Corrosion Protection of Large DiameterWelded Steel PipelinesWith Cement Mortar Coatings

摘要

Cement mortar coatings have been widely used in the Western United Statesrnsince 1935 to protect the exterior of large diameter welded steel pipelinesrn(WSP). Applications include pipe up to 144-inches in diameter and a widernrange of environmental conditions.rnThe Metropolitan Water District of Southern California has had extensivernexperience with mortar-coated WSP and at present has over 150 miles of thisrntype of pipe in its water transmission system. Characteristics of a recentlyrninstalled 7-mile, 120-inch- diameter, mortar-coated WSP pipeline arerndescribed.rnSteel coated with cement mortar is normally protected against corrosion by arnpassivating iron oxide film that forms and is maintained in a highly alkalinernenvironment of hydrated Portland cement. The potential of passive steel inrnconcrete is several hundred millivolts more positive than the potential of barernor organically coated steel. This unique property makes it possible to identifyrnareas of corrosion activity on mortar-coated WSP and areas where externalrncurrents are collecting or discharging.rnThe long-term performance of cement-mortar coatings is dependent on piperndesign, manufacture, and installation. The history of cement mortar coatingsrnis discussed, and recommendations are made for application of mortarcoatedrnWSP.