Corrosion behaviour of austenitic and duplex stainless steels in an industrial strongly acidic solution

AISI 316L metallic components of an industrial plant, producing tartaric acid as by-product of grape pressing, were subjected to a so severe corrosive attack to promote the search of alternative stainless steels for this specific application. The aggressive industrial solution is strongly acidic and it contains large amounts of chlorides and sulphates and it operates in the temperature range of 25–60 °C. As possible alternative materials to AISI 316L, two super austenitic stainless steels, AISI 904L and SanicroTM 28, and three duplex stainless steels, SAF 2707 HDTM, SAF 2507TM and SAF 2205TM, were tested. The steels were characterised by means of chemical analysis, metallographic observations and anodic polarisations in a tartaric acid saturated solution, both in presence and in absence of chlorides and sulphates. The AISI 316L alloy showed the presence of ferrite δ at the austenitic grain boundaries, which is responsible for a selective attack and severe damage of this material. Among the tested steels, as alternative to AISI 316L, Sanicro 28TM and AISI 904L can be considered promising, with a preference for Sanicro 28TM. SAF 2707 HDTM showed a passive behaviour only with a mirror surface finishing, which is not acceptable for service conditions.