Phase transformations of aged patina formed on weathering steel bridge girders

• Field investigation of patina performance on 24 weathering steel bridges with 7–61 years in service. • Identifies quantitative patina degradation thresholds: α / γ * > 2 and ( β + s) / γ * > 0.5. • Demonstrates the protective ability of patina layers on aged weathering steel through laboratory examinations. • Suggests reducing inspection frequency for bridges older than 20 years and limiting inspections to damaged areas. • Shows chloride exposure and time-of-wetness as dominant drivers of non-protective patina formation • Demonstrates XPS–XRD integration as an effective method for identifying unstable patina layers. • Provides criteria to support early detection and maintenance planning for weathering steel bridges. Weathering steel relies on the formation of a stable patina to resist atmospheric corrosion. While the protective layer performs well under normal exposure conditions, its effectiveness diminishes in environments with elevated moisture or in the presence of contaminants such as chlorides. Several recent reports of severe corrosion and section loss in conventional weathering steel structures have raised concerns about the long-term stability of the protective rust layer. This study investigates the microstructural evolution associated with non-protective patina formation and long-term field performance of the rust layer by analyzing samples collected from 24 weathering steel bridges with service lives ranging from 7 to 61 years. Phase composition and crystallographic properties were examined to identify conditions leading to active corrosion and non-protective patina using XPS, XRD, and SEM-EDS. Quantitative analysis revealed that patina degradation, characterized by non-adherent rust and progressive section loss, occurred when the ratio ( β -FeOOH+s)/ γ * exceeded 0.5 and when α / γ * was below 2. These thresholds can be adopted as a warning indicator for patina degradation. Chloride exposure and elevated time of wetness in marine-influenced environments promoted β -FeOOH and spinel iron oxide formation, directly correlating with accelerated deterioration. The research concluded that combining surface XPS analysis with bulk XRD is beneficial for screening non-protective patina. The results provide a forensic assessment of phase transformation mechanisms in long-term exposures and establish quantitative criteria for predicting service life and assessing the structural durability of aging infrastructure.