The primary element in stainless steel is chromium. However, excessive chromium can cause difficulties in manufacturing, forming, and alloy stability.

To enhance these properties, as well as improve corrosion resistance, oxidation, and other features of stainless steel, additional alloying elements are added. Below are the most important alloying elements and their effects.

Nickel

Nickel performs two critical functions in stainless steel:

1. It helps form and stabilize the austenitic structure, reduces work hardening, increases ductility, and improves mechanical properties, especially at low temperatures.
2. Nickel enhances corrosion resistance, particularly in reducing environments and mineral acids, by contributing to the formation of a protective layer.

Manganese

Like nickel, manganese is an austenite-forming element, but it is more cost-effective. When used as a partial replacement for nickel in stainless steel, it provides some of the beneficial properties of nickel while reducing the alloy’s overall cost.

Molybdenum

Adding molybdenum in moderate amounts to stainless steel greatly increases the stability of the protective layer, especially in non-oxidizing environments containing chloride ions. Molybdenum also enhances resistance to pitting and crevice corrosion. However, molybdenum-containing alloys are generally unsuitable for use in oxidizing environments, such as nitric acid and nitrates.

Silicon

Silicon improves scaling resistance at high temperatures and increases corrosion resistance in strong oxidizing chemical environments, such as hot concentrated sulfuric acid.

Titanium and Niobium

These two elements form very stable carbides with carbon, preventing chromium from binding with carbon. This helps maintain the corrosion resistance of stainless steel by ensuring chromium remains available in the alloy structure.

Copper

Copper enhances corrosion resistance in reducing environments, such as sulfuric acid solutions.

Carbon

The carbon content in stainless steels varies between 0.01% and 1%, and its impact includes the following:

1. In most stainless steels, carbon increases creep resistance and mechanical stability at high temperatures.
2. In martensitic stainless steels, carbon is the key factor for forming the martensitic phase, which results in increased hardness.
3. Although carbon does not directly affect the corrosion resistance of stainless steel, it can bind with chromium and reduce corrosion resistance, especially at grain boundaries.

Sulfur and Selenium

These elements are added to stainless steel to improve machinability.

Nitrogen

Nitrogen has several important effects:

1. In austenitic stainless steels, nitrogen increases mechanical strength (at both normal and low temperatures) and enhances resistance to pitting and crevice corrosion. It also delays the formation of brittle phases containing chromium and molybdenum.
2. In duplex stainless steels, nitrogen plays a significant role in stabilizing the austenitic phase and reduces the tendency of chromium and molybdenum to segregate.
3. In ferritic stainless steels, nitrogen has a very detrimental effect on their mechanical properties.

Source: F. Kaviani Nejad, "Stainless Steels and Heat-Resistant Steels" (2007); Stainless Steels and Their Properties, Pages 21-64.