We have discovered intergranular cracks at a certain weld location in a Monel (Ni-32Cu) vessel. An earlier failure of some Monel heat-exchanger tubes was also intergranular and appeared to be hydrogen-related. We believe the vessel is experiencing hydrogen embrittlement. Among other symptoms, the vessel hardness is now 95 HRB (it was only 80-85 HRB when it was placed in service). Can hydrogen be baked out of Monel? If so, what temperature would you recommend and for how long?

In answer to your question, it should be possible to bake out hydrogen from the Monel K400 alloy you mention. I would use a temperature slightly greater than 400ºF (say 425-450ºF), but in all cases do not exceed 800ºF. I prescribe to the philosophy that a longer bake-out time is more effective ("fast in, slow out"), so I would hold once the part reaches temperature in the order of 24 hours. Some people report doing the job in as little as 3-4 hours, but I'm not sure that there is enough evidence to back up this claim.

Here’s some additional background information:

Stress corrosion cracking (SCC) is an electrochemical process with distinct anodic and cathodic reactions. It is generally recognized that true stress corrosion cracking (as a mechanism) is controlled by the anodic reaction, and cracking, which is controlled by the cathodic reaction, is considered a form of hydrogen embrittlement. Poor cathodic protection practices could contribute to the embrittlement mechanism. Sulphide stress cracking is a form of hydrogen embrittlement and occurs in water within certain ranges of hydrogen sulfide content (for example, in “sour” gas wells). Hydrogen attack (along with decarburization) is the two most common damage mechanisms encountered by carbon or low-alloy steels involving hydrogen at elevated temperatures. “Hydrogen attack” of these materials is a mode by which carbon reacts with hydrogen to form methane that results in crack formation.

With respect to nickel alloys, they should be avoided unless the user verifies the alloy is suitable for hydrogen gas service. Monel K-500 and Inconel X-750 (which are precipitation-hardened alloys) may be used with hardness limits in subcomponents where high strength is required. Again, moderate strength levels and avoidance of high nickel levels are advantageous.

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