Grooving Corrosion of Welded Galvanized Pipes for Distribution of Warm Water

Premature failure of galvanised pipes in the distribution net of a facility supplying numerous consumers with warm water has taken place. It was caused by grooving corrosion of longitudinal weldments resulting in intensive leakage of the system. Detailed analysis of the facility and of  the way of its operation was carried out using various techniques . The analysis comprised physical and chemical data on

-          fresh water,

-          structural materials and

-          corrosion process and its products.

It was established that the water in the net is contaminated with Cu²⁺ brought into existence by corrosion of copper pipes used in the central heater of the system. On the other hand the temperature of water is approximately 70 ^oC when the net is in exploitation. Under such circumstances  reversal of polarity  between zinc coating and structural steel takes place, ie zinc becomes more noble than the underlying carbon steel so that the effect of cathodic protection of steel by zinc disappears. Simultaneously, copper is deposited on galvanised steel by ion exchange that makes possible intensive bimetallic corrosion localised on longitudinal weldments.