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This post was last edited by B0SS on 2016-10-11 at 18:04. Summary: This article analyzes the main causes of corrosion in equipment that is out of service. It introduces the corrosion conditions of 500,000-ton oil refining plant equipment, the selection and application of anti-corrosion materials, as well as the anti-corrosion mechanism, uses, types, and characteristics of vapor-phase anti-corrosion agents for equipment that is out of service. Combined with the actual usage and results of the vapor-phase corrosion inhibitor Longx-911 applied to the idle equipment in our plant in 2015, it was verified that vapor-phase corrosion inhibitors for idle equipment exhibit excellent corrosion prevention effects during the idle period of oil refining equipment. (Author: Li Mingcheng, Gao Yongwei, etc., Tasixi Petrochemical Plant, CNPC Tarim Oilfield Company) ； Li Kang, Wang Bin, et al. Lanzhou Jinlongxin Chemical Technology Co., Ltd. Keywords: standby equipment, refining equipment, vapor-phase corrosion inhibitors, corrosion prevention. 1. Introduction to the corrosion situation of the 500,000-ton refining complex at Tasixi Petrochemical Plant, analysis of the causes of corrosion, selection and application of corrosion-resistant materials. (1) Introduction to the 500,000-ton refining complex: Due to adjustments in production plans, the 500,000-ton refining equipment at Tasixi Petrochemical Plant, operated by CNPC’s Tarim Oilfield Company, was shut down at the end of 2014 and put into standby mode. Starting in September 2015, all the equipment that required corrosion protection was gradually inspected. Through on-site inspections and photographic documentation, it was found that corrosion had occurred in almost all of the equipment, with some tanks and towers being particularly affected. The tanks that store corrosive substances are severely corroded overall; their inner surfaces are completely covered in rust, with the rust layer reaching about 2 millimeters thick in some areas. The bottom of the tower is not severely corroded, with little rust; corrosion starts in the middle section where the amount of rust gradually increases. The higher up the tower, the more severe the corrosion becomes, and the thicker the layer of rust as well. In the thickest areas, the rust layer can reach about 2 millimeters thick. According to statistics, equipment without resistance to atmospheric corrosion suffers much more severe corrosion during periods of inactivity than while it is in use. If no protective measures are taken, extensive corrosion or perforations can occur, making it difficult to restart the equipment or even leading to its premature disposal and resulting in significant economic losses. Since the equipment had been out of use for up to nearly 10 months and as little as nearly 9 months prior to this anti-corrosion treatment, During prolonged periods of inactivity, some corrosion on the equipment is inevitable. We recommend that necessary anti-corrosion measures be taken immediately whenever the equipment is taken out of use in the future. (2) Analysis of corrosion causes: Atmospheric corrosion is primarily due to the depolarization process caused by oxygen; without oxygen, atmospheric corrosion of metals does not occur. However, in the presence of oxygen, the metal surface undergoes electrochemical reactions with the corrosive electrolyte, resulting in damage. Because the water film on the metal surface is very thin, oxygen in the air can easily reach the metal surface, making the cathodic process proceed readily. In atmospheric corrosion caused by moisture, the metal surface is often in a state of alternating dryness and wetness. At this point, the rust layer on the metal surface acts to accelerate corrosion; when humidity increases further, the rust layer can serve as a cathodic depolarizer together with dissolved oxygen. To reduce metal corrosion, various methods are employed to protect metals. Among them, the use of vapor-phase anticorrosive agents for equipment that is not in use is an economical and practical approach, and it has become an important method for corrosion prevention. (3) Selection and application of anti-corrosion materials: To slow down and prevent corrosion of our plant’s 500,000-ton oil refining equipment during periods of downtime, we employ various methods for corrosion protection. Following technical comparisons and product evaluations, our plant decided to use Longx-911, a gaseous anti-corrosion agent for idle equipment produced by Lanzhou Jinlongxin Chemical Technology Co., Ltd., for protecting the equipment during such periods. Following a tender process, the gaseous preservative arrived at the construction site, and the filling operations commenced gradually in September 2015. Prior to these operations, our plant’s equipment management and maintenance department, together with technical personnel from Lanzhou Jinlongxin Chemical Technology Co., Ltd., formulated a strict filling plan. During the filling process, the latter company provided on-site technical guidance. Due to the large number of devices, the filling process took a considerable amount of time; it was completed gradually between September 21 and October 20, after which the anti-corrosion period began. 2. Corrosion prevention mechanism, applications, types, and characteristics of vapor-phase corrosion inhibitors for equipment that is out of service (1) Corrosion prevention mechanism: Vapor-phase corrosion inhibitors for equipment out of service are new products developed by combining the principles of equipment corrosion, vapor-phase corrosion inhibition techniques, and corrosion protection methods. These inhibitors provide effective corrosion protection for industrial boilers, towers, reactors, tanks, pipelines, heat exchangers, and pressure vessels during periods when they are not in use. The vapor-phase corrosion inhibitor for standby equipment is a type of vapor-phase corrosion inhibitor; it consists of volatile corrosion-inhibiting substances, chelating rust removers, and various additives, and it can automatically release gases with corrosion-inhibiting, chelating, and alkaline properties at normal temperature and pressure. ①The corrosion-inhibiting gases emitted from preservatives, primarily compounds containing nitrogen-based groups, form a dense protective film on the surface of the metal equipment being protected through chemical adsorption. This film acts as a barrier, preventing corrosive substances such as air and water vapor from coming into contact with the metal substrate, thereby effectively protecting the equipment and preventing or slowing down corrosion. ②The gases emitted from preservatives have strong permeability; they can penetrate beneath rust layers and scale deposits, where they chelate with the metal ions in the equipment walls, thereby effectively preventing corrosion that occurs under the rust and scale. At the same time, through penetration and chelation, part of the existing rust layer can be removed, which helps to reduce the area of rust spots and the thickness of the rust layer. ③The weakly alkaline gases emitted by preservatives can create an alkaline environment inside the equipment, preventing electrochemical corrosion on its inner surfaces and thus achieving a corrosion-resistant effect. (2) Types and uses: The vapor-phase corrosion inhibitors for shut-down or standby equipment are available in two models, Longx-911 and Longx-901. Longx-911 is used for corrosion protection of complete industrial plants in industries such as chemicals and petroleum refining during periods of shutdown or standby, while Longx-901 is used for corrosion protection of various types of boilers and thermal piping systems during similar periods. (3) Characteristics ① This preparation automatically volatilizes at room temperature, releasing corrosive-inhibiting, chelating, and alkaline gases with a relatively high vapor pressure. The evaporated gas can provide corrosion inhibition and rust removal for metal surfaces at both short and long distances, thereby achieving corrosion prevention. ②This preparation is readily soluble in water, and its aqueous solution also has a corrosion-inhibiting effect. The residues are easy to remove after the anti-corrosion period ends. ③This formulation overcomes the shortcomings of traditional drying and nitrogen flushing methods, such as short corrosion prevention duration and poor effectiveness, as well as the issues associated with oil flushing methods, including lack of safety and cost-effectiveness. It features affordability, safety, convenience, time savings, labor savings, and high efficiency. 3. Corrosion prevention effect: Vapor-phase corrosion inhibitors are used for the corrosion prevention of these equipment that are currently out of use, with an expected corrosion prevention period of one year. Due to the need to start work in advance as part of the plan to adjust all anti-corrosion measures, equipment manholes were gradually opened on April 5, 2016, to examine the effectiveness of the anti-corrosion treatments. And a summary is drawn based on the inspection and evaluation clauses regarding the anti-corrosion protection effects in the \"Filling Plan for Longx-911 Vapor-Phase Corrosion Inhibitor as Spare Equipment for 500,000-ton Oil Refining at Tasixi Petrochemical Plant of China National Petroleum Corporation Tarim Oilfield Company\". The actual inspection and test results are as follows: ① Visually inspecting all equipment manholes revealed no significant changes in the internal surfaces of the equipment before and after anti-corrosion treatment. ② Through photographic comparison, there was no significant change in the corrosion resistance of the internal surface of the device before and after photography. It was also found that some of the rust layers formed before the addition of preservatives fell off in large areas under the effect of the preservatives. ③ The corrosion test pieces hanging inside the equipment were removed from the surface of the test plates, and visually, no significant changes were observed on the surfaces of the test plates before and after anti-corrosion treatment. While carrying out anti-corrosion treatment on the equipment, simulated monitoring tests were conducted on its exterior. The test results showed no significant changes on the surface of the corrosion coupons to which anti-corrosive agents had been added, whereas large areas of rust were observed on the surfaces of the corrosion coupons that did not contain such agents. ④ The corrosion rate is determined by weighing the corrosion coupons that have been pre-weighed and hung inside the equipment, and then calculating it using the values obtained from those weighings. The lowest corrosion rate was recorded for tower C-1002 in the atmospheric distillation unit, at 0.0005 mm/a, while the highest rate was observed in tower c-6302 of the sulfur recovery unit, at 0.0087 mm/a. The average corrosion rate was 0.0026 mm/a. The corrosion rates of all corrosion test specimens did not exceed the specified requirement of 0.01 mm/a; the average corrosion rate was merely 26% of the technical requirement. The test specimens used in the simulated monitoring test were weighed and calculated by the inspectors as follows: ① For simulated water and steam, a vapor-phase corrosion inhibitor was added; due to the high indoor temperature, the vapor-phase corrosion inhibitor evaporated too rapidly, resulting in an excessive vapor pressure inside the test bottle that forced the lid to pop off. Once the vapor-phase corrosion inhibitor had completely evaporated, a large amount of air entered the bottle. The corrosion rate of the test specimens was ultimately measured to be 0.0084 mm/a. ②By simulating water and steam and adding an adequate amount of vapor-phase corrosion inhibitor, the corrosion rate of the test piece was measured to be 0.0062 mm/a. ③In the comparative test simulating water and steam, without the addition of a vapor-phase preservative, the corrosion rate of the test specimen was measured at 0.0127 mm/a. The results of the simulated monitoring tests show that as long as a vapor-phase corrosion inhibitor is added, the corrosion rate will meet the technical requirement of 0.01 mm/a. Conclusion: \"Vapor-phase corrosion inhibitor for equipment on standby\" effectively prevented further corrosion of our plant’s 500,000-tonne oil refining installation during the half-year corrosion protection period. The corrosion rates of all corrosion test pieces met the specified requirements, with an average corrosion rate of 0.0026 mm/year, which is 26% of the technically required value. It also helped to remove the rust layers that had formed prior to the application of the corrosion inhibitor, demonstrating excellent corrosion protection effects (references omitted).