Articles techniques et présentations

Most of steel structures in Japanese harbors are becoming older, the maintenance is one of big problems in our society. As one of anti-corrosion methods for steel structures in seawater, there is a process using electro-deposition of calcareous coating by impression of electric current from power supply between the structures and electrodes in seawater. In this process, if the cathodic current is impressed to the steel structure, pH around the steel surface is increased, and then calcareous deposits is formed onto the surfaces.

But the thickness and quality of coating are known to be influenced by the electric current or potential distributions, and the distributions become non-uniform to complicated shaped structures. Then optimization about arrangement of electrodes is needed so as to set the distributions to proper and uniform conditions, numerical simulations by COMSOL Multiphysics^® simulation software are used in order to optimize arrangement of electrodes.

Here to mock-up sheet piles, optimization about the proper arrangement of electrodes were considered through both of the simulations by COMSOL Multiphysics^® and experiments.

In these cases, about 200 liters seawater were put in plastic tanks, the mock-up steel sheet piles and aluminum electrodes were set in them. Here two patterns of electrodes arrangements were tried. In one case, the electrodes were set far counter to the bare sides of sheet piles, this is general arrangement. In another case, the electrodes were set into concave parts of sheet piles each.

The process and experiments are modeled via the Secondary Current Distribution interface in Electrodeposition module offered by COMSOL Multiphysics^® in a three-dimensional space. In these simulations, seawater was set to electrolyte, and the potential distributions in seawater were solved. In these calculations, the surfaces of sheet piles set to cathodic electrodes and the ones of aluminum electrodes were set to anodic electrodes. Here the literature data was used about electrolyte conductivity of seawater, the relations between charge transfer and overpotential on the surfaces of electrodes were used the equations obtained from polarization measurements in laboratory experiments.

Below the results of simulations were shown. First in the case of general electrodes arrangement, the potential distribution was non-uniform and the potentials on the surfaces of convex sides of sheet piles face to electrodes were about 100mV lower than ones of concave sides. On the other hands, in another case, the potential distribution was more uniform than the case of general arrangement, the differences of potential between convex sides of sheet piles and concave sides were decreased by the change of electrodes arrangements.

At the next step in order to validate the results of simulations, saturated calomel reference electrodes were set on the surfaces of sheet piles, the potential were measured experimentally. As the results, potential distributions obtained from experiments were consistent with ones of simulations in both cases.

So numerical simulation model by COMSOL Multiphysics^® was validated and one example for proper electrodes arrangements was also showed. I will use COMSOL Multiphysics^® in the design of electrodes arrangements with knowledges obtained from these results.