第四章+杂散电流课件.ppt

第四章 杂散电流腐蚀第一节 概述;杂散电流腐蚀类似于电化学腐蚀 特征: 阳极区的局部腐蚀。在管线的阳极区，绝缘涂层的破损处，腐蚀破坏尤为集中。 当管地电位较正，大于-0.85V 时管道可能得不到保护； 当电位过负，如小于-1.2V 时，管道表面会析出大量氢，造成沥青类防腐绝缘层破坏，脱落，从而加剧阴极区的腐蚀破坏。 ;杂散电流腐蚀与自然腐蚀的比较;杂散电流腐蚀的基本概念;杂散电流的来源;动态干扰电流 动态干扰电流来自电力传输系统（如：地铁，火车和采矿作业），通过邻近的防腐层良好的管道网络可以传送到几英里远的地方。如果不采取措施消除，杂散干扰电流会对邻近的地下金属管线/地下结构产生非常有害的影响。 存在动态干扰电流时，管地电位也表现为偏离正常值，且随时间变化幅度较大。;第二节 直流杂散电流对埋地金属管道的腐蚀;;影响管道杂散电流腐蚀的因素;二、电解法则与电蚀系数;三、干扰源与被干扰体相互位置关系的影响;;;四、干扰腐蚀的判定方法;五、干扰的判断标准;七、杂散电流的控制措施;排流法的种类;八、现有监测杂散电流干扰方法;在没有增加电流源的情况下，管道的对地电位的提高是杂散电流进入点的迹象。管道的对地电位的下降通常为杂散电流的放电点的指示。通过电压测量发现以下问题时，说明有杂散电流存在，并通过土壤电位梯度能够分析出杂散电流流入、流出电极电流大小。 管－地电位不稳定 管－地电位严重偏离正常值 土壤电位梯度反常;Stray Current Corrosion Whenever a metallic structure is placed in the electric field between the structure and the anode, it provides an alternative route for the electron current path. Thus current can enter a foreign structure at one point and leave it at another location. At the interface on the foreign structure where the electrons move away from, corrosion is enhanced. This is known as stray current corrosion. This may be easily demonstrated in the laboratory and may be explained using a modified galvanic corrosion polarization diagram. ;;;锄屁九庆爸蒂泄词欲兰拣抓嘶销裂奠幽萎逝声转媳耀步想赚谋苟中扩禽鞠第四章+杂散电流课件第四章+杂散电流课件;Consequences of Overprotection It is possible during cathodic protection to supply excess direct current to polarize a structure below the recommended protection potential. This state of affairs is termed overprotection. There are two main consequences of overprotection, namely, waste of current and more seriously the violation of the structural integrity of the metal. The waste of current is due to the polarization of the metal below its equilibrium potential with the excess current being used to evolve hydrogen. The gas produced could cause the detachment of organic coatings and the removal of calcareous deposits in offshore structures. Hydrogen production has also adverse effects on both the corrosion fatigue life and hydrogen