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51313-02650-Unexpected Corrosion of Aluminized Steel Pipes in Limestone Backfill

Picture for Unexpected Corrosion of Aluminized Steel Pipes in Limestone Backfill
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Product Number: 51313-02650-SG
ISBN: 02650 2013 CP
Author: Mersedeh Akhoondan
Publication Date: 2013
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Aluminized steel Type 2 (AST2) ASTM A929 has a ~50 um thick coating with outer and inner layers of commercially pure Al and Fe-Al intermetallic respectively. AST2 culvert and drainage pipes are required to have long service life (e.g. 75 yr) 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 been recently 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-rich (limestone) backfill. Water in contact with limestone and allowed to equilibrate with CO2 in air (open system) tends to develop a near neutral pH compatible with a stable passive film on Al so experiments were conducted to determine if aggressive conditions took place otherwise. Simulated field fresh water conditions were created where cell water in a limestone bed was being constantly replenished at a slow rate (representing rainwater) while monitoring pH and conductivity. Under these conditions a high steady state pH > 9 developed that was aggressive to the aluminum 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 onset of severe corrosion early in the exposure confirmed by metallographic and SEM observation of loss of coating in extracted AST2 specimens. The Al-Fe intermetallic was much less attacked. Corrosion rate decreased later as a thick corrosion product layer formed. Corrosion mechanisms are discussed. These findings merit consideration in updating specifications for installation and the use of backfill materials for aluminized steel culvert pipes.
 

Key Words: Aluminized Steel Pipe Wet Limestone Backfill Water Durability Culvert Drainage Service Life Localized Corrosion
 

Aluminized steel Type 2 (AST2) ASTM A929 has a ~50 um thick coating with outer and inner layers of commercially pure Al and Fe-Al intermetallic respectively. AST2 culvert and drainage pipes are required to have long service life (e.g. 75 yr) 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 been recently 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-rich (limestone) backfill. Water in contact with limestone and allowed to equilibrate with CO2 in air (open system) tends to develop a near neutral pH compatible with a stable passive film on Al so experiments were conducted to determine if aggressive conditions took place otherwise. Simulated field fresh water conditions were created where cell water in a limestone bed was being constantly replenished at a slow rate (representing rainwater) while monitoring pH and conductivity. Under these conditions a high steady state pH > 9 developed that was aggressive to the aluminum 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 onset of severe corrosion early in the exposure confirmed by metallographic and SEM observation of loss of coating in extracted AST2 specimens. The Al-Fe intermetallic was much less attacked. Corrosion rate decreased later as a thick corrosion product layer formed. Corrosion mechanisms are discussed. These findings merit consideration in updating specifications for installation and the use of backfill materials for aluminized steel culvert pipes.
 

Key Words: Aluminized Steel Pipe Wet Limestone Backfill Water Durability Culvert Drainage Service Life Localized Corrosion
 

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