Application of molybdenum metal in stainless steel

Application of molybdenum metal in stainless steel

The passivation effect of stainless steel is formed in the oxidizing medium. Generally speaking, corrosion resistance refers to the oxidizing medium. In non-oxidizing acids, such as dilute sulfuric acid and strong organic acids, general chromium stainless steel and chromium-nickel stainless steel are not corrosion-resistant. Especially in the medium containing chloride ions (Cl), because chloride ions can destroy the passive film on the surface of stainless steel, it causes corrosion in local areas of stainless steel, that is, pitting corrosion. Adding molybdenum and copper to stainless steel is an effective way to improve the corrosion resistance of stainless steel in non-oxidizing media.

Molybdenum can promote the passivation of the surface of stainless steel and enhance the ability of stainless steel to resist pitting corrosion and crevice corrosion. If the ferritic stainless steel does not contain molybdenum, it is difficult to obtain satisfactory pitting corrosion resistance no matter how high the chromium content is, but only when it contains chromium. Molybdenum in steel can play a role.

Generally speaking, the higher the chromium content, the more obvious the effect of molybdenum on improving the pitting corrosion resistance of steel. Studies have shown that the ability of molybdenum to improve pitting corrosion resistance is equivalent to three times that of chromium. The addition of 1% molybdenum (1Cr17Mo) to 1Cr17 steel can significantly improve its corrosion resistance in organic acids and hydrochloric acid. Adding 1.5-4.0% molybdenum to 18-8 chromium-nickel steel can improve its corrosion resistance in dilute sulfuric acid, organic acids (acetic acid, formic acid, oxalic acid), hydrogen sulfide, and seawater.

Molybdenum is an element that forms ferrite. Therefore, after adding molybdenum to 18-8 chromium-nickel steel, in order to maintain the pure austenite structure, the nickel content should be increased accordingly. After adding molybdenum, the nickel content of 18-8 steel is generally increased to more than 12%, such as 0Cr17Ni12Mo2 and 00Cr17Ni14Mo2.

Molybdenum can improve the high-temperature mechanical properties of austenitic stainless steel, as shown in Table 1-4. The addition of 0.5% to 4.0% molybdenum to martensitic stainless steel can increase the tempering stability of the steel. Molybdenum can also form precipitation phases in stainless steel to improve the strength of steel, such as precipitation hardening stainless steel 0Cr17Ni5Mo3.

The influence of molybdenum on the mechanical properties of stainless steel at high temperature