Corrosion-resistant materials for use in unconventional molten carbonate electrolysis environments:

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1 Corrosion-resistant materials for use in unconventional molten carbonate electrolysis environments: Evaluation of Al-diffusion coatings for stainless steel protection in a ternary LiNaK carbonate melt at 500 C under CO 2 gas S. Frangini*, L. Turchetti, C. Felici, A. Bellucci (a) ENEA CR Casaccia, Dept. of Energy, Via Anguillarese 301, Rome, Italy (a) Centro Sviluppo Materiali (CSM), Via di Castel Romano 100, Rome, Italy * stefano.frangini@enea.it 1st International Conference on Electrolysis, Copenhagen, June

2 OUTLINE Introduction Molten Alkali Carbonate salts for advanced electrolysis processes Materials challenges in molten carbonate electrolysis environments Purpose of this work Experimental work Results Conclusions

3 Molten Alkali Carbonate salts for advanced electrolysis processes Several electrolysis processes using MC salt electrolytes are under investigation. Current studies focus on: Energy conversion and storage Hydrogen production by water electrolysis Reversible Molten Carbonate Fuel Cells Sustainable materials synthesis and processing Fe production by electrolysis of iron oxides Cement /lime production high-tech / nanostructured carbon products ( supercapacitors /Li-ion batteries) Environmental issues, i.e., CO 2 mitigation CO 2 capture and electroreduction to solid carbon or CO

4 Molten Alkali Carbonate salts for advanced electrolysis processes mostly-used operating conditions: intermediate temperature range ( C); CO 2 -rich or (CO 2 -H 2 O)-rich atmospheres interestingly, non-electrolyte applications of MC salts are also a growing area of research: Thermal energy storage (Concentrating Solar Power plants) Catalyst for biomass gasification Surface chemical conversion treatments for stainless steels (for corrosion / functional protection of SOFC steel interconnectors)

5 SOLAR HYDROGEN PRODUCTION (STAGE-STE FP7 EU Project) A low-temperature MCE of water ( C) is under investigation at ENEA for solar H2 production. The process may include oxycombustion to capture and reuse CO 2 in a closed loop system Cathode H 2 O+CO 2 + 2e = H 2 +CO 3 2- Anode CO 3 2- = CO O 2 +2e The Scientific and Technological Alliance for Guaranteeing the European Excellence in Concentrating Solar Thermal Energy

6 NOVEL CHEMICAL SURFACE TREATMENTS FOR SOFC INTERCONNECTS (SCORED2.0 FCH JU PROJECT) La-Fe perovskite conversion coatings are obtained by immersion in specially-formulated molten eutectic carbonate baths, under medium/high basicity conditions, at C. LaFeO 3 layer Steel Coatings For Reducing Degradation in SOFC

7 Materials challenges in MCE environments MCE processes involve the use of higher CO 2 gas concentrations than in MCFCs CO 2 promotes carbonate high melt acidity / melt corrosivity Commercial austenitic stainless steels (310S, 316L) and Al-diffusion coatings are resistant to corrosion at C and in air-rich gases (MCFC) But, what about metallic corrosion under the unusual combination of low T (500 C) and high melt acidity (high CO2) as considered in our solar-powered MCE process?

8 Effect of T and CO2 on stainless steel corrosion (MCFC-related conditions) AISI 310S under air-30co 2 gas: 575 C 695 C T effect Frangini et al., J. Power Sources, 160 (2006) CO 2 effect (580 C) Cr Steels do not passivate below 600 C and high CO2: Lim et al., J. Power Sources, 89 (2000) 1-6 high corrosion expected at lower Ts!

9 Materials challenges in MCE environments What about the corrosion resistance of Al-diffusion coatings? Wet-seals areas of the steel bipolar plates the most corrosive MCFC environment for steels. Direct contact with thick melt layers High-CO2 / low O2 conditions Fluctuating gas composition Al-diffusion coated steels are immune to wet-seal corrosion at 650 C due to formation of protective LiAlO2 layer! However, no studies on the effect of T and CO2

10 Purpose of the work Rapid evaluation of corrosion resistance of Al-diffusion coatings in low temperature and acidic molten carbonate salts Aluminide coated 316L stainless steel prepared by pack cementation used as model system Al-diffusion processes thermal spray PVD, CVD Ion Vapor Deposition (IVD) pack cementation slurry painting melt dipping Why the pack cementation process? cost-effective process non line-of sight process (complex shapes) inner / outer surface coatings many parts can be coated simultaneously easy scale-up to industrial production

11 Experimental work: Pack Cementation Process conditions: single-step aluminizing process at C low Al activity process (CoAl pack) Ar gas inert atmosphere NH 4 Cl activator above-the-pack arrangement Samples above the retort Pack-mix in the retort

12 Furnace Experimental work: molten salt corrosion tests Aluminide coated 316L steel samples pre-oxidation treatment in air at 750 C x 48 h partial immersion tests (for thin and thick-melt effects) (500 C x 48 h, dry CO2 atmosphere) standard ternary eutectic carbonate salt Li 2 CO 3 -Na 2 CO 3 -K 2 CO (mol %) post-test SEM-EDX analysis sample Zone in atmosphere Meniscus zone Zone immersed in the melt Alumina crucible melt

13 Effect of the pre-oxidation treatment on coating structure After preoxidation FeAl phase um thickness Fe(Al) AlN precipitate zone Fe/Al= 1.6 Thermal oxidation promotes large void formation on the surface (3-5 um); internal fine Kirkendall porosity not affected

14 Partial Immersion test results sample immersed in the melt meniscus line sample exposed to CO2 atmosphere Pitting-like attack with red rust spots: attack more intense in the meniscus line

15 Molten salt corrosion in the immersed part (thick melt) Immersed part Si-containing (Fe,Al) oxide particles Fe-rich oxides (red spots in OM) FeAl surface; Fe/Al =2 Part exposed to CO2 Fine grained structure absent Contact with the molten salt promotes formation of a fine grained structure on the coating surface Fe-rich spots areas prevalently around the surface voids

16 Molten salt corrosion in the immersed part (thick melt) Exposed to CO2 Immersed in the melt Thickness of the external FeAl layer is reduced by molten salt attack

17 Molten salt corrosion in the meniscus line (thin melt) Al-rich oxide areas Fe-rich oxide particles This means that thin melt layers promote accelerated Fe oxide dissolution much more than in the immersed part (thick melt)

18 Conclusions Al-diffusion coatings are not immune to acidic molten carbonate attack at 500 C, even for a short-term exposure The attack appeared more intense and continuous in the meniscus areas ( thin, more acidic melt) Probably, combination of an acidic melt and low Ts do not promote the formation of a protective LiAlO 2 layer (as opposed to the MCFC case) More corrosion-resistant materials than Al-diffusion coatings are required for low T / high acidic MCE environments, maybe, alumina-forming alloys (AFA) or semi-noble alloys (Cu, Ni alloys)?

19 THANK YOU FOR YOUR ATTENTION! ANY QUESTIONS??