DISSOLUTION RATES OF IRON AND CHROMIUM AND FE-CR-ALLOYS IN THE PASSIVE STATE

Samples of iron, chromium and FeCralloys were used to determine the dissolution rate in 1 N H2SO4 by Atomic Absorption Spectroscopy (AAS). Special care was taken in reducing the background levels and, therefore, rates could be determined corresponding to 2·10-8 A/cm2. The measured passive current densities in the steady state are in very good agreement with calculated values using the results of the AAS analysis and a valence of 3+ for the chromium and iron ions. The measured fraction of chromium [Cr/(Cr+Fe)] in the electrolyte is equivalent to the chromium fraction in the alloy. The chromium dissolution rate is about the same ((2.4 ± 0.6) · 10-12 g cm-2 s-1) for pure chromium and the alloys with contents as low as 1 at.%. This is considered to be direct evidence that the outermost cation layer of the passive film contains mostly chromium. Before the attainment of steady state the fraction of chromium in the electrolyte is smaller than in the alloy leading to an enrichment of chromium in the passive film. After reaching the steady state the dissolution rates were also measured during galvanostatic transients, where the currents were increased by five times the steady state values. The calculated c.d. using the AAS results were smaller than the total c.d., where the difference is assumed to be used for film formation. The ratio of the c.d. for dissolution and film formation decreases steadily for FeCr alloys by about one order of magnitude from pure iron to pure chromium. © 1990.