How to Combat Valve Corrosion?

Metal valves are often the most vulnerable components prone to corrosion failure in engineering equipment. Typically, the sealing surface, valve stem, diaphragm, small spring, and other valve parts of metal valves are made of primary materials, while the valve body and bonnet are made of secondary or tertiary materials. For valves used in high-pressure, highly toxic, flammable, explosive, or radioactive media, materials with minimal corrosiveness are selected.

Valve Corrosion

Metal valves, operating under complex conditions such as atmospheric or solution exposure, not only experience uniform corrosion on the metal surface at all times, but also are highly prone to localized corrosion such as pitting, crevice corrosion, intergranular corrosion, exfoliation corrosion, stress corrosion, fatigue corrosion, selective corrosion, abrasive wear corrosion, cavitation corrosion, fretting corrosion, hydrogen embrittlement, and other forms of localized corrosion.

Corrosion Prevention Measures for Valves

1. Selection of Corrosion-Resistant Materials Based on Corrosive Media

In practical production, media corrosion is highly complex. Even if the same valve materials are used for a single medium, the concentration, temperature, and pressure of the medium affect material corrosion differently. For every 10°C increase in medium temperature, corrosion rates typically increase by 1 to 3 times. The concentration of the medium significantly impacts valve material corrosion; for example, lead has minimal corrosion in low-concentration sulfuric acid, but corrosion sharply increases when the concentration exceeds 96%. Conversely, carbon steel experiences severe corrosion around 50% sulfuric acid concentration, but corrosion decreases sharply when the concentration exceeds 6%. Stainless steel exhibits strong corrosion resistance to dilute nitric acid, but corrosion worsens in concentrations exceeding 95%. Therefore, the selection of valve materials should consider specific factors and analyze various corrosion-influencing factors to select materials based on relevant corrosion protection manuals.

2. Use of Non-Metallic Materials

Non-metallic materials offer excellent corrosion resistance. As long as the operating temperature and pressure of the valve meet the requirements of non-metallic materials, corrosion issues can be resolved while also saving valuable metals. Valve bodies, bonnets, liners, sealing surfaces, and other components are commonly made of non-metallic materials. Gaskets and packing are predominantly made of non-metallic materials. Materials like polytetrafluoroethylene (PTFE), chlorinated polyether, natural rubber, chloroprene rubber, and butyl rubber are used for valve liners. Valve bodies and bonnets are typically made of cast iron or carbon steel to ensure strength while protecting against corrosion. Nylon, PTFE, and other plastics, as well as natural and synthetic rubbers, are increasingly used for various sealing surfaces and seals in valves. These non-metallic sealing materials not only exhibit excellent corrosion resistance but also offer superior sealing performance, making them particularly suitable for use with particulate media. However, their strength and heat resistance are relatively low, limiting their application range. The emergence of flexible graphite has allowed non-metals to enter the high-temperature field, addressing long-standing leakage issues with packing and gaskets, while also serving as an excellent high-temperature lubricant.

3. Metal Surface Treatments
Common valve connection screws are often treated with galvanizing, chromium plating, or oxidation (bluing) to enhance resistance to atmospheric and medium corrosion. Other fasteners are treated similarly or undergo surface treatments such as phosphating as needed.

For sealing surfaces and small closing parts, nitriding, boronizing, and other surface treatments are commonly employed to enhance corrosion resistance and wear resistance. Valve discs made of 38CrMoAlA undergo nitriding with a layer thickness of ≥0.4mm.

Valve stems are widely treated with nitriding, boronizing, chromium plating, nickel plating, and other surface treatments to improve corrosion resistance and wear resistance. Different surface treatments should be suitable for different stem materials and operating environments. Valves in contact with asbestos packing and water vapor in the atmosphere can benefit from hard chrome plating or gas nitriding (avoid ion nitriding for stainless steel). Valves exposed to hydrogen sulfide atmospheres exhibit good protective performance with high-phosphorus nickel plating. 38CrMoAlA treated with ion and gas nitriding also demonstrates excellent corrosion resistance and comprehensive performance, making it ideal for valve stems in various applications.

Small-bore valve bodies and handwheels are also commonly treated with chromium plating to enhance corrosion resistance and improve aesthetics.

4. Thermal Spraying

Thermal spraying is a category of processes for preparing coatings and has become one of the new technologies for surface protection. It involves heating and melting metal or non-metallic materials with high-energy density heat sources (such as gas combustion flames, arcs, plasma arcs, resistance heating, gas detonation, etc.) and then spraying them onto pretreated substrate surfaces in a mist form to form sprayed coatings. Alternatively, the sprayed coatings can be remelted on the substrate surface to form thermally sprayed layers. Most metals and their alloys, metal oxide ceramics, metal-ceramic composites, and hard metallic compounds can be coated using one or more thermal spraying methods to form coatings on metallic or non-metallic substrates.

Thermal spraying enhances surface properties such as corrosion resistance, wear resistance, and high temperature resistance, prolonging service life. Special functional coatings applied by thermal spraying possess unique properties such as thermal insulation, insulation (or heteroelectricity), grindable sealing, self-lubrication, heat radiation, electromagnetic shielding, etc. Thermal spraying can also be used for component repair.

5. Brush Coating

Coating is the most widely used corrosion prevention method, and it is an indispensable corrosion protection material and identification mark for valve products. Coatings also belong to non-metallic materials, typically formulated from synthetic resins, rubber emulsions, vegetable oils, solvents, etc., covering the metal surface to isolate it from the medium and atmosphere, achieving the purpose of corrosion prevention. Coatings are mainly used in environments with relatively weak corrosion such as water, saltwater, seawater, and the atmosphere. Corrosion-resistant paint is commonly used to brush the inner cavity of valves to prevent corrosion from water, air, and other media. Different colors are added to the paint to indicate the materials used. Valve coating is generally done every six months to a year.

6. Addition of corrosion inhibitors

Corrosion inhibitors control the mechanism of corrosion by promoting polarization of the battery. Corrosion inhibitors are mainly used in media and filler. Adding corrosion inhibitors to the medium can slow down the corrosion of equipment and valves. For example, stainless steel containing chromium and nickel exhibits severe corrosion in oxygen-free sulfuric acid within a wide solubility range, but adding a small amount of copper sulfate or nitric acid and other oxidants can transform stainless steel into a passive state, forming a protective film on the surface to prevent medium erosion. In hydrochloric acid, adding a small amount of oxidant can reduce corrosion of titanium. Water is commonly used as a medium for valve pressure testing, which can easily cause corrosion of valves. Adding a small amount of sodium nitrite to water can prevent corrosion of valves. Asbestos filler contains chloride, which causes significant corrosion to valve stems. Washing the asbestos with distilled water can reduce the chloride content, but this method is difficult to implement and cannot be widely promoted. Esters are suitable for special needs.

To protect the valve stem and prevent corrosion of asbestos filler, corrosion inhibitors and sacrificial metals are applied to the asbestos. Corrosion inhibitors such as sodium nitrite and sodium chromate can form a passive film on the surface of the valve stem to enhance its corrosion resistance. Solvents can gradually dissolve corrosion inhibitors and also provide lubrication. Adding zinc powder as a sacrificial metal to asbestos can reduce the contact between chloride in asbestos and the valve stem metal, thereby achieving the purpose of corrosion prevention. If corrosion inhibitors such as red lead and calcium plumbate are added to coatings, brushing them on the surface of valves can prevent atmospheric corrosion.

7. Electrochemical protection

Electrochemical protection includes anodic protection and cathodic protection. For example, using zinc to protect iron, where zinc acts as a sacrificial metal. In practical production, anodic protection is used less frequently, while cathodic protection is more commonly applied. This cathodic protection method is an economical, simple, and effective method, especially for large and important valves. Adding zinc to asbestos filler to protect the valve stem also belongs to cathodic protection.

8. Controlling corrosion environment

The environment refers to both a broad and narrow sense. Broadly, it refers to the environment around the valve installation location and the circulating medium inside it, while narrowly, it refers to the conditions around the valve installation location. Most environments cannot be controlled, and production processes cannot be arbitrarily changed. Only methods that do not cause damage to products or processes can be used to control the environment, such as deoxygenation of boiler water and adjusting the pH value of alkali in refining processes. From this perspective, the addition of corrosion inhibitors, electrochemical protection, etc., also belong to controlling the corrosion environment.

The atmosphere is filled with dust, water vapor, smoke, especially in production environments, such as smoke halogen, toxic gases, and fine particles emitted by equipment, all of which can cause varying degrees of corrosion to valves. Operators should follow the procedures to regularly clean, blow, and lubricate valves, which are effective measures to control environmental corrosion. Installing protective covers on valve stems, setting ground wells for globe valves, and spraying paint on valve surfaces are all methods to prevent corrosion of valves by corrosive substances. An increase in environmental temperature and air pollution, especially in enclosed environments of equipment and valves, can accelerate corrosion. Whenever possible, open-plan factories or ventilation and cooling measures should be adopted to mitigate environmental corrosion.

9. Improving processing technology and valve structure

Corrosion protection of valves begins with design considerations. A valve product with a reasonable structure design and correct processing methods undoubtedly has a good effect on slowing down valve corrosion.