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...
Corrosion of electronic components can produce a wide range of failure signatures, from intermittent electrical faults to complete functional breakdown. This paper presents an investigation on the exposure of a simple connector-coating system. The system consists of a copper contact coated with a nickel layer underneath a gold finish layer. The system was characterized using the following techniques: optical microscopy, atomic force microscopy ͑AFM͒, scanning electron microscopy ͑SEM͒, energy dispersive X-ray spectroscopy ͑EDAX͒, secondary ion mass spectroscopy ͑SIMS͒ and focused ion beam ͑FIB͒. After initial characterization, the connector was exposed to 2, 4, 7, 15, and 30 days in an aggressive environment consisting of 90% relative humidity, 40°C, and 4 ppm H 2 S. Digital images of the corrosion products that developed on the contacts after exposure clearly demonstrated localized corrosion by-products present on the connector surface. SEM, EDAX, and SIMS analysis of the corrosion sites demonstrated the presence of copper sulfide and nickel sulfur corrosion product, which suggest a two-step mechanism: first, the Ni layer is attacked by the aggressive environment at the sites where the gold layer is not available, followed by the diffusion of copper through the nickel layer. FIB cross-sectional analysis revealed that surface defects in the gold layer resulted in sites for corrosion initiation and subsequent development of a thick copper sulfide layer of approximately 5 m. It is concluded that this copper connector coating system does not prevent the formation of insulating corrosion products on the surface of the connector in a very aggressive environment.
Precision thick chip resistors are used in a variety of different industries, from telecommunications to automotive electronics, and as such can be exposed to mild and aggressive corrosive environments. This paper investigates the corrosion performance of two generic precision thick chip resistors in a controlled corrosive atmosphere consisting of 60 C, 4 ppm H 2 S and water vapor in purified air. The resistors were exposed in an environmental chamber for periods of 5, 10, 15, 30, and 60 days. Following exposure, the samples were cross sectioned and subjected to surface analysis using microscopy and microanalysis. After the initial stages of exposure, corrosion was observed on only one of the two types of resistors. The corrosion developed because H 2 S gas and water vapor diffuses through the thin protective organic layer on the resistor, and subsequently reacts with the silver conductor layer. Corrosion was facilitated by poor overlapping of the solder and nickel layer and, in particular the glass binder over the glass overcoat, which allowed silver and sulphur to diffuse along the interface. In addition, this poor overlapping allowed contact between the nickel layer and the silver layer resulting in the development of an electrochemical corrosion cell. The main corrosion products that developed were silver sulfide (Ag 2 S) and nickel sulphur residue.Index Terms-Atmospheric corrosion, precision thick chip resistors, printed circuit board (PCB), silver sulphide (Ag 2 S).
Pure copper samples were exposed in an environmental chamber for 2, 4, 7, 15, and 30 days at 90% relative humidity, 40°C, and 4 ppm hydrogen sulfide ͑H 2 S͒. Samples were subsequently subjected to microscopy and microanalysis using different techniques: scanning electron microscopy, energy analysis dispersive X-ray spectroscopy, X-ray diffraction, focused ion beam ͑FIB͒, and secondary ion mass spectroscopy. The corrosion samples were cross sectioned and the different corrosion layers were imaged using FIB. After 30 days exposure the predominant corrosion products were copper sulfide ͑Cu 2 S͒ and cuprite ͑Cu 2 O͒. Once the Cu 2 S reached a minimum thickness, the rate of growth of the layer became parabolic due to the limiting Cu + diffusion through a thickening film. As the layers reach a critical thickness ͑ϳ1000 nm͒ internal stresses and defects in the corrosion layer allow virtually free access of H 2 S and O to the underlying layers, consequently accelerating the film growth.
Imaging of two adsorbed proteins on micropatterned glass substrates in air and liquid was conducted using atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM). The micropatterned glass substrates were prepared in five steps and evaluated by NSOM at each processing step. The micropatterned samples consisted of alternating 10 µm wide laminin stripes and 20 µm wide bovine serum albumin stripes. The samples containing both protein stripes were imaged "dry" (i.e., in air) using conventional contact AFM, then imaged dry with NSOM, and finally imaged in the hydrated form (i.e., in liquid) using NSOM. When samples were imaged in air, similar topographical features were observed using both AFM and NSOM. However, after the sample was rehydrated and imaged with NSOM in solution, the topography changed significantly. Having confirmed that the "real" surface topography was obtained while the proteins were in the hydrated form, in situ NSOM was used to detect changes in surface topography with nanometer resolution as a function of the micropatterning process. Although lithographic micropatterning provides chemical cues, in situ NSOM demonstrates that topographical cues are also an important aspect of the surface that serves to regulate cellular attachment and migration.
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