Corrosion Behavior of SiCf/SiC Composites in High Temperature Fluoride Salt Environment

doi:10.15541/jim20170010

Abstract

Abstract:

Corrosion behavior of SiC_f/SiC composites, which were prepared via molten infiltration(MI), polymer impregnation and pyrolysis(PIP), and chemical vapor infiltration process(CVI), respectively, was studied by immersion in 46.5mol% LiF-11.5mol% NaF- 42.0mol% KF eutectic salt at 800℃. The evolution of phase composition and microstructure was characterized by X-ray diffraction and scanning electron microscope with energy dispersive spectrometer. The SiC matrix of SiC_f/SiC composites, derived from different preparation process, has different compositions and microstructures, leading to various compatibility with fluoride salt at high temperature. Residual free Si in MI-SiC matrix and O-contained phase in PIP-SiC matrix are the weak area with worse corrosion resistance in liquid fluoride salt and will be corroded preferentially in terms of thermodynamic. Due to the higher purity and better crystallinity over the MI-SiC matrix and PIP-SiC matrix, CVI-SiC matrix shows more excellent compatibility with fluoride salt at high temperature and has the smallest corrosion rate of 0.0445 μg/(mm²·h). Least weak area confined to the O-rich boundaries between deposited layers may account for the good corrosion resistance of CVI SiC matrix.

Key words: SiC_f/SiC composites, preparation processing, molten fluoride salt, corrosion behavior

CLC Number:

TQ174

WANG Hong-Da, ZHOU Hai-Jun, DONG Shao-Ming, WANG Zhen, HU Jian-Bao, FENG Qian. Corrosion Behavior of SiC_f/SiC Composites in High Temperature Fluoride Salt Environment[J]. Journal of Inorganic Materials, 2017, 32(11): 1133-1140.

Figures/Tables 9

Table 1 SiCf/SiC composites prepared by different processes

Process	MI	PIP	CVI
Raw materials	Si + Resin	PCS	MTS
Architecture	3D braid	Plain woven	Plain woven
Interphase	~100 nm PyC	~100 nm PyC	~100 nm PyC
Fiber volume fraction	~42%	~35%	~40%

Table 2 Impurities of molten salts FLiNaK

Impurities	Metal cations						Acid ions and chloride ions
Impurities	Mg	Ca	Al	Cr	Fe	Ni	Cl^-	NO^2-	NO^3-	PO₄^3-	SO₄^2-
Content/×10^-6	12	13	8	7	13	17	8.1	≤5.0	25.8	≤5.0	7.7

Fig. 1 Mass loss ratio and apparent mass loss rate of MI, PIP and CVI samples after corrosion in molten fluoride salt at 800℃ for 100 h

Fig. 2 XRD patterns of samples (a) MI, (b) PIP and (c) CVI before and after corrosion

Table 3 Element composition of points in MI-SiC matrix/at%

Point	1	2	3	4	5
C	-	54.87	57.59	-	54.07
Si	100	45.13	42.41	100	45.93

Fig. 3 Surface and cross-section microstructures of MI samples (a,b) before and (c,d) after corrosion

Fig. 4 Standard Gibbs free energy of formation per molecule of F2 for the salt constituents and the silicofluoride formed from Si and SiC in the tested MI-SiC matrix at 850℃

Fig. 5 Microstructure evolution of PIP samples: surface (a) before and (b) after corrosion, corrosion section (c) before and (e, f) after corrosion, and (d) EDS linerscan analysis results of dark gray area

Fig. 6 Microstructure evolution of CVI samples: surface (a) before and (c,e) after corrosion, corrosion section (b) before and (d,f) after corrosion.

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