A great deal of information is available on the atmospheric corrosion of mild steel in the short, mid and even long term, but studies of the structure and morphology of corrosion layers are less abundant and generally deal with those formed in just a few years. The present study assesses the structure and morphology of corrosion product layers formed on mild steel after 13 years of exposure in five Spanish atmospheres of different types:rural, urban, industrial, and marine (mild and severe). The corrosion layers have been characterised by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). Long-term corrosion is seen to be more severe in the industrial and marine atmospheres, and less so in the rural and urban atmospheres. In all cases the corrosion rate is seen to decrease with exposure time, stabilising after the first 4-6 years of exposure. The most relevant aspects to be noted are: (a) the great compaction of the rust layers formed in the rural and urban atmospheres; (b) the formation of hematite and ferrihydrite phases (not commonly found) in the industrial and marine atmospheres, respectively; and (c) identification of the typical morphological structures of lepidocrocite (sandy crystals and flowery plates), goethite (cotton balls structures), and akaganeite (cotton balls structures and cigarshaped crystals).Key words: A. Mild steel; B. XRD; B. SEM; C. Long-term atmospheric corrosion; C. Morphology. INTRODUCTIONSteel is the most commonly employed metallic material in open-air structures and is used to make a wide range of equipment and metallic structures due to its low cost and The atmospheric corrosion of mild steel is an extensive topic that has been studied by many authors. Useful reviews have been performed by several researchers [1][2][3][4][5].A large amount of information is available on the atmospheric corrosion of mild steel in the short and mid term. Information on long-term exposure (10-20 years) is considerably less abundant, and no consistent data is available for exposure times over 50 years [6].A great deal of effort has been dedicated to identifying corrosion products and quantifying the effect of corrosion in terms of mass loss, and a number of models have been developed to describe the influence of various environmental parameters on the corrosion rate; particularly the sulphur dioxide and chloride concentration [7]. An aspect that has been relatively little studied is the morphology of the corrosion products that grow on the steel surface in the form of thin or thick films.The present study concentrates on the morphology of corrosion product films formed on mild steel after long-term atmospheric exposure (13 years) in five Spanish atmospheres of different types: rural, urban, industrial, mild marine and severe marine.Characterisation was performed by scanning electron microscopy/energy dispersive Xray spectroscopy (SEM/EDS), X-ray diffraction (XRD), and extractable ion (SO 4 2-, Clanalysis. EXPERIMENTALMild steel test panels, ...

Extensive information on the atmospheric corrosion of weathering steel has been published in the scientific literature. The contribution of the present work is to provide a bibliographic review of the reported information, which mostly concerns the weathering steel ASTM A-242. This review addresses issues such as rust layer stabilisation times, steady-state steel corrosion rates, and situations where the use of unpainted weathering steel is feasible. It also analyses the effect of exposure conditions. Finally it approaches the important matter of predicting the long-term behaviour of weathering steel reviewing the different prediction models published in the literature.

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The atmospheric corrosion of carbon steel is an extensive topic that has been studied over the years by many researchers. However, until relatively recently, surprisingly little attention has been paid to the action of marine chlorides. Corrosion in coastal regions is a particularly relevant issue due the latter’s great importance to human society. About half of the world’s population lives in coastal regions and the industrialisation of developing countries tends to concentrate production plants close to the sea. Until the start of the 21st century, research on the basic mechanisms of rust formation in Cl−-rich atmospheres was limited to just a small number of studies. However, in recent years, scientific understanding of marine atmospheric corrosion has advanced greatly, and in the authors’ opinion a sufficient body of knowledge has been built up in published scientific papers to warrant an up-to-date review of the current state-of-the-art and to assess what issues still need to be addressed. That is the purpose of the present review. After a preliminary section devoted to basic concepts on atmospheric corrosion, the marine atmosphere, and experimentation on marine atmospheric corrosion, the paper addresses key aspects such as the most significant corrosion products, the characteristics of the rust layers formed, and the mechanisms of steel corrosion in marine atmospheres. Special attention is then paid to important matters such as coastal-industrial atmospheres and long-term behaviour of carbon steel exposed to marine atmospheres. The work ends with a section dedicated to issues pending, noting a series of questions in relation with which greater research efforts would seem to be necessary.

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The design of the first weathering steels was purely empirical and focused on a small number of conventional alloying elements such as manganese, silicon, chromium, nickel, copper and phosphorus, mainly. The environmental conditions that promote the formation of protective rust layers: existence of wet/dry cycling, absence of very long wetness times, atmospheres without a marine component, etc., were identified by trial and error. This paper makes a bibliographic review of the abundant literature that has been published on the atmospheric corrosion of weathering steels, setting out in chronological order the advances made in the scientific knowledge of important matters such as: atmospheric corrosion mechanisms of weathering steel, formation of protective rust layers, and the role played by alloying elements. The work ends with an overview of the scientific design of new weathering steels, placing special emphasis on the new compositions developed for application in marine atmospheres.

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This paper describes the peculiarities of carbon steel corrosion in very severe marine atmospheres and points out a number of uncertainties that still need to be explained, such as the following: (a) Data on the evolution of carbon steel corrosion (C) with exposure time (t) obeys the power function C ¼ At n . The variables upon which n depends are not fully known, though the salinity of the atmosphere undoubtedly plays a prominent role. (b) In marine atmospheres, the presence of akaganeite and magnetite phases among the corrosion products is especially significant. In relation with these corrosion products, a number of important questions remain unanswered: What conditions are necessary for their formation? Where are they preferentially located? Is there a critical atmospheric salinity concentration below they are unlikely to form? It is also of great interest to know the typical surface microscopic morphologies resulting from the presence of akaganeite, where the confusion among researchers is well known. (c) In very severe marine atmospheres, the morphology of the corrosion layers formed on steel can be highly unusual, such as sheet-type or mound rust. There is a lack of basic knowledge on the formation mechanisms and internal microstructure of these rust types.