The inhibition potency of expired thiamine or vitamin B1 (VB1) and riboflavin or vitamin B2 (VB2) against
SABIC iron corrosion in 0.5 M H2SO4 solutions was investigated using chemical and electrochemical
techniques. Theoretical studies such as DFT and MC simulations were performed on both VB1 and VB2
inhibitors to obtain information related to the experimental results. It has been found that the inhibition
efficacy assigned from all measurements used increases with increasing concentration of the two
expired vitamins and reduces at elevated temperatures. It reached 91.14% and 92.40% at 250 ppm of VB1
and VB2, respectively. The inhibition was explicated by the adsorption of the complex formed between
expired vitamins and ferrous ions on the SABIC iron surface. The adsorption was found to obey the
Langmuir isotherm model. Galvanostatic polarization demonstrated that the two expired vitamins act as
an inhibitor of the mixed type. These expired vitamins have proven effective in inhibiting the pitting
corrosion induced by the presence of Cl
ions. The pitting potential is transferred to the positive values
showing resistance to pitting damage. The theoretical parameter values are consistent with experimental
results.