Chrome-nickel steels
Resistant mainly to electrochemical corrosion in the environment of inorganic and organic acids, nitrogen compounds and solutions of salts and aggressive foodstuffs. They contain 18 to 25 [%]Cr and 8 to 20[%] Ni.
They have an austenitic structure. The most commonly used is 18/8 steel containing 18 C[%]r and 8[%] Ni or its modifications like 0H18N9, 1H18N9, 2H18N9. This steel is extremely resistant to corrosion, it is not affected by nitric acid, concentrated sulphuric, phosphoric acid and others. Molybdenum additives between 1,5 and 2.5 e.g. sulphuric and acetic acid additives are used to increase resistance to sulphuric [%]and acetic[%] acid. H18N10MT, H17N13M2T.
Copper additive in the amount of approx. 3 [%]reduces the tendency of these steels to stress corrosion. The addition of silicon in quanti[%]ties 2 to 3 improves resistance to hydrochloric acid and dilute sulphuric acid e.g. H18N9S. Nickel chromium steels are widely used in the construction of chemical apparatus on parts of the apparatus in the food industry. Unfortunately, the disadvantage of these steels is the tendency to intercrystalline corrosion, which occurs when they are heated to temperatures. 450 t[°C]o 750 [°C]. The cause of this type of corrosion is the secretion at the limits of grain chromium carbides causing the depletion of grain limits in chromium.
The propensity for intercrystalline corrosion can be removed by:
- reduction of carbon content to 0.02 – 0.03[%] e.g. 00H18N10,
- introduction into steel of highly carbide-forming elements such as titanium or niob e.g. 1H18N9T, 0H18N12Nb, or adequate heat treatment, which makes it difficult to create chromium carbides and this component is then concentrated in the solution.