Study Of The Crevice Corrosion Of Carbon Steel And Current Oscillations Of Iron By Numerical Reconstruction Of Digital Holograms

Digital holography, as a brand new method of obtaining optical information, is a combination of the traditional optical holography and digital technique. It is based on the theory of optical holography. And it 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. With the fast development of computer science and high resolution image sensor, the advantage of digital holography is becoming more and more obvious. Today, it has been successfully applied in particle field testing, surface shape measurement, biology, chemistry and other fields.In this paper digital holography has been used to record dynamic electrochemical processes of the electrode/solution interface. The principle of the experiment bases on the relationships between phase of object wave, solution refractive index and solution concentration. The concentration of the electrode/solution interface will change during dynamic processes of electrochemical reaction. The concentration change (AC) of the solution at the interface leads to the refractive index variation (Aw), which then brings about the change of phase (△φ) of the object wave. In a certain range of concentration change, the relationships between these three are linear. The measurement of concentration changes can be transformed to be the measurement of phase variation. 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 crevice corrosion of carbon steel electrode in NaCl solution by numerical reconstruction of digital hologramsThe concentration changes of the electrode/solution interface during dynamic processes of the crevice corrosion of carbon steel electrode in NaCl solution was studied by numerical reconstruction of digital holograms. Compared crevice electrode with non-crevice electrode by the different concentration changes of the electrode/solution interface, we studied the mechanism of the initiation and the propagation of crevice corrosion. The result showed that the change of concentration at the interface was uniform during the process of the corrosion of the non-crevice electrode, it was uniform corrosion, while the change of concentration at the interface was not uniform during the process of the corrosion of crevice electrode in NaCl solution. We can see clearly from the reconstructed planform images that the concentration at the crevice mouth increased most obviously during the process of the corrosion of the crevice electrode. Up to now, there hasn't been any report about crevice corrosion of metal studied by numerical reconstruction of digital holograms.2. Study of the current oscillations of iron in sulfuric acid solution by numerical reconstruction of digital hologramsThe concentration change of the electrode/solution interface during the whole dynamic processes of the current oscillations of iron in sulfuric acid solution was studied by numerical reconstruction of digital holograms. A detailed explanation of this process was carried out. The result showed that the periodicity, the amplitude and the shape of the oscillations are all closely related to the rate of mass transport. Fe²⁺, produced by the electrodissolution of iron will lead the change of pH, which cause the formation of passive film at the electrode/solution interface. It may be the film of Fe(OH)₂. The clear concentration diffusion layer outside the electrode detected from the reconstructed images is the key feature of the current oscillation phenomenon. The reconstructed images supply more visual information for better analyzing and understanding the whole process of solution concentration change and thus study how the related electrochemical oscillation works.