Protective Aluminide Based Coating

High temperature steel corrosion is common in today’s power generation systems, causing the failure of heat exchangers, heat recovery systems, boilers and turbine blades. Corrosion costs US industries ~$250 B per year [1]. Further, the impact of high temperature corrosion spans the field of new power generation technologies, and is only increasing in next-generation power systems.

In the case of solid oxide fuel cells (SOFCs), the key barrier to technology commercialization is the high system cost, which is driven by materials selection. The U.S. DOE cost goal of $700/kW for SOFC power generation systems requires the selection of common grade stainless steel wherever possible. However, without protective coatings, the air-facing surfaces of steel components oxidize and volatize chromium species that poison the stack. Similarly, air/exhaust heat exchangers must be designed to prevent chromium volatilization; on the fuel side, oxidation (due to high steam content gas streams), carburization and coking are all catastrophic failure mechanisms for metals facing high carbon fuel streams.

Metallic Interconnects: Ferritic stainless steel interconnects (ICs) are used in SOFCs because they offer lower cost and simpler manufacturability than alternatives. However, these alloys cannot achieve the target service lifetimes without protective coatings to prevent oxidation and chromium volatilization.

Balance of Plant: In addition to metal interconnects and the conductive stack gasket, there are numerous electrically insulating metallic components which require oxidation protection and chrome containment. These sub-systems include fuel stream conditioning, thermal management equipment, residual fuel recirculation/consumption systems, as well as piping and control systems.

NexTech has demonstrated the feasibility of a spray-based, low-cost aluminization process for applying alumina/aluminide coatings to metallic components. For the non-active (sealing surfaces) areas of metallic interconnects, a modified firing process has been developed compatible with the processing of the manganese cobaltite (MCO) active area coating. This has enabled the aluminide coating and MCO coating to be co-deposited, allowing significant cost reductions to be realized. Interconnects have been successfully dual-coated with MCO/aluminide coatings using an Aerosol Spray Deposition (ASD) process.

Aluminide Based Coatings for the Protection of Metallic Components in SOFC Systems Acknowledgements: NexTech gratefully acknowledges the support of the U.S. Department of Energy, Contract Number: DE-SC0008203

For further details please contact: Neil Kidner, Sr. Research Engineer, NexTech Materials (614) 842-6606 x126. n.kidner@nextechmaterials.com

Aluminide coatings oxidize to form self-repairing alumina scales that enhance alloy corrosion resistance in high temperature applications. Conventionally, aluminide coatings are produced by vapor deposition or pack-cementation approaches -- batch processes that require controlled atmospheres [2].

For aluminide diffusion coatings to be commercially viable for SOFC applications, lower cost coating processes are required. Researchers at Pacific Northwest National Laboratory (PNNL) have recently demonstrated a reactive air aluminization (RAA) process which does not require a controlled atmosphere heat treatment [3,4]. Outside SOFCs, similar air-fired spray/slurry aluminization processes have been reported by the PARTICOAT consortium in Europe to deposit complete thermal barrier coatings (TBC) systems on nickel-based superalloys [5,6].

Low-cost aluminization approaches

P N N L R e a c t i v e - A i r A l u m i n i z a t i o n ( R A A ) [ 3 ]

PA R T I C O AT m u l t i f u n c t i o n a l a l u m i n i d e b a s e d T B C [ 5 ]

1. http://www.bccresearch.com/report/north-american-ceramic-coatings-market-tech-avm015f.html

2. V. Rohr, PhD thesis, Institut National Polytechnique de Toulouse (2005).

3. J. P. Choi, K. S. Weil, Aluminization of metal substrate surfaces, US Patent Application, US2010/097341 A1

4. J. P. Choi et al., Reactive Air Aluminization, PNNL Report 20859 (2011)

5. X. Montero et al., A Single Step Process to Form In-Situ an Alumina/Aluminide TBC System for Alloys in

Extreme Environments at High Temperatures, Surf. Coat. Technol. 206 (2011) 1586.

6. M. Juez-Lorenzo et al., Diffusion Aluminide Coatings using Spherical Micro-Sized Aluminium Particles, Defect

and Diffusion Forum, 289-292 (2009) 261

7. H. Arabi et al., Formation Mechanism of Silicon Modified Aluminide Coating on Ni-Base Superalloy, Int. J. Eng.

Sci. 19(5-1) (2008) 39.

8. B. A. Pint et al, Performance of Al-Rich Oxidation Resistant Coatings for Fe-Base Alloys, Mat. Sci. Corr. 62(6)

(2011) 549

Thick chromia scale Significant spallation

Thick carbon deposit

Coated Uncoated Coated Uncoated

Pre

Post

Aluminum enriched, chromium depleted sub-surface diffusion zone

10-50 m

Thin, alumina surface layer

Coated Bottom Manifold

Coated Top Manifold

Flaked deposit from bottom manifold

Seal Material (PNNL G18 glass)

Discolored glass on non-coated tube

S u b s t r a t e : F e r r i t i c s t a i n l e s s s t e e l A L 4 4 1 H P A l u m i n i d e c o a t i n g : 2 0 m f i r e d | S t a n d a r d P r o c e s s i n g C o n d i t i o n s

Cross-Section Aluminide Coating Al and Cr Compositional Profiles

Long-Tem Microstructural Analysis

S u b s t r a t e : F e r r i t i c s t a i n l e s s s t e e l A L 4 4 1 H P A l u m i n i d e c o a t i n g : 2 0 m f i r e d | S t a n d a r d P r o c e s s i n g Te s t C o n d i t i o n s : 9 0 0 ° C | A i r | 1 0 0 0 h o u r s

Post-mortem microstructural analysis after 1000 hours at 900 °C confirms that the aluminide coating is effective at preventing the thick chromia scale present on uncoated stainless steel from forming. Instead a thin alumina scale forms, reducing chromium volatility and improving oxidation resistance. The coating can be fired in either oxidizing or reducing environments, offering flexibility to combine the aluminization process with other coating operations.

Coating/Seal Interactions Extension to Balance of Plant Alloys

Manufacturing Improvements

References

S u b s t r a t e | 3 0 4 a u s t e n i t i c s t a i n l e s s s t e e l A l u m i n i d e c o a t i n g | 2 0 m S t a n d a r d P r o c e s s i n g O x i d a t i o n Te s t i n g | 5 0 0 h o u r s , H u m i d i f i e d A i r, 9 0 0 ° C

14th Annual SECA Workshop | Pittsburgh | July 23-24th 2013

Coating | Localized Heat Treatment Al, Cr, and Fe Compositional Profiles

S u b s t r a t e : F e r r i t i c s t a i n l e s s s t e e l A L 4 4 1 H P A l u m i n i d e c o a t i n g : 2 0 m f i r e d | S t a n d a r d P r o c e s s i n g C o n d i t i o n s C o m p o s i t e g l a s s s e a l ( 8 0 % a l u m i n a , 2 0 % P N N L G 1 8 g l a s s ) A g e i n g | 4 h o u r s | 8 5 0 ° C

S u b s t r a t e : F e r r i t i c s t a i n l e s s s t e e l | G l a s s : P N N L G 1 8 g l a s s w i t h a n a p p r o p r i a t e b i n d e r A l u m i n i d e c o a t i n g : 2 0 m f i r e d | S t a n d a r d P r o c e s s i n g

S u b s t r a t e : F e r r i t i c s t a i n l e s s s t e e l A l u m i n i d e c o a t i n g : 2 0 m L o c a l i z e d H e a t Tr e a t m e n t

Coating of Complex Component (1/4” 316 Stainless Steel Coil)

S u b s t r a t e : ¼ ” 3 1 6 s t a i n l e s s s t e e l c o i l A l u m i n i d e C o a t i n g : 2 0 m f i r e d | S t a n d a r d P r o c e s s i n g

Significantly enhanced oxidation and chromium volatility resistance has been demonstrated for aluminide coated (fired both in air and with the MCO coating compatible firing process) versus uncoated ferritic stainless steel.

Uncoated Coated | Air Firing Coated | Reduction Firing

Chromia scale

Alumina scale

Alumina scale

The aluminide coating reduces interactions between the seal material (alkaline aluminosilicate glasses) and chromium, present in chromia forming stainless steels. These interactions form low thermal expansion chromates (i.e., SrCrO4 and BaCrO4) that cause failure during thermal cycling.

Uncoated

Coated

XRD Spectra of Composite Glass Seals in contact with uncoated and Coated Ferritic Stainless Steel

By reducing chromate formation, a significant improvement in thermal cycling performance of coated manifolds (glass seal, comprising PNNL G18 glass, binder) was demonstrated using a seal-test stand.

Seal Testing

The aluminide coating process readily translates to other alloys including ferritic (430), austenitic (304, 316) stainless steel and nickel based alloys. The coating microstructures are similar to those produced by traditional pack-cementation/ vapor deposition techniques.

Coating Microstructures versus Traditional Coating Techniques

I n c o n e l 6 1 7 N i - b a s e d a l l o y I N 7 3 8 L C P a c k C e m e n t a t i o n [ 7 ]

G r a d e 3 0 4 G r a d e 3 0 4

C V D p r o c e s s [ 8 ]

Austenitic Stainless Steel (Grade 304) Nickel Based Alloys

Introduction and Motivation Manufacturing (coating application, firing and post-firing cleaning) strategies have been developed to scale the coating technology to coat large, complicated components including 5 foot long piping. A surface pre-treatment has been developed to allow the coating to be uniformly applied to weldments.

Post Coating Post Firing

Interior Surface | Edge

Interior Surface | Center of Coil

S u b s t r a t e : N i c k e l b a s e d a l l o y A l u m i n i d e c o a t i n g | 2 0 m | S t a n d a r d P r o c e s s i n g W e l d p r e - t r e a t m e n t

Application of the Coating to Weld Areas

S u b s t r a t e | 3 0 4 a u s t e n i t i c s t a i n l e s s s t e e l A l u m i n i d e c o a t i n g | 2 0 m S t a n d a r d P r o c e s s i n g C o k i n g Te s t | 2 4 h o u r s | G a s C o m p o s i t i o n : 1 8 % C H 4 4 5 % C O 2 3 7 % C O | 6 0 0 ° C

Oxidation Resistance

Aluminide Coating Microstructure | Compositional Analysis

Aluminum rich interface

Chromium penetrating into the glass

Adherent top layer

Aluminum diffusion layer

Non-uniform coating - formation of large islands

Chromate XRD peaks

S u b s t r a t e | 3 1 6 a u s t e n i t i c s t a i n l e s s s t e e l A l u m i n i d e c o a t i n g | 2 0 m S t a n d a r d P r o c e s s i n g

NexTech is developing a low-cost alternative process to conventional aluminization to provide a protective coating to improve high temperature oxidation/corrosion and chromium volatility resistance of metallic components used in SOFC systems.

Coking Resistance

Interfacial layer

Diffusion layer

Isothermal Oxidation | Weight Gain Chromium Volatility Resistance

S u b s t r a t e : F e r r i t i c s t a i n l e s s s t e e l A L 4 4 1 H P A l u m i n i d e c o a t i n g : 2 0 m | S t a n d a r d P r o c e s s i n g O x i d a t i o n : 1 0 0 0 h r | A i r | 9 0 0 ° C

S u b s t r a t e : F e r r i t i c s t a i n l e s s s t e e l A L 4 4 1 H P A l u m i n i d e c o a t i n g : 2 0 m | S t a n d a r d P r o c e s s i n g C r v o l a t i l i t y : 1 0 0 0 h r | a i r | 9 0 0 ° C C r v o l a t i l i t y : E D S o f C r c a p t u r i n g m a t e r i a l