Laboratory corrosion tests were performed to clarify effects of relative amounts of fused salts in tube deposits on corrosion rates of super heater materials in WTE plants. All test exposures were at 550°C and of 100hour duration. The nine synthetic ashes used as corrodents consisted of mixtures of chlorides, sulfates and oxides. The test materials were alloy steel T22, stainless steels TP347H, TP310HCbN, and alloys HR1lN and 625. The gas atmosphere consisted of 500 to 3000 ppm HCl- 30ppm S02-10%02-10%COz-20%H20-bal.Nz.
Generally, the relative amount of fused salts in non-fused ash constituents at 550~ increased with increasing the chlorine content of the ashes. The corrosion rate of T22 steel did not depend directly on ash chlorine content, but for ashes of 7.7 wt.% Cl,the corrosion rate depended on the calculated amount of fused salt at 500~. The corrosion rates of TP347H steel and alloy 625 were maximum for ashes of 6-8 Wt.% Cl.For ashes of 7.7 Wt.% Cl, the corrosion rates of T22 steel, stainless steels, and alloys increased with ashes having higher amounts of fused salts. Increased HC1content of the gas caused higher corrosion of the stainless steels and high-nickel alloys, but there was no clear corrosion-exacerbating effect with T22 steel.
Super heater corrosion in waste-to-energy (WTE) plants has been extensively investigated to mitigate fireside wastage of super heater alloys at high steam temperatures. The corrosion is considered to result horn the deposition of fuel-slag condensates of sulfate and chloride mixtures on boiler tube surfaces. The deposits contain alkali metals (N~K) and heavy metals, such as zinc, which can form low melting point compounds at operating tube metal temperatures. This corrosion can be viewed as a form of hot corrosion2 caused predominantly by low melting point fused salts in slag deposits. With respect to fused salt corrosion, basicity defined as the activity of oxide anion (02-) in fused salts2,can affect the corrosion of super heater tubes. The basicity of fused salts is also influenced by the gaseous species HC1and H20 in the flue gas3.
In previous studies by the authors, it was conjectured that the relative amounts of fused salts in slag deposits at metal temperatures can affect super heater tube corrosion. These studies involved investigation of tube deposits taken from corrosion probes exposed at 550°C for 3000 hours in three operating WTE plants. The corrosion rates of TP347H stainless steel and alloy 625 probes were higher when the deposits contained greater amounts of fused salts at 55012. The severity of the corrosion environment for boiler tubes in real incinerators could be correlated not only with the chlorine content of tube deposits (considered the primary corrosion factor), but also with the relative amounts of fused salts present as a molten phase among non-fused ash particles in the deposits. The objective of the present laboratory study is to substantiate the effect of the relative amounts of fused salts formed at realistic tube metal temperatures on corrosion of super heater steels and alloys. The results from these ongoing studies are expected to assist not only alloy selection for super heaters but also super heater design in high-efficiency WTE plants under development.
