What are the precautions for using austenitic stainless steel?

A.) Testing is not required for austenitic stainless steel in the following situations: solutions such as common alcohols, aldehydes, ketones, ethers, benzene, phenols, alkanes, gasoline, and chemically pure acetic acid, as well as gaseous media, as well as in some decorative applications. B.) Intergranular corrosion testing is necessary in the following situations: common industrial acetic acid (except chemically pure acetic acid), formic acid, chromic acid, lactic acid, nitric acid (except dilute nitric acid at room temperature), oxalic acid, phosphoric acid, hydrochloric acid, sulfuric acid, sulfurous acid, carbamates, urea, and other reactive media. Tests should be carried out in accordance with GB4334.1–4334.9 <Methods for Testing Intergranular Corrosion of Stainless Steel>. From this, we can deduce that the main measures to prevent intergranular corrosion include: (1) solution treatment; (2) reducing the carbon content in the steel; (3) adding elements that stabilize carbides. If the medium has a tendency to cause intergranular corrosion in austenitic stainless steel, solution treatment must be carried out after welding the equipment. This is because the effects of welding heating temperatures and holding times (slow cooling within the range of 450–850°C) often result in chromium carbides precipitating preferentially at the grain boundaries, leading to chromium deficiency in those areas and thus causing intergranular corrosion in the welds and their vicinity. For equipment for which solution treatment is not possible, ultra-low carbon stainless steel or elements that stabilize carbides should be used instead. After the surface of austenitic stainless steel is passivated by oxidizing media, the passive film formed on its surface can be easily damaged in the presence of halide ions such as Cl-, Br-, and F-, resulting in a loss of its original corrosion resistance and decorative properties. Among them, Cl- has the strongest destructive effect on the passivation film of stainless steel. Moreover, Cl- is commonly present in water; this is one of the reasons why, during the use of austenitic stainless steel equipment, the concentration of Cl- is required to be no more than 25 μg/g. It is also required that austenitic stainless steel not be pickled using acids with high chloride ion contents such as hydrochloric acid. After chemical cleaning of austenitic stainless steel or after hydrostatic testing of pressure vessels, if the residual water cannot be removed promptly, it is also necessary to use water with a Cl- concentration of no more than 25 μg/g to rinse the equipment.

1. Be aware of polythionic acid stress corrosion cracking: Austenitic stainless steels, when exposed to high temperatures, H2, and H2S over prolonged periods, develop iron sulfide. During reactor shutdowns and maintenance, this iron sulfide reacts with oxygen in water and moist air to form polythionic acid; the presence of certain stresses can lead to SCC. 2. Hydrogen embrittlement cracks: Due to hydrogen diffusion, weld areas are prone to cracking when cooled rapidly after shutdown. 3. Delamination of the cladding layer: Rapid cooling can cause the base material and the cladding layer to separate from each other

I agree with the view expressed on the 3rd floor. For high-pressure hydrogen systems, attention must be paid to polysulfuric acid stress corrosion cracking (PSCC). When austenitic stainless steel containers and equipment need to be opened during shutdowns and maintenance, inert gas protection or neutralizing cleaning must be employed to prevent the formation of sulfurous acid and polysulfuric acid; or any existing amounts of sulfurous acid and polysulfuric acid should be neutralized through cleaning, in order to effectively avoid polysulfuric acid stress corrosion

It is very important to prevent chloride corrosion; I once saw pictures on Hai Chuan showing how liquid containing chlorides corroded storage tanks, and it was a shocking sight

Personally, I think the views from the second and third floors are the most comprehensive. For austenitic stainless steels, corrosion caused by chloride ions and polythionic acids is generally what needs to be prevented. As for the factors that generate chloride ions, there are many of them, and further analysis is required. And polydithionic acid is especially severe; you won’t know what it’s like until you experience it.

We have strict control over the chloride ion content here. Replace the stopped operation units, strictly control the reaction temperature to prevent temperature spikes, especially in new plants where the catalysts have high activity and are prone to such spikes. When starting up or shutting down, the rate of temperature increase and decrease must be controlled.

1. Chloride corrosion: Halides present in the aqueous phase (chlorides are usually the most significant cause) and tensile stress can lead to stress corrosion cracking in austenitic stainless steels. This cracking occurs mainly within the crystal and depends on time, temperature, and chloride concentration. Therefore, measures should be taken to minimize the chloride content in the process materials that come into contact with austenitic stainless steel equipment. Under normal shutdown conditions, as long as chlorides are not allowed to accumulate or concentrate in the heat equipment, and measures are taken to keep chloride levels low during any flushing, cleaning, or neutralization processes, chloride-induced corrosion and cracking will not be a problem. 2. Poly-sulfuric acid corrosion: As long as the equipment is in use during production, even if the sulfur content in the raw materials is low, it should be assumed that a layer of ferrous sulfide rust has formed on all parts made of austenitic stainless steel. Even though in many cases these rust layers are thin, they pose a potential risk to the steel beneath them. Under the action of water and oxygen, this iron sulfide rust forms weak sulfuric acid, commonly known as polythionic acid. Polyoxysulfuric acid can erode austenitic stainless steel, leading to intergranular corrosion and cracking. These stainless steels are prone to damage due to this type of corrosion, especially in areas with residual stress and in regions where intergranular carbon deposits are present, such as the heat-affected zone near welds. Therefore, special procedures should be employed to protect austenitic stainless steel in such corrosive environments.

Pay attention to controlling the temperature, pressure, and speed during heating and cooling to prevent hydrogen embrittlement and hydrogen bubbling