Abstract
Following a previous report on effects of pH and oxidizing agents on pitting of aluminum, the present study was conducted to investigate influence of concentration of anion inhibitors such as sulfate, silicate and phosphate and of free chlorine on initiation and propagation of pitting. Testing solutions were prepared by adding 5ppm of chloride, 0.1-50ppm of the inhibitor and 0.5-5ppm of free chlorine to deionized water. The pH of the solutions were adjusted to 6.0-7.0 with dilute hydrochloric acid or sodium hydroxide solutions. Corrosion tests were conducted in stagnant water at 35°C for 100hours. The results obtained are as follows:
(1) The increase of free chlorine concentration had more effective influence on the initiation of pitting than on the propagation, while the incrcase of concentration of every inhibitor had a considerable effect on the propagation of pitting.
(2) Logarithm of the average pit depth was linearly related to that of the concentration of each inhibitor. However, the average pit depth had no relation to the free chlorine concentration.
(3) The distribution curve of the number of pits versus the pit depth had a long tail in the side of the deeper pit. This fact suggested that the distribution was expressed in an exponential type function.
(4) The effects of the concentration of a particular inhibitor on the propagation of pitting were discussed by applying the extreme value analysis, proposed by Aziz, to the maximum pit depth data.
