A Brief Analysis of the Safety Hazards Caused by Corrosion of Equipment and Pipelines in Chemical Enterprises and the Corresponding Control Measures

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In recent years, safety accidents caused by corrosion and leakage in the equipment and pipelines of chemical enterprises have occurred from time to time. If monitoring, prevention, and control measures are not taken in a timely manner, this can lead to serious hazards, resulting in significant financial losses or casualties for these enterprises. The severe explosion that occurred at Liaoning Panjin Haoye Chemical Co., Ltd. on January 15, 2023, serves as another warning to us. Corrosion and leakage prevention management must receive great attention from chemical enterprises. The author analyzes the mechanisms of corrosion as well as preventive and control measures, thereby providing suggestions for the safe production management of such enterprises. Corrosion morphology: Corrosion morphology can be divided into general corrosion, local corrosion, stress corrosion, hydrogen corrosion, etc. I. General corrosion: General corrosion is a type of corrosion that occurs over a large area of the equipment’s pipes, with a similar degree of corrosion throughout that area. As a result, the metal thins out significantly over a wide range, leading to its destruction and rendering it unusable. For example, the corrosion of carbon steel in strong acids and strong bases is a type of general corrosion. For general corrosion, the corrosion rate can be calculated through coupon testing or regular inspections, allowing an estimation of the service life of metal structures or equipment; as a result, the risk of sudden failures and leakage incidents is relatively easy to manage. II. Compared to general corrosion, local corrosion is far more hazardous. Failure incidents caused by local corrosion often occur without any warning signs; they are sudden failures that are generally difficult to predict. Controlling damage resulting from local corrosion is also challenging, which leads to inadequate control measures and poor implementation of these measures. This can result in serious leaks that may cause fires or injuries. If pitting can cause perforation and leakage in containers or pipes, stress corrosion can lead to a **reduction in the load-bearing capacity** of components. Among local corrosion, pitting, crevice corrosion, stress corrosion, and corrosion fatigue are the most prominent forms. 1. Pitting corrosion: A type of corrosion that occurs in a very small area on the metal surface and penetrates deep into the metal; it generally has a small diameter but great depth. Characteristics of the hazards caused by pitting: (1) Once pitting occurs, the dissolution rate inside the pores is quite high, often leading to accidents suddenly ； (2) Pitting often occurs in metals or alloys with self-passivating properties, and it is more likely to happen in media containing chloride ions; for example, austenitic stainless steel pipes are most prone to pitting when transporting media containing chloride or bromide ions. (3) Pitting usually occurs in stagnant solutions; the presence of flow or an increase in flow rate can often reduce or prevent pitting. According to investigations into cases of failure caused by corrosion, general corrosion accounts for only about 20%; the remaining approximately 80% is due to localized corrosion damage. For example, on June 21, 2019, an explosion occurred in the hydrofluoric acid alkylation unit at a refinery owned by Energy Solutions Inc. in Philadelphia, USA, resulting in 5 injuries. The direct cause of the accident was that the pipe elbow thinned due to corrosion, resulting in a rupture; this led to a leak of propane from the pipe and consequently a fire and explosion. 2. Crevice corrosion: Localized corrosion that occurs on the metal surface in a corrosive medium, in crevices and other concealed areas. Crevice corrosion is generally attributed to the principle of a concentration cell, that is, it results from differences in oxygen concentration or metal ion concentration between the solution inside the crevice and the surrounding solution. Crevice corrosion occurs in many media, but it is most severe in chloride-containing solutions. Its mechanism involves not only the effect of oxygen concentration cells but also autocatalytic effects similar to those in pitting corrosion. When the material transported through pipes is an electrolyte solution, crevice corrosion occurs in the gaps on the inner surface of the pipes, such as at flange gaskets and in areas where single-sided welding has not been completed properly. Some ductile metals such as stainless steel, aluminum, titanium, etc., are prone to crevice corrosion. Crevice corrosion is characterized by its stealthy and sudden nature. For example, on April 22, 1992, an explosion occurred in a pipeline carrying crude oil in Guadalajara, Mexico, resulting in 206 deaths, 1,470 injuries, and many people going missing. The direct cause of the accident was gap corrosion and leakage in the pipeline, which created a flammable gas mixture that reached its explosive limit; this mixture was ignited by an unknown source of fire, resulting in an explosion. III. Stress corrosion is the damage to metals in a corrosive medium under tensile stress. Stress corrosion cracking of metals in alkaline solutions is known as alkali embrittlement ； Chloride stress corrosion cracking of stainless steel ； Sulfide corrosion and cracking of metals in media containing both hydrogen sulfide and water, etc. At 23:55 on November 6, 2011, an explosion occurred in the reflux tank at the top of the deethanizer tower at Jilin Songyuan Petrochemical Co., Ltd., resulting in 4 deaths and 1 serious injury ； The direct cause was hydrogen sulfide stress corrosion, which led to the formation of microcracks in the head of the reflux tank. As these tiny cracks grew larger, the strength of the tank head decreased sharply, resulting in its sudden complete failure. Stress corrosion can be prevented from occurring through the following approaches. (1) Try to avoid using materials sensitive to stress corrosion ； (2) When designing the equipment structure, efforts should be made to ensure rationality, in order to minimize stress concentration and the accumulation of corrosive agents ； (3) Pay attention to eliminating residual stress when manufacturing processing equipment. IV. Hydrogen corrosion: Under high temperature and pressure, hydrogen in the gas phase penetrates into the steel in the form of hydrogen atoms, where it combines with carbon in the steel to form methane. This leads to decarburization of the steel’s surface, reducing its strength and ductility; in severe cases, it can cause bubbling or cracking on the surface. Hydrogen corrosion is divided into hydrogen blistering, hydrogen embrittlement, and hydrogen erosion. (1) Hydrogen bulging: Hydrogen atoms diffuse into the voids within steel and combine there to form hydrogen molecules. Since these hydrogen molecules cannot diffuse, they accumulate, creating high internal pressures that cause the surface of the steel to bulge or even crack; this phenomenon is known as hydrogen bulging. (2) Hydrogen embrittlement: When hydrogen atoms enter the metal, they increase lattice strain, thereby reducing toughness and ductility and causing embrittlement. (3) Hydrogen embrittlement: Under high temperature and pressure conditions, hydrogen enters the metal and reacts chemically with a certain component or element, leading to the degradation of the metal. Safety accidents caused by hydrogen corrosion occur from time to time both domestically and internationally, resulting in serious consequences; for example, on April 2, 2010, a leak in a heat exchanger at the U.S.-based company TESORO led to a fire and explosion that killed 7 people ； The direct cause is that the heat exchanger experienced carbon steel embrittlement and reduced strength due to high-temperature hydrogen corrosion, leading to brittle fracture under high temperatures. Since the manifestations of hydrogen corrosion are rather subtle, if regular monitoring and management are not strengthened, potential hazards can easily be overlooked, leading to accidents that result in losses of life and property. Therefore, we must pay close attention to measures aimed at preventing uncontrolled corrosion. Every accident should serve as a reminder and prompt us for reflection; such accidents are closely related to a lack of emphasis on inherent safety within enterprises, inadequate routine testing and monitoring, and insufficient implementation of control measures. How to implement effective safety precautions will be explained below from the aspects of engineering technology, management, and emergency response. I. Engineering and technical measures 1. Design aspects. Select a material that meets the requirements based on factors such as medium properties, operating temperature, pressure, flow rate, and impurity content. Materials with high cost-performance are selected by taking into account factors such as the load distribution of equipment and pipelines, temperature ranges, and the size of corrosion areas. 2. Construction management. Develop construction plans and technical measures for equipment and pipeline installation, organize training for employees to ensure they are familiar with and proficient in these technical measures, so that they can be effectively implemented during the construction process. 3. Medium treatment. The process medium is treated using methods such as dehumidification and dust removal, oxygen removal, and desalination, in order to avoid or reduce corrosion of the equipment and pipelines by the medium, and to minimize the damage caused by such corrosion. 4. Cover layer. Anti-corrosion measures such as coating the equipment pipes with paint (e.g., anti-rust paint), metal coatings (e.g., galvanizing), lining, or anti-corrosion layers are applied to protect the actual structure of the equipment pipes. 5. Electrochemical protection. By applying an external current to change the potential of the equipment’s pipelines, the corrosion of metal products can be slowed down or prevented; this is categorized into cathodic protection, anodic protection, etc. 6. Add a corrosion inhibitor. One or several substances are added to the corrosive medium in order to prevent the medium from corroding the metal, without altering other properties of the medium. II. Management Measures: Enterprises should pay attention to risk analysis of the factors that cause corrosion in equipment and pipelines, formulate targeted and actionable control measures, clarify management responsibilities, and ensure their implementation. In terms of specific management measures, attention should be paid to the following points. 1. Enterprises should establish anti-corrosion and anti-leakage management systems, specifying the specific management contents and requirements. Detection plans and schedules are established for the critical areas of equipment pipelines, specifying the locations to be inspected, the timing, and the responsible persons. In accordance with established regulations and these plans, the corrosion status of the equipment pipelines is monitored at various levels. 2. Conduct regular inspections. Enterprises should, in accordance with the requirements of their management systems, conduct regular inspections for leakage detection on equipment and pipelines that pose a high risk due to the hazards associated with the media involved, and keep accurate records of such inspections. This allows them to identify leakage points as soon as possible and take effective control measures as well as emergency response actions, thereby eliminating potential safety hazards at an early stage. For equipment pipelines where corrosion-induced thinning can lead to serious consequences, it is necessary to enhance corrosion monitoring and shorten the inspection intervals, especially in areas such as tees and elbows along the pipelines that are prone to erosion and corrosion. This helps to identify and eliminate potential safety risks associated with thinning of the pipeline walls due to corrosion. Prevent the increasing leakage of harmful substances, which could eventually get out of control and lead to safety accidents. At the same time, enterprises should continuously improve their testing and diagnostic technologies, by using advanced instruments such as portable temperature sensors and infrared leak detectors, to more accurately and promptly identify early signs of minor leaks in equipment and pipelines. 3. Carry out preventive maintenance on equipment and pipelines. Through regular monitoring and testing, an accurate, complete, and effective database is established. The monitoring data is analyzed to determine the corrosion rate and remaining service life of equipment and pipelines, allowing for the formulation of maintenance or replacement plans. Process control measures are strengthened to facilitate \"preventive\" or \"predictive maintenance\", thereby avoiding or reducing \"reactive maintenance\" and lowering safety risks as well as uncontrollable factors. 4. Optimize process operation parameters. Where process conditions permit, strive to reduce the operating temperature, flow rate, concentration, etc., of corrosive media such as acids ； Control the content of corrosive impurities in the raw materials. Special attention must be paid to equipment pipelines operating under high temperatures, high pressures, and high flow rates, with increased frequency of inspections and tests. 5. Strengthen training. Companies should provide targeted training for professionals and employees at all levels on corrosion and leakage prevention management systems, inspection procedures, and preventive maintenance of equipment and pipelines. This is aimed at enhancing the sense of responsibility among these professionals and employees, as well as ensuring the effective implementation of job responsibilities ； Second, it is necessary to continuously improve the competence and skills of professionals and employees, ensure the proper use of advanced instruments and equipment, so as to obtain accurate, valid, and reliable testing data. Additionally, these testing data should be analyzed to develop effective control measures, which must then be put into practice. III. Emergency management measures 1. Develop an emergency response plan for leaks. Enterprises should organize specialized departments and personnel in areas such as processes, equipment, technology, and safety to develop specific emergency plans for leaks in locations and facilities where such incidents may occur. The formulation of a contingency plan must take into account the actual conditions of the enterprise, avoiding mechanical application and uncritical adoption of external models. The plan should be comprehensive, targeted, guiding, and practical. 2. Provide training on emergency response plans for leaks. After the leakage emergency response plan is approved and issued, it is necessary to develop training plans for the relevant professionals and employees in those positions, organize training according to these plans, and verify the effectiveness of such training. This ensures that every employee is familiar with and understands the contents and requirements of the emergency response plan. For employees who fail the training, corrective measures must be taken and their progress monitored. 3. Organize emergency drills for leak handling. Develop an exercise plan and organize relevant employees to participate in practical drills; any issues identified during these drills should be addressed and improved promptly. Only through continuous drilling and reflection can employees’ emergency response capabilities be continuously enhanced. Only in this way can one remain calm and carry out emergency responses in an organized manner when an accident occurs. 4. Prepare emergency rescue supplies. In accordance with the emergency response plan and the actual requirements on site, emergency supplies should be properly stocked, and dedicated personnel should be assigned to carry out regular inspections and maintenance to ensure their effectiveness and reliability. Any emergency supplies that are found to be defective or damaged during inspections should be replaced promptly, in order to prevent accidents from escalating or even safety incidents from occurring due to the failure or damage of such supplies during emergency responses.