Inspectioneering Journal

99 Diseases of Pressure Equipment: Liquid Metal Cracking

By John Reynolds, Principal Consultant at Intertek. This article appears in the November/December 2003 issue of Inspectioneering Journal.

Liquid Metal Cracking (LMC) (aka "liquid metal embrittlement") is another insidious form of cracking that strikes when you least expect it. It most commonly afflicts austenitic stainless steels, but can afflict other copper, nickel and aluminum alloys. LMC occurs when molten metals come in contact with susceptible materials. One of the more common such occurrences is during a fire when molten zinc from galvanized steel parts or inorganic zinc coatings drips down on SS equipment. For example, take the case where a fire on a steam-methane reformer furnace caused zinc from galvanized gratings to drip down on Incoloy pigtails and crack them suddenly and badly. Cracking rates can be exceedingly rapid and failure can occur within seconds. Any stainless steel that will be used in furnaces or a temperatures over about 780 F should not be allowed to come in contact with anything that has been galvanized or coated with zinc containing coatings.

Liquid mercury contamination of some crude oils can also cause LMC in crude overhead condensers as well as the overheads of depropanizers and debutanizers. Mercury has also cracked aluminum core exchangers in ethylene plants. Cadmium plated bolts (common in non-pressure retaining applications) can also crack and fail by LMC when heated above 450 degrees F. So great care must be taken to avoid getting cadmium plated bolting material mixed up with flange bolting that could see elevated temperatures.

Do you know if you may have exposure to LMC in your plants? And do you take appropriate precautions to avoid LMC?

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