No significant difference was observed for metallic and oxide coatings between laboratory and field exposure. However, a significant difference was observed for the hardmetal coatings because the field test caused less corrosion degradation. A possible explanation may be provided by the following two facts:
a) The actual ash deposit in the boiler is composed of a mixture of salts, not all as corrosive as KCl. In other words, the ash deposits are less corrosive than pure KCl.
b) The metal temperature was lower in the field tests (525 °C vs. 550
°C).
The milder corrosive environment in the boiler gave enough time for the reactive secondary carbide precipitates to ripe (as in Ostwald ripening i.e. disappearance of small precipitates and growth of large ones), as described in section 9.2.1. The result was a more resistant microstructure in a less corrosive environment. All these factors together prevented the fast corrosion mechanism involving the secondary carbide precipitates, as described in section 10.6.
11 CONCLUSIONS
This work evaluated the high-temperature corrosion resistance of nickel alloys and different hardmetal coatings under KCl salt deposit in the laboratory and in an actual boiler environment. The study aimed to evaluate the effect of high-temperature chlorine-induced corrosion on the complex microstructure of thermally sprayed coatings.
The novelties introduced by this work can be summarised as follows:
a) Better understanding of the effect of the microstructural and compositional features of NiCrMoNb and chromium carbide-based coatings in terms of their high-temperature corrosion behaviour.
b) The combination of thermal analyses and oven corrosion tests allowed the evaluation of onset temperatures and the corrosion mechanism for high-temperature chlorine-induced corrosion in chromium carbide-based thermally sprayed coatings.
c) New insights on the high-temperature corrosion properties of different ceramic coating compositions under KCl deposit.
Below, the above-mentioned novelties are presented in more detail by answering the research questions posed in section 1.2.
1) Chlorine-induced high-temperature corrosion of thermally sprayed Alloy 625 coatings
a) How do the different microstructural features, resulting from different thermal spray coating methods, affect the corrosion resistance?
The coatings showed excellent corrosion resistance both in the laboratory and in the actual environment. The HVOF and TWAS NiCrMoNb coatings outperformed the HVAF and CS coatings. In the HVAF coatings, unmelted particles hindered proper intersplat contact and, therefore chlorine was able to penetrate the coating surface via intersplat crevices and porosity, which were identified as microstructural weak
points. Better intersplat contact was observed in the TWAS and HVOF coatings thanks to the more efficient melting of the coating particles, while the consequently increased amount of oxides at the particle interfaces appeared to be unharmful.
b) How differently are alloying elements such as Mo and Cr subjected to high-temperature corrosion in Alloy 625 coatings?
Different elements were observed to provide different levels of protection based on local environmental conditions. Specifically, Ni and Cr ensured excellent protection in the oxidizing-chloridizing atmosphere present on the coating surface, while Mo proved to be beneficial at the particle interface underneath the surface, where oxygen is scarce but chlorine could be abundant.
2) Chlorine-induced high-temperature corrosion of thermally sprayed chromium carbide-based hardmetal coatings
a) What is the onset temperature for chlorine-induced high-temperature corrosion of chromium carbide-based coatings?
Different types of hardmetal coatings were studied. Namely, HVOF- and HVAF- sprayed Cr3C2-25NiCr, Cr3C2-50NiCrMoNb, and Cr3C2-37WC-18NiCoCr coatings.
The coatings were studied with thermal analysis as a screening method to identify the onset temperature of chlorine-induced high-temperature corrosion. The results were validated by oven tests. This method allowed the effective estimation of the onset temperature for hardmetal corrosion. The results indicated that 450 °C could be considered as the highest temperature for safe use under concentrated KCl deposit for Cr3C2-25NiCr and Cr3C2-37WC-18NiCoCr coatings, whereas under the same conditions, Cr3C2-50NiCrMoNb coatings could be used safely up to 500 °C.
b) What is the interaction of the carbide particles with the metal matrix and h ow does it affect the corrosion mechanism?
The corrosion properties of Cr3C2-25NiCr and Cr3C2-50NiCrMoNb coatings, especially in the short term, greatly depend on the interaction between the carbide particle and the metal binder. Specifically, carbide dissolution in the metal matrix that occurs during spraying, which leads to the formation of carbon-saturated regions in the metal binder. Exposure to high temperatures triggers the precipitation
of fine secondary carbides from these regions, which form an interconnected network. The secondary carbide precipitates are very reactive with chlorine, which will rapidly decompose them and leave a network of fine voids in a sponge-like metal matrix structure. The voids increase the active surface of the metal binder, thus accelerating the corrosion rate of the metal itself.
c) How do the different microstructural features, resulting from different thermal spray coating methods, affect the corrosion resistance?
The chemical composition and microstructure of the coatings had the most significant impact on their corrosion resistance, where high alloying and low carbide dilution were beneficial. As for the carbide type, WC was more stable than Cr3C2
below 550 °C. Although the reason is still unclear, the TGA results seem to indicate that HVAF spraying could produce more corrosion-resistant hardmetal coatings with higher onset temperature, possibly due to lower carbide dissolution.
3) High-temperature corrosion properties of ceramic oxide coatings under KCl deposit
a) Are some oxide coatings more stable than others?
HVOF and APS Cr2O3-, TiO2- and Al2O3-based coatings were studied at different temperatures under KCl deposit. The chemical stability of the oxide composition in KCl can be ranked as follows:
Al2O3-based oxides > TiO2 >> Cr2O3-based oxides
where Al2O3-based oxides are the most unreactive under KCl deposit.
b) Are the oxide coatings stable in KCl-rich high-temperature environments?
Al2O3-based oxides were inert up to 720 °C, TiO2 coatings showed only a mild superficial material loss while Cr2O3-based coatings suffered severe degradation at temperatures as low as 550 °C. Cr2O3 suffered high corrosion degradation due to the formation of a low-melting eutectic compound between the corrosion product K2Cr2O7 and the KCl deposit.
Interestingly, the severe degradation of the Cr2O3 coating could be greatly mitigated by a small addition of TiO2, as little as 3 wt.%, to the coating composition. This aspect might have implications in the design of metal alloys which rely on oxide scale formation to ensure high-temperature corrosion protection. For example, chromia-forming alloys, such as stainless steels and NiCr alloys, might be improved if engineered in a way to induce the formation of a small amount of titania in the chromia scale.
c) Are thermally sprayed oxide coatings viable coating materials for high-temperature corrosion protection?
Al2O3, ZrO2 and TiO2 ceramic coatings are chemically stable high temperature under KCl induced corrosion. It was evident that they ware significantly more stable than carbides. This makes them promising reinforcement candidates for replacing carbides where the susceptibility of carbides to chlorine limits their application.
4) Field test in an actual boiler
a) How do the laboratory and long-term field test results compare?
In general, the corrosion rates observed in boiler were remarkably lower than in laboratory scale, presumably due to the lower corrosivity of the real ash deposit compared to the KCl deposit used in laboratory. Therefore, the most inert coatings such as Alloy 625 and the oxide ceramics were subjected to negligible corrosion damage while the carbide-based coatings reported only minor damage.
b) How do the corrosion mechanisms differ compared to that observed in laboratory tests?
The most significant difference between laboratory and field test was observed for the Cr2C3-NiCr coatings as the attack to fine carbide precipitates did not occur and the sponge-like morphology did not form. The reason for this was attributed to the lower corrosivity of the actual ash deposit compared to pure KCl. The lower corrosivity gave time for the fine secondary carbides precipitates to ripe into bigger particles, which reduced the active surface and disrupted the precipitate network.
Therefore, the actual boiler corrosion resulted in being relatively mild. The use of thermodynamic calculations supported the analysis of the results.
12 SUGGESTIONS FOR FUTURE WORK
Improvements of the high-temperature corrosion properties of carbide-based hardmetal coatings under KCl deposit could be achieved if further research focused on limiting the carbide dissolution in the metal binder, by studying alternative metal and reinforcement compositions, manufacturing techniques and process parameters.
For example, it appears that finer powders could facilitate the formation of a better intersplat contact, especially in cold spray and HVAF coatings. Therefore, the coating performance could be improved significantly by powder feedstock optimization.
The new insights on the stability of thermally sprayed oxide materials could inspire further research on TiO2 and Al2O3 as a hard phase replacement for carbide-based hardmetal coatings for corrosive KCl-bearing high-temperature environments.
The protective properties of thermally sprayed oxides are promising, and further research could even enable their actual application as reliable high-temperature corrosion protective coatings.
Moreover, given the improved properties of Cr2O3 with small additions of TiO2, further research could be carried out to study the protective properties of chromia-titania scale on metal alloys.
13 BIBLIOGRAPHY
[1] European environmental agency, EEA Report No 3/2017, Renewable energy in Europe 2017:
recent growth and knock-on effects, Copenhagen, 2017.
[2] T. Abbasi, S. Abbasi, Biomass energy and the environmental impacts associated with its production and utilization, Renewable and Sustainable Energy Reviews, Vol. 14, No.
3, 2010, pp. 919-937.
[3] M. Varela, Y. Lechón, R. Sáez, Environmental and socioeconomic aspects in the strategic analysis of a biomass power plant integration, Biomass and Bioenergy, Vol. 17, No.
5, 1999, pp. 405-413.
[4] P. Nuss, S. Bringezu, K.H. Gardner, Waste-to-materials: the long-term option, in: Waste to Energy, Springer, 2012, pp. 1-26.
[5] G.A. Antaki, Piping and pipeline engineering: design, construction, maintenance, integrity, and repair, CRC Press, 2003.
[6] L. Pawlowski, The science and engineering of thermal spray coatings, John Wiley & Sons, 2008.
[7] G.Y. Lai, High-temperature corrosion and materials applications, ASM International, 2007.
[8] Y. Kawahara, Application of high temperature corrosion-resistant materials and coatings under severe corrosive environment in waste-to-energy boilers, Journal of Thermal Spray Technology, Vol. 16, No. 2, 2007, pp. 202-213.
[9] R. Jafari, E. Sadeghi, High-temperature corrosion performance of HVAF-sprayed NiCr, NiAl, and NiCrAlY coatings with alkali sulfate/chloride exposed to ambient air, Corrosion Science, Vol. 160, 2019, pp. 108066.
[10] M. Mandø, Direct combustion of biomass, in: Biomass Combustion Science, Technology and Engineering, Woodhead Publishing, 2013, pp. 61-83.
[11] C. Yin, L. A. Rosendahl, S. K. Kær, Grate-firing of biomass for heat and power production. Progress in Energy and combustion Science, 34(6), 2008, pp 725-754.
[12] L. Tobiasen, B. Kamuk, Waste to energy (WTE) systems for district heating. In Waste to Energy Conversion Technology, Woodhead Publishing, 2013, pp. 120-145.
[13] D.M. Tagare, Thermal Power Generation, Steam Generators, 2011.
[14] I.P. Prevention, Control Reference Document on the Best Available Techniques for Waste Incineration, European Commission, Brussels, August 2006.
[15] R. Bender, M. Schütze, The role of alloying elements in commercial alloys for corrosion resistance in oxidizing‐chloridizing atmospheres. Part II: Experimental investigations, Materials and Corrosion, Vol. 54, No. 9, 2003, pp. 652-686.
[16] M. Schütze, M. Malessa, V. Rohr, T. Weber, Development of coatings for protection in specific high temperature environments, Surface and Coatings Technology, Vol. 201, No. 7, 2006, pp. 3872-3879.
[17] N. Otsuka, Chemistry and melting characteristics of fireside deposits taken from boiler tubes in waste incinerators, Corrosion Science, Vol. 53, No. 6, 2011, pp. 2269-2276.
[18] N. Otsuka, Effects of fuel impurities on the fireside corrosion of boiler tubes in advanced power generating systems—a thermodynamic calculation of deposit chemistry, Corrosion Science, Vol. 44, No. 2, 2002, pp. 265-283.
[19] A. Roine, HSC chemistry, Outokumpu Research Oy, Pori, Finland, 2002.
[20] H. Nielsen, F. Frandsen, K. Dam-Johansen, L. Baxter, The implications of chlorine-associated corrosion on the operation of biomass-fired boilers, Progress in energy and combustion science, Vol. 26, No. 3, 2000, pp. 283-298.
[21] F.J. Frandsen, Utilizing biomass and waste for power production—a decade of contributing to the understanding, interpretation and analysis of deposits and corrosion products, Fuel, Vol. 84, No. 10, 2005, pp. 1277-1294.
[22] R. Saidur, E. Abdelaziz, A. Demirbas, M. Hossain, S. Mekhilef, A review on biomass as a fuel for boilers, Renewable and Sustainable Energy Reviews, Vol. 15, No. 5, 2011, pp. 2262-2289.
[23] M. Spiegel, A. Zahs, H. Grabke, Fundamental aspects of chlorine induced corrosion in power plants, Materials at high temperatures, Vol. 20, No. 2, 2003, pp. 153-159.
[24] H. Grabke, E. Reese, M. Spiegel, The effects of chlorides, hydrogen chloride, and sulfur dioxide in the oxidation of steels below deposits, Corrosion Science, Vol. 37, No. 7, 1995, pp. 1023-1043.
[25] N. Folkeson, T. Jonsson, M. Halvarsson, L. Johansson, J. Svensson, The influence of small amounts of KCl (s) on the high temperature corrosion of a Fe‐2.25 Cr‐1Mo steel at 400 and 500° C, Materials and Corrosion, Vol. 62, No. 7, 2011, pp. 606-615.
[26] Y. Alipour, High temperature corrosion in a biomass-fired power boiler: Reducing furnace wall corrosion in a waste wood-fired power plant with advanced steam data, KTH Royal Institute of Technology, 2013.
[27] A. Phongphiphat, C. Ryu, Y.B. Yang, K.N. Finney, A. Leyland, V.N. Sh arifi, J.
Swithenbank, Investigation into high-temperature corrosion in a large-scale municipal waste-to-energy plant, Corrosion Science, Vol. 52, No. 12, 2010, pp. 3861-3874.
[28] P. Rademakers, W. Hesseling, J. Van de Wetering, S.C. Brussel, L. Tange, M. R.
Montaigne, Review on corrosion in waste incinerators, and possible effect of bromine, TNO Industrial Technology, 2002, pp. 18-25.
[29] H. Kinnunen, D. Lindberg, T. Laurén, M. Uusitalo, D. Bankiewicz, S. Enestam, P. Yrjas, High-temperature corrosion due to lead chloride mixtures simulating fireside deposits in boilers firing recycled wood, Fuel Processing Technology, Vol. 167, 2017, pp. 306-313.
[30] J. Niemi, H. Kinnunen, D. Lindberg, S. Enestam, Interactions of PbCl2 with Alkali Salts in Ash Deposits and Effects on Boiler Corrosion, Energy & Fuels, Vol. 32, No. 8, 2018, pp. 8519-8529.
[31] H. Kinnunen, M. Hedman, M. Engblom, D. Lindberg, M. Uusitalo, S. Enestam, P. Yrjas, The influence of flue gas temperature on lead chloride induced high temperature corrosion, Fuel, Vol. 196, 2017, pp. 241-251.
[32] A.L. Robinson, H. Junker, L.L. Baxter, Pilot-scale investigation of the influence of coalbiomass cofiring on ash deposition, Energy & Fuels, Vol. 16, No. 2, 2002, pp. 343 -355.
[33] A.L. Robinson, H. Junker, S.G. Buckley, G. Sclippa, L.L. Baxter, Interactions between coal and biomass when cofiring, Symposium (International) on Combustion, Elsevier, pp. 1351-1359.
[34] L.A. Hansen, H.P. Nielsen, F.J. Frandsen, K. Dam-Johansen, S. Hørlyck, A. Karlsson, Influence of deposit formation on corrosion at a straw-fired boiler, Fuel Processing Technology, Vol. 64, No. 1, 2000, pp. 189-209.
[35] M. Zevenhoven, P. Yrjas, M. Hupa, Ash‐Forming Matter and Ash‐Related Problems, Handbook of Combustion: Online, 2010, pp. 493-531.
[36] J. Oh, M. McNallan, G. Lai, M. Rothman, High temperature corrosion of superalloys in an environment containing both oxygen and chlorine, Metallurgical Transactions A, Vol. 17, No. 6, 1986, pp. 1087-1094.
[37] A. Zahs, M. Spiegel, H. Grabke, The influence of alloying elements on the chlorine‐ induced high temperature corrosion of Fe‐Cr alloys in oxidizing atmospheres, Materials and Corrosion, Vol. 50, No. 10, 1999, pp. 561-578.
[38] Y. Shinata, Accelerated oxidation rate of chromium induced by sodium chloride, Oxidation of Metals, Vol. 27, No. 5-6, 1987, pp. 315-332.
[39] J. Pettersson, H. Asteman, J. Svensson, L. Johansson, KCl induced corrosion of a 304-type austenitic stainless steel at 600 C; the role of potassium, Oxidation of Metals, Vol. 64, No. 1-2, 2005, pp. 23-41.
[40] C. Pettersson, L. Johansson, J. Svensson, The influence of small amounts of KCl (s) on the initial stages of the corrosion of alloy Sanicro 28 at 600° C, Oxidation of Metals, Vol. 70, No. 5-6, 2008, pp. 241-256.
[41] T. Jonsson, N. Folkeson, J. Svensson, L. Johansson, M. Halvarsson, An ESEM in situ investigation of initial stages of the KCl induced high temperature corrosion of a Fe–
2.25 Cr–1Mo steel at 400 C, Corrosion Science, Vol. 53, No. 6, 2011, pp. 2233-2246.
[42] M. McNallan, High-temperature corrosion in halogen environments, Materials Performance;(United States), Vol. 33, No. 9, 1994.
[43] J. Indacochea, J. Smith, K. Litko, E. Karell, A. Rarez, High-temperature oxidation and corrosion of structural materials in molten chlorides, Oxidation of Metals, Vol. 55, No. 1-2, 2001, pp. 1-16.
[44] S.D. Cramer, B.S. Covino, ASM Handbook Vol. 13 A Corrosion: Fundamentals, Testing, and Protection, Materials Park, OH: ASM International, 2003.1135, 2003.
[45] P. Vuoristo, Thermal Spray Coating Process, Comprehensive Materials Processing, 2014.
[46] E. Sadeghi, N. Markocsan, S. Joshi, Advances in Corrosion-Resistant Thermal Spray Coatings for Renewable Energy Power Plants. Part I: Effect of Composition and Microstructure, Journal of Thermal Spray Technology, 2019, pp. 1-40.
[47] R. Maev, V. Leshchynsky, Cold-Spray Coatings: Recent Trends and Future perspectives, Low-Pressure Cold Spray, 2018, pp. 95-142.
[48] H. Koivuluoto, Microstructural Characteristics and Corrosion Properties of Cold-Sprayed Coatings, Tampereen teknillinen yliopisto.Julkaisu-Tampere University of Technology. Publication; 882, 2010.
[49] A. Verstak, V. Baranovski, Activated combustion HVAF coatings for protection against wear and high temperature corrosion, Thermal Spray, Vol. 1, 2003, pp. 58.
[50] C. Lyphout, S. Björklund, M. Karlsson, M. Runte, G. Reisel, P. Boccaccio, Screening design of supersonic air fuel processing for hard metal coatings, Journal of Thermal Spray Technology, Vol. 23, No. 8, 2014, pp. 1323-1332.
[51] A. Milanti, V. Matikainen, H. Koivuluoto, G. Bolelli, L. Lusvarghi, P. Vuoristo, Effect of spraying parameters on the microstructural and corrosion properties of
HVAF-sprayed Fe–Cr–Ni–B–C coatings, Surface and Coatings Technology, 277, 2015, pp.
81-90.
[52] M. Oksa, E. Turunen, T. Suhonen, T. Varis, S. Hannula, Optimization and characterization of high velocity oxy-fuel sprayed coatings: techniques, materials, and applications, Coatings, Vol. 1, No. 1, 2011, pp. 17-52.
[53] J.R. Davis, Handbook of thermal spray technology, ASM international, 2004.
[54] web page. Available (accessed 02/29/2019): https://hausnerinc.com/industrial-hard-chrome-plating-services/twas/.
[55] J. Davis, Stainless steel cladding and weld overlays, ASM Specialty Handbook: Sta inless Steels G, Vol. 6398, 1994, pp. 107-119.
[56] ASM International. Handbook Committee, ASM Handbook: Corrosion: Environments and Industries, ASM International, 2006.
[57] G.Y. Lai, Corrosion Mechanisms and Alloy Performance in Waste-To-Energy Boiler Combustion Environments, 12th Annual North American Waste-to-Energy Conference, American Society of Mechanical Engineers Digital Collection, pp. 91-98.
[58] N. Imoudu, Y. Ayele, A. Barabadi, The characteristic of cold metal transfer (CMT) and its application for cladding, 2017 IEEE International Conference on Industrial Engineering and Engineering Management (IEEM), IEEE, pp. 1883-1887.
[59] V. Wesling, A. Schram, M. Kessler, Low heat joining–manufacturing and fatigue strength of brazed, locally hardened structures, Advanced Materials Research, Trans Tech Publ, pp. 347-374.
[60] David French, Metallurgical failures in fossil fired boilers, John Wiley & Sons, 1993.
[61] U. Brill, J. Kloewer, D.C. Agarwal, Influence of alloying elements on the chlorination behavior of nickel-and iron-based alloys, 1996.
[62] L. Nylöf, E. Häggblom, Corrosion of experimental superheater alloys in waste fuel combustion, 1997.
[63] S. Cho, J. Hur, C. Seo, S. Park, High temperature corrosion of superalloys in a molten salt under an oxidizing atmosphere, Journal of Alloys and Compounds, Vol. 452, No.
1, 2008, pp. 11-15.
[64] P. Elliott, A. Ansari, R. Prescott, M. Rothman, Behavior of Selected Commercial Alloys during High Temperature Oxychloridation, Corrosion, Vol. 44, No. 8, 1988, pp. 544-554.
[65] G. Sorell, The role of chlorine in high temperature corrosion in waste-to-energy plants, Materials at high temperatures, Vol. 14, No. 3, 1997, pp. 207-220.
[66] P. James, L. Pinder, The impact of coal chlorine on the fireside corrosion behavior of boiler tubing: A UK Perspective, Corrosion97, OnePetro, 1997.
[67] S. Brooks, D. Meadowcroft, The influence of chlorine on the corrosion of mild and low alloy steels in sub-stoichiometric combustion gases, Corrosion resistant materials for coal conversion systems, 1982, pp. 105-120.
[68] T. Flatley, E. Latham, C. Morris, Co-extruded tubes improve resistance to fuel ash corrosion in UK Utility Boilers, Materials Performance, Vol. 20, No. 5, 1981, pp. 12-17.
[69] M. Uusitalo, P. Vuoristo, T. Mäntylä, Elevated temperature erosion–corrosion of thermal sprayed coatings in chlorine containing environments, Wear, Vol. 252, No.
7, 2002, pp. 586-594.
[70] M. Uusitalo, High temperature corrosion and erosion-corrosion of coatings in chlorine-containing environments, Tampere University of Technology Tampere, 2003.
[71] M. Uusitalo, R. Backman, L. Berger, P. Vuoristo, T. Mäntylä, Oxidation resistance of carbides in chlorine-containing atmospheres, High Temperature Materials and Processes, Vol. 21, No. 5, 2002, pp. 307-320.
[72] M.A. Uusitalo, R. Backman, L. Berger, P.M. Vuoristo, T.A. Mäntylä, Stability of Carbides in Chlorine-Containing High-Temperature Environments, Key Engineering Materials, Trans Tech Publ, pp. 497-500.
[73] H. Rojacz, A. Zikin, C. Mozelt, H. Winkelmann, E. Badisch, High temperature corrosion studies of cermet particle reinforced NiCrBSi hardfacings, Surface and Coatings Technology, Vol. 222, No. 0, 2013, pp. 90-96.
[74] H.S. Sidhu, B.S. Sidhu, S. Prakash, Hot corrosion behavior of HVOF sprayed coatings on ASTM SA213-T11 steel, Journal of Thermal Spray Technology, Vol. 16, No. 3, 2007, pp. 349-354.
[75] B. Wang, K. Luer, The relative erosion-corrosion resistance of commercial thermal sprayed coatings in a simulated circulating fluidized bed combustor environment, 1994.
[76] B.Q. Wang, Chromium–titanium carbide cermet coating for elevated temperature erosion protection in fluidized bed combustion boilers, Wear, Vol. 225, 1999, pp.
502-509.
[77] B. Wang, S.W. Lee, Erosion–corrosion behaviour of HVOF NiAl–Al2O3 intermetallic-ceramic coating, Wear, Vol. 239, No. 1, 2000, pp. 83-90.
[78] K.U. Kainer, Metal matrix composites: custom-made materials for automotive and
[78] K.U. Kainer, Metal matrix composites: custom-made materials for automotive and