Research Progress on Corrosion and Anticorrosion of Steel Structures in Humid and Hot Atmospheric Environment

Research Progress on Corrosion and Anticorrosion of Steel Structures in Humid and Hot Atmospheric Environment

Sijia Chen

Department of Architecture and Civil Engineering Huaiyin Institute of Technology, Huai’an

Weidong Zhang

Department of Architecture and Civil Engineering Huaiyin Institute of Technology, Huai’an

Minghao Dong

Department of Architecture and Civil Engineering Huaiyin Institute of Technology, Huai’an

Suning Lv

Department of Architecture and Civil Engineering Suqian University, Suqian

Abstract – Steel structures are widely used worldwide for their high strength, light weight, excellent toughness, ductility and convenient construction, yet their inherent susceptibility to corrosion poses numerous challenges. This paper summarizes the research advances in steel structure anticorrosion, reviewing the corrosion mechanisms and influencing factors of steel structures in humid and hot atmospheric environments, and discussing the degradation effect of corrosion on their mechanical properties. It analyzes two main corrosion types, chemical and electrochemical corrosion, and how temperature, humidity, atmospheric pollutants and other factors accelerate the corrosion process. The paper also introduces the research progress in testing methods for steel structure corrosion, including the advantages and disadvantages of outdoor exposure tests, indoor simulation tests and electrochemical methods. In addition, it explores steel structure corrosion time-varying models that predict the temporal evolution of corrosion depth and rate in corrosive environments, and discusses the negative impacts of corrosion on mechanical propertiesparticularly how pitting corrosion and stress corrosion, two forms of local corrosion, cause reduced bearing capacity, local stress concentration and crack formation.

1. INTRODUCTION

   Steel structures are widely applied in the fields of architecture, bridges, industrial facilities and so on due to their light weight, high strength and high construction efficiency. Calculations show that for bearing the same load over the same span, the mass of steel roof frames is at most only 1/4 to 1/3 of that of reinforced concrete [1]. However, the performance and durability of steel structures are facing severe challenges under the action of various environmental factors, especially chemical and electrochemical corrosion in humid and hot atmospheric environment. Globally, the economic loss caused by material corrosion accounts for about 35% of the gross national product every year. In China, the economic cost invested in corrosion control also reaches 3.4 5% of the national GDP [2]. Steel corrosion not only causes huge economic losses to the national economy of various countries,

   affects the appearance and functionality, but also may lead to the decline of structural strength, and even trigger catastrophic accidents. For example, in 2019, the Nanfang’ao Cross-sea Bridge in Taiwan collapsed due to corrosion fatigue of steel hangers, causing a major safety accident with 6 deaths and more than 20 injuries [3].

   Starting from the corrosion mechanism of steel structures, this paper elaborates on the research progress in the corrosion mechanism, influencing factors, research methods, time- varying models and mechanical property degradation of steel structures in humid and hot environments. It points out that temperature, humidity and atmospheric pollutants are the key factors affecting the corrosion rate, and discusses the advantages and disadvantages of research means such as outdoor exposure tests, indoor simulation tests and electrochemical methods. Meanwhile, it lists the time-varying models of steel structure corrosion in a chronological order to predict the degradation of material properties over time. The paper also emphasizes that future research should focus on the corrosion fatigue mechanism, the development of new anticorrosive materials and the application of intelligent detection technologies to realize more efficient and environmentally friendly corrosion control strategies. The purpose is to promote the development of steel structure anticorrosion technology, effectively address the steel structure corrosion problem, and thus enhance the implementation, management, maintenance and safety of engineering projects.

2. CORROSION MECHANISM OF STEEL

   STRUCTURES

   Steel structure corrosion is mainly divided into two types: chemical corrosion and electrochemical corrosion.

   Chemical corrosion refers to the oxidation reaction between steel and the surrounding medium under certain temperature and humidity conditions. This corrosion process does not involve the generation of electric current and is a direct chemical reaction. In this process, the metal reacts directly with

   the corrosive medium, resulting in the damage or deterioration of the metal.

   Electrochemical corrosion is the main form of metal corrosion, which involves the contact between metal and electrolyte solution to form a galvanic cell. This corrosion process must involve the participation of electrolyte solution and water, accompanied by the generation of electric current. In the atmosphere, water vapor is adsorbed on the steel surface to form an extremely thin water film. This water film can dissolve CO and SO in the atmosphere to form a weakly acidic electrolyte solution. Due to the presence of the water film, many tiny electrochemical cells are formed on the steel surface. Among them, iron (Fe) in the steel is oxidized to ferrous ions (Fe²), and oxygen (O) is reduced to hydroxide ions (OH) at the same time. These Fe² and OH react to form ferrous hydroxide (Fe(OH)), an unstable substance that will continue to oxidize in the electrolyte solution and eventually form stable ferric hydroxide (Fe(OH)) on the steel surface, leading to the continuous dissolution of steel [4]. These reactions take place in the electrolyte film formed on the steel surface, which is a more common form of steel structure corrosion.

   Other corrosion behaviors can be regarded as extensions of these two types of corrosion, such as stress corrosion. Zhao Qian [5] et al. found that under the action of applied stress, stress concentration is easily induced at the tip of corrosion pits on Q235 hot-dip galvanized steel sheets, leading to the rupture of the corrosion product film and the re-exposure of fresh Zn, forming stress corrosion cracks, whose essence is chemical corrosion accelerated by stress. Another example is microbiologically influenced corrosion. Qi Peng [6] et al. summarized that chemical substances produced by the metabolism of different microorganisms form a concentration gradient on the material surface, and the local cell effect accelerates corrosion at the same time, so it is still a coupling effect of chemical and electrochemical corrosion.

3. INFLUENCING FACTORS OF STEEL STRUCTURE CORROSION

   Temperature and humidity are important factors affecting chemical corrosion and electrochemical corrosion. Figure 1 shows the 4-year average corrosion rate prediction data of Q235 steel in the atmospheric environment provided by the National Material Environmental Corrosion Platform, among which the areas with relatively fast average corrosion rates are Sichuan-Chongqing, the coastal areas of Qilu, the coastal areas of Jiangsu-Zhejiang-Shanghai and their surrounding regions. Figure 2 is the national average humidity distribution map, which shows that the humidity in coastal areas and the Sichuan Basin is relatively high. Figure 3 is the natioal average temperature distribution map; compared with the north, the south has a lower latitude, and compared with the west, the east has a lower altitude, resulting in more frequent high- temperature weather. Combining Figure 2 and Figure 3, it can be seen that the humid and hot atmospheric environmental conditions may be the main reason for the fast corrosion rate in some areas.

   Figure 1 average corrosion rate

   Figure 2 average humidity

   Figure 3 average temperature

   Relative humidity (RH) and temperature are the main influencing factors of the humid and hot atmosphere. Relative humidity is a physical quantity reflecting the degree to which the water vapor content in the air approaches the saturation state. It refers to the ratio of the actual pressure of water vapor in the air to the saturated vapor pressure of water at a certain temperature, simply speaking, a physical quantity reflecting the humidity of the air. Studies have shown that the corrosion rate of steel structures increases significantly when the relative humidity is higher than a certain critical value, which is generally considered to be between 60% and 80% [7-8]. When the air humidity is high, water vapor molecules in the air are more likely to condense into liquid water on the steel structure surface. This corrosive water film provides an electrolyte environment for steel structures, promotes electrochemical

   corrosion, especially by accelerating the contact between oxygen and the steel surface, enhancing the oxygen polarization process, destroying the original oxide film on the metal surface, and thus accelerating the corrosion rate. The electrochemical corrosion of steel can only occur on the premise of the existence of a water film, so relative humidity provides the prerequisite for the occurrence of steel corrosion. The water film on the surface of steel structures in high- humidity areas is usually thicker, and the corrosion rate increases correspondingly with the increase of water film

   thickness [9]. Due to the influence of natural environmental factors, the thickness of the water film on the steel structure surface also changes constantly, thus affecting the corrosion rate of steel structures.Table 1 lists the relevant data on corrosion rates in the specification Classification of Atmospheric Environment Corrosivity (GB/T 15957), from which it can also be clearly seen that the classification of corrosion performance is closely related to environmental humidity.

   Table 1 Corrosivity grades of atmospheric environment

   Corrosivity Grade Name		Corrosion Rate (mm/a)	Gas Type	Annual Average Environmental Relative Humidity	Atmospheric Environment
   	No corrosion Mild corrosion	<0.001 0.001~0.025	A A	<60 60~75	Rural atmosphere Rural atmosphere,

   0.025~0.050	B A B	<60 >75 60~75	Urban atmosphere Rural atmosphere, Urban atmosphere,
   	C	<60	Industrial atmosphere
   	B	>75	Urban atmosphere,
   0.050~0.200	C	60~75	Industrial atmosphere,
   	D	<60	Marine atmosphere

    

   1. slight corrosion moderate
   2. corrosion
   3. moderately

      strong corrosion

      0.200~1.00 C

      D

      >75 60~75

      Industrial atmosphere, Marine atmosphere

   4. Severe corrosion 1,00~5.00 D >75 Marine atmosphere

   On the other hand, temperature has an obvious impact on chemical reaction processes, and the reaction rate generally increases with the rise of temperature. The same is true for the corrosion process of steel structures, and studies have found that the influence of temperature is more complex, largely due to the changes of the above-mentioned water film under temperature [7]. It is generally believed that with the rise of temperature, the reaction rate increases accordingly, and the corrosion rate decreases because the oxide protective film is formed faster on the steel structure surface [9]. However, temperature changes affect the formation and evaporation of the water film on the steel structure surface, the pH value of the water film, the solubility of oxygen, the performance of coatings, microbial activity, as well as a series of chemical reaction rates and physical processes such as thermal expansion and contraction, condensation and freeze-thaw temperature difference corrosion, so the actual corrosion rate is often much more complex. In a general atmospheric environment, due to the relative humidity being lower than the critical relative humidity of steel structures, the corrosion of steel structures is still slight with the rise of temperature due to the dry environment. However, in corrosive environments such as marine atmosphere, the air humidity is high and often higher than the critical relative humidity for steel corrosion, so the rise of temperature will significantly aggravate the marine atmospheric corrosion [10]. For general chemical reactions, according to the Arrhenius equation, the reaction rate increases by more than twice for every 10 rise in temperature. Therefore, similar to the annual average corrosion in China shown in Figure 1, affected by the average temperature in Figure 3, the tropical marine atmosphere is generally the most corrosive, followed by the temperate marine atmosphere, and the polar regions with low temperatures are the least corrosive. Seasonal changes in the same region will affect the corrosion

   rate, and the higher the temperature, the stronger the corrosivity [11].

   Figure 4 Industrial corrosion process

   Figure 5 marine corrosion process

   High temperature and humid environment will increase the corrosion rate, while acidic environment will aggravate chemical corrosion. Among them, atmospheric pollutants containing sulfur dioxide (SO) and chloride ions (Cl) have

   the most significant impact on steel structure corrosion. Liu Shinian [12] et al. found that sulfur dioxide has a prominent harmful effect on steel bars in the initial stage. It reacts with water vapor in the atmosphere to form sulfuric acid, which will accelerate the corrosion process on the steel surface. Especially in the industrial atmosphere with a high content of sulfur dioxide, the corrosion hazard to steel cannot be ignored. Chen Huiling [13] et al. inferred that the corrosion of carbon steel in SO gas mainly proceeds in the way shown in Figure 4.

   Chloride ions have strong corrosivity; they can penetrate the protective layer on the metal surface, reach the metal surface and accelerate the corrosion process. In the marine atmosphere, due to the presence of sea salt particles, the content of chloride ions is high, which accelerates the corrosion rate of metal structures in the marine atmospheric environment, and the reaction process is shown in Figure 5. Tian Yuxiao [14] investigated that the steel structure platforms of buildings in the coastal areas of Hainan are facing harsh environmental conditions such as alternating high and low temperatures at sea, high humidity and high salinity, and analyzed and discussed the characteristics, mechanism and anticorrosion methods of marine corrosion.

4. PROGRESS IN RESEARCH METHODS FOR STEEL STRUCTURE CORROSION
   1. Research Progress on the Influence of Temperature on Steel Structure Corrosion

      In recent years, significant progress has been made in the research on how temperature affects steel strucz`ture corrosio. Researchers have adopted a variety of methods to explore the relationship between temperature and corrosion, each with its unique advantages and disadvantages. For example, outdoor exposure tests can provide corrosion data closest to the natural environment, but these tests have a long cycle and are easily affected by specific geographical locations and climatic conditions, limiting their universal applicability. For this reason, many scholars at home and abroad have carried out long-term exposure tests in different regions to obtain the real data of steel structure corrosion rate. Foreign scholars such as Larrabee [15] and Shastry [16] conducted a 16-year exposure test of various types of steel in the humid and hot atmosphere. Knotkova [17], Wei [18] and others carried out an 8-year exposure test in different regions and climatic zones. China has also started the research on atmospheric exposure tests of steel since the 1960s. Hou Wentai [19], Liang Caifeng [20] and others [13-14] jointly conducted 4-year, 8-year and 16-year atmospheric exposure corrosion tests, focusing on the influence of Cl and SO in the atmosphere on the corrosion rate. Wang Chuan [21] et al. [15] studied the corrosion laws of carbon steel and weathering steel in tropical rainforests (high temperature and high humidity) by means of atmospheric exposure tests. Nowadays, with the development of big data, outdoor exposure tests and the establishment of mathematical models are more complementary. For example, many scholars such as Miao Hao

      [22] adopted outdoor exposure tests to obtain corrosion data in the actual environment, thus establishing corrosion life prediction models. The database of the National Corrosion and Protection Science Data Center for Materials and some real- time monitoring systems are also developed based on these data, which provide a strong support for the further in-depth research

      in the field of steel structure corrosion in the future.

      In contrast, indoor simulation tests can control and adjust test conditions such as temperature and humidity, thus accelerating the corrosion process and obtaining data quickly, but they may not fully simulate all actual environmental factors and have certain deviations. Some researchers believe that the laboratory simulated accelerated corrosion tests have a good correlation with the results of actual atmospheric corrosion tests in the initial stage, but this correlation weakens with the extension of time [23]. Other researchers hold that the current correlation between laboratory simulated accelerated corrosion tests and actual atmospheric corrosion tests is not ideal, because the corrosion mechanisms of the two may be different, and it is difficult to establish a mathematical relationship between the laboratory simulated corrosion rate and the actual atmospheric environmental factors [24-25]. At present, the common methods for indoor accelerated tests to study humid and hot atmospheric corrosion include damp heat tests, salt spray tests, cyclic spray composite corrosion tests, dry-wet cyclic immersion tests and multi-factor cyclic composite corrosion tests. Reference [26] lists the specific contents, applicable environments and respective characteristics of these simulation test methods. For example, Liu Yuxi [27] carried out indoor accelerated tests using a low-temperature coolant circulating pump and found that the corrosion resistance of offshore steel with different strength grades is weak at low temperatures, and the corrosion rate at low temperatures is generally higher than that at room temperature.

      Electrochemical methods provide an in-depth understanding of the corrosion mechanism by monitoring the changes of current and potential during the corrosion process. Xue Wei [28] used electrochemical workstations (such as PARSTAT 2273 workstation) to conduct electrochemical tests to study the corrosion behavior of container steel plates in the humid and hot marine atmospheric environment; Pu Shi [29] et al. used the CHI660d electrochemical workstation to conduct polarization curve and AC impedance tests, which help to evaluate the corrosion behavior and corrosion resistance of materials. Microscopic analysis methods, such as scanning electron microscopy (SEM), which is commonly used, can reveal the microstructure of corrosion products, but usually require destructive sampling. Therefore, in-situ non-destructive testing is also developing steadily. Zhou Shijie [30] designed and developed a set of automatic laser ultrasonic scanning detection system, which can realize the rapid and accurate detection of corroded materials. This system combines laser ultrasonic technology with Lamb wave theory for the detection and evaluation of corrosion damage, improving the efficiency and accuracy of non-destructive testing technology.

      In these research methods, temperature, as a key variable, has been extensively studied. For example, in outdoor exposure tests, the natural fluctuation of temperature is used to study its influence on long-term corrosion behavior, which helps to collect and analyze the long-term mechanical property changes of materials in a specific natural atmospheric environment; in indoor simulation tests, temperature is precisely controlled to study its influence on accelerated corrosion, which can well achieve the goal of rapid screening of materials or coatings. Microscopic analysis methods can be used to study how temperature affects the rate and mechanism of the corrosion reaction process over time, and obtain the morphological changes of corrosion products at different temperatures.

   2. Research Progress on the Influence of Humidity on Steel Structure Corrosion

      As we all know, dry-wet cycles not only affect the stability of the electrolyte layer, but also are directly related to the corrosion rate of steel structures, which is an important factor that cannot be ignored in corrosion control. Because when the environmental humidity increases, a thicker water film will form on the steel structure surface, which is rich in electrolytes and provides the necessary medium for electrochemical reactions, thus accelerating the corrosion process. On the contrary, when the environment becomes dry, the water film becomes thinner or disappears, and the reduction of the electrolyte layer limits the electrochemical reaction. On the one hand, the thickness of the electrolyte layer affects the transfer rate of oxygen to the steel structure surface, thus indirectly controlling the corrosion rate of steel structures; on the other hand, with the evaporation of the electrolyte layer, when the thickness of the electrolyte layer is excessively reduced resulting in a high ohmic resistance, the anodic dissolution reaction of steel structures is limited or even suspended. Therefore, the periodic dry-wet alternation on the steel structure surface will cause changes in the steel corrosion rate, ultimately leading to a complex corrosion state [27]. Under dry- wet cycle conditions, a thicker, looser rust layer with pores and cracks is more likely to form on the steel structure surface, while the rust layer formed on the specimens under immersion conditions in the same corrosion cycle is relatively thin [31]. When the rust layer is thick, its water-holding capacity can prolong the wetting time of the steel structure surface, thus

      to an exponential relationship, indicating that the influence of relative humidity on the corrosion rate is more significant in this temperature range. Cao Gongwang [38], Zheng Guo [39] and others also pointed out that temperature and humidity usually interact with each other and jointly affect the corrosion process. Under high temperature and high humidity conditions, the corrosion reaction is more intense, because high temperature accelerates chemical reactions, and high humidity provides more water, which helps to form the electrolyte environment required for corrosion. These factors act together on the steel surface and accelerate the corrosion process.

5. p>PROGRESS IN RESEARCH ON TIME-VARYING MODELS OF STEEL STRUCTURE CORROSIONThe time-varying model of steel structure corrosion is a mathematical model used to describe and predict the changes of mechanical properties of materials with time in corrosive environments. Such models are usually based on experimental data, and establish the relationship between material performance degradation and corrosion time by statistically analyzing key parameters in the corrosion process, such as mass loss rate and corrosion depth. This model is very helpful for predicting the corrosion rate and residual life of structures in humid and hot atmospheric environments. Compared with calculating the corrosion depth, the weight loss method for calculating mass loss is not only intuitive and easy to operate, but also can conveniently calculate the corrosion rate, and its calculation formulas are as follows:

   accelerating the corrosion of steel structures [32]. Dry-wet tests generally adopt cyclic spray tests or cyclic immersion tests.

   × 100% (1)

   Although Wen Bangwei [33] confirmed that cyclic spray tests have a good correlation with actual environmental exposure tests, most scholars now adopt cyclic immersion tests [34-36] because cyclic immersion tests are easier to control temperature

   Here, is the weight loss rate of the specimen after t hours of corrosion; is the mass of the specimen after t hours of corrosion and complete removal of corrosion products;

   changes and have relatively simple equipment and operation by comparison.

   1. Research Progress on the Coupling Effect of Temperature and Humidity on Steel Structure Corrosion

   At high temperatures, if the relative humidity is low, the corrosion rate of steel plates may be relatively slow. This is

   In the formula, 0 is the initial mass of the specimen before corrosion. d(T) is the average corrosion depth of the specimen; is the density of steel, equal to 7.85g/cm³; 0 is the surface area of the specimen.

   because although high temperature is conducive to accelerating chemical reactions and promoting the corrosion process,

   without enough water to form a continuous thin electrolyte liquid film (the aforementioned water film), the corrosion reaction lacks the necessary medium, thus limiting the development of corrosion. At low temperatures, if the relative humidity is high, the corrosion rate of steel structures may accelerate. A high-humidity environment is conducive to the formation of a stable water film on the steel structure surface; even at low temperatures, as long as there is enough water, the electrochemical corrosion process can be maintained, and the corrosion rate will increase accordingly. Li Ziyun [37] et al. found in the indoor simulated corrosion test of cold-rolled steel plates that the corrosion rate of cold-rolled steel plates accelerates with the increase of relative humidity. Under high humidity conditions, a continuous water film is more likely to form on the steel structure surface, providing the necessary conditions for electrochemical corrosion. They also found that when the temperature is higher than 35, the relationship between the rust initiation time and relative humidity changes

   Here, r is the corrosion rate of the specimen; is the indoor accelerated corrosion time of the specimen.

   The corrosion weight loss of steel in humid and hot atmospheric environment is a complex process, which usually shows a nonlinear time-varying law. In the initial stage, the steel surface may react rapidly with oxygen and water molecules in the environment, resulting in a relatively fast corrosion rate. With the passage of time, an oxide film will form on the steel surface, which helps to slow down the corrosion rate and make the corrosion enter a relatively stable state. However, if this protective film is damaged or the concentration of corrosive substances (such as chlorides) in the environment increases, the corrosion rate may accelerate again. Jia Chen [40] also confirmed this law in the test of corrosion damage of Q690E steel plates and their mechanical properties after corrosion by comparing the different corrosion rates caused by different tension and compression stresses in Group F and

   Group H.

   In addition, environmental factors such as temperature, humidity, pollutant concentration, as well as the composition and surface treatment of steel, will have a significant impact on the time-varying law of corrosion weight loss. Scientists have proposed different corrosion time-varying models [44] by establishing statistical and probabilistic models [41-42], combining long-term monitoring data [15-22] and through extreme value theory [43]. Li Yongchao [45] established the change law model between steel corrosion time and converted elastic modulus through the accelerated corrosion test data of artificial aerosol simulating acidic atmospheric environment, and used a power function to describe the change law of

   corrosion depth with time.

   Although the relationship between the mass loss rate and average corrosion depth (macroscopic corrosion parameters) and corrosion time can be well fitted by the power function model and exponential function model in empirical formulas, accurate prediction cannot be made in the later stage with the increase of corrosion degree and considering the coverage of corrosion products, so more refined time-varying models are constantly being proposed.

   Table 2 Steel structure corrosion time-varying models

   Year Author Object Formula Type Corrosion depth d(T)0.076 + 0.038T Linear model

   1979 Southwell[46-

   47]

   Correction of corrosion depth

   0.09 0 < 1.46

   d(T) = {d(T)0.076 + 0.038T 1.46 < 16

   Bilinear model

   Power

   Paik[48]

   Corrosion depth loss

   function model

   1998

   Loseth[49]

   Power

   Emi[50]

   Corrosion rate

   function

   1999 Melchers[51] Corrosion depth d(T) = 0.12070.6257

   model

   Power function model

   Exponential

   Corrosion depth loss d(T) {

   0

   [1 exp ( )]

   model

   1999 Soares[52]

   max

   0

   Corrosion rate r(T) = exp ( 0) >

   Exponential

   0

   model

   Liang C F [19-

   Corrosion depth

   Power

   n

   2003

   20]

   development model D=At

   0 < 0

   function model

   Corrosion depth

   d(T) {

   0

   2003 Qin S P [53]

   development model

   max [1 exp (

   ) ] 0

   Weibull corrosion model

   Corrosion rate

   0 < 0

   r(T) =

   1

   development model

   max (

   {

   0)

   exp[ (

   0) ] > 0

   Corrosion depth development model

   Corrosion depth regrowth

   max {1 Bexp[( 0)]1} 0

   Richards

   2022 Chen Y [54]

   beyond asymptote

   Corrosion rate development model

   r(T)

   max {1 Bexp[( 0)]1} > 0

   corrosion model

   = {max

   1 exp [( 0)][1 Bexp[( 0)]]1 > 0

6. PROGRESS IN RESEARCH ON MECHANICAL PROPERTY DEGRADATION OF CORRODED

   STEEL STRUCTURES

   In a high-humidity environment, steel structures may

   undergo general corrosion, that is, uniform corrosion on the entire surface. In a low-humidity environment, corrosion may tend to be local corrosion, such as pitting corrosion and stress corrosion cracking. All these will led to the degradation of mechanical properties to some extent.

   With the formation of corrosion products on the steel surface, it is generally believed that its effective cross-sectional area decreases, the bearing capacity declines, and the tensile strength value decreases. However, the yield strength is affected by corrosion before the reduction of the effective cross-sectional area, which is because corrosion leads to changes in the internal tissue structure of steel structures. Inside the steel structure, there is a fine structure called grains, which are composed of some metal ions. The reason why steel has elasticity is that the grains are independent of each other and arranged randomly; when an external force is applied to the steel structure, it can deform and resist the further application of external force. However, the long-term corrosive environment may affect the microstructure of the material, the grain boundaries may migrate, and the grains may coarsen or refine, thus leading to the reduction of the strength of steel structures [55].

   In addition, corrosion may also affect the toughness of steel, making it more prone to fracture when subjected to tension or overload [56]. This is usually due to the formation and propagation of cracks caused by local stress concentration, thus reducing the toughness and ductility of the material and increasing the risk of brittle fracture of the structure. Pitting corrosion can lead to the formation of stress concentration areas on the material surface, which may become the starting points of stress corrosion cracks and gradually affect the mechanical properties of steel structures. The dry-wet alternation process is accompanied by the evaporation of the water film and the accumulation of chloride ion content; when the chloride ion concentration exceeds the critical value, pitting corrosion will occur on the steel structure. Li Maolin [57] believes that pitting corrosion is one of the main reasons for the failure of marine steel structures, which has a serious impact on safety, including the impact on thickness and fracture. The degradation of steel structures generally occurs in the oxide layer on the steel surface; with the deepening of corrosion, the oxide layer is damaged and local penetration occurs, forming some tiny depressions on the surface of the steel matrix, which then develop and expand in depth on the steel surface to form large depressions, and then form macroscopic corrosion pits, thus developing into stress corrosion cracks, resulting in uneven corrosion. Jia Chen [58] studied the low-cycle fatigue life, cyclic softening performance and energy dissipation performance of steel plates before and after corrosion, analyzed the random distribution law of the development of uneven corrosion depth, and thus established a regression relationship between degradation and corrosion damage degree.

   The uneven surface formed by pitting corrosion may lead to uneven stress distribution and increase the areas of stress concentration. In these areas, if tensile stress exists, it may promote the generation of stress corrosion cracks, which is the so-called stress corrosion. Stress corrosion means that when the metal has residual stress or applied stress, some tiny surface defects may be generated on its surface due to corrosion, and these defects will further expand and form cracks under the action of stress. With the passage of time, these cracks will gradually expand and penetrate into the material, leading to the reduction of the strength and toughness of the material and eventually fracture. Welded structures are prone to stress corrosion cracks under the combined action of residual stress and corrosive environment, becoming a high-incidence area for structural stress corrosion cracking and failure. For this reason,

   Bai Linyue [59] et al. carried out corrosion tests on welded specimens using natural seawater and found that coarse widmanstatten structure is the main cause of corrosion grooves, and the higher the residual stress level, the deeper the corrosion grooves at the fusion line and the more the metal loss. Therefore, residual stress is an important reason why welded structures become high-incidence areas for stress corrosion cracking and failure.

7. EXISTING PROBLEMS AND PROSPECTS OF STEEL STRUCTURE CORROSION RESEARCH

   To sum up, although great progress has been made in the research on steel structure corrosion, there are still many urgent problems to be solved in the following aspects:

   1. Corrosion fatigue in humid and hot atmospheric environment is an important cause of structural failure. The research on the mechanism, influencing factors and mechanical property degradation law of corrosion fatigue is not in-depth enough, and more experimental and theoretical analyses are needed.
   2. In actual engineering, steel structures in humid and hot atmospheric environment are simultaneously subjected to the combined action of corrosion and load. Under such stress coupling conditions, the discrimination and evaluation of structural damage modes and the establishment of time-varying models of corrosion rate need to be more accurate and reliable, the process of corrosion affecting microstructure changes and mechanical property degradation needs to be more detailed and clear, and the influence of stress on the stability and performance of anticorrosive coating protective layers should also be corresponding step by step.

On the other hand, although some corrosion protection technologies have been developed, how to effectively apply these technologies in humid and hot atmospheric environments and how to develop new protection technologies to adapt to specific environmental conditions are still research hotspots. It can be imagined that temperature changes will affect the evaporation, condensation and fluidity of the water film on the steel structure surface, which also indirectly affects the corrosion rate of steel structures. In addition, thermal expansion and contraction, changes in microbial activity and pH value caused by humid and hot fluctuations all need in-depth and detailed research and development.

Looking forward to the future, research should focus on developing more efficient and environmentally friendly new anticorrosive materials and technologies to address the corrosion of steel structures in humid and hot atmospheric environments, such as water-based coatings and self-expanding flame-retardant epoxy resin-based coatings, to improve the corrosion resistance of steel structures and reduce maintenance costs. At the same time, the development of intelligent detection technologies, such as laser ultrasound, acoustic emission and magnetic measurement methods, will provide more accurate data support for the early diagnosis and real-time monitoring of steel structure corrosion. Interdisciplinary research will help to comprehensively understand and solve the steel structure corrosion problem, and research on economy and sustainability will ensure the practical application value of anticorrosion technologies.

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