Thermometric,gravimetric,and potentiometric study of corrosion of iron under conditions of reaction Fe + 2Fe 3+ = 3Fe 2+

Abstract

The corrosion of iron in slightly acid solutions under conditions of the reaction Fe + 2Fe³⁺ = 3Fe²⁺ was studied by the Mylius thermometric technique. The temperature rises steadily with time to reach a maximum and then decreases slowly. The way in which δT and the reaction number vary with the Fe(III) ion content depends on the type of ferric salt used. The results allow distinction between partially retarded dissolution in FeCl₃, autocatalytic dissolution in Fe(NO₃)₃, and dissolution leading to passivation in Fe₂(SO₄)₃, Weight loss determination under thermometric and isothermal conditions and corrosion potential measurements give independent support to this conclusion. The effect of the concentration of gelatin, Stannine LTP, and Armohib on the rate of dissolution of iron in 0·1M HCl free from and containing increasing amounts of FeCl₃ was investigated by thermometric and gravimetric techniques. The three additives act as strong corrosion inhibitors in pure acid solutions but exert low inhibition in the presence of Fe(III). The difference in behaviour is related to the kinetics of the two partial cathodic reactions. Comparison is made between the integral heat evolved during a thermometric experiment and that calculated from the dissolution of iron in chloride, nitrate, and sulphate solutions. Agreement between these values is within ±2%, which strongly recommends the Mylius thermometric method as an accurate, reliable, and simple technique for corrosion assessment.