• The forms of the metal valve corrosion and their prevention methods

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The forms of the metal valve corrosion and their prevention methods

  • Jun 21, 2019
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Corrosion is the destruction and deterioration of materials under various environmental conditions. Metallic corrosion is mainly caused by chemical corrosion and electrochemical corrosion. Non-metallic material corrosion is generally caused by direct chemical and physical damage.
 
Corrosion is one of the important factors that cause valve damage. Therefore, in the use of valves, corrosion protection is the first consideration.

The following are the forms of the valve corrosion.
Metal valve corrosion has two forms, namely uniform corrosion and local corrosion. The rate of uniform corrosion can be evaluated by the annual average corrosion rate. Metal materials, graphite, glass, ceramics and concrete, according to the corrosion rate, are divided into 4 grades: excellent with corrosion rate less than 0.05mm/a; good with corrosion rate of 0.05~0.5mm/a; still available with corrosion rate of 0.5~1.5mm/a; not applicable with corrosion rate greater than 1.5mm/a. Valve parts such as sealing surfaces, valve stems, diaphragms, and small springs are generally made of first-grade materials. Secondary or tertiary materials are suitable for valve bodies and bonnets. For valves used in high pressure, highly toxic, flammable, explosive, and radioactive media, materials with low corrosivity are used.

1. Uniform corrosion
Uniform corrosion is performed on the entire surface of the metal. For example, metallic materials like stainless steel, aluminum, titanium can create a protective film in the oxidizing environment; the metal under the film is uniformly corroded. There is also a phenomenon in which the metal surface is corroded and peeled off. This kind of corrosion is the most dangerous.
 
2. Local corrosion
Localized corrosion occurs at local locations of the metal. Its forms are pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, vibration corrosion, etc.
 
Pitting corrosion usually occurs on the metal with the passivation film or the protective film. Due to defects on the metal surface, the active ions of the passivation film can be destroyed in the solution, and the passivation film is partially destroyed. Then, the solution intrudes into the metal to become an etched hole. It is one of the most metallic destructive corrosion forms of hidden danger.
 
Crevice corrosion occurs under welding, rivets, gaskets or deposits and is a special form of pitting corrosion. The prevention method is to eliminate the gap.
 
Intergranular corrosion penetrates into the interior of the metal from the surface along the grain boundary, causing the grain boundary to be reticularly corroded. Intergranular corrosion is caused by the deposition of impurities in the grain boundary, or mainly due to improper heat treatment and cold working. Austenitic stainless steel welds on both sides are prone to chromium-depleted areas and are subject to corrosion. Intergranular corrosion of austenitic stainless steels is a common and most dangerous form of corrosion. The method for preventing intergranular corrosion of austenitic stainless steel valve parts is as follows: "solution hardening" treatment, that is, heat valve parts to about 1100 °C for water quenching and then use austenitic stainless steel containing titanium and niobium and containing less than 0.03% carbon to reduce the generation of chromium carbide.
 
The delamination corrosion occurs in the layered structure. The corrosion first develops vertically inward, and then the material which is parallel to the surface is corroded. Under the expansion force of the corrosive material, the surface peels off layerwise.
 
Stress corrosion occurs in the crack generated by the simultaneous action of corrosion and tensile stress. Methods for preventing stress corrosion; eliminating or reducing welding, stress generated in cold working by heat treatment, improving unreasonable valve structure, avoiding stress concentration, adopting electrochemical protection and spraying anti-corrosion coating, adding corrosion inhibitor, applying compressive stress and other measures.
 
Abrasion corrosion is a form of corrosion caused by the alternating action of fluids on metal wear and corrosion. It is a common type of valve corrosion that occurs at the sealing surface. Prevention method: use corrosion-resistant and wear-resistant materials, improve structural design, and adopt cathodic protection.
 
Frictional corrosion or vibration corrosion is the damage of the contact surface due to vibration and slip when the two parts are in contact with each other and are loaded at the same time. Frictional corrosion occurs at the bolted joint, between the stem and the closure, between the ball bearing and the shaft. Prevention method: use lubricating grease to reduce friction and surface phosphating; use hard alloys and cold working treatment to increase surface hardness.