The Application Of Finite Element Method In Calculating Cathodic Protection Fields Of Steel In Marine Environment

It is difficult to design an effective cathodic protection system and a corrosion monitoring system for complicated and huge steel structures in marine environment. So it is valuable to calculate the electrochemical field of cathodic protection. During the design of cathodic protection system in real marine environment, the traditional methods based on experiential estimate will lead to over-protection or defective protection in local area, meanwhile redundant cathodic nodes will result in destructive stress. Moreover, during corrosion monitoring for cathodic protection system, since it is difficult to measure the potential of each point on the surface of steel structures in marine environment, the monitoring data is incomplete. Using finite element method during the design of cathodic protection system can optimize the cathodic protection system and forecast the distribution of potential and current in real marine environment which can give best engineering parameters. Using finite element method in corrosion monitoring will work out the potential and current of each point on the surface of steel structures or in the corrosion medium, so FEM can measure potential of cathodic protection without contact. As all mentioned above, FEM can optimize cathodic protection system, enhance the precision of corrosion monitoring, evaluate corrosion status and study the law of corrosion. In this paper, the author introduces the fundamental knowledge and electrochemistry parameters about cathodic protection briefly, then discusses influencing factors of the distribution of electrochemistry field. In the first section, the author discusses the progress of computation about electrochemistry field of cathodic protection and enumerates a great deal of applied examples. Especially, the author gives the merits and defects of FEM, FDM, BEM used in the computation of electrochemistry field. This paper mainly includes the following parts: the construction and computation examples of 2-D FEM model, the construction and computation examples of 3-D FEM model, the design of experimentation device and measuring experiment. The 2-D finite element method was an effective method to calculate the potential and current density distribution of cathodic protection fields. The author deals with solution field with simple triangle unit and develops 2-D mathematical model through the mathematical method of classical electromagnetic field. The author developed mathematical model of 2-D finite element, especially discussed how to deal with no-linear boundary conditions of electrochemical fields and used iterative method to solve no-linear equation groups. Several simulated CP examples were analyzed through this model and proved that the finite element was a powerful tool. Those examples show that distribution pattern, number and position of anodes can result in differences in electrochemical fields. Besides, layered or subsection corrosion medium and other steels can also influence in the distribution of potential or current. Finite element technique can be used to predicting distributions of CP electrochemical field in complex system. To solve LAPLACE equation, cubic unit with 8 points and appropriate function were applied to discrete structure surface and its environment. Through analysis of each unit and synthesis in the whole area, mathematical model of 3-D finite element and a computing program were constructed. The author also gave potential and iso-current density distributions and proved that the result complied with classical electric principles. At last, the author designs a set of automatic measuring system for potential on steel surface and compare the experimental results with the values from numerical computation.