Under suitable conditions the sulphur oxides carried by fl ue gas can condense, as sulphuric acid, onto the surrounding surfaces. In fl ues and chimneys, where condensation of sulphuric acid occurs sporadically, weathering steels have been found to stand up very well to chemical stress. They have attained, therefore, a strong position as a material for steel chimneys and central heating furnaces. Several research projects have shown that weathering steels have higher resistance to fl ue gases compared to general structural steels and, in some cases, even to stainless steels. The condition of specimens with a painted and scratched surface after a two-year exposure to a corrosive environment. It can be seen that undercutting of the paint layer has not proceeded in the CORTEN A specimen as it has in the ordinary steel specimen. COR-TEN A GENERAL STRUCTURAL STEEL The special alloying formula (with Cr, Ni, Cu) of weathering steel improves its corrosion resistance under sulphuric acid dew point conditions and its resistance to scaling at high temperatures up to 600 – 650°C.