| Abstract | Atmospheric Corrosion Resistant (ACR) steel, also commonly known as weathering steel, has become a preferred material for bridge construction across Canada and internationally due to its ability to form a self-protecting oxide layer that reduces maintenance requirements and extends service life. Its benefits include eliminating periodic repainting, minimizing traffic disruption, and providing long-term structural resilience. However, its performance is highly dependent on local micro-climatic conditions, particularly exposure to de-icing salts. Premature corrosion failures in weathering steel bridges are most often caused by environmental and detailing factors rather than material deficiencies. This guide develops a framework for the selection, design, and maintenance of weathering steel bridges suited to Canadian environments, bridging the gap between laboratory corrosion science and practical field application. Weathering steels are low-alloy structural steels containing copper, chromium, nickel, phosphorus, silicon, and manganese in small proportions. Their corrosion resistance arises from the formation of a dense, adherent patina that limits oxygen and chloride penetration. Copper and nickel are critical in producing a compact oxide structure, while chromium contributes to the formation of a secondary passive film. Common bridge grades include ASTM A588, A242, and A709 50W, with enhanced corrosion-resistant grades such as ASTM A1010 offering superior resistance in chloride-rich environments through the formation of chromium oxide rather than a traditional patina. Although A1010 has a higher initial cost, lifecycle cost analysis demonstrates its economic advantage in aggressive environments by eliminating coating maintenance. Patina formation depends on alternating wet and dry cycles that stabilize the protective goethite phase. In chloride-rich conditions, the less protective akaganeite phase forms instead, creating a porous oxide that accelerates corrosion. Continuous moisture, poor drainage, and persistent chloride exposure prevent proper patina development and result in corrosion rates similar to carbon steel. Using the ISO 9223 classification, unpainted weathering steel is suitable up to Category C4, while C5 environments require coatings or alternative materials. In Canada, de-icing salts raise local corrosivity beyond regional averages. Sodium chloride increases time of wetness and encourages akaganeite formation, while infiltration through leaking joints and traffic spray produces localized corrosion zones, especially on horizontal surfaces and girders near traffic lanes. It is a common and good industry practice to incorporate an additional coating protection system (paint system) locally under expansion joints (to enhance steel protection in these vulnerable areas) and above supporting concrete piers to prevent rust staining of the concrete surfaces. A 10-year field exposure study conducted under the Highway 404 and Bloomington Road overpass, Ontario, tested seven weathering steel alloys under severe de-icing conditions. Results confirmed that chloride contamination fundamentally alters patina chemistry and structure. The modified GM 9540P cyclic corrosion test was found to best replicate field conditions, while the ASTM G85-A5 (Prohesion) test overestimated corrosion rates. The electrochemical method known as linear polarization resistance used in the study was found unreliable for predicting atmospheric durability because it lacks drying phases essential to patina stabilization. X-ray diffraction and electron microscopy confirmed a dominant presence of akaganeite ranging from 44 to 70 percent across all alloys, with incomplete patina stabilization and chloride incorporation in every sample. A direct correlation between patina thickness and corrosion rate was established, enabling non-destructive field estimation of corrosion using portable gauges. Live in-service corrosion and environmental monitoring at the Highway 401 in Lancaster, Ontario, were validated by laboratory experiments. Electrical resistance sensors provided reliable real-time data, slightly conservative but within expected variance, proving effective for continuous structural health monitoring. The monitoring program identified three distinct tiers of micro-environmental corrosivity: the highest corrosion occurred on downward-facing surfaces above traffic lanes, reaching up to 95 µm per year; intermediate corrosion occurred on vertical splash-zone surfaces at 56 to 60 µm per year; and the lowest rates were found on sheltered abutments around 29 µm per year. These results demonstrate that bridges over a road located in regions classified as C2 or C3 as per ISO 9223 can experience localized C4-level corrosion in specific structural zones, emphasizing the need for micro-zone-specific design and maintenance strategies rather than relying on a single macro-environmental classification. |
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