Corrosion inhibition of SLM 17-4 pH stainless steel in neutral saline solution by naphthalene-derived Schiff bases (NAFS and NATS): Experimental and theoretical insights

Selective laser melted 17–4 precipitation-hardening stainless steel has garnered significant attention owing to its critical importance in the aerospace and biomedical fields. However, microstructural defects that occur during production adversely affect its applicability in structural applications, particularly in terms of corrosion resistance. The application of corrosion inhibitors for safeguarding metals is a widely discussed topic. This study evaluates the corrosion inhibition performance, behavior, and mechanisms of naphthalene-derived Schiff base derivatives, (E)-1-(((furan-2-ylmethyl) imino) methyl) naphthalen-2-ol and (E)-N'-((2-hydroxynaphthalen-1-yl) methylene) thiophene-2-carbohydrazide. The corrosion inhibition efficiencies of these compounds were evaluated for selective laser melted 17–4 precipitation-hardening stainless steel in a neutral simulated seawater solution containing 3.5 wt% sodium chloride at room temperature. Electrochemical methods, including potentiodynamic polarization and electrochemical impedance spectroscopy, were employed to assess their inhibition performance. To elucidate the underlying inhibition mechanisms, complementary surface analyses and theoretical studies were performed using scanning electron microscopy, atomic force microscopy, density functional theory, Monte Carlo and molecular dynamics simulations, and projected density-of-states analysis. The electrochemical findings indicated that the inhibition efficiency values achieved 87.81% and 95.54%, with surface coverage values of 0.83 and 0.98, respectively, at a concentration of 200 parts per million, suggesting that the inhibitive effect of (E)-N′-((2-hydroxynaphthalen-1-yl)methylene)thiophene-2-carbohydrazide surpasses that of (E)-1-(((furan-2-ylmethyl)imino)methyl)naphthalen-2-ol. Surface characterization studies, utilizing scanning electron microscopy and atomic force microscopy, demonstrated that following the incorporation of inhibitors, the surface roughness of the steel plate decreased to 267.0 nm and 152.0 nm, respectively, resulting in a pit-free surface, suggesting effective geometric coverage adsorption. The findings from theoretical studies demonstrate that the added inhibitors chemisorb on the surface of selectively laser-melted 17–4 precipitation-hardening stainless steel via parallel adsorption, forming a protective molecular film. This phenomenon is linked to the π-electrons in conjugated structures and to the lone-pair electrons on heteroatoms such as sulfur, nitrogen, and oxygen that form complexes with iron atoms, thereby alleviating chloride-induced surface degradation. Nonetheless, the naphthalene-derived Schiff base with thiophene and carbohydrazide groups demonstrates superior inhibition performance, thanks to the synergistic effect of sulfur atoms and multidentate functional groups, which promote electron delocalization and facilitate the formation of a dense protective film that effectively restricts the migration of corrosive species.