The synergistic effect of cavitation erosion and corrosion of nickel-aluminum copper surface layer on nickel-aluminum bronze alloy

Accepted Manuscript The synergistic effect of cavitation erosion and corrosion of nickel-aluminum copper surface layer on nickel-aluminum bronze alloy Qin Luo, Qi Zhang, Zhenbo Qin, Zhong Wu, Bin Shen, Lei Liu, Wenbin Hu PII:

S0925-8388(18)30971-X

DOI:

10.1016/j.jallcom.2018.03.103

Reference:

JALCOM 45333

To appear in:

Journal of Alloys and Compounds

Received Date: 8 December 2017 Revised Date:

25 February 2018

Accepted Date: 9 March 2018

Please cite this article as: Q. Luo, Q. Zhang, Z. Qin, Z. Wu, B. Shen, L. Liu, W. Hu, The synergistic effect of cavitation erosion and corrosion of nickel-aluminum copper surface layer on nickel-aluminum bronze alloy, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.03.103. 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.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT The synergistic effect of cavitation erosion and corrosion of nickel-aluminum copper surface layer on nickel-aluminum bronze alloy Qin Luoa, Qi Zhanga, Zhenbo Qina, Zhong Wu c,Bin Shena, Lei Liu*a, b,Wenbin Hu a,c State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200240, China b

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a

Collaborative Innovation Center for Advanced Ship and deep-Sea Exploration, Shanghai, 200240, China c

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School of Material Science and Engineering, Tianjin University, Tianjin, 300072, China

Abstract

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A completely nickel-aluminum-copper (Ni-Al-Cu) layer was obtained through thermal diffusion process on a nickel-aluminum-bronze (NAB) substrate. The cumulative mass loss of the NAB alloy during the cavitation erosion tests was about

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6.4 times and 5.5 times as large as that of the Ni-Al-Cu layer in distilled water and 3.5 wt.% sodium chloride (NaCl), respectively. Electrochemical measurements under quiescence and cavitation erosion conditions were conducted to investigate the

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synergistic effects of cavitation erosion and corrosion. The results showed that the

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synergism had measurable effect on the cavitation erosion-corrosion process for both the NAB alloy and the Ni-Al-Cu layer. The total contribution of the cavitation erosion component including WE and WCIE was more than 80% and WE occupied the largest percentage. Thus, the improved cavitation erosion resistance of the Ni-Al-Cu layer was mainly due to mechanical factors, which implied a homogeneous and refined microstructure and the formation of hardened Ni3Al phase.

ACCEPTED MANUSCRIPT Keywords:

Nickel-aluminum

bronze

alloy;

Ni-Al-Cu

layer;

Cavitation

erosion-corrosion; Synergism

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1. Introduction

Cavitation erosion is a common phenomenon during material degradation in hydrodynamic systems, such as propellers and liquid-handling equipment. Cavitation

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is defined as the generation and collapse of bubbles in liquid caused by pressure fluctuations arising from changes in fast flow or vibrations [1]. In marine

simultaneously

for

engineering

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environments, the electrochemical corrosion and mechanical erosion exist alloys—namely,

cavitation

erosion-corrosion

processes. The overall damage resulting from cavitation erosion-corrosion includes

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corrosion, cavitation erosion, and their synergism. Many studies have reported the cavitation erosion-corrosion resistance of ship propeller materials, such as steels [1, 2] and copper (Cu)-based alloys [3]. Nickel-aluminum-bronze (NAB) alloy is one of the

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most widely applied Cu-based alloys for ship propellers because of its good

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mechanical properties and excellent resistance against cavitation erosion-corrosion [4, 5]. According to Al-Hashem [6], the multiphase and inhomogeneous microstructure had a negative effect on cavitation erosion-corrosion resistance because of the selective corrosion around α phase and κ precipitates. This selective corrosion resulted in larger cavities and material loss. To further improve the cavitation erosion-corrosion resistance and prolong the service life of the NAB alloy components, some surface modifications or treatments have been employed.

ACCEPTED MANUSCRIPT The friction stir process (FSP) surface modification can homogenize and refine the microstructure to improve cavitation erosion-corrosion [7]. The cavitation erosion resistance of FSP-NAB alloy increased only by about 1.5 times, and the limited effect

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was probably due to the unchanged composition of the treated surface [8]. The laser surface modification could somewhat improve the cavitation erosion-corrosion resistance of the NAB alloy [3, 8]. Conversely, the use of laser surface modifications

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is limited because of its lower efficiency in treating large surfaces and lower

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absorptivity with the metallic surfaces [9]. Thus, the surface modification layer of the NAB alloy can improve cavitation erosion-corrosion resistance, but it is not completely understood.

As is known to all, the surface hardness rather than bulk mechanical properties is

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more important in resisting cavitation erosion [8]. Intermetallic compounds of NiAl and Ni3Al are used extensively in cavitation erosion environments because of their

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work-hardening ability [10]. In addition, Ni and Al oxidations, such as Al2O3 and NiO2, are more compact, and the binding strength with the substrate is much higher

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than Cu oxidation, such as Cu2O [5]. Therefore, increasing the Ni or Al to form NiAl or Ni3Al compounds in the surface layer can enhance the cavitation erosion-corrosion resistance of the NAB alloy.

Hearley [11] proposed a thermal spraying NiAl coating to improve the erosion resistance, but it was limited by edge spallation. The deposited NiAl coating was obtained from a cathodic arc plasma ion plating process , facing the problem of energetic requirements and limited modified depth [12]. Considering these

ACCEPTED MANUSCRIPT disadvantages, a thermal diffusion layer was proposed at industrial temperatures (675°C) for industrial utilization [13].

In our previous work, a thermal modified layer consisted of ~70 wt.% Ni, ~20

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wt.% Cu, and ~10 wt.% Al was obtained in the middle of the diffusion layer and improved the corrosion resistance of the NAB substrate [14]. The goal of this work is

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to create a completely homogenous Ni-Al-Cu layer on the NAB alloy via thermal diffusion. The cavitation erosion experiments were performed on the layer and the

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NAB alloy in distilled water and 3.5 wt.% NaCl, respectively. Electrochemical tests under quiescence and cavitation erosion conditions were also conducted to determine the effect of cavitation erosion on corrosion. Furthermore, the synergistic effects between cavitation erosion and corrosion for both the Ni-Al-Cu modified layer and

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2.1 Materials

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2. Experimental

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the NAB substrate were discussed.

The NAB alloys were characterized by X-ray fluorescence spectroscopy with a

chemical composition (wt.%) of Al 9.85, Ni 3.75, Fe 3.86, Mn 1.03 and Cu balance. The Ni-Al-Cu layer was developed by thermal diffusion process of Ni coating and the NAB matrix at 675°C [14]. The nanocrystalline Ni coating was electrodeposited on the

NAB

alloy

before

thermal

diffusion

in

the

electrolyte

containing

Ni(SO3NH2)2·4H2O (350 g/L), NiCl2·6H2O (15 g/L), C12H25-OSO3Na (0.1 g/L), and

ACCEPTED MANUSCRIPT H3BO3(40 g/L) with pH of 4.0±0.1 using NH3·H2O. The electrodeposition time was 10 min, and the thermal diffusion duration time was 12 h.

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2.2 Microstructure analysis

The cross-section microstructure of the surface-modified NAB alloy was observed via scanning electron microscope (SEM; JEM-2100F, JEOL, Ltd., Japan)

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with energy dispersive spectroscopy (EDS). Before metallographic observations, the specimens were metallographically prepared and etched in solution of 5 g FeCl3 + 2

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ml HCl + 95 ml C2H5OH for the NAB alloy [7] and nitrohydrochloric acid for the Ni-Al-Cu layer. The microstructure of the NAB alloy and the Ni-Al-Cu layer was determined by X-ray diffraction spectroscopy (XRD; D8 ADVANCE DAVINCI,

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Bruker, Germany) with a Cu-Kα target (λ = 0.15405 nm). The 2θ scanning was from 30° to 90° at scan speed of 2°/min.

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2.3 Nano-indentation tests

Nano-indentation experiments were performed on cross section of the modified

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NAB alloy with a Berkovich indenter. All samples were polished to a mirror bright surface with SiC (1 um) sprayer. Once the tip contacted the polished surface, a constant force of 100 mN was applied holding for 30 s, and the load/unload speed was 3 mN/s. The hardness (H) values were obtained by the average value of the three replicate tests.

2.4 Cavitation erosion tests

ACCEPTED MANUSCRIPT Cavitation erosion experiments were performed in distilled water and 3.5 wt.% NaCl solution with an ultrasonic vibration apparatus following ASTM G32 standard [15]. Before testing, the samples were pretreated by grinding and mechanical

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polishing, cleaned with acetone in an ultrasonic bath, and dried in hot air. During testing, the samples were fixed coaxially with the horn and held 0.5 mm from the horn. The ultrasonic horn was operated at 20 kHz with an amplitude of 20 µm. The samples

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were then immersed in electrolytes at 25 ± 1°C using a water-cooling system. After

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testing, the specimens were degreased, rinsed, dried, and weighed by an analytical balance with accuracy of 0.1 mg to calculate mass loss. Each sample was repeated three times to ensure accuracy. The eroded surfaces and cross sections were also observed by SEM.

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2.5 Electrochemical measurements

The potentiodynamic polarization curves of the NAB alloy and Ni-Al-Cu layer

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under quiescence and cavitation erosion condition were carried out in 3.5 wt.% NaCl

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solution through a CHI 660E electrochemical system from China. Traditional three-electrode system was applied in the measurements: the specimen as the working electrode, a saturated calomel electrode as reference electrode, and a platinum electrode as the counter electrode. The polarization curves were swept from −0.8 V to 1 V at the rate of 1 mV/s. In addition, the corrosion potential (Ecorr) and corrosion current density (icorr) were obtained via tafel extrapolation technique [2, 16]. 3. Results and discussion

ACCEPTED MANUSCRIPT 3.1 Microstructure characterization

Fig. 1(a) shows the cross-sectional microstructure of the modified NAB alloy with EDS line scan results. The thickness of surface layer was about 15 µm and it was

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uniformly distributed with Ni, Al and Cu elements. The layer was well bonded with the NAB substrate and showed a smooth transition at the interface (Fig. 1(a)). The

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following cavitation erosion and cavitation erosion-corrosion process also required good adhesion of the surface layer to the substrate [17]. The microstructure of the

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modified layer (Fig. 1(b)) is clearly different from the NAB alloy (Fig. 1(c)). The NAB alloy was multi-phased and composed of a coarse α phase with grain size of more than 20 µm as well as β' phase and κ particles, similar to Song et al. [18] and

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Wharton et al. [4].

Conversely, the microstructure of the modified Ni-Al-Cu layer was homogeneous and refined with grain sizes near 3 µm in contrast to the complex and heterogeneous

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microstructure of the NAB alloy. The XRD spectra of the NAB alloy and Ni-Al-Cu

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layer is shown in Fig. 2. The microstructure of the NAB alloy consisted of major α phase (fcc structure) and a very small amount of β phase (bcc structure), which is consistent with Tang et al. [3]and Lv et al. [19]. However, the major peaks of the Ni-Al-Cu layer were Ni3Al and some NiCu and Ni peaks. Ni and Cu are infinite solid solution elements and diffused faster than Al [14]. However, Al preferentially affixed to Ni to form intermetallic compounds because of the lower enthalpy of formation (∆H) of the Ni-Al compounds [13]. Similar results have been reported by Chang et al. [12] in improving the cavitation erosion resistance with Ni-Al intermetallic coatings.

ACCEPTED MANUSCRIPT 3.2 Nano-indentation hardness

The nano-indentation tests on a longitudinal cross section of the Ni-Al-Cu layer and the NAB substrate were performed. The inter-distance of each adjacent two

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indents was set at 5 µm to avoid the influence of the indentations on each other. The hardness profile in Fig. 3 shows that the average micro-hardness of the Ni-Al-Cu

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layer was about 5.5 Gpa, which is almost 2.2-fold that of the NAB alloy (~2.5 Gpa). It was attributed to the grain refinement and the formation of hard Ni3Al phases

3.3 Cavitation erosion behaviors

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compared with the Cu-based alloys [10, 20]

Fig. 4 shows the cumulative mass loss and mass loss rate as a function of

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cavitation erosion time for the specimens in distilled water and 3.5 wt.% NaCl solution. The measured mass loss data in Fig. 4(a) was the average value of the three measurements. The cumulative mass loss of the Ni-Al-Cu layer exhibited much lower

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values than the NAB alloy both in 3.5 wt.% NaCl solution and distilled water. This is

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consistent with Song [7] and Chang [12]. Besides, the mass loss of the NAB alloy in 3.5 wt.% NaCl solution obviously increased because of the effect of electrochemical corrosion and the synergism on the damage of the material, whereas the Ni-Al-Cu layer showed a small increase. The contribution of corrosion and pure mechanical attack will be discussed later.

In the 3.5 wt.% NaCl solution, the mass loss of the NAB substrate was 21.5 mg after cavitation erosion for 12 h. This is almost 5.5-fold as much as that of the

ACCEPTED MANUSCRIPT Ni-Al-Cu layer (3.9 mg). In distilled water, the cumulative mass loss after 12 h of cavitation erosion was 16.6 mg and 2.6 mg for the NAB alloy and the Ni-Al-Cu layer, respectively. In addition, the cumulative mass loss rate of the Ni-Al-Cu layer in 3.5

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wt.% NaCl solution and distilled water reached a maximum of 0.325 mg/h and 0.22 mg/h at 4 h. It then decreased slightly or leveled off at later times. In contrast, the cumulative mass loss rate of the NAB alloy under both conditions was bigger and

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almost increased linearly throughout the cavitation erosion process. This indicated

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that the cavitation erosion resistance of the Ni-Al-Cu layer was much better than that of the NAB alloy whether in distilled water or in 3.5 wt.% NaCl solution.

The SEM micrographs of cavitation erosion surface of Ni-Al-Cu layer and the NAB alloy at different time intervals in distilled water are shown in Figs. 5–6. The

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deformation of the NAB alloy covered the entire surface because of the mechanical attack (Fig. 5b), whereas the Ni-Al-Cu layer still retained most original surface

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morphology after cavitation erosion for 0.5 h (Fig. 5a). At higher magnification, the surface of the Ni-Al-Cu layer had plastic deformation in the form of undulations at the

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grain boundaries as well as twin boundaries (Fig. 5(c)). This corresponded to surface damage from the Ni-based Hastelloy C-276 alloy [21] and Ni-Fe alloys [22]. The NAB alloy in Fig. 5(d) had surface cracks at the phase boundaries between the α and κ phases because of cavitation erosion stress [7].

As cavitation erosion time increased to 2 h, the plastic deformation increased in the Ni-Al-Cu layer. It diverged from the grain boundaries and twin boundaries; most of the original surface remained (Fig. 6(a)). It was attributed to the fact that the

ACCEPTED MANUSCRIPT fatigue limit was exceeded [22]. In the NAB alloy at identical time, cracks were observed between the α phase and the κ particles and some κII particles peeled off in Fig. 6(e). After cavitation erosion for 4 h, the damaged surface of the Ni-Al-Cu layer

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showed cavities and micro-cracks at the grain boundaries. This resulted from materials being removed from the grain boundaries with only some original surface remaining (Fig. 6(b)). In addition, some cavities connected to form a large cavity

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resulting in additional mass loss. This agrees with the mass loss rate curve in Fig. 4(b)

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that the mass loss rate increased to a maximum at 4 h. For the NAB alloy, the deformation covered the entire surface, which was due to the propagation of the cracks and material mass loss (Fig. 6(f)). As the cavitation erosion time increased to 8 h, the damaged surface of the Ni-Al-Cu layer in Fig. 6(c) shows that the small cavities

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continued to connect around the large cavities with only a trace of the original surface remaining. This led to more material being removed. Fig. 6(g) shows that the deformed surface of the NAB alloy was severe with large and deep cavities. This

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resulted in much rougher surface. At 12 h, the damaged surface covered the entire

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surface of the Ni-Al-Cu layer; it was still relatively smooth in Fig. 6(d). On the contrary, deep and macroscopic cavities were observed on the surface of the NAB alloy (Fig. 6(h)). When the surface was damaged, the surface roughness was positively related to the propagation and extendibility of cavities [7]. Thus, the smoother surface of the Ni-Al-Cu layer indicated improved cavitation erosion resistance compared with the NAB alloy.

ACCEPTED MANUSCRIPT The cavitation erosion process was more complex in natural seawater because of corrosion, erosion, and their synergism. Thus, tests were also performed in 3.5 wt.% NaCl as shown in Fig. 7. The NAB alloy suffered worse damage than the Ni-Al-Cu

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layer. For the Ni-Al-Cu layer in Figs. 7(a, b, c, d), the collapsing of the material around the grain boundaries and the expansion of small cavities into large ones were obviously observed as the cavitation time increased. The damage covered the entire

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surface of the NAB alloy after 2 h and deep cavities were visible after 8 h (Figs. 7(e, f,

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g, h)). Combined with Fig. 4 and Fig. 6, it’s concluded that the effect of corrosion on cavitation erosion damage is important.

For the NAB alloy, the different potential between different phases caused galvanic corrosion in the corrosive medium [7]. In 3.5 wt.% NaCl, the α phase was

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preferentially corroded because of the anodic potential compared with the κ phase. Thus, the κ II and κIV phases were removed easily under cavitation erosion attack with

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continuous dissolution of the α phase. This resulted in more vulnerable sites for attack. For the Ni-Al-Cu layer, corrosion occurred because of a de-alumination reaction

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resulting in pit formation [13]. The rough surface caused by corrosion provided more active sites for cavitation, which in turn accelerated the material loss. This explains why the mass loss in the 3.5 wt.% NaCl solution increased. The microstructure of the Ni-Al-Cu layer, however, was refined and homogenized. The galvanic corrosion did not dominate relative to the NAB alloy. Therefore, damage to the Ni-Al-Cu layer in 3.5 wt.% NaCl solution showed little difference compared with that in distilled water; the NAB alloy suffered much more serious damage in 3.5 wt.% NaCl solution.

ACCEPTED MANUSCRIPT The damaged surfaces of Ni-Al-Cu layer observed by laser confocal microscope in distilled water and 3.5 wt.% NaCl solution were presented in Fig. 8. The Z-axis showed subtle change with the increase in cavitation erosion time, indicating that few

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deeper cavities were formed, but surface roughness increased. It could also confirm that the cavitation erosion resistance of Ni-Al-Cu layer in both conditions showed little difference. The 3D morphology of the damaged surface of the NAB alloy in both

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condition was shown in Fig. 9. The Z-axis and surface roughness increased with the

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increase in cavitation erosion time, and the damage in 3.5 wt.% NaCl solution (Fig. 9(e, f, g, h)) was much more serious than that in distilled water (Fig. 9(a, b, c, d)). After cavitation erosion for 12 h, the Z-axis used in 3.5 wt.% NaCl solution was 42.13 µm and some larger cavities were clearly visible in Fig. 9(h). And the Z-axis in

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distilled water was 33.34 µm and smaller cavities were observed. Besides, the Z-axis of the NAB alloy was much higher than the Ni-Al-Cu layer in Fig. 8. It indicated the

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increasing cavitation erosion resistance of Ni-Al-Cu layer.

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3.4 Electrochemical measurements

Fig. 10 illustrates the potentiodynamic polarization curves of the NAB alloy and

the Ni-Al-Cu layer for Tafel extrapolation under quiescence and cavitation erosion condition in 3.5 wt.% NaCl solution. The corresponding electrochemical parameters are summarized in Table 1. The corrosion potential tended to increase under cavitation erosion condition. These shifts were consistent with AISI 1050 with surface alloying of the Al alloy [23]. Besides, the corrosion current density (icorr) of the NAB alloy in quiescent condition was 3.985E-6 A cm-2, which was much lower than that

ACCEPTED MANUSCRIPT value obtained under cavitation (1.469E-4 A cm-2). For the Ni-Al-Cu layer, the icorr values under quiescence and cavitation conditions were 6.487E-7 A cm-2 and 2.578E-5 A cm-2, respectively. Clearly, cavitation had a strong influence on the

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electrochemical behavior.

Table 1 Electrochemical parameters (Ecorr and icorr) of the NAB alloy and the

conditions.

Material Ecorr (mV)

−0.289

Ni-Al-Cu layer

−0.404

icorr (A cm-2)

Ecorr (mV)

icorr (A cm-2)

3.985E-6

−0.260

1.469E-4

−0.387

2.578E-5

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NAB

Cavitation

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Quiescence

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Ni-Al-Cu layer in 3.5 wt.% NaCl solution under quiescent and cavitation erosion

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6.487E-7

The effect of cavitation on corrosion behavior originates from increasing oxygen

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supply and removing protective film or corrosion products film[24], which are corresponding to cathodic corrosion and anodic corrosion, respectively. According to Kwok [23], the increase in oxygen supply results in the corrosion potential increasing, while the removing of corrosion product film or protective film causes the corrosion potential decreasing. The noble shift of corrosion potential was due to the oxygen supply dominated under cavitation erosion condition. For the NAB alloy, the anodic corrosion mainly occurred the dissolution of Ni, Cu and Al as follows:

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Cu → Cu2+ + 2e−

(2)

Al → Al 3+ + 3e−

(3)

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Ni → Ni2+ + 2e−

which resulted in the formation of corrosion products film on the surface. The cathodic corrosion of the NAB alloy came from the oxygen reduction as follows:

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O 2 +2H 2 O+4e − → 4OH −

(4)

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It is noted in Fig. 10(a) that the current density of the NAB alloy at given cathodic potentials under quiescent condition was much lower than that obtained under cavitation erosion condition, and the effect of cavitation on anodic current density was relatively small. It is due to the fact that the corrosion products film on the NAB alloy

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was discontinuous and less protective than that of the Ni-Al-Cu layer [13, 18]. Thus, cavitation had little effect on the anodic corrosion, while any increasing of oxygen

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supply could promote the cathodic reaction and increase the corrosion rate. It indicates that the cavitation erosion-corrosion process of the NAB alloy was

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dominated by cathodic reaction and mechanical impact. For the Ni-Al-Cu layer, the cavitation affected both the cathodic reaction process and the anodic reaction process as shown in Fig. 10(b). The anodic reactions of the Ni-Al-Cu layer were mainly the dissolution of Ni, Al and Cu. However, the corrosion product film formed during the anodic corrosion process was more compact and protective compared to that of the NAB alloy [13]. In the presence of cavitation, the anodic dissolution was accelerated by elimination of the passive film because of impact pressure, which exposed the bare

ACCEPTED MANUSCRIPT metal surface with more active sites for corrosion attack and increased the corrosion rate. In the cathodic reaction process, the cavitation increased the oxygen supply, which could also increase the oxygen reduction reaction and the corrosion rate. The

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increasing corrosion potential of the Ni-Al-Cu layer under cavitation erosion condition indicates that the effect of the cavitation on cathodic process was bigger. It means that the cavitation erosion-corrosion process of the Ni-Al-Cu layer was also

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dominated by cathodic reaction and mechanical impact.

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3.5 Synergistic effect of cavitation erosion and corrosion

When the cavitation erosion attack occurred in the corrosion medium, mechanical erosion and electrochemical corrosion coexist, and they may accelerate

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the mass loss and the degradation of the material. Thus, their synergistic effect could not be ignored. Generally, the synergism of cavitation erosion and corrosion contains two factors: corrosion-induced erosion and erosion-induced corrosion. The mass loss

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of total cavitation erosion (WT) can be described as follows:

WT = WC + WE + WS

(5)

WS = WCIE + WEIC

(6)

where WC is the mass loss of pure corrosion; WE is the mass loss of pure mechanical erosion; WS is the mass loss of the synergism of cavitation erosion and corrosion; WCIE is the mass loss of corrosion-induced erosion; and WEIC is the mass loss of erosion-induced corrosion.

ACCEPTED MANUSCRIPT The value of WT and WE in this work were measured under cavitation erosion in 3.5 wt.% NaCl and distilled water, respectively. WC was calculated from the corrosion current density according to the Faraday’s law [25]:

i nF

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WC = M

(7)

where M is the molar weight of the metal; i is the current density; n is the number of

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valence electrons transferred by each metal atom in corrosion process; F is Faraday’s

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constant. WEIC was calculated via the following equation:

WEIC = WC' − WC

(8)

where WC' was calculated from the corrosion current density under cavitation erosion.

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The parameters described are listed in Table 2. Apparently, the Ni-Al-Cu layer had much higher cavitation erosion resistance both in distilled water and 3.5 wt.% NaCl solution because of the lower values of WE and WT. To analyze the components of

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cavitation erosion-corrosion process, the fractional contributions are presented in

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Table 3. The contribution of the cavitation erosion component including WE and WCIE was more than 80% and WE occupied the largest percentage. It indicated that the mechanical effect dominated the overall cavitation erosion-corrosion process for the NAB alloy and the Ni-Al-Cu layer, which meant that the damage was mainly attributed to the pure cavitation erosion. However, the WE for the NAB alloy was almost 6.4-fold that of the Ni-Al-Cu layer, indicating the better mechanical properties of the Ni-Al-Cu layer, as discussed earlier. The fraction contribution of the corrosion

ACCEPTED MANUSCRIPT components (WC+ WEIC) of the Ni-Al-Cu layer was lower. This was mainly due to the higher corrosion resistance of the Ni-Al-Cu layer compared with the NAB alloy [13].

Table 2 Mass loss of pure corrosion (WC), pure erosion (WE), corrosion-induced

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erosion (WCIE), erosion-induced corrosion (WEIC), and total cavitation erosion (WT) for the NAB alloy and the Ni-Al-Cu layer in 3.5 wt.% NaCl solution.

Material WT

WC

WCIE

NAB alloy

21.5

0.12

16.6

0.902

3.878

Ni-Al-Cu layer

3.9

0.017

2.6

0.636

0.647

WEIC

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WE

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Mass loss (mg)

Table 3 Contribution of pure corrosion (WC), pure erosion (WE), corrosion-induced

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erosion (WCIE), and erosion-induced corrosion (WEIC) on total cavitation erosion (WT)

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for the NAB alloy and the Ni-Al-Cu layer in 3.5 wt.% NaCl solution.

Contribution (%)

Material

WC/ WT

WE/ WT

WCIE/ WT

WEIC/ WT

NAB alloy

0.558

77.21

4.20

18.04

Ni-Al-Cu layer

0.44

66.67

16.30

16.59

ACCEPTED MANUSCRIPT 4. Conclusions The Ni-Al-Cu surface modification layer on NAB alloys was obtained via thermal diffusion process. The microstructure of the Ni-Al-Cu layer was

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homogeneous with a refined grain size of about 3 µm. Nano-indentation test results indicate that the hardness increase in this layer was due to formation of a hardened

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Ni3Al phase.

After cavitation erosion for 12 h, the cumulative mass loss of the NAB substrate

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was 6.4- and 5.5-times that of the Ni-Al-Cu layer in distilled water and 3.5 wt.% NaCl solution, respectively. The percentage of the erosion component of the NAB alloy and the Ni-Al-Cu layer was over 80%, which indicated that the main damage was caused

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due to the erosion factor under cavitation erosion-corrosion process. The improving cavitation erosion-corrosion resistance of the Ni-Al-Cu layer was mainly due to material characteristics—homogeneous and refined microstructure with increasing

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hardness. Besides, the synergism had measurable effect on the cavitation

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erosion-corrosion process for the NAB alloy and the Ni-Al-Cu layer. And the synergism of WCIE+WEIC for the Ni-Al-Cu layer was bigger than that of the NAB alloy.

Acknowledgments

ACCEPTED MANUSCRIPT The authors acknowledge the Major State Basic Research Development Program of China (973 Program; No. 2014CB046701) and the National Natural Science Foundation of China (No. 51601114, No. 51771117).

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[8] C.H. Tang, F.T. Cheng, H.C. Man, Laser surface alloying of a marine propeller bronze using aluminium powder, Surf. Coat. Technol. 200 (2006) 2602-2609.

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[9] C.T. Kwok, H.C. Man, F.T. Cheng, K.H. Lo, Developments in laser-based surface engineering processes: with particular reference to protection against

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cavitation erosion, Surf. Coat. Technol. 291 (2016) 189-204.

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nickel aluminide intermetallic composites and matrix composites with TiC reinforcement, Surf. Coat. Technol. 201 (2006) 1289-1295.

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ACCEPTED MANUSCRIPT [14] G.W.L. F. Hasan, N. Ridley, Tempering of cast nickel-aluminium bronze, Met. Sci. 17 (1983) 289-295.

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apparatus, Annual book of ASTM standards, ASTM, Philadelphia, PA, (2009)

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3.5 wt% NaCl Solution, Corros. 71 (2015) 606-614.

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mechanical properties of hot worked NiAl bronze alloy with different deformation degree, Mater. Sci. Eng. A 643 (2015) 17-24.

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solution, Electrochim. Acta. 51 (2006) 5341-5348.

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Figures

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Captions

Fig. 1 Cross-sectional microstructure of (a) the surface modified NAB alloy, (b) the

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microstructure of NAB substrate, and (c) the Ni-Al-Cu layer.

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Fig. 2 XRD patterns of the NAB alloy and the modified Ni-Al-Cu layer.

Fig. 3 Hardness values as a function of the distance to the Ni-Al-Cu layer/NAB

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substrate interface.

Fig. 4 Cumulative (a) mass loss and (b) mass loss rate of the NAB substrate and the Ni-Al-Cu layer under cavitation erosion condition in distilled water and 3.5 wt.% NaCl.

Fig. 5 Damage morphologies under cavitation erosion in distilled water for 0.5 h: (a) and (c) Ni-Al-Cu layer, (b) and (d) NAB alloy.

ACCEPTED MANUSCRIPT Fig. 6 Typical SEM micrographs of the eroded surface of the Ni-Al-Cu layer after cavitation erosion in distilled water for (a) 2 h, (b) 4 h, (c) 8 h, and (d) 12 h and NAB alloy for (a) 2 h, (b) 4 h, (c) 8 h, and (d) 12 h.

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Fig. 7 Typical SEM micrographs of the eroded surface of the Ni-Al-Cu layer after cavitation erosion in 3.5 wt.% NaCl solution for (a) 2 h, (b) 4 h, (c) 8 h, and (d) 12 h

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and NAB alloy for (a) 2 h, (b) 4 h, (c) 8 h, and (d) 12 h.

Fig. 8 3D surface morphology of Ni-Al-Cu layer measured by laser confocal microscope after cavitation erosion in distilled water (a, b, c, d) and 3.5 wt. % NaCl solution (e, f, g, h): (a, e)2 hours; (b, f) 4 hours; (c, g) 8 hours; (d, h) 12 hours.

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Fig. 9 3D surface morphology of NAB alloy measured by laser confocal microscope after cavitation erosion in distilled water (a, b, c, d) and 3.5 wt. % NaCl solution (e, f,

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g, h): (a, e)2 hours; (b, f) 4 hours; (c, g) 8 hours; (d, h) 12 hours.

Fig. 10 The potentiodynamic polarization curves of the NAB alloy and Ni-Al-Cu

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layer in 3.5 wt.% NaCl under quiescence and cavitation erosion: (a) the NAB alloy, and (b) the Ni-Al-Cu layer.

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Highlights 1. Surface modification with Ni-Al-Cu layer was obtained through thermal diffusion. 2. Mechanical properties of Ni-Al-Cu layer was improved compared to NAB

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substrate. 3. The better properties was due to the refined and harden intermetallic particles.

4. Cavitation erosion-corrosion resistance was about 5.5 times compared to NAB

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alloy.