Exopolysaccharides from Rhizobium pusense as bioinhibitor on mild steel corrosion in 0.5 N sulphuric acid medium exploration of biofilm, adsorption, mechanistic insights

Extracellular polymeric substances (EPS) are complex mixtures of saccharides that contribute to the structure of biofilm environments via the formation of microbial aggregates. The biofilm-forming ability of the Rhizobium pusense (R.P) culture on corrosion inhibition of mild steel (MS) in 0.5 N sulphuric acid (H₂SO₄) was studied using spectral, analytical, and electrochemical techniques. EPS-biofilm exhibits the highest percentage of MS corrosion inhibition efficiency of 89.07% in 2.5 mL of inhibitor concentration and the deviations of inhibition efficiency while changing the temperatures were studied. The obtained IE, % value (89.07%) at 298 K in 0.5 N H₂SO₄ containing 2.5 mL concentration of R.P is accordance with the inhibition efficiency values (IE= 85.86% and 89.25%) obtained for the same concentration by electrochemical impedance (EIS) and polarizations studies. Tafel polarization study reveals that the R.P exhibits a mixed type of inhibition against MS in 0.5 N H₂SO₄. The R.P adsorption on the MS surface follows physisorption and obeys the Langmuir isotherm model. Surface analysis (FE-SEM, AFM), UV–visible, FTIR, and GC-MS studies confirmed the interaction between the inhibitor and MS surface and supports the biofilm formation in this study. Based on the experimental data, a suitable electrostatic attraction-based adsorption mechanism between R.P and active mild steel surface in 0.5 N H₂SO₄ was discussed.