Corrosion prevention methods can generally be divided into two categories:
- Proper selection of corrosion-resistant materials and other preventive measures.
- Selection of reasonable process operations and equipment structures.
Strict adherence to process regulations in chemical production can eliminate corrosion phenomena that should not occur. Even with the use of good corrosion-resistant materials, failure to follow proper operational procedures can result in severe corrosion.
Currently, the commonly used corrosion prevention methods in chemical production include the following:
- Proper material selection and design:
Understanding the corrosion resistance of different materials and selecting corrosion-resistant materials correctly and reasonably is the most effective method. It is well known that there are many types of materials, and their corrosion rates vary in different environments. The selection personnel should choose materials with low corrosion rates, relatively affordable prices, and physical and mechanical properties that meet design requirements for a specific environment. This ensures that the equipment achieves an economic and reasonable service life.
- Environmental adjustments:
If it is possible to eliminate various factors in the environment that cause corrosion, then corrosion can be terminated or slowed down. However, most environmental factors cannot be controlled, such as moisture in the atmosphere and soil, and oxygen in seawater. Additionally, it is not feasible to arbitrarily change the chemical production process. However, certain local environmental conditions can be adjusted. For example, if the oxygen is removed from the water entering a boiler (using deoxidants like sodium sulfite and hydrazine), it can protect the boiler from corrosion. Similarly, removing moisture from the air before entering a sealed warehouse can prevent metallic components stored within from rusting.
To prevent scaling and perforation in heat exchangers and other equipment caused by cooling water, adjusting the pH level to an optimal range (close to neutral) can be achieved by adding alkaline or acidic substances to the water. In refining processes, alkalis or ammonia are often added to maintain a neutral or alkaline environment. If the temperature is too high, cooling the vessel walls or lining them with refractory bricks for insulation can be effective solutions. These methods aim to change the environment without affecting the products or processes. Within permissible limits, it is recommended to use milder media instead of highly corrosive ones in the process.
- Adding Corrosion Inhibitors
In general, adding a small amount of corrosion inhibitors can significantly mitigate metal corrosion in corrosive environments. Corrosion inhibitors can be classified into three categories: inorganic, organic, and vapor-phase inhibitors, each functioning through different mechanisms.
① Inorganic Corrosion Inhibitors
Some corrosion inhibitors slow down the anodic process and are called anodic inhibitors. They include oxidants that promote anodic passivation (such as chromates, nitrites, iron ions) or anodic film-forming agents (alkalis, phosphates, silicates, benzoates). These inhibitors mainly react in the anodic region, promoting anodic polarization. Anodic corrosion inhibitors generally form protective films on the surface, which can provide effective corrosion inhibition. However, there is a potential risk if the dosage is insufficient, as it may lead to incomplete protection film formation. The presence of defects in the film exposes a small area of bare metal, leading to an increased anodic current density and a higher risk of localized corrosion.
Another type of corrosion inhibitor acts on the cathodic reaction, such as calcium ions, zinc ions, magnesium ions, copper ions, manganese ions, etc. These ions react with the cathode to generate hydroxide ions, forming insoluble hydroxides that cover the cathodic surface in the form of thick films. This impedes oxygen diffusion to the cathode and increases concentration polarization. Additionally, there are mixed-type inhibitors that simultaneously inhibit the anodic and cathodic reactions. However, the dosage of these inhibitors generally needs to be determined through experimentation.
② Organic Corrosion Inhibitors
Organic corrosion inhibitors are adsorption-type inhibitors that adsorb onto the metal surface, forming a thin, invisible film, usually consisting of a few molecules thick. They can simultaneously inhibit the anodic and cathodic reactions, although their influence on both processes may vary. Common organic corrosion inhibitors include compounds containing nitrogen, sulfur, oxygen, and phosphorus. The adsorption types of organic inhibitors vary depending on the molecular configuration and can be classified into electrostatic adsorption, chemical adsorption, and π-bond (non-directional electronic) adsorption. Organic inhibitors have rapidly developed and are widely used. However, their use also has some drawbacks, such as potentially contaminating products, especially in the food industry. While corrosion inhibitors may be beneficial in certain parts of the production process, they can become detrimental in other stages. They may also inhibit desired reactions, such as slowing down the removal rate during pickling.
③ Vapor-Phase Corrosion Inhibitors
Vapor-phase corrosion inhibitors are highly volatile substances that contain corrosion inhibiting groups. They are commonly used to protect metal components during storage and transportation, often in solid form. When the vapor of these inhibitors comes into contact with moisture in the air, it gets hydrolyzed and forms effective corrosion inhibiting species, which adsorb onto the metal surface, thus slowing down the corrosion process. Vapor-phase inhibitors are also considered adsorption inhibitors, eliminating the need for rust removal from the protected metal surface.
- Cathodic Protection
Cathodic protection is a method of reducing or eliminating corrosion on a metal by applying a direct current or sacrificial anode to make the protected metal the cathode. Prior to implementing cathodic protection, most corroding metal structures have both cathodic and anodic areas. If all the anodic areas can be converted into cathodic areas, the entire metal component becomes the cathode, which achieves the goal of corrosion elimination. When selecting a cathodic protection system for a specific project, several factors need to be considered:
- Required total protective current
To implement cathodic protection, it is crucial to determine the required total current. This can be determined using temporary test arrangements. If the required protective current is small (less than 1.5-2A), sacrificial anode protection is preferred. However, if a larger protective current is needed, an impressed current system is more economically feasible.
- Environmental variations
In poorly aerated soil, metal tends to polarize relatively easily. In soil where oxygen can readily access the structure’s surface, more significant currents are required for polarization. Additionally, the areas with the lowest soil resistivity are the most suitable for installing ICCP (impressed current cathodic protection) or sacrificial anode systems. Water movement plays a significant role, where stagnant water requires a lower protective current. Conversely, turbulent water can flush the structure’s surface, necessitating a stronger mechanical depolarization effect.
- Electrical shielding
For closely spaced and structurally complex components under cathodic protection, electrical shielding can occur. Current coming from the remote cathodic protection power source can easily be absorbed by the outermost components, with only a small amount reaching the inner components. This creates an electrical shielding effect. In such cases, the number and configuration of cathodes should be designed to be roughly equidistant from different parts of the protected structure, ensuring a more uniform distribution of current.
In conclusion, cathodic protection is an effective method of mitigating corrosion by altering the electrical potential of metal structures. Proper consideration of factors such as required current, environmental conditions, and electrical shielding is crucial for the successful implementation of a cathodic protection system.
④ Economic Factors
When implementing cathodic protection, it is important to consider the economic feasibility of the system. If cathodic protection is a cost-effective solution to corrosion issues, then the chosen system should have the lowest cost. This includes considering the design and installation costs, power source costs, and system maintenance costs.
⑤ Protection Life
During the design phase, it is crucial to determine the expected service life of the structure being protected. In practical applications of cathodic protection, the design life of the system should match the life expectancy of the structure. If the design life is too short, the effectiveness of the protection will be compromised, while an excessively long design life can increase costs and cause waste.
⑥ Influence of Stray Currents
Before designing a cathodic protection system, it is necessary to assess whether there are stray currents present in the area. Stray currents primarily originate from direct current sources such as electrified railways, mining machinery, and welding. Stray currents can cause rapid corrosion of the protected structure and often exacerbate corrosion compared to other environmental factors. Therefore, when designing cathodic protection, it is important to carefully select the location of anodes, aiming to avoid stray current interference.
⑦ Temperature
Temperature can affect the resistance of the electrolyte. In general, the resistivity of soil and water decreases as temperature increases. This principle explains why tropical seawater has lower resistivity compared to seawater in colder regions.
⑧ Sacrificial Anode Materials
Suitable materials for sacrificial anodes include aluminum, magnesium, and zinc. Anodes can be cast into various shapes to meet the requirements of cathodic protection designs.
⑨ Impressed Current Anodes
For impressed current cathodic protection systems, it is preferable to use anodes with a practical minimum corrosion rate when supplying output current. Scrap steel pipes, rods, and similar materials can be used as anodes for impressed current protection systems, although they consume more material. However, these materials are widely available. In general, cathodic protection is suitable for moderately corrosive media such as seawater, soil, and neutral salt solutions. In highly corrosive media, the energy consumption and protective material consumption are usually too high, making cathodic protection impractical.
Translation to English:
④ Economic Factors
When using cathodic protection, the economic viability of the system should be considered. If cathodic protection is an economically viable solution for addressing corrosion issues, the chosen system should be the most cost-effective, taking into account design and installation costs, power source costs, and system maintenance costs.
⑤ Protection Life
During the design phase, it is important to know the expected service life of the protected structure. In practical applications of cathodic protection, the design life of the system should match that of the structure being protected. A design life that is too short will result in ineffective protection, while an excessively long design life will increase costs and lead to unnecessary waste.
⑥ Influence of Stray Currents
Before designing a cathodic protection system, it is essential to determine if there are any stray currents in the area. Stray currents primarily come from direct current sources such as electric railways, mining machinery, and welding. Stray currents can accelerate corrosion of the protected structure and often cause more severe corrosion compared to other environmental factors. Therefore, when designing cathodic protection, the placement of anodes should be carefully selected to avoid stray currents as much as possible.
⑦ Temperature
Temperature can affect the electrical resistance of the environment. Typically, the resistivity of soil and water decreases as temperature increases. This principle explains why tropical seawater has lower resistivity compared to colder regions.
⑧ Sacrificial Anode Materials
Aluminum, magnesium, and zinc are suitable materials for sacrificial anodes. Anodes can be cast into various shapes to meet the requirements of cathodic protection design.
⑨ Impressed Current Anodes
For impressed current cathodic protection systems, it is preferable to use anodes that have a practical minimum corrosion rate when supplying output current. Scrap steel pipes, rods, and similar materials can be used as anodes for impressed current protection systems, although they consume more material. However, these materials are widely available. In general, cathodic protection is suitable for moderately corrosive media such as seawater, soil, and neutral salt solutions. In highly corrosive media, the energy and protective material consumption are usually too high, making cathodic protection impractical.
- Anodic Protection:
Anodic protection involves using the equipment itself as the anode and applying an external current. Generally, this would accelerate corrosion, and the corrosion current would increase with anode polarization. However, for metals that can passivate, a different situation occurs. As the potential rises with the current, reaching the passivation potential, the corrosion current rapidly decreases, even by several orders of magnitude. As the potential continues to rise, the current remains constant until it reaches the passive region. By utilizing this principle, the equipment to be protected is used as the anode, and a current is supplied to maintain the potential in the mid-range of passivation. This keeps the corrosion rate very low, and the applied current represents the corrosion rate of the equipment.
- Alloying:
By adding alloying elements that promote passivation to the base metal, materials with excellent corrosion resistance can be obtained. For example, adding more than 12% chromium to iron results in stainless steel. Adding nickel to chromium steel can expand the passivation range and improve mechanical performance. Similarly, adding 14% silicon to iron produces high-silicon iron with excellent acid resistance, and so on.
Additionally, adding trace amounts of cathodic noble metals with a low overpotential to some active metals can promote passivation. For example, stainless steel and titanium are active in certain concentrations and temperatures of sulfuric acid. When 0.1-0.15% palladium or platinum is added to the base metal, it forms numerous micro-cathodes on the alloy surface, promoting the operation of localized corrosion cells. This leads to a rapid increase in cathodic current, quickly reaching the passivation region, thereby enhancing the metal’s corrosion resistance.
- Surface Treatment:
Before being exposed to the operating environment, metals can be treated with passivating agents or film-forming agents to generate stable and dense passive films, significantly increasing corrosion resistance. Unlike the application of corrosion inhibitors, surface treatment does not require additional corrosion inhibitors to be added to the environment during usage. For example, aluminum undergoes anodization treatment, resulting in a more compact film than that formed in the atmosphere. Such films exhibit excellent corrosion resistance in mild corrosive environments. The bluing of steel surfaces also operates on this principle.
- Metal Coatings and Claddings:
A thin layer of a more corrosion-resistant metal can be applied to protect the underlying steel or iron. Electroplating is a commonly used method, typically involving 2-3 layers that are only a few tens of micrometers thick, inevitably containing micro-pores. The solution can permeate these micro-pores, leading to a galvanic cell between the coating and the underlying layer. If the coating is a precious metal with a higher potential than iron, it becomes the cathode and accelerates the corrosion of the underlying iron.
Therefore, these coatings are not suitable for highly corrosive environments. However, they can be used in atmospheric or water environments, where the slowly generated corrosion products can block the micro-pores, increase electrical resistance, and provide a certain service life. If a cheap metal is used as the coating, the polarity of the galvanic cell formed will be opposite to the scenario described above, protecting the steel as the cathode and providing a longer lifespan. In addition to electroplating, other methods such as hot-dip galvanizing, flame spraying, vapor deposition, and cladding with metal sheets are commonly used. The latter, due to the absence of micro-pores, offers stronger corrosion resistance and longer service life, albeit at a slightly higher cost.
- Coatings
Using organic coatings to protect metal structures in the atmosphere is the most common method of corrosion prevention. Coatings are applied to the metal surface and, once dry, form a porous film. While this film does not completely isolate the metal from the environment, it increases the diffusion resistance and solution resistance through the micropores, thereby reducing the corrosion current.
In mild environments such as the atmosphere and seawater, the corrosion of the metal beneath the micropores is slow, and the corrosion products can block the micropores, leading to a long service life. However, coatings are not suitable for strong corrosive solutions because the metal corrodes rapidly, accompanied by the generation of hydrogen gas, which can cause the film to rupture.
- Rubber Lining
Anti-corrosion lining refers to the use of uncured, pre-cured, or vulcanized rubber sheets or blocks to prevent corrosion of equipment. The technique involves creating a continuous isolating cover on the working surface of metals or other materials using lining rubber sheets, and it is known as rubber lining. The rubber materials used for lining can be categorized as soft rubber, hard rubber, and semi-hard rubber, generally manufactured using natural rubber (NR), chloroprene rubber (CR), styrene-butadiene rubber (SBR), etc., based on the operating conditions. The manufacturing process of lining includes surface treatment of the metal substrate, processing of lining rubber sheets, cutting, fitting, and vulcanization. Rubber lining sheets are widely used as corrosion and mechanical wear-resistant materials in chemical industry equipment lining, as well as lining for mining, metallurgical slurry pumps, flotation machines, grinding mills, cement mills in the building materials industry, and other equipment.
