Application of Cathodic Protection to Coated Steel in Atmosphere and Corrosion Controll

概要

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A catholic protection technique in the atmosphere has been applied to organic coated steel. This technique differs from the usual catholic protection because catholic current flows through thin water film layer on the coated steel. The steel plate's coating for this technique consisted of three layers. These were an insulating layer, a conductive layer containing carbon powder and a top coat. The cathodic protection current was applied from the conductive layer to steel. Defects were made on these plates in the size of 1×10mm. Various amount of salt were put on defects. Corrosion test was performed under the condition of wet and dry cycles in applied voltage 5V. During the test period, the current through defect area was meseared. As the amount of salt in the defect increased, the measuring current increased. The sample having the greater current showed more protective appearance. In the case of the smaller amount of salt in the defect, the thickness of electrolyte solution film decreased, and the resistance of the defect area increased, then the cathodic current decreased. However, the defect area of this sample showed protective appearence. It is estimated that the steel under the defect was covered with passivation film which was generated by alkalinity due to catholic reaction.
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