Analysis of Corrosion Products Formed on Some of Ontario's Weathering Steel Bridges

摘要

Ministry of Transportation Ontario has been using weathering steel for bridge girders since1968. The main advantage of using weathering steel is that, under normal weathering cycles, it will form a tough outer oxide layer referred to as 'patina' that will protect the steel from unabated corrosion. However, recently the ministry has observed that the patina formed on girders over the driving lanes is de-bonding from the parent steel material and often has to be physically removed before it becomes a safety hazard. In contrast, other locations of the girders (between the ends of girders and the driving lanes) exhibited a more stable patina. Hence, in order to determine the root cause for de-bonding of the patina, steel core samples and corrosion products (scraped oxide layer) were collected from several locations of interest. These samples were examined using scanning electron microscopy, energy dispersive X-ray analysis, inductively coupled plasma/mass spectrometry, laser Raman spectroscopy and Mossbauer spectroscopy. The results from these investigations indicate that the patina at de-bonded regions exhibited greater amounts of chloride species and lower levels of sulphur species compared to more stable patina areas. Moreover, the de-bonded patina regions exhibited greater amounts of akaganeite (β-FeOOH-CI), lepidocrocite (γ-FeOOH) and hydrated iron (III) oxyhydroxide, while greater amounts of goethite (α-FeOOH) were observed in the other regions. The corrosion product analysis data are presented in this paper and their significance towards the root cause failure analysis is discussed.