The economics favoring cathodic protection of cross country pipelines are so overwhelming, particularly on high pressure gas and oil lines, that practically every new line of consequence is provided with cathodic protection almost immediately after completion. The Department of Trans-portation has passed Federal legislation requiring that all oil, gas and gas products pipelines be cathodically protected and that the level of protection meets designated standards and regulations.
New structures are generally provided with a good, high resistivity coating that is applied with techniques that leave almost negligible amounts of the surface exposed to the soil. However, it is recognized that a coating, no matter how good or how well applied, is never perfect.
The corrosion protection afforded by the coating must be supplemented with cathodic protection in order to achieve complete mitigation of corrosion. It is important to understand that coated structures develop leaks within a shorter period of time than do uncoated structures. This is true even though the total metal loss on a coated structure is appreciably less than on a bare structure. All of the corrosion activity is concentrated at the holidays or breaks in the coating rather than evenly dispersed over the entire surface, thus accelerating the corrosion rate at the holiday locations.
Fortunately for the structure owner, coating and cathodic protection work very well together. When a tank or pipe is coated with one of the high quality materials and closely controlled application techniques that are available today, a relatively small magnitude of current can provide complete cathodic protection for tanks and their associated piping.
Although protection of cross country pipelines and existing rural tank farms is usually provided with the rectifier type systems, the use of such systems in congested areas is often very difficult because of the many interference problems created on nearby structures. Therefore, in congested areas, sacrificial anode type systems are more often used.
One example was of a well coated 10,000 gallon underground storage tank located in Detroit, Michigan. It was amply protected with one anode installed on one end of the tank with a total current output of less than 10 milli-amperes of current. The fact that sacrificial anodes have been installed on over 200,000 well-coated underground storage tanks without a single corrosion related product discharge is a testament to the effectiveness of this approach.
In many instances, spacing of anodes can be extended to 100 - 500 feet or more on small diameter buried piping depending on the quality of the coating and environmental conditions. As a consequence, many companies in recent years have established programs in which magnesium anodes are installed on pre-selected spacings as the well-coated piping is laid.
Source:www.bushman.cc/pdf/corrosion_theory.pdf
New structures are generally provided with a good, high resistivity coating that is applied with techniques that leave almost negligible amounts of the surface exposed to the soil. However, it is recognized that a coating, no matter how good or how well applied, is never perfect.
The corrosion protection afforded by the coating must be supplemented with cathodic protection in order to achieve complete mitigation of corrosion. It is important to understand that coated structures develop leaks within a shorter period of time than do uncoated structures. This is true even though the total metal loss on a coated structure is appreciably less than on a bare structure. All of the corrosion activity is concentrated at the holidays or breaks in the coating rather than evenly dispersed over the entire surface, thus accelerating the corrosion rate at the holiday locations.
Fortunately for the structure owner, coating and cathodic protection work very well together. When a tank or pipe is coated with one of the high quality materials and closely controlled application techniques that are available today, a relatively small magnitude of current can provide complete cathodic protection for tanks and their associated piping.
Although protection of cross country pipelines and existing rural tank farms is usually provided with the rectifier type systems, the use of such systems in congested areas is often very difficult because of the many interference problems created on nearby structures. Therefore, in congested areas, sacrificial anode type systems are more often used.
One example was of a well coated 10,000 gallon underground storage tank located in Detroit, Michigan. It was amply protected with one anode installed on one end of the tank with a total current output of less than 10 milli-amperes of current. The fact that sacrificial anodes have been installed on over 200,000 well-coated underground storage tanks without a single corrosion related product discharge is a testament to the effectiveness of this approach.
In many instances, spacing of anodes can be extended to 100 - 500 feet or more on small diameter buried piping depending on the quality of the coating and environmental conditions. As a consequence, many companies in recent years have established programs in which magnesium anodes are installed on pre-selected spacings as the well-coated piping is laid.
Source:www.bushman.cc/pdf/corrosion_theory.pdf
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