Many people don't understand the reasons for the corrosion of flanged gate valves. In fact, these valves are prone to corrosion if they have not undergone anti-corrosion measures. Therefore, we need to understand the reasons for flanged gate valves and then make improvements.
In order to predict the effects of high temperature oxidation, we need to examine these data: metal composition, atmosphere composition, temperature and exposure time.
However, it is well known that most light metals (those that are lighter than their oxide) form a non-protective oxide layer that gets thicker and flakes off over time. There are also other forms of high temperature corrosion including sulfidation, carburization, etc.
The physical force of fracture from abrasion, which dissolves metals through protective corrosion. The effect depends mainly on force and speed. Excessive vibration or bending of metal can have similar results.
Cavitation is a common form of corrosion in pumps, stress corrosion cracking, high tensile stress and corrosive atmospheres can cause corrosion of flanged gate valves.
Under the action of static load, the tensile stress of the metal surface exceeds the yield point of the metal, and the corrosion action concentrates on the area where the stress acts, and the result shows a localized corrosion.
Localized corrosion or pitting occurs when the protective film is destroyed or the corrosion product layer decomposes. The rupture of the film forms the anode and the unruptured film or corrosion product acts as the cathode, effectively establishing a closed circuit.
In the presence of chloride ions, some flanged gate valves are prone to pitting corrosion. When corrosion occurs, the metal surface or rough parts are due to these non-uniformities.
When two dissimilar metals are in contact and exposed to corrosive liquids and electrolytes to form a galvanic cell, the current increases the current causing the anode piece to corrode. Corrosion is usually localized near the contact points. Corrosion reduction can be achieved by electroplating dissimilar metals.
This happens in the crevice, which hinders the diffusion of oxygen, resulting in high and low oxygen areas, forming a difference in solution concentration.
In particular, there may be a narrow gap at the defect of the connector or welded joint. The valve gap is wide enough to allow the electrolyte solution to enter, so that the metal in the gap and the metal outside the gap form a short-circuit primary battery, and localized corrosion of strong corrosion occurs in the gap.
Intergranular corrosion occurs for a variety of reasons. The result is almost the same mechanical damage along the grain boundaries of the metal. If there is no proper heat treatment or contact sensitization, the intergranular corrosion of flanged gate valves of austenitic stainless steel at 427–816 °C is affected by many corrosive agents. This situation can be cleaned by pre-annealing and quenching at 1093 °C, using low carbon stainless steel ( c-0.03max) or stabilized niobium or titanium.