Alloys containing molybdenum

At temperatures below 120°C, carbon steel can be used up to high pressures. At elevated temperatures and significant pressures hydrogen will penetrate carbon steel and react with the carbon to form methane. This results in a loss of ductility and cracking or blistering of the steel. For high temperature applications steel alloys containing molybdenum and steel are satisfactory. 

Titanium alloys suffer crevice corrosion in hot aqueous chloride media, but the alloy containing molybdenum, 3A1-8V-6 Cr-4Zr-4 Mo has good resistance to crevice corrosion and is successfully used in hot sour well and geothermal brine... 

Alloys containing molybdenum can potentially experience catastrophic oxidation. The super-austenitic stainless steels such as Alloy AL-6XN, a 21Cr-25Ni-6.5Mo-N alloy (UNS N08367), are an example. A heavy molybdenum oxide scale forms, usually as a result of an improper heat treatment or a severe thermal excursion. Removal of such scales prior to service (or return to service), usually by pickling, prevents this problem. 

R.C. Newman, The dissolution and passivation kinetics of stainless alloys containing molybdenum— II. Dissolution kinetics in artificial pits, Corros. Sci. 25 (1985) 341-350. 

Stainless steels exposed to seawater develop deep pits within a matter of months, with the pits usually initiating at crevices or other areas of stagnant electrolyte (crevice corrosion). Susceptibility to pitting and crevice corrosion is greater in the martensitic and ferritic steels than in the austenitic steels it decreases in the latter alloys as the nickel content increases. The austenitic 18-8 alloys containing molybdenum (types 316,316L, 317) are still more resistant to seawater however, crevice corrosion and pitting of these alloys eventually develop within a period of 1-2.5 years. 

The duplex stainless steels are alloys whose microstructures are a mixture of austenite and ferrite. Type 32550 and related alloys containing molybdenum have an atmospheric corrosion resistance superior to type 316 in high-chloride environments. The duplex grade would be used when the greater strength of these alloys is required for application in a marine atmosphere. 

When examining alloys containing molybdenum, it must be remembered that molybdates also give a violet color with diphenylcarbazide. See page 191 regarding the masking of molybdates by oxalic acid. 

In industrial or marine environments the corrosivity is more severe. Table 1-9 compares the performance of several stainless steels after 15 years exposure to a marine atmosphere 250 m from the ocean at Kure Beach, North Carolina, USA (International Nickel Company, 1963). Rust staining was extremely slight for alloys containing molybdenum, e.g. X2CrNiMol8-14-3 (AISI 316/317), and the surface could easily be cleaned to reveal a bright surface. 

Alloys containing molybdenum, specifically t) es 316 and 317 austenitic stainless steels containing 2 and 3% molybdenum, respectively, are subject to catastrophic oxidation. Under some conditions, there is a selective oxidation of molybdenum with rapid loss of volatile M03. 

In high nickel alloys containing molybdenum, such as 15% Cr, 15% Mo, 4% W, 5% Fe, 0.06% C and balance Ni, intergranular attack is generally attributed to the depletion of molybdenum and chromium, because of the formation of M C (molybdenum carbides and chromium carbides (M23C6). 

He further observed that alloys containing molybdenum were more sensitive to... 

W. C. Leslie and M. G. Fontana, Mechanism of the Rapid Oxidation of High Temperature, High Strength Alloys Containing Molybdenum, Trans. Am. Soc. Metals 41, 1213 (1949). 

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