A critical study on corrosion and corrosion control methods for piping systems in Hong Kong

Abstract

The present project aims at investigating the corrosion and cavitation erosion behaviors of common piping materials in Hong Kong tap water, which is employed as drinking water and as cooling water. Corrosion and erosion both lead to loss of materials from the piping systems, giving rise to functional deterioration and damage, and also resulting in health hazards in the case of drinking water. The corrosion and cavitation erosion behaviors of the piping materials, both qualitatively and quantitatively, depend on the water chemistry and the operation conditions. In other words, such behaviors are, to some extent, region-specific. The present project is an endeavor to study these behaviors for tap water in Hong Kong, a study that has not been systematically conducted so far. The corrosion rates of the common piping materials in Hong Kong tap water, as determined in the present project by electrochemical methods, were comparable to those in the western countries. Cavitation erosion, which occurs in some parts of a water supply system, has been rarely studied in such a context. It was shown in the present study that cavitation erosion could contribute a substantial fraction to material loss in view of the low hardness of the common piping materials. Cavitation erosion inevitably removes the passive film and activates the surface relative to the neighboring areas that are not under cavitation attack. Thus these adjacent locations will form galvanic couples and corrosion is enhanced. Such an erosion-induced effect was studied in the present project to assess the severity. It was found that in the worst case, the average galvanic current density was 0.276 times the static corrosion current density. Another important source of galvanic effect, that due to soldering of copper pipes, was also investigated. To reduce material loss due to corrosion and erosion in some critical pats of a water supply system, laser surface modification was attempted. The corrosion resistance and cavitation erosion resistance after laser surface melting were improved by a factor in the range l.6 to 8.0 and 1 .4 to 3. 1, respectively. The effect of inhibitor concentration on the corrosion behavior of common piping materials in Hong Kong tap water was investigated using electrochemical impedance spectroscopy. The effect was most pronounced for brass, where the corrosion rate was reduced by a factor of 88.3 % and least for bronze, where a reduction of 38.5 % was recorded.