You may have heard about, or even seen, the effects of what is termed “Catastrophic Oxidation”.
This is a rapid localised metal loss due to oxidation associated with the exposure of Molybdenum bearing alloys to elevated temperatures, usually above 1038° C. More often, this condition develops during production heat treatments, than under actual service conditions.
The basic 300 series stainless materials like 304L and 316L have an austenitic microstructure and are non-magnetic. In the annealed condition they are essentially free of ferrite, which is magnetic. Cast products of these alloys typically have some ferrite
The AL-6XN® (a 6% Mo alloy) fitting shown here suffered catastrophic oxidation when it was solution annealed at 1177° C after forming.
Catastrophic oxidation occurs in Molybdenum bearing alloys when the elevated temperature atmosphere at the metal surface forms the gaseous MoO₃ compound that acts as a catalyst to promote oxidation of other elements in the base metal, or destroys protective oxides like Cr₂O₃. The MoO₃ compound can form at temperatures as low as 510° C, but the volatility and rate of reaction increases with increasing temperature.
The MoO₃ can also play a role in the formation of other low melting compounds that may lead to more rapid dissolution of an Iron substrate. These include Vanadium pentoxide (V₂O₅), Tungsten oxide (WO₃) and Bismuth oxide (Bi₂O₃).
With Molybdenum trioxide, the damage can be minimised by maintaining sufficient air circulation to keep MoO₃ concentrations low and moving away from the metal surface. This permits the protective Chromium oxide to stabilise on the surface. If air circulation is poor, or the volume of air to metal surface is low, the MoO₃ will have a chance to concentrate and promote catastrophic oxidation. The tendency of course, when running a furnace load of parts, is to load as many pieces into the furnace as will fit. This results in a low air volume to surface ratio and promotes stagnant conditions inside the furnace. This creates a greater probability that catastrophic oxidation will develop. The worst wastage is often found where parts were in contact with one another or with furnace floors, walls or fixtures.
Any stainless or Nickel alloy containing Molybdenum is susceptible to catastrophic oxidation. Even an instance of the catastrophic oxidation of 316L (less than 3% Mo) has been encountered. Examples with Alloy X (9% Mo), Alloy 625 (9% Mo) and Super Duplex stainless ZERON® 100 (4% Mo) have also been seen. This may be surprising to some since Alloy X in particular is used very successfully for high temperature oxidation resistance (Alloy 625, too). The difference is that in service there is plenty of air flow to whisk away any MoO₃ that may form at the surface.
Note that Molybdenum does oxidise very easily (almost as easily as Chromium) and if unalloyed Molybdenum is used at elevated temperatures, it must be under a vacuum or inert atmosphere. Even high temperature materials alloyed with Molybdenum (such as Alloy 625 or Alloy X) give up a little in terms of oxidation resistance to gain elevated temperature strength. At temperatures above about 1121° C, these alloys will have oxidation rates that are higher than non-Molybdenum or lower Molybdenum bearing alloys like 601 or RA333®. However, this type of more rapid oxidation should not be confused with catastrophic oxidation which is localised and non-uniform.