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1、翻译部分英文原文31Corrosion control in electric power systemsAbstractThe electrochemical mechanism of aqueous corrosion is outlined and it is demonstrated that the process depends upon a complex interaction between the material and its environment given the particular circumstances of exposure. Failure to r
2、ecognize the full implications of the environment or the circumstances will often lead to unexpected corrosion failure. The techniques forcontrolling corrosion are subdivided into the following: material selection, coatings, electrical insulation, cathodic protection, etc. Each technique is describe
3、d and explained. The need to apply a considered corrosion control package, rather than the use of a single technique, is stressed.1 IntroductionRust never sleep!, so says a popular song. A moments thought will confirm that for most metals, the chemically combined state does indeed appear to be prefe
4、rable to the metallic state. Thus, metals occur in nature predominantly in the form of oxides, sulphides, etc. Equally, freshly extracted metal, as well as finished metallic products, tend to deteriorate when exposed to moist air, particularly in the presence of pollutants. We are therefore left wit
5、h the conclusion that corrosion, for that is what this deterioration is, is all but inevitable. This impression is confirmed by thermodynamic analysis that shows that only the most noble metals approach immunity to corrosion and even they are susceptible given appropriate circumstances. Thus, corros
6、ion of most metals is inevitable and corrosion prevention all but impossible. Fortunately corrosion control is possible and depends on actively setting out to limit corrosion in any given case.This paper presents an outline of the stratagems and techniques available for corrosion control and its app
7、lications in the field of electric power systems. More detailed treatment of each method will be found in the publications listed in the references.2 Corrosion theory2.1 Basic corrosion cellThe following four components must always be present in the basic corrosion cell:(a)anode;(b)cathode;(c)electr
8、olyte;(d)connecting conductor. If any of these four components are absent, an electrochemical cell cannot exist. Anode, an electrode which is losing metal or corroding; cathode, an electrode which is having metal or hydrogen plated upon it; electrolyte, chemicals in solution contacting both the anod
9、e and cathode; connecting conductor, a low resistance conductor connecting the anode and cathode.Fig.1 shows the galvanic cell which has a homogeneous electrolyte and differences in the metal of the anode and cathode.Fig.2 shows the concentration cell which has variations in the electrolyte and no d
10、ifferences in the anode and cathode metal. Measurable potential differences up to several hundred millivolts can be caused by both types of electrochemical cells. When grounding is considered, electrolysis from galvanic current generated in galvanic cells of dissimilar metals is more prevalent than
11、from natural current generated in concentration cells.2.2 Equi6alent electrical circuit for a corrosion cellFig.3 shows the equivalent electrical circuit for a corrosion cell. Metal ions leave the anode by way of the electrolyte, and electrons travel from the anode to the cathode by way of the metal
12、lic path. The anode suffers corrosion attack but the cathode is protected. Very noble (least reactive) metals such as platinum and some nonmetals such as carbon may function as anodes without corroding. The current flows as a result of potential difference (E) between the anode and cathode. The amou
13、nt of current (I) flow in a corrosion cell (and thus the amount of corrosion at the anode) depends on the magnitude of the driving voltage and the total effective resistance.E=IR, where:R=R1+R2+R3+R4R1=metallic path resistance (usually low and often neglected).R2=electrolyte path resistance.R3=appar
14、ent or effective boundary resistance at the anodeR4=apparent or effective boundary resistance at the cathode.2.3 Corrosion theoryThere are two basic mechanisms by which metals in electrolytes corrode, namely, galvanic corrosion; and electrolytic corrosion.2.3.1 Gal6anic corrosionGalvanic corrosion i
15、s, in effect, a battery resulting from the connection of two dissimilar metals immersed in an electrolyte (soil) generating the galvanic current. These natural direct currents or galvanic currents are produced in the basic corrosion cell, sometimes referred to as an electrochemical cell, where a gal
16、vanic couple of two dissimilar metals, with an electrical conductor connecting them, is submerged in an electrolyte. Most soils contain a mixture of moisture and mineral salts and thereby, satisfy the requirements of a good electrolyteFig.1.Basic corrosion galvanic cellFig.2.Basic corrosion concentr
17、ic cellIf a copper ground rod and some part of a steel piping system to which the ground rod is electrically connected are both buried in the soil, the electrolytic nature of the soil gives rise to a galvanic action in which the copper acts as a cathode and the steel as an anode. The electrolytic ci
18、rcuit is completed, and a direct current will flow through the soil from the steel to the copper. The iron will discharge current to the soil, and wherever the current leaves the buried iron, an electrochemical equivalent amount of metal is dissolved, resulting in a corrosion pit. This galvanic corr
19、osion occurs only at the anode(steel piping system)and never at the cathode(copper ground rod).Fig.3.Corrosion cell as an electrical circuit.In addition to galvanic currents generated between various dissimilar metals, local galvanic currents may be generated between various parts of the same metall
20、ic structure. These may be due to differences in the surrounding soil or moisture content rather than differences in the metal. Some of the corrosion experienced with buried metal pipes, conduits, lead sheathed cable, etc. This is most likely caused by local variation in the soil characteristics as
21、the source of galvanic currents instead of dissimilar metals or stray currents.Fig.4.Impressed current cathodic protection (rectifier type)2.3.2 Electrolytic corrosionElectrolytic corrosion is the result of current coming from outside sources, entering and then leaving a particular metal structure b
22、y way of the electrolyte. Where a current enters the structure, that structure is usually unaffected or is provided with some degree of protection. Where the current leaves the structures, corrosion occurs. In underground work, this type of corrosion is often referred to as stray current corrosion.A
23、lthough damage by alternating (ac) is less than by direct current (dc), the resultant corrosion is usually greater for lower frequency and less for higher fre-quency currents. According to, it is estimated that for metals like steel, lead, and copper,60-Hz ac current causes less than 1% of the damag
24、e caused by equivalent dc current.Fig.5.Galvanic anode cathodic protection (sacrificial anode)Many industrial installations will have some sort of stray current problem if no attention has been given to its control. The source of these dc stray currents may be welding equipment, electroplating proce
25、sses, battery charging apparatus, motor generators, dc control circuits, uninterruptable and interruptable power systems, inverter systems, nearby impressed current cathodic protection systems, dc transit systems, etc. A common practice with welding equipment that often causes trouble is to ground o
26、ne welding electrode to a metal building frame. This superimposes dc on the grounding system of the building and, if the current which leaves the grounding electrode is large enough, considerable corrosion will result.Table 1Comparison of characteristicsGalvanic(sacrificial)Impressed(rectifier)1.Req
27、uires no external power1.External power required2.Fixed driving voltage2.Voltage can be varied3.Limited current3.Current can be varied4.Usually used where current requirements are small4.Can be designed for almostany current requirement5.Usually used in lower resistivity electrolytes5.Can be used in
28、 almost any resistivity environment6.In underground applications interference with neighboring structures must be considered6.In underground application interference with neighboring structures is usually negligibleThe bus structure in a large electroplating equivalent will have many leakage paths o
29、ver wet insulators if situated in continuously moist atmospheres. Battery charging and similar equipment also is likely to suffer from dc leakage where the path to earth is of low resistance as a result of acid fumes. Direct current control circuits, with either pole grounded, can superimpose dc on
30、the grounding system.Fig.6.Cathodic protection of direct buried transformersFig.7.Fossil-fueled power plant3 Corrosion controlThe most common methods of corrosion control involve material selection, coatings, electrical insulation and cathodic protection. Each of these measures has distinct advantag
31、es and disadvantages. All should be considered when planning a comprehensive corrosion control program3.1 Material selectionMaterial selection needs to be addressed during pro-ject design. Through the use of non-metallic components, corrosion can be avoided. Most often, however, nonmetallic material
32、s have operating limitations that necessitate the use of metals. Under these circumstances, similar metals should be incorporated in the design to avoid the interconnection of materials that are not close to each other on the galvanic series (energy hill).This serves to reduce the driving force betw
33、een the anode and cathode in the corrosion cell and thereby decreases the rate of corrosion. The proper selection of materials for the environment is very important. Many materials perform well in one environment but corrode very rapidly in another. Aluminium, for example, is extremely resistant to
34、corrosion in the atmosphere because it forms a protective oxide layer that inhibits further corrosion. In salt water or soil environments, aluminum rapidly corrodes because the oxide layer that forms does not have the same protective properties. Stainless steel performs similarly. When stainless ste
35、el is placed in an oxidizing environment, a very thin oxide forms which passivates the metal surface. If this film is damaged by such things as abrasion or the presence of chloride ions, the stainless steel will corrode at approximately the same rate as mild steel.The careful selection of materials
36、can reduce but cannot prevent corrosion. Differences in the electrolyte and differences along the surface of the metal will still create an energy imbalance which results in corrosion. This cannot be entirely avoided.3.2 CoatingsCoatings are often used for corrosion control. The purpose of a barrier
37、 coating is to insulate the anode and cathode of the corrosion cell from the environment. By insulating the metallic surface from the electrolyte, coatings prevent the corrosion reaction from occurring. Selection of a coating for a particular application requires detailed knowledge of the environmen
38、t and the type of service. After a coating is selected, great care must be exercised to make certain the coating is properly applied. A coating failure usually occurs due to improper application and not to a breakdown of the coating material. Surface preparation is critical to coating performance.Th
39、e proper selection and application of a coating system will reduce the total metal loss but will not prevent corrosion. Unfortunately, coatings concentrate the corrosion activity at the imperfections. This intensifies the corrosion and results in penetrations more rapidly than if the structure were
40、uncoated.3.3 Electrical insulationThe principles of cathodic protection can be best understood by examining the corrosion cell. At the anode, metal is consumed, but at the cathode, the energy level is maintained. By making the entire structure a cathode, all surfaces that receive sufficient current
41、can maintain their position on the energy hill. This is cathodic protection.There are two methods of applying cathodic protection. The first method, called impressed current cathodic protection, uses anodes which are energized by an external power source as shown in Fig.4.In this type of cathodic pr
42、otection system, anodes are installed in the electrolyte and are connected to the positive terminal of the DC source; the structure which is to be protected is connected to the negative terminal of that source. Due to the power source almost being a rectifier unit, this type of system is referred as
43、 a rectifier type system.The second method of protection makes use of galvanic anodes which have a natural difference of potential with respect to the structure to be protected as shown in Fig.5.These anodes are made of a material, such as magnesium or zinc, which is anodic with respect to the prote
44、cted structure and these anodes are connected directly to that structure. This is galvanic anode cathodic protection.In most cases, the rectifier type system is designed to deliver relatively large currents from a limited number of anodes, and the galvanic anode type system is designed to deliver re
45、latively small currents from a large number of anodes. Each method of applying cathodic protection has characteristics which make it more applicable to a particular problem than the other. A comparison of these characteristics is shown in Table 1.Irrespective of the type of system used, in all syste
46、ms, current flows from the anode to the structure to be protected. When current flows onto a structure, from the surrounding electrolytes, the potential is made more negative, and cathodic protection is achieved when the change in potential is sufficient to arrest corrosion.4 Application4.1 Corrosio
47、n resistance of transmission structures fabricated from weathering steelWeathering steel introduced to utility appeared to offer a way to reduce structure weight and maintenance of lattice towers through the application of bare, high strength steel that had natural corrosion resistance. Weathering s
48、teel found wide application in lattice and tubular transmission structures. Through its service life, however, the weathering steel showed evidence of continuing corrosion rather than the expected protection from corrosion. A consortium of utilities was formed to investigate the impact on structure
49、reliability of the continuing corrosion of the steel beyond initial expectations. The investigation has shown that, the weathering steel will remain practical for new lattice and tubular structures provided steps are taken during the design process to minimize the retention and collection of moisture between and around metal contact surfaces and during the operation of the line to minimize vegetation encroa