Effects of residual carbon deposits and water quality on corrosion behavior of copper tubes for air conditioning systems have been investigated by field tests and electrochemical measurements. Copper tubes with/without residual carbon deposits were filled with tap or circulated water in storage tank for 60 days. Patina and slight corrosion attack only occurred on the surface of copper tube, which has the residual carbon film and is filled with the circulated water. The corrosion potential of the copper tube with high carbon deposits was higher than that with low carbon deposits. The galvanic current density from copper tube with high carbon deposits to copper tube with low carbon deposits shows a gradual increase. This means that the high residual carbon area of surface could be the catholic area when the corrosion occurs. It is found that the combinations of residual carbon deposits and contaminated water are detrimental to the corrosion of copper tube.
Copper plates were exposed both in telephone switching centers and their outdoors to investigate the corrosion behavior of copper. We selected eight telephone switching centers and outdoor locations in the Kanto area including Tokyo (urban), Saitama (urban), Ibaraki (rural), and Tochigi (hot spring). The copper plates were exposed for a month and the experiments were conducted over the course of a year. After exposure, the amounts of sulfur and chlorine on the copper plates were determined by means of X-ray fluorescence analysis (XRF). We also analyzed the corrosion products formed during outdoor exposure using X-ray diffraction (XRD). Our results can be summarized as follows. 1) The amounts of sulfur and chlorine on the copper plates exposed in the telephone switching centers were one to two orders of magnitude lower than that on the plates exposed outdoors. 2) The amounts of sulfur and chlorine on the copper plates exposed outdoors in Tokyo and Saitama (urban areas) exhibited significant seasonal variations. The seasonal change in the sulfur was caused by the change in the sulfate ion concentration in fine airborne particles. By contrast, we attributed the seasonal change in the chlorine to changes in the sea salt aerosol concentration. 3) The corrosion products on the copper plates exposed in Tokyo, Saitama, and Ibaraki in summer were found to be cuprite (Cu2O) and posnjakite (Cu4SO4(OH)6⋅H2O). We found that the formation of posnjakite depended on the amount of sulfur on the copper plate.
Effects of Fe (III) ions on corrosion of stainless steel were examined in highly concentrated HNO3 solutions at high temperature, using mass loss measurements, electrochemical measurements, and quantitative analysis of NOx evolved during immersion. Addition of Fe(NO3)3 into HNO3 caused the corrosion potential of stainless steel to become nobler gradually from the passive to trans-passive potential regions during immersion experiments. The average corrosion rate of stainless steel for 360ks immersion increased with increasing Fe(III) concentration. Quantitative analysis of NOx evolved during immersion suggested that Fe(III) ions adsorbed on the specimen surface act as catalytic centers in the reduction of HNO3.
A newly designed working electrode on which occurrence of crevice corrosion is prevented with the flushed port cell has been developed in order to measure the critical pitting temperature, CPT, and pitting potential, Epit, for stainless steels. In the solution temperature range from 25°C to 100°C, the difference between the electrode surface temperature and bulk solution temperature could be maintained within 0.3°C in measuring CPT by using a pre-heating flushed water system. The measured values of CPT as well as Epit which were obtained from 12 trials of the measurement under the same condition, were found to obey the normal distribution. Epit at the constant temperature equivalent to CPT was found not to coincide with the setting potential for CPT due to the film formation in the CPT measurement. The mean value and deviation of Epit obtained by using the crevice corrosion preventing electrode were almost equal to those of Epit measured in the JIS method (JIS G 0577-1981). The newly developed method would be successfully applied to evaluate the difference in pitting corrosion resistance arising from the difference of surface finishing such as 2D, 2B, and BA, and the surface treatment of stainless steel.
To eliminate the risk of AC corrosion of buried pipe lines, measurements for AC probe current density as well as conventional DC probe current density, and on and off potentials must be taken refer to criteria for AC corrosion control with steel probes. Considering this situation, the authors have developed the instrument for field use to measure AC probe current density as well as DC probe current density corresponding to protective current density, and on and off potentials. The new instrument is available that can help CP inspectors improve the reliability and efficiency of their pipe line maintenance programs. With this instrument, an increased level of safety is attained. This paper describes the basic concepts and measuring methods for the new instrument, and an example of the measurements.