Corrosion Protection of Large Diameter Welded Steel Pipelines with Cement Mortar Coatings

Cement mortar coatings have been widely used in the Western United States since 1935 to protect the exterior of large diameter welded steel pipelines (WSP). Applications include pipe up to 144-inches in diameter and a wide range of environmental conditions. The Metropolitan Water District of Southern California has had extensive experience with mortar-coated WSP and at present has over 150 miles of this type of pipe in its water transmission system. Characteristics of a recently installed 7-mile, 120-inch- diameter, mortar-coated WSP pipeline are described. Steel coated with cement mortar is normally protected against corrosion by a passivating iron oxide film that forms and is maintained in a highly alkaline environment of hydrated Portland cement. The potential of passive steel in concrete is several hundred millivolts more positive than the potential of bare or organically coated steel. This unique property makes it possible to identify areas of corrosion activity on mortar-coated WSP and areas where external currents are collecting or discharging. The long-term performance of cement-mortar coatings is dependent on pipe design, manufacture, and installation. The history of cement mortar coatings is discussed, and recommendations are made for application of mortar-coated WSP.