Corrosion Causes and Preventive Measures of Tube Bundle of Air Cooler

1 Overview

As a large-scale heat exchange equipment, the air cooler is widely used in the petrochemical industry. However, the tube bundle of the air cooler is often used in high temperature, high pressure, and high corrosion conditions, so it has a great test on its corrosion performance and directly affects the safety of the equipment. , stable operation, so the common corrosion types of tube bundles are analyzed, and protective measures are proposed to improve the service life of the product.

2 Common corrosion types of air cooler tube bundles

(1) Pitting corrosion. After a certain period of time, the pressure-bearing material of the air cooler tube bundle is in a medium containing dissolved oxygen and harmful anions (mainly CL ions). After a certain period of time, most of the surface does not corrode or the corrosion is relatively light. Cavities or pits appear in individual points or small areas. As time goes by, the corrosion pits continue to develop in the depth direction, forming small hole-like corrosion pits, which is pitting corrosion. When the corrosion points are severe, the equipment can be perforated, so Also known as pinhole corrosion or pitting corrosion.

Important factors in the design and manufacture of air-cooled heat exchanger tube bundles include:

a. Influence of heat treatment aging temperature: For stainless steel tube bundles, heat treatment is generally not performed after welding for the main pressure-bearing parts, but austenitic stainless steel has the best pitting corrosion resistance after solid solution treatment. For other stainless steel materials, heat treatment such as annealing or tempering at certain temperatures will produce precipitated phases, thereby increasing the tendency of pitting corrosion.

b. With the improvement of the surface finish of the metal material of the tube bundle, it will increase the pitting corrosion resistance, but when cold working causes cold work hardening on the metal surface, the pitting corrosion resistance will decrease. Therefore, for unused materials and working conditions, pitting corrosion test evaluation is generally carried out: generally can be divided into three categories: chemical immersion method, electrochemical measurement method and field test method. The content of pitting corrosion resistance evaluation includes: pitting depth, pitting corrosion Density, corrosion rate, corrosion area, etc. And combine the corrosion rate with the pit distribution, shape, size, density, depth, etc.

(2) Crevice corrosion is caused by the formation of tiny gaps (generally 0.025-0.1mm) between metals or between metals and non-metals, the heat exchange medium stays in the gap, and there are harmful anions in this medium. There are many forms of corrosion, and the corrosion results will lead to the reduction of material strength, the increase of local additional stress, and the reduction of material bearing capacity.

In air-cooled heat exchangers, crevice corrosion usually occurs at the plate lap, flange sealing surface connection, gasket seal, base pipe and liner or rust layer, among which the most likely to occur between the base pipe and liner Crevice corrosion. Therefore, in the product design, the base pipe and the liner are often expanded to eliminate the gap between them. The air-cooled heat exchanger usually adopts a wire plug sealing structure, which is sealed by a metal gasket, and the two sides of the gasket are respectively in contact with the wire plug and the tube sheet. As the heat exchange medium corrodes, due to pressure, temperature, vibration and other factors, there is crevice corrosion at the sealing tip, which may easily lead to gasket seal failure.

(3) Erosion corrosion, including erosion and abrasion corrosion, is the metal damage caused by high-speed relative motion between the metal surface and the fluid medium. Erosion-corroded metal surfaces generally bear grooves, valleys, and teardrops, and the surface is bright and free of corrosion deposits. Compared with corrosion under other stresses, the factors affecting erosion corrosion are more complex. In addition to the chemical composition, structure, mechanical properties, surface roughness, and corrosion resistance of the material itself, it is also related to the medium temperature, pH value, and dissolved oxygen. , solid phase particle size and hardness, as well as the shape and structure of the flow-through components, the flow rate and flow state of the fluid have a great influence. In the air-cooled heat exchanger, the most prone to erosion corrosion is the base pipe and tube sheet welding. Since the base pipe and tube sheet are generally welded in a single layer or double layer, the height of the weld leg is between 1.5 and 2mm. Erosion will thin the weld seam until it leaks. Therefore, a reasonable design of the structure type of the heat exchanger can effectively reduce the erosion corrosion rate. Usually, the form of the return pipe box is used to change the fluid flow trajectory, or a choke zone is added to the base pipe. It can be used as a buffer component to reduce the flow rate of the medium and change the flow of the medium.

(4) Intergranular corrosion, a local selective corrosion of metal materials along the grain boundaries in a specific corrosive medium. Grain boundaries are the junctions between different grains. Since the crystal grains have different orientations, the arrangement of atoms at the junction must gradually transition from one orientation to another. Therefore, the grain boundary is actually a kind of "surface" incomplete structural defect.

Conditions for intergranular corrosion: a. The metal or alloy contains impurities, or the second phase precipitates along the grain boundary. b. The difference in chemical composition between the grain boundary and the grain forms a corrosion battery in a suitable medium. The grain boundary is the anode, the grain is the cathode, and the grain boundary produces selective dissolution. c. There is a specific corrosive medium. In some alloy-medium systems, severe intergranular corrosion often occurs. For example, austenitic stainless steel may have severe intergranular corrosion in weakly oxidizing media or in specific corrosive media with strong oxidizing media.

Intergranular corrosion test method and its evaluation method: There are basically five types currently listed in the standard: oxalic acid electrolytic etching method, boiling 65% nitric acid method, boiling sulfuric acid-ferric sulfate method, boiling sulfuric acid-copper sulfate method and nitric acid-hydrogen Hydrofluoric acid method.

(5) Stress corrosion, the damage caused by the combined action of stress and corrosive environment is called stress corrosion. As a brittle failure mode, it is extremely harmful. The damage of materials under the action of stress factors alone belongs to mechanical fracture or Mechanical fatigue fracture; the material is corroded by the medium alone in a corrosive state, which belongs to environmental corrosion, and the corrosion damage caused by the joint action of stress factors and the corrosive environment leads to fracture and cracking, which is called corrosion cracking.

For air-cooled heaters, leakage is often caused by corrosion cracking, and the most common corrosion cracking is hydrogen sulfide stress corrosion cracking (SSCC), which refers to the brittle fracture phenomenon of the pressure-bearing material of the tube bundle under the action of tensile stress in the sulfide medium, causing Stress corrosion cracking usually has three conditions: a. The metal has stress in the medium environment

The tendency of corrosion cracking; b. The structure composed of pressure-bearing materials is in contact with or in a selective corrosive medium; c. There is a tensile stress above a certain level.

3 Corrosion rate test

Corrosion rate is an important index to evaluate the corrosion resistance of metals. During the process of metal corrosion, its weight, structure, external dimensions, surface state, mechanical properties, etc. will all change to a certain extent. The macroscopic and microscopic change rates can be used to reflect the degree of metal corrosion, so there will be different corrosion rate tests. Methods, commonly used methods for air cooler tube bundles include resistance method, weight loss method and linear polarization method.

The resistance method is based on the principle that the cross-sectional area of the metal sample is reduced due to corrosion, resulting in an increase in resistance. Using this method, the corrosion status of the equipment can be continuously monitored during the operation of the air cooler tube bundle, and it can accurately reflect the corrosion of the equipment. The corrosion rate and its change at each stage of operation can be applied to various media. The resistance method is fast, sensitive, and convenient, and can monitor the corrosion rate under the condition of large.

The linear polarization method is a very suitable method for monitoring. It has the characteristics of high sensitivity and fast response to changes in corrosion conditions. It can obtain instantaneous corrosion rates and can reflect changes in the working conditions of the tube bundle in time. However, this method is The corrosion rate is obtained based on steady-state conditions, so the measured object is uniform corrosion or general corrosion, and cannot be applied to local corrosion.

The weight loss method is a commonly used method for measuring corrosion rate, which is simple and easy to implement, and the result is reliable. Usually, the coupon test is used, and the material of the coupon is consistent with the pressure-bearing material of the main body of the air-cooled heat exchanger. The shape and size of the coupon can be determined according to the test situation and needs of the sample. Before the test, the surface treatment of the coupons should be carried out. It must first be sanded to remove rust and polish, degrease and degrease with acetone and other solvents, rinse with distilled water, and dry at 50°C. The coupon is immersed in the corrosion solution, and the corrosion rate is measured by detecting the weight of the coupon.

4 Corrosion protection measures

4.1 Reasonable selection of materials

Correct material selection is one of the key links in corrosion control, and the following principles should be followed:

(1) The corrosion resistance of the material should meet the requirements of the environment in which the equipment is used. According to the characteristics of the corrosion type, sensitivity, and corrosion rate of the medium, the environment for use should be analyzed, and materials should be selected reasonably;

(2) The physical properties, mechanical properties and processing performance of the material should meet the design and processing requirements of the equipment, such as the necessary strength, hardness, impact toughness, fatigue performance, heat resistance and weldability, etc. For air-cooled tube bundles in the condition of hydrogen and sulfur-containing media, nickel-based alloys and duplex stainless steel materials are usually used, among which duplex stainless steel is a new type of material with excellent corrosion resistance, high strength and easy manufacturing and processing. Its physical properties are between austenitic stainless steel and ferritic stainless steel, and are closer to ferritic stainless steel and carbon steel.

4.2 Rational design of structure

Product design must start from the perspective of anti-corrosion, strictly calculate and determine the use stress according to the material and environment, and carry out reasonable anti-corrosion structural design. It needs to pay attention to the following points: (1) The shape should be as simple as possible; (2) Prevent water and dust accumulation; (3) Try to avoid sharp corners, grooves and gaps; (4) Try to avoid metal connections with large potential differences; (5) Avoid using stress, assembly stress and residual stress to superimpose in the same direction.

4.3 Manufacturing Process Improvement

Materials may also cause corrosion or leave hidden dangers of corrosion during processing and assembly engineering, such as: (1) Residual stress is likely to be generated during machining;

(2) The heat treatment specification should be carefully selected to avoid intergranular corrosion, stress corrosion, hydrogen embrittlement, etc. caused by improper heat treatment, and try to avoid heat preservation in the sensitization temperature zone. For heat treatment processes that may generate large residual stress, there should be elimination of residual measures of stress;

(3) Different welding methods have different corrosion sensitivities. In order to reduce the stress after welding, attention should also be paid to the design of the welding sequence to minimize the deformation of the workpiece. The residue at the weld seam after welding should be cleaned up in time. to avoid localized corrosion;

(4) Degrease and clean the workpiece after welding and processing to avoid residual night corrosion of the workpiece. Under certain working conditions, the tube bundle of the air cooler can be treated with nickel-phosphorus plating for internal anti-corrosion treatment;

(5) During equipment assembly

Excessive assembly stress should not be caused, the precision should be improved and the tolerance should be reduced during design, and reasonable assembly methods should be adopted during assembly to avoid stress concentration.

5 Conclusion

Only by in-depth understanding of equipment corrosion types, effective control and protective measures can effectively ensure the safe and stable operation of air cooler tube bundles and improve service life. Therefore, research on corrosion of materials and equipment is one of the important research topics in the industry. .