No abstract

An austenitic stainless steel (TP 347H FG) was coated with a synthetic deposit and exposed, under laboratory conditions simulating straw-firing at 560 °C, for 1 week. Microscopic, diffraction, and spectroscopic techniques were employed for cross-sectional and plan view “top-down” microstructural characterization of the corrosion products. The corrosion products consisted of three layers: (i) the outermost layer consists of a mixed layer of K2SO4 and Fe x O y on a partly molten layer of the initial deposit, (ii) the middle layer consists of spinel (FeCr2O4) and Fe2O3, and (iii) the innermost layer is a sponge-like Ni3S2-containing layer. At the corrosion front, Cl-rich protrusions were observed. Results indicate that selective corrosion of Fe and Cr by Cl, active oxidation, and sulfidation attack of Ni are possible corrosion mechanisms.

The variable flue gas composition in biomass-fired plants, among other parameters, contributes to the complexity of high-temperature corrosion of materials. Systematic parameter studies are thus necessary to understand the underlying corrosion mechanisms. This paper investigates the effect of water (H 2 O) vapor content in the flue gas on the high-temperature corrosion of austenitic stainless steel (TP 347H FG) under laboratory conditions, to improve the understanding of corrosion mechanisms. Deposit-coated and deposit-free samples were isothermally exposed for 72 h in a synthetic flue gas atmosphere containing either 3 or 13 vol % H 2 O vapor. Comprehensive characterization of the corrosion products was carried out by the complementary use of microscopic, spectroscopic, and diffraction-based techniques. To evaluate the effect of the exposure time, results were compared to previous results with longer isothermal exposure over 168 h and indicated that the development of a Ni-rich layer as a result of selective attack was time-dependent. The increase in the water vapor decreased the measurable corrosion attack, and in addition, decreased sulfation was observed. Results from the current investigation and from previously reported findings suggest that an increase in the water vapor content will cause competitive adsorption on active sites.

In biomass fired power plants, deposition of alkali chlorides on superheaters, as well as the presence of corrosive flue gas species, give rise to fast corrosion of superheaters. In order to understand the corrosion mechanism under this complex condition, the influence of the flue gas composition on high temperature corrosion of an austenitic superheater material under laboratory conditions mimicking biomass firing is investigated in this work. Exposures involving deposit (KCl)‐coated and deposit‐free austenitic stainless steel (TP 347H FG) samples were conducted isothermally at 560 °C for 72 h, under both oxidizing and oxidizing‐chlorinating atmospheres, and the resulting corrosion products were comprehensively studied with scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS), and X‐ray diffraction (XRD) techniques. The results show that deposit‐free samples suffer grain boundary attack only in an oxidizing‐chlorinating atmosphere, otherwise corrosion results in formation of a duplex oxide. Corrosion attack on deposit‐coated samples was higher than on deposit‐free samples irrespective of the gaseous atmosphere. Specifically, severe volatilization of alloying elements occurred on deposit‐coated samples under oxidizing‐chlorinating atmosphere due to enhanced impact of KCl and HCl.

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 o C (from 83.5 h to 672 h), and at 600 o C (from 83.5 h to 168 h) were conducted in a gas mixture comprising of O 2 , H 2 O, CO 2 , HCl and SO 2. In addition, samples were subjected to temperature variations between 560 o C and 600 o 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