Detection of Interactions among Localized Pitting Sites on Stainless Steel Using Spatial Statistics

Abstract

Spatial point pattern analysis methods [ A Population Perspective-Statistical Ecology, Kluwer Academic Publishers , Boston (1998) ; Statistics for Spatial Data, Wiley-Interscience Publications , New York (1993) ; Statistical Analysis of Spatial Point Patterns, 2nd ed. , Arnold Publishers , London (2003) ; Spatial Statistics, John Wiley & Sons , New York (1981) ] were used to characterize the spatial patterns of pitting sites on AISI 316 stainless steel. Populations of artificial pitting sites were generated (e.g., clustered, anti-clustered, random, or periodic) to test the ability of the selected spatial statistics methods to characterize these patterns. Experimental pitting patterns on AISI 316 stainless steel analyzed by the same methods indicated that interactions occur between micrometer scale pits over multiple micrometer distances. Spatial statistics results indicated that positive (e.g., clustering of pit sites) occur between pitting sites when they are grown potentiodynamically. Processes associated with the acid/halide pitting mechanism have been shown to promote interactions between pit sites that give rise to such spatial patterns [ J. Electrochem. Soc. , 151 , B233 (2004) ; Paper presented at The Electrochemical Society Meeting, Salt Lake City, UT, Oct 20-25, 2002 ; Electrochem. Solid-State Lett. , 3 , 271 (2000) ; J. Electrochem. Soc. , 149 , B163 (2002) ; Science , 305 , 1133 (2004) ]. Positive interactions were eliminated by stirring, indicating that local acid/halide accumulation was the source of positive interactions.