Green Corrosion Inhibitors for Mild Steel in Sulphuric Acid: Performance Evaluation of PEG (Polyethylene Glycol) and Alkali-Treated Starch Blends

Abstract

The corrosion inhibition performance of native starch (NS), alkaline-modified starch (MS), polyethylene glycol (PEG), and their blends on mild steel in acidic medium was evaluated using weight loss measurements over 120 h. Single inhibitors exhibited moderate inhibition efficiency at 24 h, with PEG showing the highest initial performance, but all single systems experienced a decline in efficiency with increasing exposure time due to desorption. Adsorption behavior at 24 h followed the Langmuir isotherm with R² > 0.99 for all inhibitors, indicating monolayer adsorption. Calculated adsorption equilibrium constants followed the trend NS > MS > PEG, while standard free energy values ranged from −19.6 to −20.8 kJ/mol, suggesting mixed physisorption and chemisorption mechanisms. Synergy analysis revealed that most blends exhibited antagonistic behavior at 24 h due to competitive adsorption. However, the 0.1 g/L MS + 0.4 g/L PEG blend developed strong synergy over time, with a synergy parameter of 1.64 at 120 h and inhibition efficiency increasing from 46.4% to 89.5%. This indicates that MS pre-adsorbs on the steel surface and facilitates the formation of a stable PEG protective film, leading to sustained corrosion inhibition. The results demonstrate that strategic blending of inhibitors with complementary adsorption properties can provide effective long-term corrosion protection in acidic environments.