Synergy of novel solid inhibitor system on N80 steel in Maleic–NH ₄ HF ₂ acid: Experimental and theoretical study

This study presents a systematic investigation into the design, optimization and mechanism of high-performance solid inhibitors for low-carbon steel in a high-temperature maleic-acid (ML)–ammonium hydrogen fluoride (FRC) solid mud-acid system. Multiple approaches were used, including weight-loss tests, controlled-variable method, orthogonal response surface methodology, electrochemical analysis, SEM–EDS characterization, and molecular-dynamics (MD) simulations. The results show that, at 120 °C in a 17% ML + 1.5% FRC solution, a composite inhibitor with the composition hexamethylenetetramine (HMTA):potassium iodide (KI):sulfonate (HS):tetradecyltrimethylammonium chloride (JA) = 0.5%:0.3%:0.25%:0.3% reduces the corrosion rate of N80 steel to 1.98 g/m ² ·h and achieves an inhibition efficiency of 99.44%, meeting relevant industry requirements. Electrochemical data, SEM–EDS analyses and MD simulations jointly indicate that promoters addition lowered steric hindrance among HMTA molecules and increased the density of the adsorption film at the interface; a complex, compact adsorption layer composed of HMTA, promoter molecules and ML-species forms on the steel surface and suppresses corrosion processes. The combined experimental and theoretical evidence demonstrates effective metal protection and provides a novel solid corrosion inhibition system with sufficient potential for ML–FRC acidification.