长距离石油埋地管线阴极保护技术.pdf

1 [CATHODIC PROTECTION/BM] CATHODIC PROTECTION P E FRANCIS CONTENTS 1 INTRODUCTION............................................................................................................2 2 PRINCIPLES OF CATHODIC PROTECTION..........................................................3 3 S OF APPLYING CATHODIC PROTECTION........................................5 3.1 IMPRESSED CURRENT..........................................................................................5 3.2 SACRIFICIAL ANODES..........................................................................................6 4 DESIGN............................................................................................................................8 4.1 PROTECTION POTENTIALS .................................................................................8 4.2 CURRENT DENSITY...............................................................................................8 4.3 COATINGS...............................................................................................................9 4.4 CALCAREOUS SCALES.........................................................................................9 4.5 CHOICE OF CATHODIC PROTECTION SYSTEM..............................................9 4.6 ANODE RESISTANCE..........................................................................................10 4.7 DANGERS TO BE AVOIDED...............................................................................12 2 [CATHODIC PROTECTION/BM] CATHODIC PROTECTION P E FRANCIS 1 INTRODUCTION The first practical use of cathodic protection is generally credited to Sir Humphrey Davy in the 1980s. Davy’s advice was sought by the Royal Navy in investigating the corrosion of copper sheeting used for cladding the hulls of naval vessels. Davy found that he could preserve copper in sea water by the attachment of small quantities of iron or zinc; the copper became, as Davy put it, “cathodically protected”. The most rapid development of cathodic-protection systems was made in the United States of America 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. For that purpose the was well established in the United States in 1945. In the United Kingdom, where low-pressure thicker-walled cast-iron pipes were extensively used, very little cathodic protection was applied until the early 1950s. The increasing use of cathodic protection has arisen from the success of the used from 1952 onwards to protect about 1000 miles of wartime fuel-line network that had been laid between 1940 and 1944. The is now well established. Cathodic protection can, in principle, be applied to any metallic structure in contact with a bulk electrolyte. In practice its main use is to protect steel structures buried in soil or immersed in water. It cannot be used to prevent atmospheric corrosion. Structures commonly protected are the exterior surfaces of pipelines, ships’ hulls, jetties, foundation piling, steel sheet-piling, and offshore plats. Cathodic protection is also used on the interior surfaces of water-storage tanks and water-circulating systems. However, since an external anode will seldom spread the protection for a distance of more than two or three pipe-diameters, the is not suitable for the protection of small-bore pipework. w w w . b z f x w . c o m 3 [CATHODIC PROTECTION/BM] Cathodic protection has also been applied to steel embedded in concrete, to copper-based alloys in water systems, and, exceptionally, to lead-sheathed cables and to aluminium alloys, where cathodic potentials have to be very carefully controlled. 2 PRINCIPLES OF CATHODIC PROTECTION Corrosion in aqueous solutions proceeds by an electrochemical process, and anodic and cathodic electrochemical reactions must occur simultaneously. No nett overall charge builds up on the metal as a result of corrosion since the rate of the anodic and cathodic reactions are equal. Anodic reactions involve oxidation of metal to its ions, e.g. for steel the following reaction occurs. Fe Fe2 2e 1 The cathodic process involves reduction and several reactions are possible. In acidic water, where hydrogen ions H are plentiful, the following reaction occurs. 2H 2e H2 2 In alkaline solutions, where hydrogen ions are rare, the reduction of water will occur to yield alkali and hydrogen. 2H2O 2e H2 2OH- 3 However, unless the water is deaerated reduction of oxygen is the most likely process, again producing alkali at the surface of the metal. O2 2H2O 4e 4OH- 4 Reactions 1 and 2 are shown schematically in Fig 1 where anodic and cathodic sites are nearby on the surface of a piece of metal. We can change the rate of these two reactions by w w w . b z f x w . c o m 4 [CATHODIC PROTECTION/BM] withdrawing electrons or supplying additional electrons to the piece of metal. It is an established principle that if a change occurs in one of the factors under which a system is in equilibrium, the system will tend to adjust itself so as to annul, as far as possible, the effect of that change. Thus, if we withdraw electrons from the piece of metal the rate of reaction 1 will increase to attempt to offset our action and the dissolution of iron will increase, whereas reaction 2 will decrease. Conversely, if we supply additional electrons from an external source to the piece of metal, reaction 1 will decrease to give reduced corrosion and reaction 2 will increase. The latter case will apply to cathodic protection. Thus, to prevent corrosion we have to continue to supply electrons to the steel from an external source to satisfy the requirements of the cathodic reaction. Note that the anodic and cathodic processes are inseparable. Reducing the rate of the anodic process will allow the rate of the cathodic process to increase. These principles may be expressed in a more quantitative manner by plotting the potential of the metal against the logarithm of the anodic and cathodic reaction rates expressed as current densities. Typical anodic and cathodic curves are illustrated in Fig 2. The corrosion current, Icorr, and the corrosion potential, Ecorr, occur at the point of intersection of the anodic and cathodic curves, i.e. where anodic and cathodic reactions rates are equal. If electrons are “pumped” into the metal to make it more negative the anodic dissolution of iron is decreased to a negligible rate at a potential EI, whereas the rate of the cathodic current is increased to I1. Hence, a current I1 must be supplied from an external source to maintain the potential at E1 where the rate of dissolution of the iron is at a low value. If the potential is reduced to E2 Fig 2 the current required from the external source will increase to I2. Further protection of the metal is insignificant, however, and the larger current supplied from the external source is wasted. The metal is then said to be over-protected. In aerated neutral or alkaline solutions the cathodic corrosion process is usually the reduction of oxygen. The kinetics of this cathodic process are controlled by the rate at which oxygen can diffuse to the surface of the metal, which is slower than the rate of consumption of oxygen by the cathodic reaction. Thus, the rate of this reaction does not increase as the potential of the metal is made more negative but remains constant unless the rate of supply of oxygen to the surface of the metal is increased by, for example, increase fluid flow rate. The influence of flow velocity on cathodic protection parameters is illustrated in Fig 3. A current of I1 is initially required to maintain the metal at the protection potential E1. However, if the w w w . b z f x w . c o m 5 [CATHODIC PROTECTION/BM] flow rate is increased the limiting current for the reduction of oxygen is increased dotted line and the current required to maintain the metal at the protection potential is increased by ∆I. Thus, the current density required to maintain the correct protection potential will vary with service conditions. Clearly, cathodic current density is not a good guide as to whether a structure is cathodically protected. The correct protection potential must be maintained if corrosion is to be prevented. If the structure is over-protected and the potential is reduced to a potential region where reduction of water reaction 3 can take place, further current will be required from the external source and current will be wasted. In Fig 3 reducing the potential from E1 to E2 will increase the current required from the external source from I1 to I2 as a result of an increased rate of reduction of water. Excessive negative potentials can cause accelerated corrosion of lead and aluminium because of the alkaline environments created at the cathode. These alkaline conditions may also be detrimental to certain paint systems, and may cause loss of the paint film. Hydrogen evolution at the cathode surface may, on high-strength steels, result in hydrogen embrittlement of the steel, with subsequent loss of strength. It may also cause disbanding of any insulating coating the coating would then act as an insulating shield to the cathodic- protection currents. 3 S OF APPLYING CATHODIC PROTECTION Cathodic protection may be achieved in either of two ways. By the use of an impressed current from an electrical source, or by the use of sacrificial anodes galvanic action. 3.1 IMPRESSED CURRENT The arrangement for protecting a buried pipeline is illustrated in Fig 4. The buried pipe receives current from a DC power source via an auxiliary inert electrode buried in the ground. The pipe becomes the cathode and the auxiliary electrode the anode. The auxiliary electrode sometimes consists of scrap iron. In this case the iron will dissolve from the anode by reaction 1 and the electrode is described as a consumable anode. If the anode is a noble w w w . b z f x w . c o m 6 [CATHODIC PROTECTION/BM] metal or an electrochemically inert material, the surrounding environment will be oxidised and in water reaction 2H2O O2 4H 4e 5 will occur. In saline solutions, however, chlorine may be produced at the anode. This may present problems in confined spaces. 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, 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 ing resistive barrier oxide layers, etc. 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. In practice, voltages up to 100 V and high current densities are possible on impressed-current anodes see Table 1. Thus, large areas of a structure can be protected from a single anode and, because of the high driving voltage, the anode can be placed remote from the structure. 3.2 SACRIFICIAL ANODES To understand the action of sacrificial anodes for cathodic protection it is necessary to have in mind the galvanic series of metals. The galvanic series for a few selected metals in sea w w w . b z f x w . c o m 7 [CATHODIC PROTECTION/BM] water is shown in Table 2. When the tendency for metal to go into solution as metal ions increases leaving an excess of electrons on the metal surface, i.e. M M e 6 the metal becomes more electronegative. Thus, since zinc, aluminium and magnesium are more electronegative than steel they are increasingly able to supply electrons to the more electropositive steel when in electrical contact in water, and will effect cathodic protection of the steel surface. Clearly, if steel was coupled to copper ins ea water, steel would supply electrons to copper which would become cathodically protected, and the corrosion of the steel would be enhanced. The cathodic protection of a steel pipe with sacrificial anodes is illustrated in Fig 5. Electrons are supplied to the steel pipe, via the electrical connection, and a corresponding amount of anode material goes into solution as metal ions, according to the laws of electrolysis. 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. Some anode properties are shown in Table 3. The driving voltage of sacrificial anodes is now compared with impressed-current anodes, and sacrificial anodes must be located close to the structure being protected. Although almost any piece of zinc etc could provide cathodic protection over a short period of time, cathodic protection schemes are usually required to operate over periods of several years. Anodes can lose their activity and become passivated, developing a non-conducting film on their surfaces so that they no longer are able to supply current. This can be avoided by careful control of the concentrations of trace impurities in the anode materials, and by alloying. For zinc anodes the level of iron, for example, must be kept below 0.005 for satisfactory long-term operation of the anodes. To prevent passivation of aluminium anodes,alloying with, for example, indium has been found to be successful. The previously successful alloy with mercury is now disliked on environmental grounds. w w w . b z f x w . c o m 8 [CATHODIC PROTECTION/BM] 4 DESIGN 4.1 PROTECTION POTENTIALS In practice, the structure-to-electrolyte potentials are measured using a standard reference electrode based on copper/copper sulphate, silver/silver chloride, or pure zinc. The reference electrode should be very close to the surface whose potential is being measured. For steel in an aerobic electrolyte of nearly neutral pH a commonly accepted protection potential is -850 mV; when exposed to sulphate-reducing bacteria a potential of –950 mV would be required. Both values are referred to a copper/copper sulphate electrode. Results of a laboratory determination of the protection potential for steel are shown in Fig 6. Some potential values for protection of other metals are shown in Table 4. Values for lead an