Study on Distribution Characteristics of Metro Stray Current and Evaluation of Cumulative Corrosion Effect

Stray current directly affects the regular operation of electrical equipment and facilities in the subway DC traction power supply system. Therefore, it is worthwhile to study the stray current distribution characteristics during train operation and the quantitative corrosion of buried pipelines. This paper introduces the traction characteristics of power carriages and power wheelsets of subway vehicles into the DC traction process. A finite element model considering the dynamic distribution of stray current under the actual operation of subway vehicles is established. The interference characteristics of stray current and the contribution of power sources under the multiparticle model are analyzed. The rail insulation damage caused by long service time and the quantitative calculation of rail and buried pipeline corrosion is considered. The model results show that the stray current in the buried pipeline under the multiparticle model is more accurate and more suitable for the protection in the actual subway. The quantitative corrosion of the buried pipeline is stronger than the partial insulation damage environment when the rail is not insulated. The rail and buried pipeline corrosion at both ends of the insulation damage position is relatively severe. The stray current distribution model established in this paper gives full play to the solution advantages of the finite element method and provides a new idea for the quantitative calculation of buried pipeline corrosion.

Electrochemical trepanning of blades made of Inconel 625

In the aerospace field, difficult-to-machine materials are used widely to improve engine performance. As a nickel-based material that performs well in all aspects, Inconel 625 is used for the blisks of aircraft engines, and electrochemical trepanning (ECTr) is used widely to fabricate such blisks because of its unique advantages regarding ruled surface parts. In this study, to investigate the performance of Inconel 625 in ECTr, measurements were made of the electrochemical characteristics firstly, specifically the anodic polarization curve and the actual volumetric electrochemical equivalent curve. Then, via dynamic electric-field simulations, the processes for forming Inconel 625 blades using ECTr were examined under direct voltage (DV) and pulsed voltage. The contours and current density distributions of formed blades at different times were obtained under different duty cycles. With decreasing duty cycle, the forming accuracy improved gradually and the stray current was reduced. To verify the simulation results, ECTr experiments with Inconel 625 were performed under different voltage conditions. With DV and 90% and 80% duty cycle, the taper angles of the machined blades were 7.784°, 6.278°, and 5.191°, respectively, and the surface roughness ( Ra) values were 0.95, 0.81, and 0.72 μm, respectively. With DV, there were obvious flow marks and gullies on the microscopic surface. With decreasing duty cycle, stray corrosion was reduced effectively and the state of the flow field was improved. Overall, the simulation results were verified effectively.

Method for Mitigating Stray Current Corrosion in Buried Pipelines Using Calcareous Deposits

Stray current corrosion in buried pipelines can cause serious material damage in a short period of time. However, the available methods for mitigating stray current corrosion are still insufficient. In this study, as a countermeasure against stray current corrosion, calcareous depositions were applied to reduce the total amount of current flowing into pipelines and to prevent corrosion. This study examined the reduction of stray current corrosion via the formation of calcareous deposit layers, composed of Ca, Mg, and mixed Ca and Mg, at the current inflow area. To verify the deposited layers, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were performed. The electrochemical tests revealed that all three types of calcareous deposits were able to effectively act as current barriers, and that they decreased the inflow current at the cathodic site. Among the deposits, the CaCO3 layer mitigated the stray current most effectively, as it was not affected by Mg(OH)2, which interferes with the growth of CaCO3. The calcium-based layer was very thick and dense, and it effectively blocked the inflowing stray current, compared with the other layers.

Integrity Challenges Associated with Stray Current Corrosion Across Monolithic IJ's and Resolutions

Corrosion and subsequent failures is one of the main factors affecting uninterrupted operations of Oil & Gas Industries. Pipelines are considered as most convenient means of crude and gas transportation in Oil & Gas Industry. Buried pipelines generally made of low carbon steel material are protected externally by coating and applying impressed current cathodic protection (ICCP). Monitoring and maintaining adequate level of Cathodic Protection (CP) for such pipelines remains challenging for corrosion engineers due to increased level of field congestion, complexity in accurate current mapping and mitigation of corrosion phenomena. Failure of pipelines due to corrosion can be catastrophic with following consequences: Loss of containment fluid and thereby probable fatalityDamage to asset/company reputationSafety and Environment (Fire, Toxic gases and Oil Spill)Resource and downtime cost impact Isolation joints IJ's are designed with very high insulating material at mating areas and installed on pipelines by welding to avoid loss of Cathodic Protection (CP) current. Due to high electrical insulation, a potential difference is formed across of IJ's due to applied CP current and stray currents. In upstream Oil & Gas Industry, multiphase crude transported via pipeline will have certain percentage of water and will induce an internal conductive path across the IJs resulting in ionic current discharge at anodic areas within internal surface. This study focuses on factors contributing to such internal stray current corrosion, limitation in monitoring methodology and mitigation programs. The study concludes with recommendations such as design modifications, improvement in internal lining properties and improved installation guidelines. The study practically illustrates effectiveness of combined resistive bonding and zinc earthing cells installation for controlling stray current propagation in order to reduce the corrosion rate so as to maintain Integrity of pipelines.