Microstructure and Corrosion Properties of La2zr2o7/Nicoaly Thermal Barrier Coatings Deposited on Inconel 718 Superalloy by Laser Cladding

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Article Microstructure and Corrosion Properties of La2Zr2O7/NiCoAlY Thermal Barrier Coatings Deposited on Inconel 718 Superalloy by Laser Cladding

Kaijin Huang 1,2,3,*, Wei Li 2, Kai Pan 1, Xin Lin 3 and Aihua Wang 2

1 Key Laboratory of Low Dimensional Materials & Application Technology, Xiangtan University, Xiangtan 411105, China; [email protected] 2 State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] (W.L.); [email protected] (A.W.) 3 State Key Laboratory of Solidiﬁcation Processing, Northwestern Polytechnical University, Xi’an 710072, China; [email protected] * Correspondence: [email protected]; Tel.: +86-27-87543676

Abstract: In order to improve the seawater corrosion resistance of Inconel 718 superalloy, a La2Zr2O7/ NiCoCrAlY thermal barrier coating corrosion resistant to 3.5 wt.% NaCl aqueous solution was prepared by laser cladding on Inconel 718 superalloy. X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and electrochemical techniques were used to study the microstructure and the corrosion performance of the coating in 3.5 wt.% NaCl solution. The results show that the thermal

barrier coating is mainly composed of primary La2Zr2O7 phase and γ + laves/δ phase eutectic structure. The corrosion potential and corrosion current of the coating in 3.5 wt.% NaCl solution are higher and lower than that of the Inconel 718 substrate, respectively, indicating that the corrosion performance of the coating is better than that of the Inconel 718 substrate. The presence of La2Zr2O7 Citation: Huang, K.; Li, W.; Pan, K.; phase in the thermal barrier coating is the main reason for its corrosion resistance to 3.5 wt.% Lin, X.; Wang, A. Microstructure and NaCl solution.

Corrosion Properties of La2Zr2O7/

NiCoAlY Thermal Barrier Coatings Keywords: laser-cladding; La2Zr2O7/NiCoCrAlY thermal barrier coating; Inconel 718 superalloy; Deposited on Inconel 718 Superalloy corrosion resistance by Laser Cladding. Coatings 2021, 11, 101. https://doi.org/10.3390/ coatings11010101 1. Introduction Received: 23 December 2020 It is well-known that thermal barrier coating (TBC) is widely used in aircraft engines, Accepted: 13 January 2021 turbines, and turbine blades to protect the high-temperature alloy substrate from high- Published: 18 January 2021 temperature oxidation and corrosion, and play a role in heat insulation, increasing engine inlet temperature and engine thrust-to-weight ratio. At present, the most common thermal Publisher’s Note: MDPI stays neutral barrier coatings are composed of bond coat and ceramic working layer. Among them, the with regard to jurisdictional claims in published maps and institutional affil- bond coat material is usually MCrAlY (M = Ni and/or Co), and the working layer material iations. is mainly ceramics based on 8 wt.% Y2O3 stable ZrO2 (8YSZ) [1–4]. However, 8YSZ coating cannot be applied for a long time under high temperature conditions above 1200 ◦C, the reason is that when the long-term service temperature is higher than 1200 ◦C, YSZ will undergo phase transition (t’-ZrO2 → t-ZrO2 + c-ZrO2) and sintering, accompanied by thermal physical and mechanical property degradation, strain tolerance reduction, and Copyright: © 2021 by the authors. crack generation, and eventually lead to coating failure [2,5]. Therefore, in order to meet the Licensee MDPI, Basel, Switzerland. higher service temperature requirements of aircraft engines, turbines and turbine blades, it This article is an open access article distributed under the terms and is urgent to develop new ceramic coatings for ultra-high temperature and high thermal conditions of the Creative Commons insulation thermal barrier. However, as a thermal barrier coating ceramic layer material, Attribution (CC BY) license (https:// it is required to have high melting point, low thermal conductivity, high temperature creativecommons.org/licenses/by/ phase stability, chemical stability, corrosion resistance, thermal expansion coefficient to 4.0/). match the thermal expansion coefficient of the substrate, strong bonding strength with the

Coatings 2021, 11, 101. https://doi.org/10.3390/coatings11010101 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 101 2 of 12

metal substrate, good sintering resistance, and other properties. At present, La2Zr2O7 is a new thermal barrier coating ceramic material that can basically meet the performance requirements, and its long-term use temperature can exceed 1300 ◦C[2,6,7]. On the other hand, the current commonly used methods for preparing thermal barrier coatings mainly include plasma spraying and electron beam-physical vapor deposition [1]. The former is easy to operate and cheap, but the adhesion between coating and substrate is not strong, the coating microstructure is lamellar and there is a large porosity. The latter coating microstructure is columnar, coating performance is better than the former, but the equipment is expensive. Therefore, a new thermal barrier coating preparation method, laser cladding method [8–12], has been developed. The coating prepared by this method has ﬁne grain microstructure, strong adhesion between coating and substrate, better coating performance than plasma spraying method, close to electron beam–physical vapor deposition method, and easy to realize automation. To our best knowledge, there are no other literature reports on the preparation of La2Zr2O7 thermal barrier coating by laser cladding except for our report [9]. However, there have been a large number of literature reports on the preparation of other thermal barrier coatings by laser cladding [8,10–12] and the preparation of La2Zr2O7 thermal barrier coatings by plasma spraying [13–16] and electron beam–physical vapor deposition [17–21]. In addition, there are a large number of literature reports on the corrosion performance of La2Zr2O7 thermal barrier coatings under oxygen [9,16,20,21], molten salts or oxides (such as Na2SO4,V2O5, etc.) [4,22–24], CaO–MgO–Al2O3–SiO2 (CMAS) [4,25–27], simulated rain water after CMAS corrosion [28], and high-temperature water vapor [4,29], etc., but there are few literature reports on the corrosion performance of La2Zr2O7 thermal barrier coating in seawater corrosive medium [30]. Considering that aircraft engines, turbines and turbine blades may be stored in a marine environment [28], it is necessary to study the corrosion resistance of La2Zr2O7 thermal barrier coating in this case. In this paper, a La2Zr2O7 thermal barrier coating containing NiCoCrAlY bond coat was prepared on Inconel 718 superalloy with inner layer of NiCoCrAlY powder and outer layer of La2Zr2O7 powder by laser cladding. The microstructure characteristics and corrosion properties of the coating in 3.5 wt.% NaCl solution were studied. The results show that the corrosion performance of the La2Zr2O7 thermal barrier coating containing NiCoCrAlY bond coat is better than that of the Inconel 718 substrate.

2. Materials and Methods Inconel 718 (supplied by Shanghai Tengsi Special Alloy Company Limited, Shanghai, China) was selected as the substrate material, its size was 100 mm × 50 mm × 8 mm, and its chemical composition is shown in Table1. The bond coat powder was NiCoCrAlY, with a purity of 99.0 wt.% and a particle size of 37–75 µm, and its chemical composition is shown in Table2. La 2Zr2O7 was selected as a thermal barrier coating ceramic powder with a purity of 99.5 wt.% and a particle size of 37–75 µm. Figure1 is the schematic diagram of the pre-set powder layer before laser cladding (the second powder layer was applied only after the ﬁrst powder layer was naturally dry). The binder for the pre-set powder was 4% Polyvinyl Alcohol (PVA) solution. Among them, the thickness of NiCoCrAlY powder layer was about 0.1 mm, the thickness of La2Zr2O7 powder layer was about 0.2 mm, and the total thickness of the preset powder layer was about 0.3 mm. The LDF8000- 60 semiconductor laser produced by Laserline (Augsburg, Germany) was used for laser cladding. The optimized laser cladding parameters were as follows: laser power P = 3.5 kW, laser scanning speed V = 10 mm/s, square laser spot area S = 5 × 5 mm2, laser spot overlap rate f = 30%, and the Ar gas ﬂow with protective effect was 16 L/min.

Table 1. Chemical composition of the Inconel 718 superalloy.

Element Cr Ni Nb Al Co Mo Ti C Fe wt.% 18.8 52.7 5.3 0.5 0.02 2.9 0.9 0.03 Bal. Coatings 2021, 11, x FOR PEER REVIEW 3 of 12

Table 1. Chemical composition of the Inconel 718 superalloy. Coatings 2021, 11, 101 3 of 12 Element Cr Ni Nb Al Co Mo Ti C Fe wt.% 18.8 52.7 5.3 0.5 0.02 2.9 0.9 0.03 Bal. Table 2. Chemical composition of the NiCoCrAlY powder. Table 2. Chemical composition of the NiCoCrAlY powder. Element Ni Co Cr Al Y Element Ni Co Cr Al Y wt.% 43.0 24.0 20.0 12.0 1.0 wt.% 43.0 24.0 20.0 12.0 1.0

FigureFigure 1.1. SchematicSchematic diagramdiagram ofof pre-setpre-set powderpowder layerlayer beforebefore laser laser cladding. cladding.

TheThe coatedcoated samplessamples werewere cutcut intointo small small samples samples of of 8 8 mm mm× ×8 8 mm mm× × 88 mmmm byby wirewire cuttingcutting method, method, which which were were used used to to study study the the microstructure microstructure and and corrosion corrosion resistance resistance of the of coating.the coating. It should It should be pointed be pointed out thatout that both both the laserthe laser clad clad coating coating and and Inconel Inconel 718 samples718 sam- haveples have undergone undergone processes processes such such as silicon as sili carbidecon carbide sandpaper sandpaper grinding, grinding, diamond diamond abrasive abra- pastesive paste polishing, polishing, running running water water washing, washing, cold cold air air drying, drying, and and putting putting into into a a dryer dryer for for standbystandby use.use. The The phase phase analysis analysis of of the the coat coatinging was was performed performed on onX’ X’Pert Pert PRO PRO X-ray X-ray dif- diffractometerfractometer manufactured manufactured by byPANalytical PANalytical B.V. B.V. (Almelo, (Almelo, The The Netherlands). Netherlands). The The test test pa- ◦ parametersrameters were were as as follows: follows: Cu Cu target target was was used, used, start start position position 2 θ was 20.019720.0197°,, endend positionposition ◦ ◦ 22θθwas was89.9467 89.9467°,, stepstep sizesize 22θθwas was 0.0390 0.0390°,, scan scan type type was was continuous, continuous, divergence divergence slit slit size size ◦ waswas 0.43540.4354°.. TheThe microstructuremicrostructure ofof thethe cross-sectioncross-section coating coating etched etched with with an an aqua aqua regia regia was was observedobserved and and analyzed analyzed by by Quanta Quanta 650 650 scanning scanning electron electron microscope microscope with with energy-dispersive energy-disper- X-raysive X-ray spectroscopy spectroscopy produced produced by FEI by (Quanta FEI (Qua 650nta FEG, 650 FEG, Eindhoven, Eindhoven, The Netherlands).The Netherlands). TheThe electrodynamicelectrodynamic polarization polarization curves curves of of the the coating coating and and InconelInconel 718 718 substratesubstrate werewere completedcompleted onon thethe CS350HCS350H electrochemicalelectrochemical workstationworkstation manufacturedmanufactured byby WuhanWuhan Corrtest Corrtest InstrumentsInstruments Co.Co. Ltd. (Wuhan, (Wuhan, China) China) A A standard standard three-electrode three-electrode test test system system was was used used in inthe the experiment, experiment, with with the the coating coating as the as theworkin workingg electrode, electrode, platinum platinum electrode electrode as the as aux- the auxiliaryiliary electrode electrode and and saturated saturated ca calomellomel electrode electrode as as the the reference reference electrode. TheThe effectiveeffective 2 exposureexposure area area of of the the coating coating and and Inconel Inconel 718 718 substrate substrate samples samples was was 64 64 mm mm.2. 3.5 3.5 wt.% wt.% NaCl NaCl aqueousaqueous solutionsolution waswas usedused forfor corrosioncorrosion tests.tests. SoakSoak thethe samplesample forfor 3030 minmin beforebefore startingstarting thethe measurement.measurement. TheThe parametersparameters used used in in thethe measurementmeasurement were were as as follows: follows: the the scanning scanning − speedspeed waswas 11 mV/s,mV/s, and the scanning range was −2.52.5 to to 1.0 1.0 V. V. The surface morphologies of the corroded specimens were observed using a Quanta The surface morphologies of the corroded specimens were observed using a Quanta 650 scanning electron microscope with an energy spectrum (FEI Quanta 650 FEG, Eind- 650 scanning electron microscope with an energy spectrum (FEI Quanta 650 FEG, Eind- hoven, The Netherlands). hoven, The Netherlands). 3. Results and Discussion 3.1. Microstructure Figure2 shows the SEM morphology of laser clad cross-section coating. Figure3 shows the XRD patterns of the laser clad coating. In Table3, the results of the elemental EDS analysis of the laser clad coating at different positions are given. It can be seen from Figure2a that the laser clad coating is composed of a laser modified layer and a substrate, of which the thickness of the laser modified layer is about 1738–2024 µm (The measurement method provided in literature [31] was adopted.). According to Figure2b–e and Table3, the laser modified layer is composed of the La2Zr2O7 layer and the laser alloying layer, of Coatings 2021, 11, 101 4 of 12

which the thickness of La2Zr2O7 layer is about 34–47 µm. The interface of La2Zr2O7 layer and the laser alloying layer is well combined without cracks and pores. The alloying layer is mainly composed of γ + Laves/δ phase eutectic structure. The reasons are as follows: (1) Table3 shows that EDS results at position D are mainly composed of O, Zr, and La. Combined with XRD calibration results in Figure3, it can be seen that the coating is mainly La2Zr2O7 (JCPDS 01-073-0444) phase. The other small amounts of Al, Ni, Nb, Cr, Ti, Fe, Co, Mo, and Y in position D should come from the laser-heated molten bond coat NiCoCrAlY powder and partially melted Inconel 718 substrate. This is caused by the rapid laser heating and solidification during laser cladding, and the mixed alloy elements have no time to separate. (2) Table3 shows that the Nb content of Spot3 and Spot6 exceed 10 at.% and they are probably Laves phases. Since Laves phase is generally A(Fe, Ni, Cr)2B(Nb, Mo, Ti) type, they might be Cr2Nb phase in combination with EDS results of Spot3 and Spot6. This is consistent with the results reported in literature [32–35]. (3) Table3 shows that the Nb content of Spot2 and Spot4 is 8.51 at.% and 5.62 at.%, respectively. They are probably δ phase (Ni3Nb). The reason is that the solidification temperature of δ phase is 860–995 ◦C and Nb concentration requirement is 6–8 at.% [35]. This is consistent with the results reported in literature [32]. (4) Table3 shows that Fe, Ni and Cr contents in Spot1, Spot5, and Spot7 are dominant, which should be γ phase ((Fe, Ni, and Cr) solid solution). (5) A large number of literature studies have shown [36–38] that the solidification process sequence of Inconel 718 substrate is L → γ → γ + NbC (high temperature) eutectic reaction → γ + laves (low temperature) eutectic reaction. According to EDS results in Table3, in the laser cladding process, the chemical composition of the laser heating melted preset powder layer, and partially melted Inconel 718 substrate deviated from the chemical composition of Inconel 718 substrate due to mixing, but the degree of deviation should be small, because the chemical composition of the added NiCoCrAlY bond coat powder is basically the composition of Inconel 718 substrate. Therefore, we believe that the solidification process of the laser modified layer is as follows: ◦ The high melting point La2Zr2O7 (2300 C) solidified first, and the remaining alloy liquid followed the solidification sequence of Inconel 718 alloy: L → γ → (γ + NbC) eutectic reaction → (γ + Laves) eutectic reaction. In other words, the solidification of remaining alloy liquid was mainly caused by L → γ reaction at the beginning. As time went on, Nb, Mo, Ti, C, and other elements were enriched between dendrites, leading to the eutectic reaction L → (γ + NbC) and the consumption of a large number of C atoms in the alloy. As the L → γ reaction continued and progressed, further enrichment of the interdendrite liquid solute atoms occurred until the eutectic reaction L → (γ + Laves) occurred and the solidification process was complete. At the same time, due to the characteristics of rapid laser heating and fast solidification, Nb element segregation was serious, and part of the regions with low Nb content generated δ phase instead of Laves phase, because Nb content in δ phase is usually 6–8 at. %, while Nb content in Laves phase usually exceeds 10 at. % [32–35]. It should be pointed out that there exists a large temperature gradient on the upper and lower surface of the molten pool during the laser action, thus forming an extremely steep surface tension, which causes a strong convection in the molten pool, and the convection in the molten pool can produce a strong stirring force, so that the liquid metal in the molten pool mixed evenly. The convection in the molten pool mainly comes from three aspects: First, the convection in the molten pool comes from the energy distribution of the laser beam, and the Gaussian laser beam used in this experiment can cause severe convection in the molten pool. The second factor is the protective gas that blows out to prevent the oxidation of liquid metals at high temperature. This high-pressure protective gas produces a stirring effect that promotes the diffusion of the liquid metal. Thirdly, at high temperature, the liquid metal expands when heated to form a certain density difference in the molten pool, thus generating buoyancy in the pool, the existence of which will cause the natural Coatings 2021, 11, 101 5 of 12

Coatings 2021, 11, x FOR PEER REVIEW convection of the melt [39]. It is these three factors that make the multi-layer preset powder 5 of 12 mode also form the microstructure obtained by the single-layer preset powder mode, thus reducing the mixing time of powder, and improving the production efﬁciency.

(a) (b)

(e)

Figure 2. SEMFigure images 2. SEM imagesof the oflaser the laserclad clad La2 LaZr22ZrO27/NiCoCrAlYO7/NiCoCrAlY coating: coating: (a(a)) whole, whole, (b) ( atb) position at position A, (c) atA, position (c) at position E, (d) at E, (d) at position B,position (e) at position B, (e) at position C. (A, C. B (A, and B andC represent C represent the the top, middle middle and and bottom bottom of the of coating, the coating, respectively. respectively. D and E represent D and E represent the locallythe locally amplified ampliﬁed La La2Zr2Zr2O2O7 layer7 layer and and alloyingalloying layer layer at positionat position A, respectively. A, respectively. Numbers Numbers 1–7 represent 1–7 substratesrepresent or substrates or particlesparticles at corresponding at corresponding positions positions of of the the coating.).

Figure 3. XRD patterns of the laser clad coating.

Coatings 2021, 11, x FOR PEER REVIEW 5 of 12

(a) (b)

(e)

Figure 2. SEM images of the laser clad La2Zr2O7/NiCoCrAlY coating: (a) whole, (b) at position A, (c) at position E, (d) at Coatingsposition2021, 11 B,, 101 (e) at position C. (A, B and C represent the top, middle and bottom of the coating, respectively. D and E repre-6 of 12 sent the locally amplified La2Zr2O7 layer and alloying layer at position A, respectively. Numbers 1–7 represent substrates or particles at corresponding positions of the coating.).

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Figure 3. XRD patterns of the laser clad coating. FigureTable 3. XRDEDS results patterns of ofthe the coating laser cladat different coating. positions of Figure 2 (at.%).

TableElement 3. EDS resultsO Zr of the La coating Ni at different Nb Cr positions Ti ofFe Figure Al 2 (at.%).Co Mo Y Possible Phase Area D 58.2 20.4 15.6 0.1 0.0 0.4 0.8 0.1 3.5 0.0 0.0 0.9 La2Zr2O7

ElementSpot1 11.5 O Zr4.1 0.2La 32.2 Ni 0.4Nb 18.1 Cr 0.4 Ti 30.5 Fe 1.9 Al 0.1 Co 0.5 Mo 0.1 Y Possibleγ Phase AreaSpot2 D 58.2 21.1 20.4 5.2 15.6 0.2 22.80.1 8.50.0 14.80.4 1.80.8 21.20.1 2.83.5 0.1 0.0 1.0 0.0 0.5 0.9 La2Zrδ 2O7 Spot1 11.5 4.1 0.2 32.2 0.4 18.1 0.4 30.5 1.9 0.1 0.5 0.1 γ Spot2Spot3 21.1 28.0 5.2 4.9 0.2 13.6 22.8 11.08.5 14.819.4 1.21.8 14.621.2 2.12.8 0.2 0.1 4.3 1.0 0.5 0.5 Lavesδ Spot3Spot4 28.0 19.9 4.9 4.6 0.2 22.4 13.6 5.611.0 19.4 19.8 1.01.2 20.214.6 2.62.1 0.2 0.2 3.0 4.3 0.5 0.5 Lavesδ Spot4 19.9 4.6 0.2 22.4 5.6 19.8 1.0 20.2 2.6 0.2 3.0 0.5 δ Spot5 12.2 12.2 4.3 4.3 0.2 31.1 31.1 0.5 18.1 0.60.6 29.929.9 2.22.2 0.1 0.1 0.6 0.6 0.2 0.2 γγ Spot6 28.7 28.7 5.3 5.3 0.2 13.6 13.6 10.910.9 18.8 0.90.9 14.414.4 2.22.2 0.2 0.2 4.3 4.3 0.5 0.5 LavesLaves γ Spot7 13.3 13.3 4.5 4.5 0.2 31.2 31.2 0.6 18.0 0.50.5 28.728.7 2.12.1 0.0 0.0 0.7 0.7 0.2 0.2 γ

Finally,Finally, combiningcombining thethe resultsresults ofof FiguresFigures2 2and and3 and3 and Table Table3, we3, we give give a schematic a schematic diagramdiagram of of the the phase phase distributiondistribution in in differentdifferent regions regions of of the the laser lasermodiﬁed modified layer, layer, as as shown shown inin Figure Figure4 .4.

FigureFigure 4. 4.Schematic Schematic diagram diagram of of phase phase distribution distribution in in different different areas areas of of the the laser laser modiﬁed modified layer. layer. 3.2. Corrosion Properties It should be pointed out that there exists a large temperature gradient on the upper and Figurelower5 surface shows theof potentiodynamicthe molten pool polarizationduring the laser curve action, of laser thus clad Laforming2Zr2O7 an/NiCoCrAlY extremely coatingsteep surface and Inconel tension, 718 which substrate causes in 3.5 a wt.%strong NaCl convection aqueous in solution. the molten Table pool,4 shows and thethe cor-con- rosionvection potential in the molten and current pool can of the produce laser clad a strong coating stirring and Inconel force, so 718 that substrate the liquid in 3.5 metal wt.% in NaClthe molten aqueous pool solution. mixed evenly. It can be The seen convection from Figure in5 the and molten Table 4pool that mainly the corrosion comes potentialfrom three ofaspects: laser clad First, coating the convection is 0.22 V higherin the molten than that pool of comes Inconel from 718 the substrate, energy anddistribution the corrosion of the currentlaser beam, is about and one the eleventhGaussian of laser that beam of Inconel used 718in this substrate. experiment According can cause to the severe principle convec- of “thetion higher in the themolten corrosion pool. potentialThe second ofthe factor material, is the theprotective smaller gas the corrosionthat blows current out to andprevent the betterthe oxidation the corrosion of liquid resistance”, metals at thehigh corrosion temperature. resistance This high-pressure of laser clad coating protective in 3.5 gas wt.% pro- NaClduces aqueous a stirring solution effect isthat better promotes than Inconel the diffusion 718 substrate. of the Theliquid reason metal. is thatThirdly, the surfaceat high layertemperature, of laser clad the coatingliquid metal (Figures expands2b and 3when) is mainly heated composed to form a of certain dense density La 2Zr2 Odifference7 phase, whichin the hasmolten excellent pool, thus corrosion genera resistanceting buoyancy to 3.5 wt.%in the NaClpool, aqueousthe existence solution of which [30]. will cause the natural convection of the melt [39]. It is these three factors that make the multi-layer preset powder mode also form the microstructure obtained by the single-layer preset powder mode, thus reducing the mixing time of powder, and improving the production efficiency.

3.2. Corrosion Properties Figure 5 shows the potentiodynamic polarization curve of laser clad La2Zr2O7/NiCoCrAlY coating and Inconel 718 substrate in 3.5 wt.% NaCl aqueous solution. Table 4 shows the corrosion potential and current of the laser clad coating and In- conel 718 substrate in 3.5 wt.% NaCl aqueous solution. It can be seen from Figure 5 and Table 4 that the corrosion potential of laser clad coating is 0.22 V higher than that of In- conel 718 substrate, and the corrosion current is about one eleventh of that of Inconel 718 substrate. According to the principle of “the higher the corrosion potential of the material, the smaller the corrosion current and the better the corrosion resistance”, the corrosion resistance of laser clad coating in 3.5 wt.% NaCl aqueous solution is better than Inconel

Coatings 2021, 11, x FOR PEER REVIEW 7 of 12

Coatings 2021, 11, 101 718 substrate. The reason is that the surface layer of laser clad coating (Figures 2b and7 of 12 3) is mainly composed of dense La2Zr2O7 phase, which has excellent corrosion resistance to 3.5 wt.% NaCl aqueous solution [30].

Figure 5. Potentiodynamic polarization curve of laser clad La2Zr2O7/NiCoCrAlY coating and Inconel Figure 5. Potentiodynamic polarization curve of laser clad La2Zr2O7/NiCoCrAlY coating and Inconel718 substrate 718 substrate in 3.5 wt.% in 3.5 NaCl wt.% aqueous NaCl aqueous solution. solution.

TableTable 4. 4.Corrosion Corrosion parameters parameters of of different different samples samples in in 3.5 3.5 wt.% wt.% NaCl NaCl solution. solution.

2 Specimen Ecorr (V) Icorr (A/cm2 ) Specimen Ecorr (V) Icorr (A/cm ) Inconel 718 −1.13 1.44 × 10−4 Inconel 718 −1.13 1.44 × 10−4 Laser clad coating −0.91 1.29 × 10−5 Laser clad coating −0.91 1.29 × 10−5

It should be noted that the corrosion potential and corrosion current of the pure It should be noted that the corrosion potential and corrosion current of the pure La2Zr2O7 coating prepared by plasma spraying in 3.5 wt.% NaCl aqueous solution are La Zr O coating prepared by plasma spraying in 3.5 wt.% NaCl aqueous solution are −0.662 2V 7and 5.4 × 10−7 A/cm2, respectively, as reported in literature [30]. However, in the −0.66 V and 5.4 × 10−7 A/cm2, respectively, as reported in literature [30]. However, in La2Zr2O7 coating prepared by laser cladding method in this paper, due to a small amount the La Zr O coating prepared by laser cladding method in this paper, due to a small of alloy2 elements2 7 dissolved in the coating (Table 3), the corrosion potential and current amount of alloy elements dissolved in the coating (Table3), the corrosion potential and are not as good as those reported in literature [30], but they are still better than those of current are not as good as those reported in literature [30], but they are still better than Inconel 718 substrate (Table 4). This indicates that the La2Zr2O7/NiCoCrAlY coating pre- those of Inconel 718 substrate (Table4). This indicates that the La Zr O /NiCoCrAlY pared by laser cladding is more resistant to corrosion by 3.5 wt.% NaCl2 2 aqueous7 solution coating prepared by laser cladding is more resistant to corrosion by 3.5 wt.% NaCl aqueous than Inconel 718 substrate. This is also evidenced by the surface morphology after corro- solution than Inconel 718 substrate. This is also evidenced by the surface morphology after sion in Figure 6. It can be seen from Figure 6 that the surface of Inconel 718 substrate is corrosion in Figure6. It can be seen from Figure6 that the surface of Inconel 718 substrate seriously damaged after the corrosion of 3.5 wt.% NaCl aqueous solution (Figure 6a), is seriously damaged after the corrosion of 3.5 wt.% NaCl aqueous solution (Figure6a), while the surface of laser clad coating is slightly damaged (Figure 6b). In addition, EDS while the surface of laser clad coating is slightly damaged (Figure6b). In addition, EDS resultsresults in in Table Table5 5show show that that oxygen oxygen elements elements appear appear in in the the surface surface chemical chemical compositions compositions ofof bothboth samplessamples afterafter corrosion corrosion in in 3.5 3.5 wt.% wt.% NaCl NaCl aqueous aqueous solution. solution. Combined Combined with with the the EDSEDS results results in in Tables Tables1 and1 and5 and 5 and the the SEM SEM morphology morphology after after corrosion corrosion in Figure in Figure6, it can 6, it be can seenbe seen that that in Figure in Figure6a, areas 6a, areas A and A B and should B shou be theld be surface the surface of the originalof the original Inconel Inconel 718 alloy 718 thatalloy has that not has been not corrodedbeen corroded by 3.5 by wt.% 3.5 wt.% NaCl NaCl aqueous aqueous solution solution and theand exposed the exposed surface sur- offace Inconel of Inconel 718 alloy 718 alloy after beingafter being corroded corroded by 3.5 by wt.% 3.5 wt.% NaCl NaCl aqueous aqueous solution, solution, respectively. respectively. The original La2Zr2O7 coating surface not corroded by 3.5 wt.% NaCl aqueous so- The original La2Zr2O7 coating surface not corroded by 3.5 wt.% NaCl aqueous solution shouldlution should be at area be at C area in Figure C in Figure6b. Among 6b. Among them, them, the oxygenthe oxygen inthe in the surface surface of of the the laser laser clad coating is mainly from La2Zr2O7 phase, while the oxygen in the corrosion surface of clad coating is mainly from La2Zr2O7 phase, while the oxygen in the corrosion surface of InconelInconel 718718 substratesubstrate isis mainlymainly fromfrom the the electrochemical electrochemical reactionreaction (no(no oxidation oxidation film) film) or or chemicalchemical reaction reaction (presence (presence of of oxide oxide film) film) of of Cr, Cr, Ni, Ni, Nb, Nb, Al, Al, Co, Co, Mo, Mo, Ti, Ti, and and Fe elementsFe elements in Inconelin Inconel 718 718 substrate substrate in 3.5 in wt.% 3.5 wt.% NaCl NaCl aqueous aqueous solution solution containing containing oxygen. oxygen. The detailed The de- electrochemicaltailed electrochemical or chemical or chemical reactions reactions of Inconel of Inconel 718 substrate 718 substrate are as follows: are as follows: (1) When there is no oxidation film, the following electrochemical reactions occur in 3.5 wt.% NaCl aqueous solution containing oxygen for Cr, Ni, Nb, Al, Co, Mo, Ti, and Fe elements in Inconel 718 substrate: For Cr element: Negative: Cr − 3e− = Cr3+ Positive: O + 2H O + 4e− = 4OH− 2 2 Total reaction equation of galvanic cell: 4Cr + 3O2 + 6H2O = 4Cr(OH)3. Coatings 2021, 11, 101 8 of 12

For Ni element: Negative: Ni − 2e− = Ni2+ − − Positive: O2 + 2H2O + 4e = 4OH Total reaction equation of galvanic cell: 2Ni + O2 + 2H2O = 2Ni(OH)2. For Nb element: Negative: Nb − 5e− = Nb5+ − − Positive: O2 + 2H2O + 4e = 4OH Total reaction equation of galvanic cell: 4Nb + 5O2 + 10H2O = 4Nb(OH)5. For Al element: Negative: Al − 3e− = Al3+ − − Positive: O2 + 2H2O + 4e = 4OH Total reaction equation of galvanic cell: 4Al + 3O2 + 6H2O = 4Al(OH)3. Due to the instability of Al(OH)3, it is finally decomposed into water and alumina: 2Al(OH)3 = Al2O3 + 3H2O. For Co element: Negative: Co − 2e− = Co2+ − − Positive: O2 + 2H2O + 4e = 4OH Total reaction equation of galvanic cell: Co + O2 + 2H2O = Co(OH)4. For Mo element: − 6+ − − Negative: Mo − 6e = Mo Positive: O2 +2H2O + 4e = 4OH Total reaction equation of galvanic cell: 2Mo + 3O2 + 6H2O = 2Mo(OH)6. For Ti element: Negative: Ti − 4e− = Ti4+ − − Positive: O2 + 2H2O + 4e = 4OH Total reaction equation of galvanic cell: Ti + O2 + 2H2O = Ti(OH)4. For Fe element: − 2+ − − Negative: Fe − 2e = Fe Positive: O2 + 2H2O + 4e = 4OH Total reaction equation of galvanic cell: 2Fe + O2 + 2H2O = 2Fe(OH)2. Fe(OH)2 can also continue to react with oxygen in solution: 4Fe(OH)2 + O2 + 2H2O = 4Fe(OH)3. (2) When there is oxidation film, the following chemical reactions occur in 3.5 wt.% NaCl aqueous solution for Cr, Ni, Nb, Al, Co, Mo, Ti, and Fe elements in Inconel 718 substrate: Generally speaking, all metal elements will form oxide film on their surface at room temperature. Table6 is the Pilling–Bedworth (P-B) ratio of alloy oxides of Cr, Ni, Nb, Al, Co, Mo, Ti, and Fe. According to the criterion that “the integrity of metal oxide film is a necessary condition for its protection, while the P-B ratio is greater than 1 is a necessary condition for the integrity of metal oxide film”, it can be seen that the oxide film formed by these alloy-elements is complete. However, if the P-B ratio is too large (such as more than 2), the internal stress of the oxide film is large, and the oxide film is easy to break and lose protection or the protection is very poor. Therefore, according to the results in Tables1 and6, the presence of Nb and Mo may make the oxidation film formed on the local surface of Inconel 718 substrate worse.

Table 5. EDS results of corroded surfaces of the Inconel 718 substrate and the laser clad specimen (at.%).

Samples Position O Zr La Al Nb Mo Ti Cr Fe Co Ni Y Inconel 718 Area A 15.0 0.0 0.0 2.2 5.3 0.7 0.7 17.0 39.5 0.2 19.4 0 Inconel 718 Area B 12.5 0.0 0.0 1.4 5.4 0.5 0.9 17.8 38.8 0.4 22.3 0 Inconel 718 Spot 1 10.6 0.0 0.0 1.4 5.3 0.8 0.7 17.6 42.4 0.2 21.0 0 Coating Area C 61.5 16.0 15.1 4.6 0.2 0.1 0.9 0.5 0.1 0.1 0.2 0.7 Coatings 2021, 11, 101 9 of 12 Coatings 2021, 11, x FOR PEER REVIEW 8 of 12

(a) (b)

Figure 6. SEM images of laser clad La2Zr2O7/NiCoCrAlY coating and Inconel 718 substrate after corrosion in 3.5 wt.% NaCl solu- Figure 6. SEM images of laser clad La2Zr2O7/NiCoCrAlY coating and Inconel 718 substrate after corrosion in 3.5 wt.% tion: (a) Inconel 718 substrate, (b) laser clad coating. (A, B, and C represent the different positions of the corroded specimen.) NaCl solution: (a) Inconel 718 substrate, (b) laser clad coating. (A, B, and C represent the different positions of the corroded specimen.) Table 5. EDS results of corroded surfaces of the Inconel 718 substrate and the laser clad specimen (at.%). Table 6. Pilling–Bedworth ratios of different metal oxides [40]. Samples Position O Zr La Al Nb Mo Ti Cr Fe Co Ni Y InconelMetal Cr Ni Co Fe Fe Ti Nb Al Mo Area A 15.0 0.0 0.0 2.2 5.3 0.7 0.7 17.0 39.5 0.2 19.4 0 718oxide Cr2O3 NiO CoO FeO Fe2O3 TiO2 Nb2O5 Al2O3 MoO3 InconelP-B ratio 2.07 1.65 1.86 1.70 2.14 1.73 2.68 1.28 3.4 Area B 12.5 0.0 0.0 1.4 5.4 0.5 0.9 17.8 38.8 0.4 22.3 0 718 InconelOn the one hand, all the surface metal oxide ﬁlms will react with the water in 3.5 wt.% Spot 1 10.6 0.0 0.0 1.4 5.3 0.8 0.7 17.6 42.4 0.2 21.0 0 NaCl718 aqueous solution as follows: Coating Area C 61.5 16.0 15.1 4.6 0.2 0.1 0.9 0.5 0.1 0.1 0.2 0.7 Cr2O3 + 3H2O → 2Cr(OH)3 (1) (1) When there is no oxidation film, the following electrochemical reactions occur in 3.5 NiO + H2O → Ni(OH)2 (2) wt.% NaCl aqueous solution containing oxygen for Cr, Ni, Nb, Al, Co, Mo, Ti, and Fe elements in Inconel 718Nb substrate:2O5 + 5H 2O → 2Nb(OH)5 (3)

For Cr element: Al2O3 + 3H2O → 2Al(OH)3 (4) Negative: Cr − 3e− = Cr3+ CoO + H2O → Co(OH)2 (5) Positive: O2 + 2H2O + 4e− = 4OH− Total reaction equationMoO of galvanic3 + 3H2O cell:→ Mo(OH) 4Cr + 3O62 + 6H2O = 4Cr(OH)3. (6) For Ni element: TiO2 + 2H2O → Ti(OH)4 (7) Negative: Ni − 2e− = Ni2+ Positive: O2 + 2H2O + 4eFeO− = 4OH + H−2 O → Fe(OH)2 (8) Total reaction equation of galvanic cell: 2Ni + O2 + 2H2O = 2Ni(OH)2. Fe2O3 + 3H2O → 2Fe(OH)3 (9) For Nb element: − On theNegative: other hand, Nb − due5e− = to Nb the5+ presence of Cl ions with a small radius in the NaCl − corrosion solution, Cl ions will− adsorb− and penetrate into the metal surface oxidation Positive: O2 + 2H2O + 4e = 4OH ﬁlm and diffuse into the interior and have a chemical reaction with the internal metal (see Total reaction equation of galvanic cell: 4Nb + 5O2 + 10H2O = 4Nb(OH)5. EquationsFor Al (10)–(17)), element: thus reducing the protective effect of the outermost metal oxidation ﬁlm and increasingNegative: Al the −corrosion 3e− = Al3+ rate of the Inconel 718 substrate. On the contrary, since 2− La2Zr2O7 is oxygen non-permeable− material− [2,7], and the radius of O ion is 140 pm, Positive: O2− + 2H2O + 4e = 4OH− and the radius of Cl ion is 181 pm, Cl cannot penetrate the dense La2Zr2O7 coating. Total reaction equation of galvanic cell: 4Al + 3O2 + 6H2O = 4Al(OH)3. Due to the Therefore, the laser clad coating is more resistant to corrosion by 3.5 wt.% NaCl aqueous instability of Al(OH)3, it is finally decomposed into water and alumina: 2Al(OH)3 = solution than Inconel 718 substrate. Al2O3 + 3H2O. For Co element: − Cr(s) + 3Cl (aq.) → CrCl3 (10) Negative: Co − 2e− = Co2+ − − − Positive: O2 + 2H2O + Ni(s)4e = 4OH + 2 Cl (aq.) → NiCl2 (11) Total reaction equation of galvanic cell: Co + O2 + 2H2O = Co(OH)4.

Coatings 2021, 11, 101 10 of 12

− Nb(s) + 5Cl (aq.) → NbCl5 (12) − Al(s) + 3Cl (aq.) → AlCl3 (13) − Co(s) + 2Cl (aq.) → CoCl2 (14) − Mo(s) + 6Cl (aq.) → MoCl6 (15) − Ti(s) + 4Cl (aq.) → TiCl4 (16) − Fe(s) + 3Cl (aq.) → FeCl3 (17) In conclusion, no matter whether oxidation film is formed on the surface of Inconel 718 substrate, chemical reaction (oxidation film exists), or electrochemical reaction (no oxidation film exists) will occur in 3.5 wt.% NaCl aqueous solution, thus leading to corrosion of Inconel 718 substrate. On the contrary, since Cl- ions could not penetrate the dense La2Zr2O7 coating prepared by laser cladding, the corrosion resistance of the coating in 3.5 wt.% NaCl aqueous solution is superior to that of the Inconel 718 substrate.

4. Conclusions In order to improve the seawater corrosion resistance of aircraft engines, turbines, and turbine blades, a La2Zr2O7/NiCoCrAlY coating was prepared by using the pre-set powder layer method and laser cladding, and the following two main conclusions are drawn:

(1) The outermost layer of the coating is La2Zr2O7 layer, and the secondary layer is mainly composed of γ + laves/δ phase eutectic structure. (2) The corrosion resistance of the coating is superior to that of Inconel 718 substrate in 3.5 wt.% NaCl aqueous solution, and the presence of La2Zr2O7 phase is the main reason for the improvement of the corrosion resistance of the coating.

Author Contributions: Investigation—K.H., W.L., K.P., X.L., and A.W.; writing—original draft preparation, W.L.; writing—review and editing, K.H. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Open Project Program (No. KF20180201) of Key Labora- tory of Low Dimensional Materials and Application Technology, Ministry of Education, Xiangtan University, and the Fund (Grant No. SKLSP201913) of the State Key Laboratory of Solidification Processing in NWPU. Data Availability Statement: The data of this study are available from the corresponding author upon reasonable request. Acknowledgments: The authors are also grateful to the Analytical and Testing Center of Huazhong University of Science and Technology. Conflicts of Interest: The authors declare no conflict of interest.

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