What is the applicable scope of common surface treatment methods on control valve castings

The Importance of Surface Treatment for Control Valve Castings
Control valve castings are widely used in industries such as petrochemicals, power generation, metallurgy, pharmaceuticals, and marine engineering. They operate in diverse environments, potentially involving high pressure, high temperature, highly corrosive media, and fluids containing particles. Casting surfaces come into direct contact with the process fluid and, without effective protection, are susceptible to corrosion, wear, or fatigue damage. Surface treatment technologies can enhance the corrosion resistance, wear resistance, and service life of control valve castings, making them a crucial step in ensuring long-term, stable valve operation.

Applications of Nickel Plating
Nickel plating is a common electroplating or chemical plating process, categorized as electroplating and chemical plating. Electroplating is suitable for control valve castings with simple structures and regular shapes, forming a dense, uniform coating that improves corrosion resistance. Chemical nickel plating, on the other hand, does not rely on an applied current and can form a uniform coating on complex shapes and internal cavities. It is therefore suitable for valve bodies with complex internal passages or highly corrosive fluids. The nickel plating layer offers excellent wear resistance, extending service life in neutral or mildly corrosive environments such as those in the oil and gas and chemical industries.

Applications of Thermal Spraying Technology

Thermal spraying is a process in which a molten or semi-molten material is sprayed onto the surface of a casting using a high-velocity flame, arc, or plasma to form a coating. Common spray materials include tungsten carbide, aluminum oxide, and chromium oxide. This method is suitable for control valve castings requiring high hardness and wear resistance, such as in slurry pipelines, coal chemical plants, and high-temperature gas environments. Thermal spray coatings offer high bond strength and controllable thickness, significantly improving not only wear resistance but also corrosion resistance to a certain extent.

Applications of Coating Spraying

Coating spraying uses epoxy resin, polyurethane, or fluorocarbon coatings to form a protective film on the surface of a casting. Its primary function is to protect against atmospheric and media corrosion while also ensuring aesthetics and identification. Epoxy coatings are suitable for indoor and general industrial pipeline valves, offering excellent acid and alkali resistance; polyurethane coatings offer good weather resistance and are suitable for outdoor use; and fluorocarbon coatings offer excellent weather and corrosion resistance, making them suitable for coastal environments and chemical plants. This process is simple to apply and relatively low-cost, making it a common choice for mass-produced valves.

Applications of Nitriding and Carburizing

Nitriding and carburizing are surface heat treatment processes that form a strengthening layer on the surface of castings. Nitriding forms nitrides on the steel substrate, increasing surface hardness and wear resistance while retaining core toughness. This makes it suitable for use in high differential pressure and particulate-laden fluids. Carburizing, by increasing the carbon content in the surface layer, increases surface hardness and wear resistance. It is often used on control valve seats or valve plugs where high wear resistance is required. Both effectively improve surface properties, but their effect on corrosion resistance is limited, and they are generally used in conjunction with other anti-corrosion coatings.

Applications of Stainless Steel Passivation

Chemical passivation is commonly used on stainless steel control valve castings to form a stable, dense passivation film. This process is primarily used on austenitic and duplex stainless steel valve bodies, significantly improving corrosion resistance and preventing pitting and crevice corrosion. Passivation is commonly used in industries such as food, pharmaceuticals, water treatment, and marine engineering, where cleanliness and corrosion resistance are paramount. Passivated valve bodies perform stably in corrosive environments such as chloride ions, acids, and alkalis, significantly extending their service life.

Applications of Phosphating
Phosphating is a surface conversion coating commonly used on carbon steel control valve castings. By forming a phosphate film on the surface, it improves the corrosion resistance of the substrate and the adhesion of the coating. While the phosphate layer itself has limited protective properties, it can significantly improve the adhesion of subsequent paint or coatings. Phosphating is often used in the pretreatment of carbon steel control valves, particularly in processes requiring epoxy, polyurethane, and other coatings. Phosphating can enhance overall corrosion protection.

Applications of Electrogalvanizing
Electrogalvanizing is suitable for low-alloy steel or carbon steel control valve castings. The zinc layer provides cathodic protection through sacrificial anode action, slowing corrosion of the substrate. This method is low-cost and suitable for valve body components in atmospheric environments, but is not suitable for use in strong acidic or alkaline environments. Electrogalvanizing is commonly used in low-corrosive environments, such as building water supply systems and general industrial plant piping and valves.