In Situ High-Resolution Microscopy on Duplex Stainless Steels

Abstract

Characterization of materials in situ offers a new window of opportunity for several areas of science, including electrochemistry, biology, chemistry, physics, and medicine. At the present time, few research groups are working toward building more versatile microscopes that extend modern microscopy to studies in situ in liquids. Smyrl's group, for example, demonstrated that by using a modified near-field scanning optical microscope (NSOM), with a home-built tuning fork, it is possible to obtain in situ topography in liquids together with functional imaging (FI) on some model systems. 1 William's group 2 modified an atomic force microscope (AFM) to study the onset of pits on austenitic stainless steels, by using it as a scanning electrochemical microscope (SECM). They also modified a confocal microscope to obtain a photoelectrochemical microscopy (PEM) and reported images with resolution of the order of 0.5 m. 3 Smyrl's group has also used PEM to study precursor sites for pitting in polycrystalline Ti with a resolution in the order of 100 nm. 4 In the present work, we show that by using a modified near-field scanning optical microscope (NSOM), with a home-built tuning fork, one can obtain high resolution optical images concurrently with the topography of different systems while the substrate (sample) and the probe are immersed in the liquid. This technique has been used to show, for the first time, metastable pitting experiments in a 25% Cr duplex stainless steel (DSS) UNS S32550.It is well established that MnS inclusions are the main cause of pitting in commercial stainless steels. 2,3,5,6 However, in highly alloyed steels (as in the case of duplex stainless steels), the cause for pitting is a more complex phenomenon, mainly because of the presence of different precursors (not only MnS inclusions) and because of the development of metastable pits preceding stable pitting. 7 In the present work we briefly describe the method for obtaining concurrent topography and optical imaging by using an optical fiber with an apex smaller than 100 nm. The optical image is obtained by using a three-way connector that allows the focusing of a laser light through the fiber to illuminate the sample, and at the same time, that allows the collection of the reflected light. After finding, analyzing, and isolating the particles in some areas of a DSS sample, prior to the corrosion tests, they were imaged in situ in both 3.5% NaCl and 1 M HCl solutions. Then, the metastable pitting process was initiated with further imaging of the selected areas. ExperimentalTip preparation.-All NSOM probes were prepared from type F-AS optical fiber (3.7 m in core diameter with 125 Ϯ 2 m of cladding and 245 Ϯ 15 m of polymer coating), which were obtained from Newport Corporation. The optical fibers were pulled with a commercial pulling machine (from Sutter). After pulling the fibers they were observed under a 63 times magnification microscope to check the shape of the fiber before it could be mounted in the fork. The apex of the fibers were 100 nm or l...

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