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66 NOVEMBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 11 centrations of SO 4 2– , the crevice width became noticeably greater because SO 4 2– has a high degree of mobility and does not gather in the crevice, which promoted increased crevice width rather than con- tribute to increased crevice depth. Two-Ion Solutions In the two-ion solutions (Solutions IV, V, and VI), crevice corrosion did not occur in all concentrations of mi xed solutions. When the mixed solutions of Cl – /SO 4 2– were 0.034 mol/L Cl – /0.012 mol/L SO 4 2– and 0.034 mol/L Cl – /0.36 mol/L SO 4 2– , the copper sur- face showed obvious crevice corrosion FIGURE 1 Corrosion rates of specimens exposed to Solutions I, II, and III. Values are defined in Table 1. FIGURE 2 Corrosion rates of copper in solutions with (a) two ions (Solutions IV, V, and VI); and (b) three ions (Solution VII). Values are defined in Table 1. attack, and the average depth of the crevice corrosion and the deepest depth were smaller in the former than the latter. In addition, when the concentrations were 1.02 mol/L Cl – /0.012 mol/L SO 4 2– and 1.02 mol/L Cl – /0.36 mol/L SO 4 2– , a step could be observed along the crevice mouth. When immersed in the 0.34 mol/L and 1.02 mol/L Cl – solutions, the corrosion type was mainly concentrated outside the crevice. When the concentrations of the mixed solution of Cl – and CO 3 2– were 0.034 mol/L Cl – /0.023 mol/L C O 3 2– and 1.02 mol/L Cl – /0.69 mol/L CO 3 2– , copper crevice corro- sion occurred. The average depth of the crevice corrosion and the deepest depth were smaller in the former than the latter. When the combined solution was 0.034 mol/L Cl – /0.69 mol/L CO 3 2– , no trace of cor- rosion was observed at the edge of the crev- ice. In addition, when the combined solu- tion was 1.02 mol/L Cl – /0.023 mol/L CO 3 2– , a step formed between the interior and the exterior of the crevice. No crevice corrosion was observed on the copper surfaces in the different solu- tions of SO 4 2– /CO 3 2– . Three-Ion Solutions In the three-ion solutions, when the ratio of Cl – , SO 4 2– , and CO 3 2– were 0.034 mol/L Cl – /0.012 mol/L SO 4 2– /0.023 mol/L CO 3 2- and 1.02 mol/L Cl – /0.012 mol/L SO 4 2– / 0.69 mol/L CO 3 2– , there were obvious crev- ice corrosion attacks on the surface of the copper. The average depth of the crevice corrosion and th e deep est depth were smaller in the former than the latter. When th e c o n c e n tra ti o n s w e re 0 . 0 3 4 m o l / L Cl – /0.36 mol/L SO 4 2– /0.69 mol/L CO 3 2– , no corrosion trace was observed at the edge of the crevice. In addition, when concentra- tions were 1.02 mol/L Cl – /0.36 mol/L SO 4 2– / 0.023 mol/L CO 3 2– , a step formed between t h e i n t e r i o r a n d t h e e x t e r i o r o f t h e crevice. Crevice Corrosion Analysis These experimental results demon- strated that anions affect the corrosion process during crevice corrosion. Low con- centrations of Cl – ions by themselves pro- MATERIALS SELECTION & DESIGN