API RP 1632-2010 pdf free download
API RP 1632-2010 pdf free download.Cathodic Protection of Underground Petroleum Storage Tanks and Piping Systems.
2.2.1.2 Both metal composition and environmental fac- tors may determine which areas on a metal surface become anodes or cathodes. Steel is an inherently nonhomogeneous material, and for a particular environment, potential differ- ences between adjacent areas can result from uneven distri- bution of alloying elements or contaminants within the metal structure. Differences between the weld material and the steel plate can also cause corrosion cells in welded structures. 2.2.1.3 Physical and chemical properties of the soil (elec- trolyte) may also inßuence the location of anodic and cathodic areas on the metal surface. For example, differing oxygen concentrations at different areas on a buried steel structure may generate potential differences. Areas with lower oxygen concentrations become anodic areas, and areas with higher oxygen concentrations become cathodic areas. This may result in more severe corrosion attack at the bottom of a buried tank than at the top of the tank since oxy- gen concentration in soil is primarily dependent on diffusion from the soil surface (see Figure 2). The same mechanism can also contribute to corrosion in areas where clay or debris contact a steel tank buried in a sand backÞll, or where a tank is buried in two different types of soil (see Figure 3). 2.2.1.4 Soil characteristics substantially affect the type and rate of corrosion occurring on buried structures. For example, dissolved salts inßuence the current-carrying capacity of the soil electrolyte and help determine reaction rates at anode and cathode areas. Soil moisture content, pH on buried steel structures. Like galvanic corrosion, these corrosion processes also involve electrochemical reactions. 2.2.2.2 Stray currents are electric currents that travel through the soil electrolyte. The most common and poten- tially the most damaging stray currents are direct currents.
2.3.2 SACRIFICIAL OR GALVANIC ANODES a. Driving potential is limited, and current output is low. b. The method may not be practical for use in soils with 2.3.2.1 SacriÞcial or galvanic anode systems employ a metal anode more negative in the galvanic series than the metal to be protected (see Table 1 for a partial galvanic series). The anode is electrically connected to the structure to be protected and buried in the soil. A galvanic corrosion cell develops, and the active metal anode corrodes (is sacri- very high or very low resistivity. c. The method is not applicable for protection of large bare- steel structures. d. Anode life may be short when protecting large surface areas of bare steel. Þced) while the metal structure cathode is protected. As the 2.3.3 IMPRESSED CURRENT protective current enters the structure, it opposes, over- comes, and prevents the ßow of any corrosion current from the metal structure. The protective current then returns to the sacriÞcial anode through a metallic conduc- tor (see Figure 6). 2.3.2.2 Advantages of sacriÞcial anode cathodic protec- tion systems include the following: a. No external power supply is necessary. b. Installation is relatively easy. c. Costs are low for low-current requirement situations. d. Maintenance costs are minimal after installation. e. Interference problems (stray currents) on structures other than the one being protected are rare. f. SacriÞcial anodes may be attached directly to new coated tanks by tank manufacturers.
2.3.3.1 The second method of applying cathodic protec- tion to a buried metal structure is to use impressed current from an external source. Figure 7 illustrates a typical instal- lation of this type using an AC power supply with a rectiÞer. The DC current from the rectiÞer ßows through the soil to the structure from a buried electrode. Impressed-current anodes are made of relatively inert materials, such as car- bon or graphite, and therefore have a very low rate of corrosion. 2.3.3.2 Advantages of impressed-current cathodic protec- tion systems are as follows: a. Availability of large driving potential. b. High-current output capable of protecting other under- ground steel structures with a low operating cost. c. Possibility of ßexible current output control.
