In Phase I a variety of developmental and commercial tubing alloys and claddings were exposed to laboratory fireside corrosion testing simulating a superheatez or reheater in a coal-tired boiler. Phase II (in situ testing) has exposed samples of 347, RA- 85H, HR3C, 253MA, Fe3Al + 5Cr, 310 modified, 800HT, NF 709,690 clad, and 671 clad for over 10,000 hours to the actual operating conditions of a 250-MW coal-fired boiler. The samples were tiled on an air-cooled, retractable corrosion probe, installed in the reheater cavity, and controlled to the operating metal temperatures of an existing and advanced-cycle, coal-fired boiler. Samples of each alloy will be exposed for 4000, 12,000, and 16,000 hours of operation.
High-temperature fireside metal wastage in conventional coal-tired steam generators can be caused by gas-phase oxidation or liquid-phase coal-ash corrosion. Gas-phase oxidation is usually not a problem if tube and support materials are selected for their oxidation resistance at operating temperatures and for spalling, flaking, or other reactions to their environment. Coal-ash corrosion, on the other hand, usually results in accelerated attack and rapid metal wastage-even of stainless steels. The cause of this type of corrosion is generally accepted as the presence of liquid sulfates on the surface of the metal beneath an overlying ash deposit.1-4While substantial progress has been achieved through laboratory testing, add utility service exposures are evidently necessary to verify any conclusions drawn from laboratory testing. A number of important environmental parameters cannot be fully simulated in the laboratory5:
The actual composition of the deposits formed on the tubes is more complex than the composition of the simulated ash
The S03, formed by heterogeneous reaction on cooled surfaces, is variable.
Very large temperature gradients occur within the ash deposits
The ash and fuel gas move past tubes at high velocity; the rate varies with design.
The composition of the corrosive deposits changes with time.
Metal and flue gas temperatures fluctuate.
Fly-ash erosion removes the protective oxides, exposing a clean surface to fresh ash.
Foster wheeler Development Corporation (FWDC) has performed a number of literature reviews and recent updates discussing the variables affecting the corrosion mechanism6-8. Additionally, Foster Wheeler is conducting two sizable research projects-one a laboratory and in situ field testing at three utilities of commercially available alloys5,9,15 and this program (ORNLFW2), combining laboratory and field testing to more completely cover the controlling variables and additional alloys for a long testing duration10.
PHASE I RESULTS
In Phase I of this ORNL program, “Fireside Corrosion Testing of Candidate Superheater Tube Alloys, Coatings, and Cladding,” 20 commercial and developmental alloys listed in Table 1 were evaluated10. The coupons of the metals were exposed to multiple types of synthetic coal ash and synthetic flue gases at 650 and 700º C (1202 and 1292ºF) for up to 800 hours. The aver. age thickness loss rate for 14-to 48-percemt chromium-containing alloys and the intermetallic aluminides are shown in Figures I through 3.
