Self-formation of the protective film on mild steel surface in hydrochloric acid medium containing Papaya resin extract

Plant extracts have been considered to be an effective corrosion inhibitor due to their high inhibition performance, huge amount, regeneration, and nature, as well as biological sources. However, several steps in the plant extract processes and extracted solvents are still questionable. Therefore, Papaya resin - water extract (PRE) was recommended, produced, and characterized as a mixed-type inhibitor for corrosion of mild steel in 0.1M hydrochloric acid medium. The results indicated that the 2000 ppm of PRE inhibitor has significant effects on the inhibition efficiency, reaching a remarkable value of 98.08% for mild steel in the investigated solution. The potentiodynamic polarization results showed that the PRE inhibitor acts as a mixed-type inhibitor with a dominant anodic inhibitor for mild steel. Moreover, electrochemical impedance spectroscopy and linear polarization resistance results demonstrated that PRE molecules can form protective film, and increase charge transfer and polarization resistances, suggesting the improved steel corrosion resistances. The scanning electron microscope results indicated less occurrence of corrosion on the steel surface was exposed to the inhibitor system. Meanwhile, X-ray photoelectron spectroscopy results demonstrated that PRE’s molecules were absorbed and created reactions with iron products to form a protective film on the steel surface. Furthermore, density functional theory (DFT) and molecular dynamics simulation were also used to calculate, and the results suggested that the conjugation effect of two double bonds in PRE’s molecules creates an electron-withdrawing effect on the reactive space around. Moreover, the results also revealed the good adhesion and thermal stability of the absorbent protective layer, indicating an agreement with experimental characteristics. Therefore, this result suggests a contemporary design for developing the self-corrosion inhibitor based on natural products with less steps in the synthetic process.