Corrosion behavior of zirconium diboride coated stainless steel in molten 6061 aluminum alloy

Accepted Manuscript Corrosion behavior of zirconium diboride coated stainless steel in molten 6061 aluminum alloy

Qian Wang, Wen Jun Wang, Hui Jun Liu, Chao Liu Zeng PII: DOI: Reference:

S0257-8972(17)30069-5 doi: 10.1016/j.surfcoat.2017.01.069 SCT 22045

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

2 December 2016 16 January 2017 19 January 2017

Please cite this article as: Qian Wang, Wen Jun Wang, Hui Jun Liu, Chao Liu Zeng , Corrosion behavior of zirconium diboride coated stainless steel in molten 6061 aluminum alloy. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi: 10.1016/j.surfcoat.2017.01.069

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Corrosion behavior of zirconium diboride coated stainless steel in molten 6061 aluminum alloy

Qian Wang a,b, Wen Jun Wang a, Hui Jun Liu a *, Chao Liu Zeng a Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese

PT

a

School of Materials Science and Engineering, University of Science and Technology

SC

b

RI

Academy of Sciences, Shenyang, 110016, P R China.

of China, Hefei, 230026, P R China

Tel.: +86-24-23904553;

NU

* Corresponding author.

AC

CE

PT E

D

MA

E-mail address: [email protected] (H. J. Liu).

2

Fax: +86-24-23904551.

ACCEPTED MANUSCRIPT Abstract A zirconium diboride coating on 201 stainless steel prepared by electrodeposition in NaCl-KCl-K2ZrF6-KBF4 molten salts and the stability and corrosion mechanism of the stainless steel with and without ZrB2 coating in molten aluminum alloy at 800 oC

PT

are investigated by immersion tests. The results show that the stainless steels suffer

RI

severe corrosion by aluminum after only 2 h immersion, while the ZrB2-coated

SC

stainless steels show high corrosion resistance to aluminum attacks after 120 h immersion. It can be inferred that surface modification of stainless steel by

NU

electrodepositing ZrB2 coating in molten salts is an effective way to protect substrate

MA

materials from degeneration in molten aluminum.

PT E

1. Introduction

D

Keywords: ZrB2; stainless steel; coating; molten aluminum alloy; corrosion

Aluminum and its alloys have been widely applied in the industries like transportation,

CE

aerospace, metallurgy, electronics for their excellent comprehensive properties [1-4].

AC

But as one of the most corrosive liquid metals, the corrosion and erosion caused by molten aluminum greatly shorten the service life of the iron-based materials that are used in crucibles, pumps and dies of aluminum die-casting [5-7]. To solve this problem, one economic and effective way is to use surface treatments to protect the substrate materials against the attack of molten aluminum. Actually, some investigations have been focused on the surface treatments of iron-based materials to improve the corrosion resistance and prolong the service

3

ACCEPTED MANUSCRIPT duration in molten aluminum [8-14]. WC-Co sprayed coatings were prepared on carbon steel substrates by using high velocity oxygen-fuel (HVOF) techniques by Lopez et al. [8] and the coatings maintained integrated after 24 h immersion in molten aluminum at 700 oC. Molinari et al. [9] used different treatments to modify the surface

PT

of AISI H11 hot work tool steel and found that a double-layer PVD-prepared

RI

CrN/ZrN coating with a plasma nitriding pre-treatment showed the best anti-corrosion

SC

behavior against molten aluminum at 740~760 oC. Lin et al. [10] developed CrN and CrN/TiN multilayer coatings by cathode arc evaporating method and conducted

NU

dipping tests in molten A356 aluminum alloy at 700 oC. It was found that both

MA

coatings were locally attacked after a certain incubation period but still remained compact after 21 h dipping. HVOF techniques were also applied by Salman et al. [12]

D

to prepare Ti(Al, O)/Al2O3 and TiAl(O)/Al2O3 composite coatings. The results

PT E

showed that TiAl(O)/Al2O3 composite coatings exhibited better anti-corrosion behavior but molten aluminum infiltration still happened after 22 h immersion. Tsipas

CE

et al. [11] immersed the surface-boronized carbon and high alloy steels into molten

AC

aluminum at 630 oC. The results showed that the boronization treatment had greatly enhanced the anti-corrosion behavior of the substrates, though slight intergranular attacks were observed after 120 h immersion. Zr(B, C) and Zr(B, C, N) coatings were synthesized by chemical vapor deposition methods [13] and the corrosion behavior of the coatings in molten aluminum were also tested by rotating the samples for 20 min at 700 oC. It was found that the optimum erosion resistance coatings should be nitrogen rich and carbon poor.

4

ACCEPTED MANUSCRIPT Among these researches, ceramic-based coatings showed excellent anti-corrosion potential in molten aluminum, but it should be mentioned that local attacks happened to most coating systems after certain time of immersion [8-10, 12]. This is mainly attributed to the defects of the coatings and the thermo-mechanical stress induced

PT

during dipping [8, 10]. Besides, it has to be marked that the reported immersion time

RI

in the literatures is comparatively short. Therefore, long-time tests are needed to

SC

further evaluate the corrosion behavior of the coatings in the molten aluminum and explore their potential applications in aluminum industry. Thus, it is very necessary to

NU

develop a compact and adherent coating that exhibits favorable corrosion resistance

MA

against molten aluminum. It was reported that all the borides may be regarded stable in liquid aluminum up to a temperature of 1000 oC [15]. Besides, the contact angle

D

between ZrB2 and molten aluminum was 106°~60° at 900~1250 oC according to the

PT E

literature [15], indicating that ZrB2 would not be wetted by aluminum under certain conditions. Therefore, it is potential to utilize ZrB2 as barrier layer against aluminum

CE

attacks. In our previous study [16], ZrB2 coatings have been successfully prepared on

AC

stainless steel substrates by electrochemical deposition in molten salt system. Therefore, the aim of the present study is to investigate the corrosion behavior of the ZrB2 coatings in molten aluminum and explore their potential applications in aluminum industry.

2. Experimental 2.1 Preparation of ZrB2 coatings

5

ACCEPTED MANUSCRIPT 201 stainless steels (1Cr17Mn6Ni5N) were used as substrate materials for the deposition of ZrB2 coatings. The samples were machined to the size of 20 mm×10 mm×2 mm and ground to 2000 grit, ultrasonic cleaned with acetone and alcohol, respectively, then dried. ZrB2 coatings were electrochemically deposited using a 80 g

PT

eutectic NaCl-KCl (supplied by Sinopharm Chemical Reagent Co., Ltd. analytical

RI

grade ≥ 99.5%) molten salt with the addition of electroactive species, a mixture of

SC

2.87 mol % K2ZrF6 (supplied by Shanghai SSS Reagent Co., Ltd. analytical grade≥99.5%) and 7.19 mol % KBF4 (supplied by Shanghai SSS Reagent Co., Ltd.

NU

analytical grade≥99.8%) in the molar ratio of 1:2.5. The salts were dried under

MA

vacuum for 24 h at 200 oC to remove the residual water. Galvanostatic deposition, in which the graphite crucible acted as anode and 201 stainless steel as cathode, was

D

performed with the current density of 100 mA·cm-2 at 750 oC using the molten salt

PT E

reaction apparatus we had stated [17]. After electrodeposition, the samples were rinsed in distilled water and then dried for the corrosion tests in molten aluminum.

CE

2.2 Immersion tests in molten aluminum

AC

Aluminum used in the experiment was a 6061 aluminum alloy with the chemical composition shown in Table 1. Aluminum was melted in a corundum crucible, which was placed in the bottom of a closed stainless steel chamber as shown in Fig. 1. The reaction chamber was heated with a well furnace and aerated with flowing high-purity argon. The temperature was controlled by a PtRh-Pt thermocouple with the accuracy of ±1 K. Uncoated and ZrB2-coated samples were statically dipped into the molten aluminum and held for different time at 800 oC. For the stainless steel substrates, the

6

ACCEPTED MANUSCRIPT immersion time was 2 and 4 h. For the ZrB2-coated samples, the immersion time ranged from 4 to 120 h. After immersion, the samples were taken out of the chamber and air cooled. Table 1

PT

Fig. 1.

RI

2.3 Sample characterization

SC

The as-prepared coatings were characterized by scanning electron microscopy (SEM, FEI FP 2031/11 inspect F) and X-ray diffraction (XRD, PANalytical X’Pertpro) to

NU

obtain the microstructures, morphologies and phase structures. After the immersion

MA

tests, the aluminum that adhered to the surface of the samples was kept to study the interface between the solidified aluminum and the coatings. Macrographs of the

D

uncoated and coated samples were taken by a digital camera (Nikon COOPLEX

PT E

P330). SEM was used to characterize the surface and cross-sectional morphology of the as-immersed samples. Energy dispersive X-ray spectrometer (EDX, Oxford) was

CE

also employed to determine the chemical composition of the phases that formed

AC

during immersion tests in molten aluminum.

3. Results and Discussion 3.1 The preparation of ZrB2 coatings The influences of current density, temperature and electrolysis time on the electrodeposition of ZrB2 coatings on 201 stainless steels in NaCl-KCl-K2ZrF6-KBF4 molten salt have already been discussed in our previous study [16]. Therefore, the

7

ACCEPTED MANUSCRIPT optimum parameters are chosen to obtain the dense and coherent ZrB2 coatings. Fig. 2(a) shows the typical morphology of the electrodeposited coating on the stainless steel, which is homogeneous, lamellar-crystallized and compact. From the cross-section image given in Fig. 2(b), it can be seen that the coating is dense and

PT

adherent to the substrate with an average thickness of 25 μm. The XRD pattern of a

RI

representative sample is also illustrated in Fig. 3, which indicates that the coating is

Fig. 2

NU

Fig. 3

SC

consisted of pure phase of ZrB2.

MA

3.2 Corrosion behavior of the stainless steel in molten aluminum Fig. 4 shows the macrographs and cross-section images of the stainless steels after 2

D

and 4 h immersion in molten aluminum. It can be seen that after 2 h immersion, the

PT E

surface of the steel is covered with a thick layer of solidified aluminum and two layers (outer layer and inner layer) appear in the cross-section image shown in Fig. 4b. Inner

CE

layer is about 20 μm in thickness and adhered well to the substrate, while outer layer

AC

is comparatively porous and less homogeneous with the thickness of 50 μm. Besides, some plate-shaped particles exist between the interface of the outer layer and the solidified aluminum. Fig. 4 The specimen after immersion for 4 h exhibits the similar corrosion behavior as 2 h, but it has to be pointed out that the substrate has partly dissolved into the molten aluminum as shown in Fig. 4(c), which indicates that the steel has been severely

8

ACCEPTED MANUSCRIPT attacked by aluminum. What’s more, the thickness of both of the outer and inner layer increase to 70 and 50 μm for 4 h immersion compared to 50 and 20 μm for 2 h immersion. In addition, from the EDX linear scanning results shown as the insert in Fig. 4(b) and 4(d), it can be seen that both outer layer and inner layer are consisted of

PT

intermetallic compounds of Al, Fe and Cr. However, the outer layer possesses a higher

RI

Al content and relatively lower Fe and Cr compared with the inner layer.

SC

To further analyze the composition of the intermetallic compounds, EDX point scannings were conducted by choosing different positions from both layers and

NU

average concentrations in weight percent are given in Table 2.

MA

Table 2.

From the EDX results, it can be seen that the average composition of the inner layer is

D

approximately equal to 54 wt. % Al, 32 wt. % Fe, 10 wt. % Cr, 3 wt. % Ni and 1 wt. %

PT E

Si. For this, Wu et al. [18] reported that the (Fe, Cr)2Al5 phase mainly consists of 52-56 wt. % Al, 34-38 wt. % Fe and about 10 wt. % Cr. Zhang et al [7] also measured

CE

the inner layer of several duplex stainless steels dipped in molten aluminum was

AC

composed of 61 wt. % Al, 28 wt. % Fe and 10 wt. % Cr. Thus, it can be concluded that the inner layer is composed of (Fe, Cr)2Al5 intermetallic compound. Furthermore, the average composition of the outer layer is 70 wt. % Al, 21 wt. % Fe, 5 wt. % Cr, 2 wt. % Ni and 2 wt. % Si, which would be (Fe, Cr)Al3. The compositions of the plate-shaped phases were also examined by EDX and found to be Al-Fe-Cr intermetallic compound with higher Al content of 78 wt. %. Therefore, the corrosion mechanism of the stainless steel in molten aluminum can be

9

ACCEPTED MANUSCRIPT summarized as follows. Chemical reactions between Fe, Cr and Al happen on the solid-liquid interface and (Fe, Cr)Al3 forms when molten aluminum wets the steel substrate, which is prone to nucleate comparing with other Al-Fe-Cr intermetallic compounds [5, 7]. After an outer layer of (Fe, Cr)Al3 forms, (Fe, Cr)2Al5 starts to

PT

nucleate on the interface between (Fe,Cr)Al3 and the substrate due to the composition

RI

undulation and finally leads to the formation of the inner layer [7, 18].

SC

3.3 Corrosion behavior of ZrB2-coated stainless steel in molten aluminum Basing on the upper discussions, it can be concluded that the stainless steels show

NU

poor resistance to molten aluminum attacks. Therefore, ZrB2-coated stainless steels

MA

were prepared and immersed into molten aluminum at 800 oC for 4, 8, 24, 48, 72, 96 and 120 h to evaluate their corrosion resistance. Fig. 5 shows the macrographs of

D

ZrB2-coated specimens before and after immersion for 4, 8 and 24 h in molten

PT E

aluminum. The results show that the surfaces of the ZrB2-coated substrates are hardly covered with aluminum after 4 h and 8 h immersion, while only a small amount of

CE

solidified aluminum presents when the immersion time is prolonged to 24 h. Besides,

AC

the covered aluminum is easily removed from the surface after air cooling, which indicates very poor adherence between the ZrB2 coating and the aluminum. Therefore, molten aluminum can hardly wet the surface of ZrB2-coated steel at the initial period of immersion, which is beneficial to the protection of the substrate against aluminum attacks. From the surface morphologies of the immersed specimens presented in Fig. 6, it can be seen that the ZrB2 coatings show no signs of degradation or detachment from the substrates. No obvious cracks appear after 4, 8 and 24 h immersion, but

10

ACCEPTED MANUSCRIPT some white color particles (red arrow shown in Fig. 6(c)) emerge after 24 h. The particles are consisted of 43.58 wt. % Zr, 36.88 wt. % B, 2.32 wt. % Al, 15.61 wt. % O, 1.06 wt. % Mg and 0.56 wt. % Fe. Fig. 5

PT

Fig. 6

RI

The cross-section images of the ZrB2-coated stainless steels after 4, 8 and 24 h

SC

immersion are shown in Fig. 7(a-c). The results reveal low adhesion of aluminum, which are in coincidence with the macrographs and the surface morphologies. There

NU

are no signs of aluminum infiltration into the susbtrates, indicating that ZrB2 coating

MA

is an effective barrier against aluminum attacks. Fig. 7

D

To further evaluate long-term resistance of ZrB2 coatings against molten aluminum

PT E

attacks, we prolong the immersion time to 48, 72, 96 and 120 h. Fig.8(a) and (c) shows the macrographs of ZrB2-coated stainless steels after 48 and 72 h immersion.

CE

Comparing with the samples after 4~24 h immersion disscussed above, ZrB2 coatings

AC

are more inclined to be wetted by molten aluminum, which may be attributed to the change of the surface state after long-time immersion. From the low magnitude cross-section images revealed in Fig. 8(b) and (d), it can be seen that ZrB2 coatings still stay integrated and perform as protection barriers against molten aluminum attacks. Fig. 8 When prolonging the immersion time to 96 h, the ZrB2-coated specimen still

11

ACCEPTED MANUSCRIPT maintains its integrity as shown in Fig. 9(a-b). However, some localized attacks are formed at the corner of the specimen, which are pointed as area 1 in Fig. 9(b). The amplification image of area 1 is shown in Fig. 9(c). It can be seen clearly that molten aluminum has infiltrated into the ZrB2 coatings and the outer (Fe, Cr)Al3 layer and the

PT

inner (Fe, Cr)2Al5 layer are formed. Therefore, with the immersion time increasing,

RI

some parts of ZrB2 coatings may peel off and then fresh stainless steel substrates are

SC

exposed to aluminum attacks and deep corrosion region forms, while unpeeled parts of the coatings are filled with corrosion products, as shown in Fig. 9(c), and lead to

NU

the separation of coatings from the stainless steel substrates.

MA

Fig. 9

After 120 h immersion in molten aluminum, the specimen is complete as a whole seen

D

from Fig. 10(a-b), but cracks are observed in the ZrB2 coatings and the stainless steel

PT E

substrates are also attacked, as shown in Fig. 10(c). In addition, the failure modes of ZrB2-coated substrates under aluminum attacks are carried out in two different ways

CE

as shown in Fig. 10(c) and (d). One way is that molten aluminum infiltrates into the

AC

substrate thorough penetrating cracks in the ZrB2 coating and corrosion layers form below the perceived ZrB2 coatings (Fig. 10(c)), while the other way happens due to the separation of ZrB2 coatings from the substrates, as shown in Fig. 10(d). The formation of the corrosion products causes the volume expansion, which gives rise to the spallation of the ZrB2 coatings. Fig. 10 3.4 Failure mechanism of ZrB2 coatings in molten aluminum

12

ACCEPTED MANUSCRIPT From the above discussions, it can be seen that ZrB2 coatings are effective potective barriers against long-time molten aluminum attacks. The contact between ZrB2 coatings and aluminum can be concluded as follows. During the initial period of immersion, ZrB2 coated specimens can be hardly wetted by molten aluminum, which

PT

is in agreement with the reported contact angle between ZrB2 and molten aluminum

RI

[15]. With the time passing by, the surface condititon of ZrB2 coatings gradually

SC

changes and turns to be wetted by aluminum. Although ZrB2 coatings contact with molten aluminum, the coatings still remain compact and protect the substrate against

NU

aluminum attacks.

MA

However, after 96 h immersion in molten aluminum, localized attacks start to emerge in places like corners of the specimens as shown in Fig. 9(c), at which residual

D

stresses tend to concentrate. Furthermore, attacks happen at other regions after 120 h

PT E

immersion. The failure mechanism of ZrB2 coatings can be concluded as follows: aluminum starts to infiltrate into the weak regions like corners, defects and cracks in

CE

the ZrB2 coatings with the increasing immersion time in molten aluminum. When

AC

aluminum has penetrated into the substrates, intermetallic compounds like (Fr, Cr)Al3 and (Fe, Cr)2Al5 begin to form under the ZrB2 coatings. The thermo-mechanical stresses difference between the ZrB2 coatings and the intermetallic compounds lead to the separation of coatings from the stainless steel substrates [9, 10, 19]. Besides, the formation of corrosion products can also cause the volume expansion, which can also gives rise to the spallation of the ZrB2 coatings as shown in Fig.10(d). More fresh stainless steel substrates are exposed to molten aluminum and the process inevitably

13

ACCEPTED MANUSCRIPT causes larger areas of corrosion. However, the maintained ZrB2 coatings can still function as effective barrier against aluminum attacks.

Conclusions

PT

Dense, adherent and homogeneous ZrB2 coating has been successfully synthesized

RI

using galvanostatic depositions in NaCl-KCl-K2ZrF6-KBF4 molten salts. The

SC

uncoated substrates are vulnerable to aluminum attacks and thick corrosion layer forms only after 2 h immersion in molten 6061 aluminum alloy at 800 oC. However,

NU

the ZrB2-coated stainless steel can remain complete and unattacked after 72 h

MA

immersion. Although localized attacks happen after 96 h and 120 h tests, the ZrB2-coated stainless steel still remain integrated and show good resistance to molten

D

aluminum. Therefore, ZrB2 coating will be an effective way to protect substrate

CE

Acknowledgements

PT E

materials from degradation in molten aluminum.

AC

This work is supported by National Natural Science Foundation of China Grant No. 51271190 and 51501205.

References [1] A.K. Vasudevan, R.D. Doherty, Aluminum alloys: contemporary research and applications, Nuclear Physics B - Proceedings Supplements, 174 (1989) 213–216. [2] E.A. Starke, J.T. Staley, 24–Application of modern aluminium alloys to aircraft,

14

ACCEPTED MANUSCRIPT Fundamentals of Aluminium Metallurgy, (2011) 747-783. [3] A. Heinz, A. Haszler, C. Keidel, S. Moldenhauer, R. Benedictus, W.S. Miller, Recent development in aluminum alloys for aerospace applications, Mater. Sci. Eng. A, 280 (2000) 102-107.

PT

[4] G.B. Burger, A.K. Gupta, P.W. Jeffrey, D.J. Lloyd, Microstructural control of

RI

aluminum sheet used in automotive applications, Mater. Charact., 35 (1995) 23-39.

SC

[5] K. Bouché, F. Barbier, A. Coulet, Intermetallic compound layer growth between solid iron and molten aluminium, Mater. Sci. Eng. A, 249 (1998) 167-175.

NU

[6] R. Yang, P. Zhang, J.H. Wu, The corrosion mechanism of a stainless steel

MA

electrode in the dynamic melting of Al at high temperature, Mater. Sci. Eng. A, 499 (2009) 134-137.

D

[7] X. Zhang, X. Li, W. Chen, Interfacial reactions of duplex stainless steels with

PT E

molten aluminum, Surf. Interface Anal., 47 (2015) 648-656. [8] A.J. López, J. Rams, Protection of carbon steel against molten aluminum attack

CE

and high temperature corrosion using high velocity oxygen-fuel WC–Co coatings,

AC

Surf. Coat. Tech., 262 (2014) 123-133. [9] A. Molinari, M. Pellizzari, Corrosion behaviour of a surface-treated AISI H11 hot work tool steel in molten aluminium alloy, Surf. Coat. Tech., 126 (2000) 31-38. [10] C.S. Lin, C.S. Ke, H. Peng, Corrosion of CrN and CrN/TiN coated heat-resistant steels in molten A356 aluminum alloy, Surf. Coat. Tech., s 146–147 (2001) 168-174. [11] D.N. Tsipas, G.K. Triantafyllidis, J.K. Kiplagat, P. Psillaki, Degradation behaviour of boronized carbon and high alloy steels in molten aluminium and zinc,

15

ACCEPTED MANUSCRIPT Mater. Lett., 37 (1998) 128-131. [12] A. Salman, B.L. Gabbitas, P. Cao, D.L. Zhang, The performance of thermally sprayed titanium based composite coatings in molten aluminium, Surf. Coat. Tech., 205 (2011) 5000-5008.

PT

[13] K.T. Rie, C. Pfohl, S.H. Lee, C.S. Kang, Development of zirconium and boron

SC

MO–PACVD, Surf. Coat. Tech., 97 (1997) 232-237.

RI

containing coatings for the application on aluminium diecasting tools by means of

[14] D. Wang, Z. Shi, L. Zou, A liquid aluminum corrosion resistance surface on steel

NU

substrate, Appl. Surf. Sci., 214 (2003) 304-311.

MA

[15] G.V. Samsonov, A.D. Panasyuk, M.S. Borovikova, Contact reaction between refractory compounds and liquid metals, Soviet Powder Metallurgy and Metal

D

Ceramics, 12 (1973) 403-407.

PT E

[16] Q. Wang, Y.L. Wang, H.J. Liu, C.L. Zeng, Electrochemical Deposition of Zirconium Diboride Coatings in NaCl-KCl-K2ZrF6-KBF4 Melts, J. Electrochem. Soc.,

CE

163 (2016) D636-D644.

AC

[17] Q. Wang, Y. Wang, H. Liu, C. Zeng, Electrochemical behavior of B(III) on a molybdenum electrode in NaCl-KCl eutectic molten salt, Rsc Advances, (2016). [18] X. Wu, D. Weng, S. Zhao, W. Chen, Influence of an aluminized intermediate layer on the adhesion of a γ-Al2O3 washcoat on FeCrAl, Surf. Coat. Tech., 190 (2005) 434-439. [19] C.M.D. Starling, J.R.T. Branco, Thermal fatigue of hot work tool steel with hard coatings, Thin Solid Films, 308 (1997) 436-442.

16

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

17

ACCEPTED MANUSCRIPT Figure captions

Fig. 1 Schematic diagram of the apparatus for immersion tests Fig. 2 Surface(a) and cross-section(b) morphology of the electrodeposited ZrB2

PT

coating on stainless steel substrate with the current density of 100 mA·cm-2 at 750 oC

RI

for 30 min.

SC

Fig. 3 The X-ray diffraction pattern of the ZrB2 coating.

Fig. 4 Macrographs, cross-section images of uncoated stainless steel after immersion

NU

in molten aluminum at 800 oC for different time (a,b) 2 h; (c,d) 4 h. (The insert shows

MA

EDX analysis).

Fig. 5 Macrographs of ZrB2-coated stainless steels after immersion in molten

D

aluminum at 800 oC for different time (a) 0 h; (b) 4 h; (c) 8 h; (d) 24 h.

PT E

Fig. 6 Surface morphologies of ZrB2-coated stainless steels after immersion in molten aluminum at 800 oC for different time (a) 4 h; (b) 8 h; (c) 24 h.

CE

Fig. 7 Cross-section images of ZrB2-coated stainelss steels after immersion in molten

AC

aluminum at 800 oC for different time (a) 4 h; (b) 8 h; (c) 24 h. Fig. 8 Macrographs and cross-section images of ZrB2-coated stainelss steels after immersion in molten aluminum at 800 oC for different time (a-b) 48 h; (c-d) 72 h Fig. 9 Macrograph and cross-section images of ZrB2-coated stainelss steels after immersion in molten aluminum at 800 oC for 96 h (a) macrograph; (b) low magnification image; (c) amplification image of area 1 Fig. 10 (a-b) Low magnification cross-section images; (c-d) amplification images of

18

ACCEPTED MANUSCRIPT attacked regions of ZrB2-coated stainelss steels after immersion in molten aluminum

AC

CE

PT E

D

MA

NU

SC

RI

PT

at 800 oC for 120 h

19

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 1

20

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 2

21

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 3

22

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 4

23

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 5

24

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure6

25

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 7

26

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 8

27

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 9

28

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 10

29

ACCEPTED MANUSCRIPT Table captions

Table 1 Chemical composition of 6061 aluminum alloy (wt %) Si

Cu

Fe

Cr

Mn

Ti

Al

0.89

0.65

0.25

0.07

0.07

0.03

0.02

bal.

SC

RI

PT

Mg

NU

Table 2

EDX results (in wt. %) of the specimens in different diffusion layers shown in Fig. 4 layers

Al

Fe

Cr

Ni

Si

Inner Layer

54.81

31.51

9.62

2.97

1.09

Outer Layer

68.37

5.64

1.56

2.73

21.70

Inner Layer

53.94

32.09

9.94

3.16

0.87

Outer Layer

69.76

20.89

4.98

1.95

2.42

PT E

D

2h

MA

Specimens

AC

CE

4h

30

ACCEPTED MANUSCRIPT Highlights  Dense, adherent and homogeneous ZrB2 coating is electrodeposited on stainless steel;  The stainless steel suffered severe corrosion in molten aluminum after only 2 h immersion;

PT

 The ZrB2-coated stainless steel showed high corrosion resistance to molten aluminum attack after 120 h immersion;

RI

 The corrosion failure mainly happens at defects and the stress concentration area

AC

CE

PT E

D

MA

NU

SC

of the coating.

31