Corrosion of Copper Tubes in Fresh Water

Abstract

This paper reviews the corrosion problems of copper tube experienced in fresh water service. Following three distinct types of corrosion are discussed; (1) dissolution of copper, (2) pitting corrosion, and (3) erosion corrosion.
Dissolved copper ion, which may cause green stains by forming insoluble cupric soap or accelerate the corrosion of other metals, is associated with the cupro-solvency of water. Decrease of pH and/or increase of carbon dioxide raise the dissolution rate of copper.
Pitting corrosion of copper tubes is generally classified in two types, i. e., Type I pitting experienced mainly in cold water in Europe and USA, and Type II pitting experienced mainly in hot water in Japan, Sweden and Germany. Type I is attributed to carbon film on the tube surface and/or to water quality. Carbon film can raise the potential of copper tube higher than the critical pitting potential, 100 to 170mV vs. SCE. Type II pitting is associated with chemical composition of water, i. e., pH and the ratio of HCO₃^-/SO₄^2-. Excess of chlorination also plays an important role, which can raise the potential of copper tube higher than the critical pitting potential, 150mV vs. SCE, to cause Type II pitting corrosion.
As regards the erosion corrosion of copper tubes, importance of two factors must be stressed; one is fluid dynamic conditions which affect mechanical breakdown of the protective film on the tube surface, and the other is chemical composition of water which affects the stability of the film formed on the tube surface.