Study On The Influence Of Stray Current On The Metal Corrosion Behavior Under Disbonded Coating

At present,buried pipelines were usually prevented corrosion by combination of anticorrosion coating and cathodic protection.It was easy to form a s disbonded crevice with the metal substrate when the coating existed defects and occured failure.The corrosive media in the soil through the defect into the crevice so that the metal in the disbonded zone had serious corrosion.For the actual laying environment of the pipeline,there was a widespread stray current in the earth,and the inflow of stray current was bound to have a more detrimental effect on the corrosion behavior of the pipeline under disbonded coating.The experimental study of the influence of stray current on the metal corrosion behavior under disbonded coating with cathodic polarization and without in this paper was conducted by adopting rectangular coating disbonded simulation device and using the electrochemical method.The influence of stray current on the corrosion behavior of the metal under disbonded coating can be analyzed by measuring the polarization curve and electrochemical impedance spectroscopy(EIS)of various points of the metal in the disbonded crevice,and observing the corrosion morphology of the metal in the crevice.The results showed that:(1)In the natural corrosion state,the self corrosion potential of the metal in the disbonded crevice first shifted towards positive again shifted towards negative with the increase of coating disbonded distance in the same experimental time,and the corrosion current density decreased first and then increased again;the self corrosion potential of the metal in the disbonded crevice with cathodic polarization was negative shifted obviously than that without cathodic polarization,and the corrosion current density decreased significantly,cathodic polarization can enable the metal under disbonded coating to get certain protection;when the cathodic polarization potential was-1.0V,the protection distance of the metal under disbonded coating reached the maximum value of 90 mm,and the corrosion current density of the metal in the crevice was smaller,which was better for the protective effect of the metal.(2)With the presence of AC stray current,the range of self corrosion potential of the metal in the disbonded crevice was larger than that of the natural corrosion state,and the corrosion current density of the metal in the crevice was also much larger than that of the natural corrosion state;when the AC interference was 1V~5V,the corrosion rate of the metalin the crevice was still lower whether there with cathodic polarization or without;when the cathodic polarization potential was-1.2V,the protection distance of the metal under disbonded coating reached the maximum value of 80 mm,and the corrosion current density of the metal in the crevice was smaller.Compared with other cathodic polarization situations,the polarization potential of-1.2V was better for the protective effect of the metal.(3)With the presence of DC stray current,when the interference voltage was smaller than 3V,the corrosion current density of the metal in the disbonded crevice after applying cathodic polarization was less than that without cathodic polarization.And within the range of polarization potential was-0.9V~-1.2V,the protection degree of the metal in the disbonded crevice was the same;when the interference voltage was larger(?3V),the corrosion current densities of the metal in the disbonded crevice were smaller with the cathodic polarization potentials were-1.0V and-1.2V,and the metal corrosion rates were relatively slower;within the range of DC interference was 1V~10V,when the cathodic polarization potential was-1.2V,the protection distance of the metal under disbonded coating can reach the maximum value of 80 mm.Compared with other cathodic polarization situations,the polarization potential of-1.2V was better for the protective effect of the metal.In conclusion,regardless of whether the corrosion system was applied cathodic polarization or not,the corrosion of the metal under disbonded coating in the interference of stray current was more serious than that of the natural corrosion of the metal in the crevice;the presence of AC stray current and DC stray current both narrowed the protection distance of the metal in the disbonded crevice at different cathodic polarization potentials.And enabled the polarization potential to shift towards negative properly will increase the protection effect of the metal in the crevice;in addition to DC interference was 1V,the corrosion rate of the metal under disbonded coating in the DC stray current was much larger than that of the AC stray current;after the cathodic polarization was applied,the cathodic polarization can obviously enhance the corrosion resistance of the metal in the disbonded crevice under the AC stray current,and the protective effect was better than that of the DC stray current.