Microstructure and corrosion behaviours of composite coatings on S355 offshore steel prepared by laser cladding combined with micro-arc oxidation

Applied Surface Science 497 (2019) 143703

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Microstructure and corrosion behaviours of composite coatings on S355 offshore steel prepared by laser cladding combined with micro-arc oxidation

T

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X. Hea,b,c, R.G. Songa,b,c, , D.J. Kongb,d a

School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou, Jiangsu 213164, China c Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China d School of Mechanical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Corrosion Composite coating S355 offshore steel Laser cladding Micro-arc oxidation

The composite coatings were prepared on S355 offshore steel by laser cladding combined with micro-arc oxidation technology. The microstructure and corrosion behaviours such as immersion corrosion, corrosive wear of the as-prepared composite coatings were investigated, and the corrosion properties of the coatings were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The composite coating was well bonded with the substrate layer and showed good mechanical properties. The interaction between corrosion and wear was mainly corrosion accelerating abrasion in the substrate, while it was wear accelerating corrosion in the coating. The immersion corrosion mechanism of the cladding coating was pitting corrosion. When the current density was 5A·dm−2, the composite coating could significantly improve the corrosion resistance of the substrate.

1. Introduction S355 offshore steel was mainly used on offshore platforms, and it was vulnerable to corrosion in the marine environment. Therefore, the heavy corrosion resistance of S355 steel has become the attention focus in recent years [1–2]. In the past, thermal spraying metal coating was mainly used for anticorrosion of offshore platforms, but offshore platforms put forward higher requirements for the protection of structural steel surface, except for the anticorrosion effect of the coatings, it also includes coating matching, thickness and surface treatment, and so on. It was required that the coating and the steel surface have good adhesion, anti-aging, salt fog resistance, corrosion resistance, and can form a suitable elastic coating, which can be used with cathodic protection. This not only makes the process complex, but also the effect was often not up to expectations [3]. Micro arc oxidation (MAO) is a new technique capable of in situ growth of coatings on a surface consisting of the oxide of substrate and can improve the properties of coatings. However, the micro-arc oxidation technology is currently mainly applied to valve metals (aluminum, titanium, magnesium) and their alloys [4–9]. At present, the main method for preparing ceramic layers on the surface of steel is hot-dip aluminizing technology combined with micro-arc oxidation technology [10]. Niu et al. prepared alumina ceramic layer on worn stainless steel work piece was gotten using ultrasonic vibration

⁎

aided hot-dip aluminizing and micro arc oxidation. The results show that the coating showed high hardness and wear resistance [11]. Zhao et al. obtained ceramic coating by hot-dipped aluminum (HDA) combined with micro-arc oxidation on the ductile iron [12]. However, the coatings prepared by these two techniques have many defects. First, the bonding strength is not high, and the second process is complicated and the cost is high [13–14]. Therefore, we consider to combine another surface modification technology with the lower cost and simple process. Laser cladding (LC) is an advanced processing technology [15–17]. The cladding powder can be added to the surface of the substrate in different ways and is later fused by laser irradiation. The cladding powder is then solidified as a thin layer; it is metallurgically bonded to the substrate by rapid solidification. Thus, laser cladding is a surfacestrengthening method for improving the wear resistance, corrosion resistance, fatigue resistance and oxidation resistance of the surface of a substrate [18–22]. Therefore, we use laser cladding combined with micro-arc oxidation technology to prepare a composite ceramic coating on the surface of offshore steel. Since the carbide ceramic powder has excellent properties such as high hardness, wear resistance and corrosion resistance, it is often used as a cladding powder material [23], the melting point, elastic modulus and thermal expansion coefficient of the ceramic material are greatly different from the matrix, which may result in the cladding layers having defects such as cracks. CeO2 has the

Corresponding author at: School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China. E-mail address: [email protected] (R.G. Song).

https://doi.org/10.1016/j.apsusc.2019.143703 Received 15 March 2019; Received in revised form 5 June 2019; Accepted 14 August 2019 Available online 16 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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cladding coating as a base layer [29–30]. The microstructure evolution and corrosion behaviour of the composite coating under different current densities were studied.

Table 1 Laser cladding process parameters. Parameters

Values

laser power/W laser scanning rate/mm·min−1 Powder feeding rate/g·min−1 Argon gas velocity/L·min−1 Spot diamteter/mm

1200 360 8 15 3

2. Materials and experimental 2.1. Materials The experimental materials and contrast material were European standard S355 steels. The cladding powder materials used were Al powder (purity 99.0%, average diameter 50 μm–95 μm), TiC powder (purity 99.5%, average particle size 40 μm), Ni powder (purity 99.5%, average diameter 1.5 μm). The Al powder, TiC powder and Ni powder were mixed according to the mass ratio of 6:3:1, then the 1% CeO2 (purity 99.0%, average particle size 20 nm) was added into the mixture, and the mixture was fully and evenly mixed with the QM3SP04L planetary ball-milling machine.

Table 2 MAO electrolyte formulation. Electrolyte composition

Electrolyte concentration (g/L)

Na2SiO3 KOH NaF SiO2 TiO2

12 5 0.5 3 9

2.2. Laser cladding

functions of refining crystal grains, improving grain boundary state, reducing internal stress, and suppressing columnar crystal growth [24]. He et al. showed that the hardness and corrosion resistance of the coatings were the best when the CeO2 content was 1%. At the same time, Ni has good wettability and oxidation resistance, which can effectively reduce the porosity of the coating [25]. Tao et al. have shown that when the Ni content is 5%, the coating has good wear resistance in 3.5% NaCl solution [26]. At present, Yang et al. successfully prepared a double layer composite coating system on the surface of AZ31 magnesium alloy by MAO plus electrostatic powder spraying (EPS) technique. The results indicated that the corrosion resistance of AZ31 Mg alloy was significantly improved by MAO + EPS composite coating with the excellent binding force [27]. Xiong et al. prepared a composite bio-coating on AZ80 magnesium (Mg) alloy by using micro-arc oxidation (MAO) under the pretreatment of laser shock peening (LSP). The results indicated that LSP/MAO composite bio-coating can not only improve the corrosion resistance of Mg alloy substrate evidently but also increase the mechanical properties [28]. The research above on the combination of laser processing and micro-arc oxidation technology is mainly focused on light alloys, and there are few studies on the preparation of doublelayer composite ceramic coatings on the surface of marine steel by laser cladding and micro-arc oxidation technology. Therefore, the Al-Ni-TiCCeO2 coating was prepared on the surface of S355 steel by laser cladding technology in the present work, and then a composite ceramic coating was prepared by MAO in a silicate electrolyte using the

The laser cladding experiment machine adopts a ZKSX-2008 all solid-state laser and the cladding method adopts synchronous powder feeding and cladding. The mixed powder is driven by argon and ejected through the powder feeding nozzle to reach the surface of the substrate, and the laser beam will follow the powder nozzle synchronously. Before cladding, we will dry the powders to prevent it from gathering and clogging the nozzle, the surface of the sample should be ground with metallographical sandpaper and cleaned with alcohol, then the S355 steels were treated by cladding after drying. The technological parameters of laser cladding are shown in Table 1.

2.3. Micro-arc oxidation After the cladding test is completed, the obtained pattern is cut into a size of 30 mm × 25 mm × 3 mm by wire cutting. The surface of the cladding coating is sanded step by step with water proof sandpaper until the surface is smooth and flat. Then, it was mechanically polished with an Al2O3 polishing solution. Except for the surface of the cladding layer, the other surfaces are sealed with epoxy resin and curing agent for micro-arc oxidation. The model number of micro-arc oxidation equipment is JHMAO-380/20H and the working mode adopts the constant current mode. The current density is 3A·cm−2, 5A·cm−2, 8A·cm−2, the oxidation time is 30 min, and the electrolyte temperature is controlled at 30°. The electrolyte formulation is shown in Table 2.

Fig. 1. XRD patterns of the coatings prepared by different methods (a) LC, (b) LC + MAO. 2

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(a)

(b) MAO coating

Porous layer 14µm 6µm

Substrate

Cladding layer

Compact layer

20µm

Crack LC

(d)

(c) Porous layer

MAO

19µm

16µm

Crack 8µm

Crack

9µm

Compact layer

28µm 24µm

LC

MAO

LC

MAO

(e)

Fig. 2. Sectional morphology of the composite films at different current densities and plane scan of composite films cross-section: (a) Overall morphology of the section (b) 3 A·dm−2 (c) 5 A·dm−2 (d) 8 A·dm−2 (e) plane scan.

JSM- 6510 scanning electron microscope (SEM). The elements of the coatings were analyzed by using an energy dispersive spectrometer (EDS). The phase composition of the coatings was analyzed using an Xray diffractometer (XRD, Rigaku D/max-2500). The surface chemistry of the sputtered coating was examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI). The porosity of the coatings was mainly measured by Image J software, and measure 5 times to take the average value as the final effective porosity. The microhardness of four

2.4. Characterisation After the experiment is completed, the cladding coating (contrast sample) and micro-arc oxidation ceramic layers are obtained. The obtained cladding coating is polished and smoothed by water proof sandpaper, and the micro-arc oxidation ceramic layers section is polished step by step with water proof sandpaper. The micro-morphology of the surface and cross section of the coatings was characterized by 3

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(a)

Ra=21.376 μm

(b)

Ra=1.036 μm

(c)

Ra=0.745 μm

(d)

Ra=1.735 μm

Fig. 3. three dimensional surface morphologies of cladding layer (a) and composite films at different current densities: (b) 3 A·dm−2 (c) 5 A·dm−2 (d) 8 A·dm−2.

test, the data were fitted by Tafel. All the measurements were performed after 30 min immersion in 3.5% NaCl solution until open circuit potential (OCP) was stable.

kinds of coatings were measured by HMV-1 T digital microhardness tester. The loading load was 200 g and the loading time was 15 s. One point was measured every 50 μm on the coating cross section, and the hardness was measured 3 times at the same level of depth and the results were averaged. The coalescent strength of ceramic coating was tested by a WS-2005 coating scratch tester. The indenter used for the scratch device is a diamond cone (120°) indenter with a load range of 0.01 to 200 N. The loads of ram squeeze head reached 40 N with a speed of 10 N min−1. The moving speed of ram squeeze head was set at 0.1 mm s−1 with the scratch distance left on sample surface of 4 mm. After scratch testing, the microscopic appearance of the scratch was characterized by a microscope. Immersion corrosion analysis was carried out by KD-60E corrosion tester. Seal surfaces other than test surfaces with epoxy resin, and PH is maintained at about 6, the immersion medium was replaced every 2 days during the test, The corrosion products on the surface of the sample were washed and dried after the test was completed. The morphology, electrochemical impedance and weight loss of the coating were measured. The corrosive wear test was carried out with CFT-1 type surface property tester. The 3.5%NaCl solution was used as wear medium and SiC ceramic ball was used as grinding material. The load was 200 g and the speed of motor was kept at 500 r/min. The test was carried out in reciprocating sliding mode. The wear mark radius was 3 mm and the running time was 30 min. At the same time, the corrosion potential and current in the process of corrosion and wear were measured by CS350 electrochemical workstation. The measurement was completed using a BT25S electronic analytical balance to measure the weight loss. The CS350 electrochemical workstation was used for electrochemical testing, the sample size for the electrochemical test was 10 mm × 10 mm × 3 mm. All surfaces except coating surface were sealed with epoxy resin and curing agent and then electrochemical testing was carried out. The test medium was 3.5% NaCl solution and the testing area of the sample was 1 cm2. The electrochemical workstation system consists of saturated calomel electrode as reference electrode and Pt electrode as auxiliary electrode. The test sample is used as working electrode. In the experiment, the potentiodynamic scanning range is −0.5–0.5 V, and the scanning rate is 1 mV/s. Sampling frequency of 0.5 Hz, and the test time is 30 min. The EIS test signal is a sine wave, the amplitude is 10 mV, the frequency range is 10−1–105 Hz, and the test time is 5 min. After the

3. Results and discussion 3.1. XRD analysis of coatings Fig. 1 shows the XRD patterns of the composite coatings at different current densities. The main phase of the cladding coating is composed of reinforced phase TiC and continuous phase, including the AlFe3 phase, the AlNi3 phase, the Al2O3 phase and the AlFeNi phase. As shown in Fig. 1a. The main phase of the composite film is Al2O3, and the other phases include AlF3 and Al(OH)3 phases. These phases indicate that the F− in the electrolyte reacts with O2 and Al in the solution under high pressure discharge [31]. In addition, there are SiO2, TiO2 and Al2Ti7O15 phases on the surface of the composite coating. The appearance of SiO2 and TiO2 phases is caused by additives in electrolyte. With the micro-arc oxidation reaction, SiO2 is gradually deposited on the coating surface. TiO2 participates in the micro-arc oxidation reaction with O2 and Al3+ in solution and deposited on the coating surface with Al2Ti7O15 [32], as shown in Fig. 1b. 3.2. Microstructure analysis of coatings Fig. 2 shows the cross-sectional morphologies and plane scan of a composite ceramic film. As shown in Fig. 2a, the composite coating section can be seen as three distinct regions, which in turn are the substrate, the cladding layer, and the micro-arc oxidation ceramic layer. It can be seen that there is slight oxidation in the matrix region, the cladding region is relatively flat, and there are three obvious cracks. The thickness of the micro-arc oxidation coating is about 20 μm. The commonly prepared micro-arc oxidation ceramic film layer is divided into a two-layer structure of an inner dense layer outer loose layer, as shown in Fig. 2b and c. The sawtooth metallurgical bond is formed between the ceramic layer and the substrate, and the interface is well combined. When the current density is 3 A·cm−2, the thickness of the outer layer is 14 μm, and the micropores are distributed more, the inner dense layer is 6 μm, and the number of micropores is small. When the 4

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12000

(a)

Al

Element C O Al Ti Fe Ni

(b)

10000

Counts/cps

8000

Wt% 2.27 14.28 49.15 19.84 10.03 4.43

At% 5.54 26.14 52.63 8.12 5.36 2.21

6000

4000

Ti 2000

Ti Fe Ni

O Ni C 0 0

1

2

3

4

5

6

7

8

9

10

Energy/keV

(c)

Al

12000

Element O Al Si Ti F

(d)

Counts/cps

10000

Wt% 25.06 54.34 17.38 2.07 1.15

At% 36.51 46.94 14.43 1.83 0.56

8000 6000 4000

O Ti Si

2000

F 0 0

1

2

3

4

5

6

7

8

9

10

Energy/keV

(e)

7000

Al

(f)

6000

Counts/cps

5000

Element

Wt%

At%

O

21.98

30.19

Al

57.47

46.94

Si

11.09

8.68

Ti

8.34

13.8

F

1.13

0.52

4000 3000 2000 1000

O Ti

Si F

0 0

1

2

3

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8

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Energy/keV 10000

(g)

Al

(h)

8000

Element

Wt%

At%

O

26.17

38.09

Al

55.69

48.06

Si

10.63

8.81

Ti

6.43

4.21

F

1.08

0.84

Counts/cps

6000

4000

O 2000

Ti

Si F

0 0

1

2

3

4

5

6

7

8

9

10

Energy/keV

Fig. 4. Surface morphology and EDS analysis result of the composite coating: (a, b) cladding layer; (c, d) 3 A·dm−2 (e, f) 5 A·dm−2 (g, h) 8 A·dm−2.

5

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There are many micropores distributed on the surface of the membrane. And there are obvious differences in the number of micropores and pore diameter. In Fig. 4c, the micropores on the surface of the sample are fine and uneven, and some microcracks appear on the surface of the coating. The layers in Fig. 4e and g are characteristic of typical lava solidification. This is mainly due to the short breakdown time in the micro-arc oxidation process, the concentration of heat is easy to form a molten micro-region, and the molten alumina is formed, and the gas pressure and discharge pressure generated by the reaction in the discharge channel are raised, this causes the partially melted alumina to be ejected from the discharge channel, thus forming a morphology of lava solidification around it. At this time, the number of micropores on the coating surface is reduced, but the pore diameter is increased. Fig. 4d, f and h show the surface energy spectra of the corresponding MAO samples. It can be seen that the main elements on the surface of the coating are O, Al, Si, Ti, F. As the current density increases, the contents of Si and F gradually increase. This is because the thickness of the coating increases with increasing current density, so the coating has more room to accommodate such materials [36]. Fig. 5 shows the porosity and pore size distribution of the ceramic layer. The surface porosity increases first and then stabilizes. When the current density was 3 A·cm−2, the number of micropores with a pore size of less than 1 μm is the largest, accounting for about 56%. In addition, as the current density increases, the oxidation reaction rate increases, the thickness of the ceramic layer increases rapidly, and discharge breakdown phenomenon is difficult to occur. Therefore, a certain degree of energy accumulation will occur in some tiny pores until the discharge breakdown occurs again, causing the pore size of some micropores to become larger [37], and the proportion of pore distribution within the pore diameter of 1 μm is gradually reduced. The coating adhesion is an important indicator of the bonding strength of the MAO film. Fig. 6 shows the acoustic signal and friction curves of the MAO film at different current densities and corresponding scratch images. Fig. 6a shows the bond strength of a ceramic layer prepared with a current density of 3 A·cm−2. It can be seen that the coating emits a distinct acoustic signal when it is scratched, and the friction curve rises sharply, as shown by the blue coil in the figure. In the scratched photograph of Fig. 6b, it can also be seen that when the indenter is drawn to a short distance, a metallic color region appears at the bottom of the scratched groove, indicating that the coating has been scratched at this time [38]. combined with the acoustic emission signal, the frictional force emission signal, and the photomicrograph, the value of the ceramic layer Lc having a current density of 3 A·cm−2 was 15.2 N. When the current density reaches 5 A·cm−2, the acoustic signal fluctuates greatly when the loading force reaches 28.4 N, and the ceramic layer has a short penetration distance. As shown in Fig. 6c and d, the bonding strength of the coating is better. When the current density is further increased to 8 A·cm−2, the Lc value of the coating drops to 15.8 N, and the scratching position is significantly advanced, indicating that the bonding strength of the coating is decreased, as shown in Fig. 5e and f. Fig. 7 shows the microhardness distribution curve of the longitudinal section of the composite coating. It can be seen that the hardness of composite coatings prepared with different current density has the same trend (first increased from the outside to the inside and then gradually decreased). When the current density is 3 A·cm−2, In the range of 0 to 10 μm from the surface, the hardness of the coating increases gradually, and the maximum hardness reaches 1267 HV0.2 about 12 μm away from the surface. The maximum hardness of the composite coating is 31.4% higher than that of the cladding coating (964.3 HV0.2), which is about 3.29 times of the substrate hardness (384.4 HV0.2). When the current density reaches 5 A·cm−2, the microhardness of the coating is higher than that of the ceramic layer with a current density of 3 A·cm−2. The maximum hardness of the film is 1424.3 HV0.2 at 13 μm from the surface. When the current density is further increased, the maximum microhardness of the ceramic layer is

Fig. 5. Surface porosity and pore size distribution of composite films.

current density rises to 5A·cm−2, the thickness of the outer loose layer is 16 μm, the number of micropores is small, but the pore size becomes larger, the thickness of the inner dense layer is 8 μm, and the number of micropores is small. When the current density is further increased to 8 A·cm−2, the film layer is ablated, the number of micropores is reduced, and the thickness of the ceramic layer is increased to 28 μm. This is mainly due to the large current density and the high voltage on the surface of the sample. The electric field strength and the magnetic field strength on the ceramic coating are also increased, so that the driving force in the micro-arc oxidation reaction is increased, thereby further increasing the thickness of the film layer [33], and on the other hand, the excessive current density promotes the melt oxidation. The ceramic layer of the object and the surface layer is remelted, and rapidly solidifies under the cold quenching action of the electrolyte, so that the reaction product is gradually deposited on the inner wall of the micropores, so the micropores are gradually filled [34]. Fig. 2e shows the plane scan analysis of composite coating, the plane scan spectrum of Al element have three obvious boundary, S355, cladding layer and MAO coating, respectively. The Al element and O element was compact at MAO coating zone than that the cladding layer zone, after micro-arc oxidation treatment, the dense Al2O3 was formed on the surface of the cladding coating. It can be seen that there are Fe and Ni element in the micro-arc oxidation region. The results show that the elements of the cladding coating diffuse into the composite coating, which enhances the interfacial bonding ability of the two coatings. Fig. 3 shows the surface profile morphology and surface roughness of different coatings. The surface roughness and the pore size was larger for cladding coatings. However, the surface roughness of the composite ceramic film is obviously lower than that of the cladding coating. When the current density was 5 A·cm−2, the surface of the coating was smoother than other MAO coatings, which indicates that the current density has a great influence on the roughness of the coating. Fig. 4 shows a surface morphology of the cladding coating and composite coating and the corresponding EDS spectrum. It can be seen that there are large holes on the surface of the cladding coating, and a distinct microcrack appears, and the TiC distribution of the reinforcing phase is uniform. The TiC morphology is mainly fine particles, and there is a tendency for the connection to grow, as shown in Fig. 4a. Fig. 4b shows the results of the corresponding point spectrum analysis. The surface elements of the coating are mainly Al, C, Ti, Fe, O and Ni, which are also consistent with the XRD results tested. The Fe element in the coating mainly comes from the matrix and the diffusion layer, indicating that the coating forms a better metallurgical bond with the matrix [35]. Fig. 4c, e and g show the morphology of the composite ceramic layer. The three ceramic layers show typical loose morphology. 6

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(b)

friction force signal intensity

(a)

700

Friction force/N

600 500 400 300 200 100

210 µm

0 0

5

10

15

20

25

30

35

40

Force/N

800

friction force signal intensity

(c)

(d)

Friction force/N

700 600 500 400 300 200 100

310 µm

0 0

5

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20

25

30

35

40

Force/N

friction force signal intensity

600

(e)

(f)

Friction force/N

500

400

300

200

100

250 µm 0 0

5

10

15

20

25

30

35

40

Force/N

Fig. 6. Relationship curves of Si and Ff with the change of loading force (Fn) and the morphology of the scratch on MAO coatings under different current density: (a, b) 3 A·cm−2 (c, d) 5 A·cm−2 (e, f) 8 A·cm−2.

3.3. Corrosive wear

further increased to 1496.7 HV0.2. The phase composition and compactness of the ceramic layer are the main factors affecting the hardness of the film. As the current density increases, the thickness of the ceramic layer increases gradually, and the thickness of the dense layer and the loose layer also increases. At the same time, there are more αAl2O3 phases in the dense layer, which increases the hardness of the films. On the other hand, with the increase of current density, the ceramic layer shows an inward growth mechanism, which makes the transition region of the film smaller, and the hardness decreases more obviously. [39]. Fig. 7b shows the surface indentation morphology of the coatings. The indentation of composite coating is lower than that of cladding coating, and the indentation face decreases with the increase of current density, which indicates that the hardness increases gradually.

Fig. 8 shows the morphology of the cladding coating and the MAO coating after abrasion in a 3.5% NaCl solution. It can be seen that the cladding coating has a wear scar width of 250 μm, localized corrosion around the wear scar, equal grain-like wear debris on the surface, and a small amount of microcracks. This is because after the surface of the coating is crushed, the surface of the material becomes fragile and gradually peels off. There is a shallow furrow in the spalling zone. Under the promotion of corrosive medium Cl−, the mechanism of material loss gradually changes from shaping to deformation. Mild ploughing to brittle flaking, reflecting the acceleration of corrosion on wear. On the other hand, the coating surface passivation film and the reinforcing phase TiC inhibit partial corrosion and promote its antiwear ability, resulting in less corrosion product buildup [40–41], as shown in Fig. 8a. For the micro-arc oxidation composite coating, the 7

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(a) Microhardness/HV

1400

Microhardness/HV0.2

3A/dm

1500

0.2

1600

1200

5A/dm 8A/dm

1400

2

(b)

2 2

1300

1200

1100

1000 1000

Porous layer 0

800

5

10

15

3A/dm2

Cladding layer

Compact layer 20

25

30

Distance from coating surface/μm

600 400

MAO

LC

S355

200 0

100

200

300

400

500

8A/dm2

5A/dm2

600

Distance from coating surface/μm Fig. 7. Microhardness distribution of the cladding coating and composite films.

(b)

(a)

190 m

250 m

(d)

(c)

150 m 130 m

Fig. 8. The morphology of the surface wear scar on the cladding coating (a) and MAO coating (b, c, d) after corrosive wear in 3.5% NaCl solution: (b) 3 A·cm−2 (c) 5 A·cm−2 (d) 8 A·cm−2.

coating has a wear width of about 190 μm at a current density of 3 A·dm−2, and the corrosion around the wear scar is slight, but microcracks (8b) appear. Under the action of frictional shear stress, a small amount of corrosive solution enters the inside of the coating, causing damage to the coating. When the current density is 5A·dm−2, the wear scar of the coating is shallow, the width of the wear scar is about 130 μm, there is almost no corrosion trace around the wear scar, and there is no micro crack. This is because the surface hardness of the coating is high, and only a slight scratch can be produced on the abrasive material, which makes it difficult for the corrosion solution to enter the inside of the film layer, and thus exhibits good abrasion resistance [42]. When the current density further increased to 8A·dm−2, the width of the wear scar showed a certain rise to 150 μm, and local

Table 3 Mass loss, wear rate and relative wear resistance of specimens after corrosive wear. Sample

Wear loss (g)

Wear rate (g/ min)

Relative wear resistance

LC C + MAO(3A·dm−2) LC + MAO(5A·dm−2) LC + MAO(8A·dm−2) S355

8.8 × 10−4 5.8 × 10−4 5.1 × 10−4 6.5 × 10−4 1.26 × 10−3

2.93 × 10−5 1.93 × 10−5 1.7 × 10−5 2.1 × 10−5 4.2 × 10−5

1.43 2.17 2.47 1.94 1

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Fig. 9. Tafel polarization curves of the substrate and coatings under steady (a) and (b) wear conditions.

Fig. 10. Synergetic contributions of corrosion and wear to each other and total material loss of coatings after abrasion within 3.5% NaCl solution (a) Synergetic effect, (b) Wear rate, (c) Corrosion rate, (d) Total loss rate.

Fig. 9 is a Tafel polarization plot of the substrate and coating measured in static and worn state. It can be seen that in static state, the self-corrosion potential of the substrate is −0.801 V, the corrosion current density is 2.77 × 10−6 A·cm−2, and in the worn state, the selfcorrosion potential is −0.552 V, and the corrosion current density is 2.58 × 10−6 A·cm−2. The potential is positively shifted, the active peak is lowered, and the passive current density is decreased, which means that the corrosion resistance of the substrate is improved by the shearing force, indicating that the corrosion accelerates the wear. For the cladding coating, the self-corrosion potential is −0.756 V in steady

corrosion occurred on the surface of the coating, but the corrosion was slight and no microcracks appeared. Table 3 shows the weight loss, wear rate and relative wear resistance of the sample before and after wear. It can be found that the wear loss of the coating pattern is an order of magnitude smaller than that of the substrate, and the wear amount of the MAO coating pattern is 26% to 42% less than that of the simple cladding coating pattern, and the current density is 5 A·dm−2. The wear rate and relative wear resistance of the layers are both optimal. It shows that the MAO coating has a certain anti-wear effect. 9

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(b)

(a) Pitting holes

(d)

(c)

(e)

(f)

(g)

(h)

Fig. 11. The corrosion morphology and EDS point spectrum results of coatings immersed in 3.5% NaCl solution (a, b) cladding coating (c, d) 3 A·cm−2 (e, f) 5 A·cm−2 (g, h) 8 A·cm−2.

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Fig. 12. The XRD patterns of coating surfaces immersed in 3.5% NaCl solution for 192 h: (a) cladding coating (b) MAO coatings.

Fig. 13. XPS results of the cladding coating surfaces immersed in 3.5% NaCl solution for 192 h: (a) XPS survey; (b) high-resolution spectrum of Fe2p; (c) highresolution spectrum of Al2p.

state, the corrosion potential is −0.952 V in wear, the potential is negatively shifted, the active peak is increased, and the stable passive current density is increased. This means that the coating has a reduced corrosion resistance under the action of surface shear, indicating that the wear accelerates the corrosion. For micro-arc oxidation coatings, the coating has a high self-corrosion potential when static, showing good corrosion resistance. In the worn state, the coating showed slight corrosion, the self-corrosion potential was negatively shifted, and the

passive current density increased, indicating that the wear accelerated the corrosion. Since the interaction usually manifests as acceleration, the interaction between corrosion and wear is illustrated by the following formula:

11

S = ΔCW + ΔWC

(1)

C = C0 + ΔCW

(2)

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Fig. 14. XPS results of the MAO coatings surfaces immersed in 3.5% NaCl solution for 192 h: (a) XPS survey; (b) high-resolution spectrum of Al2p. 0.2

corrosion and wear, the variation in corrosion rate induced by wear (ΔCw) and the variation in wear rate induced by corrosion(ΔWc), C is the total corrosion rate, W is the total wear rate, the total synergism factor and the wear augmentation factor as well as the corrosion augmentation factor calculated through ST, SW and SC respectively[43]. Fig. 10 shows the interaction between corrosion and wear. Since the acceleration factors used are all greater than 1, it indicates that both corrosion and wear are mutually reinforcing, as shown in Fig. 10a. In addition, the corrosion acceleration factor of the coating is greater than the wear acceleration factor, indicating that the degree of wear-promoting corrosion is greater than the degree of corrosion-promoted wear, showing the acceleration of wear on corrosion [44]. The corrosion acceleration factor of the matrix is less than the wear acceleration factor and thus exhibits an acceleration of corrosion to wear. It can be seen from Fig. 10b and c that the corrosion rate and wear rate of the coating are significantly weaker than those of the substrate. The microarc oxidation coating has a lower rate than the cladding coating, especially the corrosion rate, and there are significant changes in the values of the different coatings ΔWc and ΔCw. It can be seen from Figs. 10d that the total mass loss of the matrix material is large, about twice the mass loss of the coating material. Since the interaction of wear and corrosion (S) plays an important role in the total loss of material, the loss of corrosion material accounts for about 15% of the total loss of material, and the loss of wear material accounts for about 85% of the total material loss, This shows that the synergistic effect of corrosion and wear greatly contributes to the material loss. The material loss mainly comes from mechanical wear. When the surface of the material is subjected to micro-arc oxidation treatment, the proportion of wearaccelerated corrosion will gradually decrease.

2

LC+MAO(8A/cm ) 2 LC+MAO(3A/cm ) 2 LC+MAO(5A/cm ) LC

0.0

Potential(vsSCE)/V

-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -9

-8

-7

-6

-5

-4

-3

-2

-2

lg[I/(A⋅cm )] Fig. 15. The potentiodynamic polarization curves of the coating immersed for 192 h in 3.5% NaCl solution. Table 4 Electrochemical measurement parameters of coatings immersed for 192 h in a 3.5% NaCl solution. Sample

Ecorr (V)

icorr (A/cm2)

LC 3A·dm−2 5A·dm−2 8A·dm−2

−0.76751 −0.38486 −0.40479 −0.53862

6.8802 × 10−7 2.3968 × 10−8 1.2106 × 10−8 1.3244 × 10−8

3.4. Immersion corrosion

W = W0 + WW

(3)

T=C+W

(4)

ST = T C0 + W0

(5)

SW = W W0

(6)

SC = C C0

(7)

T = C0 + W0 + S

(8)

Fig. 11 shows the corrosion morphology of the coating immersed in 3.5% NaCl solution for 192 h. It can be seen that there is a slight pitting hole on the surface of the cladding coating. The pitting hole has a larger aperture and less corrosion products (Fig. 11a). According to the energy spectrum analysis of Fig. 10b, the main elements on the surface of the coating after immersion are Al and O. This is because the surface of the cladding layer forms a dense passivation film γ-Al2O3, the Cl− in the corrosive medium adsorbs on the surface of the passivation film, destroying the protective effect of the film. In this case, the current near the corrosion hole increases rapidly, resulting in local pitting. Fig. 11c shows the corrosion morphology of the ceramic film prepared by current density 3A·dm−2. It can be seen that there are many micropores on the surface of the film, but there are few corrosion products and no obvious microcracks. The main elements on the surface of the coating

where, T is the total material loss rate, C0 is static corrosion rate without wear, W0 is the pure mechanical wear rate without corrosion, S is the synergistic component obtained from the interactions between 12

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Fig. 16. The Nyquist plots of the coatings (a) and substrate(b) immersed for 192 h in 3.5% NaCl solution.

are O, Al and Si. Due to the low Cl− content (Fig. 11d), it is not enough to damage the film layer, so there is no pitting hole on the surface of the coating. When the current density is 5A·dm−2, the corrosion products did not change significantly in the surface of the coating, but the number of pores in the coating layer is gradually reduced (Fig. 11e), which may be due to the filling of a part of the corrosion products. The main elements on the surface of the film are O, Al and Si, and the concentration of Cl− is increased (Fig. 11f), but this is still insufficient to cause damage to the film layer, and the film layer shows good corrosion resistance. When the current density further rises to 8A·dm−2, the microcracks appear on the surface of the film and the corrosion products gradually increase (Fig. 11g). This is mainly due to the large pore size of the membrane layer, Cl− can easily penetrate the membrane layer into the interior of the coating, resulting in a gradual increase of the Cl− concentration inside the coating, so the membrane layer is gradually destroyed. This is also consistent with the results obtained from the Fig. 11h energy spectrum. XRD was used to identify the constitutive phases in the coatings after immersion in 3.5% NaCl solution. The cladding layer consisted mostly of Al and Al2O3 after immersion in 3.5% NaCl. In addition, the surface of the cladding coating also contains a small amount of AlOOH and FeOOH, This is mainly caused by the hydrolysis reaction with Al2O3 and Fe2O3, as shown in Fig. 12a. Fig. 12b shows the surface XRD of MAO ceramic layer after immersion. Besides the matrix Al phase, there are Al2O3 phase, AlOOH phase and AlF3 phase on the surface of MAO ceramic coating. The main reaction mechanism is as follows:

Fig. 17. Bode plots of the substrate and coatings immersed for 192 h in a 3.5% NaCl solution.

NaCl solution

Rs

CPEb Rb

CPEt

(9)

4Al + 3H2 O + 3O2 → 2Al(OH)3 + Al2 O3

Rt

Al(OH)3 →

Al3 +

+

3OH−

(10)

Al2O3 + H2 O–2e− → Al2 ++AlOOH

(11)

To verify the surface chemistry of the coating, an XPS analysis was performed on the top layer of the samples. Typically, O, Ti, Cl, Ca, Si, Al, and Fe were all present in coating layers (Fig. 13a). Fig. 13b presents that the two binding energy peaks of Fe2p3/2 and Fe2p1/2 locate at around 711.5 eV and 719.9 eV, respectively. Thus, the Fe2p is fitted with two peaks: valence states of Fe2+ and Fe3+, in which one peak

Fig. 18. Equivalent circuits of the EIS plots for coating in the 3.5%NaCl solution.

Table 5 Equivalent circuit data of coatings immersed for 192 h in a 3.5% NaCl solution. Sample

Rs (Ω·cm2)

Qb (Ω−1sncm−2)

nb

Rb (Ω·cm2)

Qt (Ω−1sncm−2)

nt

Rt (Ω·cm2)

LC 3A·dm−2 5A·dm−2 8A·dm−2

4.67 5.33 6.24 6.03

6.06 × 10−6 8.35 × 10−6 7.47 × 10−6 6.12 × 10−6

0.88 0.83 0.75 0.85

1.95 × 103 5.32 × 103 8.48 × 103 6.36 × 103

4.48 × 10−6 3.31 × 10−6 4.11 × 10−6 5.92 × 10−6

0.87 0.79 0.83 0.76

1.28 × 104 5.85 × 104 8.24 × 104 6.08 × 104

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1

(a) MAO

(b) 2

1

LC

2

S355

Pitting hole

Fig. 19. Cross-section morphology of composite coating after immersion corrosion: (a) Composite coating (b) Details of area 1 and 2.

arc reaches the maximum. When the current density increases to 8A·dm−2, the radius of capacitive reactance arc decreases sharply. The larger the radius of capacitive reactance arc is, the greater the resistance of the coating is, which can effectively protect the substrate from corrosion damage. On the one hand, when the current density is 8A·dm−2, the ablation phenomenon has occurred and the bonding strength of the coating has decreased obviously. On the other hand, because of the large pore size on the surface of the coating, Cl− is more likely to penetrate the film and enter the interior of the coating, which leads to the increase of Cl− concentration in the coating and the damage of the coating. Fig. 17 shows the bode plots of the coatings immersed for 192 h in a 3.5% NaCl solution, At low frequency, the impedance modulus of LC coating is 103.7 Ω·cm2, while that of LC + MAO composite coating is between 104.5 and 105.3 Ω·cm2, the impedance modulus of the composite coating is significantly higher than that of the LC coating by one or two orders of magnitude. This shows that the corrosion resistance of the coating can be further improved after micro-arc oxidation treatment, which is consistent with the morphology observed after immersion corrosion. According to the characteristics of the corrosion process of the electrochemical reaction system, the equivalent circuit diagram corresponding to the electrochemical impedance spectroscopy of the coatings was established, as shown in Fig. 18. The phase component is characterized by Q and n, and When n is 1, CPE is regarded as ideal capacitance. Rs represents the NaCl solution resistance between the sample and the reference electrode, and Rb represents the film defect resistance in parallel with the constant phase element CPEb, similar to holes and cracks. So Rb represents the resistance of pores and cracks in the LC coating, and the resistance of the outer loose layer in the MAO coating. Rt mainly refers to the contact resistance of the surface of the film and the substrate. It refers to the resistance of the barrier layer in the LC coating and the dense layer resistance in the MAO coating. The CPEt is a constant phase element parallel to the Rt [49–50]. Table 5 is the sample EIS fitting parameters. It can be seen that the outer layer resistance of LC + MAO coating is 2–4 times higher than that of LC coating, and its maximum value is 8.48 × 103 Ω·cm2. The maximum value of inner layer resistance is 8.24 × 104 Ω·cm2, which is one order of magnitude higher than the outer layer resistance. This

corresponded to FeCl2, and another peak corresponded to FeOOH, the single Al2p peak at 72.9 eV, typical for Al2O3, can be broken down into two peaks at 73.2 eV and 74.3 eV, this is also consistent with the XRD result in Fig. 11. Fig. 13a and b shows the XPS survey spectra and highresolution spectrum of Al2p of MAO ceramic coatings. In Fig. 14a, it is showed that the coating consists of O, F, Na, Al, Ti, Cl. In order to further clarify the corrosion products in the composite coating, the specific electronic binding energy of aluminum in the coating was analyzed in more detail by XPS. Fig. 14b shows the single Al2p peak at 73.2 eV, typical for Al2O3 and AlF3, can be broken down into two peaks at 72.7 eV and 73.8 eV [45–47]. The XPS results are well consistent with the XRD analysis above. Fig. 15 shows the polarization curves of the coating after immersion in 3.5% NaCl solution for 192 h. The determined parameters related to potentiodynamic polarization curves are listed in Table 4. From Fig. 15 and Table 4, it can be found that there are obvious passivation phenomena in different coatings. The LC coating has a low passivation current density (about 10−4 A/cm2), a passivation interval width of 0.2 V and a pitting potential of about −0.2 V. For the composite coatings, the passivation current density ranges from 10−5 to 10–5.5 A/cm2, and the width of passivation region is obviously longer than that of LC coating. The pitting potential of the three composite coatings changes little and is about 0.1 V, which is 0.3 V higher than that of the LC coating. The results show that the corrosion resistance of the composite coating is better than that of the LC coating. Fig. 16 shows the Nyquist plots of the samples immersed for 192 h in NaCl solution. Z' as the real part of impedance and Z" means the virtual part. The overall impedance modulus (|Z|) is defined as:

|Z| =

Z ′2 + Z ″2

(10)

It can be seen that a large capacitive arc appears in the high frequency region of the coated specimen. For the cladding coating, the capacitive arc appears to be a semicircle, but the radius of the capacitive arc is small. Under the combined treatment of LC + MAO, the arc shape of capacitive reactance is a flattened arc, showing obvious “dispersion effect”[48]. With the increase of current density, the radius of capacitive reactance arc increases gradually. When the current density of micro-arc oxidation is 5A·dm−2, the radius of capacitive reactance 14

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indicates that the corrosion resistance of the film mainly depends on the inner film which is closely connected with the substrate. The higher the Rt value is, the better the corrosion resistance of the film is. Therefore, when the current density is 5 A·dm−2, the composite coating has the best corrosion resistance. Fig. 19 shows the cross section morphology of the composite film after immersion 192 h. The black pits in the figure indicate the corrosion area. It can be seen that the matrix has been seriously corroded and the corrosion layer has been exfoliated seriously. There are many pits in the cross section of the LC coating and the corrosion mechanism is mainly pitting corrosion, which is due to the large cracks and pores on the surface of the LC coating, a large amount of Cl− entered the coating, resulting in the gradual destruction of the coating, corrosion resistance gradually reduced. For the composite film, it can be seen that the surface of the film shows local corrosion characteristics. When the current density is 5A·dm−2, the corrosion resistance of the inner dense layer increases with the increase of the thickness and the density of the ceramic layer, which results in the increase of corrosion resistance of the ceramic layer as a whole, so that there are few corrosion pits on the surface of the coating.

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4. Conclusions In this study, we prepared composite ceramic coating by laser cladding combined with micro-arc oxidation technology and compared its corrosion behaviour (immersion corrosion and corrosive wear) to the cladding coating and S355 offshore steel. (1) The composite ceramic coating was prepared on the marine steel by laser cladding combined with micro-arc oxidation. The bonding strength between the coating and the substrate was high. The loose layer was mainly composed of γ-Al2O3 and the dense layer was mainly composed of α-Al2O3. With the increase of current density, the thickness of the film increases, the number of micropores decreases and the pore size increases. (2) With the increase of current density, the composite ceramic coating shows an inward growth mechanism, the hardness of the composite coating is 31. 4% higher than that of the cladding coating, which can significantly improve the mechanical properties of the substrate. (3) The coatings and the substrate promote each other in the interaction between corrosion and wear. In the interaction, the coating is mainly wear accelerated corrosion, and the matrix is mainly corrosion accelerated wear. When the coating surface was treated by micro-arc oxidation, the corrosive wear resistance is significantly improved. (4) The corrosion mechanism of the cladding coating is pitting corrosion, the corrosion of the composite coating is slight, and the corrosion resistance of the substrate can be improved obviously. When the current density is 5A·dm−2, the corrosion resistance of the composite coating is the best. Acknowledgments The authors gratefully acknowledge the financial supports from the Key Research and Development Project of Jiangsu Province (BE2016052), the National Natural Science Foundation of China (Nos. 51371039 and 51871031) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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