| Abstract: | Influence of phosphates and/or silicates on copper ion induced pitting of galvanized hot water tubing In connection with studies on the copper-induced pitting of galvanized steel tubes the inhibition by phosphates and/or silicates should be investigated. The investigations were carried out in Berlin tap water, at temperatures of 65°C. Inhibitors used were o-phosphate, p-phosphate, combinations of o/p-phosphate and phosphate/silicate as well as two silicates. All these inhibitors reduced the general corrosion of galvanized steel and copper. Best results were obtained with p-phosphate. Protective layers on both metals were produced not before a time of 150–200 days. Number and depth of pits were positively influenced by nearly all the inhibitors used. But only the combination phosphate/silicate lead to a complete prevention from copper-induced pitting. O-phosphate showed the lowest efficiency. These investigations once again pointed to the importance of the phase boundaries air/water/tube or deposits/water/tube as nuclei for pitting in tubes. A second layer containing calcium and inhibitors was formed on the primary layer of zinc corrosion products. These secondary layers are responsible for corrosion protection by the inhibitors. |
