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 temperature corrosion of the austenitic stainless steel, TP 347H FG under conditions mimicking biomass firing is enhanced both by the presence of a deposit (KCl) and HCl in the flue gas. While chlorination in the absence of a deposit induces grain boundary attack, enhanced chlorination in the presence of a deposit results in severe volatization of chlorides of the alloying elements.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Laboratory testing of selected chromia‐forming alloys was performed to rank the materials and gain further knowledge on the mechanism of KCl‐induced high temperature corrosion. The investigated ...alloys were stainless steels EN1.4021, EN1.4057, EN1.4521, TP347H (coarse‐grained), TP347HFG (fine‐grained), Sanicro 28 and the nickel‐based alloys 625, 263 and C276. Exposure was performed at 600 °C for 168 h in flowing N
2
(g)+5%O
2
(g)+15% H
2
O(g) (vol.%). Samples were covered with KCl powder prior to exposure. A salt‐free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffractometry (XRD). It was observed that in the salt‐free exposure, stainless steels TP347H (coarse‐grained) and EN1.4521 failed to form a thin protective oxide layer compared to the oxide formed on the other alloys. In the presence of solid KCl, all the alloys showed significant corrosion. Measurement of corrosion extent indicated that alloys EN1.4057, Sanicro 28 and 625 show a better performance compared to the industrial state of the art material TP347HFG under laboratory conditions. An additional test was performed with KCl vapor in static air for the same duration and at the same temperature. This was undertaken to investigate the role of the vapor phase and revealed that KCl vapor at 600 °C can initiate attack.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
In biomass fired power plants, the fast corrosion of superheaters is facilitated by the presence of corrosive flue gas species, for example, SO2, which are released during combustion. To understand ...the role of the gas species on the corrosion process, comparative laboratory exposures of deposit (KCl)‐coated and deposit‐free austenitic stainless steel (TP 347H FG) samples to gas mixtures containing SO2 was carried out, under conditions relevant to biomass‐firing. Exposures were conducted isothermally at 560 °C for 72 h, in oxidizing‐sulphidizing, and oxidizing‐sulphidizing‐chlorinating gas mixtures containing 60 ppmv SO2. Scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffraction (XRD) techniques were complimentarily applied to characterize the resulting corrosion products. A partially molten K2SO4‐layer formed on KCl coated specimens, and corrosion resulted in localized broad pits containing sulphides and oxides. The severe pitting attack was decreased by the presence of HCl in the gas mixture.
The effect of SO2 in the flue gas is investigated to evaluate its role on high temperature corrosion of superheaters under conditions mimicking biomass firing. Results show that when a deposit is absent, SO2 minimizes corrosion attack. Deposit (KCl) particles are sulphated due to the presence of SO2 in the flue gas and the resulting corrosion attack on deposit‐coated samples involves severe local pitting, which is minimized when HCl is added to the flue gas.
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Laboratory testing on selected alumina and silica‐forming alloys was performed to evaluate their performance against high temperature corrosion induced by potassium chloride (KCl). The alloys studied ...were FeCrAlY, Kanthal APM, Nimonic 80A, 214, 153MA and HR160. Exposure was conducted at 600 °C for 168 h in flowing N
2
(g)+5%O
2
(g)+15%H
2
O(g) (vol.%) with samples covered under KCl powder. A KCl‐free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffractometry (XRD). It was observed that alloying with aluminum did not lead to the formation of protective alumina for the studied alloys. The silicon containing stainless steel 153MA showed an analogous performance to low‐silicon austenitic stainless steels of similar chromium and nickel contents. For alloy HR160, a potassium‐chromium‐silicon‐oxygen containing layer forms as the innermost corrosion product. The layer was uniformly distributed over the surface and appears to render some protection as this alloy exhibited the best performance among the investigated alloys. To reveal further aspects of the corrosion mechanism, Nimonic 80A was exposed in static laboratory air for the same duration and temperature with either KCl or K
2
CO
3
deposits. Comparison of results obtained with these experiments showed that both potassium and chlorine can play a role in material degradation by KCl.
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Pack cementation was used to produce Fe1−xAl and Fe2Al5 diffusion coatings on ferritic‐martensitic steel P91 and a Ni2Al3 diffusion coating on pure nickel. The performance of diffusion coatings ...against high‐temperature corrosion induced by potassium chloride (KCl) was evaluated by exposing the samples at 600 °C for 168 h in static lab air under KCl deposit. In addition, a salt‐free experiment was performed for comparison. Microstructure, chemical and phase composition of the samples were analyzed with scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffractometry (XRD) before and after the exposures. It was found that all the diffusion coatings formed protective oxides under salt‐free exposure in air. Under the salt deposit, Fe1−xAl showed local failure while on large parts of the sample a protective layer had formed. Fe2Al5 was attacked over the entire surface and the dominant mode of attack was selective aluminum removal. Ni2Al3 showed excellent performance and no sign of attack was observed anywhere on the sample.
Pack cementation was used to produce Fe1−xAl and Fe2Al5 diffusion coatings on ferritic‐martensitic steel P91 and a Ni2Al3 diffusion coating on pure nickel. The performance of diffusion coatings against high temperature corrosion induced by KCl was evaluated by exposing the samples at 600 °C for 168 h in static lab air under KCl deposit. Ni2Al3 showed excellent performance and no sign of attack was observed anywhere on the sample.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Laboratory testing of selected chromia‐forming alloys was performed to rank the materials and gain further knowledge on the mechanism of KCl‐induced high temperature corrosion. The investigated ...alloys were stainless steels EN1.4021, EN1.4057, EN1.4521, TP347H (coarse‐grained), TP347HFG (fine‐grained), Sanicro 28 and the nickel‐based alloys 625, 263 and C276. Exposure was performed at 600 °C for 168 h in flowing N2(g)+5%O2(g)+15% H2O(g) (vol.%). Samples were covered with KCl powder prior to exposure. A salt‐free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffractometry (XRD). It was observed that in the salt‐free exposure, stainless steels TP347H (coarse‐grained) and EN1.4521 failed to form a thin protective oxide layer compared to the oxide formed on the other alloys. In the presence of solid KCl, all the alloys showed significant corrosion. Measurement of corrosion extent indicated that alloys EN1.4057, Sanicro 28 and 625 show a better performance compared to the industrial state of the art material TP347HFG under laboratory conditions. An additional test was performed with KCl vapor in static air for the same duration and at the same temperature. This was undertaken to investigate the role of the vapor phase and revealed that KCl vapor at 600 °C can initiate attack.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Laboratory testing on selected alumina and silica‐forming alloys was performed to evaluate their performance against high temperature corrosion induced by potassium chloride (KCl). The alloys studied ...were FeCrAlY, Kanthal APM, Nimonic 80A, 214, 153MA and HR160. Exposure was conducted at 600 °C for 168 h in flowing N2(g)+5%O2(g)+15%H2O(g) (vol.%) with samples covered under KCl powder. A KCl‐free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffractometry (XRD). It was observed that alloying with aluminum did not lead to the formation of protective alumina for the studied alloys. The silicon containing stainless steel 153MA showed an analogous performance to low‐silicon austenitic stainless steels of similar chromium and nickel contents. For alloy HR160, a potassium‐chromium‐silicon‐oxygen containing layer forms as the innermost corrosion product. The layer was uniformly distributed over the surface and appears to render some protection as this alloy exhibited the best performance among the investigated alloys. To reveal further aspects of the corrosion mechanism, Nimonic 80A was exposed in static laboratory air for the same duration and temperature with either KCl or K2CO3 deposits. Comparison of results obtained with these experiments showed that both potassium and chlorine can play a role in material degradation by KCl.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Oxidation of 99·99% pure nickel was studied with and without 0·10 mg cm
−2
KCl(s) in an environment containing 5 vol.-%O
2
, 40 vol.-%H
2
O and 55 vol.-%N
2
at 600°C for up to 168 h. Oxide ...microstructure was investigated by X-ray diffraction (XRD), focused ion beam (FIB), broad ion beam (BIB) and SEM/EDX. Oxidised nickel shows an approximately parabolic oxide growth rate. The oxide scale is dense with some pores at the oxide/metal interface. Adding small amounts of KCl does not result in a faster corrosion rate of nickel. However, the surface morphology changes and small oxide crusts were observed in the vicinity of former KCl particles. This is proposed to be the result of a NiCl
2
-KCl eutectic on top of the oxide scale formed above 514°C. The oxide scale formed in the presence of KCl contains more and differently distributed voids than the scale formed without KCl.
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BFBNIB, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Transistor aging is a major reliability concern for nanoscale CMOS technology that can significantly reduce the operation lifetime of very large-scale integration chips. Negative bias temperature ...instability (NBTI) is a major contributor to transistor aging that affects pMOS transistors. On the other hand, leakage power is becoming a dominant factor of the total power with successive technology scaling. Since the input combinations applied to a logic core have a significant impact on both NBTI and leakage power, input vector control can be used to optimize both phenomena during idle cycles. In this paper, we present an efficient input vector selection technique based on linear programming for cooptimizing the NBTI-induced delay degradation and leakage power consumption during standby mode. Since the NBTI-induced delay degradation and leakage power are not affected by the input vector in the same direction, we provide a pareto curve based on both phenomena. A suitable point from such a pareto curve is chosen based on circuit conditions and requirements during runtime.
Oxidation of Ni
2
Al
3
produced by pack aluminizing of pure nickel was studied with and without 0.10 mg cm
−2
KCl(s) deposit in an environment containing 5% O
2
, 40% H
2
O and 55% N
2
at 600 °C for ...up to 168 h. Oxide microstructure and composition was investigated by SEM/EDX, BIB, TEM and GDOES. Oxidised Ni
2
Al
3
shows minimal weight gain, while adding KCl results in a small weight loss consistent with evaporation of KCl. On the surface of samples exposed to the gas environment only, a 30 nm oxide of Al oxide was present, but where KCl was present as deposit, 50-250 nm thick nodules form that are enriched in K, O and Al.
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