To elucidate the pit initiation behavior of sensitized stainless steels, the anodic polarization of a single grain boundary was examined in 0.1 M NaCl (pH 5.4) at 298 K using a micro-electrochemical system. For Type 304 heat-treated at 923 K for 2 h, no pitting was initiated on a small area (ca. 100 μm × 100 μm) with a sensitized grain boundary without MnS inclusions. However, stable pitting was observed on the electrode area that was larger than ca. 200 μm × 200 μm. In situ microscopy revealed that the first step in corrosion was a spherical pit generated at a MnS inclusion at a sensitized grain boundary, and that intergranular corrosion started at the pit. The local depassivation of the Cr-depleted zone along the sensitized grain boundary was thought to be introduced by the dissolution of the inclusion. The co-existence of the MnS inclusion and the Cr-depleted zone was considered to be the critical factor in the pit initiation of sensitized stainless steels in NaCl solutions. Stainless steels are widely used in chloride environments because of their high corrosion resistance, which is attributed to their high Cr content, at above 12 mass%.1 When stainless steels are heated to around 900 K (e.g., welding), sensitization occurs due to the formation of Cr-depleted zones at the grain boundaries. The susceptibility to intergranular corrosion and pitting at the grain boundaries increases as a result of sensitization-treatments. 2To assess the degree of sensitization and/or the mechanisms of intergranular corrosion, acidic solutions are usually used. ASTM A262 testing specification contains the following intergranular corrosion tests: 1) Huey test (boiling HNO 3 ), 2) Strauss test (boiling H 2 SO 4 + CuSO 4 + metallic Cu), 3) and Streicher test (boiling H 2 SO 4 + Fe 2 (SO 4 ) 3 ). After immersion, the specimens are visually examined and/or measured for weight loss. These methods have been widely and successfully used in corrosion engineering. ISO 12732 specifies a method that uses the double loop electrochemical potentiodynamic reactivation test (based onĈihal's method 3 ). In this method, the standard solution is 0.5 M H 2 SO 4 -0.01 M KSCN, and the degree of sensitization can be evaluated quantitatively from the comparison of the peak current densities between the anodic scan and the subsequent cathodic scan.Although acidic solutions are generally used to evaluate the degree of sensitization, in practical applications, pitting and stress corrosion cracking occur in stainless steels in near-neutral pH environments. NaCl solutions are used to evaluate the pitting corrosion resistance and the susceptibility of sensitized stainless steels to stress corrosion cracking (SCC). Cheng et al. studied the pitting corrosion of sensitized Type 304 stainless steel under wet-dry cycling and demonstrated that both the probability of pitting and the average size of the pit increased as a result of sensitization. 4 It was suggested that the observed change in the pit morphology from round to an irregular shape could be attributed t...
Cyatheaceae (tree ferns) appeared during the Jurassic period and some of the species still remain. Those species may have some morphological and/or physiological characteristics for survival. A tree fern was observed to suppress the growth of other ligneous plants in a tropical forest. It was assumed that the fern may release toxic substances into the forest floor, but those toxic substances have not yet been identified. Therefore, we investigated the phytotoxicity and phytotoxic substances of Cyathea lepifera (J. Sm. ex Hook.) Copel. An aqueous methanol extract of C. lepifera fronds inhibited the growth of roots and shoots of dicotyledonous garden cress (Lepidum sativum L.), lettuce (Lactuca sativa L.), and alfalfa (Medicago sativa L.), and monocotyledonous ryegrass (Lolium multiflorum Lam.), timothy (Phleum pratense L.), and barnyardgrass (Echinochloa crus-galli (L.) P. Beauv.). The results suggest that C. lepifera fronds may have phytotoxicity and contain some phytotoxic substances. The extract was purified through several chromatographic steps during which inhibitory activity was monitored, and p-coumaric acid and (-)-3-hydroxy-β-ionone were isolated. Those compounds showed phytotoxic activity and may contribute to the phytotoxic effects caused by the C. lepifera fronds. The fronds fall and accumulate on the forest floor through defoliation, and the compounds may be released into the forest soils through the decomposition process of the fronds. The phytotoxic activities of the compounds may be partly responsible for the fern's survival.
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Pitting corrosion on stainless steel is one of the most serious problems caused by seawater intrusion into power plants because it possibly perforates boundary structures, such as piping or containers, and leads to solution leakage. Although the evaluation of the pit growth rate in accordance with the corrosion condition is required to secure the plant’s safety, the dependence of the pit growth rate on various corrosion conditions is not fully understood. In this study, the effects of applied potential and chloride concentration on the pit growth rate on type 304L stainless steel were assessed with and without fluid flow by performing the pit penetration test with in situ observations of pit initiation and penetration. An artificial one-dimensional pit experiment was performed to obtain fundamental understanding of pit growth with and without fluid flow. In addition, three-dimensional pit growth behavior was further analyzed by measuring dissolution current of a single three-dimensional pit. In the pit penetration test, the effects of applied potential and chloride concentration on the pit growth rate differed depending on the fluid flow condition; the pit growth rate is almost independent of the applied potential and chloride concentration without fluid flow, whereas the growth rate was decreased by decreasing the potential and chloride concentration with fluid flow. For the results of the artificial one-dimensional pit experiment, regardless of the fluid flow condition, stable pit growth in the depth direction was suggested to be diffusion-controlled. Measuring the time variation of the dissolution current of a single three-dimensional pit indicated that the three-dimensional pit growth involves temporary passivation and reactivation, which depend on the fluid flow condition and applied potential. This suggested pit growth in the pit penetration test proceeded discontinuously depending on the corrosion conditions, resulting in a dependence of the pit growth rate on the corrosion conditions.
When austenitic stainless steels are heated at around 900 K, sensitization occurs and Cr-depleted areas are formed at grain boundaries. As a result of sensitization, the localized corrosion resistance of the stainless steels decreases1. The mechanism of intergranular corrosion of sensitized stainless steels was well understood with the results of immersion tests in acidic solutions, but the mechanism of pitting corrosion and the pit initiation sites for sensitized stainless steels in near-neutral solutions have remained unclear. Micro-electrochemical measurements are promising techniques for investigating the pit initiation process. Chiba et al. developed a micro-electrochemical system for in situ high-resolution optical microscopy2. In this study, we applied this technique to investigate the pit initiation of sensitized stainless steel. In addition to this, the electrochemical properties of single grain boundary were measured. A commercial 18Cr-8Ni stainless steel sheet (0.06 %C, 0.39 %Si, 1.1 %Mn, 0.003 %S, 8.0 %Ni, 18.0 %Cr, 0.13 %Mo, 0.22 %Cu, 0.002 %Al, 0.04 %N, and 0.003 % O) was used as specimens. The steel was solution-treated at 1373 K for 0.5 h and was quenched in water. As a sensitization-treatment, the steel was heat-treated at 923 K for 2 h and then quenched in water. After that, the steel surface was polished down to 1 μm by a diamond paste. Potentiodynamic anodic polarization was carried out in naturally aerated 0.1 M NaCl at 298 K. The electrode areas were ca. 1 cm × 1 cm (macro-scale) and 100 μm × 100 μm (micro-scale). The solution was naturally aerated 0.1 M NaCl. All potential were measured against a Ag/AgCl(3.33 M KCl) electrode. In anodic polarization, electrode potential was scanned at 23 mV s-1. The results of macro-scale anodic polarization for the solution-treated and sensitized specimens are shown in Fig. 1. For the sensitized specimen, pitting occurred at around 0.4 V, and the morphology of the pit was different from that formed on the solution- treated specimen. Grain boundary corrosion and pitting were generated. Micro-scale polarization (electrode area: ca.100 μm × 100 μm) was conducted to elucidate electrochemical properties of individual grain-boundaries of the sensitized stainless steels. Figure 2 shows the preparation procedure for the electrode area with a single grain-boundary. To know the location of grain boundaries, electrolytic etching in oxalic acid was performed for 10 s, and the indentations with a Vickers hardness tester were formed on the both sides of the selected grain boundary segment. To prevent the overlapping of grain boundary and masking, both sides of the grain boundary were solution-treated using a micro spot TIG welder (Fig. 2a). Then specimen was mirror polished again and masked (Fig. 2b). The anodic polarization behavior of a single grain-boundary in the sensitized stainless steel is shown in Fig. 3. Figure 3b is the composite image of the as-polished electrode area and the etched surface. This figure indicates that the electrode area contained single grain-boundary. Figures 3c and 3d are the optical micrographs of the electrode area before and after polarization. These results indicate no pit was initiated at the grain boundary, and this means that not all grain boundaries act as a pit initiation site even in the case of the sensitized stainless steels. References; U. Kamachi Mudali, R. K. Dayal, J. B. Gnanamoorthy, and P. Rodriguez, ISIJ Int., 36, 799-806 (1996) Aya Chiba, Izumi Muto, Yu Sugawara, and Nobuyoshi Hara, J. Electrochem. Soc., 159, C341 (2012) Figure 1
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