Atmospheric corrosion of steel by the exposure test avoiding rain

Weathering steel has been widely used in many structures for its good corrosion resistance in unpainted usage. The reason is that a dense and tight rust layer is formed on the surface of weathering steel during a long service life. It is, however, known that such a protective rust layer can not be created in the case of usage at the seaside area. Especially, corrosion of the inside of the structure was more severe than of the outside. It is estimated that the difference is caused by the washing effect of rain. Sheltered exposure stands have, therefore, been utilized to clarify the effects of cain. Mild steel and weathering steel samples were exposed at the six weathering sites having various sea salt amounts in the atmosphere. The results of five years of exposure tests show that the amounts of corrosion were higher in the case of sheltered exposure tests than for ordinary exposure tests. Furthermore, the difference of the amounts of corrosion in the two types of exposure tests increased, as the amounts of chloride in the atmosphere increased. The samples corroded in the sheltered exposure tests accumulated more chloride ions than those in the ordinary exposure tests. The amounts of chloride ions on the samples at the sheltered exposure tests were accumulated without the effect of washing by rain. This accumulation of sea salt accelerates the corrosion rates in both mild steel and weathering steel. It is, therefore, realized that the portion of the seaside structure where the washing effect is not formed by rain is an extremely severe corrosion environment. Furthermore, it is also found that the modified weathering steel has better corrosion resistance in such a severe environment.