The Investigation Of Galvanic Corrosion Under The Organic Coating On Ship Steel

Various materials are be used in the mechanical structure of great coastal engineering projects (e.g., offshore oil rig, sea-crossing bridge, ships, etc). Under the action of corroding medium, such as wet air and seawater, these materials will emerge varied galvanic corrosion. Organic coatings have been used extensively for corrosion protection of metal. However, most outdoor exposure and indoor accelerated corrosion tests were performed using bare metal, which are fare away from the real working condition, and can't reflect the discipline of the real environment completely.The protective function of the organic coating plays an importantly role during the galvanic corrosion in real system. The galvanic corrosion of the coupled system was mainly influenced by the loss of adhesion between coating and metal , electrochemical reactions at interface, and the stripping of coating from surface of metal, etc. Consequently, the main researches in this article are listed as following:(1) The galvanic corrosion process of coating/steel-steel system was studied by EIS, LPR, galvanic potential and galvanic current. According to the EIS responses at every stage, the equivalent circuit models were established. Meanwhile, the relevance between electrochemical parameters and degradation of the organic coating was analyzed. Within this context, several appearances have been found: the current density of galvanic nearly reached zero, when resistance of the coating was very large, and meanwhile the bare steel was under free-corrosion state; when electrochemical reactions happened under the coating, the current density of galvanic increased quickly and the corrosion rate of the steel was accelerated; at the beginning of bare steel to protect the coating system, the resistance of the coating changed slowly and never decreased during the experiment period, until the current density of galvanic became steady. (2) For coating steel and bare steel, by using EIS and WBE technology, the relevance between coating degradation and distribution of current and potential was also researched. It was studied that some otherness existed during the galvanic corrosion because of the space difference of the steel wire. For the bare system, the driving force of the couple and the galvanic current density were decreased by the polarization. The zone of the anode and the cathode changed during the immersion. For the coating system, the current distribution results of the WBE show that the high anode current existed at the defect of the coating, the cathodic current dispersed over the coating. The anode current was found in the region of negative Ecorr with the development of the corrosion. Although the early defect region was still anode area, the current density diminished, and the cathode current expanded to the area near the defect. It showed that in the late immersion, the negative part would protect the positive part. During the entire coating deterioration process, the EIS diagrams were dominated by the substrate corrosion process of the defect, the coatings and the underlying electrochemical process were"averaged"out. However, through detecting the current distribution information on the metal surface, the local electrochemical process occurring in the coatings and the underlying could be monitored.