Temperature Coefficient of Corrosion Inhibition^*

The inhibition of steel corrosion in mineral acids by simple amines is shown to follow the Langmuir adsorption isotherm over a wide concentration range (10^–5 to 10^–3 M). From a comparison of activation energies of uninhibited and inhibited corrosion reactions, heats of adsorption of amines may be obtained. These fall in the range 5,000-10,000 calories/mole and combine with the activation energy of the uninhibited reaction in the inhibited rate equation to give a negative temperature coefficient of inhibition (i.e., a higher percent protection at lower temperatures). However, at the very highest inhibitor concentrations, this temperature coefficient becomes positive, which may be explained by assuming that che corrosion reaction at very high coverages is no longer the simple metal/acid reaction, but involves the adsorbed species directly.