High temperature corrosion of austenitic TP347H steels with two grain sizes was studied at 600-750 8C in dry air. A relationship between oxidation rate and grain size was found. The surface oxides consisted of iron enriched (Fe,Cr) 2 O 3 nodules and a (Cr,Mn) 2 O 3 scale. Initial oxidation with both grain sizes followed cubic kinetics, a combination of fast oxide formation via oxide grain boundary diffusion at the beginning of oxidation and parabolic kinetics combined with oxygen dissolution to the steel through breakaway oxidation nodules. Above 650 8C, the breakaway oxidation became more severe in coarse grained steel and fine grained steel started to destroy the nodules. Better oxidation resistance was achieved with smaller grain size, especially at higher temperatures. Reducing the TP347H steel grain size lowers the critical chromium concentration needed for the formation and recovering of the protective oxide scale.

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Flue dust produced by copper flash smelting causes problems by forming corrosive melts on the walls of heat recovery boiler used to recover the process heat value and to separate flue dust from SO 2 -rich gas, derived from smelting sulphidic copper ore, before manufacture of sulphuric acid. The corrosion phenomena of molybdenum containing low and high nickel stainless steels AISI 316 and Sanicro 28 were studied by simulating the conditions prevailing in the heat recovery boiler in laboratory in the temperature range of 250-350°C. ZnCl 2 in copper smelter flue dust resulted in partial melting of the deposit containing sulphates and oxides of copper, zinc, arsenic, iron and lead, which increased the rate of corrosion dramatically. Chlorination of the steel was the dominant corrosion mechanism. The high nickel steel corroded more than low nickel steel due to reactivity of nickel under molten sulphates.

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