1、1,第六章 防腐方法Corrosion Control,2,It has been estimated that a quarter of all corrosion problems could be prevented easily by using well established techniques. By retarding either the anodic or cathodic reactions the rate of corrosion can be reduced.Various corrosion control measures are available, one
2、 or more of which might be appropriate.,3,Three ways of Corrosion Control,Conditioning the MetalThis can be sub-divided into two main groups:Coating the metal, in order to interpose a corrosion resistant coating between metal and environment.Alloying the metal, to produce a more corrosion resistant
3、alloy.,4,Conditioning the Corrosive EnvironmentRemoval of OxygenBy the removal of oxygen from water systems in the pH range 6.5- 8.5 one of the components required for corrosion would be absent.Corrosion InhibitorsA corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous
4、 environment, reduces the rate of metal wastage.,5,Electrochemical ControlCathodic and anodic protectionThis is the control of metal potentials to reduce the corrosion rate. It is suitable for immersed and underground conditions for plant, recirculatory systems and natural environments. This can be
5、done by either using sacrificial electrodes or using an impressed current.,6,第一节 电化学保护Electrochemical Control,1.Cathodic Protection,7,1.History,The first reported practical use of cathodic protection is generally credited to Sir Humphrey Davy in the 1820s. Davys advice was sought by the Royal Navy i
6、n investigating the corrosion of copper sheeting used for cladding the hulls（船体） of naval vessels.,8,Davy found that he could preserve copper in seawater by the attachment of small quantities of iron, zinc or tin. The copper became, as Davy put it, “cathodically protected”. It was quickly abandoned
7、because by protecting the copper its antifouling防污的 properties became retarded, reducing the streamline of the ships, as they began to collect marine growths.,9,The most rapid development of cathodic-protection was made in the United States of America and by 1945, the method was well established to
8、meet the requirements of the rapidly expanding oil and natural gas industry, which wanted to benefit from the advantages of using thin-walled steel pipes for underground transmission.,10,In the United Kingdom, where low-pressure, thicker-walled cast iron pipes were used extensively, very little cath
9、odic protection was applied until the early 1950s. The increasing use of cathodic protection in modern times has arisen, in part, from the initial success of the method as used from 1952 onwards to protect about 1000 miles of wartime fuel-line network.,11,The method is now well established and is us
10、ed on a wide variety of immersed and buried facilities and infrastructure基底结构, as well as reinforced concrete structures, to provide corrosion control.,12,If electrons are passed into the metal and reach the metal/electrolyte interface (a cathodic current) the anodic reaction will be stifled while t
11、he cathodic reaction rate increases. This process is called cathodic protection.,2.Principles,13,I保IK1-IA1,IA0,14,The two main methods of achieving this goal are by either:Using sacrificial anodes with a corrosion potential lower than the metal to be protected. (SCP) Using an impressed current provi
12、ded by an external current source. (ICCP),3.Methods,15,An impressed current cathodic protection system is so named because the power is provided by a conventional power source (the local utility company). The current is impressed on the corroding structure through the use of relatively inert materia
13、ls.,16,Protection Of A Buried Pipeline Using Impressed Current,17,A range of materials have been used as non-consumable anodes for impressed-current systems. The sort of properties required by these anodes are：a. good electrical conduction,b. low rate of corrosion,18,c. good mechanical properties, a
14、ble to stand the stresses which they may be subjected to during installation and in service,d. readily fabricated into a variety of shapes,e. low cost,f. able to withstand high current densities at their surfaces without forming resistive barrier oxide layers, etc.,19,The following materials have be
15、en used as anodes: magnetite, carbonaceous materials (graphite), high silicon iron (14-18% Si), lead/lead oxide, lead alloys, platinised materials (such as tantalum, niobium, titanium). Platinum, with its high resistance to corrosion, would be an ideal anode material but has the major disadvantage o
16、f very high cost.,20,A galvanic (or sacrificial) cathodic protection system - the electrical currents are generated by dissimilar metals in a common electrolyte. When dissimilar metals are placed in an electrolyte (and are connected by a metallic return path) a current will flow from the metal with
17、the higher potential, through the electrolyte, to the metal with the lower potential.,21,Protection of an Oil Production Platform Using Sacrificial Zinc Anodes,22,Some anode material is lost by self-corrosion, and the anodes are not converted to electrical energy with 100% efficiency. Zinc, aluminiu
18、m and magnesium area the metals commonly used for sacrificial cathodic protection.,23,4.Advantages,The main advantage of cathodic protection over other forms of anti-corrosion treatment is that it is applied simply by maintaining a dc circuit and its effectiveness may be monitored continuously.,24,C
19、athodic protection is commonly applied to a coated structure to provide corrosion control to areas where the coating may be damaged. It may be applied to existing structures to prolong their life.,25,Structures that are commonly protected by cathodic protection are the exterior surfaces of:Pipelines
20、Ships hullsStorage tank basesJetties码头 and harbour structuresSteel sheet, tubular and foundation pilingsOffshore platforms, floating and sub sea structures,26,Cathodic protection is also used to protect the internal surfaces of:Large diameter pipelinesShips tanks (product and ballast)Storage tanks (
21、oil and water)Water-circulating systems.,27,5.Design,Effectiveness of cathode protection: ,V0- corrosion rate before protection;V- corrosion rate after protection.,28,From the basic electrochemical theory absolute protection (zero corrosion rate) is achieved if the structure is polarized to the reve
22、rsible electrode potential of the anodic reaction. Epr = Eea,1)Protection Potentials,29,（1）海水指洁净，并未稀释的海水（2）铝进行阴极保护时，电位不能太负，否则会加速腐蚀，产生负保护效应,一些金属的保护电位 (单位：V),30,2)Current Density,The current density required to maintain the protection potential is very dependent on local conditions. Increased availabi
23、lity of oxygen at the surface of the metal will directly increase current density.,31,Current densities to structures in sea water, rivers, etc are likely to vary continuously. The pH of the environment will also be important. The presence of coatings, marine fouling, and calcareous石灰质的 deposits wil
24、l have a profound effect on current density.,32,Current densities required to protect steel,33,The total anode current can be determined from the area of the structure. The size of the anodes can then be determined if a sacrificial anode scheme is to be employed, taking into account the working life
25、 of the protected structure or the period required between refits.,34,3)distribution of current,When current flows from a small anode to a large metal structure, the current density is at a maximum near the surface of the anode. Hence, a major portion of the potential drop between anode and structur
26、e occurs in the vicinity（邻近） of the anode.,35,Current and potential distribute on along a pipe,36,Clearly, to ensure that the ends of the pipe are protected, the centre of the pipe, nearest the anode, must be overprotected to some degree. The effect can be minimised by using several anodes spaced al
27、ong the pipe, but this will greatly increase installation costs.,37,阴极保护中电流遮蔽作用的实例,引自电化学保护在化肥生产中的应用P83.84,(c)有突出部分结构,(a)对管内壁保护,(b) 管束间实施保护,- +,38,4)COATINGS,The provision of an insulating coating to the structure will greatly reduce the current demand for cathodic protection. When first applied, coa
28、tings will often contain flaws, and in service, further defects will develop over a period of time. The conjoint use of coatings and cathodic protection takes advantage of the most attractive features of each method of corrosion control.,39,Thus, the bulk of the protection is provided by the coating
29、 and cathodic protection provides protection to flaws in the coating. As the coating degrades with time, the activity of the cathodic protection system develops to protect the deficiencies in the coating. A combination of coating and cathodic protection will normally result in the most economic prot
30、ection system.,40,5)CALCAREOUS SCALES,In sea water, cathodic protection of bare steel is economic because of the formation of calcareous deposits. The alkali formed at the surface of a protected structure reacts with bicarbonate ions present in sea water to form carbonate ions.,41,The carbonate ions
31、 in turn precipitate as insoluble calcium carbonate on the surface of the metal.,42,Anodic Protection,43,In contrast to cathodic protection, anodic protection is relatively new. Edeleanu first demonstrated the feasibility of anodic protection in 1954 and tested it on small-scale stainless steel boil
32、ers used for sulfuric acid solutions.,44,1.PRINCIPLES,The corrosion rate of an active-passive metal can be significantly reduced by shifting the potential of the metal so that it is at a value in the passive range. The current required to shift the potential in the anodic direction from the corrosio
33、n potential Ecorr can be several orders of magnitude greater than the current necessary to maintain the potential at a passive value.,45,46,Anodic protection possesses unique advantages. For example, the applied current is usually equal to the corrosion rate of the protected system. Thus, anodic pro
34、tection not only protects but also offers a direct means for monitoring the corrosion rate of a system.,47,Anodic Protection of a Steel Tank,To potentiostat,Reference electrode,Current flow,Salt bridge,Cathode,Acid,Steel tank,-,+,48,2.PARAMETERS,致钝电流密度i致：为使金属钝化所需的外加阳极极化电流密度 。维钝电流密度i维：钝化区所对应的阳极极化电流密度
35、。i维 用于维持金属的钝态，在阳极保护中反映日常的电耗和钝化后金属的腐蚀速度。i维 越小，阳极保护的效果越好。维钝区电位范围EppEtp： 反映金属钝态的稳定程度，钝化区电位范围越宽，说明金属钝化后不容易活化或过钝化。,49,Combination of AP and CP,CP of carbon steel(CS) head boxes,AP of 316L SS shell and tubes,acid cooler,50,Anodic protection can decrease corrosion rate substantially. The primary advantages of anodic protection are its applicability in extremely corrosive environments and its low current requirements. Anodic protection has been most extensively applied to protect equipment used to store and handle sulfuric acid.,51,Comparison of AP and CP,52,53,防护技术,
