Dichromate Reduction Rate at a Steel Surface in Air-Free, Acetic Acid Solution

Abstract

A system was devised in which aqueous solutions of K2Cr2O7 and of acetic acid could be brought in contact with steel coupons in the absence of air. The rate of reduction of the dichromate was determined as a function of time by titration and spectrophotometrically. Changes in pH were also followed. Rates were measured with various initial concentrations of dichromate and of acetic acid and for several areas of steel surface. The effects of temperature, presence of normalNaCl , presence of oxygen, and order of addition of the solutions were studied. There was measurable loss of dichromate in all cases except those in which air was deliberately added. In general, the rate curves became level before all of the dichromate was reduced, with the maximum loss about 60 per cent. It was concluded that dichromate at 400 ppm was a fairly effective inhibitor to the corrosion of steel down to pH 3.0 in the absence of oxygen even with 1000 ppm normalNaCl present. A mechanism for the reaction and of the corrosion inhibition is proposed.