Degradation found in carbon steel Service Water piping is typically characterized as localized corrosion or “pitting.” Localized corrosion or pitting is identified as small sections of thinning that deviate from the average wall thickness. In its most basic sense, a pit is simply a geometric entity that is abrupt and distinct that can range in size, extent, and density throughout the system. This raises the question of how one defines a pit and what geometric characteristics can be used to create this definition.

Research has been conducted to help provide answers to this question through detailed study of wall thickness examination results on operating service water piping in a nuclear power plant. This analysis has determined that the primary geometric characteristics are the depth-to-extent ratio, the extents of thinning, the shape of the pit, and the distance between adjacent pits. Statistical distributions of these characteristics for the evaluated data sets were developed to understand the overall trends of the pitting. With further research, the results of this evaluation can be used to extrapolate these trends of pitting characteristics, with high statistical confidence, to sections of service water piping that have little to no detailed non-destructive examination data. The objective of this paper is to present the preliminary findings of the statistical study on “pitting” and provide a set of geometric characteristics that best define a pit.

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