Corrosion and Protection: [Weekly Topic] What are the basic requirements and principles for material selection in equipment corrosion prevention design? (2011.05.23-29)

Basic requirements for material selection: 1. First, consider the properties, temperature, and pressure of the medium. Whether the medium is oxidizing or reducing, and what its concentration is ； Are there any impurities? What are the properties of these impurities? Do they slow down or accelerate corrosion? If they accelerate corrosion, what is the reason for this acceleration? In addition, it is necessary to fully understand the conductivity of the medium, its pH value, and the properties of the corrosion products formed ； 2. The temperature and pressure of the medium are also important factors to consider ； 3. When selecting materials, consideration should also be given to environmental corrosion of the materials as well as the specific requirements of the product ； 4. Secondly, the properties of the material must also be considered; as a structural material, it is generally required to possess certain strength and ductility ； 5. When selecting materials, one must also consider their price and availability; an economic perspective is necessary. Basic principles for material selection: 1. Service properties of materials ① Analyze the operating conditions of the component (pressure, temperature, medium, etc.) to determine the service properties required of it (strength, stiffness, plasticity, toughness, etc.). ②Through failure analysis, the main service performance of the component is determined. ③Based on the main performance requirements for components, requirements for the material’s performance are proposed. 2. Machinability of the material 3. Economic viability of the material 4. The selection of material shall meet the requirements of relevant standards and specifications

Answer: Performance in use: 1 Chemical properties: oxidation resistance and corrosion resistance. 2 Physical properties: density, melting point, coefficient of thermal expansion, thermal conductivity, and elastic modulus, etc. 3 Mechanical properties: elasticity, plasticity, strength, and toughness. Process properties: weldability, forgeability, machinability, formability, and heat treatment properties. I believe that water treatment equipment has higher requirements regarding the strength, stiffness, and corrosion resistance of metal materials. Based on the actual engineering requirements, the optimal material is selected through a comparison of different materials.

Corrosion protection design methods 1. Procedure for corrosion protection design The corrosion protection design process should include: the proper selection of materials and their manufacturing methods； Requirements are specified for material testing and corrosion testing ； Analyze potential corrosion, wear, or damage that may occur during the service life of equipment, in order to select appropriate protective measures ； Evaluate the strength of the structure from a corrosion prevention perspective, and review the shape and combined design of the structure and its components ； The maintenance of the device and its expected lifespan also need to be considered. 2. Principles for material selection during design When selecting materials at the design stage, it is necessary to have comprehensive information and data on various materials, as well as their corrosion characteristics in specific media, and to make full use of proven work experience and procedures. (1) Understanding of corrosion environmental conditions Since the properties of materials vary significantly depending on the operating conditions, it is first necessary to understand the environmental factors that cause corrosion, including: 1) Chemical factors such as the composition of the medium (including impurities), pH value, oxygen content, and potential chemical reactions ； 2) Physical factors: temperature of the medium, flow rate, heating and cooling conditions, type and magnitude of forces, etc. Special attention should be paid to environmental conditions such as high temperature, low temperature, high pressure, vacuum, impact loads, and alternating stresses. (2) General sequence for material selection: 1) Preliminary selection – Based on past failure experiences, consult authoritative material manuals; if there are any doubts, seek advice from experts in corrosion and protection ； Determine the possible types of corrosion that may occur, and conduct a preliminary selection of materials ； Taking into account the complexity of the actual equipment and requirements regarding processability—such as weldability, formability, castability, and surface treatment—and after considering the costs, several candidate materials are selected for further screening. 　　2) Corrosion testing: If there is no sufficient experience regarding the operating conditions, corrosion testing should be conducted to obtain reliable data that can serve as a basis for material selection. This is also necessary to further verify the preliminary election results. In addition to laboratory tests, corrosion testing for particularly important equipment sometimes requires supplementary field simulation tests under actual operating conditions, such as coupon testing on equipment in use or tests simulating small devices. Building on the aforementioned work, the corrosion resistance, mechanical properties, processability, and cost of the materials are taken into comprehensive consideration, thereby balancing durability and economic efficiency to properly select the materials and their manufacturing processes. (3) Consideration of the expected service life of the equipment: When designing equipment, the determination of its service life should generally take into account: ① A lifespan that meets the requirements of the entire production facility ； ②It is hoped that the materials in all parts of the device will degrade as evenly as possible ； ③Material costs/construction costs/maintenance costs, taking comprehensive optimal economic considerations into account. For general corrosion, the expected service life can be estimated based on the corrosion rate ； For various types of localized corrosion, life prediction and estimation must be carried out based on a thorough understanding of their initiation and propagation mechanisms, as well as the acquisition of relevant data. The corresponding corrosion equipment life prediction technology is one of the key areas that should be studied and developed in corrosion prevention design techniques. 3. General principles for the corrosion-resistant design of engineering structures. In the design of corrosion-resistant structures, it is essential to consider whether the shape of the structures and components, as well as their mutual arrangement, meet the requirements for corrosion resistance, particularly with regard to preventing various types of local corrosion. This includes not only the design of the equipment but also the way in which the equipment is installed and the layout of the piping systems – what is known as system design issues. The most general rational design rule for corrosion prevention is to avoid the occurrence of uneven and multiphase corrosion. The following general rules should be given special attention in corrosion-resistant design: 1) The shape of structural components should be as simple and rational as possible. Components with simple shapes are easy to take anti-corrosion measures, facilitate fault resolution, and are easy to inspect and maintain. Dead corners and gaps can easily lead to the accumulation and concentration of corrosive agents, thereby causing corrosion, which should be avoided. Where possible, especially for vulnerable components, designing them as a detachable structure facilitates maintenance and replacement. 2) Try to avoid stress concentration and reduce or eliminate residual stresses, in order to reduce or prevent hazards such as stress corrosion cracking. 3) Prevent galvanic corrosion. Avoiding contact between dissimilar metals and the formation of electrolytes on surfaces, as well as preventing contact with materials capable of absorbing moisture, are all important measures. 4) Avoid gaps to prevent gap corrosion. Replacing riveting with welding, using butt welding and continuous welding as much as possible instead of lap welding, spot welding, or indirect welding, and selecting solid, elastic, non-absorbent gasket materials all help to avoid gaps. 5) Prevent erosion corrosion caused by turbulence, vortices, etc. Apply fluid mechanics theory to guide component design in order to avoid this type of flow. 6) Prevent corrosion caused by potential differences resulting from temperature differences, poor ventilation, differences in solution concentration, etc. 7) If there is a choice regarding the location of equipment and buildings, locations with less natural corrosion should be selected, in order to avoid accelerated corrosion caused by marine atmospheres, industrial wastewater, and dust from chemical plants. When designing corrosion-resistant structures, it is also necessary to consider appropriate corrosion prevention measures and the feasibility and convenience of implementing them, such as the installation of electrochemical protection systems, the selection and application of coatings, and systems for adding and replenishing corrosion inhibitors. 4. Corrosion resistance strength design: In the strength design, it is necessary to consider whether the strength of the materials and structures in a corrosive environment meets the requirements for corrosion resistance; this is also known as \"environmental strength design\". Under corrosive media conditions, it is generally insufficient to consider only safety factors and allowable stresses; it is necessary to take into account the impact of the environment on strength and carry out appropriate calculations, which is particularly important for stress corrosion, corrosion fatigue, etc. If the selected material exhibits only uniform corrosion in the medium used by the equipment, then in the design process, the issues of corrosion and strength are often treated separately. Specifically, when determining the strength requirements, the dimensions and wall thickness of the components are chosen first; subsequently, based on the average annual corrosion rate and the expected service life, an additional wall thickness—referred to as the “corrosion allowance”—is calculated and added to the original wall thickness. Considering that under external forces, changes in potential and corrosion current density may occur when metal undergoes plastic deformation, design should aim to avoid abrupt size changes and sharp transitions as much as possible, so as to prevent local stress concentration and locally accelerated corrosion. The failure processes caused by the combined action of media and mechanics, such as stress corrosion cracking, corrosion fatigue, and hydrogen-induced cracking, are highly dangerous and represent key issues in the design of corrosion resistance. (1) Stress corrosion cracking (SCC): SCC is a form of failure that occurs in materials under the combined effect of tensile stress and a specific corrosive environment; cracking takes place without significant overall surface corrosion, making it a dangerous type of corrosion. During design, efforts should be made to avoid material-environment combinations that can lead to SCC, as well as to reduce or eliminate sources of stress, including external loads, deformation caused by cold and hot working, residual stresses from welding or heat treatment, and thermal stresses resulting from temperature differences. Through experiments or by consulting manuals, it is important to obtain as much data as possible regarding the critical stress for stress corrosion σSCC, the critical strength factor for stress corrosion KISCC (defined respectively as the maximum initial stress or stress strength factor under which no SCC occurs within a certain time period), and the subcritical growth rate of stress corrosion cracks da/dt. This information is crucial for the design of corrosion-resistant structures, the assessment of their strength, and the estimation of the expected service life of equipment. When using these data, the mode of force application and its changes over time should also be taken into account. The testing techniques developed in recent years for measuring the stress and strain at the onset of stress corrosion cracking, along with the associated data, provide new parameters for designing corrosion resistance, which are also very useful for engineering design. (2) Corrosion fatigue (CF): Corrosion fatigue is a type of corrosion damage caused by the combined action of a corrosive medium and alternating or pulsating stresses. Corrosion fatigue is different from stress corrosion; it does not involve the issue of a specific combination of medium and material ； Corrosion fatigue also differs from ordinary fatigue in that it has no true fatigue limit. Given a sufficiently large number of cyclic repetitions, corrosion fatigue failure can occur even under very low stress levels. Therefore, the limit values for corrosion fatigue are often associated with a certain number of cycles, which is referred to as the apparent fatigue limit; typically, the fatigue stress at 107 cycles is used as the standard value. Then it is divided by the safety factor to obtain the corrosion fatigue value to be used in design. When applying the corrosion fatigue limit values, the effects of stress waveform, frequency of stress fluctuations, material strength and composition, combined stresses, and corrosive environment must be taken into account. If these factors reduce the fatigue life, then in the equipment design phase, the safety factor can be appropriately increased or a lower allowable stress value can be adopted for strength calculations, depending on the degree of such reduction. 　　 -

Code for Design of Anti-Corrosion in Industrial Buildings (GB 50046-95) – Standards and Regulations...... 3 Basic Provisions 3.1 Classification of corrosiveness 3.1.1 Corrosive media are classified into five categories based on their state and site of action: gaseous media, highly corrosive substances, acid/base/salt solutions, solid media, and contaminated soil. Various media are further classified according to their properties and content. For media listed in the specifications, the designer shall determine the category based on similar media, taking into account factors such as the properties and content of the media. During design, the category of corrosion behavior should be determined based on the production process conditions; if experience or insufficient information is available, it can also be determined according to Appendix A. Appendix A lists the main components of buildings and structures in corrosive production facilities across various industries, as well as the categories of corrosive agents in the outdoor atmosphere. However, due to continuous updates in production processes and differences in management levels, factors such as the concentration of corrosive media and the degree of leakage may vary. Therefore, the type of corrosion should also be determined based on actual conditions. 3.1.2 The corrosion severity level of a medium on building materials is determined based on the category of the medium, as well as factors such as environmental humidity and the amount of exposure, and is classified into four levels: high, medium, low, and none. Conceptually, it can be understood as follows: under highly corrosive conditions, materials corrode rapidly; therefore, components must be protected by surface isolation measures to prevent direct contact between the corrosive medium and the components ； Under these conditions, if possible, it is advisable to switch to other materials with less corrosiveness. Under moderate corrosion conditions, some corrosion of the materials occurs. In such cases, it is sometimes possible to improve the quality of the components themselves (e.g., increasing the density of concrete, thickening the concrete cover over steel reinforcement, or raising the grade of bricks and mortar in masonry), or applying simple surface protection measures. Under conditions of mild corrosion, the material corrodes slowly, but certain measures still need to be taken ； Generally, improving one’s own quality is sufficient. Under non-corrosive conditions, material corrosion is very slow or there are no noticeable signs of corrosion; therefore, the components do not require the protective measures specified in these standards. The corrosion rating is primarily based on practical engineering considerations; in addition to chemical corrosion, it is determined by taking into account adverse factors such as alternating wet and dry conditions in the affected area and crystalline corrosion. Building materials are the commonly used materials for constructing components of the superstructure: reinforced concrete, plain concrete, brick masonry, wood, steel, and aluminum. Among them, brick masonry combines the corrosion resistance properties of both fired clay bricks and cement mortar. The corrosion behavior of prestressed concrete is essentially the same as that of reinforced concrete, but there are some differences. Due to a lack of data, it is temporarily determined based on the corrosion classification of reinforced concrete. 3.1.3 The value of humidity is primarily determined based on the critical humidity of steel. The humidity range in which rebar in steel and concrete is most prone to rusting is between 70% and 80% relative humidity, while the rusting process slows down when the relative humidity is below 60%. The corrosion of other building materials such as bricks, concrete, and wood—non-metallic materials—also follows a similar relationship with humidity. Therefore, humidity is divided into three categories: less than 60%, 60%~75%, and greater than 75%. The value of relative humidity in the environment should be used, but it needs to be adjusted under the following circumstances: In areas with heavy rainfall, the relative humidity in the environment surrounding outdoor components increases due to rain, and therefore it should be set at a level slightly higher than the average annual relative humidity ； When the production environment has an impact on relative humidity, the values from the actual environment should be used ； For components where dew formation is inevitable, the relative humidity should be set above 75%. 3.1.4 Gaseous media include various corrosive gases, acid mists, and alkali mists (including alkali-containing water vapor), which primarily affect the components of upper building structures both indoors and outdoors ； Its corrosivity is mainly related to the properties and concentration of the medium, as well as the relative humidity of the environment. Acid mist and alkali mist are originally aerosols with a liquid as the dispersed phase, but their corrosive characteristics and sites of action are more similar to those of gaseous media; therefore, they are classified under gaseous media. The values for the medium content are derived from: 1. Thousands of gas concentration data collected over the past decade from production and maintenance processes in factories in industries such as chemicals, petrochemicals, non-ferrous metallurgy, machinery, and textiles; these data were organized and analyzed before being compared with standard values. 2. Foreign building standards and codes: \"Corrosion Protection of Building Structures\" (Former Soviet Union) CH11, II2-03-11-85 ； Former East German **Standard TGL33408/01/81 ； Bulgarian **standard BAC9075-71. Among them, the standards of the former Soviet Union were established on the basis of extensive testing and hold significant reference value; this specification draws on most of the data from those standards. 3. The management team of these specifications commissioned relevant units to conduct gas corrosion tests in order to verify certain data on gas concentrations. 4. Refer to the data on the classification of corrosive gases in specifications for other disciplines such as electrical engineering. 5. Compare and verify with the allowable values for gas concentrations in workshops specified in the current **standard, namely the ‘Hygienic Standards for Industrial Enterprises’. There is no experimental data available regarding the concentrations of acetic acid mist, sulfuric acid mist, and alkali mist; these substances are not included in the former Soviet Union standards. The measured values in China show a high degree of variability. Nevertheless, these substances are important and cannot be ignored. Therefore, this specification uses qualitative descriptions rather than quantitative values. Most gas contents are divided into two grades, which is simpler than the 3–4 grades in foreign standards. This is because the more levels there are, the harder it is to determine the design ； In reality, regardless of how many levels there are, when combined with humidity, they ultimately result in just four levels of corrosiveness to materials. In the standards of the former Soviet Union, the highest grade is not permitted from a health perspective and actually occurs very rarely, while the lowest grades are basically free from corrosion. The second-grade content specified in these standards is roughly equivalent to the intermediate second grade in the former Soviet Union’s standards. Among them, ammonia and hydrogen fluoride in gases have little corrosive effect on building materials; moreover, due to their harmful effects on humans, their concentrations are not allowed to be too high, so only one grade is specified. 3.1.5, 3.1.6 Corrosive water and acid, alkali, and salt solutions all belong to liquid media; the former refers to industrial water or groundwater that has been contaminated during the production process. Due to its low purity, it is classified based on the concentration of corrosive ions in the water ； The latter are mainly the production media that act directly or leak out, and are classified into solutions of different properties and concentrations. There is a certain continuity between the two types of liquids. Corrosive water, as well as acid and alkali salt solutions, affect buildings in similar ways; however, corrosive water primarily affects underground structures and sewage treatment tanks, while these solutions mainly affect storage tanks, floors, and baseboards. Corrosive water does not include environmental water; therefore, HCO3 media present in underground aquifers are not included. In the table, corrosive media are listed in their individual forms; however, in actual engineering applications, polluted water usually contains multiple types of media. Solutions can consist of a single medium, mixed media, or media that act alternately ； When determining the corrosion grade, the higher value should be adopted; however, when selecting protective materials, the corrosivity of all fluids in contact must be taken into account. The criteria for the content of corrosive water are mainly in line with the national standard \"Code for Geotechnical Investigation\", and indicators from relevant domestic and international standards have also been taken into consideration. However, in this specification, corrosive water applies not only to underground structures but also to tanks and floors; therefore, the environmental condition factor is not included. The determination of the indicators also takes into appropriate account the conditions in different areas, as well as the consistency among the indicators for acid, alkali, and salt solutions. Acid, alkali, and salt solutions have a strong corrosive effect on building materials such as concrete and brick masonry. With the exception of a few cases where dense concrete can withstand such effects, corrosion-resistant materials must be used for covering in all other cases. Therefore, it is meaningless to classify and quantify these solutions; classification is generally done on a qualitative basis. In inorganic acids, a very low concentration is sufficient to achieve a pH value of 1; therefore, the acidic solutions in Table 3 are defined by a pH value of less than 1. However, the evaluation of medium corrosionity based on pH is not applicable to high-concentration organic acids; therefore, organic acids are classified according to their concentration. The action of moisture is an important factor in corrosion, but areas where liquid media such as foundations, storage tanks, sewage ponds, and drainage ditches are present are subject to continuous exposure as well as alternating wet and dry conditions; only a small portion of the surface experiences minor and occasional exposure to moisture. Therefore, the corrosion grades in the table are all based on regular exposure, taking into account the crystalline degradation caused by the alternation of dry and wet conditions involving alkalis and salts. Building materials are classified into reinforced concrete, plain concrete, and brick masonry based on the location where the liquid acts. The substrates for foundations, floor surfaces, storage tanks, septic tanks, and similar structures are mostly concrete or reinforced concrete, while the substrate for baseboards is brick masonry. When the foundation is a stone masonry made of cement mortar, its corrosivity depends on the cement mortar, and is determined according to the corrosivity of concrete. The corrosion resistance of steel to liquid media is relatively complex, and it is not used in many applications in construction; therefore, it has not been included. The corrosion grades are all determined based on media at normal temperatures; under the action of media at temperatures above 40°C, the corrosivity of various solutions changes in different ways. For example, in the case of sodium hydroxide solutions, their corrosivity increases sharply as the temperature rises. Hot alkalis and molten alkalis exert significant corrosive effects on materials that are resistant to normal-temperature alkalis, such as concrete, acid-resistant bricks, and granite. Therefore, when selecting protective linings for tanks and sewage ponds, attention should be paid to temperature changes in the solution. The “%” in Table 3 refers to the percentage by mass of the solution in the medium. 3.1.7 Solid media include bases, salts, corrosive dusts, and aerosols in which solids are the dispersed phase. Solid media mainly act on the ground, walls, and the components of building structures above the ground. Solid media only cause corrosion to building materials after they dissolve; therefore, the degree of corrosion is related to water and environmental humidity. Insoluble and poorly soluble solids are essentially non-corrosive, while solids that are completely dissolved are evaluated for corrosivity as if they were liquid media. In an anhydrous environment, it depends on the hygroscopicity of the solid and the relative humidity of the environment. Generally, solids that are prone to absorbing moisture will absorb moisture and deliquesce to a semi-liquid state or dissolve partially when the relative humidity is above 60%. Under humid conditions, the corrosion of steel structures caused by dust is generally greater than that caused by gases. Soluble solids in the outdoor portion are considered as a liquid medium due to the effect of rainwater. Apart from humidity, the corrosivity of solid media is directly related to their properties; their chemical corrosivity is essentially the same as that of similar liquid media. Another characteristic of soluble salts is that, after dissolving, they can revert to a solid crystalline hydrate at a certain temperature. At this point, the volume will increase by multiples of its original value. Therefore, when dust on the surface of bricks or concrete absorbs moisture and deliquesces, penetrating into the pores of the material, it causes recrystallization and expansion, which puts pressure on the inner walls of the pores and leads to the destruction of the material. The greater the volume expansion, the stronger the destructive force. The severe corrosion of brick masonry by sodium sulfate and sodium carbonate in salts is due, in addition to chemical corrosion factors, also to physical damage as an important cause. For example, after sodium sulfate is dissolved, at a transformation temperature of 32.3°C, the co-strongly hydrated compound occupies 311% of its original volume. 3.1.8 Contaminated soil, as referred to in this specification, mainly denotes the contamination of the foundation soil at the site under consideration due to production activities, with the affected area being underground structures. When no groundwater is found at the location of the underground structure, the corrosion grade of the material shall be determined based on the substances contained in the contaminated soil. In current domestic and international standards, few corrosion parameters for soil media are specified; basically, these include parameters such as SO4, Cl-, and the pH value of hydrogen ions. **In the standard \"Code for Geotechnical Investigation,\" the corrosion index of soil (mg/kg of soil) is determined by multiplying the corrosion index of groundwater (mg/L) by a coefficient of 1.5. Actual engineering investigations conducted under this standard group have yielded some data on soil corrosion, but it is not yet appropriate to include a large number of substances in the standards. Therefore, only those that are more well-established and common, namely SO4, Cl-, and hydrogen ion concentration (pH value), have been selected and included after ensuring their consistency with the criteria specified in the \"Geotechnical Investigation Standards\". The corrosion of plain concrete and reinforced concrete by alkalis is less severe in soil, once the environmental factor of alternating wet and dry conditions is removed. Moreover, in underground areas, sodium hydroxide corrodes soil more than it does concrete. Therefore, the pH value is listed only up to acidic corrosion, without considering alkaline corrosion. 3.2 General Layout and Building Arrangement 3.2.1 Practical experience shows that production units that emit large amounts of corrosive gases and dust can have an impact on adjacent buildings as well as the equipment and instruments in those units. A reasonable general layout is highly beneficial for reducing corrosion, with wind direction and frequency being the main factors to consider. In addition to considering the impacts between various areas within the plant, the mutual influences between adjacent factories must also be taken into account. Practice has shown that under normal conditions, the impact of groundwater diffusion is minimal, so it is not emphasized. 3.2.2 Large storage tanks for corrosive solutions have experienced leakage incidents; if such tanks are located within a factory building or near the foundation, leaks can cause severe corrosion, and the consequences often include settlement or swelling of the foundation, making repair and reinforcement very difficult. The containment dike is a measure taken to prevent secondary disasters caused by the spread of acid liquid in the event of a sudden, large-scale acid leak. The retaining wall does not need to be made of corrosion-resistant materials, but it must be able to prevent the solution from leaking away in large quantities over a short period of time, so that recovery measures can be taken promptly. 3.2.3 The area where the spray-type cooling coils and tanks are located is filled with mist, with water all over the ground. When installed indoors under the action of corrosive media, corrosion is significantly accelerated. In recent years, design practices have drawn on experience by moving the exhaust pipes and tanks outdoors; however, they are placed too close to the factory building, and water mist still has a significant corrosive effect on the walls. The pool should be at a distance of no less than 4m from the exterior wall of the building, which helps to reduce the impact. 3.2.4 Open and semi-open factory structures help dilute corrosive gases, thereby reducing corrosion. However, in addition to meeting the requirements for production and maintenance, attention must also be paid to the effect of rainwater after the facility is made open; especially in the presence of corrosive dusts, this can exacerbate corrosion. 3.2.6 Investigations show that on floors exposed to liquid media, leaks can easily cause corrosion to the ceilings and walls of lower floors, as well as to equipment and electrical wires. If the control room and power distribution room are directly connected to the corrosive plant area, gases and dust can enter these rooms, while liquids can get in as well, contaminating the floors inside. The instruments and wiring in the control room and power distribution room are quite sensitive to corrosion, and its consequences can be severe once it occurs. 3.2.7 The floor level of the basement is low, making it difficult to remove corrosive liquids from the surface; moreover, the ventilation conditions are poor, which hinders the removal of corrosive gases or dust. Therefore, placing corrosive equipment in the basement objectively makes it difficult to prevent corrosion. 3.2.8 Local reinforcement and the adoption of isolation measures are both aimed at reducing the impact of corrosion and narrowing the scope of protection required. Gaseous media and solid dust are mainly separated by partitions, while liquid media are primarily protected from water with barriers placed on the floor. 3.2.9 Grouping devices that are exposed to similar corrosive agents together can reduce or prevent the alternating action of different corrosive media, simplify protective measures, and ease material selection. 3.2.10 Solutions such as sulfuric acid, sodium hydroxide, sulfuric anhydride, and sodium sulfate are highly corrosive to foundations. Numerous cases have shown that when these substances penetrate the foundation of a building, they can easily cause deformation of the foundation and cracking in the building. To prevent this phenomenon from occurring, it is required that the horizontal distance between the pipeline carrying the aforementioned liquid and the factory foundation be no less than 1 m. To facilitate leak detection, it is also required that the pipes be installed in pipe trenches. 3.2.11 Floor openings are weak points susceptible to corrosion by liquid media, and wall openings are not conducive to protection. Concentrating various types of pipelines and reducing the number of openings facilitates protection. 3.2.12 Cable trenches in chemical plant buildings are often filled with surface water; laying pipeline cables along corrosion-resistant floors or trenches exposes them to corrosion by the substances present in the floor or trench, which hinders construction and maintenance work. Experience shows that installing all cables and pipelines overhead helps prevent corrosion. 4 Structures 4.1 Concrete structures 4.1.1 The durability of concrete structures, apart from being ensured by the quality of materials, should also be guaranteed through the selection of appropriate structures and components, crack control, construction measures, and surface protection. Among these, the selection of structures and components plays a decisive role. The specification incorporates lessons learned both domestically and internationally, and puts forward several requirements. 1. The cast-in-place frame structure features good integrity and ease of protection. It has no weak points that might arise from steel embedment or prefabricated joints; therefore, its durability is relatively high. The prefabricated integral frame structure, due to the use of wet joints made of high-strength concrete at the connections between components, which seal off any embedded parts or welded joints, performs well in corrosive environments as well. Therefore, the above two types are recommended. 2. Composite structures made of reinforced concrete and steel, although they can leverage the respective advantages of both materials and offer benefits such as material savings and ease of construction, exhibit unique corrosion characteristics in corrosive environments. This is because different materials have varying sensitivities to corrosive agents. According to surveys conducted at certain plants, the corrosion of composite structures is sometimes more severe than that of reinforced concrete or steel structures alone, especially at the interface where concrete and steel come into contact. Therefore, such structures can be used in environments with low corrosivity levels, but are not suitable for environments with high or moderate corrosivity levels. 3. Prestressed concrete components feature high strength grades, good density, and excellent crack resistance. According to foreign tests, regarding the corrosion under stress conditions in concrete, the tensile zones are more susceptible to corrosion than the compressive zones. Therefore, from a durability standpoint, prestressed concrete components are superior to reinforced concrete components. Post-tensioned components assembled from blocks have joints or gaps between them. It is more difficult to seal than gaps, and corrosive substances can seep in through these gaps to corrode the prestressed rebar. In a factory, a composite trapezoidal roof truss with a 21-meter span suffered corrosion of its prestressed steel bars due to corrosive substances seeping in through the joints. After 10 years of use, the prestressed bars broke and suddenly fell. Therefore, post-tensioned members with block combinations should not be used under corrosive conditions. Components reinforced with cold-drawn steel wires, indented steel wires, carbon steel wires, or steel strands are prone to stress corrosion, as the wires are under high stress. Additionally, due to their relatively small diameter, even slight corrosion results in a significant reduction in their cross-sectional area. Therefore, such components should not be used in highly corrosive environments. 4. The cross-section of the columns should be of a solid-web type. The purpose is to minimize the exposed area subject to corrosion; moreover, a regular cross-section facilitates protection measures. I-shaped columns with double limbs and web openings have a large surface area, making them prone to corrosion; therefore, they are not suitable for use. 4.1.2 For the calculation of internal forces in indeterminate structures, if the redistribution of plastic internal forces resulting from inelastic deformation is taken into account, it is possible to make full use of the material to save steel and simplify the reinforcement layout. However, when plastic zones form, the deformation and cracking of the members become significant; under the influence of corrosive environments, this can affect the durability of the structure. Moreover, the large cracking can also cause cracks in the surface protective layer. Therefore, when calculating the internal forces of statically indeterminate structural members, the redistribution of plastic internal forces should not be taken into account. 4.1.3 The width of transverse cracks in components has a certain impact on durability; excessive width can lead to rusting of the rebar. In the past, domestic and international standards imposed relatively strict limits on crack width. However, the exposure tests conducted on-site have shown that the relationship between the width of transverse cracks and steel bar corrosion is not as severe as once thought; as a result, codes both domestically and internationally now generally allow for wider crack widths, which helps to save steel. Results of exposure tests on components conducted at some production sites show that, within a certain range (such as less than 0.3 mm), there is no obvious direct causal relationship between steel bar corrosion and crack width. Simulation tests on the relationship between crack width in components and steel bar corrosion under different corrosive gases (HCL, SO2, and NO2) as well as various relative humidities show that crack width has little impact on steel bar corrosion; it is generally believed that when the crack width is no more than 0.2 mm, it has virtually no effect on steel bar corrosion. The reinforcement in prestressed concrete members is subjected to high stress levels, and high-strength steel is often used for it; such steel is relatively sensitive to corrosion. Under the combined effect of corrosive environments and tensile stresses, stress corrosion tends to occur. If the concrete cracks are too large, the degree of corrosion in prestressed concrete members is more severe than that in reinforced concrete members; therefore, strict control is necessary. The crack control levels are classified, based on the corrosion severity level and the type of reinforcement, into those that require absolutely no cracks and those where it is generally sufficient to avoid cracks; these correspond to Level 1 and Level 2 crack control levels as specified in the current **Code for Design of Concrete Structures**. The stress limit coefficient for the normal section of concrete and the allowable maximum crack width specified in these codes are comparable to those in the current **Code for Design of Concrete Structures** for outdoor or indoor high-humidity environments; however, they are slightly higher for the heat-treated rebars and steel wires used in prestressed concrete structures. 4.1.4 Some tests show that the compactness of the original Concrete No. 200 (C18) is lower than that of Concrete No. 300 (C28); its resistance to carbonation is approximately half that of Concrete No. 300 (C28) and 1/8 that of Concrete No. 400 (C38). According to the current **standard, the Code for Design of Concrete Structures**, for structures located in outdoor or indoor environments with high humidity, the concrete strength grade should not be lower than C25. Therefore, this specification specifies the concrete strength grade for critical components ； It is C25 for reinforced concrete, and C35 for prestressed concrete. 4.1.5 The corrosion of components by corrosive media generally progresses from the outer surface toward the interior. The impermeability of concrete has a significant impact on the corrosion rate ； The impermeability of concrete is primarily determined by its density, and what controls this density are the water-cement ratio and the amount of cement used, among which the water-cement ratio plays the most important role. There is an approximate linear relationship between the water-cement ratio and the carbonation coefficient ； There is also an approximately linear relationship between the cement content and the carbonation coefficient, but the coefficient increases significantly when the value is below 300 kg/m3. Design codes for concrete durability, both domestically and internationally, specify limits for the maximum water-cement ratio and the minimum amount of cement to be used. The water-cement ratio is generally kept around 0.55 (for a water resistance grade equivalent to 0.6 MPa), while it is around 0.45 for prestressed concrete (for a water resistance grade equivalent to 0.8 MPa). 4.1.6 Adding rust inhibitors to concrete is an effective supplementary measure to prevent or slow down the corrosion of rebar, with particularly significant protective effects against the corrosive action of chloride ions. This material has been extensively studied abroad and has been commercialized. Some domestic steel bar rust inhibitors have performance comparable to Japanese products. The composite rust inhibitor eliminates the drawbacks associated with the previous use of sodium nitrite alone; it also has water-reducing and strength-enhancing effects. The Ministry of Metallurgy issued the \"Technical Regulations for the Use of Reinforcement Rust Inhibitors\" (YBJ231-91), providing a technical basis for the use of such inhibitors. Currently, there are many domestic manufacturers of steel bar rust inhibitors. Due to the varying performance and quality of these products, it is necessary to evaluate their properties before using them; they can be employed only after it has been confirmed that they have no adverse effects on the physical-mechanical properties and chemical characteristics of concrete. 4.1.7 The impact of foreign admixtures on the durability of reinforced concrete is still under investigation, but the corrosive effect of chloride-containing admixtures on steel rebar is already well recognized. Many regulations both at home and abroad impose varying degrees of restrictions on the use of admixtures containing chloride ions. Under the action of corrosive media, chloride ions accelerate the corrosion of steel; therefore, they should not be used in reinforced concrete and prestressed concrete structures. It is still unclear whether other types of admixtures, such as those containing sulfate ions, have an impact on the durability of reinforced concrete; however, sulfate ions can accelerate the corrosion of the reinforcing steel after the concrete becomes neutralized. Therefore, the use of admixtures should be approached with caution; they can only be used after it has been confirmed that they have no impact on durability. 4.1.8 To protect the rebar, in addition to concrete with a certain density, a protective layer of sufficient thickness is also required. According to investigations, if the thickness of the protective layer is reduced by 1/4, the time it takes for the concrete neutralization layer to reach the surface of the reinforcing steel can be halved. To facilitate the use of standard atlases, calculation programs, and charts, the values adopted in this specification are comparable to those in the current **Standard for Design of Concrete Structures** for outdoor or high-humidity environments. For post-tensioned prestressed members, the clear distance from the ducts to the edge of the member is slightly larger than that specified in the current **Code for Design of Concrete Structures**. This is mainly due to the importance of the prestressing steel bars and the potential defects that may occur during duct grouting; it is also determined with reference to domestic port engineering codes and relevant foreign regulations. 4.1.9 Around the openings in equipment containing liquid or solid media, due to factors such as leakage and flushing, these beams and slabs may be frequently exposed to liquid or solid media, resulting in relatively severe corrosion. To protect the side beams from corrosion, they can be positioned away from the edge of the hole with the plate protruding outward; this arrangement has proven effective in copper electrolysis plants. 4.1.10 The embedments for fixing pipelines and equipment supports are severely corroded. If the embedment is welded to the load-bearing rebar of a component, it will cause corrosion of that rebar. The steel hooks on the beams that are embedded directly also suffer from severe corrosion, which can sometimes cause cracking in the concrete surrounding those hooks. Corrosion-resistant sleeves are embedded in the beam, allowing steel hooks to be fixed through these sleeves; this facilitates replacement without affecting the beam, resulting in a good outcome. 4.11 Once embedded components are corroded, they are difficult to repair or replace, posing various risks and even potentially affecting the components themselves. For the protection of embedded parts, based on engineering experience, coating with ordinary coatings or glass flake coatings can be used for protection. Composite coating protection, which involves applying paint over a metal coating of aluminum or zinc that has been sprayed (or plated), can be used in cases where corrosion is severe. 4.12 In prefabricated structures, the connection points between components—such as the joints where large roof panels are connected to roof trusses or beams, the joints between skylight frames and roof trusses, and the joints between roof trusses and columns—are key elements that ensure the structural integrity of the whole structure. During the inspection, it was found that both the welds and the embedded parts were corroded to varying degrees, with severe cases showing complete corrosion. Therefore, it must be carefully protected; covering it with concrete or polymer cement mortar is a good option. The exposed metal anchors in post-tensioned prestressed concrete, as well as the exposed parts of the end rebar in pre-tensioned concrete, are all critical components; they are covered with concrete to ensure their reliability. 4.2 Steel Structures 4.2.1 4.2.2 The forms of steel structural members and bars have a significant impact on the corrosion rate of the structures or bars. According to the principle of material concentration, the smaller the ratio of the perimeter to the area of the cross-section, the higher the corrosion resistance. The walls of thin-walled steel sections and light steel structures are relatively thin; even slight corrosion can have a significant impact on their load-bearing capacity ； The cross-sections of the members in lattice structures are small, and with numerous straps and plates, their surface area is large, which makes corrosion prevention difficult ； The T-shaped cross-section composed of two angle steels has a corrosion rate that is twice that of tubular shapes or 1.5 times that of ordinary I-beams. Moreover, the gap formed between the two angle steels cannot be protected, resulting in areas where corrosion occurs intensively; therefore, codes restrict the use of such structures and members. The cross-section of the members should preferably be solid or closed-type; if members with composite cross-sections are to be used, the gap width between the steel sections must meet the requirements for inspection and maintenance during the installation of protective layers. 4.2.3 To ensure the durability of steel components, certain thickness requirements must be met. Too thin members lose their load-bearing capacity quickly once corroded. The minimum values specified in the standards are determined based on practical experience. 4.2.4 Welds, due to the presence of slag on their surface and their uneven texture, are prone to attracting corrosive agents. Residual stress generally exists at welds at present; therefore, welds often corrode before the base material. The weld seam is a critical part for transmitting forces and ensuring the integrity of the structure, and there must be minimum requirements for its root dimension. Intermittent welds are prone to crevice corrosion; if intermittent welds are used for the joints in closed-sections, corrosive agents and moisture can easily penetrate inside through the gaps in the welds. Therefore, continuous welds are used for the welds of critical members and closed-section members. 4.2.5 The connection materials of components, such as welding electrodes, bolts, and joint plates, shall have a corrosion resistance that is not lower than that of the main structural materials, in order to ensure the integrity of the structure. 4.3 Masonry Structures 4.3.1 Both silicate bricks and fly ash bricks contain a certain amount of lime-based binders; moreover, due to their large pores and high water absorption, these materials should not be used in load-bearing masonry in corrosive environments. The performance of load-bearing clay hollow bricks is comparable to that of solid clay bricks. To improve the durability of masonry structures, the current **standard, the Code for Design of Masonry Structures, requires that for exterior walls in humid rooms or those with a ceiling height greater than 6 meters, the brick strength grade should be MU10. **Authorities such as the Building Materials Bureau and the Ministry of Construction have repeatedly issued notices calling for restrictions on the use of solid clay bricks of grade MU10 or lower, in order to promote energy efficiency in construction. Therefore, this specification requires that the strength grade of bricks in load-bearing structures should not be lower than MU10. Given that the grade of cement is currently high, mortars with low strength grades contain too little cement, resulting in poor density and increased susceptibility to corrosion; therefore, it is required that the strength grade of the mortar be at least M5. 4.3.2 Clay bricks and blocks are porous materials that readily absorb corrosive liquids; under conditions of alternating dryness and wetness, crystalline expansion corrosion can occur, leading to rapid destruction of the masonry ； It should not be used under the above conditions. Independent brick columns have a small cross-section and are subjected to single types of loads; moreover, as they are exposed to corrosion on all four sides, they are not safe for use under corrosive conditions, which is why their use is restricted. 4.4 Wood Structures 4.4.1 To ensure the safety and reliability of buildings, and in line with the principle of conserving timber, the use of wood structures is appropriately restricted. 4.4.2 Laminated wood structures are recommended as they do not contain steel components, which makes them resistant to corrosion. 4.4.3, 4.4.4 The joints of wooden structural members and steel components represent weak points in terms of protection; moreover, corrosive substances tend to accumulate at these joints and connections, leading to severe corrosion. Corrosion of steel components can cause the joints to loosen, increasing the deformation of the components; therefore, the use of steel components should be minimized, and they should not be used under conditions of severe corrosion. 4.5 Foundation 4.5.1 The current **standard, the ‘Code for Geotechnical Investigation’, already includes requirements for the investigation and evaluation of contaminated soil; therefore, this code only specifies the corresponding design requirements. The effect of corrosive liquids on soil can have the following impacts: 1. Acids such as sulfuric acid, and substances like sodium hydroxide, sodium sulfate, and sulfuric anhydride react with certain components in the soil, resulting in the formation of new salts or altering the physical properties of the soil through ion exchange. As a result of these reactions, the soil tends to become expandable. Another scenario is where the medium crystallizes within the soil pores, causing the soil to expand. Both of these situations will cause the superstructure to deform upward and crack. 2. The loss of soluble corrosion products resulting from the action of corrosive substances on the soil leads to an increase in the soil’s porosity ； Or, the dissolution of certain cementing salts in the soil reduces or eliminates its chemical cohesion. This may lead to changes in the physical and mechanical properties of the soil; the void ratio increases, and the particles become finer. The bearing capacity and compressive modulus may decrease, resulting in foundation settlement and cracking of the superstructure. 3. When constructing new factories on contaminated sites, changes in production conditions can lead to alterations in the hydrogeological conditions, thereby disrupting the original equilibrium and causing the contaminated soil layers to expand or subside. During the design phase, based on the evaluation framework for contaminated soil, and taking into account factors such as the specific conditions of the building, the nature and concentration of corrosive media, and the production environment, measures should be taken by drawing on existing experience and considering the aforementioned influences. Regarding the treatment of contaminated soil foundations, the commonly used methods in engineering at present are as follows: 1. When the swelling or settlement amount of the contaminated soil is relatively small and the soil layer is relatively thick, or when it is difficult to excavate all of it for other reasons, partial excavation can be carried out, followed by backfilling with sand or stone materials. However, the thickness of the retained soil layer should be determined through calculations of expansion or settlement deformation, so that the deformation of the superstructure remains within allowable limits. 2. Completely excavate the contaminated soil layer. This is the most effective and reliable method, but it must be determined after technical and economic evaluations. 3. In certain contaminated soil layers with low bearing capacity and compressive modulus but no expansibility, corrosion-resistant sand and stone piles can be used for reinforcement. When the stabilized soil layer is a poorly permeable layer, while the underlying soil is a more permeable layer, it is not advisable for the pile to penetrate through the stabilized layer. This prevents the upper-layer medium from diffusing into deeper layers through the pile, thereby avoiding an expansion of the contamination area. 4. When the layer of contaminated soil is relatively thick and cannot be completely excavated, and the building in question is of great importance, prefabricated reinforced concrete pile foundations should be used to penetrate through the contaminated soil layer and rest on the uncontaminated soil layer. The pile bodies must undergo necessary protective treatments. 4.5.2 When selecting a foundation reinforcement method under corrosive conditions, the following factors should be considered: 1. Corrosion-resistant reinforcement materials should be chosen; for instance, in the presence of acidic media, carbonate-based sand and stone piles should not be used. Under conditions that are corrosive to cement, do not use cement as a curing agent in mixing piles. 2. The reinforced soil layers formed by certain chemical reinforcement methods may lose their effectiveness due to new chemical reactions that occur under the influence of surrounding media. For example, in the silicification reinforcement method, sodium silicate dissolves and leaches out under the action of an alkaline medium. 3. The salts produced from stone-like materials under the action of acids or sulfates; some of these salts exhibit expansive properties, while others prevent lime soil from consolidating, thus rendering it ineffective as a stabilizer. 4.6 Foundations 4.6.1 The medium acting on the ground may penetrate into the foundation through trenches, the ground surface, and drainage facilities, thereby causing corrosion to the foundation. However, the amount of penetration is limited; therefore, its corrosivity level is determined by reducing one grade according to Table 3. 4.6.2 Foundations of thin-walled types such as shells and folded plates should not be used, as they have thin walls, complex stress conditions, and are difficult to protect; there is also no experience in using them in corrosive environments. 4.6.3 Rough-hewn stone concrete, concrete, and reinforced concrete have high density and integrity, as well as a smooth surface that is easy to protect; therefore, they are recommended for use. Brick foundations have poor durability; they feature many complex joints and are difficult to protect, making them unsuitable as foundation materials in environments with corrosive agents. 4.6.5 When media such as sulfuric acid and sodium hydroxide penetrate into the soil, they can cause the foundation soil to expand, leading to cracks in the superstructure. Burying the foundation sufficiently deep can reduce or eliminate this effect. It is generally difficult to ensure complete leaklessness in storage tanks or pits; in order to prevent the soil layer beneath the foundation from being corroded, the bottom surface of the foundation should be below the bottom surface of the storage tank or pit. 4.6.6 Foundations are important components of buildings, and since they are buried deep underground, it is difficult to carry out regular inspections and maintenance. To ensure safety, surface protection should be applied in areas with high or moderate corrosion levels. Surface protection layers using asphalt mortar have many years of usage experience and demonstrate good performance. To address the difficulties of hot construction and working on wet substrates, wet-curing epoxy asphalt coatings can be used. The foundation beams are located near the ground surface, suffer from severe corrosion, and have a small cross-section; therefore, their protection requirements should be higher than those for ordinary foundations. 4.7 Pile Foundations 4.7.1 The top of the pile is generally at a height of not less than 1.5 m above the ground level, and it is protected by a cap; therefore, for pile foundations, only the corrosive effects of contaminated soil and groundwater are taken into account, while the corrosive effects of substances present on the surface are ignored. 4.7.2 Precast reinforced concrete piles have high concrete density, their quality is easy to control, and they are also easy to protect. Poured piles come into contact with the medium before the concrete has hardened, making protection difficult; moreover, there is a lack of experience in using them in gymnasium settings. Steel piles lack experience in use under corrosive conditions, making it difficult to determine the corrosion margin; therefore, they cannot be included for now. Piles are not included either, as they are used very rarely; to save wood. 4.7.3 The self-protection performance of precast reinforced concrete piles plays an important role in the durability of the piles. Therefore, high requirements are placed on the strength grade of the concrete, the water-cement ratio, and the cover layer. The values specified in these standards are generally consistent with relevant regulations at home and abroad. 4.7.4 When the corrosion level is severe or moderate, relying solely on the concrete’s own protective properties is insufficient; additional protective measures must be taken. The protection measures for piles mainly fall into three categories: 1. Selecting cement varieties with good corrosion resistance; under conditions of pure SO4 corrosion, piles can be made from sulfate-resistant cement or ordinary Portland cement with a tricalcium aluminate content of no more than 5%, and no protective measures are required on the surface of the piles. 2. Under corrosion conditions caused solely by Cl- ions, steel reinforcement rust inhibitors can be added to the concrete, allowing the pile shaft to remain without additional protective measures. 3. When the corrosion grade in acidic media (pH less than 4.5) is severe or moderate, surface protection measures must be applied to the piles. The surface of the piles can be coated with asphalt mortar, epoxy coal tar pitch, or oil-soluble polyurethane (cyanate curing). These coatings have been used domestically, and in strata with fine-grained soil, they generally do not wear out during pile driving. 5 Building Protection 5.1 Floor 5.1.1 The surface materials of floors are subject to various forms of mechanical wear or impact from time to time, in addition to being affected by corrosive substances. Various surface materials have their own characteristics. Sodium silicate concrete, although it has good acid resistance, higher mechanical strength, and tolerance to high temperatures, it is not resistant to hydrofluoric acid or alkaline media; its impermeability is poor, and its water resistance is also inadequate. Resin-based surface layers have advantages such as resistance to acids of moderate concentration, alkali resistance, density, and high strength; however, they are not resistant to strong acids or high temperatures, and they may not be resistant to certain types of chemicals. Soft PVC surface layers are resistant to acids of moderate concentration, alkali, and water, but they have poor wear resistance, are not resistant to impacts, and tend to age easily. Therefore, designers are required to correctly select the surface material based on the usage conditions, taking advantage of its strengths and avoiding its weaknesses. Compared with the original specifications, the following changes have been made to the floor surface materials: 1. The fiberglass reinforced plastic surface layer has been removed. Due to its poor wear resistance, this surface layer does not perform well, and it is only recommended for use as a barrier layer. In fiberglass insulation and energy guidance applications, it can be used as a surface coating for resin-based paste, resin mortar, and blocks. 2. Sulfur-based materials have been removed. This material has advantages such as acid resistance, density, and the ability to be used without any special treatment after installation. However, sulfuric acid-based materials have poor adhesion to the base layer and building blocks; their adhesion decreases over time, they exhibit high shrinkage during curing, and there are few successful applications in practice. Therefore, they were not included in this standard during its revision. They will be added to the standard only after significant progress is made in experimental research and sufficient experience is gained from practical applications. 3. “Acid-resistant stones” include granite, quartz stone, etc. These stones possess excellent corrosion resistance as well as good physical and mechanical properties, and are widely used in engineering projects; they are collectively referred to as “acid-resistant stones” in the relevant standards. 5.1.2 Thickness of acid-resistant stone: Thanks to the development of the stone industry, mechanical cutting techniques are now used by many stone manufacturers, allowing for a wider range of thicknesses; stones with thicknesses ranging from 20 mm to 100 mm can be ordered. Thinning the stone thickness can reduce the load on the floor and lower transportation costs. Due to mechanical cutting, the surface smoothness of the stone is also **improved**, which not only reduces the amount of mortar needed for construction and lowers costs, but also enhances the quality of the floor. Resin mortar and resin thin-set finish coats have been widely adopted; there is some experience in their use, but there are also many cases of failure. The quality of this surface layer, and whether it is successful or not, is closely related to the quality of the resin and the quality of the installation process. 5.1.3 When different materials are used for the grout joints and the bonding layer in a slab floor, slabs with rigid materials used for filling the joints should not have a flexible bonding layer; otherwise, when the floor is subjected to gravitational forces, cracks may occur at the joints. 5.1.4 The use of an isolation layer can enhance the water resistance of the floor surface and compensate for any shortcomings in the surface layer, thereby improving the reliability of corrosion-resistant floor systems as a whole. Due to their poor water resistance, water glass mortars, grouts, and concrete require an isolation layer when used as surface layers or bonding layers. 5.1.5 The isolation layer material should be a waterproof material with certain corrosion resistance. In addition to various resin-based fiberglass materials, corrosion-resistant waterproof materials can also be used. The main types that can be used as isolation layers are the following two. : 1. Asphalt-based products: various polymer-modified asphalt sheets, rubber-modified asphalt sheets, asphalt-glass cloth sheets, etc. 2. Synthetic polymers: polyvinyl chloride sheets, chlorinated polyethylene sheets, ethylene propylene diene monomer sheets, polyurethane sheets, polyethylene sheets, neoprene sheets, and various polymer-based waterproof coatings, etc. Ordinary paper-based asphalt shingles have low tensile strength, poor corrosion resistance, and inferior performance, so their use is not recommended. 5.1.7 In the past, anti-corrosion floor underlays generally used 100-grade concrete with a thickness of 100 mm. In actual engineering projects, due to reasons such as construction, the stiffness of the cushion layer is insufficient, resulting in some problems. Following the practices used in foreign-related projects, the thickness of the cushion layer is generally 200–250 mm, with 200-grade concrete and double-direction reinforcement using bars of φ8–φ12@150. Due to the high quality of the cushion layer, cracking and damage to the anti-corrosion surface layer occur less frequently. Therefore, the code increases the minimum thickness of the cushion layer to 120 mm, with a strength of not less than C15. For outdoor surfaces that are large in area or where the foundation may experience uneven deformation, cracking and deformation are likely to occur; hence, the use of reinforcement is required. For resin-based monolithic floors, due to the significant shrinkage stresses generated by the curing of the surface material, higher requirements are placed on the underlayment; therefore, the use of reinforcement is necessary. For outdoor surfaces, according to relevant ground standards, a frost protection layer is required only when the depth of freezing in the ground is greater than 600 mm. However, anti-corrosion surfaces have high requirements regarding crack resistance. To meet these strict requirements and prevent frost heave, all outdoor surfaces in frozen areas should be equipped with a frost protection layer with a thickness of no less than 300 mm. Freezing expansion also occurs if water accumulates within the anti-freeze layer. Resin mortar, resin grout, and soft PVC sheet surfaces often experience flaking, one of the reasons being that the capillary action of groundwater reduces the adhesion between the surface layer and the substrate. Therefore, when the groundwater level is high, it is necessary to take waterproof and moisture-proof measures for the cushion layer. 5.1.8 Laying the surface layer directly on the precast slab easily leads to cracks at the joints between the slabs; therefore, it is specified to use a monolithic poured layer with reinforcement to ensure its integrity. 5.1.9 On surfaces exposed to corrosive liquids, a slope should be provided to allow the fluid to drain away quickly, preventing water accumulation on the surface and thus reducing corrosion. A large ground slope is beneficial for corrosion prevention, but an excessively large slope also has various disadvantages. According to engineering investigations, it is reasonable for the floor slope to be 1% or greater, and the slope of the ground floor to be 2% or greater. If the slope of the floor surface is less than 1%, drainage will be poor; when there are no vehicles moving around in the facility, the slope of the ground floor can be increased to 3%–4% as well. 5.1.10 Generally, the underlying surface is sloped using native soil, as this is the simplest and most reasonable approach ； Floor surfaces are usually leveled using a leveling compound, but this requires more material and results in greater load ； Using structural slope formation saves materials and reduces load, but the structural design and construction are more complex; it can be adopted when conditions permit. 5.1.11 Actual investigations show that drainage ditches and floor drains are prone to leakage, causing significant corrosion to nearby structures. To prevent damage to nearby critical components, minimum distances are specified between the drainage ditch and walls or columns, as well as between the center of the floor drain and the edges of structures such as walls, columns, and beams. 5.1.12 Floor drains are important components for the floor surfaces on upper floors or the ground floor in corrosive workshops. According to investigations, there are very few effective and complete floor drains in the production facilities; over 95% of the floor drains are defective, and they cause blockages and leaks during use, leading to severe corrosion of the surrounding floor slabs. Therefore, the floor drain should be made of materials that are resistant to corrosion and have sufficient strength; its dimensions should be larger than those of ordinary drainage floors, and its construction must be tight, with a focus on preventing leaks at the connection points. 5.1.13 Water-blocking devices are used to prevent the spread of corrosive liquids or their leakage to lower layers; therefore, all openings must be equipped with such devices. 5.1.14 The installation of footplates is intended to prevent corrosive liquids on the ground from damaging the walls and the bases of columns; its height should be determined based on the possible height to which liquids may splash, as well as taking into account the size of the building blocks. 5.1.15 Corrosion-resistant bases are provided at the bottoms of steel piles, steel ladders, and barrels to prevent direct exposure of the steel components to corrosive agents in the ground. 5.1.16 The expansion joint for outdoor floors should not exceed 20 m, which is the lower limit value specified in the \"Code for Design of Building Floors\"; for indoor floors, this value should not exceed 30 m, and it is determined by referring to the maximum spacing of 30 m for expansion joints in indoor basement walls as specified in the \"Code for Design of Concrete Structures\". 5.1.17 Ground deformation joints are weak points in terms of corrosion resistance; corrosive substances can easily seep through these areas and cause corrosion, so strict measures must be taken to prevent leakage. Generally, a deformable expansion strip is installed at the bottom of the seam, with a corrosion-resistant, elastic material possessing good adhesion properties embedded in it. Asphalt mortar was used in the past, but its durability was very poor, so it is no longer recommended. Chlorosulfonated polyethylene putty, polyvinyl chloride putty, and polyurethane sealant are recommended. Components such as expansion pieces may come into contact with corrosive substances; therefore, corrosion-resistant materials should also be used. 5.2 Equipment Foundations 5.2.1 The underground portion of wall-type and frame-type equipment foundations should be protected in accordance with the standards for factory building foundations, as these foundations are usually made of reinforced concrete, have a small cross-section, and are of great importance. The underground part of the foundation for small equipment does not need to be protected, as such foundations are generally made of plain concrete in solid form; even if the surface is slightly corroded, it does not affect its functionality. 5.2.4 One of the purposes of using strip foundations for large metal storage tanks is to facilitate leak detection; therefore, the spatial dimensions between the strip piers should meet the requirements for leak detection and maintenance. 5.2.6 The bolt holes in the equipment foundation shall be filled with corrosion-resistant mortar, primarily to prevent the penetration of corrosive substances, while also ensuring the anchoring strength of the bolts. 5.3 Ditches and pits 5.3.1 Corrosive liquids generally act on trenches and pits for extended periods of time, and leaks often occur as well. To ensure the safety of the load-bearing structure and protect it from corrosion, it is stipulated that walls, columns, and foundations shall not be used as the side walls and floor slabs of ditches or pits. 5.3.2 Ditches generally have only simple anti-corrosion measures and do not meet the requirements of drainage ditches. If pipes are laid in drainage ditches, they will corrode, and the rigid fixing points of the pipes will also damage the integrity of the anti-corrosion coating. Therefore, trenches should not be used as drainage ditches. 5.3.4 Drainage ditches must be waterproof and moisture-proof; this is mainly to ensure that the anti-corrosion lining is not damaged due to water seepage from the base layer. 5.3.5 Deformation joints should not be provided in gutters, as they represent weak points that are prone to leakage. Numerous engineering practices have shown that it is possible to achieve this without seams, as long as appropriate design measures are taken. When joints must be created due to unavoidable circumstances, strict and multi-layered leak prevention measures are required. 5.3.6 Drainage ditches and sump pits are exposed to liquid media for extended periods, and sedimentation such as mud and sand occurs, requiring cleaning; they are prone to mechanical damage, and their operating conditions are more severe than those on the ground surface. An isolation layer is installed to improve their impermeability. 5.3.7 Open drains for drainage are used to facilitate cleaning; covering them with lids is necessary for safety and operational purposes. 5.3.9 When a ditch passes through the foundation of a building, if a joint is created near the foundation, the leaked medium may corrode the foundation. The space reserved between the trench and the foundation is intended to prevent the trench from being stressed and breaking under the load caused by the settlement of the factory building. 5.4 Surface protection of components 5.4.1 Under the action of gaseous media, brick or concrete walls generally suffer only corrosion of moderate severity or less; however, when condensation forms on the wall surface or water comes into contact with solids, liquid corrosion occurs. Therefore, the wall may be subjected to three types of media: gaseous, liquid, and solid. Severe corrosion of floor surfaces can occur under the following conditions and in the following areas: where liquid media are present (such as areas where condensation forms on walls, areas that remain damp frequently, and around exhaust vents), or in areas where solid media such as alkaline dusts or sodium sulfate come into contact with water. 5.4.2 Dew formation on the building envelope occurs frequently inside buildings in humid and cold regions; the areas where dew forms cause gaseous or solid substances to transform into liquid form, thereby accelerating corrosion. In a nickel electrolysis plant, for example, the walls surrounding the side windows often develop condensation, and the walls are severely damaged as a result of the alternating effects of wetness and dryness as well as the crystallization of sulfates. It is difficult to completely prevent dew formation in a small number of workshops that are frequently exposed to steam and high humidity; therefore, the codes recommend strengthening protection in areas where dew formation is likely to occur. 5.4.3 The protection of concrete members is implemented in a differentiated manner based on the importance of the members and their corrosion severity level, with emphasis placed on areas that require greater protection. Coatings are an important protective measure for concrete components. Their advantages include good corrosion resistance and relatively easy application; however, their drawbacks are high cost and the need for rework after several years. Polymer cement slurry has inferior corrosion resistance compared to anti-corrosion coatings, but it exhibits good adhesion to concrete surfaces and is dense enough to resist corrosion from moderately corrosive gaseous media. Rust inhibitors protect the rebar in concrete. Therefore, when the corrosion conditions for ordinary concrete members are moderately corrosive and a corrosion inhibitor is added, no surface protection is necessary. 5.4.4 Wall skirts are generally subjected to liquid or solid media, but the degree of exposure is less severe than that of floors; especially with liquid media, such exposure cannot persist for long periods, so the requirements for protective materials and construction are relatively low. Generally, in an acidic medium, ceramic tiles can be used, or fiberglass can be employed, or soft PVC sheets can be applied to meet the anti-corrosion requirements. For skirtings in alkaline environments, cement mortar, polymer cement mortar, or coatings are sufficient to meet the requirements. 5.4.5 In the design and construction of anti-corrosion projects, great attention is paid to the protection of floor surfaces; however, insufficient attention is given to their detailing. For instance, the protection of the sides and bottom surfaces around openings in the floor slabs is often neglected. In fact, medium flow often occurs in these areas, causing corrosion of the floor slabs and edge beams. Based on practical experience, fiberglass or coatings can be used for protection, depending on the level of corrosivity. 5.4.6 The effectiveness of coating protection for steel structures is highly dependent on the rust prevention of the base surface. For substrates with different rust removal effects, the service life of their coatings can differ by 2 to 3 times. The rust grade and derusting grade of the mesh material are in accordance with the national standard \"Rust Grades and Derusting Grades of Steel Surfaces before Painting\". The requirements for the rust removal grade are related to the type of coating and the importance of the component. Some coatings, such as zinc-rich primers and ethylene phosphating primers, require a high level of rust removal from the substrate; sandblasting or shot blasting should be used for this purpose ； For asphalt coatings, chlorinated rubber coatings, and epoxy-asphalt coatings, the requirements for rust removal from the substrate can be slightly relaxed. Of course, no matter what type of coating is used, it’s better to remove rust from the substrate as thoroughly as possible. However, the higher the rust removal standards, the greater the cost and the more difficult it becomes to achieve them. Therefore, the determination of rust removal standards must take into account both practical construction considerations and economic factors. 5.4.7 The thickness of the coating applied to steel structures shall be determined based on the importance of the component and its corrosion resistance rating. For areas that are difficult to repair (such as high elevations) and outdoor components, the coating thickness should be increased appropriately. The minimum coating thickness specified in this article. The minimum coating thickness specified in this clause is greater than the actual coating thickness used in current general building corrosion protection projects, as insufficient coating is a major cause of poor coating durability. 5.4.8 For particularly important steel structures that are difficult to maintain, such as tall towers, metal can be sprayed onto the surface of the steel (such as galvanizing), followed by the application of anti-corrosion coatings (such as chlorinated rubber paint, chlorosulfonated polyethylene paint, and glass flake paint). In recent years, many television towers have adopted this composite coating, with good results. 5.4.9 Wood structures are rarely used in critical structures, and generally not in areas subject to severe corrosion; therefore, ordinary protective coatings or surface treatment agents can be used for their protection as well. 5.5 Doors and Windows 5.5.1 Sliding doors and metal roller shutters are types of hung folding doors; their metal components can corrode, which may make it difficult to open them, so they are not suitable for use. 5.5.2 Plastic windows possess excellent corrosion resistance. Standard drawings for corrosion-resistant plastic windows are available, and they are used in many corrosive environments; therefore, they are included in the specifications. At present, there are no standardized fiberglass windows available in China; moreover, they have numerous issues in practical use, so they have not been included in the specifications. Weather-resistant steel windows have better corrosion resistance than ordinary steel windows, are not expensive, and have been widely adopted in the Shanghai area. 5.6 Roofing 5.6.1 The purpose of using organized drainage is to prevent rainwater containing corrosive substances from flowing over and corroding the wall surfaces. Surveys show that after parapets are installed on the rooftops of buildings that emit large amounts of corrosive dust, a significant amount of dust accumulates at those parapets, making it difficult to remove and exacerbating corrosion. 5.6.2 The selection of roofing materials should be determined by considering the corrosive media present in the environment. Since roofs are frequently exposed to rainwater, any presence of corrosive gases or dust on the roof makes it more susceptible to corrosion than other areas. Therefore, the choice of materials should be based on the nature of these media. Experience has shown that fittings made of thin steel sheets or galvanized iron sheets for roofs are prone to corrosion in any corrosive atmospheric environment; therefore, they are not suitable for use.

Reply 1# eastchen: In the design for corrosion protection of equipment, the basic requirements regarding material selection are as follows: the selected materials must be able to resist corrosion from the environment and from the media involved, meet the requirements of the manufacturing process, comply with safety standards and regulations, and at the same time, the cost of these materials should be kept as low as possible to reduce the manufacturing costs of the equipment. Basic principles for material selection: Reduce equipment corrosion to extend its service life, meet safety standards and regulations; while ensuring that the manufactured equipment fulfills requirements related to manufacturing processes, corrosion prevention, and safety, efforts should also be made to reduce the costs associated with equipment production.

Does anyone know what measures can be taken to prevent corrosion of stainless steel products, especially when it comes to corrosion caused by acidic or alkaline gases?