It has been about fifteen years since a new type pitting corrosion of copper tubes which has been called ‘moundless type pit’ was discovered in Japan. This type of pitting corrosion has some unique morphological features which differ from ordinary types of pitting corrosion such as type I or type II. First, this type of pitting corrosion has no mound of corrosion products which covers on the entrance of a pit. Second, there is a glassy verdigris around the pit. Third, the size of the pit is less than 1 mm in diameter but very deep. We reproduced moundless type pits by a field test in the area where moundless type pits have often occurred. Moundless type pits were also generated in synthetic freshwater dissolving some chemicals in a laboratory test. This synthetic freshwater contains 40 ppm SiO2, 50 ppm SO42–, 10 ppm Cl– and 10 ppm HCO3–. Surface analysis of the moundless type pits revealed that the mouth of the pits was covered with a thin film which was mainly composed of silica. The initiation and propagation mechanisms of moundless pitting are discussed on the basis of the experimental results.
In order to discuss the behavior of accumulating ions on inside-ward surface of a girder of the bridge, composition of accumulating ions on the girder are examined. Abundance of accumulating ions are high in winter and spring, and low in summer and autumn. The fluctuation of the abundance is associated with the quantities of air born salt in each season. The abundances of Na+ and Cl– on the inside-ward surface of girder decrease during days with a hard rain and high humidity. While the inside-ward surface of the girder can not be washed by rain, many traces of dew can be found on the girder. Therefore, the decrease of Na+ and Cl– on the girder is considered to result from that accumulating NaCl with high solubility are partly wash out by dew. Hence, chemical composition of salts on girders is influenced not only by the quantities of air born salts, but also by the quantities washed out by dew. As a result, the abundances of ions on girders are not proportional to length of period that ions have accumulated.
The effect of anodic oxide film structure, barrier type and porous type on galvanic corrosion resistance of aluminum in 0.5 kmol/m3 H3BO3 / 0.05 kmol/m3 Na2B4O7 with NaCl was examined by electrochemical random signal (electrochemical noise) analysis technique. When pitting corrosion occurs, rest potential suddenly decreases and current increases rapidly, and rest potential and current change with fluctuation during galvanic corrosion. The galvanic current change correlates closely with corrosion potential change. As the thickness of both types of anodic oxide films and decreasing of chloride ion concentration, the incubation time of pitting corrosion became longer, and incubation time of pitting corrosion of specimen formed porous type anodic oxide film is longer than that of specimen formed barrier type anodic oxide film. The inverse of incubation time increase with chloride ion concentration to the power of n, [Cl–]n. The value of n is between 1.2 and 2.5 in both oxide films.