Acid-resistant steel
What is acid-resistant steel?
Acid-resistant steel is an austenitic stainless steel alloyed with molybdenum to improve corrosion resistance, especially in acidic and chloride-containing environments. It belongs to the group of stainless steels but stands out from conventional stainless steel thanks to its superior resistance to pitting and crevice corrosion. In machine shops, acid-resistant steel is used in applications where parts are exposed to chemical stress, moisture, or marine climates.
Typically, acid-resistant steel contains about 16–18% chromium, 10–14% nickel, and at least about 2% molybdenum. Chromium forms a passive oxide layer on the steel surface that protects the base material from corrosion, and molybdenum strengthens this protection, particularly against chloride ions.
Structure and corrosion resistance of acid-resistant steel
Acid-resistant steel is structurally an austenitic steel, which means its microstructure remains tough and ductile even at low temperatures. Compared with ferritic or martensitic steels, the austenitic structure improves weldability and formability.
Corrosion resistance is based primarily on the passive layer formed by chromium. Molybdenum increases resistance to pitting corrosion, which is particularly common in seawater and in chemical process industries. The performance of acid-resistant steel is often evaluated using the PREN value (Pitting Resistance Equivalent Number), which takes into account the influence of chromium, molybdenum, and nitrogen. The higher the PREN value, the better the resistance in chloride-rich environments.
The most common acid-resistant steel grades in machine shops
In the engineering workshop industry, the most commonly used acid-resistant steel is AISI 316, also known by its EN designation 1.4401. Its low-carbon version AISI 316L (1.4404) is particularly popular in welded structures, because the low carbon content reduces sensitization risk and intergranular corrosion.
In more demanding chemical environments, higher-alloyed grades such as 904L steel are used, with higher nickel and molybdenum contents. For extremely demanding applications, duplex and Super Duplex steels can be selected, combining high strength with excellent corrosion resistance.
Machinability of acid-resistant steel
Acid-resistant steel is a tough and work-hardening material, which places specific demands on machining. During cutting, a work-hardened surface easily forms if the depth of cut and feed rate are too low. For this reason, the machinist must choose the correct cutting parameters, sufficient depth of cut, and suitable cutting tools, such as carbide inserts.
The cutting speed is generally lower than with carbon steels, and an effective cutting fluid helps control the thermal load and extend tool life. When machining acid-resistant steel, the sharpness of the cutting tool, proper cooling, and rigid clamping are essential to avoid vibration and surface quality deterioration.
Applications of acid-resistant steel in the engineering workshop industry
In machine shops, acid-resistant steel is used especially where long-term corrosion resistance is required without surface coating. Typical applications include process industry pipelines, tanks, shafts, flanges, valve bodies, and various machined components.
In marine environments, acid-resistant steel withstands corrosion caused by seawater better than conventional stainless steel. In food and pharmaceutical industry equipment, the material is well suited for hygienic structures because its surface is dense and easy to clean.
Acid-resistant steel and the difference to stainless steel
All acid-resistant steels are stainless steels, but not all stainless steels are acid-resistant. The key difference lies in the molybdenum alloying, which improves performance in aggressive environments. Conventional stainless steel, such as AISI 304, is suitable for many general-purpose applications, but in chloride-containing conditions, acid-resistant steel provides a significantly longer service life.
Summary
Acid-resistant steel is a molybdenum-alloyed austenitic stainless steel that provides excellent corrosion resistance, particularly in acidic and chloride-rich environments. In machine shops and machining workshops, it is a key material for components used in the chemical industry, marine environments, and the food industry. With properly selected cutting parameters and suitable cutting tools, acid-resistant steel can be machined to high quality and cost-effectively for demanding applications.
