Corrosion Resistance of Chromium
Corrosion Resistance of Chromium
Effects of Additions of Platinum Metals
The addition of small percentages of platinum, palladium or other noble metal to readily passivated metals such as titanium and stainless steel has been shown to be capable of greatly improving their corrosion resistance towards non-oxidising acids. A series of papers by Tomashov and coworkers on stainless steel and one by Stern and co-workers on titanium were reviewed and summarised in this journal (1, 2). Now Greene, Bishop and Stern (3), in a paper presented to the Detroit meeting of the Electrochemical Society, have reported the extension of the work to chromium, a metal in which considerably greater interest is being taken than was formerly the case.
Noble metal additions appear to have two main actions: first, by providing points of low hydrogen overpotential they induce a high anode current density at a high potential over the bulk of the alloy surface, so leading to passivation, and secondly, by somewhat blocking the main part of the surface they reduce the overall anode current density required for passivation.
For chromium, Greene and his colleagues obtain the data summarised in Tables I and II. It may be seen that the corrosion resistance in hot sulphuric and hydrochloric acids is markedly increased by noble metal additions. The authors suggest that noble metals accumulate on the surface during dissolution (whether by remaining undissolved or by being re-precipitated), so leading generally to both the effects described above, just as in the previously demonstrated improvement of titanium and stainless steel. In chromium- gold alloy the first effect predominates; in chromium-iridium the second effect.
Chromium, however, exhibits transpassivity, dissolution to CrVI at very high anode potentials. Consequently its corrosion rate in strongly oxidising nitric acid is increased by noble metal additions, except by those of palladium, osmium and rhodium; palladium and osmium are soluble in nitric acid and thus do not accumulate on the surface, while rhodium, although it is not soluble and accumulates, may be a poor catalyst for the cathodic reduction of nitric acid.
Greene and his colleagues also point out that chromium can be passivated even in hydrochloric acid and shows no evidence of any tendency to localised attack when passivity breaks down: these results have much interest apart from the general theme of the paper.
The results generally confirm and extend the theory of Tomashov and of Stern on the mechanism of the action of platinum metal additions. Experimentally, Tomashov has recently reported (4) full confirmation of the Stern school’s findings concerning titanium. A further publication by Greene and his colleagues on the effect of noble metal additions on high chromium alloys is promised. All in all, the principle of noble metal additions for increasing the acid resistance of easily passivated metals appears to be now very well established and worthy of extended practical trial.
T. P. Hoar Platinum Metals Rev., 1958, 2, 117
T. P. Hoar Platinum Metals Rev., 1960, 4, 59
N. D. Greene,, C. R. Bishop and M. Stern Paper presented to Electrochem. Soc., Detroit, October 1961
D. Tomashov Lecture to Gordon Conference on Corrosion, August 1960