Evaluation Of Metal Corrosion And Coating Delamination In Water-Line Area By WBE Technique

The wire beam electrode (WBE) method was used to study water-line corrosion ofcarbon steel and water-line corrosion of steel under defective coatings in3.5%NaClsolution. The current distribution and EIS data was analyzed to evaluate the status ofcorrosion and coating deterioration. Through the research, the mechanism ofcorrosion and coating deterioration with defect in water-line area could be madeclearer.The wire beam electrode, in the shape of square（11by11）, was made of Q235carbon steel. The diameter of each wire is1.6mm. This electrode was used toresearch the water-line corrosion of steel. In the initial period, It was found that,down from the waterline, the anodic current increased gradually. This indicated thata oxygen concentration cell had been formed already. But the cathode region andanode region were mixed up in this period. With the development of corrosion, thecathode region was located near the waterline and the anode region was locatedunder the cathode region. As the cathodic reaction above the waterline speeding up,the corrosion extended from the bottom of the electrode to the waterline and thecorrosion rate of whole WBE increased. In the stabilization period of corrosion, themain cathode region was located above the waterline and the corrosion rate gotstable.The wire beam electrode, in the shape of rectangle(6by20), was made of Q235carbon steel. Each single electrode was in the shape of square(1cm by1cm).Thiselectrode was used to research the water-line corrosion of steel compared with thesmaller one. In the initial period of immersion, the cathode region and anode regionwere mixed up. Down from the waterline, the corrosion products increased gradually.The anode current at the bottom of the electrode was higher than other parts’. This indicated that a oxygen concentration cell had been formed already. With thedevelopment of corrosion, the cathode region was located near the waterline and theanode region was located under the cathode region. Meanwhile, the corrosion ratedecreased and the corrosion products appeared under and near the waterline. Afterrenewing the electrolyte solution, the anode current increased rapidly and soon fellwithin couple of days. In the whole experimental stage, the current distribution andcorrosion rate was stable except near the time after renewing electrolyte solution.The length of the bigger WBE under the waterline was30cm. Compared with thesmaller one, it could be made clear that if the size of electrode can effect thewater-line corrosion. In addition, it was found that there was no apparent cathodecurrent above the waterline of the bigger electrode. It showed that the thinelectrolyte film above the waterline was too thin to play a role in the oxygenconcentration cell. So the effect of thin electrolyte film on water-line corrosion canalso be demonstrated by comparing with the smaller electrode. During the period ofcorrosion developing, there was obvious difference between the bigger electrode andthe smaller one. The corrosion rate of the smaller one had been increasing gradually,however the bigger one’s was stable in most of the time. Besides, the corrosion rateof each line under the waterline varied with the depth, the size of the electrode hadan effect on the location of max corrosion rate.The wire beam electrode, in the shape of square（11by11）, was made of Q235carbon steel and the diameter of each wire is1.6mm.The surface was painted withAlkyd resin varnish. After the paint had been dry, an artificial defect was made onthe middle three wires of line8under the waterline. This electrode was to researchthe process of defective coating deterioration. In the initial period of immersion,there was a cathodic current point appearing above the defective area. It indicatedthat the paint of the point had original defect, so the electrolyte could reach thesubstrate metal fast and the wire occurred cathodic reaction. Meanwhile, thedefective wire appeared both anode current and cathode current. As the time of immersion going, the cathode current mainly extended around the defective area andup from the original defective point. In the later stage, the anode current of defectivearea kept increasing, most of the other parts of the WBE appeared cathode current.However, there was a phenomenon that a few wires’ cathode current turned to anodecurrent in this period. The earlier the cathode current appeared, the lower the coatingimpedance was and the earlier the coating bubbled. It demonstrated that, in thisexperiment, the coating with defect in water-line area happened cathodic disbonding.Just a few wires above the waterline bubbled in the end of the experiment, it wasbecause that the water diffusion was hard in the coating above the waterline.The same WBE, as described above, was used to research the deterioration processof coating with artificial defect at the waterline. The paint of the middle three wiresof line4was damaged. In the initial period of immersion, the galvanic cell wasformed between the defective wires. In the64days before, there was cathode currenton the defective wire all the time. As the time of immersion going, the cathodecurrent mainly extended around the defective area. Meanwhile there was cathodecurrent appearing at the bottom of the electrode. In the later stage, with the coatingbubbling developing, the cathode current of defective wire turned to anode current.However, there was a phenomenon that the cathode current of a few wires turned toanode current in this period. The coating impedance of wires appearing cathodecurrent was lower than those of wires appearing no current. The order of appearingcathode current corresponded to the order of coating bubbling.Compared with the coating with defect under the waterline, the stripping rate of thecoating with defect at the waterline was lower. It was because the defective wires atthe waterline had strong capability of cathodic reaction, so the anode current waslower. In addition, the situation of coating bubbling was different. On the WBE withdefect under the waterline, there was just a few wires that happened coatingbubbling above the waterline. However, on the WBE with defect at the waterline,the wires happening coating bubbling above the waterline was more than those under and near the waterline. The reason why this difference occurred was that, thedefective wires could destroy the coating surrounding them, so the water diffusionbecame easier in the coating. Besides, the wires above the waterline had strongcapability of cathodic reaction, so the coating above the waterline inclined tocathodic disbonding.This work confirms the capability of the WBE method in water-line corrosion study.This method makes up for the shortcomings of traditional long vertical steel strip. Itcan give current distribution and its changing process of whole electrode.