The analysis of the composite and distribution of iron oxide due to pit corrosion in saline solutions (3.4% NaCl) is of great importance. Many electrochemical methods, XPS, and other photoelectronic techniques have been applied to this domain. Surface?enhanced Raman scattering (SERS) spectroscopy can provide the vibrational information of surface species with high sensitivity and is very useful in the study of surface chemistry of iron. Notwithstanding, the application of SERS in iron corrosion is hampered for its SERS?inactivity. Efforts have been devoted to extend the use of SERS to the study of iron. Approaches include the deposition of particles or layers of silver or gold on iron surface to obtain surface Raman spectra from species in the vicinity  . However the application of the substrate was limited to potential region in which the silver or gold is thermodynamically stable in the aqueous solution. Another method is to deposit thin films of iron onto surfaces of silver to obtain the enhanced Raman scattering of species through the electromagnetic long?range effect. But the analysis of the spectra is difficult due to the existence of pinhole. The most liable method would be to obtain the surface Raman spectra directly from the bare iron surfaces. By using proper roughening pretreatments and high?sensitivity confocal Raman system (LabRam I), Tian and coworkers now can obtain directly from bare iron electrode surface Raman spectra of some adsorbates  . This enables us to further study the surface chemical reaction of iron. This paper reports our preliminary results in the study of the distribution of surface species caused by pit corrosion of iron in a simulated saline solution. Fig. 1 represents the normal image of a roughened iron surface with an area of 100×100 μm 2 under white lamp. The electrode potential was held at -0.2 V (vs. SCE)in a saline solution during the measurement. The darker region (inserted box in Fig. 1) refers to the hole of pit corrosion. Raman spectra of surface species in this region have been acquired in two?dimension, a typical Raman spectrum is shown in Fig. 2. The results clearly show the complexity in composition of the corrosion products. The band at ca. 660 cm -1 was assigned to magnetite; the bands at 402 and 292 cm -1 may be accounted for the existence of Fe 2O 3. The Raman image of the same region at 660 cm -1 was displayed in Fig. 3. The scanning area was 30×30 μm 2. The brighter regions in the Raman image correspond to regions with higher Raman intensity. It can be found that the Raman intensity varies largely over the pit corrosion region, showing that the distribution of magnetite was not uniform.The success in obtaining surface Raman spectra and Raman image of the pit corrosion region on iron surface enable us to further study the corrosion mechanism. Obviously, the potential?dependent Raman images will inevitably provide more information. This work was still in progress.
Online Available Date
Pei-gen CAO, Jian lin YAO, Jun wei ZHENG, Bin REN, Ren-ao GU, Zhong qun TIAN.
Two-dimensional Surface Raman Imaging of a Roughened Iron Electrode in Saline Water[J]. Journal of Electrochemistry,
7(1): Article 19.
Available at: https://jelectrochem.xmu.edu.cn/journal/vol7/iss1/19
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