1998 Volume 47 Issue 5 Pages 320-325
Pitting corrosion properties of a tin-coated copper tube in hot water were investigated in this paper. The long-term looping corrosion test had been conducted in hot water containing 1.5-3.0mg/L residual chlorine at 60°C for 745 days. The pitting corrosion life of a tin-coated copper tube could be estimated 2.5 times as long as that of a copper tube by applying the extreme-value statistical analysis to the measured maximum pit depth data. The excellent pitting corrosion resistance for a tin-coated copper tube was attributable to the fact that the corrosion potential, ECORR, values for the pitting corrosion growing period were approximately 30mV less noble than these of a copper tube. It is found that the difference of these ECORR values was caused by the reason of followings: (1) Whenever pitting occurred on the copper tube, the unattacked area on the inner surface was covered with corrosion products film consisting of cupric oxide and silicate precipitated into the film in order to ennoble ECORR up to the pitting potential. (2) The tin-coated layers prevented the formation of cupric oxide and the precipitation of silicate.