Studies on the Corrosion of Pure Metals due to V₂O₅ Melts

Abstract

It is known that the corrosion rate of metals is accelerated in the oxidative corrosion due to V₂O₅ melts. Although a relatively large number of papers dealing with oil-ash corrosion of commercial heat resistant alloys have been published, little progress has been made to elucidate the mechanism of corrosion, because of the complexity of factors encountered in the corrosive environment. Here, a simple corrosion model was proposed on the basis of the assumption that both growth and dissolution of the corrosion layer at the V₂O₅/oxide interface occur simultaneously. According to the model, the equations on the rate of metal recession were derived. These equations were applied to the corrosion of Fe, Cr and Ni, and the rate of metal recession estimated was in good agreement with experimental data. The parabolic growth rate constant of the corrosion layer (k_d) and the dissolution rate constant (k_s) were calculated from another equation derived from the same model, which relates the thickness of corrosion layer to k_d and k_s. The results obtained were as follows:
(1) The k_s is dependent on the species of metals. The k_s of iron is twice as large as that of chromium or nickel.
(2) The k_d of the corrosion layer on chromium or nickel is much larger than that of Cr₂O₃ or NiO on the surface of the metals in the atmosphere. The k_d on chromium or nickel is about one-tenth compared to that on iron which is 5×10⁻¹³ m²/s.
As a result, the weight loss of chromium or nickel due to V₂O₅ melts becomes one-fifth compared to that of iron.