API RP 1632:1996 pdf download.Cathodic Protection of Underground Petroleum Storage Tanks and Piping Systems
1.1 Scope
This recommended practice covers two methods of pro- viding cathodic protection for buried steel petroleum stor- age and dispensing systems. Its intent is to provide information specific to buried steel structures such as motor fuel storage tanks and delivery piping, waste oil tanks, heat- ing-oil tanks, and automobile lifts installed at service sta- tions. Information presented for service stations is not necessarily applicable to buried tanks and piping used for other purposes. This recommended practice is intended to serve only as a general guide to marketers, architects, and engineers interested in cathodic protection of underground petroleum storage and dispensing systems. Specific cathodic protection designs are not provided. Such designs should be developed or adapted by a qualified corrosion engineer or a person thoroughly familiar with cathodic protection practices.
2.1 Introduction
Corrosion may be defined as the deterioration of metal due to a reaction with its environment. External corrosion of buried steel structures is an electrochemical process. For the process to occur, areas with different electrical potentials must exist on the metal surface. These areas must be electri- cally connected and in contact with an electrolyte. There are, therefore, four components in each electrochemical cor- rosion cell: an anode, a cathode, a metallic path connecting the anode and cathode, and an electrolyte (see Figure 1). The role of each component in the corrosion process is as follows: a. At the anode, the base metal goes into solution (corrodes) by releasing electrons and forming positive metal ions. b. No metal loss occurs at the cathode. However, other chemical reactions occur that consume the electrons released at the anode. C. Positive current flows through the metal path connecting the cathode and anode. Electrons generated by the chemical corrosion reactions at the anode are conducted through the metal to the cathode where they are consumed.
2.2.1.3 Physical and chemical properties of the soil (elec- trolyte) may also infuence 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 backfll, or where a tank is buried in two different types of soil (see Figure 3).
API RP 1632:1996 pdf download
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