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Surface treatment of valves（1）

一、the importance of valve surface treatment

Valve as a piping system to control the flow of fluids in the key components, its surface treatment has a vital role. Good surface treatment can improve the valve's corrosion resistance, abrasion resistance, abrasion resistance, enhance the sealing of the valve, extend the service life of the valve, but also to enhance the appearance of the valve quality to meet the needs of different working environments and users.

  二、The common valve surface treatment methods

1. Plating

Definition and principle: electroplating is the use of electrolysis in the valve surface deposition of a layer of metal or alloy method. By using the valve as the cathode, into the electrolyte containing metal ions to be plated, after the power supply is turned on, the metal ions on the surface of the valve to get the electronic reduction into metal atoms, and gradually form a layer of uniform, dense coating.

Common plating types and characteristics

Nickel plating: nickel plating has good corrosion resistance, wear resistance and gloss. It can protect the valve substrate in a variety of media, especially in some non-oxidising acids and alkali solutions show good stability. At the same time, the nickel plating layer can improve the hardness of the valve surface and enhance its ability to resist abrasion. Nickel plating process can be divided into dark nickel, bright nickel and chemical nickel plating according to nickel layer characteristics. Chemical nickel-plated phosphorus alloy layer porosity is less than 0.1 /cm², in the field of marine engineering, the use of chemical nickel-plated phosphorus alloy treatment of the valve, in the salt spray test after 1000 hours of testing without significant corrosion, while ordinary galvanised valves can only withstand 240 hours. For example, in some chemical pipelines, the medium has a certain corrosive, nickel-plated valves can effectively resist corrosion, to ensure the normal operation of the valve.

Chromium plating: Chromium plating has high hardness, good abrasion resistance, excellent corrosion resistance and decorative properties. Chrome plating can form a hard protective film on the surface of the valve, effectively preventing the valve from wear and corrosion. Moreover, chrome-plated valves have a bright and beautiful appearance, and are often used in some occasions with high requirements for appearance, such as building water supply and drainage systems, high-grade sanitary equipment. Take hard chrome plating as an example, its process includes degreasing → pickling → pre-plating copper → chrome plating → post-treatment, and each link has a decisive impact on the quality of the plating layer. In the aerospace field, after hard chrome plating on titanium alloy valves, the surface hardness is increased from HV300 to HV1000, and the abrasion resistance is increased by 5 times, which effectively solves the wear problem of valves in aviation fuel system.

2. Chemical plating

Definition and principle: Chemical plating is a plating method in the absence of applied current, the use of reductants to reduce the metal ions in solution and deposited on the valve surface. It relies on a specific chemical reaction, so that the metal ions in the valve surface under the catalytic effect of the reduction of metal monomers, the formation of plating.

Features and Advantages: The uniformity of the plating layer of chemical plating is good, no matter how complex the shape of the valve, it can obtain a uniform thickness of the plating layer. Moreover, chemical plating does not require an external power source and is relatively simple to operate, making it suitable for valves made of various materials.

Chemical conversion film treatment can generate a dense oxide film on the valve surface. Anodic oxidation of aluminium valves, the film thickness of up to 25μm, corrosion resistance increased by 10 times in the building water supply and drainage system, the anodic oxidation of aluminium valves, chloride corrosion resistance significantly enhanced.

Chromate passivation treatment is limited due to environmental issues, but in some military fields, the generation of rainbow-coloured passivation film is still excellent corrosion resistance (neutral salt spray test ≥ 500 hours) to occupy a place.

In addition, the plating formed by chemical plating has good corrosion resistance and wear resistance, some chemical plating alloy layer also has special physicochemical properties, such as chemical plating nickel-phosphorus alloy layer with high hardness and good corrosion resistance, in some cases can replace the electroplating hard chrome.

The following section describes three other methods of valve surface treatment: thermal spraying, painting and nitriding.