Three-Dimensional Analysis of Corrosion Surface on Outer Surface of Piping by Use of Fractal Concept
In this study, the three-dimensional geometry of simulated corrosion surface was compared with that of actual corrosion surface by use of fractal concept that was useful to understand the feature of self-similar pattern. To discuss the difference, fractal dimension (Fd) and fractal boundary (λb) are used. The method to make the simulated surface was that the nitric acid was dropped on steel (JIS SB410) plate to make a dimple on it by corrosion. The actual corrosion surface was provided from the piping which used in the actual chemical plant. To calculate Fd and λb of these corrosion surfaces, three-dimensional geometry was measured by the 3D-geometry measurement system (Keyence VR-3000). The measured size was respectively about 90×60mm (4000×3000 pixels). Next, wavelet transform was applied to the three-dimensional geometry. Fd and λb were calculated from the wavelet coefficient instead of the power spectrum. The mother wavelet was the Gabor function that is the combination of sinusoidal wave and Gaussian function. In the result, the difference of Fd between the simulated surface and the actual surface was obtained. The average Fd of the simulated surface was about 2.2. On the other hand, the average Fd of the actual surface was more than 2.6. It means the actual surface is more complex than the simulated surface. The λb of the actual surfaces was not obtained, because the linier relationship was satisfied in every frequency. However, the λb of the simulated surface was about 1cm. It was similar to the dimple’s diameter. Therefore, there is a difference between the simulated surface and the actual surface by applying the concept of fractal though they look similar by the human observation. The relationship between the burst pressure, Fd and λb will be discussed.