Corrosion of Type 430 Stainless Steel: Effects of Mo-Pd and Other Addition Pairs

Abstract

Continuing previous work on the effects of single additions, anodic and cathodic polarization measurements were performed at room temperature in N H2SO4, with and without 0.5N NaCl, on Type 430 ferritic stainless steels containing addition pairs of Mo, V or W with each of Pd, Ge or Re. Weight loss corrosion tests were also carried out in the solutions used in the polarization measurements, in N HCl, in N FeCl3, and in H2SO4 over a range of temperatures and concentrations. Except in sulfuric acid, in which the Pd bearing alloys showed extremely good resistance, corrosion behavior of alloys containing the various addition pairs was generally inferior to that of the corresponding alloy containing Mo, V or W only. Of the Pd bearing alloys, the Mo-Pd combination showed the best corrosion behavior in H2SO4 at the lower acid concentrations. As indicated by potential measurements, in steels having the nominal additions 3% Mo-1/2% Pd and 2% Mo-1 % Pd, comparatively large regions of spontaneous passivity were observed at concentrations of 25% H2SO4 or less, and temperatures extending nearly to the boiling point. In N H2SO4, these Mo-Pd steels were shown to passivate even when 2 to 3% NaCl was added to the acid, thus showing much greater tolerance to chlorides than observed for a steel containing a nominal 1% Pd only. However, at higher chloride levels than mentioned above, i.e., in N HCl, the Mo-Pd and indeed all other Pd bearing alloys depassivated and showed extremely high corrosion rates. Some exploratory experiments on Pd free Type 430 steels showed that surface deposition of Pd, by treatment in dilute PdCl2, produced steels which passivated spontaneously when subsequently exposed to N H2SO4.