Electrochemical and Surface-Analytical Investigation of Sodium Gluconate–Mn2+ Synergism for Carbon-Steel Corrosion Control

Abstract

Groundwater serves as an important source of water for industrial
applications; however, its dissolved oxygen and salts can accelerate the
corrosion of carbon steel. This study investigates the synergistic corrosion
inhibition effect of sodium gluconate (SG) combined with Mn²⁺ ions on carbon
steel immersed in groundwater collected from Tirunelveli, Tamil Nadu, India.
The inhibition efficiency and protective film characteristics were assessed using
weight-loss measurements, potentiodynamic polarization, electrochemical
impedance spectroscopy (EIS), Fourier-transform infrared spectroscopy
(FTIR), and surface microscopy. The groundwater exhibited a pH of 8.0, a total
dissolved solids concentration of 810 mg L⁻¹, and chloride and sulphate
concentrations of 15 and 17 mg L⁻¹, respectively. According to the original
experimental dataset, the SG–Mn²⁺ formulation demonstrated pronounced
synergistic inhibition, achieving a maximum inhibition efficiency of 97.56% at
concentrations of 200 ppm SG and 50 ppm Mn²⁺ after three days of immersion.
Under these conditions, the charge-transfer resistance increased from 5.12 to
298.345 Ω cm², whereas the double-layer capacitance decreased from 1.7 × 10-2
to 1.3 × 10-5 F cm⁻², indicating the formation of a more resistive protective
layer on the metal surface. FTIR analysis suggested the formation of an Fe²⁺–
gluconate surface complex in association with Mn (OH)₂. These findings are
considered in the context of recent advances in sustainable corrosion protection
and synergistic inhibitor formulations for carbon steel in aqueous
environments. Overall, the SG–Mn²⁺ combination shows promise as a
comparatively low-dose corrosion inhibitor and warrants further investigation
for potential applications in groundwater systems and industrial cooling-water
environments.