CORROSION INHIBITION BEHAVIOUR OF CALF THYMUS GLAND DNA ON MILD STEEL IN SULPHAMIC ACID

Abstract

Inorganic acid cleaners based on sulphamic acid are frequently employed in industrial equipment cleaning, descaling and acidizing. This application of sulphamic acid in industrial cleaning is not entirely without its drawback as the cleaning action usually leads to dissolution and loss of base metals. The addition of corrosion inhibitors is one of the industrial practices employed to minimize equipment corrosion damage. The purpose of this work was to assess the viability of deoxyribonucleic acid (DNA), extracted from calf thymus gland, as an inhibitor for mild steel corrosion in sulphamic acid medium, and in comparison, with salmon Fish DNA and INDION 5489, a commercial inhibitor. The inhibition process was investigated using weight loss, potentiodynamic polarisation, SEM/EDX and FTIR measurements. Response surface method (RSM) and artificial neural network (ANN) were employed to determine the optimum corrosion inhibition conditions. The weight loss measurements obtained the highest inhibition efficiency of 82.71% at 303 K and immersion time of 6 h by addition of 2.5 mg/L of calf thymus DNA, CTGDNA. The corrosion rate was also observed to decrease with an increase in inhibitor concentration. Potentiodynamic polarisation curves showed a shift in Ecorr < 85 mV an indication that CTGDNA is a mixed inhibitor, suppressing both cathodic and anodic reactions. An RSM generated polynomial model obtained an optimum efficiency of 72% at 303 K, 5.5 mg/L after 2.12 h immersion. Estimation by ANN, with minimal errors, and a higher R2 of 0.983 in comparison to 0.925 for RSM were close to the experimental inhibition efficiency. CTGDNA adsorption on mild steel modelled the Langmuir isotherm with a linear regression coefficient of 0.99. The increase in the activation energy from 37.54 kJ/mol to 52.5 kJ/mol after 2 h immersion; with a similar trend for 4 and 6 h demonstrated that addition of CTGDNA favoured physioisorption. The small and negative value of entropy was an indication that the adsorption of CTGDNA was spontaneous. FTIR confirmed the presence of protective film formed by CTGDNA inhibitor on the mild steel surface at various concentration. SEM images showed reduction in the degradation of mild steel surface in the uninhibited solution after addition of CTGDNA. The comparative studies obtained a weight loss of 0.0036, 0.0047, 0.0072 and 0.0086 mg in 10% sulphamic acid in the presence of CTGDNA inhibitor, salmon fish DNA, conventional cleaning solution and blank solution of 10% sulphamic acid without an inhibitor, respectively. This confirmed that the CTGDNA inhibitor enhanced the 10% sulphamic acid cleaning solution as a suitable and viable cleaning agent for mild steel in comparison with INDION 5489.