Codes and Standards

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I. INTRODUCTION

Since Volume 1 of the SSPC Manual was first published, general knowledge of cathodic protection and its practical application have increased greatly. Its importance can be seen in the federal regulation that all new construction for hazardous pipelines must include the use of effective coatings and cathodic protection.

Because a comprehensive discussion of cathodic pro-tectton would be voluminous, this chapter presents only a general introductory account. Other sources provide more specific treatment^1.2.

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II. HOW CATHODIC PROTECTION COMPLEMENTS A COATING

Coatings used on pipelines and other underground structures frequently need to protect very large areas of underground metal, especially with major cross-country pipelines. For instance, only 10 miles (16.09 km) of 48" (1219.2 mm) diameter steel pipeline has an exterior surface area of approximately 15.2 acres (6.2 hectares).

Such large areas, when coated and buried, cannot remain permanently free of all pinholes, developed defects, or outside damage. Even though the coating may, initially, be free of holes in the film, pipe movement with temperature variations, soil stresses, and damage from outside sources (such as excavation work on other projects) will ultimately expose bare metal to the corrosive effect of the surrounding environment (soil or water). The amount of metal exposed will be determined by the quality of the coating used and the severity of the hazards working to damage the coating.

Even though 99.9999% of the surface area might remain fully and effectively protected by the coating, the remaining 0.0001% could be a problem. On the 10 miles of 48" pipe mentioned earlier, this represents some 0.6635 square feet of exposed metal. Serious damage could occur on this much exposed surface if corrosive effects are not controlled. To put the 0.6635 square feet in better perspective, it represents in the order of 50 one-half-inch diameter holes in the coating for each mite of the 48" diameter pipe. A tot can happen at these locations.

Although the coating-cathodic-protection combination is used widely on pipeline exterior surfaces, this working team can also be used on the interior surfaces of pipelines carrying electrically conductive materials such as water or other conductive solutions. Although there may be unique design considerations when planning a cathodic protection system for pipeline interiors, such a system can work very effectively.

Cathodic protection current must be able to reach all of the pipeline's exposed metal. If, for example, a coating material is not bonded to the pipe surface, it may permit water to reach the metallic pipe surface under the disbond-ed coating around coating holidays.

Where the disbonded coating has a high electrical resistance, cathodic protection current cannot flow through it to reach the pipe surface, and active corrosion may result.

While cathodic protection current can enter the space between the disbonded coating and pipe surface, the thin water film there may prevent the protection current from penetrating in sufficient quantity for adequate protection.

If the space between a disbonded coating and pipe surface remains dry, there will normally not be a corrosion problem. If water does enter the space, however, an electrically insulating coating acts as an "electrical shield" preventing the effective cathodic protection of the pipe at that location. Electrical potential measurements may indicate that the pipe is cathodically protected in accord with an accepted criterion at the disbonded area, whereas it may, in fact, be corroding.

The pipeline operator will normally find that a top grade coating will give the best practicable corrosion control for his metallic pipe when it is complemented by a cathodic protection system which has been properly designed, installed and maintained.

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III. RELATION BETWEEN COATING CHARACTERISTICS AND CATHODIC PROTECTION

The characteristics of a coatings system determine the requirements for cathodic protection.

If a coating on a buried or submerged pipeline forms a high electrical resistance barrier between the pipe and surrounding earth or water, the electric current needed to provide cathodic protection will be less than with a coating barrier having a lower effective electrical resistance, and lower current requirements for cathodic protection mean a lower investment for the cathodic protection system.

The designer and operator of the cathodic protection system for a coated pipeline are concerned with three major coating characteristics:

1) Effective coating resistance;

2) Bond between coating and protected structure as discussed in the preceding section;

3) Coating stability.

Of these three, "coating stability" is perhaps the most important. A stable coating has effective resistance and bond for a long period of time.

A coating with a high effective resistance will be chosen for pipeline application. This effective resistance per average square foot depends upon the basic resistivity of the coating material itself, the coating thickness, number and size of holidays in the coatings, deleterious effect of the environment on the coating, the resistivity of the conducting environment in which the pipeline is buried or submerged, and the bond between pipe surface and coating. If effective resistance is unstable, the electric current needed for cathodic protection may double every few years, causing increased costs for installation of new cathodic protection facilities, maintenance, and energy.

Resistance almost always declines as additional coating defects are generated through environmental effects. While the cathodic protection engineers are able to measure the effective resistance of a coating on a pipeline, this measurement can be misleading if the pipe has been installed in dry earth and not given time enough for the backfill to settle and for moisture to permeate all existing coating pinholes and holidays. Measurements made under these conditions will normally indicate a higher effective resistance (possibly much higher) than what actually exists. Thus, experience is required to judge the validity of coating resistance measurements and 10 use them for calculating the design of cathodic protection systems.

There is a great difference between the resistance of a perfect pipeline coating and one with even just a very few small pinholes. To illustrate: the resistance across a 3/32" (2.38 mm) thick completely pinhole-free coal tar enamel coating having a volume resistivity of 10¹³ ohm-centimeters on a ten-mile (16.09 km) length of 36" (914.4 mm) diameter pipeline in a 1000 ohm-cm environment would be approximately 5148.2 ohms. This is equivalent to an effective coating resistance of 2.56 x 10⁹ ohms per average square foot. However, if there were just one 1/16" (1.59 mm) diameter pinhole filled with the 1000 ohm-cm environment, the resistance across the 3/32" (2.38 mm) length of the pinhole would be 12,026 ohms. This resistance in parallel with the 5148.2-ohm coating resistance would be approximately 3605 ohms, which is equivalent to an apparent coating resistance on the ten-mile (16.09 kin) section of 1.79 x 10⁹ ohms per average square foot -- a 30% reduction from the "perfect coating" condition.

Under practical conditions, the chances are that there would be many more than one pinhole in a ten-mile pipeline section. Assuming there were fifty pinholes of the size stated in the example, the parallel resistance across the fifty pinholes would be 240.52 ohms. This figure, in turn, in parallel with the 5148,2-ohm coating resistance would give a net resistance across the ten mile (16.09 kin) length of 229.8 ohms which is equivalent to an apparent coating resistance on the ten-mile (16.09 kin) section of 114.3 x 106 ohms per average square foot. Although this is still a high resistance figure (and indicates a very low electrical energy requirement for cathodic protection), it nevertheless represents a 95.5 reduction from the "perfect coating" condition.

Coating bond also effects resistance particularly adjacent to any pinholes or holidays in the in-place coating. Any lifting of the coating because of disbondment at such openings in the coating increases the amount of pipe metal exposed to the surrounding environment, reduces the el; fective coating resistance, increases the electrical current requirements for cathodic protection and introduces the possibility of under-film corrosion that is electrically shielded from the beneficial effects of cathodic protection.

It doesn't take much disbondment to double the area of exposed metal at the usual small coating defect. For example, doubling the exposed metal area at the base of a ¼" (6.35 mm) diameter hole in a pipe coating involves a disbondment lifting of only approximately 52 mils (1.32 mm) from the edges of the hole. Disbonding effects can be far worse than this.

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IV. BASIC THEORY OF CATHODIC PROTECTION

To understand how cathodic protection works, it is necessary to understand corrosion. On a pipeline, corrosion causes a flow of direct current between the elements of a corrosion cell on the pipeline or between the pipeline and some external entity which may be affecting the pipeline. Fig. I illustrates this. As shown by the figure, areas where corrosion is occurring are called "anodic", which means that they are discharging corrosion current to a conductive electrolytic environment, earth or water. When direct current leaves the surface of pipe metal to enter such an electrolytic environment, it takes the metal with it, and the pipe suffers corrosive deterioration.

On the other hand, when direct current flows from a surrounding electrolytic environment onto the pipe surface at cathodic areas, there is no corrosive damage.

If there were a way to convert all anodic areas on a buried or submerged pipe surface to corrosion-free cathodic areas, corrosive damage would be eliminated. This is exactly what is accomplished with a properly designed, installed and maintained cathodic protection system. Basically, this is done by using some external source of direct current to neutralize and counteract the natural corrosion currents discharging from anodic areas.

Figure 2 illustrates the concept of cathodic protection. The figure shows a ground connection established separate from the pipeline. A source of direct current metallically connected between the pipe and ground connection forces the ground connection to discharge current. The system is designed to regulate the amount of current discharged (cathodic protection current) so that it eliminates the flow of corrosion cell current from anodic areas by converting them to cathodic areas. A net flow is established from the conductive environment onto the previously anodic areas.

Because the cathodic protection system ground connection (also known as "ground bed" or "anode bed") is discharging current to do its job, it is subject to corrosion.

Thus, a cathodic protection system, although it renders a protected structure surface free of corrosion, does not eliminate corrosion -- it transfers the corrosive effect from critical operating structures such as pipelines to known locations (the ground connections) where replacements may be made periodically (10 to 15 years or more) without making it necessary to take the protected Operating pipeline out of service.

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