Microstructural Evolution of Slurry Fe Aluminide Coatings During High Temperature Steam Oxidation A. Agüero, M. Gutiérrez, R. Muelas and seibersdorf research Ein Unternehmen der Austrian Research Centers. K. Spiradek, S. Höfinger HTCPM Les Embiez, May 2008
Future steam power plants will increase their operating temperature from the current 550º C to 600-650º C in order to increase efficiency. Steam oxidation of steels becomes significant at these higher temperatures
P91 SAMPLES FROM KOMET 650 (BYPASS OF A STEAM POWER PLANT- Alstom). 3000 h AT 642 C
Main issues related to oxidation - Reduction of component cross section (important for critical components) - Thermal insulation in heat exchangers causing overheating and higher energy requirements - Erosion damage of down-stream components caused by spallation - Efficiency reduction by blockage
Results of steam oxidation are costly After Masuyama, 2004
Efforts to develop steels with both high creep strenght and oxidation resistance at 650º C are underway. Findings so far indicate that 9-10 wt. % in Cr seems to be the upper limit to produce creep resistance materials. However, in order to provide adequate oxidation resistance the Cr % needs to be higher
Coatings could allow the use of high strengh ferritic/martensitic steels with low steam oxidation resistance Within EC COST actions 522 and 536 as well as the EC project SUPERCOAT, different type of coatings have been explored for this application. Other groups in the EC, USA and Japan are also active in this field
Explored coatings are based mostly, on protective oxide (Cr, Al, Si) former alloys or intermetallics deposited by: CVD pack cementation slurry application thermal spray, etc.
Slurry coatings are economical, practical and can be deposited at the component manufacturing or assembly site
STEAM TURBINE
Results available to present indicate that slurry aluminide coatings do not affect the mechanical properties of the substrate (creep strenght and thermo-mechanical fatigue) Field tests (Westfallen) at 640 º C carried out by Alstom indicate protective behaviour for at least 21,000 h
SLURRY ALUMINIDE COATING EXPOSED TO SUPERCRITICAL STEAM AFTER AT 640º C (Turbine Bypass) SLURRY ALUMINIDE COATING EXPOSED TO SUPERCRITICAL STEAM AFTER AT 640º C
COATING CHARACTERIZATION AT DIFFERENT EXPOSURES TO STEAM AT 650º C
SLURRY DEPOSITION
Slurry Applied Aluminide Coating on P92 P92 = C: 0.1, Mn: 0.5, Si: 0.03, Cr: 8.8, Ni: 0.06, Mo: 0.4, W: 1.8, V: 0.20, S: 0.006, N: 0.046 Fe: bal (wt. %,) Al slurry 100 µm As deposited After heat treatment at 700º C for 10 h
COATING CHARACTERIZATION SEM-EDS (INTA) XRD (INTA) TEM-ELECTRON DIFFRACTION (SIEBERDORF)
STEAM OXIDATION LABORATORY TEST (ATMOSPHERIC PRESSURE)
Slurry Aluminide Coating before exposure Outer Layer Fe 2 Al 5 Fe 2 Al 5 FeAl wt.% 47.5 Al 49.4 Fe 4 Cr 100 µm stress relieving cracks appear on cooling
Slurry Aluminide Coating before exposure Outer Layer Al 9 Cr 4 precipitates Fe 2 Al 5 100 µm stress relieving cracks appear on cooling
Slurry Aluminide Coating - before exposure Intermediate Layer Fe 2 Al 5 FeAl The top layer was partially removed by grinding 100 µm 3 µm FeAl wt.% Al: 28.2 Fe: 67.0 Cr: 4.8
Slurry Aluminide Coating before exposure Bottom Zone FeAl 1 µm SE Fe 3 µm AlN precipitates Al N 100 µm 3 µm
Slurry Aluminide Coating before exposure Al 9 Cr 4 precipitates 3 µm Fe 2 Al 5 3 µm FeAl AlN precipitates 100 µm FeAl Al: 28.2, Fe: 67.0 and Cr: 4.8 w% stress relieving cracks appear on cooling
Slurry Aluminide Coating Exposed to Steam at 650º C 58 P-92 Weigth Variation (mg/cm2) 48 38 28 18 8 Pure Aluminide Sup15 6 y cos 175-2 0 10000 20000 30000 40000 50000 60000 Time (h)
Samples exposed to steam at 650º C were characterized after 24, 4600, 8000, 17000 and 40000 h and 50000 h
Slurry Aluminide Coating 24 h under steam at 650º C Outer Layer Kirkendall porosity -2-21 0-21 Fe 2 Al 5 Same grain, TEM fo tilted ~20 020-201
Slurry Aluminide Coating 24 h under steam at 650º C Outer Layer Al 9 Cr 4-2-22 2-31
Slurry Aluminide Coating 24 h under steam at 650º C Top Oxide Hexagonal χ-al 2 O 3!!! 0-11 -210 Minour amounts of γ- and α- Al 2 O 3 could also be observed
Slurry Aluminide Coating 4600 h under steam at 650º C Kirkendall porosity FeAl Fe 2 Al 5 + FeAl FeAl 100 µm AlN precipitates Kirkendall porosity develops Fe 2 Al 5 slowly transforms into FeAl
Slurry Aluminide Coating 8000 h under steam at 650º C Kirkendall porosity Fe 2 Al 5 + FeAl FeAl 100 µm AlN precipitates Kirkendall porosity increases Fe 2 Al 5 further transforms into FeAl
Slurry Aluminide Coating 17000 h under steam at 650º C FeAl wt.% 9 8 1 µm AlN precipitates 50µm 1 µm Only FeAl is left Al Cr
Slurry Aluminide Coating 17000 h under steam at 650º C β-feal 50 µm
Slurry Aluminide Coating 17000 h under steam at 650º C 50 µm 111 311 200 500 nm AlN [0,-1,1]
Slurry Aluminide Coating 17000 h under steam at 650º C 1 µm 200 nm 003 2-13 600 nm α-al2o3 2-13
Slurry Aluminide Coating 40000 h under steam at 650º C Al 2 O 3 10µm 7 wt.% Al 8 wt.% Cr Al 2 O 3 The protective oxide fills the cracks. The AlN precipitation zone has significantly increased in thickness
Slurry Aluminide Coating 50000 h under steam at 650º C 4 wt.% Al 8 wt.% Cr 10 µm COS 175_ 50000h Mixed Al,Fe Oxide 20 µm The protective oxide has significantly increased in thickness Kirkendall porosity degree appears to decrease
SUMMARY 1 Slurry aluminide coatings protect ferritic steels from steam oxidation up to at least 55,000 h at 650ºC
SUMMARY 2 Degradation mostly occur by interdiffusion Aluminium inwards diffusion causes: - depletion of Al beneath the protective scale until it reaches a critical value below which Fe, Al mixed oxides form instead of Al 2 O 3 - precipitation of AlN as it advances within the N containing sustrate Fe outwards diffusion causes: - reduction of the Al % beneath the protective scale - development of Kirkendall porosity at the coating substrate interface
SUMMARY 2 (cont) MICROSTRUCTURE EVOLUTION AT 650ºC 30µm 30µm 30µm 0 h 10000 h 20000 h
SUMMARY 3 Protective Al 2 O 3 forms at the initial stages at 650º C under steam and slowly transforms from χ to α phase χ-al 2 O 3 is also protective Protective oxide spallation does not appear to be significant At 4 < Al < 7 wt. %, Fe Al mixed oxides begin to develop χ- Al 2 O 3 α - Al 2 O 3 (Al, Fe) oxide Early stages (Al wt. % 7) (Al wt. % < 7)
SUMMARY 3 (cont.) PROPOSED Al 2 O 3 TRANSFORMATION MECHANISM Al(OH) 3 χ-al 2 O 3 γ-al 2 O 3 α-al 2 O 3 (Al, Fe) oxides Very Fast slow (7 > Al > 4 wt. % )
ACKNOWLEDGEMENTS the members of the Area of Metallic Materials at INTA the Spanish Inter-Ministerial Commission of Science and Technology (CICYT) for financial support the Austrian Research Promotion Agency Ltd. (FFG) the Austrian Research Centers Seibersdorf for financial support the European Commission for providing funds for the COST 536 action the COST 536 members
THANK YOU!!!!!!!
BACKUP UNDIFFUSED AL SLURRY COATING EXPOSED TO STEAM Oxidation Diffusion Before exposure After 1000 h of exposure to steam at 650ºC Diffusion competes with steam oxidation through the as deposited coating porosity
CREEP TEST OF P91 COATED WITH ALUMINIDE AT 650ºC 650 ºC
THERMO-MECHANICAL FATIGUE TESTING 1600 Max Temperature : 650 C Min Temperature : 200 C Phase Shift : 180 Heating/Cooling Rate : 5º/s Mechanical Strain at 650 C = 0 Gauge diameter: 4.5 mm Nf 1400 1200 1000 800 600 400 Uncoated Slurry Aluminide 200 0 0.6 0.5 0.3 Strain, % Substrate vs. Slurry aluminide