Pitting Corrosion in Copper Tubes – Cause of Corrosion and Counter-Measures

Abstract

An investigation of failures of hard-drawn copper water pipes (phosphorus-deoxidised copper) in service due to pitting corrosion was conducted from November, 1962 to February, 1965. Fifteen cases were reported. All those about which information could be obtained came from hot water installations and occurred in water with a low pH (≤7) and a HCO_3- content of, at the most, 100 mg/l but generally below 50 mg/1. Failures not due to pitting corrosion (i.e. caused by erosion and corrosion or corrosion fatigue) occurred in waters with a higher pH and higher HCO₃- content.

A laboratory investigation into the ability of the corrosion products to counteract further corrosion in different types of water was also carried out, using an electrolytic cell which, in principle, was a model of an active pit in a copper tube. This led to the following conclusions, which are in good agreement with the results obtained from the examination of service failures:

If the pH value of the water is high enough, the copper dissolved by the corrosion can be precipitated as basic copper salt. At low pH values such precipitation does not take place.

If the [HCO₃ ⁻]/[SO₄ ²⁻] ratio in the water is high, dissolved copper can be precipitated as basic copper carbonate in the neighbourhood of the corrosion site and counteract further corrosion.

At a low [HCO₃ ⁻]/[SO₄ ²⁻] ratio, crusts of basic copper sulphate will be precipitated at some distance from the corrosion site and may lead to a high corrosion rate.

Pitting is not likely to occur in hot water tubes of hard copper if the pH is ≥ 7·4 and the [HCO₃ ⁻]/[SO₄ ²⁻] ratio ≥1 (the concentrations given in mg/1). The critical values mentioned are approximate and may be adjusted in the light of future experience.