Numerical Simulation Of Cathodic Protection And Transient Impulse Influence For Structures In Seawater

There are many researches on the potential distributions of marine steel structure under cathodic protection (CP) condition, such as pipelines, hulls, oil tanks and oil/gas platforms in marine environment. These researches were focused on the study of potential distributions and current distributions of marine structures which were under steady state condition of CP, and other transient impulses were not considered. It is difficult to design an effective cathodic protection system and corrosion detection and monitoring system for complicated and huge steel structures in marine environment. The engineering will cost a lot of time and money with a lot of works. Numerical simulation method could decrease over-protection or defective protection in local area of the traditional methods based on experiential estimate in real marine environment. The calculation could offer lost collecting data of detection and monitoring, and could get the data which experiments could not test. The study of the paper follows the following order: theory analysis, pre-experiments, numerical simulation, and verification test. The study has learned Q275 steel protected by aluminum sacrificial anode, copper protected by medium carbon steel, crevice corrosion, small scale electrochemical transient impulse test, and large scale electrochemical transient impulse test. The highlights are summarized as follows:1. Physical models of cathodic protection for Q275 steel by aluminum sacrificial anode in seawater were built, under different conditions of normal cathodic protection, over-protection and under-protection, respectively. And the weak form of Laplace equation was deduced to make finite element method (FEM) numerical calculation conveniently. The comparison of FEM data and measured experimental data was made by Kriging gridding interpolation. The results show that the potential distributions under three protection conditions simulated by FEM approximated to measured experimental results.2. Six different 2-D physical models for cathodic protection of copper by medium carbon steel as sacrificial anode in seawater are built in this paper. Potential distribution of various physical models was computed by FEM, and followed by experimental measurements for validation. The results show clearly that potential distribution of the cathodic protection system could be well simulated by the 2-D FEM solution. The distance of the galvanic couples is not a key factor influencing potential distribution in small range. Typical simulation data (along X-axis and Y-axis) of different models are consistent with the experimentally measured results. Therefore, it should be feasible to protect copper with medium carbon steel as sacrificial anode, and FEM could afford well a basis for cathodic protection design.3. A mechanistic model was presented to describe the chemical reactions within the corroding thin and narrow crevice. In the mathematical model, a two-dimensional steady-state was used to predict the crevice pH profile by taking into account dissolved oxygen and hydrogen ions within the crevice. It consists of six parallel electrochemical reactions: multi-anodic reactions (Fe, Cr, Ni dissolution reactions) and three cathodic reactions (the oxygen reduction, the hydrogen reaction and water dissociation). Current density distributions and oxygen concentration distribution were determined to be corresponding to the evolution of potential distribution within the crevice. The contribution of each metal reaction to the overall corrosion process was in proportion to the mole fraction, and the simulation provided a good agreement with published experimental results for the crevice corrosion of stainless steel in sodium chloride solution.4. The electrochemical response behaviors on marine structures by electrochemical transient impulse were studied in this paper. The law of potential and current density distributions of steel structure surface with coulostatic impulse response was discussed. The numerical simulation models of potential vs. time for 304 stainless steel and Q235 steel were established in seawater. Its boundary includes different geometry structures and sizes. The self-made array electrodes and multi-channel corrosion monitor were used for tests. Potential distribution of steel structures was impulsed by electrochemical transient response. And all models were calculated by numerical simulation.5. The paper has accomplished the tests of potential responses for large scale steel frame which were with or without cathodic protection influenced by electrochemical transient impulse. Different transient impulse models of experiments were respectively established. The variation laws of potential and current distributions of steel frame surfaces as time delayed were studied by numerical simulation.