Time and Temperature Effects on Alkali Chloride Induced High Temperature Corrosion of Superheaters during Biomass Firing

The high content of alkali chloride in deposits which form during biomass firing in power plants contributes significantly to corrosion of the superheaters. In order to understand the influence of time and temperature on high temperature corrosion under such harsh conditions, laboratory scale studies as a function of time and temperature were carried out using KCl coated samples of the austenitic stainless steel (TP347H). To understand the progress of corrosion with time, isothermal exposures at 560 degrees C (from 83.5 to 672 h) and at 600 degrees C (from 83.5 to 168 h) were conducted in a gas mixture comprising of O-2, H2O, CO2, HCl, and SO2. In addition, samples were subjected to temperature variations between 560 and 600 degrees C to gain insights on the influence of temperature. The microstructure and elemental composition of the corrosion products resulting from the exposures were studied with scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The results show that corrosion attack progressed with time such that the thickness of the consistently identified three regions of corrosion products increased with time, therefore suggesting that the corrosion products were not protective. Also, exposures under varying temperature conditions revealed that an increased corrosion attack would occur once the superheater experiences a higher temperature, because a memory effect from prior exposure at higher temperature propagates more corrosion attack during subsequent exposure to a lower temperature.