Please give me some advice to see if this is corrosion.
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Dear Masters, the following 3 pictures are the coloring and flaw detection pictures of 2 reactors of our company. They are colored with a huge colorant. The material of the kettle is 321, the reaction temperature of the kettle is 45℃, and the pressure is 0.05MPa. Please take a look at the coloring results. Is corrosion occurring? What type of corrosion is it?
My personal feeling is that it is stress corrosion of austenitic stainless steel.； I don’t know if the analysis is correct, I hope the experts in the forum can give more answers. ； And what is the medium in the reactor?
In this working condition, there is no need to choose stainless steel!
From the photos, the colorant display shows that the color is darker along the weld, which can definitely be stress corrosion.
The low-stress brittle fracture phenomenon that occurs in metal under the combined action of stress and specific chemical media after a period of time is called stress corrosion cracking (Stress Corrosion Crack, abbreviated as SCC). The temperature at which stress corrosion occurs is generally between 50 and 300°C.
This post was last edited by Shutong on 2020-7-21 21:09 The conditions for stress corrosion cracking used to be believed that stress corrosion cracking could only be caused by tensile stress, but in recent years it has also been found that compressive stress can cause it in stainless steel. This tensile stress can be the stress caused by external loads, but it is mainly various residual stresses, such as welding residual stress, heat treatment residual stress and assembly stress. According to statistics, more than 80% of stress corrosion cracking accidents are caused by residual stress, while less than 20% are caused by working stress.
The media that cause stress corrosion are generally specific, that is to say, each material is only sensitive to certain media, and this medium may have no obvious effect on other materials. For example, brass is prone to stress corrosion in an ammonia atmosphere and stainless steel is in a corrosive medium with chloride ions. However, stainless steel is not sensitive to ammonia and brass is not sensitive to chloride ions.
Austenitic stainless steel will cause stress corrosion to acidic and neutral chloride solutions, seawater and marine atmosphere, hot NaCl, HS aqueous solutions, severely polluted industrial atmosphere, etc.
The crack expansion rate of stress corrosion is small, a bit like fatigue, and is gradual and slow. This subcritical expansion condition reaches a certain critical size, and when the remaining section cannot bear the external load, a sudden fracture occurs.
Measures to prevent stress corrosion: 1. Reasonably select metal materials 2. Reduce or eliminate residual tensile stress in parts 3. Improve chemical media 4. Use electrochemical protection
This post was last edited by Shutong on 2020-7-21 21:23. Our company has experienced many situations that are obviously stress corrosion cracking.: Methanol purification condenser tube plate weld cracks. The condenser tube plate and heat exchange tube weld cracks in the tower are all made of 304 stainless steel; the cracks are all at the welds of the tube plate and heat exchange tubes and the welding heat affected zone, and there are welding residual stresses; the shell side is full of circulating water, and the heat exchange temperature exceeds 60°C. Chloride ions are enriched in the gaps between the tube plates and heat exchange tubes. High concentrations of specific alloys, certain stresses, and specific corrosion environments have caused stress corrosion cracking.