Azo-film inhibitors of carbon steel corrosion in HCl: A theoretical and experimental investigation of 4-(aryl) diazenyl-1-naphthol coating

This study focuses on the synthesis and evaluation of the corrosion protection properties of three 4-[(Aryl)diazenyl]-1-naphthol compounds (NAZ-1, NAZ-2, and NAZ-3) for carbon steel in 1.0 M HCl. Based on electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation (PDP), all inhibitors exhibited excellent performance, achieving inhibition efficiencies of 93.2% for NAZ-1, 95.5% for NAZ-2, and 97.7% for NAZ-3 at a concentration of 1.0 mM and 25 °C. Thermodynamic studies revealed that the adsorption behaviour of all compounds adhered to the model of Langmuir isotherm, supported by high adsorption equilibrium constants ( K ads = 5.80 × 10 4 , 8.05 × 10 4 , and 12.97 × 10 4 L/mol for NAZ-1, NAZ-2, and NAZ-3, respectively) and negative Gibbs free energy values ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:msubsup> <mml:mi>G</mml:mi> <mml:mrow> <mml:mi>a</mml:mi> <mml:mi>d</mml:mi> <mml:mi>s</mml:mi> </mml:mrow> <mml:mn>0</mml:mn> </mml:msubsup> </mml:math> = −37.11, −37.93, and −39.12 kJ/mol), suggesting spontaneous adsorption relating both physical and chemical interactions. The inhibition mechanism was further clarified through Density Functional Theory (DFT) calculations and Monte Carlo (MC) simulations, which confirmed the strong affinity between the inhibitor molecules and the carbon steel interface. Overall, the combined experimental and computational results demonstrate that NAZ-1, NAZ-2, and NAZ-3 are highly efficient and stable corrosion inhibitors with strong potential for sustainable industrial applications. The novelty of using these materials as corrosion inhibitors lies in several advantages: (i) they are very easy and low-cost to synthesise; (ii) they contain multiple active adsorption sites (–N = N– and –OH groups); and (iii) they exhibit a pronounced ability to mitigate the corrosion of carbon steel.