摘要
为添补管线钢在内蒙古地区典型土壤环境的腐蚀性研究的空白。在包头土壤模拟溶液中,研究了X70管线钢带有涂层缺陷及楔形缝隙的腐蚀行为。
研究了X70钢在包头土壤模拟溶液中的基本电化学性质。X70钢在实验溶液中极化曲线的测定结果表明:在敞开体系中,随着溶液中HCO_3~-离子浓度的升高,钢的腐蚀程度增加;固定Cl-和HCO_3~-浓度时,随着溶液中SO_4~(2-)的加入,钢的腐蚀程度增加;模拟溶液比土壤浸出液腐蚀电流密度大,腐蚀速度快。
X70管线钢为基体,用环氧煤沥青制成不同种类和不同破损尺寸的涂层缺陷,浸泡在包头土壤模拟溶液中,腐蚀在缺陷处发生。自腐蚀电位和交流阻抗的测定结果表明,不同种类涂层缺陷的X70管线钢中剥离与破损涂层缺陷腐蚀最轻微,剥离次之,破损腐蚀最严重。随着破损涂层缺陷直径的增大,X70管线钢的腐蚀越来越严重。腐蚀介质顺着涂层与基体金属的接触处向涂层下扩散。
为了研究金属缝隙腐蚀的溶液化学和金属的电化学成行为,本文使用了一种楔形模拟缝隙构型。测试模拟缝隙自腐蚀状态下,X70管线钢的电极电位和实验结束时缝内溶液的pH值。测试对模拟缝隙构型施加不同程度阴极极化时,缝隙内X70管线钢的电极电位、电流密度以及实验结束时缝隙内溶液的pH值。
结果表明,自腐蚀状态下,模拟溶液中X70管线钢试样在缝隙中从缝口向缝尖电极电位越来越负,pH值越来越小,总体趋势是腐蚀敏感性越来越强。
外加阴极极化时,随极化电位降低,缝隙内电位相应降低,极化电流逐渐增大,pH值上升;极化电位小于-925mVSCE时,缝隙内各试样都能得到保护,不发生缝隙腐蚀;极化电位为-775mVSCE时,未达到保护电位,产生缝隙腐蚀。
Studying the corrosion behaviors of X70 pipeline steel with coating defects and wedge-shaped simulated crevice in the simulated solutions that are base on the Baotou area soil in order to replenish the blank of pipeline steels causticity investigation in Inner Mongolia Autonomous Region .
Electrochemical behaviors of X70 steel were studied in the simulated solutions.X70 steel’s polarization curves of measurement result indicate : in open system, the corrosion degree of X70 steel increases with the increasing of HCO_3~- concentration in the solution. Fixed the Cl- and HCO_3~- concentration, with the SO_4~(2-) joining, the corrosion degree of the steel increases. comparing in the simulate solutions and the soil lixivium, corrosion degree of X70 steel is bad in the simulate solutions.
The X70 pipeline steel as the body, The samples with the different sort and the different size circle defect which was made with epoxy coal pitch were immerged in the simulated solution, corrosion happened in the defects. Result of Ecorr and EIS diagram indicate the corrosion degree of X70 steel’s samples with disbonded and broken coating defects was most slight, disbond take second place, it is worst that the corrosion degree of X70 steel’s samples with broken coating defect.In the several different sizes of circle broken defects, following the diameter of defects increasing, the corrosion degree worse and worse. Corrosion medium was diffused from the joint between coating and base metal.
A self-made wedge-shaped simulated crevice was used to investigate the solution chemistry and the electrochemical behavior of the metal in the crevice. Measured distribution of pole potential and pH value of the solution in the crevice under the natural corrosive state. Measured distribution of pole potential, current and pH value of the solution in the crevice forcing different cathodic polarization potential.
The result shows: in all of simulated solutions, the electrode potential turned more and more negative , the pH value turned more and more small from the mouth of the crevice to the tip of the crevice under the natural corrosive state. the total trend is to the sensitivity of decay more and more strong.
Under cathodic polarization condition,following the polarization potential negative, the electrode potential corresponds to negative , the current density gradual enlarge and pH value rise in the crevice. When the polarization potential is less than -925mVSCE, the every sample in the crevice can get protection, don't take place the gap corrosion; While the polarization potential is -775mVSCE, miss to the protective potential, bring the gap decay.
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