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Ominous corrosion statistics from a utility survey led CALCET to develop comprehensive guidelines for corrosion control and monitoring
Corrosion problems in fire protection water systems (FPWS) have been the subject of Nuclear Regulatory Commission investigation for some time. In the worst case, FPWS corrosion could, if undetected and untreated, lead to failure of a plant's fire protection system (FPS). The permanent or periodic water exposure of low-carbon-steel piping components' internal surfaces subjects them to inevitable cumulative damage by initiating and sustaining the corrosion process. It is imperative that nuclear facilities, as well as other utilities, have in place a corrosion monitoring and prevention program that specifically addresses FPWS corrosion. Ignoring this widespread problem can be very costly for utilities that must finally repair or replace these systems.
Low-carbon-steel pipes are affected by several different corrosion types: uniform corrosion, which extends over a large portion of the pipe's internal surface; galvanic corrosion, produced at the interface of dissimilar metals and at welded joints; crevice corrosion, localized at sharp surface discontinuities, weld form defects or inorganic and organic buildup deposition; pitting corrosion, characterized by the insidious formation and penetration of pin holes; and microorganism-influenced corrosion, characterized by self-formed crevices as a result of the metabolic activity of certain microorganisms.
Survey says ...
A recent CALCET study surveyed 18 utilities with a variety of water supplies for corrosion in their FPWS. The plants reporting corrosion used water from wells, rivers or domestic sources.
Only one plant with corrosion damage experienced flow blockaes. This particular plant reported corrosion problems severe enough to require significant piping replacement, changing water supplies and chemically treating the new supply. Two other plants developed flow restrictions. One plant's fouling was due to marine growth and the other's was due to silting at the pump pit.
At another plant, preliminary investigations identified corrosion damage in pipes, fittings, tanks, valves and pumps. Investigators attributed the damage to water's corrosiveness and, especially, to the presence of microorganism activity. Engineers conducted coupon tests on system components and determined the corrosion rate was more than three times the acceptable limit. The engineers also determined that observed microorganism-influenced severe pitting was a potential source of through-wall holes even in short-operation service.
Eight surveyed plants provided cathodic protection on...