Corrosion of some stainless and duplex stainless steel alloys in phosphoric acid plants

Author(s): Sh.A.Abdel-Kader, O.E.Abdel-Salam, S.A.Foad

Corrosion behavior of austenitic 316L, 904L alloys and a new type of super duplex stainless steel alloy, namely, Ferralium255 SD50 was studied in aqueous phosphoric acid solution and its mixtures with sulfuric acid and chlorides at different concentrations and temperatures using electrochemical techniques and surface morphology technique, namely, scanning electron microscopy (SEM). Results showed that the corrosion rate of these alloys increases with the increase of temperature, Ferralium alloy which contains chromium,molybdenumand nitrogen in sufficient quantities exhibited lower corrosion rates than the other alloys at higher temperatures, whereas 904L loses its stability at higher temperature. The alloy 316L had the lowest corrosion resistance at all temperatures and concentrations. In addition, phosphoric acid showed lower corrosive action than sulfuric acid at all studied temperatures. Mixing sulfuric acid with phosphoric acid induced an acceleration of corrosion reaction of 316L. On the other hand this addition decreased the corrosion rates of 904L and Ferralium alloys at all tested temperatures and formed a stable passive layer of corrosion product. Pitting tests in the presence of chlorides confirmed that Ferraliumalloy had the best performance with highest pitting potential, highest stable passivity, and highest repassivation ability followed by 904L and 316L. In erosion-corrosion tests, experimental set up allowed to followthe electrochemical behavior of tested samples. Under simulated conditions as inAbo-Zabaal plant for phosphate fertilizers, also Ferralium was superior to the others alloys. Surface morphology tests confirmed the results obtained by electrochemical methods.

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