New chemical mechanism explaining the breakdown of protective oxides on high temperature steels in biomass combustion and gasification plants

Biomass is considered a replacement fuel over fossil fuels to mitigate climate change. The switch to biomass in the combustors changes the inorganic chemistry of the flue gases and leads to more severe corrosion of the construction materials of the combustors. The integrity of most high temperature steels relies on the formation of a protective Cr2O3 layer on the steel surface at a high temperature environment. The ash compound found on the heavily corroded steel in biomass combustion and gasification plants is KCl, but the mechanism, which triggers the breakdown of the protective Cr2O3 layer under the KCl salt is not known. We studied the chemical reactions involved with furnace exposure of KCl and KOH with Cr2O3 and identified the formed reaction products with XRD analysis. The amount of reaction products was analyzed from the leachates of the salt-oxide mixtures by UV/VIS spectroscopy. We also used thermodynamic Gibbs energy minimization calculations to evaluate the evolution of reactions as a function of temperature. The results suggests that the reaction of KCl with Cr2O3 involves a KOH reaction intermediate that forms before K2CrO4 is formed. The amount of reacted potassium as a function of temperature follows the trend of KCl decomposition to KOH and HCl(g) as predicted by thermodynamics calculations. Therefore, we argue that the suggested overall reaction of KCl with Cr2O3 as found in the corrosion literature: , starting with the initiation step: KCl + H2O(g) ⇒ KOH + HCl(g) and then the formed KOH reacts with Cr2O3 to form K2CrO4. This explains the initial breakdown of the protective Cr2O3 under KCl salt in water containing high temperature atmospheres. The result is essential for the development of new alloys for biomass fired combustors.


Introduction
Burning coal is responsible for approximately 25% of anthropogenic CO 2 emissions. The largest use of coal as a fuel is in electricity and heat production, which accounts for about 16% of the total CO 2 emissions. 1 Replacing coal with sustainable biomass as the fuel in power boilers would lead to substantial reductions in the net CO 2 emission. Also, SO 2 emissions would be diminished due to the generally much lower sulphur content of biomass based fuels compared to coal. However, biomass combustion leads to more severe fouling and corrosion issues of the heat transfer surfaces in boilers. [2][3][4][5] One way to minimize these problems is to operate the boilers with lower steam temperatures, but that lowers the efficiency of the Rankine cycle leading to higher emission factors per unit of electricity produced.
Potassium behaviour in the boiler has been identied as the major cause of slagging, fouling and corrosion in biomass red boilers. 6 Potassium is found associated in the fuel moisture as water soluble salts, or reacted with the organic functional groups (carboxylic, alkoxy, phenolic) of the lignocellulosic matrix. 7 During combustion, potassium is released in the gas phase as elemental potassium, potassium chloride or potassium hydroxide as measured by mass spectrometric or optical methods. [8][9][10] Due to its extreme reactivity, elemental potassium is expected to react to compounds in the vicinity of the biomass particle it is released from. Therefore, the major gas phase potassium compounds in the biomass boiler ue gases are KCl(g) and KOH(g). These compounds may react further with other fuel elements, such as silicon or sulphur to form potassium silicates or potassium sulphate. K 2 O-SiO 2 silicate formation plays a major role in deposit formation in the furnace (slagging) and in bed agglomeration issues in uidized bed boilers, where silica containing sand is oen used as the bed material. [11][12][13] Formation of K 2 SO 4 plays a major role in aerosol and deposit formation further downstream of the boiler. 14,15 K 2 SO 4 formation may decrease the corrosion rate of the heat exchangers forming a less corrosive deposit than KCl or KOH/ K 2 CO 3 . Its formation may however, also increase fouling rate of the heat exchangers forming tenuous deposits that are difficult to be removed by soot blowing, even though the corrosion rate under the deposits may still be low. 16,17 During their path through the boiler, KCl(g) and KOH(g) in the ue gases condense out when the gas temperature is decreased below their dew points. Condensation can happen directly on the heat exchanger surfaces or on the ash particles present in the ue gas ow. Purely homogeneous nucleation is also possible in conditions where the ue gas does not contain enough foreign surfaces that can act as nucleation sites for heterogeneous nucleation. 18,19 Aer condensation KCl(s,l) can react heterogeneously further to K 2 SO 4 (s,l) and KOH(s,l) can react to K 2 SO 4 (s,l), KCl(s,l) or K 2 CO 3 (s,l) as predicted by thermodynamic stabilities of the compounds. 20,21 KCl induced corrosion of Fe-Cr steels has been studied extensively in the scientic literature. [22][23][24][25][26][27][28] Recently, the effect of K 2 CO 3 on the high temperature corrosion has gained more interest. It appears both potassium and chlorine are important in the corrosion reactions with steels. It has been suggested that potassium can initiate the destruction of the protective oxide, but chlorine is needed to sustain the corrosion. 29,30 KOH induced corrosion has not been extensively studied in the context of high temperature corrosion in biomass red boilers, but earlier studies have shown that Fe-Cr alloys are unsuitable for service in KOH containing high temperature environments. [31][32][33] Earlier work of the author on the elemental balances of the deposit forming elements in biomass based fuels suggest that KOH(g) condensation may be more important in fouling and corrosion than has previously been thought. The details of K 2 CO 3 (s,l) formation on the heat exchanger surfaces has not been claried yet, but its formation has been predicted by thermodynamics when the (Cl(g) + 2S(g)) molar content in the ue gases is lower than the molar K(g) content. K 2 CO 3 (s,l) is formed on the heat exchanger surfaces likely via a surface reaction of adsorbed KOH(ads.) with CO 2 (g). Homogeneous formation of K 2 CO 3 (g) in the gas phase, followed by condensation of K 2 CO 3 (g) is less likely, because of the thermodynamic instability of K 2 CO 3 (g). 34,35 K 2 CO 3 (s,l) has also been directly detected in some boiler deposits. 17 In this work we studied in detail the reactivity of KCl and KOH towards Cr 2 O 3 and Fe 2 O 3 , the protective oxide components formed on the Fe-Cr alloys in high temperature oxidizing service conditions. The results may also be of interest for chromite ore roasting by KOH and for understanding corrosion of Cr 2 O 3 containing refractory bricks in potassium and chlorine containing environments. 36 molar ratios were 1 and K 2 CrO 4 /Cr 2 O 3 molar ratio was 0.5 in order to have the same K/metal molar ratio in all of the mixtures. A 10 g batch of each mixture was prepared in one go in a screw cap sealed bottle. From the 10.0 g batch bottles, 1.00 g samples were weighted to a 10 ml sintered Al 2 O 3 crucible and then loaded immediately in a muffle furnace (Nabertherm P330) that was at the isothermal exposure temperature (100-800 C). The samples were exposed in the furnace for 2 hours in ambient air atmosphere. Aer furnace exposures, the samples were cooled in ambient air so that handling of the crucible was possible with nitrile gloves (5-10 min) and then transferred to glass bottles that were sealed with screw caps. Then the samples were placed in a desiccator cabin for storage. During the analyses, the sample exposure times to ambient air before starting the analyses were minimized by opening the cap and preparing the sample from the bottle only just before starting the analysis. However, the XRD analysis took approximately an hour per sample, therefore possible reactions during the analysis with ambient air could not be completely eliminated. KOH especially is known to be highly hygroscopic and reactive towards CO 2 during exposure to ambient air. in the samples in concentrations high enough to be visible to the naked eye.

XRD measurements
XRD analysis was done with PANalytical X'pert Pro PW 3040/60 powder diffraction spectrometer with monochromated Cu Kalpha X-ray source (a ¼ 1.5406Å). Samples were rst grinded manually in a mortar to make a visually homogeneous powder. Then the sample holder was lled evenly with the powder, pressed against a at surface to level the sample surface with the sample holder top surface and then XRD q-2q scans were recorded from 10-90 . The automatic phase identication algorithm of the X'Pert HighScore Plus program was used for preliminary identication of the phases. Then the results were checked manually and the most likely crystalline phases where manually identied using the JCPDS cards. The XRD cards used for phase identications were 00-021-0645 for KOH, 00-011-0655 for K 2 CO 3 $1.

UV/VIS spectroscopic measurements of CrO 4 2À
Approximately 0.20 g of sample powder was mixed with 20 ml of room temperature ion exchanged water. The solution was rst stirred in a beaker with a magnetic stirrer for 15 min. Then the solution was ltered using qualitative lter paper, 410 (Cat. No. 516-0802 from VWR), the residue washed two times with 10 ml of room temperature ion exchanged water. Then the ltrates were transferred to 50 ml volumetric asks and the ask was lled to the mark with ion exchanged water and mixed. reported to be the stable form regardless of the Cr-concentration. 39 Therefore, the K 2 Cr 2 O 7 possibly present originally in the samples was detected as CrO 4 2À and the concentration determined reects the sum of K 2 CrO 4 + K 2 Cr 2 O 7 originally in the sample. The pH values of the ltrates were determined to be $7 with a pH indicator sticks (Fisher Scientic number 10642751). All the KCl based samples had pH ¼ 7 and KOH based samples had pH ¼ 7 in samples exposed at $ 500 C and pH 8-12 in samples exposed at #400 C. The higher pH with low temperature samples in case of KOH mixtures was caused by the KOH that was not reacted to K 2 CrO 4 , but formed K 2 CO 3 $1.5H 2 O during the furnace/ambient exposures. When dissolving in water, K 2 CO 3 $1.5H 2 O results in a basic solution. Concentration standards were prepared by dissolving known amounts of K 2 CrO 4 powder to ion exchanged water in 50 ml volumetric asks (ESI †). Then the absorbance of the standards were measured and linear concentration-absorbance curves were established. Then the absorbance of the sample ltrates were measured and the concentrations were determined using the standard curves. In cases where the sample had CrO 4 2À concentration so high that the absorbance was higher than the standards, the sample was diluted with pure water in volumetric asks with 1 : 10 or 1 : 20 dilution ratios, which ever was suitable to bring the absorbance value in between the standards. Molar ratios reacted to K 2 CrO 4 and K 2 Cr 2 O 7 were calculated as follows: Error estimation of the above described method for K 2 CrO 4 determination was done by running a few duplicate runs with KOH + Cr 2 O 3 mixtures (2 at 300 and 400 C exposure temperatures). Because of the small number of duplicate samples, error was estimated with the range rather than with statistical methods. The error range (max-min) of the method was found to be about 0.08 (or 8%-points). This error value was assumed to be similar at other exposure temperatures and also for the leaching tests described below for Fe 3+ .
2.6 UV/VIS spectroscopic measurements of Fe 3+ 2.6.1 Water soluble Fe 3+ . Approximately 0.20 g of sample powder was mixed with 20 ml of room temperature ion exchanged water. The solution was stirred in a beaker with a magnetic stirrer for 15 min. Then the solution was ltered using qualitative lter paper 410 (Cat. No. 516-0802 from VWR), the residue washed two times with 10 ml of room temperature ion exchanged water. Then the ltrates were transferred to 50 ml volumetric asks and the ask was lled to the mark with ion exchanged water and mixed. The Fe 3+ concentrations of the ltrates were determined by UV/VIS spectroscopy using Shimadzu UV-2600 spectrophotometer. Absorbance at 225 nm was used for Fe 3+ . Fe 3+ ion in a water solution is present as Fe 3+ , Fe(OH) 2+ and Fe(OH) 2 + ions or as non-water soluble Fe(OH) 3 precipitate, depending on the pH of the solution. 40 The pH values of the ltrates were determined with a pH indicator sticks (Fisher Scientic number 10642751). All the KCl based samples had pH ¼ 7 and all the KOH based samples had pH ¼ 12. The high pH value over the entire exposure temperature in case of KOH based samples indicate that they may have contained some unreacted KOH/K 2 CO 3 or then the higher pH at ring temperatures $ 500 C (which did not reveal any KOH/ K 2 CO 3 residues by XRD) was originated from the dissolution of KFeO 2 (KFeO 2 + 2H 2 O ¼ K + + Fe 3+ + 4OH À ). In order to shi the equilibrium so that all the dissolved Fe(III) was in the Fe 3+ state, the ltrates were buffered to pH ¼ 1 by mixing 1 : 1 ratio of sample with a solution to 1 mol l À1 HCl (1 : 2 dilution). This pH stabilized solution was then used to ll the quartz cuvette in the UV/VIS absorption measurements. The added Cl-ion can also form complex ions with Fe in the form Fe( . Concentration standards were prepared by dissolving known amounts of FeCl 3 $6H 2 O powder to ion exchanged water and diluting (ESI †). The pH of the standards were buffered to 1 with 1 mol l À1 HCl before measurement as with the samples. Then the absorbance of the standards were measured and linear concentration-absorbance curves were established. Then the absorbance of the sample ltrates were measured and the concentrations were determined using the standard curves. Molar ratios reacted to KFeO 2 were calculated as follows: V ¼ volume, l; m ¼ sample mass, g; M ¼ molar mass, g mol À1 ; [Fe 3+ ] ¼ Fe 3+ ion concentration, mol l À1 . 2.6.2 Acid soluble Fe 3+ . The analysis was done in a similar way as has been discussed for the H 2 O soluble Fe 3+ determination, except that the 15 min leaching steps were done in 20 ml of 1 mol l À1 HCl in water solutions instead of pure water. Also the washing of the ltration residue was done with HCl containing water (z0.1 mol l À1 HCl, pH ¼ 1) instead of pure water. This prevented the precipitation of Fe(OH) 3 that was possible in the water leaching steps, where the pH values of the ltrates were $7. In case of 1 M HCl leaching, some ltrates had a yellow colour visible already to a naked eye, indicating that indeed the acid leaching resulted in much higher dissolution of the reaction products compared to H 2 O leaching. All the ltrates had a pH value of 1, measured with pH indicator sticks (Fisher Scientic number 10642751). Because the pH of the ltrates were stabilized to one already during the leaching step, they were used directly to ll the quartz cuvette in the UV/VIS absorption measurements. In cases where the sample had Fe 3+ concentration so high that the absorbance was higher than the standards, the sample was diluted in volumetric asks with 1 : 2, 1 : 50 or 1 : 100 dilution ratios, which ever was suitable to bring the absorbance value in between the standards. In the 1 : 50 and 1 : 100 dilution cases, the dilution was done by adding 1 ml of sample, then 5 ml of 1 mol l À1 HCl, and then lling the volumetric ask with pure H 2 O.

Thermodynamic calculations
HSC v6.12 Gibbs energy minimization soware was used in the thermodynamic calculations. 41 The input les for the calculations are presented in Table 1. Air composition 42 with z0.99 mol% H 2 O vapor (RH z 32% at 25 C) was used to simulate the humid ambient gas phase in the muffle furnace. In case of KCl based systems, HCl(g) was added as the possible gas phase Cl-compound released in the reactions. Note that each gas phase component in the atmosphere has at least several times higher absolute amounts than any of the solid phase components. This assures that the amount of the formed products was never limited by the amount of the gas phase component, thus simulating an open ambient system. The calculated systems were kept as simple as possible. Therefore, in addition to the reactants, only the phases identied in the XRD analysis were added to the solid phase input le. In the case of Fe 2 O 3 systems, K 2 Fe 4 O 7 was detected by XRD in the sample with the highest exposure temperature with KOH, but unfortunately it was not found in the HSC v6.12 database and could not be included in the thermodynamic calculation.

KOH-Cr 2 O 3 system
The powder XRD analysis of the system aer furnace exposures is shown in Fig. 1. It appears that KOH reacted partially with the air in the furnace and formed K 2 CO 3 $1.5 H 2 O in samples exposed to furnace temperatures below 500 C. Samples exposed to 500 C or higher ring temperatures did not contain any residual K 2 CO 3 $1.5 H 2 O or KOH. This was also reected by the pH of the water soluble ltrates as explained in the experimental section. It seems that the reaction of KOH with Cr 2 O 3 , forming K 2 CrO 4 is not fast enough below 500 C and competes with K 2 CO 3 $1.5 H 2 O formation in the experimental conditions used. According to XRD analysis, K 2 CrO 4 formation starts already with a solid-solid reaction at 200 C. Melting point of KOH is 406 C, so it appears that there is no need for molten phase to form in the system before K 2 CrO 4 formation proceeds. The photograph in Fig. 1 reveals the characteristic colour of the CrO 4 2À ion already appearing in the water soluble fraction of the products at room temperature exposures. However, this  water solution and therefore it is considered that the XRD analysis provides a better estimation of the onset temperature where K 2 CrO 4 formation starts. The characteristic orange colour of the Cr 2 O 7 2À ion was detected in samples red at 700 C and 800 C. K 2 Cr 2 O 7 was also detected in the XRD analysis at these ring temperatures. Thermodynamic calculations predict K 2 Cr 2 O 7 to be the most stable reaction product throughout the temperature range used as shown in Fig. 2, but its formation appears to be kinetically prevented below 700 C. The experimental results clearly show that K 2 CrO 4 formation is kinetically preferred at the studied temperature range and exposure time used. Like explained above, K 2 CrO 4 formation itself seemed also to be kinetically controlled at temperatures lower than 500 C. The maximum CrO 4 2À amount (>90% of the theoretical) in the reaction products was measured at 500 C furnace exposure, aer which the amount decreased slightly at higher temperatures. Taking into account the experimental errors associated with the CrO 4 2À analysis, it is suggested that starting at 500 C, practically all the KOH had reacted to K 2 CrO 4 and that the slightly lower amounts of CrO 4 2À detected at higher exposure temperatures can be explained by slight loss of KOH by evaporation, competing with the reaction with Cr 2 O 3 .

K 2 CrO 4 -Cr 2 O 3 system
In order to determine if the K 2 Cr 2 O 7 formation at higher ring temperatures proceeds directly from the reaction with KOH or via rst formation of K 2 CrO 4 and then further reaction with Cr 2 O 3 , the K 2 CrO 4 -Cr 2 O 3 system was studied with XRD analysis. Results are presented in Fig. 3

KCl-Cr 2 O 3 system
Compared to KOH, KCl is substantially less reactive with Cr 2 O 3 . As shown in the XRD analysis in Fig. 4, there were no clear signs of reaction with Cr 2 O 3 until the highest ring temperature, 800 C. At 800 C, an additional peak was detected at 26.95 (2q). This peak coincides with the (021) reection of K 2 Cr 2 O 7 . However, using only one peak in phase identication is not reliable, but based on the analogy with results from the KOH-Cr 2 O 3 and K 2 CrO 4 -Cr 2 O 3 systems, K 2 Cr 2 O 7 was considered to be the most likely reaction product at 800 C. CrO 4 2À was detected qualitatively and quantitatively also at lower ring temperatures as shown in Fig. 4 and 5. Thermodynamic calculation predicted again the formation of K 2 Cr 2 O 7 and K 2 CrO 4 as the major products, but now the discrepancy between the thermodynamic prediction and experimental results was higher than with KOH. CrO 4 2À was detected only in samples exposed to ring temperatures $ 400 C and even at 800 C the CrO 4 2À amount was only z16% of the theoretical maximum. Fig. 6 shows the XRD analysis of the KOH-Fe 2 O 3 system. Reaction with Fe 2 O 3 started at 200 C ring temperature, forming KFeO 2 as the reaction product. KFeO 2 stayed as the major reaction product throughout the temperature range. At 500 C an additional peak at 27.95 (2q) was detected that could not be assigned to KFeO 2 . This peak was assigned to potassium containing magnetite, K 1.75 Fe 1.25 O 4 that has the maximum intensity powder XRD peak at this position, but with one peak only, the identication remains doubtful. At 800 C ring temperature, K 2 Fe 4 O 7 could also be identied as the reaction product in addition to KFeO 2 . There was no indication of higher oxidation state than +3 reaction products of Fe, such as K 2 FeO 4 . This was consistent with the lack of any colour of the water solutions containing the dissolved reaction products shown in Fig. 6. This result reects the more favourable tendency of Cr to adapt oxidation state +6 compared to Fe. The comparison of the measured water soluble and acid soluble Fe 3+ amounts with the thermodynamic prediction are presented in Fig. 7. No water soluble Fe 3+ was found, but the amount of acid soluble Fe 3+ was very close to the thermodynamically predicted amount of KFeO 2 . This difference in the ltrates as a function of pH is caused by the fact that either KFeO 2 is not soluble in water, or that aer initial dissolution of KFeO 2 , rapid formation of insoluble Fe(OH) 3 takes place in the basic solution. The high OH À concentration precipitates the initially dissolved iron and captures it in the ltration residue. In acidic conditions, Fe(OH) 3 formation is prevented and KFeO 2 was dissolved completely in the 15 min leaching step and the dissolved iron stayed in the solution phase during the ltration step.

KCl-Fe 2 O 3 system
As with the KCl-Cr 2 O 3 system, KCl turned out to be much less reactive towards Fe 2 O 3 compared to KOH. There was no sign of any reaction in the XRD analysis or in the visual qualitative analysis of the water soluble reaction products as shown in Fig. 8. Similar to the KOH-Fe 2 O 3 case, no water soluble iron was ] in the filtrates with the thermodynamically predicted amount of the reaction products in the KCl-Cr 2 O 3 system with pure water leaching.
found, but now there was only traces of Fe 3+ detected even in the acid soluble ltrate. The results agree with thermodynamic equilibrium calculations, which predicted only slight reaction in the KCl-Fe 2 O 3 system at temperatures > 600 C to form KFeO 2 as shown in Fig. 9.

Discussion
Our results are consistent with the suggestion that the reaction of KCl with Cr 2 O 3 is initiated by K 2 CrO 4 formation as found in the corrosion literature. 22 44,45 However, combustion environments contain inevitably also H 2 O(g) in the ue gases and therefore KOH formation should not be neglected. Furthermore, many virgin biomass fuels contain very little Cl, so there is simply not much Cl 2 (g) or HCl(g) available in the ue gases when combusting these types of fuels. Still KCl layer has been found on the heat exchanger surfaces and previous work has argued that the corroding Cl 2 (g) or HCl(g) must form from the KCl layer, but the fate of the K when the Cl 2 (g) or HCl(g) forms from KCl has not been studied in detail. In waste incineration, the situation is different, because the feedstock contains typically large amounts of Cl and therefore the direct gas phase attack by Cl 2 (g) or HCl(g) may be more relevant. It may be that the chlorine corrosion  Based on the results in this study, it is proposed that the reactivity of KCl towards both oxides in H 2 O(g) containing environment can simply be explained by the thermodynamics of the decomposition reaction R2. Fig. 10 presents the thermodynamic equilibrium of R2 in ambient moist air. It is suggested that HCl(g) does not play a key role in the reaction with the oxides and that the reactive compound is the formed KOH. The reason that we did not detect any reaction between KCl and Fe 2 O 3 , but did detect K 2 CrO 4 formation with Cr 2 O 3 at >500 C can be explained by the higher thermodynamic stability of K 2 CrO 4 compared to KFeO 2 as shown in Fig. 11. K 2 CrO 4 is much more stable than KFeO 2 , therefore it is suggested that the KOH formed in the decomposition of KCl at >500 C does not form KFeO 2 as readily as K 2 CrO 4 and most of the formed KOH is lost in the gas phase by vaporization before reacting with Fe 2 O 3 . That is why KFeO 2 was not detected in the KCl-Fe 2 O 3 system. In case of Fe-Cr steel corrosion, once the protective oxide is destroyed by KOH and if the bare metals are exposed to the  If KCl diffuses through the oxide scale (grain boundary diffusion), the formation of the reactive chlorine species can also take place at the metal-scale interface: Cr + 2KCl + 2O 2 (g) / K 2 CrO 4 + Cl 2 (g) (R11) Cr + 2KCl + 4H 2 O(g) / K 2 CrO 4 + 2HCl(g) + H 2 (g) (R12) 2Fe + 2KCl + 2O 2 (g) / 2KFeO 2 + Cl 2 (g) (R13) The formed chlorine compounds can further attack the metal leading to the active oxidation mechanism by chlorine. 46 However, it is somewhat arbitrary to speculate about the corrosion reactions with elemental metals, because these reactive metals in the elemental form will thermodynamically favour reaction with practically any reactant. For example, a chlorine free corrosion mechanism involving only KOH may also be suggested:    This journal is © The Royal Society of Chemistry 2019 The formed water or the O 2 (g) diffusing through the oxide scale can further oxidize the underlying metal and form Cr 2 O 3 or Fe 2 O 3 oxides, which are then consumed by KOH according to reactions R3 and R5, and a chlorine free active oxidation mechanism is established. It is argued that KOH diffuses more readily to the metal scale interphase than KCl, because of the lower melting point of KOH (406 C) compared to KCl (773 C) and thus higher mobility at typical service temperatures (400-600 C). Because of the low melting point of KOH, diffusion can also take place in ionic form by K + and O 2À ions diffusing to the metal-scale interphase:   The schematic image in Fig. 12 summarizes the proposed corrosion reactions.

Conclusions
KOH is much more reactive towards Cr 2 O 3 and Fe 2 O 3 than KCl in ambient air environment. Thermodynamic calculations predict the difference in reactivity quite nicely above 400 C. KOH reacts with both oxides at temperatures higher than 200 C while KCl reacts with Cr 2 O 3 at temperatures > 400 C, but no reaction with Fe 2 O 3 was detected in the temperature range from room temperature to 800 C. Both KOH and KCl form K 2 CrO 4 as the reaction product when reacting with Cr 2 O 3 . K 2 CrO 4 can further react with Cr 2 O 3 and form K 2 Cr 2 O 7 at temperatures > 400 C. K 2 CrO 4 and K 2 Cr 2 O 7 reaction products can be easily leached from the mixture with water at room temperature. KOH forms KFeO 2 as the reaction product when reacting with Fe 2 O 3 . KFeO 2 cannot be leached with pure water, but requires an acidic media. In 1 M HCl solution, KFeO 2 can be easily leached at room temperature. The reactivity of KCl towards the protective oxides of Fe-Cr steels in water containing high temperature environments can be explained by its decomposition to KOH and HCl(g) and the subsequent reaction of the formed KOH with the protective oxide. These results are very valuable for the development of materials for boilers, gasiers, furnaces and gas turbines utilizing biomass derived fuels as their energy source.

Conflicts of interest
There are no conicts of interest to declare.