Corrosion Types and Protection of Mechanical Seals
Metal ring corrosion
Firstly, the surface undergoes uniform corrosion. If the surface of the metal ring comes into contact with corrosive media, and the metal itself is not corrosion-resistant, surface corrosion will occur, including leakage, early wear, damage, and noise. There are two forms of uniform corrosion on metal surfaces: film forming and non film forming. Metal corrosion without film is very dangerous, and the corrosion process proceeds at a certain speed, which is mainly caused by material selection errors. The passivation film of film-forming corrosion usually has protective properties, but the passivation film on the surface of materials used for metal sealing rings, such as stainless steel, cobalt, chromium alloy, etc., is damaged during end face friction, making it difficult to generate a new film under anaerobic conditions, exacerbating galvanic corrosion.
Secondly, stress corrosion cracking. Under the simultaneous action of corrosion and tensile stress, metals first produce cracks in weak areas, and then develop into deep cracks, known as stress corrosion cracking. When hard alloy and cast iron, tungsten carbide, titanium carbide, and other sealing rings are used for welding, stress corrosion cracking is prone to occur. Sealing ring cracks are generally radial divergent and can be one or more. These cracks communicate the entire sealing end face, accelerating wear on the end face and increasing leakage.
Corrosion of non-metallic rings
Firstly, graphite ring corrosion. There are three reasons for the corrosion of impermeable graphite rings impregnated with resin: firstly, when the end face is overheated and the temperature is greater than 180oC, the impregnated resin needs to break away from the graphite ring, reducing the wear resistance of the ring; Secondly, if the resin used for impregnation is not selected properly, chemical changes will occur in the medium, which will also reduce wear resistance; The third reason is that the resin impregnation depth is not enough, and when the impregnation layer is removed, the wear resistance decreases. Therefore, it is necessary to establish a sealed cooling system by selecting corrosion-resistant impregnating resin and using high-pressure impregnation to increase the impregnation depth.
Secondly, the oxidation of graphite rings. In an oxidizing medium, when the end face undergoes dry friction or poor cooling, a temperature of 350~40o ℃ can cause the graphite ring to react with oxygen, producing CO and gas, which can make the end face rough or even crack. Non metallic rings can also rupture under the simultaneous action of chemical media and stress.
Thirdly, the corrosion of polytetrafluoroethylene (F4) sealing rings. F4 fillers such as glass fiber, graphite powder, metal powder, etc. are used to improve their temperature resistance and wear resistance. The corrosion of the filled F4 ring mainly refers to the selective corrosion, dissolution, or deterioration of the filling. For example, in hydrofluoric acid, glass fiber molecules undergo thermal corrosion, so the filling material should depend on the specific situation.
Firstly, the surface undergoes uniform corrosion. If the surface of the metal ring comes into contact with corrosive media, and the metal itself is not corrosion-resistant, surface corrosion will occur, including leakage, early wear, damage, and noise. There are two forms of uniform corrosion on metal surfaces: film forming and non film forming. Metal corrosion without film is very dangerous, and the corrosion process proceeds at a certain speed, which is mainly caused by material selection errors. The passivation film of film-forming corrosion usually has protective properties, but the passivation film on the surface of materials used for metal sealing rings, such as stainless steel, cobalt, chromium alloy, etc., is damaged during end face friction, making it difficult to generate a new film under anaerobic conditions, exacerbating galvanic corrosion.
Secondly, stress corrosion cracking. Under the simultaneous action of corrosion and tensile stress, metals first produce cracks in weak areas, and then develop into deep cracks, known as stress corrosion cracking. When hard alloy and cast iron, tungsten carbide, titanium carbide, and other sealing rings are used for welding, stress corrosion cracking is prone to occur. Sealing ring cracks are generally radial divergent and can be one or more. These cracks communicate the entire sealing end face, accelerating wear on the end face and increasing leakage.
Corrosion of non-metallic rings
Firstly, graphite ring corrosion. There are three reasons for the corrosion of impermeable graphite rings impregnated with resin: firstly, when the end face is overheated and the temperature is greater than 180oC, the impregnated resin needs to break away from the graphite ring, reducing the wear resistance of the ring; Secondly, if the resin used for impregnation is not selected properly, chemical changes will occur in the medium, which will also reduce wear resistance; The third reason is that the resin impregnation depth is not enough, and when the impregnation layer is removed, the wear resistance decreases. Therefore, it is necessary to establish a sealed cooling system by selecting corrosion-resistant impregnating resin and using high-pressure impregnation to increase the impregnation depth.
Secondly, the oxidation of graphite rings. In an oxidizing medium, when the end face undergoes dry friction or poor cooling, a temperature of 350~40o ℃ can cause the graphite ring to react with oxygen, producing CO and gas, which can make the end face rough or even crack. Non metallic rings can also rupture under the simultaneous action of chemical media and stress.
Thirdly, the corrosion of polytetrafluoroethylene (F4) sealing rings. F4 fillers such as glass fiber, graphite powder, metal powder, etc. are used to improve their temperature resistance and wear resistance. The corrosion of the filled F4 ring mainly refers to the selective corrosion, dissolution, or deterioration of the filling. For example, in hydrofluoric acid, glass fiber molecules undergo thermal corrosion, so the filling material should depend on the specific situation.
