Case Study and Factor Analysis of Early Localized Corrosion in SUS304 Siphon Pipes of Agricultural Water Facilities— An Engineering Reassessment Based on Field Investigation Records —

This technical report presents an engineering analysis of an early case of localized corrosion in SUS304 stainless steel siphon pipes used in an agricultural water facility in Akita Prefecture, Japan. Within only a few years after installation, significant wall thinning and perforation were observed, particularly around welded regions, even though stainless steels are generally expected to provide high resistance under neutral freshwater conditions. Radiographic testing (RT) confirmed corrosion shadows and uneven thinning, and the estimated corrosion rate was several times higher than typical values reported for stainless steels in similar environments. Detailed evaluation based on inspection records, design specifications, and operating history suggests that multiple factors overlapped to accelerate the damage. Multiple overlapping factors are considered responsible, including weld-zone sensitization and stagnant/empty-pipe oxygen limitation with possible local chloride build-up. In this context, MIC is treated as a plausible, yet unproven, contributory factor. In this context, microbiologically influenced corrosion (MIC) is treated as a plausible, yet unproven, contributory factor rather than a sole cause. This case highlights that even highly corrosion-resistant alloys may fail to deliver expected performance if site-specific conditions are overlooked. The findings provide important lessons for material selection, corrosion protection, and maintenance strategies in irrigation facilities where stainless steels are widely adopted with expectations of long-term durability.