Corrosion Behavior Of Weld Joint 3PE Coating On Buried Pipelines Under Cathodic Protection

Pipeline transport is the main transportation way of oil and natural gas.But the transportation safety is worring.The particular structure of weld joint is easy to form the dead zone for cathodic protection,causing the pipe to suffering severe crevice corrosion.Soil moisture,coating status and cathodic protection effect are three factors that influence pipeline corrosion.Effects of the soil,coating status and cathodic protection on corrosion of pipelines were studied in this paper by simulating the soil environment,coating structure,status of heat shrinkable wraparound sleeve on the weld joint and cathodic protection.The three main results are listed as follows:1.A test device of pipeline accelerated corrosion in soil under cathodic protection was designed according to the actual soil condition without introduction of other ions.Here,the soil moisture,temperature,coating structure and air flow were controlled,and the natural environmental conditions of seasonal change of temperature and humidity were also simulated.This device does not change the corrosion mechanism of pipe material in the soil.And it can achieve pipeline accelerated corrosion tests in constant temperature and humidity,alternating hot and cold and wet alternation under the cathodic protection.Moreover,correlation with the actual pipeline corrosion can be verified.2.Pipeline samples with different structures were made by simulating the coating structure and the status of heat shrinkable wraparound sleeve on the weld joint of West-East gas pipeline.Influences of the soil type,soil moisture,cathodic protection,status of heat shrinkable wraparound sleeve and time on the corrosion of weld joint were investigated using the pipeline accelerated corrosion test device.The results are:(1)Compared with the soil type,the influence of the weld joint structure on the corrosion is much greater.The pitting corrosion of the weld joint with damaged coating occurring in saline soil and sandy soil is much more serious than that in yellow clay.(2)Compared to the dry and wet alternating soil environment,the cathodic protection is more efficient for breakage of heat shrinkable wraparound sleeve in the constant saturated humidity soil.But for the small cracks in heat shrinkable wraparound sleeve,cathodic protection can't work well.What's worse,the potential will accelerate the corrosion of pipeline with the lapse of time.(3)For weld joints with different statuses of heat shrinkable wraparound sleeve,the effect of cathodic protection on pipeline without bare joint is the best.The corrosion of weld joint with intact heat shrinkable wraparound sleeve is uniform,while the corrosion of breakage weld joint is the most serious.As time prolonging and soil moisture changing,cathodic protection will aggravate the corrosion of weld joint.(4)According to the magnitude and change of cathodic protection current,we can judge the status of heat shrinkable wraparound sleeve and the existence of cathodic protection dead zone.For the weld joint with a breakage,cathodic protection is effective only in a very short period of time.With time prolonging,the cathodic protection effect reduced,and the cathodic protection current accelerated the pipe body corrosion.3.Samples with different coatings were prepared by simulating the coating structure of weld joint.The corrosion behaviors of the samples in different humidity conditions were researched and discussed by testing the free corrosion potentials and AC impedance spectra of the samples under cathodic protection in the Korla soil.The results demonstrated that:(1)In saturated humidity saline soil,cathodic protection is effective in the early stage for stripping broken and damaged samples,but as time prolongs,plenty of electrolyte solution and cathodic protection will accelerate the stripping of coating and hence produce severe crevice corrosion.In high humid soil,the damage coating will cause stripping,and cathodic protection will accelerate this process.(2)In moderate humidity saline soil,the damaged coating samples can get completely cathodic protection,and the stripping place of broken and stripped coating samples will produce more serious stripping,leading to more serious corrosion.(3)In low water content soil,no corrosion will happen for all coatings in a short time.(4)In alternating dry and wet soil,the corrosion of the damaged coating samples is the most serious.As time prolonging,new stripping and damage of the coating will appear serious crevice corrosion occuring under the stripping coating.At the same time,many solid matters such as KNaCO3,KOH and NaOH appeared in the stripping place of damaged coating and broken stripped coating samples.