In this study, we report on a combined microscopic, analytical and electrochemical characterization of the nanoscopic passive layer on a tungsten‑molybdenum-containing super duplex stainless steel. We used scanning transmission electron microscopy/energy dispersive X-ray spectroscopy, scanning Kelvin probe force microscopy, scanning tunneling spectroscopy, and Mott–Schottky electrochemical impedance spectroscopy analysis to correlate the local chemical composition and electronic properties of passive layers on austenite and ferrite phases. The passive layer on the ferrite phase contains a higher amount of Mo, W, and Cr, which accommodates a higher nobility of ferrite and a higher local energy of the band gap compared to those on the austenite. The two aforementioned phases exhibit a different composition and semi-conductive properties of their passive layers leading to dissimilar local corrosion susceptibility. These findings are of pivotal importance in further studies of austenite and ferrite phase resolved corrosion resistance of duplex stainless steel demanding a dedicated alloying strategy.

In this work, response surface method (RSM) is employed to design electrochemical experiments for assessment of a green organic molecule, namely Clopidogrel from cardiovascular drugs class, as a corrosion inhibitor for mild steel in sulfuric acid solution. Mathematical models based on multiple regressions are generated to estimate the influence of the affecting factors like acid concentration, solution temperature and inhibitor concentration on the inhibitive performance of Clopidogrel. The corrosion rates are measured experimentally using potentiodynamic polarization technique. The calculated Langmuir adsorption energy reveals that Clopidogrel adsorbs both physically and chemically onto the mild steel surface and the adsorption type is almost independent of the environmental factors. The adsorption mechanism of Clopidogrel is computed through quantum chemical calculations, confirming that this compound can replace water molecules from the surface upon its adsorption to the metal substrate. The simulation results are approved experimentally using an inherently surface sensitive tool, sum frequency generation spectroscopy (SFG).