Coulometric determinations on copper samples exposed to low content propionic acid vapor environments
In the present study the effect of three propionic acid vapor concentrations (260, 380 and 680 parts by billion in volume, ppbv) on the deterioration of copper was evaluated in an air stream at 30 °C and 90% of relative humidity, using the coulometric reduction technique for the determination of the total charge required to reduce the formed copper compounds, according to ASTM B-825-97. The electrochemical tests were performed on samples exposed during 7, 14 and 21 days. In addition, gravimetric determinations for the samples with 21 days of exposure were made, according to ASTM G1-90, in order to compare these results with the electrochemical results. The corrosive environments were performed in a climatic chamber with both temperature and relative humidity electronic controls. A constant air flow 4 of L/min was maintained during the tests. The polluting agent was added to the air stream by means of emission tubes of propionic acid vapor with different emission openings. All emission tubes were gravimetrically certificated at 2650, 3750 and 6900 ng/min. The results showed statistical differences (with 95% confidence) for the charge needed for the total reduction of oxides formed on the samples with 7 and 21 days of exposure and it was found that this charge also increases with the polluting agent level, indicating that the corrosion products grow continuously with exposure time and with the increase in the propionic acid vapor concentration. In the other hand, it was found that the coulometric reduction technique is more reliable than the gravimetric test in order to evaluate the corrosion rate in these atmospheres, typical of environments of very low aggressiveness. On the gravimetric technique, the inherent removal of base metal as a result of the acid pickling procedure was in the same order of magnitude with respect to the effect of the corrosion process, leading to a high dispersion of the results.
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