Development of Corrosion Estimation System With Scanning Vibrating Electrode Technique by usint Boundary Element Inverse Analysis

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A new effective three-dimensional boundary element method of analyzing electric field produced by localized corrosions was developed to estimate the accuracy of SVET (Scanning Vibrating Electrode Technique). With decreasing height of the electrode probe from the surface of a sample in order to improve the resolution of SVET,the electrode probe disturbs the electric field and measuring errors of the current distribution increase. Although the boundary element method is suitable for estimating the error by a numerical method, the scale ratio of whole vessel of the instrument to measurement area is larger than a factor of 10^5, and the conventional BEM has a difficulty with huge number of elements. To overcome this difficulty, a new effective method classifing computation into a few steps was proposed.A method for estimating the galvanic corrosion rate from the potential values measured at some points far from the metal surfaces was developed. This inverse problem was formulated by employ … More ing the boundary element method. Since the system of linear equations obtained is ill-conditioned, direct application of conventional numerical procedures to the equation system results in an oscillatory solution. To cope with this difficulty, the polarization characteristics which represent the relationship between potential and current density was estimated first. In order to improve the accuracy, a new method using a priori information which was expressed as fuzzy membership functions was developed.The above method was generalized. The generalized method consists of the following procedures. 1.The singular value decomposition of the coefficient matrix is performed, and then the rank is appropriately reduced. 2.A set of solutions is represented by Moore Penrose's solution and null space. 3.The solution is obtained by fuzzy reasoning using the set of solutions and a priori information which is expressed as fuzzy membership functions. Since the solution is identified by taking into account a priori information, the iteration converges quickly and stably, and oscillatory solutions can be avoided.To improve the measuring accuracy of SVET,an inverse problem, in which the true current density distribution along the metal surface was estimated from the data having errors due to the probe disturbance, was solved by using the 3-D boundary element method. The data were, however, not the real experimental data, but the numerical simulation results. Further study is necessary to perform an inverse analysis using experimental data, because more complicate electro-chemical phenomena than the simulation must prevail near the surface of the metal. Less