RIASSUNTO
ABSTRACT
Temporary coatings in conjunction with cathodic protection (CP) in seawater have been used in this paper for solving the contradiction between big initial current density and small mean current density. The repairing effect of calcareous deposits produced during CP on the coating deterioration areas was studied. The morphology of the formed calcareous deposits has been evaluated through optical microscope and chemical analysis has been performed using Raman spectrum. The changes of cathodic protection current and electrochemical impedance spectroscopy with time were measured to explore the failure process of coating-metal system. It was clearly confirmed that selected temporary coating can decreased the current demand in the intimal period and the calcareous can complement the bit by bit lacking of the coating. Because of the repairing effect of calcareous deposits, the protection current had been maintained at a smaller value. Compared with the 3.5% NaCl solution, the calcareous deposits produced with CP in seawater could reduce the initial current density by more than 40%. These results implied that CP could work well with temporary coatings to maintain smaller current density.
INTRODUCTION
Corrosion of metal materials in marine environment is very serious1. Coating protection is an important anticorrosion technology, but once the coating is damaged locally, the substrate metal exposed in the damaged coating areas is unprotected and therefore susceptible to corrosion 2,3. At the same time, it is difficult to find and repair coating defects for large and complex metal structures. Cathodic protection (CP) has been used wildly to corrosion control in the marine environment, especially for complex large-scale structures. Sacrificial anode cathodic protection is often used for metal structures in the marine environment 4-6. To meet the requirement of high current density at the cathodic protection initial stage, the amount of sacrificial anode needs to be increased. However, this will not only cause waste of resources, but also make the protected body too heavy. Combining the advantages of anticorrosive coating and sacrificial anode protection, the combined protection of anticorrosive coating and cathodic protection has been widely used. Overall consideration the advantages of the two methods, the organic coatings in conjunction with CP is generally used 7-10. But for the joint protection, the coating process is complex, long cycle and high cost. In addition, the deterioration of protective organic coatings under excessive cathodic potentials is a widely reported coating failure mechanism, then shortening service life of coating 11-16. Therefore, if a simple, inexpensive and widely used organic coating is selected as a temporary coating, so in the early stage of cathodic protection, temporary coatings play a leading role and the current density is small. With the continuous deterioration of the coatings, the calcareous deposits gradually deposit in the disbonded areas and continue to provide metals, thus keeping the current density at a low value in the later stage of cathodic protection. This is of great significance for saving engineering cost and improving protection effect.