Adsorption of 2-mercaptobenzothiazole (2-MBT) at ultra-low pressure and room temperature on metallic and pre-oxidized Cu(111) surfaces and its thermal stability were investigated using X-ray photoelectron spectroscopy in order to better understand the interfacial corrosion inhibiting properties. 2-MBT is lying flat in the monolayer with two sulphur atoms bonded to Cu and decomposes partially yielding atomic sulphur when interacting with metallic copper prior to forming molecular multilayers. Decomposition is prevented by surface pre-oxidation with 2D oxide dissociation accelerating the 2-MBT initial adsorption kinetics. 2-MBT further decomposes and partially desorbs above 100 • C. A pre-adsorbed 2-MBT monolayer on metallic copper inhibits surface corrosion. sulphur in the organic molecule could improve its capacity as corrosion inhibitor by forming coordinative bonds with copper [1]. It has been shown that 2-MBT dissolved in solution can react with copper to form a complex which acts as a protective layer at the copper surface [21]. However, controversy exists on the chemical nature of the complex formed, as well as on the fundamental mechanisms of the inhibiting interaction. The resulting structures of the protective layer formed on copper are also still not clear. Most notably, the exact role of the surface oxide in the inhibiting function remains to be studied. With few exceptions [12], experimental research concerning corrosion inhibitors have been done by immersion into solutions containing the organic compounds [7-24]. However, in order to elucidate the interaction mechanisms, deposition of the molecule evaporated in vacuum could be more insightful, since it allows controlling each step of the deposition process in a well-defined environment and on a well-defined surface. Data have been reported for vacuum evaporation [25, 26] of 1,4-benzenedimethanethiol (BDMT) on Au(111), Au(110), Cu(100) and Cu(111) single crystal surfaces. The authors found that on Cu (100) and Cu(111) surfaces, which are more reactive, BDMT dissociates in the initial stage of adsorption, resulting in atomic S adsorbed on the Cu surface. This phenomenon was not observed for the adsorption of benzotriazole on Cu(100) [27].In this work, the adsorption of 2-MBT at ultra-low pressure (ULP) and room temperature on clean and pre-oxidized Cu(111) surfaces and its effect on the oxidation of copper were investigated using ultra-high vacuum (UHV) spectroscopic techniques. The results were compared to those obtained for ULP deposition on the oxide-covered Cu(111) surface prepared in air. The thermal stability of the adsorbed molecular layer under UHV was also studied. This work brings new insight into the interaction of 2-MBT with copper, which allow us to better understand its corrosion inhibition mechanisms.
Material and methodsA high purity (99.999%) Cu(111) single-crystal was used in this work. The surface was mechanically polished to 1/4 µm (diamond paste), successively rinsed with acetone, ethanol, and ultra pure (UP) water (resistivity > 18...
In situ X-ray photoelectron spectroscopy real-time measurements and angular-dependent high resolution core level analysis were used for the first time to investigate the Cr enrichment and oxide growth mechanisms on a model 304 austenitic stainless steel surface in the very initial stages of oxidation leading to pre-passivation. The oxidation kinetics was followed for increasing oxygen exposure and temperature, revealing an early nucleation regime (for exposure < 10 L) leading to the formation of a strongly Cr-enriched Cr 3+ /Fe 3+ mixed layer followed by an oxide growth regime where preferential iron oxidation takes over and mitigate the initial chromium enrichment.
Stainless steels are widely used as metal components owing to self-protection in aggressive environments, provided by an extremely thin surface oxide film enriched in chromium oxide. Yet, despite decades of research, the mechanisms distributing the chromium enrichment at small length scale are poorly understood, although it may cause loss of stability and local failure of the corrosion resistance. Here, we apply high resolution surface analysis to investigate at small time and length scales the nucleation and growth mechanisms of the surface oxide on a model stainless steel. Starting from an oxide-free surface, we report the direct observation of the oxide nucleation and local oxidation of chromium, which governs the nanoscale heterogeneity of the growing surface oxide by chromium pumping from the atomic terraces to the steps for preferential Cr(III) oxide nucleation and subsequently by segregation from the atomic planes below to grow the Cr(III) layer incompletely saturating the stainless steel surface. This work provides new insight on corrosion chemistry, by evidencing local chemical and structural defects self-generated at the nanoscale by the building process of the protective oxide barrier, and affecting the passive film stability.
Passivation mechanisms and the effects of controlled pre-oxidation, by exposure to oxygen at ultra-low pressure, on Cr and Mo surface enrichments were investigated on polycrystalline AISI 316L stainless steel surfaces with direct transfer between surface preparation and analysis by X-ray photoelectron spectroscopy and electrochemistry. Exposure to sulfuric acid at open circuit potential causes preferential dissolution of oxidized iron species, which promotes Cr 3+ and Mo 4+/6+ enrichments. Anodic passivation forces oxide film re-growth and Cr 3+ dehydroxylation with no loss of Mo 4+/6+ pre-enrichment. Ultra-low pressure preoxidation promotes Mo 4+/6+ enrichment in the exchange outer hydroxide layer of the passive film, with no Mo 0 depletion in the modified alloy region underneath the oxide film at open circuit potential, and under anodic passivation. Mo 4+/6+ enrichment improves protectiveness against transient active dissolution during the active/passive transition.
Passivation mechanisms were investigated on (100)-oriented Fe-18Cr-13Ni surfaces with direct transfer between surface preparation and analysis by X-ray photoelectron spectroscopy and scanning tunneling microscopy and electrochemical characterization. Starting from oxide-free surfaces, pre-oxidation at saturation under ultra-low pressure (ULP) oxygen markedly promotes the oxide film Cr(III) enrichment and hinders/delays subsequent iron oxidation in water-containing environment. Exposure to sulfuric acid at open circuit potential causes preferential dissolution of oxidized iron species. Anodic passivation forces oxide film re-growth, Cr(III) dehydroxylation and further enrichment. ULP pre-oxidation promotes Cr(III) hydroxide formation at open circuit potential, compactness of the nanogranular oxide film and corrosion protection.
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