"Monel or Inconel"

Question:

"I need to know which will be more suitable material for control valve bellows seal Inconel 600 or monel for service containing small percentage caustic, sulphur based compounds & parafinns based hydrocarbon with free water. For body of control valve should I use Monel or SS316. Temperature will not exceed 80 deg C. Sulphur will be h3S and some mercaptants total sulphur not exceeding 2%"

Answer:

Available data suggests that both Inconel 600 and Monel 400 may experience excessive corrosion rates (for a thin-walled component) in dilute, hot, sulfur-containing water. The exact corrosion rate will be affected by the type and amount of dissolved sulfur species and whether the amount of caustic is sufficient to maintain an alkaline pH. (Dissolved h3S will not be present in an alkaline pH water). I would consider other, more resistant alloys for your stated service. If the service is critical I suggest conducting corrosion tests of Inconel 600, Monel 400, as well as other candidates, possibly C-276 and alloy 625.

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