Determination of Corrosion and Inhibition of Mild Steel in HNO 3 by the Thermometric (Mylius) Technique

Abstract

The thermometric (Mylius) technique has been found to be suitable for determining the action of nitric acid on steel and the effectiveness of inhibitors. This technique may be usefully applied in controlling pickling and acid-cleaning baths by determining the threshold concentrations of inhibitors and checking their depletion during service.

The increase in the rate of attack on steel by HNO₃ with increase in concentration is shown by both weight-loss measurements and the value of the reaction number (R.N.) derived from the thermometric measurements. In HNO₃ at concentrations producing passivity no temperature rise occurs. Lower RN values and, accordingly, lower corrosion rates are obtained in the presence of thiourea, KI, NaCl, and HCl. The presence of an accelerator (NaNO₂) produces higher RN values. Thiourea is able to arrest corrosion occurring in uninhibited HNO₃. Progressive additions of an inhibitor and the recording of the accompanying temperature rise enable the threshold concentration of an inhibitor to be determined.