Corrosion Failure of a Tee Fitting

Wet natural gas was dried by being passed through a carbon steel vessel that contained a molecular-sieve drying agent. The drying agent became saturated after several hours in service and was regenerated by a gas that was heated to 290 to 345 deg C in a salt-bath heat exchanger. The tee joint in the piping between the heat exchanger and the sieve bed failed after 12 months. A hole in the tee fitting and a corrosion product on the inner surface of the pitting was revealed by visual examination. Iron sulfide was revealed by chemical analysis of the scale which indicated hydrogen sulfide attack on the carbon steel. The presence of oxygen was indicated by the carbon and sulfur found in the scale on the piping and in the sieves indicated that oxygen combined with moisture produced conditions for attack of hydrogen sulfide on carbon steel. Turbulence with some effect from the coarse grain size was interpreted to have contributed. The piping material was changed from carbon steel to AISI type 316 stainless steel as it is readily weldable and resistant to corrosion by hydrogen sulfide.