Study Of In-service Small Diameter Pipeline Internal Coating Evaluation Technology

As an important part of the gathering pipe network,the corrosion and protection of small-diameter pipeline has always been the research difficulty and focus in the industry.Due to the small diameter of small-diameter pipeline（≤200mm）,the pipeline material is mainly carbon steel,resulting in a certain limitation for small-diameter pipeline surface protection.In this regard,the industry has a variety of anti-corrosion measures,such as:inner coating,corrosion inhibitors and lining structure,and so on.which for small-diameter pipelines in China using technology process of the pneumatically-pig-squeezed for inner coating.But no matter which one can not solve once and for all type of small-diameter pipeline anti-corrosion,once the failure of protective measures will cause serious economic losses and even threaten personal safety.In China,there is no corresponding implementation standard for the safety evaluation of the inner coating of small-diameter pipelines,and the conventional evaluation methods will not only cause a certain degree of economic loss but also cannot well simulate the impact caused by the service environment.In response to the above problems,this paper explores the factors influencing the failure of four specimens（divided into new 1#,old 1#,new 2#and old 2#according to the before and after service with two calibers）of the inner coating of a small diameter pipe in service at a gas mine by means of adhesion,salt spray test,immersion test and electrochemical test and other performance evaluation.;combined with Ansys program,the relationship between the pressure,temperature,flow rate and the inclination of the pipeline under the coupling effect of four influencing factors on the inner coating is investigated;based on the test results,a safety evaluation system is established for the inner coating of small-diameter pipes by Fuzzy Analytical Hierarchy Process.The main research results are as follows:（1）Experiments on coating thickness,adhesion,salt spray and immersion were conducted on new/old 1#and 2#.The results show that the thickness of the coating inside the new 1#and2#pipes is>450μm,the adhesion is 4A,the coating has no leakage（qualified）,the bending test has cracks（unqualified）,the porosity of the section/bonding surface is grade 3（qualified）,the500 h salt spray test is qualified;the 6 J impact test is only qualified for the new 2#,the 90 h acid and alkali immersion test is qualified,and the salt solution immersion test is unqualified for the new 1#specimen.The performance of the coating can meet the needs of corrosion protection in small diameter pipelines.Old 1#and old 2#pipeline inner coating from the macroscopic and coating thickness test results,there is a large area of coating off and coating degeneration is brittle loose,the above tests are not qualified,this is due to the inner coating in service environment by erosion degeneration,resulting in the coating protection effect is reduced,such as long-term use will lead to corrosion of the inner wall of the pipeline.（2）the new/old 1#,2#specimens for the potentiodynamic polarization curves and electrochemical impedance spectroscopy（EIS）,the results show that:in the new 2#specimen surface coating with the increase in soaking time,E_corr has a significant negative shift,which indicates that the new 2#internal coating in the simulated solution with the increase in soaking time its corrosion tendency increased;i_corr also with the increase in time and increase,the same indicates that the longer the specimen soaking time its is the faster the corrosion rate,the worse the coating protection performance.With the increase in soaking time,these ions and water molecules penetrate the amount of increase,therefore leading to an increase in corrosion rate.In the impedance test,the impedance value of the old 1#/2#is much lower than that of the new1#/2#,which indicates that the protective performance of the coating inside the pipe after service is poor;the impedance of the new 1#/2#specimen is consistent with the polarization curve test results,and the radius of capacitive arc resistance decreases with time,and the protective performance of the coating decreases.（3）By using the orthogonal experiment to design the Finite Element Modelling（FEM）experimental program,using four factors and three levels of orthogonal test table and combined with the actual working conditions to set the value of the individual factor level,establish the geometric model of the pipe and set parameters for it,a total of nine sets of tests.The results show that the order of influence under the coupling of four factors is:pressure>temperature>flow rate>inclination;Pressure is the main reason for the deformation of the inner coating,while temperature changes will lead to changes in the coating density,elastic modulus and Poisson’s ratio,the lower the temperature the greater the elastic modulus and density of the coating leads to poor deformation and shrinkage of the inner coating,increasing the internal stress of the coating,which may cause the coating failure when the internal stress exceeds the coating adhesion.（4）Combined with the above research content to summarize the influence factors of coating inside small diameter pipeline to get 38 influencing factors,divided into:coating performance（10 kinds）,service environment（10 kinds）,coating process quality（12 kinds）and coating aging（6 kinds）four categories.The weight proportion of each influencing factor was calculated according to the principle of hierarchical analysis.A rubric set is established,and several experts are invited to evaluate the inner coating of small-diameter pipes in kind according to the influencing factors,and the final physical safety evaluation score is obtained through the step-by-step fuzzy change.The method can scientifically and effectively evaluate the safety performance of the inner coating of small-diameter pipelines,and its more objective solution to the fuzzy nature of the inner coating for multiple periods,multiple objectives and multiple criteria,providing a new method for the way of evaluating the inner coating which is currently in the laboratory stage in China.