Failure analysis of monel packing in atmospheric distillation tower under the service in the presence of corrosive gases

We have performed a failure analysis on a monel packing material subjected to sulfidation and sulfide stress cracking after a short period of service in an atmospheric distillation tower containing corrosive gases. Optical macroscopic inspections show that the failed packing material was basically covered with dark scales and corrosion products. Energy dispersive X-ray spectroscopy and X-ray diffraction spectrum show that the corrosion products contain sulfur as the main element along with oxygen indicating that oxidation may have been taken place beside sulfidation. Metallography of the failed samples reveals that cracks exist and were initiated on the surface from defects most likely developed due to the formation of porous and brittle sulfide layer accomplished by hydrogen embrittlement. Although nickel base alloys are an appropriate candidate owing to their high corrosion resistant in corrosive atmospheres, however, presence of H2S, HCl, O2 and naphthenic acid in crude oil lead to reduction in corrosion resistant of the monel 400 alloy.