An Investigation Of The Cathodic Protection Potential Distribution On The Exterior Of Steel Tank Bottom

The cathodic protection potential distribution on steel tank bottom is of great significance in both theory and practice, which is very complicated due to a large number of influencing factors, including nonuniformity of soil, polarization characteristics of tank bottom, anodes placement manners and so on. Consequently, it is very difficult to solve a 3-D mathematical model taking into account so many factors analytically, while numerical simulation, as a useful tool, can be applied to solve this problem.The cathodic protection potential distribution on the exterior of steel tank bottom is investicated using numerical simulation software, in which a 3-D mathematical model taking into account nonuniformity of medium is solved. In the model, the cathodic polarization performance is characterized based on the mixed polarization theory, which is determined experimently. The polarization characteristics of carbon steel in different surface conditions and corrosive mediums are measured, which provide appropriate boundary conditions in the numerical simulation. Two sets of cathodic protection experimental installations are established, in which the potentials at different measuring positions on the simulated tank bottom are obtained and corresponding numerical simulations are carried out. A good agreement exists between experimental data and numerical simulated results, which verifies the rationality of modeling ideas and the veracity of modeling methods.The potential distribution laws corresponding to six different kinds of anode placement manners used in practical production are obtained, including vertical anodes around tank bottom, horizontal anodes under tank bottom, deep-well anodes, inclined well anodes, tape anodes and net anodes. Besides, the influences of various factors on potential distribution of tank bottom under protection of these anode placement manners are also studied, such as soil resistivity, bitumen sand base, setting parameters of anodes, tank bottom diameter, surface condition of tank bottom metal, properties of medium contacting with tank bottom, oxygen concentration distribution, etc. The results can provide theoretical guidance for practical production.Based on the numerical simulated results, two theorectical formulas describing the cathodic protection potential difference between the center and the brim of tank bottom are derived according to the electric field superposition principle and the charge distribution law of electric disc, of which one is for anodes symmetrically placed around tank bottom and the other for one-side deep-well anodes. The calculated results agree well with the experimental data.