Casein as a natural protein to inhibit the corrosion of mild steel in HCl solution

In this research, the effects of casein on the corrosion behavior of mild steel in 0.1 M HCl were investigated by different analytical techniques. According to weight loss measurements, an increase in the concentration of casein from 50 ppm to 400 ppm resulted in a decrease in the corrosion rate of mild steel, for example, from 34.3 mpy to 3.9 mpy as compared to 98.1 mpy calculated for the casein-free solution at 298 K. However, increasing the temperature from 298 K to 343 K had adverse effects on the surface coverage, and the corrosion inhibition efficiency of casein. Based on thermodynamic calculations, in the presence of 400 ppm casein the corrosion activation energy and the enthalpy of activation increased from 8.61 kJ.mol(-1) to 45.89 kJ.mol(-1) and from 5.95 kJ.mol(-)(1) to 43.23 kJ.mol(-1), respectively. Potentiodynamic polarization experiments revealed a considerable decrease in the corrosion current density when casein was dissolved in the corrosive solution. This is in accordance with the observed increase in the charge transfer resistance from 121.4 Omega.cm(2 )to 917.9 Omega.cm(2) measured by EIS. ATR-FTIR and EDS analyses confirmed the surface adsorption of casein on mild steel. AFM also illustrated an improvement in the surface properties of the samples in the presence of casein. (C) 2018 Elsevier B.V. All rights reserved.