Improvement of Corrosion Resistance of Hastelloy-N Alloy in LiF-NaF-KF Molten Salt by Laser Cladding Pure Metallic Coatings
Abstract
:1. Introduction
2. Materials and Methods
2.1. Materials
2.2. Coating Preparation
2.3. Characterization
3. Results and Discussion
3.1. Corrosion Behavior
3.2. Phase Analysis
3.3. Microstructural Characterization
3.4. Corrosion Mechanisms
4. Conclusions
- Superior corrosion resistance caused by pure Ni and Co metallic coatings. Pure metallic coatings can effectively improve the corrosion resistance of Hastelloy-N alloy in molten fluoride salt, which was confirmed by the much lower corrosion rate of the pure Ni- and Co-coated specimens than that of Hastelloy-N alloy. Significantly, the firstly-reported Co-coated specimen exhibits the optimal stability in the molten fluoride salt without any discernible corrosion.
- Unique microstructure for treated Hastelloy-N alloy. The Hastelloy-N alloy is composed of γ-Ni and M6C, and a new Cr9Mo21Ni20 precipitate after corrosion. In contrast, the pure Ni coating is mainly composed of γ-Ni, NiO, and Cr2O3, while the pure Co coating is mainly composed of γ-Co and Cr2O3. After corrosion in molten fluoride salt, only γ-Ni phase or γ-Co phase exists in the pure metallic coatings, respectively.
- Mechanism understanding on the corrosion behavior. The elemental Cr in Hastelloy-N diffuses and dissolves into the molten fluoride salt, leading to an elemental depleted layer on the alloy surface with severe intergranular corrosion. For pure metal-coated specimens, in contrast, only metal oxides formed during laser cladding process dissolve into the molten fluoride salt. The dense pure metal (Ni or Co) coatings can effectively hinder the penetration of the molten fluoride and thus improve the corrosion resistance of the substrate remarkably.
Author Contributions
Funding
Conflicts of Interest
References
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Element | Ni | Cr | Mo | Fe | Mn | Al | W | Si | C |
Mass Fraction (wt.%) | Balance | 7.01 | 16.80 | 4.16 | 0.52 | 0.28 | 0.20 | 0.36 | 0.06 |
Specimens | Corrosion Weight Loss (mg/mm2) |
---|---|
Hastelloy-N Alloy | 0.3164 ± 0.0002 |
Ni-Coated Specimen | 0.1263 ± 0.0003 |
Co-Coated Specimen | 0.1129 ± 0.0001 |
Oxides | Reactions for Oxide Formation during the Laser Cladding Process | Gibbs Free Energy of Formation of Oxides | Reactions and Gibb’s Free Energy in 900 °C for Oxide Dissolution in the Molten Fluoride Salt |
---|---|---|---|
Cr2O3 | 2Cr + 1.5O2 = Cr2O3 | = −1056.62 + 0.25349T | Cr2O3 + 6HF = 2CrF3 + 3H2O, ΔG = −106.254 kJ/mol |
NiO | Ni + 0.5O2 = NiO | = −210.04 + 0.08467T | NiO + 2HF = NiF2 + H2O, ΔG = −55.779 kJ/mol |
CoO | Co + 0.5O2 = CoO | = −210.17 + 0.06485T | CoO + 2HF = CoF2 + H2O, ΔG = −68.379 kJ/mol |
Particles | Element (at.%) | ||||||||
---|---|---|---|---|---|---|---|---|---|
C | O | Ni | Cr | Si | Fe | Mo | Mn | Al | |
1 (Cr2O3) | 0.07 | 65.10 | 5.60 | 17.53 | 3.98 | – | – | 4.96 | 2.76 |
2 (NiO) | 0.05 | 52.60 | 29.49 | 2.01 | 7.84 | – | 0.72 | 0.72 | 6.60 |
3 (Cr2O3) | 0.06 | 66.22 | 2.52 | 23.42 | – | – | – | – | 7.77 |
4 (γ-Ni) | 0.08 | – | 95.16 | 1.90 | – | 1.81 | 1.05 | – | – |
Particles | Element (at.%) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
C | O | Co | Ni | Cr | Si | Fe | Mo | Mn | Al | |
1 (γ-Co) | 0.06 | – | 57.74 | 31.41 | 2.86 | – | 1.60 | 6.09 | – | – |
2 (γ-Co) | 0.15 | – | 58.58 | 31.43 | 3.28 | – | 1.97 | 4.59 | – | – |
3 (Cr2O3) | 0.06 | 68.89 | 2.81 | 1.05 | 11.72 | 9.83 | – | 0.42 | 3.57 | 1.65 |
4 | 0.04 | 61.98 | 4.85 | 2.40 | 4.98 | 16.03 | – | 0.81 | 5.34 | 3.56 |
5 (γ-Co) | 0.10 | – | 56.61 | 33.47 | 2.66 | – | 1.96 | 5.19 | – | – |
6 (Cr2O3) | 0.03 | 55.80 | 11.59 | 6.57 | 7.71 | 12.53 | 0.64 | 0.99 | 3.61 | 0.54 |
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Zhu, H.; Li, B.; Chen, M.; Qiu, C.; Tang, Z. Improvement of Corrosion Resistance of Hastelloy-N Alloy in LiF-NaF-KF Molten Salt by Laser Cladding Pure Metallic Coatings. Coatings 2018, 8, 322. https://doi.org/10.3390/coatings8090322
Zhu H, Li B, Chen M, Qiu C, Tang Z. Improvement of Corrosion Resistance of Hastelloy-N Alloy in LiF-NaF-KF Molten Salt by Laser Cladding Pure Metallic Coatings. Coatings. 2018; 8(9):322. https://doi.org/10.3390/coatings8090322
Chicago/Turabian StyleZhu, Hongmei, Baichun Li, Minghui Chen, Changjun Qiu, and Zhongfeng Tang. 2018. "Improvement of Corrosion Resistance of Hastelloy-N Alloy in LiF-NaF-KF Molten Salt by Laser Cladding Pure Metallic Coatings" Coatings 8, no. 9: 322. https://doi.org/10.3390/coatings8090322