A wedge-shaped crevice cell was used to model previously disbonded coating holiday of X70 pipeline steel under impressed current cathodic protection in simulated solution of Ku'erle desert soil, being composed of 0.06 M Cl-, 0.009 M SO2-4 and 0.0017 M HCO-3 ions, in Xinjiang Province of West China . The wedge-shaped crevice was 100 mm in depth, 45 mm in width, 0.15～0.45 mm in opening thickness of one end (another end sealed), and open to test solution through opening end of the crevice stretching into bulk solution compartment (100 mm×100 mm×100 mm, and 8mm in wall thickness). X70 steel electrodes mounted into the crevice were used to determine the local current densities and monitor the corrosion situation inside the crevice. The H+ sensor mounted above the steel electrode through the ports in a top plate of polymethyl methacrylate (100 mm×45 mm×10 mm) forming the crevice was used to measure the pH inside the crevice. Distributions of the local potentials, local current densities and pH inside the crevice were measured at three applied control potentials (Ek) of -0.85, -1.0, and -1.15 VCSE, and at three crevice opening thickness (δ) of 0.15, 0.30, and 0.45 mm. Distribution curves of local potentials and local current densities inside crevice became more uniform with increasing polarization time. A negative shift of control potentials and increase of opening thickness resulted in the decrease of the local potentials, and the increase of local current densities inside crevice. The pH inside crevice gradually increased with more negative control potentials and bigger opening thickness. At the same control potential, the pH from crevice opening to bottom in crevice increased first, then stabilized at a definite level. Moreover, more negative shift of control potentials and (or) bigger opening thickness can make the stable pH increase, but the increasing rate of pH gradually decreased with more negative control potentials and (or) bigger opening thickness.%以新疆库尔勒土壤模拟溶液为腐蚀介质(0.06 M Cl-+0.009 M SO2-4+0.001 7 M HCO3-),在阴极极化条件下,采用楔型缝隙模型研究了具有剥离涂层缺陷X70钢的缝隙腐蚀.楔型缝隙的一端封闭,另一开口端延伸到溶池(100 mm×100 mm×100 mm)的模拟溶液中.楔型缝隙的尺寸为深100 mm,宽45 mm,开口尺寸0.15～0.45 mm.将6条X70钢电极试样安装到缝隙中用以测定缝口不同距离处的局部电流密度、极化电位并监测试样的腐蚀状况.安装在缝隙上盖测量孔中的H+离子探头用于测量缝内的pH值.采用-0.85,-1.0和-1.15 V(相对于CSE)的阴极极化电位分别对缝口尺寸(δ)为 0.15,0.30和 0.45 mm的楔型缝隙进行阴极极化.试验结果表明,随着极化时间的延长,缝内局部电流密度和极化电位变得更为均匀,随着缝口控电位的降低以及缝口尺寸的增加,缝内的局部电流密度和pH值增加.缝口控电位相同时,从缝口到缝底的pH值先增加,然后达到一个较稳定的状态.缝口控电位越低以及缝口尺寸越大,pH值越易于达到稳定状态,且pH值开始增加的速率越小.
展开▼
