Der Einfluß von Schwefeldioxid,Schwefelwasserstoff und Kohlenmonoxid auf die Lochkorrosion von austenitischen Chrom-Nickel-Stählen mit bis zu 4 Massen-% Molybdän in 1 M Natriumchlorid-Lösung

The Influence of SO₂, H₂S and CO on Pitting Corrosion of Austenitic Chromium-Nickel Stainless Steels with up to 4 wt. % Molybdenum in 1 M NaCl Active corrosion of chromium-nickel stainless steel X 5 CrNi 189 (AISI 304) in H₂SO₄ is stimulated by H₂S as well as by SO₂ (extension of the potential range of active corrosion, increase of the maximum corrosion rate in the active state and of the passivation current density), but is inhibited by CO (decrease of both maximum active corrosion rate and passivation current density). It is investigated whether likewise stimulating and inhibiting effects are valid also in case of pitting corrosion of austenitic stainless steels with molybdenum contents ranging from about zero (material no. 1.4301) to 4 wt. % (material no. 1.4449), tested in 1 M NaCl (ambient temperature) saturated with the gases mentioned above. The pitting corrosion behaviour of the materials investigated is judged by their pitting potentials measured by potentiostatically controlled experiments (testing time 24 hrs). The pitting potentials are compared with those measured in 1 M NaCl, N₂-bubbled. Pitting corrosion is stimulated by SO₂, CO and H₂S, with the stimulating efficacy increasing in the sequence given before. No stimulation is found only in 1 M NaCl, SO₂-bubbled,-with the highest Mocontent. In all other cases, stimulation of pitting corrosion increases with increasing Mo-content of the stainless steels. In coarse approximation, the critical limiting potentials of stable pitting in 1 M NaCl, bubbled with H₂S, SO₂ CO, correspond to the critical potentials of repassivating pitting corrosion found in N₂-bubbled 1 M NaCl. The chemical reactions and reaction products of SO₂ in aqueous solution are discussed. The nature of the stimulating component of the corrosive medium is not quite clear. The stimulating effects of SO₂ or one of its reaction products and of H₂S on pitting are in keeping with their stimulating effect on active corrosion of the steels investigated. The stimulating effect of CO, however, is in contradiction to the results expected and cannot be explained.