LONG TERM TESTING OF PROTECTIVE COATINGS AND WELD OVERLAYS IN A SUPERCRITICAL BOILER, RETROFITTED WITH LOW NO_x BURNERS

摘要

From 1994-1999 several protective coatings and weld overlays were evaluated in Hatfield's Ferry #2 boiler. This boiler is a universal pressure, once through, supercritical boiler rated at 550 MWe. It burns eastern bituminous coals with a sulfur content of about 2.2 percent. Prior to the tests the boiler was retrofitted with a low NO_x cell burner system (LNCB). During the first round of the testing mainly thermal spray coatings were tested, as expected corrosion rates were relatively low (<25 mils (0.62 mm)/yr). This expectation was based on laboratory corrosion testing as well as results from field testing in the first boiler retrofitted with a low NO_x cell burner system. However, corrosion rates were found to be at least 2-3 times higher than expected and the thermal spray coating provided inadequate protection. Thus a second panel containing weld overlays, and diffusion coatings in addition to the most promising thermal spray coatings was installed and tested for 2.5 yrs. Evaluation of the coatings and weld overlays confirmed that the durability of the thermal spray and diffusion coatings was generally inadequate in the most corrosive areas. Further testing is needed to confirm their suitability for less corrosive areas, although some promising results were obtained. The durability of weld overlays depended greatly on their chemical composition, but all showed measurable wear. Weld overlays with a Cr content less than 20 percent generally corroded quite rapidly. Weld overlays with Cr content above 20 percent had generally low corrosion rates. The best performing materials had projected life expectancies 2-3 times greater than that of the poorest performing material.