Characteristics and applications of different types of stainless steel
Stainless steel is an alloy steel with high chromium and nickel content. It is generally categorized based on its corrosion resistance: stainless steel resists atmospheric corrosion, while acid-resistant steel withstands corrosion from acids and other highly corrosive media. These types are commonly referred to collectively as stainless and acid-resistant steel, or simply stainless steel.

Stainless steel pipe fittings
Chromium 13 Stainless Steel (Martensitic, Ferritic)
Chromium 13 stainless steel contains 12-14% chromium, which is the minimum amount required for stainless steel (Cr > 12%). This type of steel can be hardened through heat treatment, resulting in a magnetic martensitic structure, hence the name martensitic stainless steel. Low carbon variants are known as ferritic stainless steel. Its corrosion resistance is similar to other stainless steels, capable of resisting corrosion from the atmosphere, water, nitric acid, alkalis, salts, organic acids, organic compounds, and other oxidizing environments. However, it is not resistant to non-oxidizing acids like sulfuric acid and hydrochloric acid. In solutions containing halide ions, it may suffer from pitting and stress corrosion cracking. Due to its lack of nickel and lower chromium content, its overall corrosion resistance is lower than that of chromium 17 ferritic and chromium 18 nickel 9 austenitic stainless steel. With high strength and hardness but lower toughness, it is commonly used in applications requiring both corrosion resistance and mechanical strength, such as valve components and ball bearings. It is generally not used for chemical equipment like tanks and pipes.
Chromium 17 Stainless Steel (Ferritic)
Chromium 17 stainless steel contains 17-27% chromium, primarily exhibiting a magnetic ferritic grain structure. It cannot be hardened by heat treatment but can be hardened through cold working. Its corrosion resistance is similar to other stainless steels, with excellent atmospheric resistance and good resistance to nitric acid and other oxidizing environments, as well as alkalis, salts, water, organic acids, and organic compounds. However, it does not resist non-oxidizing acids like sulfuric acid and hydrochloric acid. Its corrosion resistance is better than martensitic stainless steel but inferior to austenitic stainless steel. It performs better than austenitic stainless steel in resisting stress corrosion cracking. This steel is mainly used in applications requiring atmospheric corrosion resistance, such as automotive and architectural components, household utensils, and equipment in the nitric acid industry. However, its welding and processing performance is far inferior to chromium 18 nickel 9 steel, limiting its use in chemical applications. Variants of this steel include titanium-containing, non-titanium, and low-carbon types, all with similar corrosion resistance, but low-carbon titanium-containing steel has strong resistance to intergranular corrosion.
Chromium 18 Nickel 9 Stainless Steel (Austenitic)
Chromium 18 nickel 9 stainless steel, also known as austenitic stainless steel, contains over 11% chromium or chromium and nickel. In oxidizing environments like the atmosphere, water, and strong oxidizing acids, it easily passivates, forming a protective chromium oxide (Cr2O3) film that gives it its stainless properties. However, at higher temperatures or in less oxidizing conditions, it can become active and more prone to corrosion. This type of stainless steel has excellent corrosion resistance to organic acids, organic compounds, alkalis, neutral solutions, and various gases but suffers significant corrosion in non-oxidizing acids like sulfuric acid and hydrochloric acid. Stainless steel equipment often experiences localized corrosion, particularly pitting in halide-containing solutions and stress corrosion cracking in solutions with stress-sensitive ions like Cl- and OH-. The sensitized zone near welds is also prone to intergranular corrosion. Chromium-nickel steel surpasses chromium steel in both corrosion resistance and mechanical properties. The addition of nickel promotes an austenitic structure, enhancing toughness and expanding the passivation range. Chromium 18 nickel 9 steel is the most widely used stainless steel, non-magnetic, and hardenable through cold working. Variants with small amounts of titanium or niobium exhibit higher resistance to intergranular corrosion and are known as stabilized steels. Higher chromium and nickel contents increase resistance to heat and corrosion. Examples include chromium 25 nickel 20 steel, which resists high-temperature oxidation, and chromium-manganese-nitrogen steels as nickel-saving or nickel-free alternatives. Molybdenum-containing chromium-nickel-molybdenum steels (2-4% Mo) exhibit superior resistance to non-oxidizing acids and halides, reducing pitting tendencies.

ASTM A312 TP304 welded steel pipes
Chromium 18 Nickel 12 Molybdenum (Titanium) Stainless Steel
This type is similar to general chromium-nickel stainless steel but contains 2-4% molybdenum, enhancing its performance in many areas, especially resistance to non-oxidizing acids, hot organic acids, and chlorides. It also exhibits superior pitting resistance. Titanium or niobium-containing variants offer strong resistance to intergranular corrosion. Molybdenum-containing chromium-manganese-nitrogen (nickel) steels have similar corrosion resistance and can serve as substitutes. Low-carbon ferritic stainless steels like Chromium 26 Molybdenum 1 (E-Brite 26-1) offer similar resistance with added advantages of low pitting and stress corrosion cracking resistance, making them excellent nickel-saving alternatives.
Chromium 20 Nickel 22-30 Stainless Steel – Alloy 20
This alloy contains high levels of chromium and nickel, providing superior corrosion resistance compared to standard stainless steel. It is suitable for handling sulfuric acid, nitric acid, phosphoric acid, mixed acids, sulfurous acid, organic acids, alkalis, salt solutions, and hydrogen sulfide. It can also withstand high temperatures at certain concentrations. However, it is not resistant to concentrated or hot hydrochloric acid, wet fluorine, chlorine, bromine, iodine, and aqua regia. Due to its high cost, it is generally used in environments with severe corrosion and for high-temperature, high-velocity applications like valves and pumps. The corrosion resistance may vary slightly with different alloy compositions, so careful selection is necessary based on specific environmental conditions.