Nickel is a pure chemical element of the periodic table. It is represented by the symbol Ni and it’s atomic number is 28. Nickel is a transition metal in the d-block with a high boiling point (14530C). It is ductile and exhibits magnetic properties at room temperature. First discovered in the 1700s, nickel is used as catalyst to various chemical reactions and one of the main alloying element in stainless steel. Nickel in Stainless Steel is a widely used constituent.
More than 66% of globally produced nickel is used to manufacture stainless steel alloy. Nickel in stainless steel improves formability, weldability and ductility. Nickel containing stainless steel is suitable for high-temperature applications. They provide very good corrosion resistance and remain ductile at low temperatures. In certain applications, nickel increases the corrosion resistance of stainless steel. Importance of nickel in stainless steel can be easily understood from the fact that nickel-containing stainless steel grades make up more than 75% of the global stainless steel production.
Role of Nickel in Stainless Steel
There are several benefits that come with addition of nickel in stainless steel.
Addition of Nickel increases the toughness of stainless steel. In general, chromium addition is steel decreases the toughness. So, that decrease in toughness is compensated by addition of Nickel. Ferromagnetism in steel is not desirable in certain applications. The chromium addition induces ferromagnetism. Proper addition of Nickel helps in countering such problems.
Nickel in stainless steel work as the austenitic stabilizer. Nickel addition in steel functions to stabilise tough and ductile, FCC austenitic structure. Conventional steel has a body centred cubic crystal known as ferritic structure. Roughly 8% nickel can stabilise the austenitic structure at room temperature. This is the reason that most stainless steel grades (Example 304 grade) contains 8 percent nickel. In the early twentieth century, this composition (18% Chromium and 8% Nickel) was one of the first to be developed in the history of stainless steel.
In recent times, Manganese and Nitrogen is used to form austenitic structure. However, still a small percentage of nickel is deliberately added to all the high-manganese austenitic grades commercially available today. Even the duplex stainless steel grades contain 1 percent or more nickel.
Nickel is also known to reduce the ductile-to-brittle transition temperature. This is the temperature below which the stainless steel or alloys have a tendency to convert into brittle. However, there are other parameters like, grain size and other alloying elements that affect ductile-to-brittle transition temperature.
Even all high strength precipitation-hardening stainless steel grades also contain nickel at appreciable amount.
Increase in nickel addition in stainless steel increases the corrosion resistance. Nickel in stainless steel reduces the active corrosion rate in crevice corrosion. The following image shows the effect of increasing nickel content in reducing the corrosion rate in a 15 percent sulphuric acid solution at 80° C. However, note that various other alloying additions, too, affect the corrosion rate.
Properties of Nickel Containing Stainless Steels
Nickel containing stainless steels provide various unique properties like
- Good weldability
- High toughness
- Good formability
- High-temperature properties
- Good Corrosion resistance
- Good surface finish
Is nickel in stainless steel harmful?
Nickel containing Stainless Steel is widely used for food and beverage industries for quite long time. Easy formability, superb corrosion resistance and excellent durability makes them an ideal choice for cookware. Due to various health concerns raised over the recent past the stainless steel is also believed to be harmful due to the presence of nickel in it. But studies has shown that unless the stainless steel is of poor quality, nickel will be solute in a very small amount that is not in the poisonous level to human. So high-quality 304 or 316 grade stainless steels does not pose severely bad effect to human health.
The following image from U.S. National Library of Medicine shows that the amount of nickel intake due to use of nickel containing stainless steel is much lower than tolerable intake limit of daily consumption.