H. LV et al.: EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS 49–56 EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS VPLIV PREDHODNE OKSIDACIJE NA PREVLEKO Al–Si ODPORNO NA KOROZIJO V VRO^EM Haishuang Lv, Yanmei Li * , Yabin Li, Naiwen Fan The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, China Prejem rokopisa – received: 2022-10-07; sprejem za objavo – accepted for publication: 2022-12-12 doi:10.17222/mit.2022.645 In this study, effects of pre-oxidation on the hot corrosion behaviour of Al–Si coatings in a Na2SO4 and 25 w/% NaCl mixture at 900 °C were investigated. The results showed that after 120 h of corrosion, a large amount of -NiAl residual was found with no obvious sulphides or oxidation in the pre-oxidised Al–Si coating. The pre-formed oxide film on the pre-oxidised coating pre- vented external O 2– and species such as S 2– and Cl – from the molten salts from directly contacting the coating and reduced the rate of TiO2 formation in the oxide layer, thus forming an Al2O3-based oxide film and reducing the growth rate of the oxide film on the coating surface. Cracking and peeling of the oxide film, caused by the internal stress owing to the difference in the ther- mal expansion coefficient between the oxides in the oxide layer, are suppressed; thus, the hot corrosion resistance of the coating is significantly improved. Keywords: superalloy, Al–Si coating, pre-oxidation, hot corrosion Namen raziskave je ocena vpliva predhodne oksidacije prevleke na osnovi Al–Si na njeno obna{anje med vro~o oksidacijo v raztaljeni me{anici soli Na2SO4 in 25 masnih % NaCl pri 900 °C. Rezultati raziskave so pokazali, da je po 120 urah korozije v vro~em nastala velika koli~ina ostankov -NiAl, toda brez o~itnega nastanka sulfidov ali dodatne oksidacije pred oksidirane prevleke Al–Si. Zaradi predhodne oksidacije prevleke je nastali oksidni film prepre~il neposredni stik zunanjih ionov O 2– in tudi drugih, kot sta S 2– and Cl – v raztaljeni soli. Tako je pri{lo tudi do zmanj{ane hitrosti tvorbe TiO2 v oksidni plasti. Pri{lo je do tvorbe oksidnega filma na osnovi Al2O3 in zmanj{anja hitrosti rasti oksidnega filma na povr{ini prevleke. Pokanje in lupljenje oksidnega filma zaradi notranjih napetosti so pripisali notranjim napetostim nastalim zaradi razlike v koeficientih termi~nega raztezka med oksidi v oksidnem filmu. Avtorji so ugotovili, da je bila zaradi postopka pred oksidacije odpornost proti koroziji v vro~em obravnavane prevleke Al–Si mo~no izbolj{ana. Klju~ne besede: superzlitina, prevleka Al–Si, predhodna oksidacija, korozija v vro~em 1 INTRODUCTION DZ417G, a nickel-based superalloy, is mainly applied to turbine blades of aircraft engines, and its working temperature is below 980 °C. In a high-temperature envi- ronment, the fuel in the engine produces large amounts of products including S, Na, V, Cl and other elements during the combustion process. These products react with salts in the marine environment to generate the cor- responding sulphate or chloride salts, and eutectics con- taining mixed salts may cause accelerated or destructive corrosion of materials, 1 having a significant impact on the performance and service life of the engine blades. Hot corrosion is divided into two main categories: high-temperature hot corrosion (type I) at 850–1000 °C and low-temperature hot corrosion (type II) below 800 °C. The corrosion rate can be reduced by forming an Al 2 O 3 or Cr 2 O 3 oxide film on the substrate surface; how- ever, the high-temperature hot corrosion resistance of the substrate is affected. Various studies have concluded that high-temperature protective coatings can effectively pro- tect nickel-based superalloys from high-temperature hot corrosion for a long time. Among them, Al–Si coatings provide the advantages of a simple process, low cost and unique corrosion resistance for types I and II. 2–4 After an Al–Si coating is subjected to hot corrosion, there is no internal vulcanisation phenomenon in the matrix, but the internal corrosion of the coating is a serious issue. How- ever, studies have shown that pre-oxidation can reduce the internal vulcanisation rate in the coatings. After a pre-oxidation treatment at a certain tempera- ture and time, a protective oxide film is formed on the material surface, preventing corrosive substances from directly contacting the coating, thus slowing down the subsequent corrosion rate. Yang et al. 5 studied the changes in the microstructures and properties of Pt–Al coating samples after the pre-oxidation treatment and found that a continuous and dense Al 2 O 3 film formed on the coating surface after the treatment. After hot corro- sion, the oxide film on the surface of the pre-oxidised Pt–Al coating was thinner and more complete, and the structure of the oxide film formed at the same time fluc- tuated greatly, indicating that the oxide grew toward the inner layer of the coating during the dissolution pro- cess. 6,7 Materiali in tehnologije / Materials and technology 57 (2023) 1, 49–56 49 UDK 669.058.5 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(1)49(2023) *Corresponding author's e-mail: liym@ral.neu.edu.cn (Yanmei Li) To date, studies on the influence of pre-oxidation on the hot corrosion resistance of Al–Si coatings only inves- tigated the functions of pre-oxidation; however, no sys- tematic study was conducted on the impacts of the pre-oxidation process parameters on the hot corrosion re- sistance. To investigate the effects of the pre-oxidation process, this study pre-oxidised Al–Si coatings at 950 °C for 7 h, and a hot corrosion experiment was performed to analyse the effects of pre-oxidation on the hot corrosion behaviour of Al–Si coatings and the corresponding mechanisms. 2 EXPERIMENTAL PART Prior to the hot corrosion experiment, an Al–Si coat- ing was pre-oxidised at 950 °C for 7 h, as shown in Fig- ure 1. Two groups of control experiments were con- ducted with three parallel samples in each group. A prepared mixed-salt solution containing Na 2 SO 4 and 25 w/% NaCl was applied on the sample surfaces with a brush to form a salt film, and each sample was dried until the salt with a load of approximately (1.0 ± 0.5) mg/cm 2 was deposited on the surface. The specimens were put into a crucible and subjected to a hot corrosion experi- ment in an SXL-1400C high-temperature furnace at 900 °C. The specimens were removed, weighed and re- corded after (24, 48, 72, 96 and 120) h. After weighing, the specimens were returned to the furnace, and the above processes were repeated until the corrosion time reached 120 h. The samples obtained after 24 h and 120 h of hot corrosion were taken out as intermediate samples, and Origin was used to plot the corrosion kinet- ics curve based on the corrosion weighing data. A ZeissUltra55 scanning electron microscope (SEM) and SmartLab9KW X-ray diffractometer (XRD) were used to observe the surface corrosion morphologies and crys- tal structures, respectively, at the early corrosion stage (24 h) and after 120 h of corrosion. The elemental distri- bution in the cross-sections of the two sample groups was conducted using a field emission electron probe (JEOL-JXA-8530F). 3 RESULTS 3.1 Hot corrosion products and microstructures after 24 h of corrosion Figure 2 shows XRD patterns of the pre-oxidised and un-pre-oxidised Al–Si coatings after hot corrosion at 900 °C for 24 h. The main corrosion products of both groups were -Al 2 O 3 and -NiAl, with a few low-inten- sity peaks corresponding to Cr 2 O 3 . The corrosion prod- ucts on the surface of the Al–Si coating without the pre-oxidation treatment contained strong '-Ni 3 Al dif- fraction peaks and small amounts of CrS. However, the intensity of the -Al 2 O 3 diffraction peaks on the surface of the pre-oxidised Al–Si coating was relatively high, and only a few low-intensity peaks corresponding to '-Ni 3 Al could be observed. The surface and macroscopic morphologies of the pre-oxidised and un-pre-oxidised Al–Si coatings after corrosion in the Na 2 SO 4 and 25 w/% NaCl mixture at 900 °C for 24 h are shown in Figure 3. The oxide films on the surfaces of both groups are relatively lamellar, with small granular oxides. The granular oxides are mainly composed of Al, O and Ti (from Al 2 O 3 and TiO 2 ), with TiO 2 growing on Al 2 O 3 . On the other hand, the sam- ples without the pre-oxidation treatment (Figure 3b) have more granular oxides on the surface. TiO 2 can ac- celerate the growth of oxide films, and the Ti in the ox- ide film promotes the transformation of metastable -Al 2 O 3 to steady-state -Al 2 O 3 , giving rise to a surface oxide film mainly composed of stable -Al 2 O 3 and its improved high-temperature oxidation resistance. 8,9 How- ever, this transformation increases the hot-compression stress, and thus the coating may flake or crack. 10 The sur- face of the pre-oxidised Al–Si coating (Figure 3b)isrel - atively flat after 24 h of corrosion. The macroscopic morphologies after 24 h of corrosion in Figures 3b and 3c show that the surfaces of the two sample groups are relatively intact. As displayed in Figure 3c, the surface of the coating without the pre-oxidation treatment shows H. LV et al.: EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS 50 Materiali in tehnologije / Materials and technology 57 (2023) 1, 49–56 Figure 2: XRD patterns of pre-oxidised and un-pre-oxidised Al–Si coatings corroded in mixed salts at 900 °C for 24 h Figure 1: Experimental process of hot corrosion of pre-oxidised Al–Si coatings slight cracking, which is mainly ascribed to the uneven oxidation caused by stress relaxation during the growth of the oxide films or the uneven thickness of the oxide films caused by the growth of the new oxide. 11 In Fig- ure 3d, no corrosion trace or cracking on the surface of the pre-oxidised Al–Si coating can be observed. Figure 4 shows cross-sectional morphologies of the two sample groups after 24 h of corrosion. A continuous oxide film, mainly composed of Al 2 O 3 , is in the outer- most layer of both coatings; however, significant mor- phological differences appear in the interdiffusion zone. The interdiffusion zone of the Al–Si coating without the pre-oxidation treatment is mainly composed of -NiAl and a small amount of '-Ni 3 Al. In addition, this inter- diffusion zone is not continuous and has an obvious black-tissue formation. According to the elemental dis- tribution in the section shown in Figure 5, the black tis- sue mainly contains the sulphide of Cr, namely, CrS. On the coating sample with the pre-oxidation treatment, the interdiffusion zone exhibits an island structure with more -NiAl and less '-Ni 3 Al and it is more continuous with no obvious vulcanization in the coating. More impor- tantly, after 24 h of corrosion, the pre-oxidised coating formed a Si- and Cr-rich layer at the interface with the matrix. Part of the Cr formed M 6 C by combining with C and other elements, while Si promoted the formation of M 6 C. The Si-rich M 6 C formed a precipitated band, which greatly reduced the Al diffusion in the coating in the sub- strate direction and reduced the Ni diffusion in the coat- ing direction. 12,13 In addition, the precipitated zone can H. LV et al.: EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS Materiali in tehnologije / Materials and technology 57 (2023) 1, 49–56 51 Figure 3: Surface and macroscopic morphologies of pre-oxidised and un-pre-oxidised Al–Si coatings etched in the mixed salts of Na 2 SO 4 and 25 w/% NaCl at 900 °C for 24 h: surface morphology for a) un-pre-oxidised coating, b) pre-oxidised coating; and macroscopic morphology for c) un-pre-oxidised coating, d) pre-oxidised coating Figure 4: Cross-sectional morphologies of Al–Si coatings corroded in mixed salts at 900 °C for 24 h: a) un-pre-oxidised coating and b) pre-oxidised coating also easily combine with the refractory particles (Mo and V) in the matrix, ensuring that the mechanical properties of the matrix are not affected when the coated sample is oxidised at high temperatures. Si can also slow down the rate of the oxide dissolution in an alkaline solution dur- ing the molten salt corrosion and reduce the rate of hot corrosion. 14–17 3.2 Hot corrosion products and microstructures after 120 h of corrosion Figure 6 shows XRD patterns of the two sample groups after 120 h of hot corrosion. -Al 2 O 3 remained dominant on the surfaces of the two groups; however, '-Ni 3 Al diffraction peaks with high intensities appeared in both groups, and some TiO 2 remained. The diffraction peak from NiAl 2 O 4 with a relatively weak intensity was observed in the un-pre-oxidised Al–Si coating, indicat- ing that the oxide films in the coating were partially de- graded after 120 h of corrosion. Compared with the un-pre-oxidised coating, the pre-oxidised Al–Si coating presented more -Al 2 O 3 and -NiAl with fewer '-Ni 3 Al diffraction peaks. Generally, the appearance of '-Ni 3 Al indicates that the coating is degraded; however, the pres- ence of -NiAl can hinder further degradation of the coating. A continuous Al 2 O 3 film was maintained on the coating surface, and the oxide-film formation and spalling rates reached a dynamic balance, which was beneficial for improving the high-temperature corrosion resistance of the Al–Si coating. Figure 7 shows microstructures and macroscopic morphologies of the two groups of Al–Si coatings after 120 h of corrosion. As shown in Figure 7c, the surface of the Al–Si coating without the pre-oxidation treatment exhibits an obvious peeling phenomenon after 120 h of corrosion, especially at the boundary. Moreover, for the surface microstructure (Figure 7a), various round or block oxides can be observed after the corrosion, mainly containing TiO 2 by analysis. As displayed in Figure 7c, TiO 2 damages the oxide film on the Al–Si coating sur- face, which is because TiO 2 nucleates, grows on Al 2 O 3 and aggregates on the sample surface, damaging the sta- bility of the oxide films. At present, there are two theo- ries regarding the effects of Ti on promoting the spalling and cracking of oxide films: 18–20 • The rapid diffusion of Ti from the matrix to the coat- ing surface leads to the formation of TiO 2 on the Al 2 O 3 film and improves the growth rate of the oxide film. • Ti promotes the formation of Ti-rich oxide bulges on the surface of metal or oxide films, complicating the stress distribution during the oxide-film growth and leading to the formation of cracks in the films. H. LV et al.: EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS 52 Materiali in tehnologije / Materials and technology 57 (2023) 1, 49–56 Figure 6: XRD patterns of the pre-oxidised and un-pre-oxidised Al–Si coatings corroded in the mixed salts of Na 2 SO 4 and 25 w/% NaCl at 900 °C for 120 h Figure 5: Cross-sectional elemental distribution of Al–Si coatings corroded in mixed salts at 900 °C for 24 h: a) un-pre-oxidised coating and b) pre-oxidised coating However, the Ti content on the surface of the pre-oxi- dised Al–Si coating was lower (as shown in Table 1), and less TiO 2 was generated on the surface after hot cor- rosion. After 120 h of hot corrosion, the surface was in- tact with no obvious spalling or cracking; only the edges and corners exhibited slight corrosion spalling, which was mainly caused by the growth stress in the oxide film and had little impact on the overall heat-resistance corro- sion performance of the sample. Figures 8 and 9 show the cross-sectional morphol- ogies and elemental distribution for the two sample groups after 120 h of corrosion. As shown in Figure 8a, a gap with a saw-tooth shape under the oxide layer is ob- served on the surface of the Al–Si coating without the pre-oxidation treatment, and the elemental distribution H. LV et al.: EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS Materiali in tehnologije / Materials and technology 57 (2023) 1, 49–56 53 Table 1: Average compositions of pre-oxidised and un-pre-oxidised Al–Si coatings after 120 h of corrosion in atomic % (x/%) Sample O Na Al Si Ti Cr Co Ni UnPreO 63.47 5.98 2.53 1.35 21.86 1.82 2.16 0.83 PreO 27.61 2.15 23.79 1.65 9.89 14.22 2.32 18.37 Figure 8: Cross-sectional morphologies of the Al–Si coatings corroded by the mixed salts of Na 2 SO 4 and 25 w/% NaCl at 900 °C for 120 h: a) un-pre-oxidised coating and b) pre-oxidised coating Figure 7: Al–Si coatings corroded in the mixed salts of Na 2 SO 4 and 25 w/% NaCl at 900 °C for 120 h with surface morphologies for: a) un-pre-oxidised coating, b) pre-oxidised coating; and macroscopic morphology for c) un-pre-oxidised coating, d) pre-oxidised coating indicates that it is a Ti-rich diffusion layer. The black area mainly contains the sulphide of Cr, namely, CrS, in- dicating that the inner coating has been corroded by the molten salts, leading to coating degradation. As dis- played in Figure 8b, no void band can be found on the surface of the pre-oxidised coating after 120 h of corro- sion, and no vulcanisation inside the coating is observed. The overall coating is relatively intact. The outer layer of the coating is an Al-rich oxide layer, and the inter- diffusion zone is composed of -NiAl and a small amount of '-Ni 3 Al, with Si- and Cr-rich diffusion bands at the interface between the coating and the matrix. 4 DISCUSSION 4.1 Corrosion kinetics analysis The hot corrosion kinetic curves for the pre-oxidised and un-pre-oxidised Al–Si coatings in the Na 2 SO 4 and 25 w/% NaCl mixture at 900 °C are shown in Figure 10. The trends of the mass gain in the two sample groups are almost identical. The corrosion mass gain for the sam- ples with the pre-oxidation treatment is significantly lower than that for the samples without the treatment. In the first 48 h, the corrosion mass gain rate for the Al–Si coating without the pre-oxidation treatment is faster be- cause a metastable -Al 2 O 3 film is mainly formed on the coating surface, which has a weak hot corrosion resis- tance. After 48 h, the corrosion mass gain rate for the pre-oxidised Al–Si coating decreases and tends to be constant; however, the rate remains faster for the samples without pre-oxidation. The growth stress in the pre-oxi- dised samples increase owing to the damage by the S 2– and Cl – from the molten salts at the early stage of corro- sion, damaging the integrity of the oxide films and mak- ing them loose and porous. 4.2 Mechanistic analysis of hot corrosion at high tem- peratures Owing to the combination of turbine gases (V, Cl and S) and elements (Na) in air, salt mixtures with low melt- ing points are formed. The melting point of pure Na 2 SO 4 is 884 °C, and NaCl can reduce the Na 2 SO 4 melting point and finally form a eutectic system with a melting point 21, 22 of 620 °C. 21, 22 The experimental temperature is 900 °C, significantly promoting the oxidation process of Na 2 SO 4 and NaCl. 23 In the early stage of hot corrosion, the main corrosion product on the outer surface of the coating is Al 2 O 3 owing to the high Al content, high oxy- gen partial pressure at the initial stage and fast oxidation rates. The specific reaction is given by Equation (1): 4/3 Al + O 2 = 2/3 Al 2 O 3 (1) In the molten state, the Na 2 SO 4 in the mixed salts penetrates the Al 2 O 3 film formed on the coating surface H. LV et al.: EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS 54 Materiali in tehnologije / Materials and technology 57 (2023) 1, 49–56 Figure 10: Hot corrosion kinetic curves of un-pre-oxidised and pre-oxidised Al–Si coatings in mixed salts at 900 °C Figure 9: Elemental distribution of the cross-sections of the Al–Si coatings corroded by the mixed salts of Na 2 SO 4 and 25 w/% NaCl at 900 °C for 120 h: a) un-pre-oxidised coating; and b) pre-oxidised coating and reacts, as shown in Equation (2). Because of the re- action between Al 2 O 3 and Na 2 SO 4 , the protective oxide film, Al 2 O 3 , is continuously dissolved during the hot cor- rosion process, leading to cracking and peeling of the ox- ide film. 24 Na 2 SO 4 +Al 2 O 3 = 2NaAlO 2 +SO 3 (2) As the surface Al 2 O 3 film is damaged by the molten salt, the -NiAl in the coating, as a source storing Al, provides Al, which repairs the cracked or peeled off Al 2 O 3 film by spreading to the outer layer of the coating, ensuring a relatively continuous and complete protective oxide film on the coating surface. With continuous corro- sion, the oxides containing Al on the surface are dis- solved and, hence, the Al element is constantly con- sumed, reducing the heat and corrosion resistance of the matrix and coating, thus promoting the oxidation and vulcanisation of the samples. Therefore, according to the above morphological characterisation and elemental dis- tribution analyses, the coating samples without the pre-oxidation treatment exhibit internal oxidation and vulcanisation, which is mainly due to the fact that with an increase in the corrosion time, alloying elements in the matrix diffuse into the coating and interact with Na 2 SO 4 , resulting in the following reactions: Na 2 SO 4 + 8/3 Al = Na 2 O+Al 2 O 3 +Al 2 S 3 (3) Na 2 SO 4 +3Cr=Na 2 O+Cr 2 O 3 +CrS 3 (4) 4.3 Mechanism of pre-oxidation effects on hot corro- sion resistance of Al–Si coatings According to Vialas and Monceau, 20 the Bedworth ra- tios (PBRs) of Al 2 O 3 and TiO 2 growing on -NiAl are different. The PBR of Al 2 O 3 on NiAl is 1.8, whereas that of TiO 2 on NiAl is 2.6. This significant difference causes the formation of TiO 2 on the oxide film, which can easily induce a large local compressive stress, leading to the tendency toward cracking and peeling of the coating. 19 However, pre-oxidation can enhance the anti-corrosion properties of the Al–Si coating at high temperatures. Af- ter the pre-oxidation treatment, a relatively stable oxide film containing -Al 2 O 3 is formed on the coating sur- face. In addition, as provided in Table 1, the content of Ti on the coating surface is low. Therefore, the pre-oxi- dation treatment reduces the impacts of Ti on the oxide films on coating surfaces. Moreover, because the forma- tion of the continuous -Al 2 O 3 on coating surfaces takes a certain time, S 2– and Cl – from the molten salts and O 2– from air enter the interior of the coating without pre-oxi- dation through diffusion during hot corrosion, leading to a reduction of the hot corrosion resistance of the Al–Si coating. In contrast, a continuous oxide film is formed on the coating surface due to the pre-oxidation treatment be- fore hot corrosion, which prevents a direct contact of the coating with the external O 2– and the elements, such as S 2– and Cl – from the molten salts, and reduces its hot cor- rosion rate. 5 CONCLUSIONS During hot corrosion in mixed salts at 900 °C, the corrosion weight of the pre-oxidised Al–Si coating was significantly reduced, and the corrosion rate was much slower than that of the un-pre-oxidised Al–Si coating. After 24 h of hot corrosion, -Al 2 O 3 formed on the surfaces of both coating groups. However, the pre-oxi- dised Al–Si coating was more intact and compact after corrosion and contained more -NiAl, ensuring the con- tinuous formation of the Al 2 O 3 film on the coating sur- face. After 120 h of hot corrosion, large amounts of TiO 2 were generated on the un-pre-oxidised Al–Si coating surface, with obvious peeling at the boundaries; CrS and other compounds were found in the coating. In contrast, the surface of the pre-oxidised Al–Si coating remained intact and was mainly composed of -Al 2 O 3 with no ob- vious sulphides in the coating. The oxide film generated by the pre-oxidation treat- ment can prevent S 2– and Cl – from the molten salts and O 2– from air from directly contacting the coating surface, reduce the rate of the TiO 2 formation in the oxide layer, and form an oxide film dominated by Al 2 O 3 , signifi- cantly improving the hot corrosion resistance of the coat- ing. 6 REFERENCES 1 G. M. Liu, F. Yu, J. H. Tian, J. H. Ma, Influence of pre-oxidation on the hot corrosion of M38G superalloy in the mixture of Na2SO4-NaCl melts, Mater. Sci. Eng.: A, 496 (2008) 1–2, 40–44, doi:10.1016/j.msea.2008.04.046 2 F. Fitzer, Aluminium and silicon base coatings for high temperature alloys – Process development and comparison of properties, Thin Solid Films, 64 (1979) 2, 305–319, doi:10.1016/0040-6090(79) 90525-X 3 F. Fitzer, J. Schlichting, Coatings containing chromium, aluminum, and silicon for high temperature alloys, NACE – International Corro- sion Conference Series, 32 (1983), 604–614 4 N. J. Simms, Erosion-corrosion modelling of gas turbine materials for coal-fired combined cycle power generation, Wear, 186 (1996) 95, 247–255, doi:10.1016/0043-1648(95)07167-9 5 Y. F. Yang, C. Y. Jiang, Z. Y. Zhang, Z. B. Bao, M. H. Chen, S. L. Zhu, F. H. Wang, Hot corrosion behaviour of single-phase plati- num-modified aluminide coatings: Effect of Pt content and pre-oxi- dation, Corros. Sci., 127 (2017), 82–90, doi:10.1016/j.corsci.2017. 08.015 6 Q. Li, D. Zhang, P. Song, Z. Li, J. Lu, Influence of Pre-Oxidation on High Temperature Oxidation and Corrosion Behavior of Ni-Based Aluminide Coating in Na2SO4 Salt at 1050 °C, Front. Mater., 5 (2021), 1–13, doi:10.3389/fmats.2021.679682 7 J. Peng, T. Huang, P. Song, W. Huang, R. Chen, Effect of platinum and pre-oxidation on the hot corrosion behavior of aluminide coating with NaCl at 1050 ?, Mater. Res. Express., 7 (2020) 11, 116–121, doi:10.1088/2053-1591/abc4b2 8 M. Göbel, A. Rahmel, M. Schötze, M. Schorr, W. T. Wu, Inter- diffusion between the platinum-modified aluminide coating RT 22 and nickel-based single-crystal superalloys at 1000 and 1200 °C, Mater. High. Temp., 12 (1994) 4, 301–309, doi:10.1080/09603409. 1994.11752534 H. LV et al.: EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS Materiali in tehnologije / Materials and technology 57 (2023) 1, 49–56 55 9 B. A. Pint, B. A. Nagaraj, M. A. Rosenzweig, Evaluation of TBC-coated -NiAl substrates without a bond coat, Office of scien- tific & technical information technical reports, 9 (1996), 51–60 10 B. A. Pint, On the formation of interfacial and internal voids in -Al2O3 scales, Oxid. Met., 48 (1997) 3, 303–328, doi:10.1007/ BF01670505 11 Q. Li, X. Yuan, D. Li, P. Song, Z. Li, T. Huang, L. Wen, Y. Liang, J. Lu, Effect of pre-oxidation treatment on the hot corrosion behavior of pack-cemented aluminide coatings on the K438 alloy in salt mix- ture, Corrosion Communications, 5 (2022) 3, 1–13, doi:10.1016/ j.corcom.2021.10.006 12 Z. L. Yang, X. F. Yu, Effect of Si Distribution on Hot Corrosion Be- havior of Al-Si Coating, Journal of Aeronautical Materials, 14 (1994) 2, 36–40 13 S. W. Yang, Y. J. Shi, T. Y. Zhang, Y. H. Wang, Element Diffusion Analysis of Al-Si Coating on K4104 Nickel Base Superalloy During High Temperature Oxidation, Development and Application of Mate- rials, 25 (2010) 5, 4–8, doi:10.19515/j.cnki.1003-1545.2010.05.002 14 K. Maki, M. Shioda, M. Sayashi, T. Shimizu, S. Isobe, Effect of sili- con and niobium on oxidation resistance of TiAl intermetallics, Ma- ter. Sci. Eng.: A, 153 (1992) 1–2, 591–596, doi:10.1016/0921- 5093(92)90256-Z 15 S. Bose, High Temperature Coatings, 2 nd ed., Butterworth-Heine- mann, Oxford 2007, 71–154 16 J. L. Murray, A. J. Mcalister, The Al-Si (Aluminum-Silicon) system, Bulletin of Alloy Phase Diagrams, 5 (1984) 1, 74–84 17 Y. L. Cai, Y. R. Zheng, L. S. Mo, Z. L. Yang, Distribution and action of silicon in Al-Si coatings: Study on Si-containing interlayer, Jour- nal of Materials Engineering, 1 (1981) 1, 39–44 18 J. F. Ackerman, Heating substrate coated with platinum and excess aluminum to diffuse metal into substrate to form aluminide with trace platinum, Patent US, US5494704 A, 1996 19 M. J. Li, X. F. Sun, H. R. Guan, X. X. Jiang, Z. Q. Hu, Effect of Pal- ladium Incorporation on Isothermal Oxidation Behavior of Aluminide Coatings, Oxid. Met., 59 (2003) 5–6, 483–502, doi:10.1023/A:1023667106068 20 N. Vialas, D. Monceau, Substrate Effect on the High-Temperature Oxidation Behavior of a Pt-Modified Aluminide Coating. Part I: In- fluence of the Initial Chemical Composition of the Coating Surface, Oxid. Met., 66 (2006) 3–4, 155–189, doi:10.1007/s11085- 006-9024-z 21 P. Hancock, Vanadic and chloride attack of superalloys, Mater. Sci. Technol., 23 (1987), 113–119, doi:10.1080/02670836.1987. 11782265 22 N. Latanision, Hot corrosion in gas turbine components, Eng. Fail. Anal., 9 (2002) 1, 31–43, doi:10.1016/S1350-6307(00)00035-2 23 R. D. Liu, S. M. Jiang, H. J. Yu, J. Gong, C. Sun, Preparation and hot corrosion behaviour of Pt modified AlSiY coating on a Ni-based superalloy, Corros. Sci., 104 (2016) 3, 162–172, doi:10.1016/ j.corsci.2015.12.007 24 Y. Pei, C. Zhou, Improved hot corrosion resistance of Dy-Co-modi- fied aluminide coating by pack cementation process on nickel base superalloys, Corros. Sci., 112 (2016) 11, 710–717, doi:10.1016/ j.corsci.2016.09.011 H. LV et al.: EFFECTS OF PRE-OXIDATION ON HOT CORROSION RESISTANCE OF Al–Si COATINGS 56 Materiali in tehnologije / Materials and technology 57 (2023) 1, 49–56