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This article introduces the successful application experience of sacrificial anode protection in Kunming’s gas pipeline network over the past 15 years since its operation, as well as its future development directions. Preface The urban gas pipeline network is one of the important infrastructure systems in a city; it is responsible for transporting energy and serves as a necessary foundation for the survival and development of the city, hence it is referred to as the city’s \"lifeline\". With the rapid economic development, urban gas pipeline networks have expanded swiftly, with various pipelines crisscrossing throughout the area. These pipelines are buried underground and are constantly subjected to severe corrosion from both the external soil and internal media, which often leads to corrosion-related leaks. This results in unplanned maintenance, replacement, and shutdown of the pipeline equipment, causing significant direct and indirect economic losses. According to statistics, in an industrially developed country, the direct losses resulting from metal corrosion account for 4% of the total value of the national economy each year. Cathodic protection refers to the process of cathodically polarizing the metal to be protected (such as gas pipelines), thereby reducing its potential to the equilibrium potential at the anode surface of the metal. This eliminates the corrosion cells caused by chemical unevenness, thus protecting the metal from corrosion by environmental agents such as soil. In other words, the corrosion of auxiliary anodes or sacrificial anode materials is used to replace the corrosion of the pipelines and equipment that are to be protected. Thus, it is possible to extend the service life of the protected pipelines and improve their safety and cost-effectiveness. Practice has shown that this is a technology with low investment costs and significant economic and social benefits. Cathodic protection technology originated over 160 years ago. In the early 1970s, it became more refined and was established as regulatory standards in countries such as Europe, the United States, and Japan. China’s early industry-standard SYJ7-84 already stipulated that oil and gas pipelines must be protected by cathodic protection. Both environmental protection requirements and the standards of the SY/T series issued by the Petroleum and Natural Gas Industry Standards of the People’s Republic of China require that cathodic protection be applied to oil and gas pipelines. Overview Cathodic protection is divided into two methods: impressed current protection and sacrificial anode protection. At present, in Kunming’s gas distribution network system, impressed current cathodic protection is used for the 17.9 KM long φ630 medium-pressure main pipeline running from Kunming Coking Plant to Area 1 of Kunming city; whereas sacrificial anode protection is commonly employed for the second main pipeline as well as the rest of the urban pipelines. It is favored for its ease of construction, high reliability, and low cost. Generally, the investment in cathodic protection systems accounts for only 3-5% of the total investment in pipeline networks. Under proper protection, it can extend the service life of pipelines by more than twice. (A cathodic protection system refers to the cathodic protection facilities: such as impressed current protection stations, sacrificial anodes, insulating flanges, monitoring piles, and the object to be protected—the gas distribution network.) As well as the corresponding testing and management software. ) By the end of 2000, the total length of urban gas pipelines was 1,113.82 kilometers; there were 380,000 urban gas users. In total, more than 1,200 sets of sacrificial anodes were used, nearly 4,000 sets of large-diameter insulated flanges were employed, 252 pressure regulation stations existed, 567 gas valves were in use, and there were 4,142 gas collection wells. The total investment in urban gas pipeline infrastructure amounted to 4,054,613,143.7 yuan. Buried gas pipelines and facilities that have been in service for over 15 years are now reaching a stage where major repairs or even partial replacement are necessary. By conducting an in-depth investigation into the causes of corrosion in the city’s gas pipelines and providing a comprehensive evaluation of the effectiveness of cathodic protection over the 15 years since the pipelines began to be in use, it is possible to identify successful practices. At the same time, it is possible to examine the problems and shortcomings in current operations, thereby improving efforts to address pipeline corrosion, predicting and preventing potential accidents in advance, and minimizing the occurrence of such accidents as well as associated economic losses. Due to soil corrosion and other disturbances affecting buried gas pipelines, statistics on the service life of such pipelines in some areas of the country show that it ranges from 3–5 years in lower cases to over 10 years in higher cases; in severe cases, the pipelines can become corroded and damaged, leading to leakage accidents within just a few months. Cathodic protection has been in use for the Kunming urban gas pipeline network project since its construction began in 1982, and to date, there have only been two incidents caused by corrosion-induced perforations and leaks. Summarizing the experience, we mainly carried out the following tasks. I. Environmental investigation and plan formulation Since the construction of Kunming’s gas project began in 1982, the gas network has continued to expand. It has been in operation since 1986, and 15 years have passed since then. In the 1988 report titled “Investigation on Corrosion of Gas Pipelines in Kunming City and the Effectiveness of Cathodic Protection,” experts from outside the province were invited to conduct an investigation into the cathodic protection of the gas network; the conclusions of this investigation were as follows: 1. The corrosivity of the soil in Kunming’s urban area: moderate corrosion accounts for 50%, while severe corrosion also accounts for 50%. Due to shallow sampling, the actual soil corrosivity is higher than the values obtained from analytical testing. 2. The maximum protection level achieved through cathodic protection is 98%, while the average protection level is 59%. 3. The ratios of the average weight loss, average maximum corrosion depth, and average maximum corrosion rate for pipes with cathodic protection to those without it are respectively 1:2.76, 1:2.52, and 1:2.49. 4. The cathodic protection technique can reduce pipe corrosion and extend their service life by 1.49–1.76 times, resulting in significant economic benefits. The urban gas pipeline network in Kunming was fully equipped with cathodic protection, which at that time represented the first large-scale application of such technology in domestic urban gas pipelines, and it was successful. Based on the survey results regarding soil corrosivity and soil resistivity in Kunming, and considering factors such as cost-effectiveness, management, and safety, we adopt external current cathodic protection for the main pipelines in the suburban areas, while using a combined protection system of magnesium and zinc sacrificial anodes for the medium and low pressure pipelines in urban areas as well as in residential complexes. II. Strengthening the anti-corrosion, insulation, and cathodic protection of pipeline networks The gas pipeline networks in the urban area of Kunming generally use asphalt-glass cloth for enhanced or extra-enhanced anti-corrosion measures; in addition, more than 10 kilometers of polyethylene tape have been tried out for anti-corrosion purposes at the Kunming Cigarette Factory and along the Longquan Road section. Due to environmental and other considerations, epoxy coal tar pitch is now being used for anti-corrosion purposes. According to regulations, an electric spark withstand test is required for anti-corrosion pipes before they leave the factory. During on-site construction, it is required to carry out on-site anti-corrosion treatment and spark testing for any damaged areas of the anti-corrosion layer as well as at the joints; there are also specific requirements regarding the backfill soil. To ensure the protective potential of the pipeline network and avoid mutual interference, we first separated the medium and low-pressure main pipelines from the urban pipeline network using insulating flanges; meanwhile, the medium and low-pressure urban pipeline network was also separated from the pipelines in residential areas and large courtyards through insulating flanges. The urban medium and low-pressure pipeline network was further divided into four sections. The voltage regulation station and valve chambers are also isolated using insulating flanges, while the pipes at both ends are connected electrically through cables. At the gas inlet risers in urban areas, φ57 insulated flanges are used for isolation in all cases. This results in a relatively higher investment, but it is also necessary to ensure the effectiveness of cathodic protection for the pipeline network. Combined protection is achieved using magnesium and zinc sacrificial anodes, with one set buried every 1 to 2 kilometers on average; simultaneously, 10 \"corrosion samples\" were buried in various areas to facilitate comparative analysis in the future. The setting of inspection piles is determined based on management requirements and the regulation of potential across various sections. III. Implementing targeted corrosion control in conjunction with major renovations At the initial stage of operation of the gas pipeline network, the quality of the gas produced by the coking plant was poor, with a high content of corrosive impurities in the form of hydrogen sulfide – around 2–4 g/Nm3. Even after passing through the desulfurization unit, the effect was insufficient, and the hydrogen sulfide concentration remained as high as 1.5 g/Nm3. The quality standards for city gas stipulate that the H2S content must not exceed 20 mg/Nm3. Furthermore, high levels of impurities such as chlorides, dissolved oxygen, sulfur dioxide, carbon dioxide, and water cause the condensate in the pipeline network to absorb large amounts of corrosive substances, which inevitably leads to corrosion of the metals. In particular, the condensate accumulates in the main pipes, as well as in the low- and medium-pressure main pipe condensers and in sections of pipes with a low slope within the urban area. In March 1989, to support the construction of the interchange at Kunming West Station, it was necessary to modify the medium-pressure gas main pipes; we took this opportunity to collect samples from the inner walls of two medium-pressure condensers as well as from the inner walls of certain sections of the pipes, and to conduct analyses and tests on them. The results showed that corrosion was most severe on the inner wall of the pipe condenser, concentrated in the liquid phase area of the condenser and at the welds (where the deepest corrosion pits on the outer wall of the liquid phase areas of the water extraction pipes inside the two condensers reached 0.95–2.88 mm per year). In the two condensers, the maximum depth of corrosion pits on the liquid-phase side walls was 2.34–3.66 mm, with an average maximum depth of 2.01–2.73 mm; the average maximum corrosion rate was 0.923–0.431 mm/year. In the two liquid-phase end caps, the maximum depth of corrosion pits was 1.61–2.80 mm, with an average maximum depth of 0.892–2.147 mm, and the maximum corrosion rate was 0.635–1.095 mm/year. The maximum corrosion rate at the gas-phase end caps was 0.351–0.356 mm/year. At the joints where the liquid-phase side walls met the end caps, the average maximum depth of corrosion pits was 2.12–5.34 mm, with a maximum corrosion rate of 1.388–2.934 mm/year. To address the corrosion issue in the liquid phase area inside the condenser, we conducted experiments by installing small-sized magnesium and zinc anodes within the condenser. One end of the anode’s steel core was welded to the lower part of the condenser’s inner wall; both ends were treated to prevent corrosion, and the bottom of the anode was insulated, in order to replace the corrosive effect of the electrolytic liquid inside the condenser on its inner walls. It has been widely applied in condensers for pipe sections of φ219 and above, and to date no accidents resulting from corrosion and perforation of the inner wall of the condensers have been reported. IV. Preventing interference from stray electricity in Gui-Kun electric locomotives and ensuring the safe operation of the main pipelines On the No. 1 φ630 medium-pressure main pipeline, which is 17.9 km long and runs from the Kunming Coking Gas Production Plant to the urban area of Kunming, we have employed external current protection, with a protection level of 98%. One section, approximately 4 km long, crosses and runs parallel to the Guiyi-Kunming electrified railway, resulting in strong stray current interference. The construction plan for the Guiyun electrified railway was approved at the ** level in 1981, ahead of the construction of the gas pipeline network. For the Guiyun electrified line, 110 kV voltage was stepped down to 27.5 kV for use on the contact network above the railway tracks. The power supply method employed was BT single-phase single-sided supply; however, this BT technology was not advanced, and there was a large amount of stray current flowing into the ground through the rails (AT power supply is more advanced). 14 traction substations will be built along the line, one of which is located in Jinma Village. The load and traction current of the Guiyun electrified railway are, in the initial stage, under single-unit operation, with an average traction current of 300A and losses of 580A. In the future, with dual-unit operation, the traction current will double, averaging 600A with instantaneous values reaching 1000A. Its locomotive traction current flows from the traction substation → overhead contact line → locomotive → rails (and soil) → return wire → traction substation. Although, per technical requirements, insulation is used between the rails and sleepers, the stray current flowing into the ground accounts for 50% of the total traction current, and this figure is even higher in the event of a short circuit. According to reports from relevant domestic authorities, electrified locomotives cause significant interference with other underground metal structures due to the leakage of stray currents; this interference extends over a range of several kilometers. Under the influence of stray currents and induced electricity, the voltage on pipelines can reach 57 V, resulting in a corrosion rate of 2 mm per year for steel pipes, with rates as high as 6 mm per year in severe cases. The Songshan section of the Northeast Oil Transport Pipeline suffered corrosion caused by alternating current stray currents of more than 5 mm. Stories of corrosion and perforation in oil and gas pipelines occur from time to time). Following discussions, Tie Er Yuan agreed to install suction devices in Jinma Village, 2400 meters in the direction of Guiyang up to Kunming Station; these devices can reduce stray electricity in the ground by 90%, thereby minimizing the impact of stray currents on gas pipelines. At the same time, Tie Er Yuan requested that grounding and current-discharge measures be implemented for the main gas pipelines. Since the corrosion rate caused by stray electrical interference is much higher than that due to soil, in order to prevent the 4 KM long main gas pipeline from suffering corrosion as a result of such interference at the section where it crosses parallel to the Guiyi-Kunming electrified railway, we installed 6 sets of magnesium anodes as grounding and current-discharge devices on the gas pipeline at that crossing point, building upon the external current cathodic protection system already in use on the original 17.9 KM long main gas pipeline. After multiple tests, and in 1993 experts from the Sanming Wireless Power Plant in Fujian were invited to conduct on-site tests to assess the impact of stray electrical interference on the main pipelines. (When trains were traveling from Kunming to Guiyang, the alternating voltage measured on the main pipelines located near the railway was between 0.46–0.01 V; the starting voltage was between 0.59–5.70 V. At the points where the main pipelines crossed the railway tracks, the voltage measured at the current protection stations was 2.4 V…) The results showed that by using suction devices and sacrificial anode systems for current dissipation, the impact of stray currents on the main pipelines could be effectively controlled, thereby ensuring the safety of the gas main pipelines. V. Analysis of the operation protection effectiveness and reasons for failure Based on cumulative test data from over a decade as well as recent data analysis, the operation protection effectiveness is as follows: 1. The average protection rate for the No. 1 main pipe with a length of 17.9 KM and a diameter of 630 is 98% ； 2. The average protection rate for the 18.5 KM long φ630 main pipe No. 2 is around 87% ； 3. The average protection level of the medium and low voltage main lines in zones 1 to 4 of the urban area is around 80%; among them, one location in zone 3 and four locations in zone 2 do not reach the protection potential of –0.85V ； 4. The average protection rate for courtyards and residential complexes in Zones 1 and 2 of the urban area is around 80% ； 5. The average protection rate for courtyards and residential complexes in the three urban areas is around 70% ； 6. The average protection rate for the courtyard communities in Zones 4 and 5 of the urban area is around 68%, with the potential at the end of 21 of these courtyards ranging from 0.592V to 0.823V ； 7. (Outside the Second Ring Road and in the suburbs) The average protection rate for medium and low voltage main lines in Zones 5, 6, 7, and 8 is around 65% ； Analysis of the causes of failure: 1. In Phase I and some sections of Phase II, proper attention was paid to the quality of the anti-corrosion coatings on the main pipelines as well as the medium and low-pressure main pipes in the urban area, and adequate production and maintenance management was in place. In particular, for the gas pipelines and sacrificial anodes in Phase I and some sections of Phase II, the construction units were all state-owned group companies that enforced strict quality controls and demonstrated a high level of responsibility. Therefore, the protection effect of the No. 1 main pipeline and the medium and low-pressure main pipelines in areas 1 to 4 is quite good. 2. In the second half of the second-phase project and the third-phase project, the construction and supervision of sacrificial anodes were somewhat separated from each other; moreover, the problems identified during inspection were not rectified in a timely manner, which led to damage to the anti-corrosion coating and consequently a decrease in the protective potential. Such as large courtyards in areas like Sunshine Community. 3. As the pipeline network and sacrificial anodes have been in use for over a decade, the asphalt anti-corrosion coating has aged; as a result, the current output of the anodes continues to increase, the protection potential tends to decline, and excessive consumption of the anodes occurs, which leads to a gradual decrease in the protection level. 4. Decrease in protective potential caused by \"short circuits\" in the pipeline network due to urban redevelopment and demolition, as well as leakage resulting from damage to the anti-corrosion coating. 5. The lack of maintenance personnel and inadequate supervision of on-site construction result in maintenance of sacrificial anodes failing to keep up with the pace of pipeline network construction; as a consequence, the sacrificial anode facilities cannot be maintained in a timely manner nor can the anode materials be replaced, which is also a factor contributing to the decline in protection potential. VI. Future Development Directions 1. Strengthen the management and maintenance of pipeline networks, striving to achieve informational management. In summary, building on the experience gained from the effective use of cathodic protection, we will strive to overcome existing problems and shortcomings, and take measures to advance work in all aspects. From the perspective of long-term benefits such as ensuring the safety of pipeline networks and extending their service life, cathodic protection serves the long-term economic and social benefits of these networks. Given the nature of the urban gas industry, “safety is also a benefit.” Safety presupposes quality assurance, whether it comes to the quality of cathodic protection works or the quality of the anti-corrosion coating ； Or from the professional competence of the management staff ； Moreover, the necessary investments in software and hardware also require standards and regulations to govern them ； There are measures in place for supervision ； There are funds to ensure it. At the same time, it is necessary to establish and improve the operation and management mechanisms for cathodic protection systems, clarify responsibilities and management scopes, and digitalized management of records and other materials should also be included on the agenda. 2. Simultaneous census and monitoring It is necessary to understand the current state of corrosion in the underground gas pipelines in our city, as this is essential for disaster prevention and responding to sudden leaks, and it plays a crucial role in ensuring the proper operation of the city’s \"lifeline\" systems. Inspect the damage to these pipelines and predict their lifespan ； Apply new technologies to carry out anti-corrosion treatment on new and existing pipelines in order to extend their service life. To prevent corrosion and perforation of pipelines, enhance the safety of pipeline network operation, and improve the efficiency of its use. Proactive prediction and prevention of corrosion and leaks in gas pipelines are essential, enabling targeted, planned, and systematic operation management, maintenance, and corrosion control monitoring. In particular, in the face of emergencies and disasters, it is necessary to make rapid and accurate decisions as well as provide effective rescue responses. Next, by investigating the corrosion status of the pipeline network and analyzing corrosion samples, the current situation and patterns of corrosion in the pipeline network will be determined; at the same time, corrosion monitoring for various parameters on both the inner and outer surfaces of the pipes will be increased. By learning from experts and peers, we have developed a cathodic protection management information system, which elevates the functions of cathodic protection beyond mere management, maintenance, and corrosion control to include corrosion monitoring and prediction of potential hazards. Recently, the company has been carrying out measurements and calculations of the thickness reduction in pipeline walls (i.e., metal loss), with the aim of establishing a database on this aspect in order to provide a basis for future plans for the repair or replacement of pipelines, thus ensuring their safe and continuous operation. 3. Differentiate the protection measures for new and old pipeline networks. Analysis of test data shows that the protection potential of some pipe sections is not uniform; using the traditional method of installing sacrificial anodes does not ensure effective protection for these pipelines. In the future, we will adopt a decentralized burial method to address this issue, and install sacrificial anodes in areas with strong stray current interference to achieve both protection and current discharge effects. Insulated flanges are used to separate the new and old pipeline networks; a comprehensive assessment is conducted on the aging of the anti-corrosion coating on the old networks, and the anodes are gradually replaced. 4. Research on gas tank protection and deep-well anode projects With the aim of ensuring economic efficiency in investment and expanding the effective coverage of pipeline protection in urban areas, we will explore the use of deep-well anodes. Based on learning from the experiences of our peers, we will include cathodic protection for gas tanks in our research agenda. Building on the successful application of impressed current protection and sacrificial anode protection for the pipeline networks in our city, we will explore deep-well anode protection and gas holder cathodic protection technologies, striving to ensure that all these protection methods can be successfully implemented in our city’s pipeline networks.