Potentialabhängigkeit der Korrosion Mo-freier und Mo-haltiger Stähle in Calciumnitrat-Lösung und Natronlauge

Effect of potential on corrosion of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide The effect of potential on the corrosion behaviour of three low alloy steels with different carbon and molybdenum contents (0.08 C, 0.01 Mo; 0.08 C, 0.98 Mo; 0.18 C, 1.10 Mo) was investigated in boiling 60 wt.% Ca(NO₃)₂ solution (DIN 50 915) and in boiling concentrated NaOH solutions (20 and 35 wt.% NaOH) by potentiodynamic and chronopotentiostatic polarization measurements (i/E curves) and chronopotentiostatic mass loss measurements (corrosion rate v vs. potential E curves). In Ca(NO₃)₂ solution, i/E measurements give no information about the effect of potential on the anodic dissolution. For the materials investigated, v/E measurements indicate the existence of potential ranges with pronounced differences of the corrosion response. It can be differentiated between active, passive, and transpassive ranges, and also a potential range of secondary passivity was established. Transpassivity and secondary passivity are markedly pronounced with the molybdenum bearing steels but not with the steel free from molybdenum. There are no hints to the occurrence of intergranular attack in the specimens which are free from of internal and external mechanical stresses, whereas such hints could be derived from the shape of the i/E curves. Nevertheless, under mechanical stresses (constant load, CERT conditions) the materials are susceptible to intergranular SCC. The conception that intergranular SCC of low alloy steels in Ca(NO₃)₂ solution is connected with a break-through potential of grain boundary corrosion and hence is to be interpreted as an intergranular attack which, under mechanical stresses, runs in a modified form as SCC with intergranular crack path, cannot be maintained in such general terms. A pronounced active/passive behaviour is observed in NaOH. The effect of potential on anodic metal dissolution which is derived from v/E curve is also established by i/E measurements. For molybdenum bearing steels, the active potential range is somewhat extended to more positive potentials. Manifestations of localized attack, e.g., intergranular corrosion, do not occur. From the investigations, no hints to the cause of the deterioration of the resistance to intergranular SCC in caustic solutions by molybdenum can be derived.