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An oxadiazole derivative with functional groups favouring its adsorption on copper surface, namely 5-(4-Pyridyl)-1,3,4-oxadiazole-2-thiol, has been explored as potential inhibitor of copper corrosion in 3.5 wt.% NaCl. Electrochemical evaluation by electrochemical impedance spectroscopy, potentiodynamic polarization and SVET reveals inhibition efficiencies exceeding 99%. Surface microscopy inspection and spectroscopic analysis by Raman, SEM-EDX and XPS highlight the formation of a compact barrier film responsible for long-lasting protection, that is mainly composed of the organic molecules. Machine Learning algorithms used in combination with Raman spectroscopy data were used successfully for the first time in corrosion studies to allow discrimination between corroded and inhibitor-protected metal surfaces. Quantum Chemistry calculations in aqueous solution and Molecular Dynamic studies predict a strong interaction between copper and the thiolate group and an extensive coverage of the metal surface, responsible for the excellent protection against corrosion.

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Three newly synthesized Schiff base derivatives, sodium (E)-4-(nitrobenzylideneamino)benzoate (SNBB), sodium (E)-4-(benzylideneamino)-benzoate (SBB), and sodium (E)-4-(hydroxybenzylideneamino)-benzoate (SHBB) were investigated as pitting corrosion inhibitors for 304 stainless steel in neutral 0.1 M NaCl. Potentiodynamic polarization evidenced major shifts in pitting potential to more positive values with increasing inhibitor concentration. The scanning vibrating electrode technique (SVET) imaged metastable pitting in 0.1 M NaCl, but not in the presence of the inhibitor, indicating that it prevented pit nucleation. The inhibition performance was established under anodic polarization conditions, because only minute local anodic activity due to metastable pit formation could be observed when the steel was exposed to SNBB-containing solution, whereas the metal would undergo pit propagation at the same potential in the inhibitor-free solution. X-ray photoelectron spectroscopy (XPS) analysis evidenced chromium enrichment at weak points (pores) of the passive film at anodic polarization condition where sudden release of Fe cations is possible. In this way, the SNBB molecules will migrate to these sites to react with the Fe ions and form a chelate compound which will deposit finally at those sites and plug them, whereas no effect occurred at the open circuit potential (OCP).

The inhibition performance of a novel anionic carboxylic Schiff base, sodium (E)-4-(4nitrobenzylideneamino)benzoate (SNBB), was investigated for various metals, namely low carbon steel F111, pure iron and copper, in neutral 10 mM NaCl solution. Potentiodynamic polarization, scanning vibrating electrode technique (SVET), quantum chemical (QC) calculation, and molecular dynamics (MD) simulation were employed. The potentiodynamic polarization data showed that SNBB acts as an effective corrosion inhibitor for both iron and F111 steel, but it is not effective for the copper. In situ spatially-resolved SVET maps evidenced a major change in surface reactivity for Fe and F111 steel immersed in 10 mM aqueous solution in the absence and in the presence of SNBB. Featureless ionic current density distributions were recorded in the presence of SNBB at both their spontaneous open circuit potential (OCP) and under mild anodic polarization conditions, while major ionic flows were monitored above the metals in the absence of SNBB. On the basis of computer simulations, it is proposed that SNBB produces a stable chelate film on iron and steel surfaces that accounts for the good corrosion inhibition efficiency observed. The different inhibition efficiencies of SNBB molecules on the iron and copper was attributed to the special chemical structure of SNBB molecule and its different chelation ability with the released metal ions on the metal surface. The QC calculations also confirmed the high corrosion inhibition efficiency of SNBB. The MD simulation indicated higher binding energy of SNBB on iron surface compared to that of copper surface. The interaction mode of SNBB on iron and F111 steel surfaces corresponds to a mixed chemical and physical adsorption, and it obeys the Langmuir isotherm.

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