Optimisation of the corrosion inhibition performance of isatin 4-ethyl-3-thiosemicarbazone for mild steel in sulfuric acid medium using response surface methodology

Sulfuric acid (H 2 SO 4 ) has been widely used in the industrial sector to eliminate rust and undesirable scales from mild steel. However, the aggressive nature of the H 2 SO 4 solution has led to severe corrosion during the acid cleaning procedure, producing hazardous waste with environmental consequences. To address these issues, this study attempts to synthesise, screen, and optimise the corrosion inhibition efficacy of isatin 4-ethyl-3-thiosemicarbazone (Is4ETSC). The weight loss technique was utilised to evaluate the potential of Is4ETSC as a corrosion inhibitor at five concentrations (1–5 mM) before optimising it using the response surface methodology (RSM). Is4ETSC was synthesised via a condensation method and yielded yellow precipitates, characterised by distinctive stretching bands at ν(C=S) [937 cm −1 ], ν(C=O) [1670 cm −1 ], and ν(C=N) [1686 cm −1 ] as confirmed by ATR-FTIR. Weight loss analysis for screening revealed that as the Is4ETSC concentrations increase, more Is4ETSC is theoretically adsorbed on the mild steel surface, resulting in higher coverage and smoother surfaces. Surface analysis via SEM–EDX revealed that mild steel treated with the inhibitor exhibited a smoother morphology compared to the untreated steel. The adsorption of Is4ETSC on the mild steel surface occurred through both chemisorption and physisorption, forming a monolayer, as validated by the Langmuir isotherm and Gibbs energy. Meanwhile, for RSM analysis, the optimum operating conditions for corrosion inhibition efficiency were identified at 8 mM inhibitor concentration, 0.5 M H 2 SO 4 concentration, and 14 h of immersion time, achieving a maximum inhibition efficiency of 98%. The overall model proved significant ( p