Corrosion Behavior of Coated 2 1/4 Cr-1 Mo and Carbon Steels in a Simulated High p/sub S sub 2 / Waste Heat Recovery System Environment of a Coal Gasifier

摘要

Corrosion tests have been conducted on coated 2-1/4Cr-1Mo and A106 carbon steels in a gaseous environment typical of that expected in a downstream waste-heat recovery system of a coal gasifier. The environment contained controlled oxygen and sulfur partial pressures, and the tests were run in a rig designed to expose specimens under simulated heat-exchanger conditions. Uncoated steel substrates exhibited breakaway corrosion behavior. Incoloy 800 also exhibited breakaway corrosion behavior. Coatings containing the stable oxide-forming elements Al and/or Cr were applied by pack-diffusion processes by several commercial vendors. Coatings containing Al contained numerous internal oxide particles and cracks while chromizing occasionally resulted in coatings containing voids. Results indicate that a minimum Cr concentration of approximately 20 to 22 wt % is required to suppress rapid corrosive degradation in the most aggressive environment used in the investigation. A minimum Al concentration could not be as closely defined, but would appear to lie in the range of 14 to 20 wt %. A minimum total concentration of combined additions of Al and Cr could not be defined. The presence of cracks in the aluminized materials promotes internal oxidation. Two Ni-base weld metals were used to simulate joining or repair operations of prior aluminized steel substrates. Both Inconel 625 and Inconel 72 exhibited breakaway corrosion behavior, the former at an unacceptable rate and the latter at a rate on the borderline of acceptability. The corrosion behavior of all coated and coated/welded materials is described and the results used to formulate a provisional specification for the procurement of coatings for eventual commercial application. 11 refs., 15 figs., 5 tabs. (ERA citation 11:026817)