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Corrosion is a problem faced by various industries and sectors. In our country, the total cost associated with corrosion amounts to about 5% of GDP, resulting in numerous casualties and economic losses. Since production in the chemical industry often takes place under high temperature and pressure conditions, and the reactants and media involved are hazardous, accidents caused by corrosion of equipment and pipelines in chemical plants cannot be ignored; therefore, it is particularly important to carry out anti-corrosion measures for equipment and to inspect for corrosion in industrial installations. Corrosion classification: To prevent corrosion, it is first necessary to understand its classification. Corrosion can be divided into metal corrosion and non-metal corrosion based on the type of material. Corrosion is classified into general corrosion and local corrosion based on surface morphology ； Local corrosion includes pitting, stress corrosion cracking, crevice corrosion, galvanic corrosion, wear corrosion, and so on ； Metal corrosion can be classified by mechanism into physical corrosion, chemical corrosion, electrochemical corrosion, etc. Physical corrosion: Damage to a material caused by purely physical effects, usually resulting from dissolution or penetration, such as the dissolution of metal containers in molten metals, or the dissolution and penetration of containers by high-temperature molten salts or caustic substances. Chemical corrosion: Damage caused by a direct chemical reaction between a metal and a non-electrolyte; no electric current is generated during the corrosion process. The corrosion process is a pure oxidation-reduction reaction; the corrosive medium collides directly with the atoms on the metal surface to form corrosion products, and no electric current is generated during this reaction, in accordance with the laws of chemical kinetics. Electrochemical corrosion: Damage caused by an electrochemical reaction between a metal and an electrolyte solution. During the reaction process, the anode loses electrons while the cathode gains electrons, along with the flow of electrons (current); this process follows the laws of electrochemical kinetics. Main causes of corrosion in chemical production processes: Many corrosive substances exist and are generated during chemical production processes. The type, chemical composition, concentration, pH value, impurities, moisture content, and oxygen level of these substances are all external factors that contribute to corrosion. The faster the flow rate of the medium, the greater the likelihood of corrosion, as the flowing medium erodes the protective film, creating vortices, turbulence, and bubbles that lead to severe impact wear and cavitation corrosion. Additionally, improper material selection can lead to surface corrosion if the equipment’s surface comes into contact with corrosive substances and the equipment itself is not resistant to corrosion. The rougher the surface, the more prone it is to corrosion; the resulting symptoms include leakage, premature wear, damage, and noise. Uniform surface corrosion can occur in two forms: with film formation and without film formation. Corrosion without film formation is very dangerous, and the corrosion process proceeds at a certain rate; this is mainly caused by incorrect material selection. Lack of anti-corrosion measures or poor construction quality all create an environment conducive to corrosion damage. Different environments require different materials. During the production of equipment, it is often not possible to balance material selection with resistance to environmental corrosion. Differences in temperature, concentration, and pressure lead to different material choices and varying levels of corrosion. Moreover, inadequate quality control during construction results in poor construction standards, making corrosion problems inevitable. Over-temperature and over-pressure during operation, inadequate equipment management, and a lack of attention are also among the causes of corrosion damage. Generally, the higher the temperature of the medium, the higher the pressure, and the faster the corrosion occurs, as corrosion is a chemical reaction; for every 10°C increase in temperature, the corrosion rate increases by 1 to 3 times. Corrosion inspection plan: Corrosion inspection involves the use of various instruments, tools, and analysis methods to determine the rate of corrosion of materials in specific process media environments. It provides engineering technicians with information on equipment corrosion, enabling them to take effective measures to slow down corrosion and prevent corrosion-related accidents. Corrosion inspection during major plant overhauls involves formulating a corrosion inspection plan tailored to the specific conditions of the plant. During the shutdown for overhauls, a specialized team is assembled to carry out inspections of the equipment and pipelines using macroscopic examination as well as other effective testing methods, in order to assess the morphology and condition of corrosion. Based on these findings, the mechanism of corrosion is analyzed, the corrosion rate is calculated, and the extent of damage is determined. 01 Purpose of corrosion inspection during major equipment maintenance: To understand the degree of equipment damage and corrosion that has occurred after one cycle of continuous operation. Evaluate the effectiveness of the equipment and process anti-corrosion measures adopted for this unit during this cycle. It verifies the accuracy of the RBI risk assessment results, providing reliable validation to guide the evaluation of pressure vessels and pressure pipelines using RBI. To provide a basis for determining the equipment and process anti-corrosion measures to be adopted for this device in the next cycle. 02 Tasks related to corrosion inspection during unit overhauls I. Collection of original data for corrosion inspection: Process flow diagrams of the unit, operating procedures for the unit, detailed lists and individual drawings of equipment and pipelines, unit overhaul plans, inspection plans for pressure vessels and pressure pipelines, historical thickness measurement records, historical maintenance and inspection records, historical laboratory test records, etc. (including: crude oil evaluation and analysis data, electrodialysis data, analysis data of three-phase condensate, etc.). Major maintenance plan (phone numbers of the responsible persons from each construction unit are preferred). Equipment operation procedures, individual equipment diagrams, equipment documentation records, evaluation and analysis of raw materials used in the previous production cycle, equipment calibration reports, historical corrosion records, completion documents (including installation details), records of maintenance or emergency repairs over the years, DCS historical data for the past six months along with records of overpressure and overheating alarms, sampling and analysis data from the past six months and the most recent equipment calibration reports, data from fixed-point thickness measurements and online corrosion monitoring. II. Formulation of corrosion inspection plans: The corrosion agents affecting different parts of the equipment are determined based on the actual conditions of the equipment (process flow, process parameters, equipment characteristics) as well as the results of the RBI risk assessment ； Possible corrosion mechanisms ； Failure modes and detection methods of possible corrosion mechanisms. Thereby determining the focus of the corrosion investigation and the inspection strategy. November 10-14 – TÜV HAZOP Leader Training Course – Shangyu, Shaoxing. III. On-site corrosion inspection: On-site corrosion inspections are carried out in accordance with the corrosion investigation plan ； Analyze the corrosion status of equipment devices ； Preliminary analysis of corrosion mechanism ； Report on Recommendations for Corrosion Protection of the Equipment ； Prepare on-site reports for corrosion inspections. IV. Comprehensive Report on Corrosion Inspection – Device Overview ； Analysis of the device’s corrosion over the years and review of its performance during the previous cycle ； Inspection status of equipment for major workshop repairs ； Overhaul details (mainly the completion status of opening containers, repairing, and modifying equipment pipelines) ； Analysis of inspection data for pressure vessels and pressure pipelines ； Analysis of thickness measurement data from major equipment overhauls ； Non-destructive testing data analysis for plant overhauls ； Detailed Table of Equipment and Pipeline Corrosion Inspections ； Report on Analysis of Typical Corrosion Cases and Corrosion Mechanisms ； Selection of monitoring points and type schemes for corrosion online monitoring ； Analysis of fixed-point thickness measurement scheme ； In-service inspection, inspection plan analysis ； Sampling point selection and analysis of testing plan ； Analysis of the overall corrosion condition of the device ； Analysis of the Current Status of Process Anti-corrosion and Optimization Strategies ； Analysis of the Current Anti-corrosion Status of Equipment and Optimization Strategies ； V. Assessment of the equipment’s safety condition: This involves determining whether, under the current material conditions and existing process-based anti-corrosion measures, the corrosion rate remains within acceptable limits. What measures can be taken to reduce the corrosion rate if it exceeds the allowable range? VI. Assessment of weak points in the installation: In accordance with the relevant regulations of Sinopec Corporation, an actual corrosion rate of more than 0.25 mm/a indicates that the material grade used at that location is insufficient, and an upgrade of the material is necessary. The parts where the corrosion rate is above the limit and thus have a higher likelihood of failure are the weak points of the device. Generally, a material upgrade plan for the next cycle should be formulated. VII. Process Adaptability Assessment In petrochemical plants, the properties of raw materials and process conditions often change; in particular, some process modifications are carried out during each major plant overhaul. It is necessary to determine what impact these changes will have on the equipment and pipelines, whether new problems will arise, and what measures should be taken to prevent them. VIII. Others includes various tests, simulations, experiments, and analyses of active sulfur carried out during the same period to complete the aforementioned report.