Corrosion Mechanism of Aluminized Steel in Limestone Backfill

Abstract

Aluminized steel Type 2 (AST2), ASTM A929, has a ~50 μm thick coating with outer and inner layers of Al alloy and Al-Fe intermetallic, respectively. AST2 culvert and drainage pipes are required to have a long service life (e.g., 75 years) in natural soil and water environments; premature pipe failures incur heavy repair and replacement costs. Performance has been adequate in many service conditions, but unexpected early corrosion of AST2 pipes has recently been observed in some Florida inland locations. A recent event with loss of the coating and local penetration of the substrate, after only ~3 years of service, was associated with the use of calcium carbonate (CaCO3)-rich (limestone) backfill. Water in contact with limestone and allowed to equilibrate with carbon dioxide (CO2) in the atmosphere (open system) tends to develop a near-neutral pH, compatible with a stable passive film on Al, so experiments were conducted to determine if the aggressive conditions took place otherwise. Simulated field fresh water conditions were created, where water in a limestone-filled cell was constantly replenished at a slow rate (representing rain-water), while the pH and conductivity were monitored. Under these conditions, a high, steady-state pH > 9 developed that was aggressive to the air-formed aluminum oxide passive film. The higher pH was ascribed to the dissolution of limestone in slowly flowing water that is not given enough time for equilibration with atmospheric CO2 (approaching a closed system). Electrochemical impedance measurements indicated the onset of severe corrosion early in the exposure, confirmed by metal-lographic and scanning electron microscopy (SEM) observation of loss of coating in extracted AST2 specimens. The corrosion rate decreased later as a thick corrosion product layer formed. Corrosion mechanisms are discussed.