Chapter Nine

Chapter Nine

Pipeline protection

The most effective of mitigating corrosion on the external surface of a buried or submerged facility utilizes a dual system of a protective coating supplemented by cathodic protection.

Corrosion

Corrosion is the destructive attack on a metal by chemical or electrochemical reaction with its environment. Deterioration by physical causes is not called corrosion, but is described as erosion, galling, or wear and is dealt with later. In some instances, chemical attack accompanies physical deterioration as described by the terms: corrosion-erosion, corrosive wear, or fretting corrosion. Non-metals are not included in this present definition. Plastics may swell or crack, wood may split or decay, granite may erode, and Portland cement may leach away but the term corrosion is presently restricted to chemical attack of metals. Rusting applies to the corrosion of iron or iron-base alloys with the formation of corrosion products consisting largely of hydrous ferric oxides. Non-ferrous metals, therefore, corrode but do not rust.

Metals occur most commonly in nature as oxide or sulphide ores which are in a higher oxidation state than that of the free metal. An extraction of the metal from the ore involves reduction of the oxidized form to free metal involving an increase in internal free energy. Consequently the metal will try to loose its excess energy by becoming oxidized again, through loss of electrons. This oxidizing tendency is the driving force for corrosion, and it is found in virtually all metals except the very noble metals such as gold and platinum.

1 Pipeline Corrosion

Having outlined the mechanism of corrosion in the general sense it is necessary to relate the problem to pipelines specifically. Internal corrosion occurs when the components for corrosion come together and primarily when the pipe is carrying water, aqueous solutions, aqueous suspensions, damp gas, or water in any form. External corrosion is related to the environment in which the pipe exists, in air with many different climatic conditions, in soil with widely varying conditions and in water with different salt contents

Internal corrosion, Apart from exceptional cases of corrosive fluid components such as H2S, is usually a gradual process resulting in a lowering of pipeline efficiency and is characterized by indentations and pigs. Regular line cleaning with scraper pigs, discussed later in chapter 10 of this project, can be used to care for the internal surface of most installations. Internal corrosion can also be controlled by injecting a corrosion inhibitor into the transported fluid. Another method of reducing internal corrosion is by internally lining the pipe while there are few examples of existing pipeline systems using epoxy coatings; they give good protection, long life, and have a low friction factor.

External corrosion, It is a major factor in the design and operation of a pipeline system in that external corrosion can reduce the life of a pipeline and impair its safety. External corrosion is mitigated by application of a pipe coating and the installation of a cathodic protection system. The external coating increases the pipe-soil electrochemical resistance, and the cathodic protection (impressed current or galvanic anodes) system makes the pipe cathodic with respect to the surrounding soil.

2 Corrosion typesCorrosion is an electrochemical phenomenon; that is, it involves chemical reactions and the flow of current. The two most common types are corrosion caused by stray currents from external sources and galvanic-type corrosion caused by different metals in an electrolyte or the same metal in different electrolytes.

1 Galvanic-type corrosion

Description, Galvanic-type corrosion occurs as the result of the tendency of metals to revert to their natural state. If this is to occur, the metals must be so arranged as to form a complete cell, which may be termed a battery or corrosion cell or galvanic cell. Since corrosion may stem from other causes, it is important to note that the type described as galvanic may be recognized from the fact that the cell provides the forces causing corrosion, rather than external currents, etc. The cell is comprised of an anode and cathode immersed in an electrolyte. When the anode and cathode are metallically connected (as when a wire is connected across the terminals of a battery), current flows and corrosion of the anode occurs. When the anode happens to be a metallic part of a structure, piping, or cable system, severe damage may result.

Natural corrosion cells, The environment for many electrical power structures provides conditions favoring formation of natural corrosion cells. The metal or metals of a structure serve as anode, cathode, and the necessary metallic conductor between the two. Water, either as such or as moisture in soil, provides the electrolyte required to complete the cell circuit. Such cells develop their driving force or electrical potential from differing conditions at the interfaces between metal and electrolyte of the anode and cathode. These differences fall into three categories: a. Dissimilar metals comprising the anode and cathode,

b. In homogeneity of a single metal, which causes one area to be anodic to another area, and

c. In homogeneity of the electrolyte. The following are a few of many possible examples in which the essential requirements of a complete cell are satisfied in a structure.

The galvanic series, The differing vigor with which different metals tend to dissolve in electrolytes provides the driving force for galvanic cells and gives rise to the galvanic series. This is a listing of metals in decreasing order of their corrosion when any two of them are the electrodes of a complete cell. That is, the metal higher on the list will be the anode and will be corroded while the lower will be the cathode and will be protected in the cell. (A galvanic series tabulation developed by the International Nickel Company is shown in table 1). This series was developed by actual field and laboratory tests using electrolytes likely to be encountered under operation conditions. It takes into account that certain metals form protective oxides which cause these metals to assume more noble positions in the series than the clean metal would have. This series, then considers practical corrosion aspects as well. However, it cannot anticipate all service conditions and reversals of position which may occur.

Corroded end (anodic or least noble)

Magnesium

Magnesium alloys

Zinc

Aluminum 2S

Cadmium

Aluminum 17ST

Steel or iron. Cast iron

Chromium-iron (active)

Ni-Resist

18-8 Chromium-nickel-iron (active)

18-8-3 Chromium-nickel-molybdenum-iron (active)

Lead-tin solders

Lead

Tin

Nickel (active)

Inconel (active)

Hastelloy C (active)

Brass

Copper

Bronzes

Copper-nickel alloys

Monel

Silver solder

Nickel (passive)

Inconel (passive)

Chromium-iron (passive)

18-8 Chromium-nickel-iron

18-8-3 Chromium-nickel-molybdenum-iron (passive)

Hastelloy C (passive)

Silver

Gold

Platinum

Graphite

Protected end (cathodic or most noble)

Table 1 Galvanized series of metals and alloys

2 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.

EXTERNAL CORROSION PROTECTION

2 Coating

The primary function of an external coating is to establish a permanent barrier between a structure and immediate environment.

The performance of any coating system is directly related to the conditions encountered during the installation and operational life of a facility. Therefore, before any coating selection is initiated, it is imperative that the environmental and construction conditions of a facility are will understood.

Based on an evaluation of the relevant chemical, electrical, environmental, and mechanical conditions experienced by a pipe coating system during its service life, an optimum external pipeline coating for buried service should exhibit the following properties:

Adhesion

Chemical resistance

Electrical resistance

Compatibility with cathodic protection

Flexibility

Soil stressing resistance

Compatible repair and girth weld coating material

Coatings and corrosion cells, Protective coatings are widely used to prevent corrosion, and they serve this function by interposing a mechanical and often electrical barrier between the metal surface being protected and the corrosive environment. As long as the barrier remains intact, corrosion usually will not progress. Viewed from the standpoint of the corrosion cell such as a battery or corroding pipeline, an organic coating acts rather as an envelope insulating an electrode away from the electrolyte, thus ideally removing that electrode from contact and breaking the electrical circuit of the cell. However, coatings may be damaged mechanically during installation, they deteriorate at varying rates with time, and high cathodic or stray currents may destroy their bond and continuity. Further, some coatings offer little or no electrical resistance. In practice, then, the corrosion cell circuit is often restored to some degree with the coating providing some measure of resistance to the flow of current.

Compatibility of protective coatings and cathodic protection, Coatings and cathodic protection complement each other and, where possible, should be used as a combination to achieve the best economy and protection. However, the coating must be compatible with cathodic protection. Certain materials, notably phenolic resin and aluminum pigment, deteriorate rapidly in the alkaline environment which cathodic protection creates where the structure is being protected. Coal-tar enamel and vinyl resins are relatively unaffected. Both high stray current voltages and excessive cathodic protection voltages may "blow off" coatings; that is, cause disbonding and rupture of the coatings. All coatings are susceptible, but high adhesion decreases the vulnerability to this effect. Since the cost of cathodic protection is a function of coating resistance, the better electrical insulator the coating is, the lower the cost. Coal-tar enamel and plastic tape coatings offer the greatest advantage from this standpoint. The preceding information should be considered in evaluating the condition of a coating where a corrosion problem exists. It may be found that providing protection may best be accomplished by restoring the continuity of an existing coating, and the condition of an existing coating will always be a factor in evaluating the desirability of installing cathodic protection. Reclamation's Paint Manual should be referred to for a discussion of the characteristics of various paints and the procedures for their application and maintenance.

3 Cathodic protection

Cathodic protection is the use of an impressed or galvanic current to reduce or prevent corrosion of a metal in an electrolyte by making the metal to be protected by the cathode of a corrosion cell. The source of the protective current is immaterial, and it may be derived from zinc or magnesium anodes or external sources of power, i.e., a rectifier. Whenever corrosion takes place at the surface of steel in contact with an electrolyte, it can be controlled by cathodic protection. It is not always the most economical method since other more corrosion-resistant materials may be applied. However, after careful study of all the factors, cathodic control of corrosion by itself or in conjunction with protective coatings will often prove to be the most efficient means of protecting buried or submerged metals. Cathodic protection is not considered a practical means for protecting the interior surfaces of smaller diameter pipelines. In this bulletin, methods of using cathodic protection by sacrificial anodes for protection of the exterior of buried pipeline installations will be described. Other applications of cathodic protection will be briefly covered, and some reference to adaptability of the systems to other structures will be made. It must be remembered that for each structure, protection is a specific problem and has to be handled as such in cathodic protection installations.

Theory of Cathodic Protection

Referring back to the Daniel cell, the measured potential between the copper and zinc electrodes, without current flow, is 1.1 Volts, If a small current is allowed to flow, the electrodes polarize and the potential difference falls to below one volt. If the electrodes are short circuited, the current rises to a maximum value known as the corrosion current and the potential difference falls to zero, at a value corresponding to the corrosion potential. It follows, therefore, that if the cathode is polarised beyond the corrosion potential to the open circuit potential of the anode by an external current, both electrodes achieve the same potential and no corrosion to the anode can occur.

This now requires to be related to a corroding pipe which contains on its surface, both anodic and cathodic areas. The pipe is connected to an auxiliary anode and a potential is applied between the two. The current enters both cathodic and anodic areas, polarizing the cathodic areas to the open circuit potential of the anodes as before, Therefore, as tong as the external current is maintained, the pipe cannot corrode, If the potential goes beyond that required for protection, protection is maintained but the extra current may have harmful effects on coatings. If the potential is just short of that required, corrosion will occur, but at a greatly reduced rate.

For steel in sea-water, the surface potential required to stop corrosion is around -0.80 volts, with respect to an AgIAgCl reference electrode. In soil, the potential is around -0.82 volts, or -0.91 volts (w.r.t.AgIAgC1) if sulphate reducing bacteria are present.

It has been suggested that part of the protective process, when the pipe surface becomes cathodic as a whole, is due to the deposition of calcium carbonate and magnesium hydroxide in soil or water containing bicarbonate salts. Also any iron lions are precipitated as iron carbonate resulting in a chalky film which would plug any gaps in a protective coating. The deposit, as well as restricting the movement of iron lions, would delay the onset of corrosion should the protective current is cut off.

There are two methods of applying a cathodic protection current:

1. Impressed current

2. Sacrificial anodes

1IMPRESSED CURRENT (RECTIFIER) SYSTEMS

Description, The corrosion situation depicted in figures 1(A and B) may also be solved by cathodic protection using an impressed current or rectifier system. Such a system is physically comparable to the sacrificial anode system in that an anode is installed in the electrolyte (soil or water) and is metallically connected to the corroding structure which is made the cathode. However, rather than rely for protection on the current which results from the anode-cathode couple, an artificial source of current is introduced into the circuit as shown in figure 2. This has several consequences. First, the galvanic potential of the anode is no longer relevant, and almost any electrode material may be used. Scrap iron, abandoned structures, driven steel anodes, etc., among sacrificial materials will suffice; or non sacrificial materials, such as high silicon iron, graphite, or platinum, may be selected as anodes. Second, a more powerful and flexible system can be designed because the artificial current source makes available higher voltages and currents which can be manipulated to advantage. For instance, anodes can be located considerable distances from a pipeline and sufficient current supplied to protect the lines for as much as an 80.5-km (50-mi) length. Also, high enough voltages can be obtained to supply necessary currents for protection n high-resistance soils where sacrificial anodes are ineffective. Third, the power potential in a rectifier system carries with it the danger that at excessive current densities, coatings on the structure may be damaged or destroyed or that accidental reversal of the polarity of the impressed current source may cause highly-accelerated corrosion of the structure instead of protecting it.

General installations procedure, Impressed current systems appropriate for protection of larger structures and are more effective in handling the more complicated corrosion problems than are sacrificial anodes. The correct installation of such systems ordinarily requires a preliminary field survey of the structure and surrounding terrain to obtain soil resistivities and other information and data. A temporary anode ground bed may be installed and a temporary source of direct current such as a welding machine used to supply current to determine current and other system requirements necessary to assure correct distribution. After the permanent system has been designed and installed, follow up measurements should be made to assure that adequate protection has been supplied where required and that no excessive voltages occur. The design and installation of an impressed current system calls for specialized knowledge and considerable experience in this field. The operation and maintenance of a rectifier system is more complex than for a sacrificial anode system, and field personnel will usually require instruction to obtain the best results.

Figure 1 Protection of buried pipe by impressed current rectifier method.

Figure 2 Artificial source of current

Figure 3 Impressed current systems to protect small tank.Advantages of an impressed current

1. Can be designed for wide range of voltage and current

2. High ampere year output available from a single ground bed

3. Large areas can be protected by single installation

4. Variable voltage and current output

5. Applicable in high-resistively environments

6. Effective in protecting uncoated and poor coated structures

Limitations of an impressed current1. Can cause cathodic interference problems with adjacent pipelines

2. Subject to power failure and vandalism

3. Requires periodic inspection and maintenance

4. Requires external power, which involves costs

5. Overprotection can cause coating damage3 Sacrificial anode systems

Theory, Sacrificial anodes, metallically connected to a corroding structure and suitably immersed in the electrolyte (water or moist soil), create a simple galvanic cell in which the structure is the cathode or protected surface. By this device, detrimental corrosion is replaced by localized and controlled corrosion of an expendable anode which can readily be examined and replaced as necessary.

Figure 4 corrosion of iron pipe

Figure 4 shows a typical problem and its solution. In Figure 4A an iron pipe with a break in the mill scale is in moist soil or water. Since the pipe metal is anodic to the mill scale and all elements of a corrosion cell are present, current flows and corrosion (formation of ferrous ion, Fe+ +) progress at the break in the mill scale as in figure 4B; and if left for a sufficient period of time, a pit is likely to develop possibly, resulting in eventual perforation and failure of the pipe. However, as shown in figure 4C installation of a magnesium anode has created a new corrosion cell in which the corrosion (formation of magnesium ion, Mg+ +) is now taking place at the anode. The iron of the pipe (as well as the mill-scale coating) has become the cathode of the new cell and is said to be catholically protected. This type of cathodic protection is easily recognizable as the sacrificial anode type since the cell generates all of the current for protection, there being no external sources involved.

Anode metals, Reference to table 1 show that magnesium heads the list as the most anodic metal and is widely separated from iron in the galvanic series. Magnesium coupled to iron provides sufficient galvanic potential to provide positive protection. An important feature of a sacrificial anode system is that it is inherently a safe system because the normal potentials generated are insufficient to damage coatings present on the surface to be protected. Because of the low potentials generated, sacrificial systems can be used only in low-resistance soils, i.e., with a resistively less than 3000 ohm-centimeters.

Assumptions of protective current requirements and bare metal areas, To obtain a starting point, certain general assumptions have been found helpful.

1. For bare metal in the ground, a current of 11 to 22 mA/m2 (1 to 2 mA/ ft2) of bare metal surface has been found adequate, except under extreme or unusual conditions. This value must then be modified to suit the particular conditions.

2. For coated pipe, the current required is difficult to estimate without field tests. The primary reason is the unknown condition of the protective coat which can vary from nearly 0 to 98 percent coverage. For a fairly new protective coat properly applied, assume 2 percent bare and 22 mA/m2 (2 mA/ft2) for use in tentative calculations. Field test may show that this figure should be modified.

3. Bare pipelines can usually be protected by 11 to 22 mA/m2 (1 to 2 mA/ ft2). This is seldom justifiable economically for extensive or long lines, however, and the necessary protection is usually afforded by the application of cathodic protection to localized areas called "hot spots."

4. Bare steel tanks are treated the same as bare pipelines. Inside steel surfaces in contact with fresh water at zero or low velocities require from 22 to 65 mA/m2 (2 to 6 mA/ ft2), depending on the nature of the water. The low value is used for water which is scale forming. That is, the water will form a calcareous coating on the surface of the metal.

5. Protecting steel surfaces in contact with water in motion presents another problem. Water in motion produces a scouring effect which prevents the formation of the above-mentioned coating and even the formation of a hydrogen film. Therefore, surfaces exposed to water in motion require a higher current density. The amount required is hard to predict. In this case, an experimental determination of the current requirement should be made.

Anode spacing, After determining the number of anodes required in an installation, the success or failure of the system is dependent primarily upon the proper location and installation of the galvanic anodes. Figure 5 show locating galvanic anodes along a comparatively short bare pipeline in homogeneous soil is quite simple as the required number of anodes can be equally spaced along the pipeline and connected to the line with insulated wires. The same thing is true of a long-coated pipeline under identical conditions. However, the problem is not usually that simple as the proper spacing along a continuous structure depends upon the varying physical condition of the structure surface and the surrounding soil. For example, an evenly-coated pipeline located in soil that changes from mildly corrosive (5,000 to 10,000 ohm-centimeters) to very corrosive (1,000 ohm-centimeters or less), the spacing of the anodes along the line would vary. Closer spacing of anodes along the pipeline would be required for the part of the pipe in the very corrosive soil to afford the same protection being received by the pipeline section in the mildly corrosive soil with greater distances between the anodes. The same closer spacing of anodes is required if the soil conditions are found to be constant, but it is known that the condition of the protective coating varies. A closer grouping of the anodes is required where the protective coating is inferior. In the case of a bare pipe of considerable length, it is usually not economical to protect the entire pipeline. Figure 5 Distribution of anodesHowever, a bare pipeline located in the above-mentioned soil condition can usually be protected economically by protecting with sacrificial anodes the pipe sections in the very corrosive soils. This type of protection is referred to as "hot spot" protection and is economically justifiable in that the useful life of the entire pipeline has been extended by the cathodic protection of the severely corroding areas. Sacrificial anodes should be placed around the structure symmetrically to provide good current distribution and to increase anode efficiency. Some structures, however, are very irregular and care must be taken to distribute the anodes to provide adequate protection to as much of the metal work as possible. Installation of the anodes should be made at a distance of 3 to 10 m (10 to 30 feet) from the structure.

Anode Installation, Anodes should be buried a minimum of 0.6 m (2 ft) into a low-resistivety material. This may necessitate deep holes to reach moist soil. Clays are common, low-resistivety materials. In order to assure minimum electrical resistance between the anode and ground, a chemical backfill is used. Anodes are sometimes supplied pre-packed, with the chemical backfill in a cloth bag around the magnesium. If pre-packed anodes, which are preferred, are used, no additional chemical backfill is required. If bare anodes are used, chemical backfill should be tamped around the bare anodes as shown in figure 6. The anode and backfill shall be placed in a water-filled hole and tamped. The anode leads should be buried a minimum of 0.5 m (18 in) and the free end should be attached to the structure by thermosetting resin, welding, or brazing. If resin is used, care must be taken to ensure a metal-to-metal contact. This connection should be protected by a suitable protective coating. Another lead should be connected to the structure in a similar manner and the other end brought to the surface to terminate in a test structure as shown in figure 7. This lead should have at least 0.3 m (1 ft) of slack to facilitate testing. This lead may be used by a corrosion engineer to determine (by a pipe-to-soil potential test) whether the cathodic protection system is working properly and whether the anodes have been consumed. The anode-to-structure lead should be constructed of No. 10 or 8 AWG type TW copper wire. Splice connections should be made using a split, bolt-type electrical connector of the proper size. The connection should be wrapped with three layer of plastic electrician's tape, followed by three layers of self-vulcanizing, rubber insulating tape and the joint encased in a suitable electrical waterproofing compound. In low resistivity soils, a resistor is often required in the anode lead to reduce the current supplied to the structure to the amount necessary to maintain the proper protection.

Figure 6 Installation of galvanic anode

Figure 7 Two types of test load insulations

Figure 8 Sacrificial anode systems

Advantages of galvanic systems

1. No external power required2. Low maintenance3. Uniform distribution4. Easy to install Limitation of galvanic systems1. Limited driving potential

2. Low / limited current output / unit

3. Can be ineffective in a high-resistivity environmentIsolation and sectionalization

Purposes, Two purposes can be served by the use of insulated joints in cathodic protection, that of isolation and that of sectionalization. Isolation is the application of insulated joints to prevent a galvanic cell from being formed with a portion of the structure being the sacrificial metal or to insulate a structure being protected from others that would add an excessive drain on a cathodic protection system. Sectionalization is the dividing of a group of structures into smaller units for cathodic protection because of different current requirements or merely to simplify a system. Although both purposes are mentioned here, the most common application in power plant work would be that of isolation.

Examples using insulating joints, A typical example of an isolation application is shown in figure 9. Figures 9(A and B) show an installation subject to severe corrosion, depending, of course, on the soil characteristics. In figure 9A the direct connecting of the dissimilar metals provides an ideal path for the flow of current from the iron to the copper through the ground (electrolyte). The iron in this case is anodic, or the sacrificial metal. The same conditions are provided in figure 9B as the copper ground mat is connected directly to the steel pipe, and both are in a common electrolyte, the ground. The flow of current in this case is from the steel pipe to the copper ground mat. The steel pipe is anodic, or the sacrificial metal. The condition depicted in figures 9(C and D) is identical to that shown in figures 9(A and B) with the exception that the wire connection in figure 9D has been isolated from the copper ground mat by the installation of an insulated joint. Under this condition, the only corrosion action is local, caused by in-homogeneity in the metal or contacting solution. Therefore, in figure 9C, the condition will still permit the flow of current from an anodic section of the steel plate to a cathodic section of the steel plate to a cathodic section of the same plate. The same is true in figure 9D One section of the pipe can be anodic with respect to another section of the pipe and, therefore, deteriorate at that point. The examples in figures 9(E and F) are identical to the examples in figures 9(C and D) except that a sacrificial anode of magnesium has been added. Since the magnesium anode is the least noble of the three metals in the ground (electrolyte), it becomes the sacrificial metal and deteriorates from the corrosive action in lieu of the iron plate and steel pipe.

Galvanic-type corrosion

Corrosion types

Stray current corrosion

Protection methods

Application of protective coatings

Install cathodic protection by impressed currents

Install cathodic protection by sacrificial anodes