Zur Lochkorrosionsneigung von austenitischen CrNiMo-Stählen in konzentrierten Ammoniumrhodanidlösungen

Pitting corrosion of austenitic CrNiMo-steels in concentrated ammoniumrhodanide solutions Quasipotentiostatic and potentiokinetic polarisation measurements at various 18 Cr-10 Ni steels with molybdenum contents up to 4,3% were performed in 25 and 45% ammoniumrhodanide solutions. It was found that pitting corrosion is caused by incomplete passivation in the potential range of –300 to +250 m V _H. At these potentials the formation of stable passive layers is hindered by the formation and local oxidative dissolution of sulfidic layers. Above +250 m V _H rhodanide ions act in these weak acidic ammoniumrhodanide solutions as agents which destroy passive layers, comparable with chloride ions. The limiting potentials for stable pitting corrosion, obtained from potentiostatic experiments, are shifted from –300 to –150 m V _H with increasing molybdenum content of the steel. The least tendency of pitting corrosion was found for that steel with the highest molybdenum content.