near the steel reaches a certain threshold,localized corrosion is triggered on the steel surface,which can eventually lead to the cracking and delamination of the concrete structures.So far,the corrosion damage of the steel rebar in concrete is generally believed to result from a pitting process or a galvanic effect.Nevertheless,there is a possibility that the pitting or galvanic corrosion stems from crevices,since in reinforced concrete numerous gaps can be formed between the steel rebar surface and the aggregates,and some of the gaps are perfect crevices to initiate a crevice corrosion process.For the local corrosion of the reinforcing steel,Many techniques have been developed to control the initiation and inhibit the development of the corrosion of reinforced concrete structures.Among these corrosion control methods,cathodic protection is considered as the most effective technique.There are basically two types of cathodic protection systems:1)impressed current cathodic protection and 2)sacrificial anode cathodic protection.The first system has been widely employed,but its maintenance is difficult,sometimes limited by the availability of electric power supply.The second system appears to be more convenient,as no external wiring nor auxiliary power supply is required,but it is not adjustable after installation and cannot provide a desired current or voltage for ideal protection.Hence,a simple sacrificial anode technique that can intelligently offer a cathodic protection for reinforced concrete is highly demanded.This paper focuses on the local corrosion(mainly crevice corrosion)of reinforcing steel in the concrete and the cathodic protection of sacrifi cial anode.Main research contents and conclusions of the thesis are listed as followings:1.The crevice corrosion was firstly proposed as one of the important mechanisms responsible for the damage of steel rebar in concrete,which was experimentally verified in simulated concrete pore solution(SCPS)and concrete blocks.The effects of chloride concentration and temperature on the crevice corrosion were comparatively investigated by means of potentiodynamic polarization curve,electrochemical impedance spectroscopy(EIS)and galvanic current measurements.Results indicated that the crevice corrosion could occur on the HRB400 steel at room temperature once the concentration of chloride ions was higher than 0.25 M in the SCPS,or at temperatures higher than 35℃ in the SCPS containing 0.10 M chloride ions.In addition,The galvanic effect resulting from the differences in electrochemistry between the interior and exterior of a crevice in the SCPS accelerated the anodic dissolution inside the crevice.This accelerating effect increased with increasing concentration of chloride ions and temperature.Finally,the possibility of the crevice corrosion in the reinforced concrete were verified in practice,and the results indicated that the crevice corrosion could also be preferentially triggered in concrete.Crevice corrosion damage that had been overlooked must be considered in reinforced concrete structures.2.Corrosivity of simulated concrete pore solution(SCPS)contaminated by chloride was automatically detected and corrosion of HRB400 steel in SCPS was intelligently prevented by magnesium(Mg)alloy.Results of the polarization curve,galvanic current,and electrochemical impedance spectroscopy(EIS)measurements indicated that the traditional sacrificial anodes Zn and Al dissolved rapidly in the SCPS without corrosive media,resulting in overprotection for the HRB400 steel.However,Mg could act as effective sacrificial anode to cathodically protect steel from corrosion attack once the SCPS was polluted by chlorides.Al alloying could enhance the sensitivity of Mg anode to chloride contamination and further improve its cathodic protection effect,and thus enhanced its intelligence level as corrosivity detector and corrosion protector.The automatic corrosivity detection and intelligent cathodic protection of the Mg alloy for HRB400 steel were also validated in concrete.3.Mg-Al alloys with different Al concentrations were prepared by using of an electromagnetic induction heating furnace.The intelligence of Mg-Al alloys in corrosivity detection of concrete and cathodic protection of HRB400 steel was systematically investigated by means of polarization curve,galvanic current and electrochemical impedance spectroscopy(EIS)measurements.Results indicated that the intelligence of a Mg-Al alloy sacrificial anode in concrete-corrosivity detection and reinforcing steel corrosion protection could be improved with appropriate alloying Al concentration.However,an excessive of Al concentration in the Mg-Al alloy sacrificial anode could trigger hydrogen embrittlement damage to the steel.The Mg-Al alloy sacrificial anode with 17 wt.%Al was the best corrosivity sensor and corrosion protector.In addition,the service life and the intelligence level of a Mg-Al alloy sacrificial anode corrosivity sensor and corrosion protector could be improved by adjusting the cathodic/anodic surface ratio of the HRB400 steel over the Mg-Al alloy.Based on the above results,the corrosion mechanisms of the Mg-Al alloy sacrificial anodes were analyzed.In the Mg-Al alloy sacrificial anode with a lower Al concentration,the a phases near to the β phases and the a phases included in the βphases were preferentially corroded,whereas both the primary a phases and the a phases included in the β phases suffered from serious corrosion damage in the Mg-Al alloy sacrificial anode with a higher Al concentration.Finally,the intelligent corrosivity detection and cathodic protection performances of the Mg-Al alloy sacrificial anodes were also verified in reinforced concrete blocks.