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What are the types of valve corrosion

1. The form of valve corrosion
There are two forms of corrosion of metal valves, namely uniform corrosion and localized corrosion.
The rate of uniform corrosion can be evaluated by the annual average corrosion rate.
Metal materials, graphite, glass, ceramics and concrete, are divided into 4 grades according to the corrosion rate:
The corrosion rate is less than 0.05mm/a is excellent;
The corrosion rate of 0.05~0.5mm/a is good;
The corrosion rate of 0.5~1.5mm/a can still be used;
If the corrosion rate is greater than 1.5mm/a, it is not applicable.
The sealing surface, valve stem, diaphragm, small spring and other valve parts of the valve are generally made of primary materials, and the valve body and valve cover are suitable for secondary or tertiary materials, which are used for high pressure, highly toxic, flammable, explosive, radioactive For the valve of the medium, the material with little corrosiveness is selected.

1. Uniform corrosion
Uniform corrosion is performed on the entire surface of the metal. Such as stainless steel, aluminum, titanium, etc., a protective film produced in an oxidizing environment, and the metal state under the film is uniformly corroded. There is also a phenomenon that the metal surface is corroded and peeled off, and this corrosion is the most dangerous.

2. Local corrosion
Localized corrosion occurs at the local location of the metal, and its forms include pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction corrosion, hydrogen eclipse etc.

Pitting corrosion usually occurs on the metal of the passivation film or protective film. It is due to the defects on the metal surface. The active ions of the passivation film can be destroyed in the solution, so that the passivation film is partially destroyed and protrudes into the metal to become corrosion holes. It is one of the most destructive and hidden corrosion forms of metals.

Crevice corrosion occurs in environments such as welding, riveting, gaskets or under deposits, and it is a special form of pitting corrosion. The prevention method is to eliminate the gap.

Intergranular corrosion is to penetrate deep into the metal along the grain boundary from the surface, so that the grain boundary is corroded in the form of a network. In addition to the precipitation of impurities at the grain boundary, the intergranular corrosion is mainly caused by improper heat treatment and cold working. Austenitic stainless steel is prone to corrosion due to chromium-poor zones on both sides of the weld. Intergranular corrosion of austenitic stainless steels is the most common and most dangerous form of corrosion. The methods to prevent intergranular corrosion of austenitic stainless steel valve parts include: “solution quenching” treatment, that is, heating to about 1100 ° C for water quenching, and selecting austenitic stainless steel containing titanium and niobium and carbon content below 0.03% , reduce the production of chromium carbide.

Delamination corrosion occurs in a layered structure. The corrosion first develops vertically and inwardly, and then corrodes the substances parallel to the surface. Under the expansion force of the corrosion, the surface is exfoliated in layers.

Stress corrosion occurs when corrosion and tensile stress act simultaneously to produce cracks. Methods to prevent stress corrosion; eliminate or reduce the stress generated in welding and cold working through heat treatment, improve the unreasonable valve structure, avoid stress concentration, use electrochemical protection, and spray anti-corrosion paint. Add corrosion inhibitor, apply compressive stress and other measures.

Corrosion fatigue occurs where the combined action of alternating stress corrosion cracks the metal. Heat treatment can be carried out to eliminate or reduce stress, surface shot peening and electro-galvanizing, chromium, nickel, etc., but it should be noted that the coating should not have tensile stress and hydrogen diffusion.

Selective corrosion occurs in materials with different compositions and impurities. In a certain environment, some elements are etched and leached, and the remaining uncorroded elements are sponge-like. Common ones include dezincification of brass, dealumination of copper alloys, and graphitization of cast iron.

Wear corrosion is a form of corrosion caused by the alternating action of fluid on metal wear and corrosion. It is a common corrosion of valves. This corrosion occurs mostly on the sealing surface. Prevention methods: choose corrosion-resistant and wear-resistant materials, improve structural design, and use cathodic protection.

Cavitation corrosion, also known as cavitation and cavitation, is a special form of wear corrosion. It is the shock wave generated by the bubbles generated in the fluid, and the pressure can be as high as 400 atmospheres when it bursts, which destroys the metal protective film and even tears the metal particles. Then it is etched to form a film, and this process is repeated continuously, causing the metal to corrode. The method of preventing cavitation corrosion can be selected from cavitation-resistant corrosion-resistant materials, processing surfaces with high smoothness, elastic protective layers and cathodic protection.

Frictional corrosion is the failure of two parts in contact with each other under load at the same time, and the contact surface is damaged due to vibration and sliding. Frictional corrosion occurs at the bolt connection, the connection between the valve stem and the closing part, between the ball bearing and the shaft, etc. It can be protected by applying lubricating grease, reducing friction, surface phosphating, selecting cemented carbide, and improving surface hardness by spraying tiles or cold working.


Post time: Dec-12-2022