Iron dissolution in acid water-methanol mixtures

Dissolution rates of Armco iron in de-aereated and aereated acid (10-3 M H2SO4) water-methanol mixtures have been evaluated by accurate weight loss measurements, by extrapolation of Tafel lines to the corrosion potential and by using the linear polarization method. The corrosion rate was found to decrease and then to increase at increasing molar fraction of methanol both in aereated and aereated solutions, showing a minimum at an intermediate solvent composition which is significantly different from the values observed in pure solvents. Furthermore the corrosion rate in de-aereated acid pure methanol is greater than that in pure water by a factor of nearly two. The experimental results have been plotted in terms of rs rw ratio versus methanol mole fraction, where rs is the corrosion rate in the mixture and rw that in pure water. A behaviour was found quite similar to that exhibited by the ratio of exchange current density for the hydrogen evolution reaction. This similarity leads to a theoretical expression for the ratio rs rw as well as to a physical explanation for the greatest iron dissolution rate in de-aereated acid methanol solution compared to that in pure water which is in satisfactory agreement with experimental results. © 1987.