混凝土模拟孔溶液中外加电流阴极保护对阳极体系的酸化侵蚀效应*

王羊洋¹, 胡捷¹^,²^,, 郭文昊¹, 赵翼¹, 韦江雄¹^,², 余其俊¹^,²

1. 华南理工大学材料科学与工程学院广州 510640

2. 广东省建筑材料低碳技术工程技术研究中心广州 510640

WANG Yangyang¹, HU Jie¹^,²^,^*^,, GUO Wenhao¹, ZHAO Yi¹, WEI Jiangxiong¹^,², YU Qijun¹^,²

1. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China

2. Guangdong Province Building Materials Engineering Technology Research Center of Low Carbon Technology, Guangzhou 510640, China

**To whom correspondence should be addressed, Tel: (020) 87114137, E-mail: msjiehu@scut.edu.cn
本文联系人: 胡捷

基金资助: * 国家高技术研究发展计划项目2015AA034701, 国家自然科学基金51572088, 广州市珠江科技新星项目201506010004和硅酸盐建筑材料国家重点实验室开放基金SYSJJ2015-08资助;

*Supported by High Technology Research and Development Program of China No. 2015AA034701 National Natural Science Foundation of China No. 51572088 Pearl River Nova Project of Guangzhou No. 201506010004 and the Open Fund of State Key Laboratory of Silicate Building Materials No. SYSJJ2015-08.;

中图分类号: 分类号 TG174.41 文章编号 1005-3093(2016)12-0931-09 文献标识码: 分类号 TG174.41 文章编号 1005-3093(2016)12-0931-09

摘要:

测定了外加电流阴极保护技术作用下混凝土模拟孔溶液的pH值变化, 建立了通电量Q与混凝土模拟孔溶液中氢氧根浓度c_OH^-的定量关系, 量化表征了钛网阳极表面的腐蚀产物。结果表明, 相同极化时间条件下, 电流密度越高, 阳极区模拟孔溶液中c_OH^-越低, 钛网表面腐蚀产物越多; 相同极化电流密度条件下, 含Cl^-模拟孔溶液中OH^-消耗速率要大于无Cl^-模拟孔溶液中OH^-消耗速率; 外加电流阴极保护过程中混凝土模拟孔溶液c_OH^-与Q之间的关系符合Logistic回归方程, 基于该方程可预测外加电流阴极保护技术中外部阳极处的pH下降程度, 进而评估其对阳极体系的酸化侵蚀作用。

关键词: 材料失效与保护 ; 外加电流阴极保护 ; 混凝土模拟孔溶液 ; 外部阳极, ; 酸化 ; 侵蚀

Abstract:

The quantitative relationship between the electric charge quantity (Q) and OH^- concentration (c_OH^-) was established by measuring the variation of pH value of simulated concrete pore solutions by impressed current cathodic protection (ICCP), and corrosion products formed on the titanium mesh electrode were quantitatively characterized. The results indicate that by the same polarization time, a lower c_OH^- was related to a higher applied current density; the accumulation of corrosion products on titanium mesh surface was also much heavier. By the same current density, the consumption rate of OH^- was larger in the chloride-containing solution rather than that in the chloride-free counterpart. The relationship between Q and c_OH^- in simulated concrete pore solution by the applied cathodic protection follows logistic regression equation, thus this equation can be used to evaluate the descent of pH near the anode, and further predict the effect of acidification of solutions on the corrosion of external anode by the applied cathodic protection.

Key words: materials failure and protection ; impressed current cathodic protection ; simulated concrete pore solutions ; external anode ; acidification ; erosion

钢筋混凝土腐蚀所带来的耐久性问题是困扰工程结构服役寿命的一大难题, 同时也是工程领域亟待解决的问题。近年来, 在沿海经济高速发展和海洋资源开发力度空前加大的战略趋势下, 服役于海洋严苛环境下的钢筋混凝土构筑物日益增加。因此, 在滨海环境下如何对钢筋混凝土构筑物的腐蚀破坏进行有效的预防和修复是目前亟需解决的重要问题。

在众多腐蚀防护技术中, 外加电流阴极保护技术以其能直接抑制钢筋自身的电化学腐蚀过程的技术优势, 被世界各地广泛研究和应用, 尤其适用于受Cl^-污染的钢筋混凝土构筑物^[1]。外加电流阴极保护技术在美国、英国、日本等许多发达国家已有较广泛的应用^[2]。在中国, 20世纪80年代中期, 洪定海等^[3]已经开展了对钢筋混凝土结构进行外加电流阴极保护的试验研究, 并于1993年在浙江北仑港码头实施了该技术。2006年, 外加电流阴极保护技术成功应用于杭州湾跨海大桥两座主塔的腐蚀防护。随后, 青岛海湾大桥、威海长会口大桥以及连接香港、珠海、澳门的港珠澳大桥等沿海大型桥梁工程都采用了该技术进行钢筋混凝土构筑物的腐蚀防护^{[4, 5]}。

当对钢筋混凝土构筑物实施外加电流阴极保护时, 钢筋在外部电源的作用下会被阴极极化至较负电位(一般小于-900mV vs. Saturated Calomel Electrode (SCE))^[6], 其表面阳极金属溶解反应受到抑制, 从而在热力学上防止钢筋锈蚀的发生。此外, 外加电流阴极保护技术还能从其它角度抑制钢筋的锈蚀: 如提高钢筋-混凝土界面的pH值并降低Cl^-浓度, 从而使钢筋的点蚀电位正移, 扩大其钝化电位的范围^[7]; 提高钢筋表面钝化膜的稳定性, 保持钢筋-混凝土界面的高碱性, 有效地保护钢筋混凝土免遭腐蚀破坏^[8]。

在外加电流阴极保护系统中, 外部阳极是该系统的重要组成部分, 它的性能对于外加电流阴极保护系统的长期稳定运行起到至关重要的作用。当对钢筋混凝土实施外加电流阴极保护时, 在外部钛网阳极区会发生如下阳极反应^[9]:

$\begin{matrix} 4 O H^{-} \to O_{2} + 2 H_{2} O + 4 e^{-} \end{matrix}$ (1)

$\begin{matrix} 2 H_{2} O \to O_{2} + 4 H^{+} + 4 e^{-} \end{matrix}$ (2)

在含有Cl^-的孔溶液中还会同时发生以下的阳极反应:

$\begin{matrix} 2 Cl \to C l_{2} + 2 e^{-} \end{matrix}$ (3)

$\begin{matrix} C l_{2} + H_{2} O \to HCl + HClO \end{matrix}$ (4)

以上反应会不断消耗外部金属阳极区砂浆孔溶液中的OH^-并生成H⁺, 引起该区域局部环境的酸化, 对外部金属阳极及其附近砂浆产生侵蚀作用, 严重影响外部阳极系统的耐久性和外加电流阴极保护技术的保护效率^[10]。此外, 酸化侵蚀过程中砂浆矿物组分的成分和形貌, 以及微观结构的变化, 也会影响砂浆的导电均匀性和保护电流的分布^[11], 从而对外加电流阴极保护的稳定性产生影响。

目前, 关于外加电流阴极保护技术所造成的酸化侵蚀的报道还较少。其中McArthur^[12]对外加电流阴极保护作用下, 钢筋混凝土模拟孔溶液pH值的变化过程及向阴极的扩散趋势进行了模拟, 发现当电流密度为20 mA/mm², 通电时间为1050 h时, 酸化效应在横截面积为4.9 mm²玻璃管中向外扩散的范围可以达到260 mm。Polder等^[13]基于Nernst-Planck方程建立了外加电流阴极保护作用下Ca²⁺、OH^-和Ca(OH)₂的共平衡关系并通过模拟得到在1 mA/m²电流密度作用10年后, 酸化效应的影响范围可以达到100 µm, 但该研究没有确定酸化反应动力学参数, 也没有考虑Na⁺, K⁺对体系的影响, 并且没有进行实验对模拟结果进行验证。

本文定量测定了不同电流密度条件下, 阴/阳极区混凝土模拟孔溶液(包括含Cl^-及不含Cl^-两种情况) pH值随时间的变化; 采用原子吸收光谱(AAS)技术研究了阴极保护作用前后阴/阳极区模拟孔溶液中Na⁺, K⁺及Ca²⁺浓度变化; 利用扫描电子显微镜(SEM)观察通电前后钛网阳极的表面形貌, 分析试样微区的元素组成, 以及利用X射线衍射(XRD)分析不同电流密度阴极保护作用后阳极表面的产物组成。在此基础上, 建立外加电流阴极保护技术作用产生的酸化侵蚀效应与通电量之间的关系。

1 实验方法

1.1 钢筋电极与钛网电极

本文所采用的钢筋电极直径ϕ14 mm, 长度为30 mm的Q235建筑光圆钢筋(化学成分见表1), 实验前依次使用200#, 600#, 1000#, 2000#水磨砂纸将钢筋电极工作面打磨至镜面, 然后将打磨好的钢筋段用酒精擦拭除油, 去除表面残留油性物质及因打磨而产生的灰层。随后在钢筋一端焊接上导线再用丙酮擦拭钢筋的焊接端面去除因焊接产生的油性物质, 最后使用环氧树脂封装并引出铜线。制备的钢筋电极的工作面积约为10.05 cm²。本文所采用的阳极材料为市售筛孔大小为830 µm钛金属网, 其尺寸为25 mm×40 mm。

Element	C	Si	Mn	S	P	Fe
Content	0.18	0.28	0.55	0.04	0.04	98.91

表1 Q235钢的化学组成

Table 1 Chemical composition of Q235 steel reinforcement (%, mass fraction)

1.2 混凝土模拟孔溶液

根据相关参考文献^[14-18], 本研究中以去离子水配制了0.002 mol/L Ca(OH)₂+0.06 mol/L NaOH+0.18 mol/L KOH溶液作为混凝土模拟孔溶液, 并用稀HNO₃溶液(0.0001 mol/L)将模拟孔溶液pH值调至13。此外, 在模拟孔溶液中掺入质量分数为3.5% NaCl, 模拟钢筋受到氯盐侵蚀的环境。配置混凝土模拟孔溶液所用试剂均为分析纯。

1.3 实验装置及试样处理

本文中实验装置图如图1所示。本实验中混凝土模拟孔溶液周围环境是模拟外加电流阴极保护实际工程中的环境条件即空气环境。将40 mL混凝土模拟孔溶液(含氯或不含氯)分别注入广口瓶中。测试前, 将钢筋和钛网分别放入相应的模拟孔溶液中并盖上橡胶塞浸泡24 h。浸泡结束后采用Corrtest CS1002恒电位/恒电流仪作为外加恒电流源, 其中钢筋电极连接电源负极作为阴极, 钛网电极连接电源正极作为阳极。阴阳极溶液之间用盐桥连接。为了加快电化学阴极保护的酸化侵蚀速度, 本试验中采用2种外加电流密度分别为100 mA/m²和200 mA/m², 同时设置参比试样(未通电试样)。本研究中试样编号如表2所示。

图1 试验装置示意图

Fig.1 Schematic diagram of experimental set-up (1-galvanostat, 2-electrolytic bridge, 3-Q235 steel reinforcement, 4-titanium mesh, 5-simulated concrete pore solution, 6-wide mouth bottle, 7- rubber plug 8-copper wire)

Sample	Designations
Without polarization (chloride-free)	Ref
With polarization at a current density of 100 mA/m²(chloride-free)	S100
With polarization at a current density of 100 mA/m²(chloride-free)	S200
Without polarization (chloride-containing)	Cl-Ref
With polarization at a current density of 100 mA/m²(chloride-containing)	Cl-S100
With polarization at a current density of 200 mA/m²(chloride-containing)	Cl-S200

表2 各试样编号

Table 2 Designations of tested samples in this study

1.4 测试方法

(1) 阴/阳极区模拟孔溶液离子浓度

试验中每隔5d 对每一个需要取样的样品使用一次性针孔注射器抽取3 mL的模拟孔溶液, 利用原子吸收光谱仪(AAS, Contra 700) 测量外加电流阴极保护作用后阴/阳极区混凝土模拟孔溶液中的Na⁺, K⁺, Ca²⁺含量, 并且对已取样的样品重新补充3 mL初始模拟孔溶液。

(2) 阴/阳极区模拟孔溶液pH值

实验中每隔24 h把广口瓶上部橡胶塞拔出, 将pH计(Mettler Toledo FE20K)直接放入阴/阳极区模拟孔溶液中, 测量阴/阳极区混凝土模拟孔溶液的pH值, 测量完成后重新盖上橡胶塞。所测值为阴/阳极区平均pH值。

(3) 电极表面形貌与成分

采用扫描电子显微镜(SEM, ZEISS EVO 18)结合X射线能谱(EDS, Oxford INCA 250, 20 kV)对外加电流阴极保护前后阳极钛网表面微观形貌进行观察, 分析试样微区的元素组成。采用X-ray衍射分析仪(XRD, PANalytical X'pert Pro)测定阴极保护作用结束后钛网表面产物组成。

2 结果与讨论

2.1 外加电流阴极保护作用下阴/阳极区模拟孔溶液中Na/K/Ca浓度变化

在外加电流阴极保护作用下, 阴/阳极区模拟孔溶液中的Na⁺/K⁺/Ca²⁺浓度变化, 如图2和3所示。随着通电时间延长, 阴极区模拟孔溶液中Na⁺/K⁺浓度呈现增大的趋势: 在100和200 mA/m²外加电流密度条件下, 通电15 d后, 不含Cl^-模拟孔溶液中Na⁺ (图2a)由1.36 g/L分别升高到1.49 g/L(100 mA/m²)和1.58 g/L(200 mA/m²); K⁺ (图2b)由5.82 g/L分别升高到6.63 g/L(100 mA/m²)和6.81 g/L(200 mA/m²)。另一方面, 阳极区模拟孔溶液中Na⁺/K⁺浓度则逐渐降低: Na⁺(图2a)由1.36 g/L分别降低到1.29 g/L(100 mA/m²)和1.25 g/L(200 mA/m²), K⁺(图2b)由5.82 g/L分别降低到5.40 g/L(100 mA/m²)和5.21 g/L(200 mA/m²)。在含Cl^-模拟孔溶液中阴/阳极区Na⁺/K⁺浓度呈现与不含Cl^-模拟孔溶液相同的变化趋势: 通电15 d后, 阴极区Na⁺(图3a)由14.14 g/L分别升高到14.30 g/L(100 mA/m²)和14.57 g/L(200 mA/m²); K⁺ (图3b)由5.67 g/L分别升高到6.08 g/L(100 mA/m²)和6.11 g/L(200 mA/m²)。阳极区模拟孔溶液中Na⁺/K⁺浓度则逐渐降低, Na⁺ (图3a)由14.14 g/L分别降低到13.55 g/L(100 mA/m²)和13.20 g/L(200 mA/m²); K⁺由(图3b) 5.67 g/L分别降低到5.54 g/L(100 mA/m²) /5.46 g/L(200 mA/m²)。这是因为Na⁺\K⁺均为荷正电粒子, 在外界电场的作用下会沿着电场线方向向阴极方向迁移。

图2 外加电流阴极保护下无Cl^-模拟孔溶液中阴/阳极区(a)Na⁺, (b)K⁺, (c)Ca²⁺含量变化

Fig.2 Changes of (a) Na⁺, (b) K⁺, (c) Ca²⁺ contents in chloride-free simulated pore solutions under the condition of impressed current cathodic protection

图3 外加电流阴极保护下含Cl^-模拟孔溶液中阴/阳极区(a)Na⁺, (b)K⁺, (c)Ca²⁺含量变化

Fig.3 Changes of (a) Na⁺, (b) K⁺, (c) Ca²⁺ contents in chloride-containing simulated pore solutions under the condition of impressed current cathodic protection

在100和200 mA/m²外加电流密度条件下, 不含Cl^-模拟孔溶液中通电15 d后, 阴极区Ca²⁺ (图2c)由35.76 mg/L分别降低到11.70 mg/L(100 mA/m²)和10.67 mg/L(200 mA/m²); 阳极区Ca²⁺(图2c)由35.76 mg/L分别降低到8.84 mg/L(100 mA/m²)和8.27 mg/L(200 mA/m²); 在含Cl^-模拟孔溶液中阴/阳极区Na⁺/K⁺浓度也呈现与不含Cl^-模拟孔溶液相同的变化趋势: 通电15 d后, 阴极区Ca²⁺(图3c)由12.06 mg/L分别降低到7.19 mg/L(100 mA/m²)和10.16 mg/L(200 mA/m²); 阳极区Ca²⁺(图3c)则由12.06 mg/L分别降低到4.49 mg/L(100 mA/m²)和3.23 mg/L(200 mA/m²)。由于本研究中混凝土模拟孔溶液体系处于大气环境, 空气中的二氧化碳对测量体系产生的碳化作用是造成阴极区Ca²⁺浓度降低的主要原因, 但氧气的存在对阳极区的反应并没有明显影响。即便如此, 阴极区Ca²⁺浓度仍然要高于阳极区。

2.2 外加电流阴极保护对模拟孔溶液pH值的影响

通电过程中阴/阳极区模拟孔溶液pH值的变化如图4所示。钢筋阴极区模拟孔溶液的pH值随时间的变化非常小(图4a)。在200 mA/m²电流密度下通电15 d后, 不含Cl^-模拟孔溶液的pH值约为12.80, 含Cl^-模拟孔溶液中pH值约为12.95(模拟孔溶液的初始pH值为13)。通电15 d后, 钛网阳极区模拟孔溶液的pH值随着通电时间的延长均有较大降低(图4 b)。同样在200 mA/m²电流密度下, 不含Cl^-模拟孔溶液的pH值下降至9.74, 含Cl^-模拟孔溶液中pH值甚至降低到9.25。随着电流密度增大, 阳极区模拟孔溶液pH值的下降幅度增大。这是因为当电流密度提高时, 外电路中的电流增大, 因此在单位时间内通过外电路的电荷量也相应增多, 外电路中转移的电子量也相应增大。因此在高电流密度下, 在相同通电时间内由于反应(1)消耗的OH^-增大, 导致阳极区模拟孔溶液的pH值比小电流密度时下降更快。

图4 电化学阴极保护对(a)阴极区和(b)阳极区模拟孔溶液pH值的影响

Fig.4 Effects of impressed current cathodic protection on pH values of simulated pore solutions (a) cathode and (b) anode zones

此外, 在含Cl^-模拟孔溶液中, OH^-不再是唯一发生阳极反应的阴离子, Cl^-也会发生阳极反应并为外电路电流传输提供电子, 如反应(3)所示。反应后生成的Cl₂在向外扩散过程中溶于水中, 生成盐酸和次氯酸(反应(4)), 从而消耗更多OH^-。因此在相同时间内, 含Cl^-模拟孔溶液pH值比不含Cl^-模拟孔溶液pH值下降更快。

2.3 外加电流阴极保护下阳极区模拟孔溶液中OH浓度变化规律

通电过程中, 阳极区模拟孔溶液OH^-浓度（由测量的pH值计算所得）变化如图5所示。经过回归分析可知, 在外加电流阴极保护的作用下, 前期模拟孔溶液中OH^-浓度随时间延长不断加快, 中期OH^-浓度消耗速率趋于稳定, 后期由于OH^-浓度过低导致外界对其影响作用较小。OH^-平均消耗速率可以用拟合后曲线的微分结果表示, 从不同条件下阳极区模拟孔溶液中OH^-浓度变化幅度可以清晰看到, 高电流密度下OH^-消耗引起的降幅要大于低电流密度下OH^-消耗引起的降幅, 也即高电流密度下要大于低电流密度下OH^-平均消耗速率, 而且含Cl^-模拟孔溶液中OH^-的降低幅度大于不含Cl^-模拟孔溶液中OH^-的降低幅度。同时可以得到在2种外加电流作用下, 不含Cl^-模拟孔溶液中OH^-平均消耗速率分别为0.0113 mol/(Ld)(200 mA/m²)(图5a)和0.0075 mol/(Ld) (100 mA/m²) (图5a); 含Cl^-模拟孔溶液中OH^-平均消耗速率分别为0.0127 mol/(L d) (200 mA/m²)(图5b)和0.0098 mol/(Ld) (100 mA/m²)(图5b), 也即无氯模拟孔溶液中OH^-平均消耗速率在200 mA/m²电流密度下的为在100 mA/m²电流密度下的1.5倍; 含氯模拟孔溶液中的OH^-平均消耗速率在200 mA/m²电流密度为100 mA/m²电流密度下的1.3倍。

Fig.5 Relationship between OH^- concentration in the anode zone and polarization time under 他the conditions of impressed current cathodic protection (a) without Cl^-, (b) with Cl^-

2.4 外加电流阴极保护产生的阳极酸化效应与通电量的关系

通电过程中, 阳极区模拟孔溶液OH^-浓度与通电量之间的关系如图6所示。经过拟合, 外加电流阴极保护作用下阳极区模拟孔溶液中OH^-浓度与通电量Q的关系符合二项分布Logistic回归模型, 如公式(5)所示:

$\begin{matrix} c = \frac{c_{0}}{1 + {(\frac{Q}{Q_{0}})}^{P}} \end{matrix}$ (5)

其中, c为OH^-浓度(单位mol/L), Q为通电量(单位C), p为回归系数, c₀表示未通电时模拟孔溶液中OH^-初始浓度, Q₀表示OH^-浓度变化幅度最大时也即拐点处的通电量。方程拟合后的曲线如图6中所示, 拟合参数如表3所示。

	Parameter	Value	Standard Error
In chloride-free simulated concrete pore solutions	c₀	0.095	0.003
Q₀	81.574	3.971	0.963
p	2.885	0.282
In chloride-containing simulated concrete pore solutions	c₀	0.096	0.003
Q₀	72.065	3.227	0.970
p	3.012	0.281

表3 Logistic方程中各参数经拟合后结果

Table 3 Fitting parameters in the Logistic equation

图6 外加电流阴极保护下阳极区OH^-浓度变化与通电量的关系

Fig.6 Relationship between OH^- concentration in the vicinity of anode and the electric charge quantity under the conditions of impressed current cathodic protection

一方面, 在含Cl^-模拟孔溶液和不含Cl^-模拟孔溶液中R²分别达到了0.963和0.970, 说明利用上述方程拟合后的相关度非常高, 拟合结果比较理想。另一方面, 拟合后方程的初始浓度c₀为0.095 mol/L(含Cl^-模拟孔溶液)和0.096 mol/L(不含Cl^-模拟孔溶液), 这也分别与实验中实际的OH^-浓度0.108 mol/L(含Cl^-模拟孔溶液)和0.111 mol/L(不含Cl^-模拟孔溶液)相接近。

可以推测, 在新建混凝土构筑物中, 阳极区OH^-浓度的变化与外加电流阴极保护通电量的关系符合公式(6):

$\begin{matrix} c = \frac{0.095}{1 + {(\frac{Q}{81.574})}^{2.885}} \end{matrix}$ (6)

在已受Cl^-侵蚀的混凝土构筑物中, 阳极区OH^-浓度的变化与外加电流阴极保护通电量的关系符合(7)式:

$\begin{matrix} c = \frac{0.096}{1 + {(\frac{Q}{72.065})}^{3.012}} \end{matrix}$ (7)

因此, 可以根据上述两式对不同环境条件、不同通电量下外加电流阴极保护所造成的酸化侵蚀效应程度进行预测和评估。

2.5 外加电流阴极保护作用前后阳极钛网的形貌和成分

外加电流阴极保护产生的阳极酸化效应除了会降低阳极区模拟孔溶液中OH^-浓度, 还会对阳极钛网产生一定影响。通电15d后钛网的表面形貌如图7和8所示。在不含Cl^-模拟孔溶液中, 且未通电条件下(图7a)钛网表面较光滑干净, 并未发现明显的腐蚀产物; 在通电情况下(图7b和c), 钛网表面变得较粗糙, 有较多腐蚀产物在钛网表面形成和累积; 此

外, 随着电流密度增大, 酸化效应对钛网表面的侵蚀作用越大, 累积的腐蚀产物越多。在含Cl^-模拟孔溶液中, 即使在未通电条件下(图8 a), 钛网表面也出现少许腐蚀产物; 在通电条件下(图8 b和c), 则有大量的腐蚀产物在钛网表面形成与累积。这主要是因为模拟孔溶液中的Cl^-对金属钛网有加剧腐蚀的作用, 并且随着电流密度的增大这种侵蚀加深的趋势会更加明显。

图7 无Cl^-模拟孔溶液中通电15 d后阳极钛网形貌

Fig.7 Morphologies of titanium mesh in chloride-free simulated pore solution after 15 d with polarization at (a) 0 mA/m², (b) 100 mA/m²and (c) 200 mA/m²

图8 含Cl^-模拟孔溶液中通电15 d后阳极钛网形貌

Fig.8 Morphologies of titanium mesh in chloride-containing simulated pore solution after 15 d with polarization at (a) 0 mA/m², (b) 100 mA/m²and (c) 200 mA/m²

对外加电流阴极保护后阳极钛网表面进行XRD分析, 结果如图9所示。钛网表面的腐蚀产物为钛氧化合物。在不含Cl^-模拟孔溶液中, 不通电时, 衍射图谱中几乎没有腐蚀产物的特征峰; 在通电情况下, 腐蚀产物的特征峰逐渐明显, 并且随着外界电流密度的提高, 腐蚀产物(钛氧化合物)的特征峰强度增大, 说明腐蚀产物增多。含Cl^-模拟孔溶液中, 即使未通电, 钛网表面仍有少许腐蚀产物的特征峰; 在通电条件下, 腐蚀产物的特征峰更加尖锐, 这说明钛网表面累积大量钛氧化合物形成的腐蚀产物, 也与前述扫描电镜观察结果(图7和图8)相一致。

图9 阳极钛网表面产物XRD结果

Fig.9 XRD results of surface products of anodic titanium mesh

3 结论

1. 电化学阴极保护会对外部阳极系统产生酸化侵蚀作用, 降低阳极区模拟孔溶液pH值, 并且该效应随着外界通电电流密度的提高而加剧。此外, 在相同通电电流密度下, 含Cl^-模拟孔溶液中OH^-的消耗速率大于不含Cl^-模拟孔溶液中OH^-的消耗速率。

2. 阳极酸化侵蚀作用会对阳极钛网产生腐蚀破坏, 在钛网表面生成以钛氧化合物为主的腐蚀产物。随着外界通电电流密度的提高, 腐蚀作用加大, 且相同电流密度下Cl^-会促进钛网表面腐蚀产物的生成和累积。

3. 根据Logistic回归模型可以模拟阴极保护技术下酸化效应所造成的OH^-浓度c的变化与外界通电量Q大小之间的对应关系。对于外加电流阴极保护作用于新建混凝土构筑物来说, $\begin{matrix} c = \frac{0.095}{1 + {(\frac{Q}{81.574})}^{2.885}} \end{matrix}$ ;

对于外加电流阴极保护作用于已受氯盐侵蚀混凝土构筑物来说,

$\begin{matrix} c = \frac{0.096}{1 + {(\frac{Q}{72.065})}^{3.012}} \end{matrix}$ 。

The authors have declared that no competing interests exist.

参考文献

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1	HONG Dinghai, Corrosion and Protection of Steel Bars in Concrete (Beijing, China Railway Press, 1998) [本文引用:1] (洪定海, 混凝土中钢筋的腐蚀与保护(北京, 中国铁道出版社, 1998))
2	R. B. Polder, W. H. A.Peelen, M. Raupach, K. Reichling, Economic effects of full corrosion surveys for aging concrete structures, Materials and Corrosion, 64(2), 105(2013) This paper investigates the economic effects of full corrosion surveys of concrete structures. The background is that the existing concrete infrastructure is aging, while being exposed to aggressive influences, which increases the occurrence of corrosion and related concrete damage over time. The central proposition is that solely relying on visual inspection for interventions (repair) may result in unnecessarily high costs and associated risks. The reason is that visual inspection can only signal deterioration that is in a relatively advanced stage of development. Consequently, heavy and costly repairs are needed, while undetected degradations still go on developing, presenting future risks. On the other hand, carrying out full surface corrosion surveys may be considerably more economic. This is because using detailed survey information, degradation can be detected at an early stage. Prevention of corrosion is generally less costly than correction. Consequently, an optimal mix of preventive and corrective measures can be applied at the right time and at the right places. These alternative approaches to inspection may be considered elements in so-called reactive and proactive strategies for maintenance of infrastructure, respectively. DOI:10.1002/maco.201206709 Magsci [本文引用:1]
3	HONG Dinghai, FAN Weiguo, A preliminary study on the applied current cathodic protection technology for the reinforced concrete superstructure(I), Corrosion and Protection, 5(4), 257(1990) [本文引用:1] (洪定海, 范卫国, 海工钢筋混凝土上部结构外加电流阴极保护技术的初步研究(I), 腐蚀与防护, 5(4), 257(1990)) URL
4	WEI Xinlai, WANG Yijie, Corrosion of steel bars in reinforced concrete bridges and cathodic protection, Vision of Sci. and Tech., 36(23), 285(2012)(魏新来, 王艺杰, 钢筋混凝土桥梁中钢筋锈蚀与其阴极保护, 科技视野, 36(23), 285(2012)) 鋼99コンクリート構造物の腐食99防食、劣化とセンシング技術の課題と展望 = Subjects and future vision of technology in corrosion, its protection and degradation of steel and concrete structure, and their sensing technology 日本鉄鋼協会育成委員会技術講座WG編（白石記念講座 / 日本鉄鋼協会編, 第64回）日本鉄鋼協会, 2012.12 URL [本文引用:1]
5	MENG Fanchao, WU Weisheng, LIU Minghu, WANG Qi, LIANG Zhang, Innovation of durability design for the bridge of the Hong Kong Macao Bridge, Prestress Tech., 18(06), 11(2010) [本文引用:1] (孟凡超, 吴伟胜, 刘明虎, 王麒, 梁张, 港珠澳大桥耐久性设计创新, 预应力技术, 18(06), 11(2010)) 以港珠澳大桥使用年限120年为例，分别对混凝土结构的碳化过程和氯离子侵入过程进行了定量分析，并详细阐述了提高其耐久性的措施，同时对钢结构中钢箱梁、桥面、斜拉索、钢锚梁、钢锚箱等钢结构组件防腐措施分别进行详细阐述，为桥梁在同类环境下耐久性设计提供一定的参考。 DOI:10.6052/j.issn.1000-4750.2013.07.0656 URL
6	L. Bertolini, F. Bolzoni, P. Pedeferri, Cathodic protection and cathodic prevention in concrete: principles and applications, J. Appl. Electrochem., 28(12), 1321(1998) [本文引用:1]
7	G. K. Glass, N. R. Buenfeld, On the current density required to protect steel in atmospherically exposed concrete structures, Corros. Sci., 37(10), 1643(1995) Abstract Mixed potential theory suggests that the current required to cathodically protect steel in atmospherically exposed concrete is strongly dependent on the corrosion rate. At modest corrosion rates, typical design current densities would not achieve the level of cathodic polarisation required by commonly accepted protection criteria. However a cathodic current effectively lowers the unprotected corrosion rate by removing chloride ions and increasing the pH at the cathode. This work suggests that the prevention of further corrosion and the achievement of an adequate level of polarisation by a cathodic protection system is strongly dependent on these protective effects. DOI:10.1016/0010-938X(95)00116-2 URL [本文引用:1]
8	J. XU, W. YAO, Characteristics and microscopic mechanism of cathodic protection of reinforced concrete, J. Build. Mater., 13(3), 315(2010) For the purpose of exploring the mechanism underlying the efficiency of cathodic protection,X-ray diffraction(XRD) combined with scanning electron microscopy(SEM) were applied for microstructural analysis of corrosion products and steel/concrete interface.The study shows that more common corrosion product Fe_3O_4 is produced in specimens if cathodic protection is not provided.Besides,corrosion product of Fe~(3+)(O,OH,Cl) can also formed when chlorides are abundant in concrete.Cathodic protection is helpful for the formation of Ca(OH)_2 crystal,thereby the high alkalinity of the steel/paste interface is retained.Chloride ions are mitigated away from the steel surface by cathodic current.These effects guarantee the efficient protection of steel reinforcement. DOI:10.3969/j.issn.1007-9629.2010.03.009 URL [本文引用:1]
9	B. Elsener, Long-term durability of electrochemical chloride extraction, Materials and Corrosion, 59(2), 91(2008) Abstract This paper presents field data from one of the first well-documented reinforced concrete structures that has been rehabilitated with electrochemical chloride extraction (ECE) in the year 1989. The results of half-cell potential mapping measured 1, 5 and nearly 20 years after the ECE treatment show that the reinforcement was completely repassivated after the ECE treatment and remained in fully passive condition. Regarding the efficiency of the ECE treatment an intermittent (current on–off) is beneficial. The question of repassivation of pre-corroded reinforcement is discussed. According to this and other results from field applications the ECE treatment is effective and – if further chloride ingress is avoided – also durable. For economical reasons it is recommended to apply the non-destructive ECE treatment in an early state, before delaminations or spalling has occurred. DOI:10.1002/maco.200804165 URL [本文引用:1]
10	L. Page, George Sergi, Development in cathodic protection applied to reinforced concerte, J. Mater. Civil Eng., 12(1), 8(2000) [本文引用:1]
11	J. Xu, W. Yao, Current distribution in reinforced concrete cathodic protection system with conductive mortar overlay anode, Constr. Build. Mater., 23(6), 2220(2009) Abstract The protection current distribution was investigated in a three-layer reinforced concrete cathodic protection system, with carbon fiber reinforced cement-based material as the conductive coating anode. The influence of steel bars' initial current state, concrete resistivity and magnitude of applied current density on the current distribution was discussed, respectively. The results show that both the initial corrosion rate of rebars and concrete resistivity have a great effect on the protection current distribution. However, the magnitude of impressed current density only affects the uniformity of current distribution to some extent when the rebars are corroding severely. This study contributes to the cathodic protection of reinforced concrete structures in field condition. DOI:10.1007/978-3-540-85168-4_52 URL [本文引用:1]
12	H. McArthur, S. D'Arcy, J. Barker, Cathodic protection by impressed DC currents for construction, maintenance and refurbishment in reinforced concrete, Constr. Build. Mater., 7(2), 85(1993) Abstract This paper models the pH changes which occur when occur when reinforced concrete structures are cathodically protected by an externally applied DC current. It is shown that the cathodic area is initially made very alkaline immediately after switch-on, and the anodic area becomes acidic in nature. This acidic area spreads out from the anodic electrode towards the cathodic area. It is found that this alkalinity is produced at the cathodically impressed rebar as the impressed current (a) uses up the dissolved oxygen; (b) requires the hydroxyl ions to carry the ionic current; (c) produces hydrogen. In conclusion, for cathodic protection to work effectively there must be a way for oxygen to diffuse to the cathodic area, so that it takes part in the cathodic reaction. The anodic area becomes acidic and the alkaline OH - ions are moved away from the rebar as a requirement for continuous current flow. DOI:10.1016/0950-0618(93)90037-D URL [本文引用:1]
13	W. H. A.Peelen, R. B. Polder, E. Redaelli, L. Bertolini, Qualitative model of concrete acidification due to cathodic protection, Materials and Corrosion, 59(2), 81(2008) Abstract In this paper a mathematical description and numerical implementation for ion transport in concrete due to current passage is developed, in which the heterogeneous equilibrium between Ca 2+ , OH 61 and the solid Ca(OH) 2 is incorporated. The description is based on the Nernst–Planck equation for ion transport, and reaction terms for the dissolution/precipitation of Ca(OH) 2 . This description was implemented in the finite element package Comsol Multiphysics. In this way Ca(OH) 2 depletion in a zone at a CP anode adjacent to a bulk of concrete with Ca(OH) 2 could be modelled in one calculation. Drawback of this model is that the kinetic parameters in the reaction terms are not known, and must be chosen high to ensure the dissolution of Ca(OH) 2 to be in equilibrium. This proved numerically challenging and sometimes caused long calculation times. The growth rate of the zone without solid depends on the current density applied, concrete cover, the pore liquid composition and the diffusion constants of Ca 2+ and OH 61 . This rate must be evaluated numerically. This qualitative model of anode acidification shows no participation of Na + ; therefore transport properties of this ion do not affect the acidification rate of concrete. The same would hold for any other ion included in the model, which is not involved in electrochemical or chemical reactions. DOI:10.1002/maco.200804106 URL [本文引用:1]
14	Andreas Leemann, Barbara Lothenbach, Cédric Thalmann, Influence of superplasticizers on pore solution composition and on expansion of concrete due to alkali-silica reaction, Constr. Build. Mater., 25(1), 344-350(2011) The use of superplasticizers (SP) is widespread in today’s concrete production. Usually, SP are used when specific demands in regard to workability, strength or durability have to be met. As they contain sodium sulphate, they have the potential to increase the alkalinity in the pore solution and the risk of damages due to alkali-silica reaction. In this study, the effect of two SP on the potential reactivity of concrete is examined by analyzing pore solutions of pastes and mortars and by measuring concrete expansion. The use of a naphthalene–sulphonate-based SP (Na content referred to liquid: 2.8%) leads to an increase in hydroxide concentration in the pore solution during the first two weeks going together with an accelerated concrete expansion during this period. The effects of a polycarboxylate-based SP (Na content referred to liquid: 0.5%) are less pronounced and do not result in an increased concrete expansion. DOI:10.1016/j.conbuildmat.2010.06.019 URL [本文引用:1]
15	F. Puertas, A. Ferna Ndez-Jime Nez, M. T. Blanco-Varela, Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate, Cement Concrete Res., 34(1), 139(2004) The nature of the alkaline activator influences the structure and composition of the calcium silicate hydrate formed as a consequence of the alkaline activation of the slag. The characteristic of calcium silicate hydrate in AAS pastes activated with waterglass is characterised by a low structural order with a low Ca/Si ratio. Besides, in this paste, Q 3 units are detected. The calcium silicate hydrate formed in the pastes activated with NaOH has a higher structural order (higher crystallinity) and contains more Al in its structure and a higher Ca/Si ratio than those obtained with waterglass. DOI:10.1016/S0008-8846(03)00254-0 URL [本文引用:0]
16	ZHAO Tiejun, ZHOU Zonghui, LIU Junchang, Conductivity of pore solution and permeability of concrete, Concrete, 41(02), 12(2000) [本文引用:0] (赵铁军, 周宗辉, 刘君昌, 孔溶液电导率与电测混凝土渗透性, 混凝土, 41(02), 12(2000)) ＡＡＳＨＴＯＴ２７７或ＡＳＴＭＣ１２０２通过对混凝土试件端加一６０Ｖ的直流电压，测量其６小时通过的电量，并以此评定混凝土抗氯离子渗透性等级。该方法本质上测量的是混产土电导率，其值的大小取决于混凝土的孔隙结构和孔溶液的化学成分。由于混凝土中的氯离子传质几乎不受孔溶液的化学成分影响，故用所测量通过混凝土的电量划分其抗氯邝子渗透性的等级是不当的。 URL
17	K. Andersson, B. Allard, M. Bengtsson, B. Magnusson, Chemical composition of cement pore solutions, Cement Concrete Res., 19(03), 327(1989) The chemical compositions of the pore solutions extracted from seven different cement pastes (one Swedish and one French standard Portland cement, sulfate resistant, blast-furnace slag, fly ash, silica, and high alumina cement) have been determined. Analyses covered Na, K, Ca, Mg, Al, Fe, and Si as well as pH and Eh (redox potential). Ionic strengths in the range 0.03–0.29 M and pH-values in the range 12.4–13.5 were obtained. Only the pore solutions from the slag cement and the French Portland cement were reducing (negative Eh). The dominating cations of standard Portland, sulfate resistant, slag, silica and fly ash cement pore solutions were Na and K, and in sulfate resistant cement also Ca. The main components in the pore solutions of aluminate cement were Na and Al. The Si and Fe concentrations were low in all pore solutions. DOI:10.1016/0008-8846(89)90022-7 URL [本文引用:0]
18	C. L. Page, Pore solution composition and chloride binding capacity of silica-fume cement pastes, Matériaux et Construction, 16(91), 19(1983) Samples of the capillary pore solution, associated with hardened cement pastes containing various proportions of silica-fume and sodium chloride, have been expressed at different stages of curing and analysed to determine concentrations of dissolved ions. The results indicate that partial replacement of Portland cement by increasing percentages of silica-fume causes a regular decrease in alkalinity of the pore solution and a reduction in the chloride-binding capacity of the material. Possible implications regarding the level of corrosion protection afforded to embedded steel are considered. DOI:10.1007/BF02474863 URL [本文引用:1]

1998

... 在众多腐蚀防护技术中, 外加电流阴极保护技术以其能直接抑制钢筋自身的电化学腐蚀过程的技术优势, 被世界各地广泛研究和应用, 尤其适用于受Cl^-污染的钢筋混凝土构筑物^[1].外加电流阴极保护技术在美国、英国、日本等许多发达国家已有较广泛的应用^[2].在中国, 20世纪80年代中期, 洪定海等^[3]已经开展了对钢筋混凝土结构进行外加电流阴极保护的试验研究, 并于1993年在浙江北仑港码头实施了该技术.2006年, 外加电流阴极保护技术成功应用于杭州湾跨海大桥两座主塔的腐蚀防护.随后, 青岛海湾大桥、威海长会口大桥以及连接香港、珠海、澳门的港珠澳大桥等沿海大型桥梁工程都采用了该技术进行钢筋混凝土构筑物的腐蚀防护^{[4, 5]}. ...

Peelen, M. Raupach, K. Reichling, Economic effects of full corrosion surveys for aging concrete structures,

2013

A preliminary study on the applied current cathodic protection technology for the reinforced concrete superstructure(I),

1990

海工钢筋混凝土上部结构外加电流阴极保护技术的初步研究(I),

1990

Corrosion of steel bars in reinforced concrete bridges and cathodic protection, Vision of Sci. and Tech., 36(23), 285(2012)(魏新来, 王艺杰, 钢筋混凝土桥梁中钢筋锈蚀与其阴极保护,

2012

Innovation of durability design for the bridge of the Hong Kong Macao Bridge

2010

港珠澳大桥耐久性设计创新,

2010

Cathodic protection and cathodic prevention in concrete: principles and applications, J

1998

... 当对钢筋混凝土构筑物实施外加电流阴极保护时, 钢筋在外部电源的作用下会被阴极极化至较负电位(一般小于-900mV vs. Saturated Calomel Electrode (SCE))^[6], 其表面阳极金属溶解反应受到抑制, 从而在热力学上防止钢筋锈蚀的发生.此外, 外加电流阴极保护技术还能从其它角度抑制钢筋的锈蚀: 如提高钢筋-混凝土界面的pH值并降低Cl^-浓度, 从而使钢筋的点蚀电位正移, 扩大其钝化电位的范围^[7]; 提高钢筋表面钝化膜的稳定性, 保持钢筋-混凝土界面的高碱性, 有效地保护钢筋混凝土免遭腐蚀破坏^[8]. ...

On the current density required to protect steel in atmospherically exposed concrete structures

1995

Characteristics and microscopic mechanism of cathodic protection of reinforced concrete, J

2010

Long-term durability of electrochemical chloride extraction,

2008

... 在外加电流阴极保护系统中, 外部阳极是该系统的重要组成部分, 它的性能对于外加电流阴极保护系统的长期稳定运行起到至关重要的作用.当对钢筋混凝土实施外加电流阴极保护时, 在外部钛网阳极区会发生如下阳极反应^[9]: ...

George Sergi, Development in cathodic protection applied to reinforced concerte, J

2000

... 以上反应会不断消耗外部金属阳极区砂浆孔溶液中的OH^-并生成H⁺, 引起该区域局部环境的酸化, 对外部金属阳极及其附近砂浆产生侵蚀作用, 严重影响外部阳极系统的耐久性和外加电流阴极保护技术的保护效率^[10].此外, 酸化侵蚀过程中砂浆矿物组分的成分和形貌, 以及微观结构的变化, 也会影响砂浆的导电均匀性和保护电流的分布^[11], 从而对外加电流阴极保护的稳定性产生影响. ...

Current distribution in reinforced concrete cathodic protection system with conductive mortar overlay anode

2009

S. D'Arcy, J. Barker, Cathodic protection by impressed DC currents for construction, maintenance and refurbishment in reinforced concrete

1993

... 目前, 关于外加电流阴极保护技术所造成的酸化侵蚀的报道还较少.其中McArthur^[12]对外加电流阴极保护作用下, 钢筋混凝土模拟孔溶液pH值的变化过程及向阴极的扩散趋势进行了模拟, 发现当电流密度为20 mA/mm², 通电时间为1050 h时, 酸化效应在横截面积为4.9 mm²玻璃管中向外扩散的范围可以达到260 mm.Polder等^[13]基于Nernst-Planck方程建立了外加电流阴极保护作用下Ca²⁺、OH^-和Ca(OH)₂的共平衡关系并通过模拟得到在1 mA/m²电流密度作用10年后, 酸化效应的影响范围可以达到100 µm, 但该研究没有确定酸化反应动力学参数, 也没有考虑Na⁺, K⁺对体系的影响, 并且没有进行实验对模拟结果进行验证. ...

Peelen, R. B. Polder, E. Redaelli, L. Bertolini, Qualitative model of concrete acidification due to cathodic protection,

2008

Influence of superplasticizers on pore solution composition and on expansion of concrete due to alkali-silica reaction

2011

... 根据相关参考文献^[14-18], 本研究中以去离子水配制了0.002 mol/L Ca(OH)₂+0.06 mol/L NaOH+0.18 mol/L KOH溶液作为混凝土模拟孔溶液, 并用稀HNO₃溶液(0.0001 mol/L)将模拟孔溶液pH值调至13.此外, 在模拟孔溶液中掺入质量分数为3.5% NaCl, 模拟钢筋受到氯盐侵蚀的环境.配置混凝土模拟孔溶液所用试剂均为分析纯. ...

A. Ferna Ndez-Jime Nez, M. T. Blanco-Varela, Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate

2004

Conductivity of pore solution and permeability of concrete,

2000

孔溶液电导率与电测混凝土渗透性,

2000

Chemical composition of cement pore solutions

1989

Pore solution composition and chloride binding capacity of silica-fume cement pastes,

1983