第四章 杂散电流2011.ppt

第四章第四章 杂散电流腐蚀杂散电流腐蚀第一节第一节 概述概述杂散电流是指在规定的电路或意图电路之外流动的电流。杂散电流是指在规定的电路或意图电路之外流动的电流。在规定的电路中流动的电流，其中一部分自回路中流出，在规定的电路中流动的电流，其中一部分自回路中流出，流入大地、水等环境中，形成了杂散电流。流入大地、水等环境中，形成了杂散电流。杂散电流腐蚀类似于电化学腐蚀杂散电流腐蚀类似于电化学腐蚀特征特征: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与自然腐蚀的比较 （1）杂散电流腐蚀是一种外部电源作用的结果，而自）杂散电流腐蚀是一种外部电源作用的结果，而自然腐蚀是金属固有的特性；然腐蚀是金属固有的特性；（2）杂散电流腐蚀实质上是金属的电解过程，作为阳）杂散电流腐蚀实质上是金属的电解过程，作为阳极金属的腐蚀量与流经的电流量和时间长短成正比，可用极金属的腐蚀量与流经的电流量和时间长短成正比，可用法拉第定律进行计算法拉第定律进行计算 （3）杂散电流腐蚀其阴极区可能发生析氢破坏，而自）杂散电流腐蚀其阴极区可能发生析氢破坏，而自然腐蚀的阴极区不会受影响。然腐蚀的阴极区不会受影响。杂散电流腐蚀的基本概念杂散电流腐蚀的基本概念干扰干扰：通常把由于杂散电流的产生，促使金属构筑物腐蚀等一系列过程或现象，称为干扰。电蚀电蚀：为了区别土壤中的自然腐蚀，称杂散电流腐蚀为电蚀，也称为干扰腐蚀。干扰源：干扰源：一般将可能产生杂散电流的电路、设备或设施称为干扰源。干扰体干扰体：受干扰源影响的金属构筑物为干扰体。杂散电流的来源杂散电流的来源直流干扰源直流干扰源 直流电解设备、电焊机、直流输电线路。交流杂散电流交流杂散电流 交流电气化铁路、输配电线路及其系统，通过其阻性、感性、容性和耦合等对管道等造成的干扰。地电流地电流动态干扰电流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。幅度较大。静态干扰电流静态干扰电流静态干扰电流通常是从外加电流系统强制施加到管道上的，如：其它行业的阳极地床，或来自其它的外部结构。存在静态干扰电流时，管地电位表现为偏离正常值，但一般比较稳定。存在静态干扰电流时，管地电位表现为偏离正常值，但一般比较稳定。第二节 直流杂散电流对埋地金属管道的腐蚀 一、直流电力系统对管道腐蚀的影响 电车、电气化铁路、以接地为回路的输电系统等直流电力系统，都可能在土壤中产生杂散电流。杂散电流腐蚀原理图杂散电流腐蚀原理图杂散电流的数值随着行驶在路上的车辆数量、车辆间的相互间距、轨道状况、土壤状况和地下管道系统的状况而变化。由管道沿线电位的变化图可以判断管道上腐蚀电池的阳极区和阴极区，以及杂散电流腐蚀最强的部位。影响管道杂散电流腐蚀的因素一是负荷电流的大小二是管道防腐层对地的绝缘性三是土壤电阻率二、电解法则与电蚀系数二、电解法则与电蚀系数电解法则：电解法则：埋地管道受到干扰腐蚀时，腐蚀量和电量之间符合法拉第定律。电蚀系数电蚀系数：计算的腐蚀量与理论腐蚀量的百分比三、干扰源与被干扰体相互位置关系的影响三、干扰源与被干扰体相互位置关系的影响典型的位置关系典型的位置关系 存在与管道平行走向和交叉（垂直）走向两种关系。为了便于分析干扰影响和防护措施的一般规律分为两个类型。平行走向：干扰可通过排流法得以缓解，称为平行走向：干扰可通过排流法得以缓解，称为A型电蚀。型电蚀。垂直交叉走向：不能通过排流等简单方法缓解的，称为垂直交叉走向：不能通过排流等简单方法缓解的，称为B型电蚀。型电蚀。四、干扰腐蚀的判定方法四、干扰腐蚀的判定方法从腐蚀部位的外观特征判断从腐蚀部位的外观特征判断从环境条件判断从环境条件判断通过测量被干扰体对地电位进行判定通过测量被干扰体对地电位进行判定五、干扰的判断标准五、干扰的判断标准 埋地管道是否受到干扰，以管地电位的变化埋地管道是否受到干扰，以管地电位的变化为判据。为判据。一般认为，管地电位正向偏移一般认为，管地电位正向偏移20mV是是干扰存在和需要采取措施的判定指标。干扰存在和需要采取措施的判定指标。英国：英国：20mV 日本：日本：50mV 德国：德国：100mV七、杂散电流的控制措施七、杂散电流的控制措施 排流法排流法。把原先相对与路轨为阳极区的管线，用导线同路轨直接相连，使整个管线为阴极性。绝缘法。绝缘法。用绝缘性覆盖层或部件来切断管线所提供的电流通道。牺牲阳极法。牺牲阳极法。在管线的阳极区埋入与其相连的长金属棒，将腐蚀集中到长金属棒上。排流法的种类排流法的种类直接排流直接排流极性排流极性排流强制排流强制排流接地排流接地排流八、现有监测杂散电流干扰方法八、现有监测杂散电流干扰方法管道对地电位测量管道对地电位测量监测现有的电气接头监测现有的电气接头电压和电流检查电压和电流检查变压器变压器/整流器电压和电流检查整流器电压和电流检查使用腐蚀取样管使用腐蚀取样管定期的漏电点调查定期的漏电点调查当地的干扰统筹委员会通知有新的外加电流系统安装当地的干扰统筹委员会通知有新的外加电流系统安装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不稳定管地电位严重偏离正常值管地电位严重偏离正常值土壤电位梯度反常土壤电位梯度反常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.Stray currents in soils could originate as well from DC electrified rail tracks.A pipeline buried nearby could suffer stray current corrosion.The influence of high AC(alternating current)voltage overhead power lines on the corrosion of nearby structures is subject to substantial investigation.Stray current corrosion refers to corrosion damage resulting from current flow other than in the intended circuit(s).For larger structures this term usually alludes to corrosion damage caused by extraneous current(s)flowing through soil and/or water.Consequences of OverprotectionIt 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 embrittlement properties of structures especially those made of high strength materials.During overprotection large amounts of hydroxyl ions are also produced.On bare surfaces immersed in seawater,these could have a beneficial effect since the hydroxyl species may passivate and/or enhance the formation of calcareous deposits which in turn will reduce the current demand.However for organically coated surfaces the strong alkali condition at the metal surface may result in loss of adhesion for the paint.This phenomenon is known as cathodic disbonding.Introduction to Stray Current Corrosion Stray currents which cause corrosion may originate from direct-current distribution lines,substations,or street railway systems,etc.,and flow into a pipe system or other steel structure.Alternating currents very rarely cause corrosion.The corrosion resulting from stray currents(external sources)is similar to that from galvanic cells(which generate their own current)but different remedial measures may be indicated.In the electrolyte and at the metal-electrolyte interfaces,chemical and electrical reactions occur and are the same as those in the galvanic cell;specifically,the corroding metal is again considered to be the anode from which current leaves to flow to the cathode.Soil and water characteristics affect the corrosion rate in the same manner as with galvanic-type corrosion.However,stray current strengths may be much higher than those produced by galvanic cells and,as a consequence,corrosion may be much more rapid.Another difference between galvanic-type currents and stray currents is that the latter are more likely to operate over long distances since the anode and cathode are more likely to be remotely separated from one another.Seeking the path of least resistance,the stray current from a foreign installation may travel along a pipeline causing severe corrosion where it leaves the line.Knowing when stray currents are present becomes highly important when remedial measures are undertaken since a simple sacrificial anode system is likely to be ineffectual in preventing corrosion under such circumstances.end