Classification of stainless steels – acid-resistant


Stal nierdzewna to określenie wspólne dla całej rodziny gatunków stali jakościowych nie podatnych na rdzewienie, kwasoodpornych charakteryzujących się kilkakrotnie większą odpornością na korozję w porównaniu z innymi grupami stali.


High corrosion resistance is obtained by adding at least 10.5% chromium to the steel. Ferritic steel has the lowest chromium content. The highest austenitic steel. Already at the content of 10.5% (ferritic steel) there is a rapid change of the electrochemical potential to a more positive one. As a result, a film of oxides invisible to the naked eye is formed on the surface of the steel, preventing corrosion. The higher chromium content and further alloying elements like nickel and molybdenum (austenitic steel) further increase the resistance in question.


Another important feature of stainless / acid-resistant steel (apart from high corrosion resistance) is the ability to regenerate the protective surface film in places of damage, e.g. scratching, bending. Of course, there are many more these features and advantages of stainless steel, although it depends on the group to which a given grade belongs due to the chemical composition or production process.


We will cover the four most commonly purchased types of stainless steel. These are ferritic, martensitic, austenitic and austenitic-ferritic steels. They can be systematized taking into account the chemical composition and internal structure (microstructure). The table available in the STANDARDS section contains the chemical compositions and physical properties of the most important and most common standard stainless steel, rolled and forged steels on the market.

Stainless steel – types:

  1. Ferritic steel;
    Due to the relatively low chromium content, the corrosion resistance of ferritic steel is limited. It works slightly worse than other species in unfavorable weather conditions or in water media. Hence the often used term for ferritic steel as a steel with “increased corrosion resistance”. A particular advantage of ferritic steel, which is not found in corrosion-resistant austenitic steel, is its greater resistance to chloride-induced intracrystalline stress corrosion.
  2. Martensitic steel;
    Austenitic steel cooled in the production process is hardened and acquires a martensitic structure. Martensitic steel is characterized by much greater hardness and abrasion resistance than ferritic steel and austenitic steel. Additionally, in the improved condition, they obtain high strength values.
  3. Austenitic steel;
    The most important property of austenitic steel is high corrosion resistance. This resistance increases with the amount of alloying elements – such as chromium and molybdenum. Unlike martensitic steel, austenitic steel is not hardened in the production process. It has elongation values almost twice as high as ferritic steel.
  4. Austenitic-ferritic steel;
    Often referred to as duplex steel. It combines the advantages of ferritic steel and austenitic steel. It has been gaining in importance recently. Due to its property profile, austenitic-ferritic steel is used in a number of applications in chemical equipment, environmental protection and marine technology.

In terms of chemical composition, stainless steels are divided into chrome, chrome-nickel and chrome-nickel steels with further alloying elements. Individual groups are further divided into grades, differing in carbon content and the type and content of alloying additives such as nickel (Ni) or molybdenum (Mo).

Changing the basic division of stainless-acid-resistant steels

Until recently, in Poland, there was a division of corrosion-resistant, stainless and acid-resistant steels. Stainless steels include ferritic and martensitic chrome steels, and acid-resistant steels – austenitic chrome-nickel steels.

Another systematics of corrosion-resistant steel, i.e. types of stainless and acid-resistant steel, was adopted in the European standard PN-EN 10088, which is already in force in Poland. According to this standard, the name “stainless steel” includes all types of corrosion-resistant steel, i.e. stainless steel and acid-resistant steels. These differences in nomenclature should be taken into account in order to avoid any confusion between suppliers and users of corrosion resistant steels.