Abstract
Recently, resulting from pitting corrosion of copper tube, leaks in air-conditioning coils for open heat-storage water tank systems are frequently happening. Water-side cause of the corrosion is the sediment on tube inner surface of either CaCO3 due to too high pH and saturation index or a compound containing PO43- or MoO42- supplied by some corrosion-inhibitor. Coil-side cause of the corrosion is the scattered thin oxide film on tube inner surface originated in oxidization by humid air at room temperature during the period from the creation of copper tube till the water pour at installation site. This oxide film shows nobler corrosion potential than ground copper by 35 to 103mV. Consequently, when the sedimentation has occurred, oxygen concentration differential cell will be formed and, increasing potential difference to the oxide film, corrosion of the neighbouring ground copper will be accelarated toward the pitting. In restraint of the oxide film formation, it may be important to keep the tube inside clean throughout the manufacturing, but, as a sure means, it is recommendable that the coil should be filled with dry nitrogen gas at the completion of manufacture, and be kept intact till the water pour at the site.
