Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 109.—Corrosion has completelydestroyed this CMP spillway conduit. Backfillmaterials that surrounded the conduit havebeen eroded by flow within the conduit.CMPs generally consists of two types: soil side or water side. Most metal lossassociated with corrosion occurs on the interior or water side of the pipe. Soil sidecorrosion is not usually a significant factor in conduit life. In the presence of oxygenand water, metal corrodes through an oxidative process that involves the formationand release of metallic ions. The water acts as an electrolyte to carry these ions,which form the basis for the corrosion of the CMP. The reaction of the metal withthe dissolved oxygen in the water causes the deterioration most visible on the waterside of the conduit (Federal Highway Administration [FHWA], 1991, p. 4). CMPsare subject to electrolytic corrosion due to galvanic action between the metal and thesurrounding soil, groundwater, and water flowing through the conduit (Kula,Zamensky, and King, 2000, p. 2). The galvanic action results in corrosion of theCMP and a gradual decrease in wall thickness and structural integrity. Over time,corrosion of the CMP will result in the reduction of wall thickness, formation of pipeperforations, and the eventual collapse of the CMP.The service life of the CMP is affected by its metallic makeup, coatings, linings, pHand resistivity of the backfill and water, moisture content of the backfill, andabrasion from material particles in the flow. Pipe manufacturers have appliedcoatings and linings to CMP to mitigate corrosion and extend the service life. CMPcoatings have included metallic coatings (zinc [galvanized] and aluminum), andnonmetallic coatings (bituminous [asphalt], cement, and polymers). CMP liningshave included asphalt and concrete. The natural scaling tendencies inherent in somewaters provide additional protection. Scaling is the deposit and adherence ofinsoluble products on the surface of the CMP, which isolate it from the water andprotect it from corrosion. The factor that most affects corrosion and scale184