What is the role of Cr, Ni, and Mo elements in stainless steel?

Chromium (Cr): It is a key element for the formation of ferrite. When combined with oxygen, it forms a corrosion-resistant Cr2O3 passivation layer; it is one of the essential elements that enable stainless steel to retain its corrosion resistance. An increase in chromium content enhances the steel’s ability to repair its passivation layer. Generally, the chromium content in stainless steel must be above 12%； Nickel (Ni): It is a key austenite-forming element that can reduce the corrosion of steel as well as the growth of grain size during heating ； Molybdenum (Mo): It is an element that forms carbides; these carbides are extremely stable and can prevent the grain growth of austenite during heating, thereby reducing the steel’s sensitivity to overheating. Additionally, molybdenum helps to make the passivation film more dense and robust, thus effectively improving the corrosion resistance of stainless steel to Cl- ；

Titanium (TI): Titanium is used to fix the carbon within the material in order to eliminate chromium depletion (sensitization) at the grain boundaries. In high-chromium stainless steels, about 5 times as much titanium as carbon is typically added; this not only improves the steel’s corrosion resistance (particularly against intergranular corrosion) but also its toughness； It can also control the grain growth tendency of steel at high temperatures and improve the weldability of steel. Chromium (CR): Provides good high-temperature oxidation resistance and resistance to oxidative corrosion, as well as enhancing the heat strength of steel. Chromium is the main alloying element in stainless steel, acid-resistant steel, and heat-resistant steel. Nickel (NI): Ni is used in stainless steel to give the steel a uniform A-phase structure, thereby improving its corrosion resistance. Ni-containing steels generally do not overheat easily, so they can prevent grain growth at high temperatures and maintain a fine-grained structure. Molybdenum (Mo): In stainless acid-resistant steel, molybdenum further enhances resistance to corrosion caused by organic acids such as formic acid, acetic acid, oxalic acid, etc., as well as hydrogen peroxide, sulfuric acid, sulfurous acid, sulfates, acidic dyes, and bleaching solutions. In particular, the addition of molybdenum prevents the tendency to pitting corrosion caused by the presence of chloride ions.