Submitted 21st August 1999
Tooru TSURU, Takao YAMAZAKI and Atsushi NISHIKATA
Dept. Metallurgy and Ceramics Science, Tokyo Institute of Technology,
2-12-1, O-okayama, Meguro-ku, Tokyo, 152-8552 JAPAN
Keywords: Cathodic protection, atmospheric corrosion, electrical conductive layer, oxygen diffusion, contamination, water film, moisture layer
Organic coatings for steels are effective to prevent corrosion in many environments by insulating the steel surface from corrosive media. When the coating layer is damaged by mechanical scratches or delamination, corrosion under the film proceeds gradually and covered by red rusts. Corrosion rate under the film is not so fast, however, appearance of rusted steel is not acceptable in usual use.
Cathodic protection for steel structures is widely applied in seawater, fresh water and soils. Since a current path for the protection current is not available in air and on coating film surface, cathodic protection is not adopted for these environments. There are some trials for cathodic protection of coated steels in air atmosphere where anode is positioned on the coating film surface. In this case, current path formed when the film surface was covered by moisture layer.
In this paper, we propose a new coating system, which more effectively protects the steel surface in atmospheric corrosion condition, and describe some results of corrosion tests in laboratory and fields.
Coating System: The coating system applied onto a steel surface consisted three layers and an electrical conductive layer was sandwiched by insulating layers. For the laboratory tests, the first insulating layer was epoxy layer of 200 mm, the second layer was polyester layer of 100 mm containing carbon powders for electrical conduction and the top layer was epoxy layer of 100 mm. Sizes of specimens were 5cm x 5cm. For field test, almost same coating but the top coat was fluoride resin type of 20mm and the size was 45cm x 90cm.
Cathodic Protection: Protection current was applied between the steel surface and the electrical conducting layer. Applied voltage was 10V or 5V.
Experimental Procedure: After insulating test, the coating of specimen was scratched as 1mm width and 10mm length or removed the coatings some other shapes. Specimens were exposed in a constant humidity or in a cyclic wet/dry condition. In some cases, defects were contaminated by sodium chloride. The cathodic protection current was monitored with temperature and relative humidity. The defect surface was checked after the experiment.
Results of Lab Test: The damaged samples with different contamination levels from 0 to 32mg/m2 were exposed in a constant humidity of 50 to 90% at 298K. Only the specimens that have wide bare surface or very low contamination level were difficult to protect from red rusting. Most of the specimens were protected perfectly. Rough estimation of the minimum cathodic current density for protection was 60 to 70mA/cm2. This is several times larger than the cathodic limiting current of oxygen diffusion in bulk solution. This is understood as an increase in the diffusion limiting current with decreasing the water film thickness.
In case of large defects, such as several mm square or diameter, the protection current is not enough at the central part of the defect especially for low moisture layer condition. It was seemed that the thickness of the first insulate layer affected the minimum thickness of the water layer.
Field Test: The specimens were exposed in urban atmosphere with applied voltage of 10V. The specimens exposed against sunshine were protected cathodically except for large defects. In case of large defects, the moisture layer dried up too fast to distribute the protection current to the bare surface apart from the edge of coating. For the specimens facing downward, the moisture layer at the defects dried up gradually, so the cathodic protection continued for longer time and more widely. When the defects were contaminated with a NaCl solution, it was found that the protection current flowed for longer time in day cycles compared with that of clean defects. The distance or location of the defects from the current drain of the coating film did not affect the current densities at the defects. This indicates that the ohmic drop is negligible within the film, and suggests that the protection system does not require so many current drains for a huge steel structure.
The results showed that the cathodic protection for new coating system is performed unless the moisture layer keeps the electrical conduction between the substrate and the electrical conductive coating layer. This coating system will be suitable to apply the steel structures close to seashore or polluted air atmosphere.
Current Needs for Protection: The minimum current density for cathodic protection in a neutral solution is nearly equal to the diffusion limiting current of oxygen. It is about 20mA/cm2 in bulk stagnant solution. The required current observed in laboratory tests was several times larger than this. In case of thin water layer, less than the thickness of the stable diffusion layer, the diffusion limiting current increases with decreasing the water layer thickness.
New coating system to be able to use cathodic protection of coating defects was proposed. An electrical conducting layer was sandwiched by insulating layers and cathodic protection current was applied between this layer and steel substrate. Coated steels with some artificial defects were tested in laboratory and field. Unless moisture layer covered the defects surface, no red rusts appeared for long period. It is suggested that the new system is applicable to steel structures at or near seashore.
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