Materials Performance

NOV 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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65 NACE INTERNATIONAL: VOL. 56, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2017 tions in Solutions I, II, and III are shown in Figure 1. In Solution I, it was observed that the corrosion rate first increased rapidly and then decreased slightly with increasing Cl – concentration. The corrosion rate rose slowly as the SO 4 2– concentration increased in Solution II. The copper exposed to Solu- tion III was almost immune to corrosion, with corrosion rates close to zero. Figure 2(a) shows the corrosion rates of copper in the two-ion solutions (Solutions IV, V, and VI). Cl – played a dominant role in the Cl – /SO 4 2– solutions (Solution IV). Com- pared with Figure 1, it was observed that the presence of SO 4 2– in the Cl – -containing solutions slightly increased the corrosion rate. When the concentration of SO 4 2– was high in Solution IV, the corrosion rate expe- rienced a small decrease. When CO 3 2– was ad ded to th e Cl – -c ontaining solutions (Solution V), the corrosion rates decreased. When the concentration of CO 3 2– was high in Solution V, the corrosion rates decreased. The results strongly indicate a competitive relationship between Cl – and CO 3 2– . The corrosion rate of copper varied little in the solutions with various concentrations of SO 4 2– and CO 3 2– (Solution VI), which was similar to the copper corrosion rate in Solution II. Figure 2(b) exhibits the corrosion rates of copper in electrolytes with three ions (Solution VII). The corrosion rate was mainly affected by the Cl – concentration, which increased with increasing Cl – , while increasing the CO 3 2– concentration pro- duced an inhibition effect on the corrosion rate. Based on these observations, the pres- ence of Cl – and SO 4 2– in a solution promoted the corrosion process of copper, and Cl – had a much greater impact than SO 4 2– . Add- ing CO 3 2– could suppress copper corrosion. Crevice Depth After the formation of crevice corro- s i o n , t h e c o r r o s i o n p r o d u c t s w e r e removed, and the crevice depth was mea- sured by the surface profiler. The statisti- cal results of crevice corrosion and crevice depth in the different solutions are pre- sented in Table 2. Single-Ion Solutions In the single-ion solutions (Solutions I, II,and III), severe crevice corrosion was ob- served in the 0.034 mol/L Cl – solution and all SO 4 2– solutions, while no crevice corro- sion occurred in the CO 3 2– solutions. Crev- ice corrosion was not observed in the 0.34 mol/L and 1.02 mol/L Cl – solutions, but they left a step between the interior and ex- terior of the crevice—the interior of the crevice was slightly corroded and the exte- rior of the crevice was severely corroded. In these cases, crevice corrosion had oc- curred; however, the most severe corrosion attack occurred outside the crevice and the corrosion depths were greater than the cor- rosion attack near the crevice mouth, so crevice corrosion could not be accurately observed on the copper surface. The average depth of the crevice corro- sion was 0.016 mm, and the deepest corro- sion was 0.032 mm after immersion in the 0.034 mol/L Cl – solution. The greatest crev- ice depth of copper exposed to the three different SO 4 2– solutions was almost the same as that of the copper exposed to the 0.034 mol/L Cl – solution . The greatest depth did not change with increases in the concentration of SO 4 2– ; however, when cop- per was exposed to the three different con- TABLE 1. ELECTROLYTE CONCENTRATIONS Ions in Solution (mol/L) Solutions NaCl Na 2 SO 4 Na 2 CO 3 Ion Concentration Multipliers I 0.034 0 0 1 0.34 0 0 10 1.02 0 0 30 II 0 0.012 0 1 0 0.12 0 10 0 0.36 0 30 III 0 0 0.0023 1 0 0 0.023 10 0 0 0.069 30 IV 0.034 0.012 0 1-1 0.034 0.36 0 1-30 1.02 0.012 0 30-1 1.02 0.36 0 30-30 V 0.034 0 0.0023 1-1 0.034 0 0.069 1-30 1.02 0 0.0023 30-1 1.02 0 0.069 30-30 VI 0 0.012 0.0023 1-1 0 0.012 0.069 1-30 0 0.36 0.0023 30-1 0 0.36 0.069 30-30 VII 0.034 0.012 0.023 1-1-1 0.034 0.12 0.069 1-30-30 0.34 0.012 0.069 30-1-30 0.34 0.12 0.0023 30-30-1

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