Corrosion Protection Measures for Metal Valves

In industrial production, metal valves are indispensable and play a key role in controlling fluid flow and ensuring smooth operation of production processes. However, due to prolonged exposure to various complex media environments, metal valves face serious corrosion challenges. Corrosion not only shortens the service life of metal valves but may also lead to severe safety incidents. Therefore, it is essential to implement effective corrosion protection measures.

Corrosion of metal valves by media is a complex process influenced by multiple factors. First, the concentration of the medium has a significant effect on corrosion. For example, lead exhibits relatively mild corrosion in low-concentration sulfuric acid; however, when the sulfuric acid concentration exceeds 96%, the corrosion rate of lead rises sharply. Carbon steel shows the opposite behavior in sulfuric acid: it corrodes more severely at around 50% concentration but the corrosion decreases drastically when the concentration exceeds 96%. Aluminum also behaves differently in nitric acid of various concentrations. In highly concentrated nitric acid (above 80%), aluminum experiences strong corrosion, but in medium and low concentrations, the corrosion is even more severe. Stainless steel offers strong corrosion resistance in dilute nitric acid but corrodes more in nitric acid concentrations above 95%. This demonstrates that different metals exhibit varied corrosion characteristics in media of different concentrations, and conclusions cannot be generalized.

In addition to concentration, the temperature of the medium is another critical factor affecting corrosion speed. Generally, for every 10°C increase in temperature, the corrosion rate increases by 1 to 3 times. Elevated temperature accelerates chemical reactions, intensifying the corrosion between metal and medium and thus worsening valve corrosion. Furthermore, the pressure of the medium also influences corrosion. Under different pressure conditions, the chemical properties and physical state of the medium may change, thereby affecting its corrosive effect on metal valves.

Due to their excellent corrosion resistance, non-metallic materials are widely used in valve corrosion protection. Compared to metallic materials, non-metallic materials often offer superior anti-corrosion performance when exposed to various corrosive media. Non-metallic materials can be used in valve bodies, bonnets, linings, sealing surfaces, and other components. Examples include plastics like PTFE (polytetrafluoroethylene) and chlorinated polyether, as well as rubber materials such as natural rubber, neoprene, and nitrile rubber—all of which can serve as valve lining materials. These linings not only effectively isolate the medium from the valve's metal body, preventing corrosion, but also retain structural strength since the valve body and bonnet are still typically made from cast iron or carbon steel.

Besides linings, gaskets and packings are common non-metallic components used in valves. These components are critical to the sealing performance of valves. Gaskets and packings made from non-metallic materials offer good corrosion resistance and excellent sealing. For instance, the advent of flexible graphite has enabled the use of non-metallic materials in high-temperature environments. Flexible graphite possesses good high-temperature resistance and sealing properties, effectively solving long-standing leakage issues in valve packings and gaskets. Additionally, it serves as an excellent high-temperature lubricant.

However, non-metallic materials are not without limitations. Their lower strength and heat resistance restrict their range of applications. Thus, when selecting non-metallic materials, one must consider the specific working conditions, corrosion resistance, strength, and thermal performance, to ensure the safety and reliability of the valve in service.

To enhance the corrosion resistance of metal valves, surface treatment is an effective method. By applying various surface treatments to metal surfaces, a protective film can be formed to isolate the medium from the metal, thus achieving corrosion protection.

Valve connecting bolts are commonly treated with zinc plating, chrome plating, or oxidation (bluing) to improve resistance to atmospheric and media corrosion. Zinc plating is a common electroplating method where a zinc layer acts as a sacrificial anode, corroding in place of the iron to protect it. Chrome plating forms a dense chrome layer with high hardness and excellent corrosion resistance, effectively preventing metal corrosion. Oxidation (bluing) treatment forms an oxide film on the metal surface through chemical reactions, which can also protect the metal from contact with external media.

In addition to bolts, other fasteners can also be treated with similar surface processes. Depending on the usage environment and requirements, phosphating and other surface treatments can further enhance corrosion resistance.

Sealing surfaces and smaller closure components often undergo surface processes like nitriding and boriding to enhance corrosion and wear resistance. For instance, valve discs made of 38CrMoAlA can achieve a nitrided layer thickness of over 0.4 mm after treatment. Nitriding forms a nitride layer on the metal surface with high hardness and corrosion resistance, significantly improving surface wear and corrosion performance. Similarly, boriding produces a boride layer on the metal surface that offers comparable protection.

The valve stem is a critical component, and its corrosion resistance is vital for valve performance. Nitriding, boriding, chrome plating, and nickel plating are commonly used for valve stem surface treatment. Different treatments suit different materials and working environments. For instance, valve stems exposed to atmospheric or steam media with asbestos packing can be treated with hard chrome plating or gas nitriding. For hydrogen sulfide environments, high-phosphorus nickel plating offers excellent protection. Valve stems made from 38CrMoAlA, after ion or gas nitriding, exhibit excellent corrosion resistance and overall performance, making the material a common choice for valve stems.

Thermal spraying is a coating preparation method and has become a new technology for surface protection. It uses high-energy thermal sources such as gas combustion flame, arc, or plasma arc to heat and melt metal or non-metal materials. These materials are then atomized and sprayed onto a pre-treated substrate surface, forming a spray coating. In some processes, the substrate is also heated so that the coating remelts and forms a welded layer.

Thermal spraying allows the use of a wide range of materials, including most metals and alloys, metal oxide ceramics, metal-ceramic composites, and hard metal compounds, to create functional coatings on metal or non-metal substrates.

This method significantly improves metal surface resistance to corrosion, wear, and high temperatures, thereby extending valve service life. Moreover, thermal spraying can create coatings with special functions such as thermal insulation, electrical insulation (or conductivity), sealing, self-lubrication, thermal radiation, and electromagnetic shielding. It can also be used to repair damaged components and restore original performance.

Coatings are a widely used corrosion protection method and are indispensable for valve products, serving both protective and identification purposes. These coatings are usually made from synthetic resins, rubber solutions, vegetable oils, solvents, and other ingredients. Once applied to the metal surface, they isolate it from the medium and atmosphere to prevent corrosion.

Coatings are primarily used in environments with water, brine, seawater, and mild atmospheric corrosion. The inner cavity of valves is often coated with protective paint to prevent corrosion from water, air, and other media. Additionally, for easy identification and distinction between different valves, various colors are added to the paint for visual marking. The typical cycle for spraying and brushing valve coatings is generally once every six months to one year.

In addition to the physical protection and surface treatment methods mentioned above, chemical methods also play an important role in valve corrosion protection. Chemical methods can directly intervene in the corrosion reaction process, thereby effectively delaying or even preventing corrosion.

Corrosion inhibitors are chemical substances that control corrosion, mainly by promoting the polarization reaction of the corrosion cell. Corrosion inhibitors are mainly used in the medium and packing. Adding corrosion inhibitors to the medium can slow down the corrosion rate of equipment and valves. For example, chromium-nickel stainless steel in oxygen-free sulfuric acid exists in an active state over a wide concentration range, causing severe corrosion. However, adding small amounts of oxidizers like copper sulfate or nitric acid can convert the stainless steel to a passive state, forming a protective film on the surface that prevents corrosion by the medium. Similarly, adding small amounts of oxidizers in hydrochloric acid can reduce corrosion of titanium.

Water is usually used as the pressure test medium for valves, but water easily causes valve corrosion. To prevent this, small amounts of sodium nitrite can be added to the water to inhibit corrosion. Asbestos packing contains chlorides, which cause significant corrosion to the valve stem. Although washing with distilled water can reduce chloride content, this method faces many practical difficulties and is hard to implement widely. To protect the valve stem from asbestos packing corrosion, 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 corrosion resistance. Solvents can slowly dissolve the inhibitors and also act as lubricants. Adding zinc powder as a sacrificial metal in asbestos causes the zinc to react first with chlorides in asbestos, reducing chloride contact with the valve stem metal, achieving corrosion protection. Additionally, adding corrosion inhibitors like red lead and lead calcium compounds into coatings sprayed onto valve surfaces can also prevent atmospheric corrosion of valves.

Electrochemical protection is a method of preventing metal corrosion based on electrochemical principles, mainly including anodic protection and cathodic protection. In production practice, anodic protection is less commonly used, while cathodic protection is more widely applied. For example, zinc protects iron by acting as a sacrificial anode and corroding first, thus protecting iron from corrosion. For large and important valves, cathodic protection is an economical, simple, and effective method. Adding zinc to asbestos packing to protect valve stems is an application of cathodic protection.

The environment has a significant influence on corrosion of metal valves. Broadly speaking, the environment includes the surrounding conditions where the valve is installed and the medium flowing inside it; narrowly, it refers only to the conditions around the valve installation site. In most cases, the environment is uncontrollable and production processes cannot be arbitrarily changed. However, if it does not harm products or processes, environmental control methods can be used to slow corrosion. For example, deoxygenation treatment of boiler water, pH adjustment by alkalinity in refining processes, etc., can partially control corrosive environments.

The atmosphere contains dust, water vapor, smoke, and especially in production environments, toxic gases and micro-particles from chimneys and equipment, all of which can cause varying degrees of valve corrosion. Therefore, operators should follow operation procedures to regularly clean, blow off, and lubricate valves—these are effective methods to control environmental corrosion. In addition, installing valve stem protective covers, setting valve pits, and painting valves can prevent corrosive environmental factors from attacking valves. Rising environmental temperatures and air pollution, especially for equipment and valves in enclosed spaces, accelerate corrosion. Thus, open plant designs or ventilation and cooling measures should be adopted to reduce environmental corrosion.

Metal valves face complex corrosive environments in industrial production, with corrosion seriously affecting valve service life and safety. To effectively prevent valve corrosion, a comprehensive application of multiple anti-corrosion measures is necessary. First, understand the corrosion characteristics and influencing factors of the medium in depth, and select valve materials and anti-corrosion methods reasonably based on the concentration, temperature, pressure, and other characteristics of different media. Second, fully utilize the corrosion resistance advantages of non-metallic materials by reasonably applying them to valve bodies, covers, linings, sealing surfaces, etc., while addressing their lower strength and heat resistance. Third, adopt advanced surface treatment technologies such as electroplating, nitriding, boriding, thermal spraying, etc., to improve the corrosion resistance of metal valve surfaces. Furthermore, rationally apply chemical methods such as adding corrosion inhibitors and electrochemical protection to control corrosion chemically. Finally, control environmental factors by regularly cleaning and blowing valves, and implementing ventilation and cooling measures to reduce environmental corrosion effects.