Introduction
The corrosion of steel bar in tunnel lining concrete under high geothermal environment is the result of the combined effect of temperature, pH value of pore solution, and erosion ions. In this paper, the critical ion concentration and corrosion behavior of steel bar under the combined action of ambient temperature (20 ℃ and 60 ℃), pH value of pore solution (12.55, 12.15, and 11.75) and erosion ions (Cl–, SO42–, and Cl–, SO42– composite) were studied by simulating concrete pore solution. The results show that the critical chloride concentration of steel corrosion is 0.04–0.06 mol/L in the saturated concrete simulated solution with temperature of 60 ℃ and pH of 12.55 during chloride corrosion. The stability of the passivation film decreased when the pH value decreased (12.15 and 11.75), and the corresponding critical chloride ion concentration also decreased (0.03–0.05 mol/L and 0.02–0.04 mol/L, respectively). The corrosion induction period of steel bar becomes longer when the steel bar is eroded by single sulfate. The threshold value of sulfate ion concentration is between 0.06–0.08 mol/L when the temperature is 60 ℃ and the pH value is 12.55 and 12.15, and it becomes 0.05–0.08 mol/L when the pH value decreases to 11.75. There is no significant difference in the sulfate concentration threshold of steel corrosion, because the gypsum covers the steel surface and delays the steel corrosion which formed by the reaction of sulfate with Ca(OH)2 in the simulated pore solution. Therefore, temperature and pH value have not significant effect on the corrosion concentration threshold caused by SO42–. Under the combined action of Cl– and SO42–, SO42– will repel the Cl–, and it reacts with Ca2+ to form CaSO4, which covers the surface of steel bar, densifies the passive film, and slows down the corrosion rate of steel bar. When the pH value is 12.55 and 12.15, SO42– reaches 0.03 mol/L and 0.04 mol/L respectively, which can inhibit the corrosion of steel bar caused by Cl–. However, when the pH value is 11.75, SO42– has not inhibition effect on the corrosion of steel bar caused by Cl–.
Methods
The electrochemical experimental instrument was Wuhan Koster electrochemical workstation, model CS1350, with a current range of 2 μA–20 A and a constant potential control range of ±10 V. A three-electrode system includes working electrode, counter electrode, and reference electrode, was chosen for electrochemical testing, that is, the steel bar sheet (10 mm in diameter and 0.785 cm2 of exposed area) as the working electrode, a platinum sheet electrode (20.0 mm×20.0 mm×0.1 mm) as the counter electrode, and a saturated copper sulfate electrode (CSE) as the reference one. The corrosion potential (Ecorr) of the steel bars was tested by the Open Circuit Potential Method (OCP). During the test, the working electrode was immersed in the simulation solution, and the test began after the corrosion potential of the steel bar was stable. Then the polarization curve of the steel bar was tested by linear polarization method (LPR), the changes of corrosion current density (Icorr) and polarization resistance (Rp) of the steel bar were analyzed. The corrosion products of steel bars under different erosion ions were studied by X-ray diffractometer.
Results and discussion
In this paper, rapid corrosion tests of steel bars in concrete simulated pore solutions under the combined effect of different temperatures, erosion ions types and concentrations and pH values were carried out. The critical chloride ion concentration and sulfate concentration thresholds for the steel to start rusting in different temperatures and pH values were clarified. Moreover, the corrosion products generated by steel corrosion under different erosion ions were investigated by XRD.
Conclusions
The results showed that the critical chloride ion concentration of steel corrosion was 0.04–0.06 mol/L in the saturated concrete simulated solution with pH value of 12.55 at 60 ℃. When the pH value reduced to 12.15 and 11.75, the stability of the passivation film decreased at the same time. The critical concentration of chloride ion also reduced to 0.03–0.05 mol/L and 0.02–0.04 mol/L. The corrosion induction period of steel bar becomes longer when the steel bar is eroded by single sulfate. The threshold value of sulfate ion concentration is between 0.06–0.08 mol/L when the temperature is 60 ℃ and the pH value is 12.55 and 12.15, and it becomes 0.05–0.08 mol/L when the pH value decreases to 11.75. There is no significant difference in the sulfate concentration threshold of steel corrosion, because the gypsum covers the steel surface and delays the steel corrosion which formed by the reaction of sulfate with Ca(OH)2 in the simulated pore solution. Therefore, temperature and pH value have not significant effect on the corrosion concentration threshold caused by SO42–. Under the combined action of Cl– and SO42–, SO42– can inhibit the corrosion of steel bar caused by Cl– when the pH value is 12.55 and 12.15, and the corresponding concentration of SO42– is 0.03 mol/L and 0.04 mol/L. However, when pH decreased to 11.75, SO42– had not inhibition effect on steel bar corrosion caused by Cl–.