Both the potential noises (PNs) and acoustic emissions (AEs) during inter-granular (IG) and trans-granular (TG) chloride stress corrosion cracking (SCC) of Type 304 austenitic stainless steel sheet in a 38 mass% MgCl2 solution were analyzed. Both AEs and PNs were observed for IG-SCC but only PNs for TG-SCC. Pitting corrosion, crevice corrosion and IG-SCC via pitting were produced by SCC test under combined thermal and applied tensile stresses. These corrosion damages produced characteristic ENs. In addition to the RD-type PNs (rapid drop to active potential) by pitting corrosion in the early stage (Zone I), RR-type PNs (rapid rise to noble potential) were observed in the middle stage (Zone II). AEs from fast inter-granular cracking were detected during the later stage (Zone III) when small-amplitude RD-type PNs with low frequency component (<0.07Hz) were observed. Timings of AEs and PNs during zone-III were classified into two types: RD-type potential shifts just after the AEs suggested an anodic current from a new-borne grain boundary. RD-type potential shifts just before the AEs were supposed to be due to the IG cracking at pit bottom. Trans-granular (TG)-SCC did not produce any AEs but produced RD-type PNs with frequency component less than 0.016Hz. Potential shift rate (0.08mV/s) of TG-SCC is 6% that (1.4mV/s) of IG-SCC.
Unlike the liner polarization technique or the electrochemical impedance spectroscopy (EIS), the electrochemical noise method (ENM) enables to measure the polarization resistance (Rp) of an electrode under the conditions that are free from electrochemical perturbations caused by the application of current. However, as well as by the liner polarization, the measured Rp by the ENM affected by solution resistance (Rs) between the electrodes. Though this might produce a substantial error in the Rp measured in lower conductivity solutions, the effect of Rs on the Rp has been scarcely investigated. In this study, the Rp of a carbon steel electrode in dilute HCl solutions were measured by both the ENM and the EIS, under the same conditions. The error of the Rp by the ENM was evaluated by comparing the Rp by the EIS; the latter is regarded as free from the effect of Rs. The Rp by the ENM was higher than that by the EIS, and their differences were decreased with the concentration of the solution. However the degree of the differences was smaller than the Rs measured by the EIS. This advantage would be brought by the specific electrode configuration used for the experiments.
Supercritical water (SCW) process is an effective method for the destruction of hazardous organic wastes and the upgrading of unused hydrocarbon resources. However, the corrosion of a reactor and a heat exchanger is known as the major drawbacks to industry application. So it is important to select corrosion-resistant materials and explicate the life span of those under SCW state. The objectives of this study are to evaluate corrosion rate of corrosion-resistant materials and to explicate corrosion mechanism under SCW and reducing environment. (1) The oxide layer of Cr-Mo steel is sensitive to environment condition, for example oxygen-hydrogen pressure, anion concentration. (2) Corrosion rate of SUS316 is little in SCW. Corrosion rate in SCW with the addition of NaCl or HCl is high, especially corrosion rate in sub-critical condition and low pH (under pH2) is remarkably high. It is difficult to apply SUS316 to equipment material in these environments. (3) Corrosion rate of Ni-base alloy is lower than that of SUS316. Ni-45Cr-1Mo (MC Alloy) containing much chromium is the most corrosion-resistant in SCW. Ni-base alloys, for example MAT21 or Hastelloy C-276, containing the moderate content of elements as Cr and Mo are more stable under reducing environment.
The electrochemical reactions of copper and carbon steel in LiBr solution have been investigated by electrochemical measurements including channel flow double electrode (CFDE) and electrochemical quartz crystal microbalance (EQCM). The active dissolution mechanism of copper in LiBr solution was proposed as follows. Cu+Br-→CuBr+e-, CuBr+Br-→CuBr2-. The disproportional reaction of copper (Cu+Cu2+=Cu+) in LiBr solution was discussed by CFDE. The disproportional reaction rate was evaluated by detecting Cu (I) emitted from Cu working electrode in LiBr solution containing Cu (II) at an open circuit condition. It was found that the disproportional reaction was accelerated by the presence of LiBr, and the total reaction was proposed as follows. Cu+Cu2++4Br-→2CuBr2-. The anodic polarization curve of carbon steel in LiBr solution shows the passivation at low anodic overvoltage region and the active dissolution by pitting corrosion at high anodic overvoltage region. Furthermore by using the quartz crystal which has 100kHz resonant frequency in EQCM, the anodic polarization curve and mass change of steel sheet electrode were measured simultaneosuly. Finally, the replacement reaction of carbon steel in LiBr solution containing Cu (II) was discussed by CFDE and EQCM.