LABORATORY STUDIES OF THE CORROSION OF NICKEL - BASED ALLOYS BY PURE CALCIUM SULPHATE - INFLUENCE OF ENVIRONMENTAL FACTORS

摘要

Several nickel - based alloys and, for comparison purposes, iron - and cobalt - based alloys have been coated with 8 -10μm Analar calcium sulphate and exposed in three types of external environments at a temperature of 850℃. Results from experiments in which the coated coupons were totally immersed in lightly compacted Analar - grade calcium sulphate powder and exposed in air for 144 hours showed that they underwent sulphidation - oxidation corrosion. Control experiments with no coating resulted in the formation of oxides alone, confirming that the presence of a compact calcium sulphate overlay was crucial to the development of the conditions necessary to cause sulphidation - oxidation corrosion. Relative extents of corrosion for the alloys showed distinct similarities with those observed in pilot scale coal - fired fluidised bed combustion systems. This fact together with the observation that corrosion morphologies for the alloys tested were similar to those observed in operating systems, demonstrates that the laboratory test method developed in this work is a suitable screening test for alloys with suitable mechanical properties for this industrial application. Subsequent sets of experiments at the same temperature, in which coated coupon surfaces were exposed to a synthetic flue gas containing 0.2% by volume sulphur dioxide for 144 hours, gave results which showed that sulphur exhaustion from the coating did not occur in the air environment. Thus, corrosion morphologies and extents of corrosion for the two different environments were effectively the same. The effect of substoichiometric conditions on the corrosion of the coated alloys was investigated by immersing the coupons in a calcium sulphate covered with finely divided carbon to give a sulphate to carbon mole ratio of 1:2. After 24 hours exposure at 850C, significant corrosion of the nickel - based alloys occurred. The greater degree of corrosive attack can be attributed to the high partial pressures of sulphur vapour which arises from the reaction between calcium sulphate and carbon.