Ni/AlN Composite Coating for Corrosion and Elements Interdiffusion Resistance in Molten Fluoride Salts System

doi:10.1007/s40195-021-01224-2

Abstract

Abstract:

The Ni/AlN composite coating was prepared for increasing corrosion and elements interdiffusion resistance of GH3535 alloy in molten fluoride salts, and the effect of mechanical interlocking on adhesion strength between AlN layer and nickel coating was also studied. Results indicated that the adhesion strength between AlN layer and nickel coating could be significantly enhanced through mechanical interlocking effect, which effectively prevented the nickel coating from flaking off at elevated temperature. Through an etching pre-treatment of AlN layer, the corrosion resistance of the Ni/AlN coated GH3535 alloy in molten FLiNaK salt was further improved, and elements interdiffusion between the substrate and nickel coating was completely suppressed. AlN layer as a diffusion barrier remained compact and continuous in Ni/GH3535 system after high-temperature molten salt corrosion. Moreover, a Ni-P layer consisting of Ni₃P and Ni phases formed in the Ni coating after corrosion.

Key words: Molten salt, Superalloy, Composite coating, High temperature corrosion

Chengxu Wang, Wei Chen, Minghui Chen, Demin Chen, Fuhui Wang. Ni/AlN Composite Coating for Corrosion and Elements Interdiffusion Resistance in Molten Fluoride Salts System[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(12): 1704-1714.

Figures/Tables 17

Table 1 Chemical composition (wt%) of as-received GH3535 superalloy

C	Mo	Fe	Cr	Mn	Si	Al	W	Ti	P	S	B	Ni
0.015	17.1	4.03	7.03	0.77	0.59	0.03	0.02	< 0.01	0.004	0.001	0.0009	Bal

Table 2 Conditions of electroless nickel-plating pre-treatment

Stage	Aqueous solution composition	Procedure (at 25 °C)
Sensitization	SnCl₂ (10 g/L), HCl 40 mL/L	Time: 3-5 min
Activation	PdCl₂ (0.5 g/L), C₂H₅OH 500 mL, H₂O 500 mL	Time: 3-5 min
Reduction	NaH₂PO₂·H₂O 30 g/L	Time: 10 s-1 min

Table 3 Chemical composition of the aqueous solution used for electroless nickel-plating

Components of solution	Content
NiSO₄·6H₂O	30 g/L
NaH₂PO₂·H₂O	20 g/L
Na₃C₆H₅O₇·2H₂O	10 g/L
NaC₂H₃O₂·3H₂O	20 g/L
H₂SO₄	To keep pH: 4-5

Fig. 1 Schematic illustration of the preparation route for AEN coated GH3535 alloy

Fig. 2 Nail used for connecting sample and equipment of Romulus Universal Materials Tester

Fig. 3 Surface morphology a, XRD pattern b, cross-sectional morphology c, STEM image and electron diffraction rings d of the as-deposited AlN interlayer

Fig. 4 Surface morphology of AlN layer after etching in 20% NaOH solution for 1 min

Table 4 Chemical composition of AlN layer after etching in 20% NaOH solution for 1 min, unit: at.%. (P1: in the pit of AlN surface, P2: at the platform of AlN surface)

Position	Al	N
P1	53.12	46.88
P2	47.30	52.70

Fig. 5 STEM image of the as-deposited AEN coated GH3535 alloy a, TEM b, HREM c images of the interface between the AlN layer and GH3535 substrate

Fig. 6 Cross-sectional morphologies of AEN a, b, AN c coated GH3535 alloys after annealing at 500 °C for 4 h

Fig. 7 Cross-sectional morphologies of AN a, AEN b coated GH3535 alloys after adhesion strength experiment

Fig. 8 Cross-sectional SEM images of AN a, AEN b coated GH3535 alloy after corrosion in molten FLiNaK salt at 700 °C for 100 h, and corresponding EDS line scans c of AEN coated GH3535 alloy after corrosion

Fig. 9 Graph of mean coefficient of linear thermal expansion of two samples of GH3535 alloy

Table 5 Chemical compositions of cross section and surface of nickel coating of AEN coated GH3535 alloy after the molten salt exposure, unit: at.%. (P3: at the center of cross section of nickel coating, P4: at surface of nickel coating)

Position	Ni
P3	100
P4	100

Fig. 10 Element mappings of the AlN/substrate interface after corrosion in molten FLiNaK salt at 700 °C for 100 h

Fig. 11 Surface morphologies of the nickel coating of AEN coated GH3535 alloy before a, after b corrosion in molten FLiNaK salt at 700 °C for 100 h

Fig. 12 Cross-sectional SEM image of AEN coated GH3535 alloy after corrosion a, STEM images of Ni coating b, Ni-P layer c, the SAED image of Ni3P d, element mapping of Ni-P layer: Ni e, P f

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