Quantitative Research on Signal of Pipeline Coating Defect Detection Based on HHT
Coating is a very important way to ensure the integrity of the pipeline. However, due to environmental or external factors, the coating will has some defects such as peeling defects, holes and so on. This paper is concentrated on axial stripping defects and holes of buried pipeline coated with anti-corrosion coating, using ultrasonic guided wave detection technology to obtain the coating detection signal to do the study of quantification, it provides a feasible method to study the level quantitative description of the anticorrosive layer peeling defects.Doing time-frequency analysis to the operating status of the coating, we will have a conclusion that when the coating has peeling defects, the waveform of the detection signal will has a nonlinear characteristic signal. According to the analysis of time-frequency distribution, we can find out that when the signal has nonlinear characteristic signal, the time-frequency distribution will have a "concave" type frequency distribution, it is the basis to judge the junction of the coating peeling defects. According to the judgment, and obtain the relative length of the coating stripping defect, it will finally obtain the degree of damage caused by peeling defects.Therefore, this paper is a post-treatment method for pipeline coating detection, providing feasible and effective signal processing methods and quantitative research methods for the engineering application of the detection technology of non-linear ultrasonic pipeline coating peeling defects.
An Electrochemical Measurement for Evaluating the Cathodic Disbondment of Buried Pipeline Coatings under Cathodic Protection
Steel pipelines are susceptible to corrosion by the action of corrosive substance in the environment and one of the most common failure modes in buried pipeline coating is cathodic disbondment. Consequently, Electrochemical Impedance Spectroscopy (EIS) was used to assess the effect of three important parameters, the thickness of coating, the artificial defect and electrolyte type, on cathodic disbondment, according to ASTM G8. Experiments were done in 3.5 % wt NaCl, KCl and CaCl2 solutions at room temperature with different coating thicknesses (354, 483 and 1014μm) and several artificial defects diameters (3, 6 and 9 mm). Immersion time was 28 days and during this period, a -1.5 V cathodic potential (vs. SCE) was applied. Further, the EIS measurements were done in Open Circuit Potential (OCP). Investigations showed that a thick polyurethane coating was highly resistant to cathodic disbondment in intact areas and that coating thickness increases caused decreases in the rate of cathodic disbondment in areas with defects. On the other hand, the evaluation on artificial defects showed disputable results, though with no logical correlations between artificial defects' diameters and any disbonded area individually. In addition, assessment on electrolyte type showed the rate of cathodic disbonding might be classified after two parameters: solubility and mobility of ions. So, the disbonded area was dependent on the cation type in the electrolyte, and the anion diffusion had less effect in the cathodic disbondment process. Furthermore, the comparison between the cations and the anions in solution showed that the most important coating disbondments occurred when cations / anions are in a 1:1 ratio.
An EIS study of ion diffusion through organic coatings
Abstract A very important mechanism of organic coating deterioration is the cathodic delamination. In this study a double electrochemical cell has been used in order to keep separated the solution in contact with an artificial defect and the solution in contact with the rest of the intact organic coating (polyester coated phosphatised steel). In this way it is possible to polarise the sample in the cathodic region and to control the environment (ions and concentration, oxygen, water activity) in the defect and around the defect. Moreover it is possible to measure electrochemical parameters in the two different areas of the coating. The cathodic delamination was measured, in different solutions, by measuring the distance of delamination from the artificial defect after different times of exposure under cathodic polarisation. The coating properties have been evaluated by analysing electrochemical impedance data obtained in the two parts of the cell. The behaviour of different cations and anions has been compared and a different diffusion mechanism was found for the cations couple Li and Na and the ions couple K and Cs. The second couple, because of the smaller dimension of the hydrated shell can diffuse through the coating, while Na and Li diffuse mainly through the defect. The anions diffusion plays an important role in the case of positive ions diffusing through the coating, partially balancing the positive charge and increasing the cations diffusion rate. For these reasons the delamination rate in KCl solution is higher that the delamination rate in KBr.