Molybdenum

Molybdenum, in contrast to chromium, has very low resistance to oxidizing solutions but does resist reducing and halogen acids. Oxidizing acids such as nitric, aqua regia, and concentrated sulfuric acids readily dissolve molybdenum metal. Hydrofluoric acid does not affect Mo, and hot (110 C) hydrochloric acid attacks molybdenum metal only slowly. In both stainless steels and nickel-base alloys, molybdenum as an alloying element is required for resistance to halogen acids and to pitting by acid or oxidizing chlorides. The amount

used ranges from 2% in 316L stainless steel up to 24–30% in alloys B through B-10. As an alloying addition, molybdenum improves the stability of the passive layer in the presence of halogens.

Tungsten at the 2–4% level is used, along with molybdenum, to improve chloride pitting corrosion resistance.

Nickel metal in general is attacked by oxidizing solutions, while reducing solutions are less aggressive. Some examples1 follow:

Nickel metal resists neutral chloride solutions, such as sodium chloride, but is attacked by acid or oxidizing chloride salts. Alloys for use in reducing acids invariably have a considerable nickel content, ranging from as little as 8% in 304L to as much as 71% in alloy B-2.

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STAINLESS STEELS
James Kelly
Rochester, Michigan




Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright  2006 by John Wiley & Sons, Inc.



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