Initaial Study Of Detecting The Scratches Corrosion And The SCC Of Alloy Materials With Digital Holography

Digital holography as a hot research topic in recent years is a combination of optic holography, computer technique and electronic imaging technique. It improved greatly with the recent ongoing development of CCD device and computer performance. I t utilizes CCD device to record holograms and fulfills the numerical reconstruction of the object wavefront by computer. Digital holography has the characteristic of non-contact, non-breakage and high precision. This technology will have great potential application in the areas such as cell viewing, grain size analysis, mechanical measurement, objects deformation measurement etc.The concentration changes at the electrode/solution interface during localized corrosion was studied by numerical reconstruction of digital holograms. It is on the basis of the digital holography. The concentration change can be measured by the change of phase variation according to the relations of the phase variation, solution refractive index and solution concentration are linear. In our experiment, phase variation was recorded and stored in computer in the form of holograms. After numerical reconstruction with the Fourier analysis method, holograms were transformed into three-dimensional images of the electrode/solution interface, which can be more visual than the two-dimensional holograms and can realize the quantitative measurement of the concentration changes. The main contents of this paper are as the following:1. Study of the localized corrosion aroused by scratches of 690 alloy electrode inH2SO4 and NaCl solution by numerical reconstruction of digital holograms The localized corrosion process of the Alloy 690 induced by scratches in H2SO4 containing NaCl was studied by the digital holographic reconstruction technique. Compared scratches electrode with non-scratches electrode by the different concentration changes of the electrode/solution interface, we studied the mechanism of the initiation and the propagation of the localized corrosion aroused by the scratches. The results showed that the change of concentration at the interface was uniform during the process of the corrosion of the electrode with no scratches. While the change of concentration at the interface was not uniform during the process of the corrosion of the electrode with scratches in H2SO4 containing NaCl solution. We can clearly see that the concentration at the scratches increased obviously during the process of the corrosion. And one protruding concentration peak can be seen in the reconstructed digital holographic images.2. Study of the SCC of C-ring in H2SO4 solution containing NaCl solution by numerical reconstruction of digital hologramsThe digital holographic reconstruction technique was used to study the concentration changes at the electrode/solution interface during the SCC of 304ss in H2SO4 solution containing NaCl solution. By this experiment we can see that before SCC the corrosion was very uniform but after SCC the concentration change at the crack was more obvious than other parts of the electrode. So we can judge the occurrence and development of SCC by the concentration changes during the SCC. The digital holographic reconstruction technique may be used to detect the SCC.3. Study of the anodic dissolution of iron in sulfuric acid solution by digital holographyThe dynamic process of the electrode/solution interface during anodic dissolution of iron in sulfuric acid solution was studied in situ by digital holographic reconstruction. The solution part and the electrode part on the electrode interface would take changes during electrochemical reactions. Digital holography has the merits of easy operation, ability to numerical simulation, good at investigating micro changes.