Hot Corrosion Resistance of Laser-Sealed Thermal-Sprayed Cermet Coatings
Abstract
:1. Introduction
2. Materials and Methods
2.1. Starting Powders and Coatings Deposition
2.2. Laser Post-Treatment
2.3. Hot Corrosion Tests
2.4. Coatings Characterization
3. Results and Discussion
3.1. Optimization of the Post-Deposition Laser Treatment
- Iincorporation of large amounts of the shielding gas inside the molten material, with formation of residual macro-porosities in the coating (mainly caused by too high a laser power, in the presence of high scan rates, where the viscosity of the molten material was drastically reduced and the interaction time was not sufficient to allow for proper degassing); Figure 2b, CRCZ coating, track 1, 800 W, 4 mm/s;
- Selective remelting or evaporation and recondensation in the shape of surface droplets of the lower melting fraction of the material (too long interaction times); Figure 2c, MSN coating, track 3, 300 W, 2 mm/s.
3.2. Hot Corrosion Results
3.2.1. Microstructure
- For the CRCZ as-sprayed coatings (Figure 3a–c), a scale of about 25 µm was formed after 100 h, made of an inner layer of about 10 µm, compact and continuous, and an outer, less dense, coat, characterized by the presence of both acicular and sharp-cornered non-cohesive phases. The same corrosion scale morphology was highlighted in the CRCZ laser-treated samples, after 100 h of exposure test (Figure 11a).
- For CRCY, the thickness of the scale increased with the number of hot corrosion cycles for, both, the as-sprayed (Figure 4a–c) and laser-treated coatings (Figure 4d–f), but in this case, a more evident beneficial effect of the sealing treatment could be observed—after 200 h the as-sprayed coatings exhibited a spalled, detached, and thick (≈50 μm) corrosion scale, whereas, the laser-treated samples showed thinner and more compact surface layers formed by hot corrosion products.
- The as-sprayed (Figure 5a,c) and laser-treated (Figure 5d–f) MSN samples did not show significant differences in terms of hot corrosion behavior—the scale thickness was comparable for each exposure time, while the corrosion layers of the laser-treated samples seemed to be slightly more compact and cohesive than the untreated.
3.2.2. Composition of the Scale
3.2.3. Recession
4. Conclusions
- The mechanism of the degradation occurring in typical type II hot corrosion conditions was not substantially altered by the formation of a surface-laser-densified layer, as confirmed by the analysis of the corrosion products of the treated and as-sprayed samples exhibiting an analogous microstructure and composition;
- The formation of surface compact layers (with overall thickness and microstructure very much dependent on coating composition) was responsible for a considerable increase of surface hardness of all coatings, and for a consistent improvement in the hot corrosion resistance, promoting the formation of thinner and more compact corrosion scales, and considerably reducing the surface recession rate (up to 60%, after 200 h exposure for Cr3C2–25% CoNiCrAlY coatings).
Author Contributions
Funding
Conflicts of Interest
References
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Name | Composition | Particle Size | Trade Name |
---|---|---|---|
CRCZ | 75 wt % Cr3C2 | −45 + 15 μm | Sulzer-WOKA 7302 |
25wt % Ni20Cr | |||
CRCY | 75 wt % Cr3C2 | −45 + 15 μm | FST-K880.23 |
25wt % CoNiCrAlY (Co 9.50 Ni 7.50 Al 1.75 Y 0.20 C10 Cr bal.) | |||
MSN | 10wt % nano-SiO2 | −53 + 20 μm (sieved) | Purposely produced |
30wt % mullite (3Al2O3-2SIO2) | |||
60wt % Ni20Cr |
Composition | CRCZ | CRCY | MSN |
---|---|---|---|
Kerosene feed rate (gph) | 6.5 | 6.5 | 6 |
O2 feed rate (scfh) | 2000 | 2000 | 1850 |
Spray distance (mm) | 350 | 370 | 350 |
Test No. | Power P (W) | Scan Rate V (mm/s) | Fluence H (J/mm2) |
---|---|---|---|
1 | 800 | 4 | 53.08 |
2 | 800 | 3 | 70.77 |
3 | 700 | 3 | 61.92 |
4 | 200 | 2 | 26.54 |
5 | 300 | 2 | 39.81 |
6 | 400 | 2 | 53.08 |
7 | 500 | 2.5 | 52.08 |
8 | 600 | 2.5 | 63.69 |
9 | 300 | 3 | 26.54 |
10 | 400 | 4 | 26.54 |
11 | 500 | 5 | 26.54 |
12 | 300 | 4 | 20.51 |
13 | 400 | 5 | 20.51 |
14 | 500 | 6.5 | 20.51 |
Test No. | Power P (W) | Scan Rate V (mm/s) | Fluence H (J/mm2) |
---|---|---|---|
1 | 800 | 4 | 53.08 |
2 | 800 | 3 | 70.77 |
3 | 700 | 4 | 46.44 |
4 | 700 | 3 | 61.92 |
Test No. | Power P (W) | Scan Rate V (mm/s) | Fluence H (J/mm2) |
---|---|---|---|
1 | 200 | 2 | 26.54 |
2 | 200 | 4 | 13.27 |
3 | 300 | 2 | 39.81 |
4 | 300 | 4 | 20.51 |
5 | 400 | 2 | 53.08 |
6 | 400 | 4 | 26.54 |
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Baiamonte, L.; Bartuli, C.; Marra, F.; Gisario, A.; Pulci, G. Hot Corrosion Resistance of Laser-Sealed Thermal-Sprayed Cermet Coatings. Coatings 2019, 9, 347. https://doi.org/10.3390/coatings9060347
Baiamonte L, Bartuli C, Marra F, Gisario A, Pulci G. Hot Corrosion Resistance of Laser-Sealed Thermal-Sprayed Cermet Coatings. Coatings. 2019; 9(6):347. https://doi.org/10.3390/coatings9060347
Chicago/Turabian StyleBaiamonte, Lidia, Cecilia Bartuli, Francesco Marra, Annamaria Gisario, and Giovanni Pulci. 2019. "Hot Corrosion Resistance of Laser-Sealed Thermal-Sprayed Cermet Coatings" Coatings 9, no. 6: 347. https://doi.org/10.3390/coatings9060347