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Journal of Chinese Society for Corrosion and protection  2023, Vol. 43 Issue (3): 587-593    DOI: 10.11902/1005.4537.2022.141
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Growth Mechanism of Aluminide Coating on T92 Steel Prepared by Slurry Aluminizing at 700 ℃
GONG Bingbing1, LIU Guangming1(), AN Chunxiang2, MEI Linbo2, ZHANG Bangyan1
1.School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China
2.Shanghai Electric Power Plant Equipment Co. Ltd., Steam Turbine Plant, Shanghai 201306, China
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Abstract  

The aluminizing process of T92 steel was conducted by slurry aluminizing method. The growth kinetics of the aluminized layer with a slurry layer thickness of about 80 μm at 700 ℃ was tested. The surface and cross-sectional morphology and composition of the aluminide coating were characterized by using SEM, EDS and XRD respectively. The results show that the active Al atoms ([Al]) diffused inward to form Fe2Al5 (η) phase in the first three hours of aluminizing. Then [Al] began to accumulate at the interface of the slurry/the formed aluminide coating, while Fe diffused outward rapidly resulted in the formation of the outer portion of the coating composed of mixed-phases Fe2Al5 (η) and FeAl3 (θ). After 10 h, the inner portion of FeAl (B2) phase could be clearly observed in the aluminized coatings. The mechanism of aluminized coating growth process and the relevant diffusion process of metallic components was discussed from the point of view of kinetics and thermodynamics. The thickness of the η-phase layer formed by the diffusion of [Al] with time can be expressed by equation of x=18.40t1/2-4.80.

Key words:  T92 steel      slurry aluminizing      kinetics      thermodynamics     
Received:  15 June 2022      32134.14.1005.4537.2022.141
ZTFLH:  TG174  
Fund: National Natural Science Foundation of China(51961028)
Corresponding Authors:  LIU Guangming, E-mail: gemliu@126.com

Cite this article: 

GONG Bingbing, LIU Guangming, AN Chunxiang, MEI Linbo, ZHANG Bangyan. Growth Mechanism of Aluminide Coating on T92 Steel Prepared by Slurry Aluminizing at 700 ℃. Journal of Chinese Society for Corrosion and protection, 2023, 43(3): 587-593.

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https://www.jcscp.org/EN/10.11902/1005.4537.2022.141     OR     https://www.jcscp.org/EN/Y2023/V43/I3/587

Fig.1  XRD patterns of T92 steel after aluminizing at 700 ℃ for 1 and 4 h
Fig.2  Cross-sectional morphologies of T92 steel after aluminizing at 700 ℃ for 0.5 h (a), 1 h (b), 2 h (c), 3 h (d), 4 h (e) and 10 h (f), and EDS analysis results for 1 h (g) and 4 h (h) aluminized samples
AreaFeAlCrW
126.1470.323.070.47
210.2834.7650.134.83
322.8774.532.60-
442.4749.597.100.84
Table 1  Concentrations of main elements in the marked areas in Fig.2 (atomic fraction / %)
Fig.3  Thickness (x) of η phase layer as a function of time
Fig.4  Thickness (x) of η phase layer in the aluminized coating as a function of square root of time
SubstanceΔH2980 / (J·mol-1)ΔS2980 / (J·K-1 mol-1)
Fe027.15
Al028.33
B2-48483.1450.74
η-201636.29153.61
θ-111368.8895.29
Table 2  Thermodynamic data of all phases in the aluminized coating[21]
ReactionT / KΔrH / (J·mol-1) (933-1298 K)
Fe+Al→B2298-933-48483.14-11.20(T-298)-0.0338(T 2-2982)
933-1298-58950.14-11.20(T-298)-0.0338(T 2-2982)
2Fe+5Al→η298-933-201636.29-109.17(T-298)-0.1152(T 2-2982)
933-1298-253971.29-109.17(T-298)-0.1152(T 2-2982)
Fe+3Al→θ298-933-111368.88-50.74(T-298)-0.0638(T 2-2982)
933-1298-142769.88-50.74(T-298)-0.0638(T 2-2982)
Table 3  Relationships between molar phase transition enthalpy and temperature for the reactions of Fe and Al
ReactionT / KΔrS / (J·mol-1) (933-1298K)
Fe+Al→B2298-933-4.74-11.20lnT / 298-0.0338(T-298)
933-1298-4.74-11.20lnT / 298-0.0338(T-298)-10467 / T
2Fe+5Al→η298-933-42.34-109.17lnT / 298-0.1152(T-298)
933-1298-42.34-109.17lnT / 298-0.1152(T-298)-52335 / T
Fe+3Al→θ298-933-16.85-50.74lnT / 298-0.0638(T-298)
933-1298-16.85-50.74lnT / 298-0.0638(T-298)-31401 / Τ
Table 4  Relationships between molar entropy change and temperature for the reactions of Fe and Al
ReactionΔrG / (J·mol-1) (298-1298 K)
Fe+Al→B2-48483.14-21.2724(T-298)+4.74+11.20TlnT / 298
2Fe+5Al→η-201636.29-143.4996(T-298)+42.34+109.17TlnT / 298
Fe+3Al→θ-111368.88-69.7524(T-298)+16.85+50.74TlnT / 298
Table 5  Relationships between molar Gibbs free energy change and temperature for the reactions of Fe and Al
Fig.5  ΔrHphase-T (a) and ΔrGphase-T (b) curves for the different reactions of Fe and Al
Fig.6  Growth model of aluminizing coating on T92 steel: (a) VD>VP, (b) VD=VP, (c) VD<VP
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