摘要
基于模拟混凝土孔隙液中的钢筋脱钝试验,揭示了钢筋电极的工作面积和模拟混凝土孔隙液的pH值对钢筋脱钝过程中开路电位、极化电阻和腐蚀电流密度的影响规律,确定了不同钢筋电极工作面积和pH值条件下钢筋脱钝的临界氯离子浓度,分别建立了钢筋电极工作面积和模拟混凝土孔隙液pH值与钢筋脱钝临界氯离子浓度之间的定量关系.结果表明:随着钢筋电极工作面积的增大,钢筋脱钝的临界氯离子浓度降低;随着模拟混凝土孔隙液pH值的增加,以自由氯离子浓度[C
钢筋腐蚀是导致混凝土结构耐久性劣化的重要原因之一.在氧气和水分充足的条件下,当混凝土中钢筋表面的氯离子浓度达到钢筋脱钝的阈值浓度(即临界氯离子浓度)时,钢筋就会发生腐蚀.由于混凝土材料具有非均质和多相性,导致难以准确测试混凝土中钢筋的腐蚀状态,所以通常采用模拟混凝土孔隙液来开展钢筋脱钝的临界氯离子浓度研
在模拟混凝土孔隙液中,钢筋的腐蚀状态通常可以利用开路电位、极化电阻和腐蚀电流密度等腐蚀电化学参数来表
鉴于此,本文基于模拟混凝土孔隙液中的钢筋脱钝试验,揭示了钢筋电极工作面积和模拟混凝土孔隙液pH值对钢筋脱钝过程中开路电位、极化电阻和腐蚀电流密度的影响规律,分别确定了不同钢筋电极工作面积和模拟混凝土孔隙液pH值条件下钢筋脱钝的临界氯离子浓度,并分别建立了钢筋电极工作面积和模拟混凝土孔隙液pH值与钢筋脱钝临界氯离子浓度之间的量化关系.
利用直径D分别为6、10、16 mm的HPB300钢筋制作长度为10 mm的钢筋电极,将钢筋电极的一端光滑面与铜芯线焊接;另一端光滑面作为测试工作面,分别用240#、400#、600#、800#、1 000#、1 200#和1 500#级的水磨砂纸逐级打磨,再将打磨后的钢筋电极在无水乙醇中进行超声波清洗,干燥保存备用.利用Ca(OH)2、NaOH、KOH 3组分体系配置pH=13.5的模拟混凝土孔隙液;利用饱和Ca(OH)2和NaHCO3配置pH值分别为12.5、11.5和11.0的模拟混凝土孔隙液.每组试验采用3个平行试样,试样编号见
基于CS3002型电化学工作站,采用经典的三电极体系开展钢筋脱钝过程的腐蚀电化学参数测试.其中,利用钢筋电极作为工作电极,利用Pt电极作为辅助电极,利用附加硝酸钾盐桥的饱和甘汞电极(简称SCE)作为参比电极.首先利用半电池电位法测试开路电位,测试时间为120 s,当测试期间开路电位最大波动幅度不超过1 mV时认为达到稳定;然后利用电化学阻抗谱法测试电荷转移电阻,频率范围为0.01 Hz~100 kHz,施加10 mV的正弦波电位信号,每个数量级测定7个数据点,共取49个对数扫描点,进而利用等效电路图拟合确定钢筋的极化电
当模拟混凝土孔隙液的pH=12.5时,对于钢筋电极D6、D10和D16(工作面积S分别为0.283、0.785、2.011 c

图1 钢筋电极工作面积对开路电位的影响
Fig.1 Influence of working area for steel electrode on open‑circuit potential
对于放置在pH=12.5的模拟混凝土孔隙液中的钢筋电极D6、D10和D16,利用电化学阻抗谱法可以测得不同氯离子浓度对应的电化学阻抗谱图,以D6-1为例,其电化学阻抗谱如

图2 不同氯离子浓度对应的电化学阻抗谱图
Fig.2 EIS plots for different chloride concentration (D6-1)

图3 钢筋电极工作面积对极化电阻的影响
Fig.3 Influence of working area for steel electrode on polarization resistance
由
根据开路电位(OCP)和极化电阻Rp判定钢筋脱钝后,利用动电位极化法测试钢筋电极的阳极Tafel斜率ba和阴极Tafel斜率bc.当模拟混凝土孔隙液的pH=12.5时,对于钢筋电极D6、D10和D16,ba分别为177.42、228.62、173.27 mV,bc分别为98.85、93.05、123.28 mV.根据阳极Tafel斜率ba和阴极Tafel斜率bc,可以计算Tafel常数B:
(1) |
对于钢筋电极D6、D10和D16,Tafel常数B分别为27.56、28.72、31.28 mV,进而利用Stern‑Geary公
(2) |
对于钢筋电极D6、D10和D16,腐蚀电流密度与氯离子浓度之间的变化关系如

图4 钢筋电极工作面积对腐蚀电流密度的影响
Fig.4 Influence of working area for steel electrode on corrosion current density
当钢筋电极直径D为10 mm(工作面积S为0.785 c

图5 模拟混凝土孔隙液pH值对开路电位的影响
Fig.5 Influence of pH value for simulated concrete pore solution on open‑circuit potential
当钢筋电极的直径为10 mm(工作面积为0.785 c

图6 模拟混凝土孔隙液pH值对极化电阻的影响
Fig.6 Influence of pH value for simulated concrete pore solution on polarization resistance
当利用开路电位和极化电阻判定钢筋脱钝后,采用动电位极化法分别测试钢筋电极的阳极和阴极Tafel斜率,根据

图7 模拟混凝土孔隙液pH值对腐蚀电流密度的影响
Fig.7 Influence of pH value for simulated concrete pore solution on corrosion current density
在模拟混凝土孔隙液中,钢筋电极同时遭受氯离子的侵蚀作用和氢氧根离子的缓蚀作用.对于pH值分别为13.5、12.5、11.5和11.0的模拟混凝土孔隙液,钢筋电极脱钝时的自由氯离子浓度[C
(4) |
(5) |
(1)随着钢筋电极工作面积的增大,开路电位、极化电阻和腐蚀电流密度发生突变时的氯离子浓度减小,钢筋电极脱钝的临界氯离子浓度降低;与开路电位和极化电阻相比,腐蚀电流密度的突变点更加明显.
(2)随着模拟混凝土孔隙液pH值的增加,开路电位、极化电阻和腐蚀电流密度发生突变时的氯离子浓度增大,以自由氯离子浓度[C
(3)本文主要基于模拟混凝土孔隙液开展研究,所揭示的钢筋腐蚀行为以及临界氯离子浓度变化规律可以为后续开展混凝土中钢筋脱钝临界氯离子浓度研究提供参考.
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