ABSTRACT

The corrosion of mild steel in chloride-rich atmospheres is a highly topical issue for industry and departments of transportation. The formation of the oxyhydroxide akaganeite (ß-FeOOH) in this type of atmosphere is associated with a notable acceleration of the steel corrosion process. The scientific literature contains many references to outdoor marine atmospheric tests, but has not yet clarified some issues regarding akaganeite, such as, the environmental conditions for its formation. Research has been performed at thirty atmospheric corrosion stations located at south Florida within 0.1 to 21 miles from the east ocean shore. Conditions in Florida generally led to the formation of akaganeite due to the presence of chlorine in the atmosphere near the ocean. Steel plate samples were exposed for evaluation of crystallographic structures of the corrosion products using the X-ray diffraction (XRD). For this investigation the corrosion products on these samples were analyzed using XRD with a cobalt anode to observe the intensities and peaks related to akaganeite, goethite, lepidocrocite and hematite products. The ratio of akaganeite to the other crystalline iron oxides was calculated from the peak positions corresponding to the respective oxides and intensities. The results show correlations between plate weight loss, distance from the shoreline and akaganeite formation.

INTRODUCTION

In the coastal regions without roadway deicing treatments, the salinity in the marine atmosphere increases the corrosion rate of steel in comparison with a benign, chloride-free atmosphere. As chloride salts in the marine atmosphere dissolve into surface moisture layers, the conductivity increases dramatically on the surface of the steel. As a result of the elevated chloride, the passivating film will be damaged and corrosion rate increases as the chloride deposition rate increases [1-3]. The high corrosion rate in marine atmospheres may be due, in part, to the higher hydroscopicity of sea salt versus sodium chloride, which increases the time of wetness for the electrochemical corrosion process. These differences can affect the solubility and properties of the corrosion products formed.

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