Evaluation of coated and clad heat exchangers for syngas coolers. Volume 2. Laboratory corrosion study of coated low-alloy steel tubes. [Low alloy steels such as T-11 (1-1/4% Cr-0. 5% Mo)]

The feasibility of coating large heat exchanger components made from low alloy steels for syngas coolers of coal gasification plants was studied. Diffused coatings applied by the pack cementation process were found the most promising. Waterwall panels and convection modules were aluminized and chromized to study the uniformity of the deposits. The thickness of commercially produced aluminized coatings was quite variable and occasionally unsatisfactory. The thickness of chromized coatings was also variable, but generally satisfactory as the coating in the thinnest areas still contained 25% Cr and was about 10 mils (250 m) thick. Welding of chromized coatings was satisfactory. Welding of aluminized coatings caused cracking in the outer, high aluminum part of the coating. High temperature corrosion studies were carried out at 400C in syngas environments using aluminized and chromized coatings and stainless steel claddings on low alloy steels such as T-11 (1-1/4% Cr-0.5% Mo). Aluminized coatings were resistant to sulfidation when their aluminum content exceeded 16 to 18%. Intergranular corrosion occurred when cracks extended into the lower part of the coating, where the aluminum content was less than 18%. This happened especially adjacent to welded areas. Chromized coatings with a chrome content of 25% or more were alsomore » resistant to sulfidation. Some Intergranular corrosion of chrome depleted areas near carbide precipitates was noted, especially near welds. Chromized coatings on carbon stabilized alloys appeared completely resistant to sulfidation and contained few carbide precipitates. Stainless steel claddings generally had low sulfidation rates, especially Nb stabilized SS310 (25 Cr-20 Ni). 15 refs., 37 figs.« less