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 or more of which might be appropriate.

3. Three ways of Corrosion Control
Conditioning the Metal
This 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 alloy.

4. Conditioning the Corrosive Environment
Removal of Oxygen
By the removal of oxygen from water systems in the pH range one of the components required for corrosion would be absent.
Corrosion Inhibitors
A corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous environment, reduces the rate of metal wastage.

5. Electrochemical Control
Cathodic and anodic protection
This 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 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. Davy’s advice was sought by the Royal Navy in 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 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 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 cathodic 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 used 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. 2.Principles
If electrons are passed into the metal and reach the metal/electrolyte interface (a cathodic current) the anodic reaction will be stifled while the cathodic reaction rate increases. This process is called cathodic protection.

13. IA＝0
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14. 3.Methods 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 provided by an external current source. (ICCP)

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 materials.

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, able 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 been 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 of 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 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, aluminium 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. Cathodic 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
Ships’ hulls
Storage tank bases
Jetties码头 and harbour structures
Steel sheet, tubular and foundation pilings
Offshore platforms, floating and sub sea structures

26. Cathodic protection is also used to protect the internal surfaces of:
Large diameter pipelines
Ship’s tanks (product and ballast)
Storage tanks (oil and water)
Water-circulating systems.

27. 5.Design Effectiveness of cathode protection: η
V corrosion rate before protection;
V----- corrosion rate after protection.

28. 1)Protection Potentials
From the basic electrochemical theory absolute protection (zero corrosion rate) is achieved if the structure is polarized to the reversible electrode potential of the anodic reaction.
Epr = Eea

29. 参 比 电 极 一些金属的保护电位 (单位：V) 金属或合金 铁与钢 含氧环境 缺氧环境 -0.85 -0.95 -0.80 -0.90
　　参　 比 　 电 极
Cu/饱和CuSO4
Ag/AgCl/
海水 (1)
饱和KCl
Zn/洁净海水
铁与钢
含氧环境
缺氧环境
-0.85
-0.95
-0.80
-0.90
-0.75
+0.25
+0.15
铅　　
-0.6
-0.55
-0.5
+0.5
铜合金
-0.5~-0.65
-0.45~-0.6
-0.4~-0.55
+0.6~+0.45
铝(2)
正的极限值
负的极限值
-1.2
-1.15
-1.1
-0.1
（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

30. 2)Current Density
The current density required to maintain the protection potential is very dependent on local conditions.
Increased availability 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 will have a profound effect on current density.

32. Current densities required to protect steel环境 条件
Ipr(mA/m²)
稀硫酸 海水 淡水
高温淡水 室温 流动
氧饱和 脱气
120
150 60
180 40
中性土壤
混凝土
细菌繁殖 通气
不通气
含氯化物
无氯化物
400 4 5 1

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 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 structure 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 along the pipe, but this will greatly increase installation costs.

37. 阴极保护中电流遮蔽作用的实例 - + - + (a)对管内壁保护 (b) 管束间实施保护 (c)有突出部分结构 － ＋ 引自«电化学保护在化
－　＋
- +
(a)对管内壁保护
(b) 管束间实施保护
- +
(c)有突出部分结构
引自«电化学保护在化
肥生产中的应用»P83.84
阴极保护中电流遮蔽作用的实例

38. 4)COATINGS
The provision of an insulating coating to the structure will greatly reduce the current demand for cathodic protection. When first applied, coatings 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 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 protection 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 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 boilers 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 corrosion potential Ecorr can be several orders of magnitude greater than the current necessary to maintain the potential at a passive value.

46. Anodic protection possesses unique advantages.
For example, the applied current is usually equal to the corrosion rate of the protected system. Thus, anodic protection 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
Anodic Protection of a Steel Tank - +
Reference electrode
Cathode
Acid
Steel tank
Salt bridge
Current flow

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。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Epp～Etp： 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. CP of carbon steel(CS) head boxes
Combination of AP and CP
acid cooler
AP of 316L SS
shell and tubes
CP of carbon steel(CS) head boxes

50. Anodic protection can decrease corrosion rate substantially
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

53. 阳 极 保 护 阴 极 保 护 相同 只能用于电解质溶液的连续液相部分；所需极化电流必须符合经济要求；设备结构不能太复杂。 不 同
防护技术
　阳　极　保　护
　　　阴　极　保　护 相同
只能用于电解质溶液的连续液相部分；所需极化电流必须符合经济要求；设备结构不能太复杂。 不 同
1、本质上的差别：阳极极化时金属腐蚀倾向增大，才只适用于能阳极钝化的体系。
1、阴极极化时：金属腐蚀倾向减小，到达稳定区则金属不会腐蚀，故原理上适用于一切腐蚀体系。
2、钝化前要经过电解腐蚀阶段维钝电流密度对应于保护下的腐蚀速度
2、不会产生电解腐蚀，保护电流和设备腐蚀速度没有直接关系。
3、控制电位要求高
3、控制电位要求低
4、致钝电流大，而维钝电流小，投资费用高，日常操作费用低。
4、极化电流变化不大，投资费用较低，而日常操作费用较阳极保护高。　
5、强氧化性介质中保护效果好
5、多用于弱和中等程度腐蚀环境，（特别是氧扩散控制体系）
6、设备不会造成氢损害
6、析氢可能造成设备材料的鼓泡或脆性，对加压和高强材料危险。 7
7、可能造成两性金属腐蚀增加，加速涂料破坏，有利Ca²+.Mg²+沉积