Because the passivation of the stainless steel plate occurs in the oxidizing medium, the corrosion resistance in the general sense refers to the corrosion resistance of the oxidizing medium. While in the non oxidative acid environment, such as sulfuric acid and strong acid, the conventional chromium stainless steel and chromium nickel stainless steel are not corrosive, especially in the medium containing chloride ions in the environment, because the passivation film of chloride ion can destroy the stainless steel board, leading to local formation of corrosion, that is the point corrosion.
Is there no way for stainless steel plates to corrode the non oxidizing medium? The answer is, it is proved by experiments and practical use that adding molybdenum and copper to stainless steel can effectively improve the corrosion resistance of stainless steel in the non oxidizing medium.
Molybdenum can make the stainless steel surface passivation reaction, can effectively improve the performance of stainless steel pitting corrosion and crevice corrosion in ferritic stainless steel, if does not contain molybdenum, chromium content high is not satisfactory pitting ability, but only in the chromium stainless steel adding molybdenum elements can play a role. Generally, the higher the chromium content, the better the corrosion resistance of the molybdenum reinforced stainless steel.
The practice shows that the ability of molybdenum to improve the corrosion resistance of stainless steel is basically three times that of the chromium element. Adding 1% of the molybdenum element in 1Cr17 stainless steel can significantly improve the corrosion resistance of organic acids and hydrochloric acid. Adding 1.5-4.0% molybdenum element in 18-8 chromium nickel stainless steel can increase the corrosion resistance of stainless steel plate in dilute sulfuric acid, organic acid, hydrogen sulfide or sea water.
Molybdenum is an element to form ferrite. Therefore, the addition of molybdenum to 18-8 chromium nickel stainless steel ensures that the pure nickel content of austenite must also be improved. After adding the molybdenum element, the nickel content of 18-8 stainless steel usually increases to 12%.
Molybdenum can also improve the mechanical properties of austenitic stainless steel at high temperature. Adding 0.5%-4.0% molybdenum element in martensitic stainless steel can also improve the tempering stability of stainless steel. Molybdenum can also precipitate precipitates in stainless steel and increase the strength of stainless steel.
Adding copper to stainless steel can improve the corrosion resistance of stainless steel in the sulfuric acid environment. The molten steel of copper containing stainless steel is more fluidity and is easier to be cast as a better quality component. Copper can also enhance the cold working performance of stainless steel.
Source: China Stainless Steel Plate Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)