| Ⅱ.外部电位和缝内电位及溶液pH的关系 |
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| 引用本文： | 左景伊,张树霞,徐玉宝,王光耀. Ⅱ.外部电位和缝内电位及溶液pH的关系[J]. 化工学报, 1982, 33(4): 302-309 |
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| 作者姓名： | 左景伊 张树霞 徐玉宝 王光耀 |
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| 作者单位： | 北京化工学院,北京化工学院,北京化工学院,北京化工学院 |
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| 摘 要： | 本文系利用作者设计的模拟腐蚀裂缝并能承受拉应力的闭塞电池,以研究Crl8Ni9Ti奥氏体不锈钢在0.5NNaCl溶液中(40℃)模拟的缝隙(闭塞区)中电位及pH随外部试件极化时的变化.发现这个体系的缝隙腐蚀(或应力腐蚀)临界电位约为-0.10至-0.20V SCE 当外部试件极化到临界电位区,闭塞区内腐蚀微小,pH不变化.当极化到临界电位以上(+0.20V以下),闭塞区内电位最终稳定在-0.10至-0.13V SCE,即保持在自由腐蚀电位(或临界电位)范围之内,但阳极极化使闭塞区通过的电流增大,促进了金属阳极溶解,金属离子水解使溶液pH下降到2—3.5之间,闭塞区由钝态变为活态.腐蚀加速.当外部试件极化到临界电位以下,闭塞区内电位与外部极化电位基本相等,一般稍正,闭塞区内处于钝态,溶液pH增大到碱性.但如极化电位下降到放氢平衡线以下,则可能产生氢脆.当闭塞区试件受拉应力(0.90_y)时,在阳极极化下,闭塞区电位变得更负,溶液pH下降也较大些.在阴极极化下应力对闭塞区的电位及pH均无影响.由于了解了缝内电位和腐蚀程度随外部电位变化的情况,所以可以通过外部极化以控制缝内电位,达到保护的目的.也可以由外部电位的变化来探测缝内腐蚀是否可能发生.
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II. Relations between potential and pH in the bulk and occluded cell |
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| Zuo Jingyi,Zhang Shuxia,Xu Yubao and Wang Quangyao. II. Relations between potential and pH in the bulk and occluded cell[J]. Journal of Chemical Industry and Engineering(China), 1982, 33(4): 302-309 |
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| Authors: | Zuo Jingyi Zhang Shuxia Xu Yubao Wang Quangyao |
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| Affiliation: | Zuo Jingyi,Zhang Shuxia,Xu Yubao and Wang Quangyao(Beijing Institute of Chemical Technology) |
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| Abstract: | A simulated occluded cell which can be loaded by lever system was designed to study the changes of potential and pH in occluded cell with polarization of bulk specimen. The system studied was Crl8Ni9Ti austenitic stainless steel-0.5M NaCl at 40℃. It was found that the critical crevice (or stress cor-rosion) potential of this system lies almost in the same range as the corrosion potentials of the occluded and bulk potential (0V to -0.2V SCE). The rule of changes of the potentials and pH values in the occluded cell could be classified into three categories as follows.1. When the bulk specimen was polarized above the critical potential, the occluded potentials did not rise with bulk potential, but remained nearly constant within corrosion potential range (-0.10 to- 0.13V SCE). Meanwhile, the pH in the occluded cell decreased rapidly from 7 to 2-3.5.2. When the bulk specimen was polarized below the critical potential, the occluded potential always kept a few mV more positive than the bulk potential, and the pH in the occluded cell increased higher than bulk pH (>7).3. When the potential of bulk specimen remained in the range of the critical potential, no change of pH and potential in the occluded cell took place within a long period.The tensile load of occluded specimen caused both the potential and pH in the occluded cell lower than those without load under anodic polarization of the bulk specimen, but no effect under cathodic polarization.The important phenomenon of incubation period for crevice or stress corrosion could be interpreted properly by the present results. The crevice or stress corrosion starts only when the bulk potential rises above the critical po-tential so that a corrosion cell of sufficient voltage difference between bulk cathode and occluded anode could be established. A rapidly accelerating process will result from pH fall and Cl- migration in the occluded cell. While the bulk potential is lower than the critical potential (cathodically polarized), the crevice or stress corrosion will not occur since no significant voltage difference exists between bulk and occluded cell.It is very interesting that the potential and pH in the occluded cell have a simple relation with the bulk potential. It could be easily to evaluate occluded potential or pH if one knows the bulk potential. By controlling the bulk potential one could make the initiation and propagation of crevice or stress corrosion prevented. Besides, one may determine whether there is danger of crevice or stress corrosion by measuring the bulk potential. |
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