Three low alloyed carbon steels were given different heat treatment before testing in a corrosion flow loop at 80°C. Both the general corrosion rate and the susceptibility to form localized attack were significantly affected by the tempering temperature and the size and distribution of the
carbides. Two of the steels contained small amount of chromium and it was seen that the presence of chromium improved the persistency of the corrosion film for all the heat treated samples. Keywords: Sweet corrosion, carbon steel, microstructure, protective film formation.