Corrosion of Stainless Steels in Boiling Concentrated Nitric Acid Containing Chromium Ions

Abstract

As a result of investigations of the mechanism through which the corrosion of stainless steels in boiling concentrated HNO3 could be accelerated the following facts were made clear.(1) The oxidizing power in terms of steady potential of a platinized platinum electrode attains its maximum at the azeotropic concentration. In case of addition of Cr6+, the power increases with the logarithmic amount added, indicating a positive slope of about 16mV which corresponds to the oxidation number 3, while in case of addition of Cr3+ a negative slope of about 33mV which corresponds to the number 1.5 is observed, the latter reason may be ascribed to partial oxidation of Cr3+ to Cr6+. Furthermore, increment or decrement of the power is dependent on the Langmuir adsorption type curve.(2) The corrosion of none-sensitized austenitic stainless steels can be accelerated either by addition of tri- or hexa-valent ions, the influence of which becomes maximum in the azeotropic acid, where the logarithmic corrosion rate is in direct proportion to the power. The cause by which the addition of Cr3+ can accelerate corrosion may be sought in the easer oxidation on the steel surfaces.(3) When Cr6+ is added, the higher the chromium content in the steels the higher becomes the corrosion rate, but when Cr6+ is not added the inverse result can be observed, i. e. the former corrosion may be in the transpassive region while the latter corrosion in the passive region. Nickel and molybdenum alloyed in the traditional amounts may, if anything, be taken as increasing the susceptibility to accelerating effect of Cr6+.(4) In the addition of Cr6+ corrosion of 18-9 and18-11-Mo type steels increases as the carbon content is decreased, whilst in no addition the state of affairs is inversed.As a rule, all above results may be influence some important factors as rate of escaping NO2 as well as the test duration, and so on.