Corrosion is the destruction and deterioration of materials under the action of various environments. The corrosion of metals is mainly caused by chemical corrosion and electrochemical corrosion and the corrosion of non-metallic materials is generally caused by direct chemical and physical actions. 

1. Morphology of Valve Corrosion

    There are two morphologies of valve corrosion, namely, uniform corrosion and local corrosion. The speed of uniform corrosion can be evaluated by the annual average corrosion rate. Metallic materials, graphite, glass, cerics, and concrete are divided into 4 grades according to the corrosion rate: those with a corrosion rate less than 0.05mm/a are excellent; those with corrosion rate between 0.05~0.5mm/a are good; those with a corrosion rate between 0.5~1.5mm/a are usable; those with a corrosion rate greater than 1.5mm/a are not applicable. Generally, primary materials are used for valve parts such as sealing surfaces, valve stems diaphragms, and small springs, while secondary or tertiary materials are suitable for valve bodies and valve covers. For valves used in high pressure, highly toxic, flable, explosive, or radioactive media, materials with very low corrosion rates should be selected.

    1. Uniform Corrosion

    Uniform corrosion occurs the entire surface of the metal. For example, a layer of protective film is produced on stainless steel, aluminum, titanium, etc., in an oxidizing environment. The metal under the film is uniformly corroded. There is also a phenomenon where the metal surface is corroded and peeled off, which is the most dangerous type of corrosion.

2. Local Corrosion

    Local corrosion occurs at local positions on the metal. Its morphologies include pitting corrosion, crevice corrosion, intergranular corrosion,amination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, vibration corrosion, hydrogen corrosion, etc.

    Pitting corrosion usually occurs metals with a passive film or protective film. It is caused by defects on the metal surface and active ions in the solution that can destroy the passive film, leading to local destruction of passive film, extending into the metal interior, and becoming pits. It is one of the most destructive and hidden corrosion morphologies.

    Crevice corrosion occurs in environments as under welds, rivets, gaskets, or deposits. It is a special form of pitting corrosion. The prevention method is to eliminate crevices.

    Intergranular corrosion penetrates the metal from the surface along the grain boundary, causing the grain boundary to corrode in a net-like pattern. The main causes of interular corrosion, in addition to the precipitation of impurities at the grain boundary, are improper heat treatment and cold working. Austenitic stainless steel welds are prone to corrosion in theium-deficient zones on both sides of the weld. Intergranular corrosion of austenitic stainless steel is a common and most dangerous corrosion morphology. Methods to preventgranular corrosion of austenitic stainless steel valves include: solid solution quenching treatment, i.e., heating to about 1100℃ and water quen, selecting austenitic stainless steel containing titanium and niobium with a carbon content of less than 0.03%, and reducing the formation of chromium carb.

    Delamination corrosion occurs in layered structures, where corrosion first develops vertically inward and then corrodes substances parallel to the surface. Under the expansive force of theded material, the surface peels off in layers.

    Stress corrosion occurs when a fracture is produced under the simultaneous action of corrosion and tensile stress. Methods to stress corrosion include: eliminating or reducing welding and cold working stresses through heat treatment, improving unreasonable valve structures to avoid stress concentration, and adopting electrochemical protection, brushing anticorros paint, adding inhibitors, applying compressive stress, etc.

    Corrosion fatigue occurs at locations subjected to the combined action of alternating stress and corrosion, causing the to break. Heat treatment can be used to eliminate or reduce stress, surface shot peening treatment, and electroplating with zinc, chromium, nickel, etc., but should be taken that the coating should not have tensile stress and hydrogen diffusion.