To correctly select valves for environments involving corrosive media, one must have a detailed understanding of corrosion; this ensures the selection of appropriate valve materials and structural designs. Metal materials experience various forms of corrosion failure in industrial production. Different materials exhibit different forms of corrosion depending on the loads applied and the specific media environments involved.

I. Classification of Corrosion

By corrosion reaction mechanism: Chemical corrosion and electrochemical corrosion.

By corrosion environment: Atmospheric corrosion, water and steam corrosion, soil corrosion, chemical media corrosion, etc.

By corrosion morphology: General corrosion and localized corrosion. Localized corrosion includes stress corrosion cracking, pitting, intergranular corrosion, galvanic corrosion, crevice corrosion, erosion-corrosion, selective corrosion, hydrogen embrittlement, cavitation corrosion, etc.

II. Introduction to Various Types of Corrosion

1. Intergranular corrosion: Corrosion occurring along grain boundaries, destroying the bonds between grains. This type of corrosion is highly dangerous; it can cause the metal to become brittle or lose strength—resulting in a loss of the characteristic metallic ring when struck—and can easily lead to sudden failures. Intergranular corrosion is the primary form of corrosion in austenitic stainless steel; it is caused by differences in composition and stress between the grain boundary regions and the grain interiors, leading to a significant drop in the electrode potential at the grain boundaries.

2. Stress corrosion: A phenomenon where metal fractures under the combined action of a corrosive medium and tensile stress (whether applied externally or present as internal stress). Stress corrosion frequently occurs in the presence of chlorides, alkaline hydroxides, or other water-soluble media and accounts for a significant proportion of equipment failures.

3. Pitting corrosion: A form of corrosive damage occurring in localized areas of the metal surface. Once formed, pits can rapidly deepen, eventually penetrating the metal entirely. Pitting corrosion is highly hazardous and is particularly detrimental to various types of vessels. Upon detection, pitted areas should be promptly ground smooth or coated with paint to prevent the corrosion from worsening.

4. Corrosion fatigue: Failure of a metal occurring under the combined action of a corrosive medium and alternating stress. It is characterized by the formation of corrosion pits and numerous cracks, which significantly reduce the steel's fatigue strength and lead to premature fracture. Unlike mechanical fatigue, corrosion fatigue lacks a distinct fatigue limit; the fatigue strength continues to decline as the number of cycles increases

5. Electrochemical corrosion: Corrosion resulting from the contact between a metallic material and an electrolyte solution, driven by electrode reactions. This process is a type of redox reaction, analogous to the reactions occurring at the anode of an electrolytic cell.

For more information on valves, please visit the WEITEN Valve official website at www.weitengroup.com.