Electrochemical Characterization And Scanning Electrochemical Microscopy Study On The Inhibitive Self-assembled Monolayers On Iron And Stainless Steel

Corrosion of metals occurs under the influence of the corrosive environment. The corrosion will not only cause huge economic losses, but also lead to a lot of catastrophic accidents. Therefore, corrosion and protection technology for metals would be an important research field related to the national economy. The traditional methods of corrosion protection have some limitations. Compared to the traditional methods, self-assembled monolayers (SAMs) are formed by spontaneous chemisorptions of active molecules from solution or the gas phase onto solid surface. As a kind of two dimensional ultrathin layers, the SAMs are formed-spontaneous, highly ordered and thermodynamic stability films. In addition, expected physical and chemical properties of the interface can be obtained by artificially designed molecular structure. Hence, SAMs provide a great application prospect in corrosion and protection field.Compared with inert metal (such as gold, silver, etc.), it is difficult for active organic molecules to assemble onto the surface of iron and stainless steel, because these metals are easily oxidized in air. Moreover, traditional electrochemical methods combined with surface analysis techniques are generally employed, lacking of in situ characterization for local electrochemical activity of electrode surface. As well known, scanning electrochemical microscopy (SECM) is a new powerful technology with high spatial resolution and electrochemical sensitivity. Compared with conventional electrochemical techniques which can only provide global and average electrochemical information, SECM can be used to characterize the topography and redox activity of the solid/liquid interface. Therefore, the SECM study on the inhibitive SAMs formed on active stainless steel and iron is of great significance.Sulfur and nitrogen atoms contain lone-pair electrons which can form coordinate bond with empty d orbital of iron atom. In this paper, the iron and 304 stainless steel were used as the working electrodes. Dodecanethiol (DT) SAMs, dodecylamine (DA) SAMs, and DT/DA mixed SAMs were prepared on iron and 304stainless steel surface. XPS and SEM were used to characterize the composition and morphology of the SAMs. EIS and polarization curves were applied to investigate not only the influences of assembling time and assembling molecular concentration on inhibition of the SAMs to iron and 304 stainless steel substrate, but also the inhibition of SAMs in different corrosive media. Furthermore, theoretical equivalent circuits were proposed to fit the experimental EIS data to calculate inhibition efficiency. Finally, the effects of assembling time, immersion time and corrosion medium on electrochemical activity of metallic substrates covered with SAMs were studied by SECM technique.The main work is as follows:1. Studies of DT assembled onto the surface of 304 stainless steelXPS was used to characterize the formation of SAMs on the stainless steel. The influences of assembling time, assembling molecular concentration on corrosion inhibition in 0.2M H2SO4 aqueous solution were studied by EIS and polarization curves. Furthermore, the inhibition of SAMs in different corrosion media was investigated. EIS was used to determine the inhibition efficiency(IE). The results showed that the inhibitive effect became stronger as the assembling time and the assembling molecular concentration increased. Compared to SO42-, Cl- was more aggressive to cause corrosion.2. Determination of the SECM parameterThe appropriate tip potentials were determined by CV. The applied tip potential of H+ reduction for approach curve was-1.2V, and The applied tip potential of Fe2+ oxidation for area scan was 0.7V.3. Studies of electrochemical activity of 304 stainless steel with DT/SAMsThe electrochemical activity of 304 stainless steel coated with SAMs was obtained by approach curve and area scan at open circuit potential. Results of the approach curve tests showed that current of the tip decreased as the assembling time increased, which indicated that the electrochemical activity of the electrode surface reduced and the blocking effect of SAMs to electron transfer enhanced as the assembling time increased. Results of the area scan experiments showed that the current fluctuation of the electrode surface became relatively small as the assembling time increased, and the electrochemical activity of the electrode surface became relatively uniform. The defects of the SAMs were gradually formed as the immersion time increased. Finally, the SAMs lost protective action. Current fluctuation became relatively large in 0.2M HC1 medium, which indicated that Cl" easily passed through the SAMs, contacted with stainless steel substrate and cause corrosion.4. Studies of DT/SAMs, DA/SAMs and DT/DA mixed SAMs on ironDT/SAMs, DA/SAMs and DT/DA mixed SAMs were prepared on iron electrode. The inhibition ability of the SAMs was evaluated by EIS and polarization curves. SECM technique was used to study the electrochemical activity of iron coated with SAMs. The results showed that, compared with Single SAMs, mixed SAMs which had a better blocking effect to electron transfer were more compact.