Corrosion Prevention Strategies for Metal Valves

In the industrial sector, metal valves are indispensable key components in many types of machinery and equipment. However, metal valves are highly susceptible to corrosion in various complex working environments, leading to failure. This article will delve into the corrosion issues of metal valves and provide a range of effective corrosion prevention strategies to help extend the service life of metal valves and ensure the stable operation of equipment.

Metal valves play a crucial role in engineering equipment, but they are also among the components most prone to failure due to corrosion. The sealing surfaces, valve stems, diaphragms, and small springs of valves are typically made from first-grade materials, while the valve bodies and bonnets are made from second or third-grade materials. For valves used to handle high-pressure, highly toxic, flammable, explosive, or radioactive media, extremely corrosion-resistant materials must be selected.

In practical applications, metal valves are not only subject to uniform corrosion but are also prone to various types of localized corrosion, such as pitting, crevice corrosion, intergranular corrosion, exfoliation corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, fretting corrosion, and hydrogen embrittlement. These corrosion problems severely affect the performance and lifespan of valves.

The corrosion of metal valves by the medium is an extremely complex process. Even in the same medium, different concentrations, temperatures, and pressures can lead to vastly different corrosion outcomes. For example, for every 10°C increase in medium temperature, the corrosion rate may increase by 1 to 3 times. The effect of concentration on corrosion is also very significant. Take sulfuric acid as an example: the corrosion of lead in low-concentration sulfuric acid is minimal, but when the concentration exceeds 96%, the corrosion increases sharply. Conversely, carbon steel experiences the most severe corrosion at around 50% sulfuric acid concentration, but when the concentration rises above 6%, the corrosion rate drops sharply. Aluminum is highly corrosive in concentrated nitric acid above 80%, but it is even more severely corroded in medium and low concentrations of nitric acid. Stainless steel has strong corrosion resistance to dilute nitric acid, but the corrosion increases in concentrated nitric acid above 95%.

Non-metallic materials are widely used in valve manufacturing due to their excellent corrosion resistance. As long as the temperature and pressure conditions of the valve's use meet the requirements of non-metallic materials, not only can corrosion problems be effectively solved, but also precious metals can be saved. The valve body, bonnet, lining, and sealing surfaces of valves are often made from non-metallic materials, while gaskets and packing are almost entirely made from non-metallic materials.

Plastics such as polytetrafluoroethylene (PTFE) and chlorinated polyether, as well as rubbers like natural rubber, neoprene, and nitrile rubber, are commonly used as lining materials for valves. These materials not only have strong corrosion resistance but also ensure the sealing performance of the valves. The design of pinch valves fully utilizes the corrosion resistance and excellent deformation properties of rubber. Nowadays, more and more valves are beginning to use plastics like nylon and PTFE, as well as natural and synthetic rubbers, to make various sealing surfaces and sealing rings. These non-metallic materials not only have good corrosion resistance but also excellent sealing performance, making them particularly suitable for handling media containing particles.

However, the strength and heat resistance of non-metallic materials are relatively low, which limits their application range. The emergence of flexible graphite has provided the possibility for the application of non-metallic materials in high-temperature fields. It not only solves the long-standing problem of leakage of packing and gaskets but also serves as an excellent high-temperature lubricant.

To enhance the corrosion resistance of metal valves, surface treatment technologies are widely applied. Valve connection bolts are usually treated with galvanizing, chrome plating, or oxidation (bluing) to improve their resistance to atmospheric and medium corrosion. In addition to the aforementioned methods, other fasteners may also undergo surface treatment processes such as phosphating, depending on specific circumstances.

Sealing surfaces and small-sized closing parts often undergo surface processes like nitriding and boriding to enhance their corrosion and wear resistance. For example, the nitriding layer thickness of valve discs made from 38CrMoAlA is typically not less than 0.4 millimeters. The corrosion protection of valve stems widely employs surface treatment processes such as nitriding, boriding, chrome plating, and nickel plating to improve their corrosion and wear resistance. Different surface treatment processes are suitable for different valve stem materials and working environments. For example, valve stems in contact with air, steam, and asbestos packing can be treated with hard chrome plating or gas nitriding (ion nitriding is not recommended for stainless steel); valve stems in a hydrogen sulfide atmosphere can be protected with electroplated high-phosphorus nickel coatings; 38CrMoAlA material, after ion and gas nitriding treatment, exhibits excellent corrosion resistance and overall performance and is commonly used for valve stems.

Small-sized valve bodies and handwheels are also often chrome-plated to enhance their corrosion resistance and for decorative purposes.

Thermal spraying is a coating preparation process and has become one of the new technologies for material surface protection. It uses high-energy-density heat sources (such as gas combustion flames, electric arcs, plasma arcs, electric heating, and gas detonation) to heat and melt metal or non-metal materials, which are then sprayed onto a pre-treated substrate surface in atomized form to form a sprayed coating. Thermal spraying technology can not only improve the surface's corrosion, wear, and high-temperature resistance but also extend the service life of components. Moreover, thermal spraying can prepare functional coatings with special properties such as thermal insulation, electrical insulation (or electrical separation), grinding sealing, self-lubrication, thermal radiation, and electromagnetic shielding, and it can also be used for component repair.

Coatings are one of the most widely used corrosion prevention methods and are indispensable for corrosion protection and identification marks on valve products. Coatings are usually formulated from synthetic resins, rubber emulsions, vegetable oils, solvents, etc., and are applied to metal surfaces to isolate them from media and the atmosphere, thereby achieving corrosion prevention. Coatings are mainly used in environments with weak corrosivity such as water, brine, seawater, and air. The interior of valves is often painted with anti-corrosion paint to prevent corrosion from water, air, and other media. To facilitate identification, different colors are added to the coatings. Valves are typically repainted with coatings every six months to one year.

Corrosion inhibitors control corrosion by promoting the polarization of the galvanic cell. They are mainly used in media and packing. Adding corrosion inhibitors to the medium can slow down the corrosion of equipment and valves. For example, chromium-nickel stainless steel experiences severe corrosion in oxygen-free sulfuric acid, but the addition of a small amount of copper sulfate or nitric acid as an oxidizing agent can transform the stainless steel into a passive state, forming a protective film on the surface to prevent medium corrosion. Adding a small amount of oxidizing agent to hydrochloric acid can reduce corrosion of titanium. Water is commonly used for valve testing, but it can easily cause valve corrosion. Adding a small amount of sodium nitrite to water can effectively prevent water corrosion of valves. Asbestos packing contains chlorides, which are highly corrosive to valve stems. Although the method of washing with distilled water can reduce the chloride content, it is difficult to implement widely in practice. To protect the valve stem from corrosion by asbestos packing, corrosion inhibitors and sacrificial metals can be added to the asbestos packing. Corrosion inhibitors such as sodium nitrite and sodium chromate can form a passive film on the valve stem surface, enhancing its corrosion resistance; solvents can slowly dissolve the corrosion inhibitors and also serve as lubricants. Adding zinc powder as a sacrificial metal to asbestos can reduce the contact between chlorides and the valve stem metal, thereby achieving corrosion prevention. If red lead or calcium plumbate are added as corrosion inhibitors to the coatings and sprayed on the valve surface, they can prevent atmospheric corrosion.

Electrochemical protection is an effective corrosion prevention method, including both anodic and cathodic protection. In production practice, anodic protection is less commonly used, while cathodic protection is widely applied. For large and important valves, cathodic protection is an economical, simple, and effective method. For example, when zinc protects iron, zinc acts as the sacrificial metal and corrodes, thereby protecting the iron from corrosion. Adding zinc to asbestos packing to protect the valve stem also falls under cathodic protection.

The corrosion problem of metal valves is a complex and severe challenge. However, by reasonably selecting materials, applying advanced surface treatment technologies, using appropriate coatings and corrosion inhibitors, and implementing effective electrochemical protection measures, we can significantly enhance the corrosion resistance of valves and extend their service life. In practical applications, the most suitable corrosion prevention strategy should be chosen based on the specific medium conditions, working environment, and valve type. Only in this way can we ensure the stable and reliable operation of metal valves under various complex working conditions, providing a solid guarantee for industrial production.

Through the detailed analysis of this article, it is believed that readers have gained a deeper understanding of the corrosion prevention strategies for metal valves. It is hoped that this practical information will help engineers and technicians better address the corrosion issues of metal valves in their actual work, thereby improving the reliability and operational efficiency of equipment.