Critical Pitting Temperature of Stainless Steel: Deterministic Vs. Probabilistic Behavior

Abstract

The concept of the critical pitting temperature (CPT), which is described as the temperature below which stable pits are not formed regardless of applied potential and exposure time, has received considerable attention in the literature. Over the past few decades, many attempts have been made to assess the CPT of stainless steel as a function of alloy composition, alloy microstructure, bulk solution composition, and surface roughness. In contrast to the pitting potential, which is known to be probabilistic in nature, CPT is believed by most to be a deterministic phenomenon that can be measured within a few degrees Celsius [1]. This notion has been validated by many authors who have observed a narrow range of CPT for several alloy/environment combinations [2,3]. Some authors have recently challenged this idea by reporting scattered CPT values [4]. This work seeks to determine whether the CPT falls within a narrow or wide temperature range (i.e., is CPT a deterministic or probabilistic phenomenon?).To answer this question, potentiostatic CPT measurements were performed using 1 cm² 904L stainless steel electrodes and an applied potential of 750 mV (Ag/AgCl). Temperature was increased at a rate of 1 °C/min, and the CPT was determined using the temperature at which current density rapidly increased. The CPT of 904L stainless steel was measured in four different conditions to evaluate the effect of bulk Cl⁻ concentration (0.05 and 1.0 M NaCl), passivation in 20 vol.% HNO3, and the presence of a corrosion inhibitor (0.02 M NaNO3).Figure 1 shows a cumulative graph of the CPT values of 904L obtained using the four different conditions. In this graph, n is the number of the n th pitted sample and N is the total number of experiments (i.e., 10). For all test conditions, the CPT was measured within ± 1.8 °C, which is in agreement with the reproducible CPT values reported by others [3]. Figure 1 shows that the CPT of 904L is independent of passivation in HNO₃. Further, decreasing bulk solution aggressiveness from 1.0 M NaCl to 0.05 M NaCl does not result in scattered CPT values. However, some authors have reported a considerably broader range (i.e., ± 10 °C) for the CPT of stainless steel (AISI 316L and 2205), as discussed previously. One possible explanation for their larger reported CPT range is the occurrence of crevice corrosion at the interface of the alloy/inert support (e.g., epoxy mount), which results in an inaccurate CPT measurement. To evaluate this hypothesis, the current density vs. temperature curves for two conditions were compared (see Figure 2). The blue and red curves show potentiostatic polarization results for pure pitting corrosion (i.e., crevice-free CPT measurement) and pitting + crevice corrosion, respectively. The gradual increase of the current density with temperature for the red curve suggests the presence of a crevice at the alloy/epoxy interface, which was confirmed using SEM to characterize the electrode post-polarization. On the other hand, when no crevice corrosion occurs, the current density shows a sudden increase at the CPT.The results of this study reveal that the CPT of 904L can be measured to within ± 1.8 °C, regardless of bulk solution aggressiveness (e.g., 1.0 M NaCl and 0.05 M NaCl solution) or passive state before polarization. This finding is in contrast to what some authors have suggested recently. The scattered CPT values reported in the literature are most likely due to contributions from both pitting + crevice corrosion rather than pitting corrosion alone. Acknowledgments Financial support provided by Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged.