904L is a low-carbon, high-nickel, molybdenum austenitic stainless acid-resistant steel. It has good activation-passivation transformation ability, excellent corrosion resistance, good corrosion resistance in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid, phosphoric acid, and good corrosion resistance in neutral chloride-containing media Excellent pitting corrosion resistance, as well as good crevice corrosion resistance and stress corrosion resistance. It is suitable for various concentrations of sulfuric acid below 70 °C, and has good corrosion resistance in acetic acid and mixed acid of formic acid and acetic acid at any concentration and temperature under normal pressure. Metallographic structure of 904L stainless steel: 904L is a completely austenitic structure. Compared with austenitic stainless steel with high molybdenum content, 904L is not sensitive to the precipitation of ferrite and α phase.
Corrosion resistance of 904L stainless steel
Because the carbon content of 904L is very low (0.020% maximum), there will be no carbide precipitation in the case of general heat treatment and welding. This eliminates the risk of intergranular corrosion that normally occurs after heat treatment and welding. Due to the high chromium-nickel-molybdenum content and the addition of copper, 904L can be passivated even in reducing environments such as sulfuric and formic acids. The high nickel content also results in a lower corrosion rate in the active state. In pure sulfuric acid in the concentration range of 0~98%, the operating temperature of 904L can be as high as 40 degrees Celsius. In pure phosphoric acid in the concentration range of 0~85%, its corrosion resistance is very good. In industrial phosphoric acid produced by the wet process, impurities have a strong effect on the corrosion resistance. Of all the various phosphoric acids, 904L is more corrosion resistant than ordinary stainless steel. In strong oxidizing nitric acid, 904L has lower corrosion resistance than highly alloyed steel grades that do not contain molybdenum. In hydrochloric acid, the use of 904L is limited to lower concentrations of 1-2%. in this concentration range. The corrosion resistance of 904L is better than that of conventional stainless steel. 904L steel has high resistance to pitting corrosion. Its resistance to crevice corrosion in chloride solutions is also very good. The high nickel content of 904L reduces the rate of corrosion in pits and crevices. Ordinary austenitic stainless steels may be susceptible to stress corrosion in a chloride-rich environment at temperatures above 60°C, and this sensitization can be reduced by increasing the nickel content of the stainless steel. Due to the high nickel content, 904L is highly resistant to stress corrosion cracking in chloride solutions, concentrated hydroxide solutions and hydrogen sulfide rich environments.