Synergistic effect between cavitation erosion and corrosion for various copper alloys in sulphide-containing 3.5% NaCl solutions

Wear 450–451 (2020) 203258

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Synergistic effect between cavitation erosion and corrosion for various copper alloys in sulphide-containing 3.5% NaCl solutions Q.N. Song a, *, Y. Tong a, N. Xu a, S.Y. Sun a, H.L. Li a, Y.F. Bao a, Y.F. Jiang a, Z.B. Wang b, Y. X. Qiao c a b c

College of Mechanical and Electrical Engineering, Hohai University, 200 Jinling North Road, Changzhou, 213022, China Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang, 110016, China College of Materials Science and Engineering, Jiangsu University of Science and Technology, 2 Mengxi Road, Zhenjiang, 212003, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Copper alloy Sulphide Cavitation erosion Synergy

Cavitation erosion-corrosion (CE-C) behaviour of various copper alloys for ship propellers were investigated in sulphide-containing 3.5% NaCl solutions. Results indicate that the CE-C resistance remarkably decreases with increase in sulphide concentration for cast and friction stir processed nickel-aluminum bronzes. CE increases the current density by approximately two orders of magnitude and causes a positive potential shift when the sulphide concentration exceeds 50 ppm. The synergistic effect between CE and corrosion contributes greatly to CE-C degradation in sulphide solutions. However, the CE-C behaviour is insensitive to sulphide concentration for manganese-aluminum bronze and manganese brass. In all solutions, CE causes a positive potential shift and increases the current density by less than one order of magnitude. Mechanical attack dominates the CE-C damage.

1. Introduction Due to the excellent corrosion resistance to seawater, copper and its alloys are widely used in marine environments, such as in the manufacturing of pipes, pumps, valves and ship propellers [1]. Pro­ pellers, as crucial propulsion parts for ships, undergo corrosion and cavitation erosion (CE) because of the high-speed revolution in seawater. CE is caused by the nucleation and collapse of bubbles due to pressure fluctuations in a liquid medium. The collapsing bubbles impact the components in the manner of a shock wave or microjet, resulting in mechanical surface deformation and fracture [2–4]. In seawater, a synergistic effect exists between corrosion and CE, resulting in acceler­ ated failure of marine components [5,6]. Hence, a good combination of corrosion resistance and mechanical properties is required for the pro­ peller materials. Typical copper alloys, which are used for making ship propellers, consist of manganese brass (MB), manganese-aluminum bronze (MAB) and nickel-aluminum bronze (NAB), as a few examples [7–10]. MB is applied for making low-rotating speed propellers because of its relatively low price with low contents of alloying Ni and Al ele­ ments. Moreover, brass is susceptive to dezincification corrosion in seawater [1], which will further weaken the mechanical properties and

CE resistance. MAB and NAB, which are two aluminum bronzes with the addition of Mn, Fe and Ni, exhibit higher corrosion and CE-corrosion (CE-C) resistance than MB and are more extensively applied for mak­ ing large-size and high-rotating speed propellers [8–10]. Large-sized ship propellers are generally made from castings with coarse and inhomogeneous microstructure and inevitable casting porosities. To prolong the service life of cast ship propellers, some surface modification methods have been applied. Friction stir process (FSP) is one of them, and it originates from friction stir welding (FSW) [11]. During FSP, the intense plastic deformation, which is induced by the rotating stir tool, refines the microstructure and enhances the mechanical properties of castings [12]. Oh-shi et al. [13,14], Ni et al. [15,16], and Lv et al. [17] applied FSP on the cast NAB (as the most widely used copper alloy for ship propellers), and confirmed that FSP remarkably refined the microstructure and improved the tensile strength and elongation of the cast NAB. It was also reported that FSP raised the sliding wear resistance [18] and enhanced the corrosion and CE resistance of the cast NAB in 3.5% NaCl solution [19,20]. It is noteworthy that copper and its alloys suffer much more severe corrosion in seawater when pollutants are present. Sulphide is one of the common pollutants introduced from industrial waste and metabolic

* Corresponding author. E-mail address: [email protected] (Q.N. Song). https://doi.org/10.1016/j.wear.2020.203258 Received 5 December 2019; Received in revised form 25 February 2020; Accepted 2 March 2020 Available online 3 March 2020 0043-1648/© 2020 Elsevier B.V. All rights reserved.

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processes of marine organisms and microorganisms [21]. In the presence of sulphide, a film, which consisted of both copper oxides and sulphides, developed on the copper alloys. It exhibited loose structure and inferior protectiveness. The higher the sulphide concentration, the higher the corrosion rates of copper and its alloys [22–24]. The tendency of stress cracking corrosion also increased with sulphide concentration in seawater for Al bronze and Al brass [25,26]. The failure analysis of a NAB impeller demonstrated that NAB underwent pitting and selective phase corrosion (SPC) in heavily polluted seawater, which was in presence of sulphide [27]. In consideration of practical working conditions, the erosioncorrosion behaviours of hydraulic component materials have been also investigated in the presence of sulphide. In a sulphide-containing aerated 3.5% NaCl solution, the local acidification caused by iron sul­ phide formation decreased the erosion-corrosion resistance of mild steel [28]. Kwok et al. also found out that an increase in sulphide concen­ tration resulted in an increased mean depth of penetration rate for super duplex stainless steels after CE in a sulphide-containing aerated 3.5% NaCl solution [29]. Although many researches have been conducted on the corrosion behaviour of the copper alloys in the sulphide-containing environments, the CE-C behaviour still lacks proper investigation. The severe corrosion damage induced by sulphide will further deteriorate the CE-C resistance of the copper and its alloys. The results of our recent research indicated the CE damage of the cast NAB was largely caused by the synergy between CE and corrosion when sulphide was present [30, 31]. In contrast, the cast MAB exhibited a mechanical attack-dominated degradation mechanism whether sulphide was present or not [32]. Thus, sulphide has different effects on the CE-C behaviour of different copper alloys. However, the synergistic effect between CE and corrosion, which is crucial to uncover the CE damage mechanism, still lacks intensive characterization. To the best of the authors’ knowledge, there are few comparative studies conducted on the CE-C behaviours of various cop­ per alloys in sulphide-containing seawater. Meanwhile, the effect of sulphide concentration on the CE degradation mechanism, and the correlation between microstructure and CE-C behaviour also need to be clarified. In the present study, the CE-C behaviour of various copper alloys for ship propellers were investigated in sulphide-containing 3.5% NaCl so­ lutions. Mass loss measurements, surface morphology observation and electrochemical tests under the CE condition were performed. The synergistic effect between CE and corrosion were emphatically analysed and discussed. This study aims to explore the CE-C degradation mech­ anism of copper alloys and provide guidance.

water were also performed in order to simulate the pure mechanical attack condition. The mean depth of erosion (MDE) was calculated based on the mass loss results using Eq. (1): MDE ¼ △m/(ρ⋅S),

(1)

where △m is the mass loss, ρ is the density (approximately 7.4793 g/ cm3 for the cast and FSPed NABs, 7.4093 g/cm3 for the cast MAB and 8.1610 g/cm3 for the cast MB) and S is the sample area (1.2 cm2 in this study). Electrochemical measurements under both the quiescence and CE conditions were conducted with a Gamry Interface 1000E potentiostat using a conventional three-electrode cell. The electrochemical mea­ surements under the CE condition were carried out by connecting the cavitation rig with the potentiostat, as shown in Fig. 1. The test sample, as the working electrode, was sealed by an epoxy resin with an exposure area of 1 cm2. The counter electrode was a platinum plate and the reference electrode was a saturated calomel electrode (SCE). All the potentials in the present study were recorded versus SCE. The open circuit potential (OCP) was recorded under the quiescence-CE alternate conditions for different copper alloys, and each condition lasted for 20 min in a quiescence-CE cycle. Potentiodynamic polarization was per­ formed from 0.25 V (vs. OCP), and the potential scanning rate was 0.5 mV/s. The results were analysed by the software CView 3.2. Electro­ chemical tests were conducted in triplicate for each copper alloy in each solution. The results were repeatable and represented the average level. The synergy between CE and corrosion can be evaluated by Eq. (2): T ¼ E þ△E þ (C þ △C) ¼ E þ C þ S,

(2)

where T is the cumulative mass loss, E represents the pure erosion mass loss, and C represents the pure corrosion mass loss under the quiescence condition. S represents the mass loss caused by CE-corrosion synergy, and it consists of two parts, i.e. corrosion-enhanced CE (△E) and CEenhanced corrosion (△C) [6,34]. The corrosion mass loss under the CE condition (C0 ) equals the sum of C and △C. C0 and C can be calcu­ lated from the corrosion current densities, which were obtained from the polarization curve results, on the basis of Faraday’s law. The CE-C degradation mechanism can be revealed based on the percentages of the above components to the cumulative mass loss. The surface and cross-sectional morphologies of eroded copper alloys were observed by scanning electron microscopy (SEM, JEOL JSM-6360LA, acceleration voltage 15 kV). 3. Results

2. Experimental

3.1. Microstructure

The materials investigated in this study included cast NAB, friction stir processed (FSPed) NAB, cast MAB and MB. Chemical compositions are shown in Table 1. The FSP process and parameters had been illus­ trated in a previous study by the present authors [19]. The test solutions were aerated 3.5% NaCl solutions with Na2S concentrations (sulphide concentrations) of 20, 50, 100 and 200 ppm. An ultrasonically vibrating device (Fig. 1) was used to perform CE tests according to the ASTM G32-10 [33]. The sample was fixed 0.5 mm directly below the vibrating horn, which was driven at a frequency of 20 kHz with a peak-to-peak amplitude of 60 μm. The sample was fixed 15 mm deep from the liquid level. The test solution was kept at 18–22 � C by cycling cooling water. The CE duration was 5 h. CE tests in distilled

Fig. 2 presents the optical microstructures of the four samples. The cast NAB consists of α, various κ and β0 phases, as presented in Fig. 2a. The κ phases are comprised of intermetallic compounds including globular κII, lamellar κIII, and fine κIV phases. The κII and κIV phases are Fe3Al-based intermetallics, and the κIII phase is a NiAl-based interme­ tallic [35,36]. The FSPed NAB contains inhomogeneous microstructure along the plate thickness, as previously reported [16]. The working surface, which is exposed to the cavitation impact, mainly consists of equiaxed α and β0 phases (Fig. 2b). The α and β0 phases are Cu-rich solid solutions, and small-sized particles, which are rich in Fe, Ni and Al, precipitate inside β’. The cast MAB consists of α matrix, β and dendritic-shaped κ phases (Fig. 2c). The α phase is a Cu-rich solid so­ lution, β is an intermediate based on Cu3Al or Cu2MnAl, and the κ phase is an Fe, Mn-rich intermetallic compound [32,37]. The cast MB contains β-solution matrix, α-solution islands and dendritic-shaped κ phases (Fig. 2d). The κ phase is also an intermetallic compound, rich in Fe and Mn [2].

Table 1 Chemical compositions of investigated copper alloys. Element (wt.%)

Al

Ni

Fe

Mn

Cu

Zn

NAB MAB MB

9.18 7.28 0.86

4.49 2.10 0.22

4.06 3.62 0.89

1.03 12.35 2.17

Bal. Bal. 55.70

Bal.

2

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Fig. 1. Schematic diagram of the cavitation rig for the electrochemical measurements under the CE condition.

Fig. 2. Optical microstructures of (a) cast NAB, (b) FSPed NAB, (c) cast MAB and (d) cast MB.

3.2. Mean depth of erosion results

concentration is less than 20 ppm, the FSPed NAB possesses the best CEC resistance, followed by the cast NAB, MAB and MB. In 3.5% NaCl solution, the MDE of the FSPed NAB is only 42.07% that of the cast MAB and 25.44% that of the cast MB, and the MDE of the cast NAB is only 62.76% that of the cast MAB and 37.94% that of the cast MB. However, the cast MAB exhibits the highest CE-C resistance once the sulphide concentration exceeds 50 ppm. In the 200 ppm sulphide solution, the FSPed NAB is even less CE-C resistant than the cast MB. The MDE of the FSPed NAB reaches 2.49 times that of the cast MAB and 1.22 times that of the cast MB, and the MDE of the cast NAB reaches 4.83 times that of the cast MAB and 2.36 times that of the cast MB. Fourth, the CE damage in corrosive solutions is greater than that in distilled water for the four copper alloys, indicating that a positive synergistic effect exists between

Fig. 3 shows the MDE results of the four copper alloys in different solutions. The following points can be drawn. First, the MDE signifi­ cantly increases with sulphide concentration for the cast and FSPed NABs. The presence of 200 ppm sulphide increases the MDE by a factor of 5.10 for the cast NAB and 3.68 for the FSPed NAB, compared with the results in the absence of sulphide. The FSPed NAB is more CE-C resistant in all solutions than the cast NAB. Second, for the cast MAB and MB, the MDE changes slightly with sulphide concentration. The error bars of MDE overlap with each other in different solutions, revealing that the cast MAB and MB are not as sensitive as the cast and FSPed NABs to the CE-C damage when sulphide is present. Third, when the sulphide 3

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the contrary, CE shifts the OCP to a more positive value when the sul­ phide concentration is higher than 50 ppm. In the presence of 20 ppm, the OCP result of the first quiescence-CE cycle is analogous to that in the absence of sulphide. However, the OCP continues to decrease when CE stops. The second CE condition causes a positive OCP shift, and this phenomenon is analogous to that when the sulphide concentration ex­ ceeds 50 ppm. By comparison, for the cast MAB and MB, CE shifts the OCP in the positive direction in all solutions, as shown in Figs. 4c and 4d. Fig. 5 shows the potentiodynamic polarization curves in different solutions under the quiescence and CE conditions. The current density (icorr) and corrosion potential (Ecorr) results are presented in Tables 2 and 3. CE increases icorr by one order of magnitude when sulphide is absent, but two orders of magnitude when sulphide is present for the cast and FSPed NABs. CE increases icorr by one order of magnitude for the cast MAB in all solutions and for the cast MB when the sulphide concentra­ tion is less than 50 ppm. However, CE causes a very slight increase in icorr of the cast MB in the presence of 100 and 200 ppm sulphide. Apparently, the corrosion rates are much more distinctly accelerated by CE in the sulphide solutions for the cast and FSPed NABs. Fig. 3. Mean depth of erosion results of different copper alloys after CE for 5 h in 3.5% NaCl solutions with different sulphide concentrations.

3.4. Synergistic effect between CE and corrosion

CE and corrosion.

Tables 4 and 5 present the synergistic effect results between CE and corrosion of the four copper alloys based on Eq. (2). According to ASTM G102-89 (2015)ε1 [38], the alloying elements (above 1 mass percent) should be included in the calculation of the corrosion rates. Therefore, the corrosion rates are calculated by Eq. (3):

3.3. Electrochemical results Electrochemical measurements were conducted to investigate the synergistic effect between CE and corrosion of the four copper alloys in different solutions. The OCP results for different solutions under the quiescence-CE alternate conditions are presented in Fig. 4. The results of the cast and FSPed NABs are very similar, as presented in Figs. 4a and b. CE shifts the OCP to a more negative value when sulphide is absent. On

(3)

C (C0 ) ¼ 37.3 � EW � icorr-quiescence (icorr-cavitation),

where EW is the alloy equivalent weight. The units of C (C ) and icorr are mg⋅cm 2⋅h 1 and A⋅cm 2, respectively. EW can be calculated by Eq. (4): 0

Fig. 4. OCP results of different copper alloys in 3.5% NaCl solutions with different sulphide concentrations under the alternate conditions of quiescence and CE: (a) cast NAB, (b) FSPed NAB, (c) cast MAB and (d) cast MB. 4

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Fig. 5. Polarization curves of different copper alloys under both the quiescence and CE conditions in 3.5% NaCl solutions with different sulphide concentrations: (a) cast NAB, (b) FSPed NAB, (c) cast MAB and (d) cast MB. Table 2 Electrochemical parameters obtained from the polarization curves for the cast and FSPed NAB in 3.5% NaCl solutions with different sulphide concentrations. Materials

Conditions

Electrochemical parameters

Sulphide concentrations (ppm)

Cast NAB

Quiescence

icorr (A⋅cm 2)

(7.79 � 1.36) � 10

Cavitation

Ecorr (mV) icorr (A⋅cm 2)

270 � 11 (2.49 � 0.31 ) � 10 5 287 � 3

Ecorr (mV) FSPed NAB

Quiescence

icorr (A⋅cm 2)

Cavitation

Ecorr (mV) icorr (A⋅cm 2)

0

(8.39 � 0.72 ) � 10 6 283 � 25 (3.00 � 1.57) � 10

Ecorr (mV)

295 � 4

6

5

20

50

100

200

(3.63 � 0.87) � 10 6 344 � 11 (2.52 � 0.05) � 10 4 349 � 22

(4.16 � 0.19) � 10 6 949 � 2 (7.01 � 2.15) � 10 4 765 � 12

(2.97 � 1.50) � 10 6 969 � 18 (9.07 � 0.83) � 10 4 794 � 8

(1.93 � 1.17) � 10 6 1000 � 6 (9.47 � 0.72) � 10 4 832 � 9

(3.76 � 0.85) � 10 6 343 � 21 (2.69 � 1.17) � 10 4 339 � 14

(4.75 � 2.25) � 10 6 942 � 16 (8.72 � 0.60) � 10 4 748 � 4

(8.00 � 3.61) � 10 6 940 � 15 (8.33 � 0.30) � 10 4 806 � 1

(8.31 � 1.81) � 10 6 959 � 18 (3.48 � 0.81) � 10 4 846 � 1

Table 3 Electrochemical parameters obtained from the polarization curves for the cast MAB and MB in 3.5% NaCl solutions with different sulphide concentrations. Materials

Conditions

Electrochemical parameters

Cast MAB

Quiescence

icorr (A⋅cm Ecorr (mV) icorr (A⋅cm Ecorr (mV)

2

icorr (A⋅cm Ecorr (mV) icorr (A⋅cm Ecorr (mV)

2

Cavitation Cast MB

Quiescence Cavitation

)

2

) )

2

)

Sulphide concentrations (ppm) 0

20

(4.94 � 2.48) � 10 429 � 25 (8.21 � 1.41) � 10 347 � 12

6

(3.56 � 0.03) � 10 498 � 19 (4.20 � 2.13) � 10 298 � 10

6

5

5

50

(2.21 � 1.07) � 10 477 � 40 (9.16 � 0.89) � 10 359 � 36

6

(4.76 � 0.55) � 10 509 � 10 (4.67 � 1.36) � 10 405 � 38

6

5

5

5

100

(3.62 � 0.80) � 10 947 � 24 (5.27 � 2.27) � 10 375 � 23

6

(2.49 � 0.36) � 10 467 � 35 (2.63 � 0.77) � 10 459 � 27

6

5

5

200

(7.29 � 2.59) � 10 956 � 4 (6.14 � 0.15) � 10 617 � 6

6

(2.13 � 0.92) � 10 541 � 11 (5.18 � 1.97) � 10 497 � 27

6

5

6

(7.81 � 1.49) � 10 993 � 30 (6.10 � 1.42) � 10 647 � 7

6

(2.30 � 0.06) � 10 590 � 3 (3.01 � 0.38) � 10 563 � 28

6

5

6

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Table 4 CE-corrosion synergy results for the cast and FSPed NABs in 3.5% NaCl solutions with different sulphide concentrations. Mass loss rate (mg⋅cm 2⋅h

1

T

E

Materials

Sulphide concentrations

) △E

△C

S

Cast NAB

0 ppm 20 ppm 50 ppm 100 ppm 200 ppm

1.4750 � 1.8042 � 3.3458 � 4.4617 � 8.9917 �

0.1296 0.1750 0.1031 0.5090 0.2239

1.0000 1.0000 1.0000 1.0000 1.0000

� 0.0236 � 0.0236 � 0.0236 � 0.0236 � 0.0236

0.0111 0.0052 0.0059 0.0042 0.0027

� � � � �

0.0019 0.0012 0.0003 0.0021 0.0017

0.4396 0.4460 1.3493 2.1724 6.6455

0.0243 0.3531 0.9906 1.2851 1.3434

0.4639 0.7990 2.3399 3.4575 7.9890

FSPed NAB

0 ppm 20 ppm 50 ppm 100 ppm 200 ppm

0.9889 � 1.6200 � 2.4722 � 3.4167 � 4.6292 �

0.0674 0.2787 0.1347 0.3909 0.4471

0.5028 0.5028 0.5028 0.5028 0.5028

� 0.1271 � 0.1271 � 0.1271 � 0.1271 � 0.1271

0.0119 0.0053 0.0068 0.0114 0.0118

� � � � �

0.0010 0.0012 0.0032 0.0051 0.0026

0.4435 0.7348 0.7298 1.7298 3.6317

0.0307 0.3770 1.2328 1.1727 0.4829

0.4742 1.1119 1.9626 2.9025 4.1146

C

Table 5 CE-corrosion synergy results for the cast MAB and MB in 3.5% NaCl solutions with different sulphide concentrations. Mass loss rate (mg⋅cm

2

⋅h 1)

Materials

Sulphide concentrations

△E

△C

S

Cast MAB

0 ppm 20 ppm 50 ppm 100 ppm 200 ppm

2.3283 2.4250 2.1117 1.6500 1.8433

� 0.3433 � 0.1280 � 0.1262 � 0.1748 � 0.2226

1.6000 1.6000 1.6000 1.6000 1.6000

� 0.0236 � 0.0236 � 0.0236 � 0.0236 � 0.0236

0.0070 0.0031 0.0051 0.0104 0.0111

� � � � �

0.0035 0.0015 0.0011 0.0037 0.0021

0.6116 0.6948 0.4368 0.0373 0.1566

0.1097 0.1270 0.0697 0.0769 0.0756

0.7213 0.8219 0.5066 0.0396 0.2322

Cast MB

0 ppm 20 ppm 50 ppm 100 ppm 200 ppm

4.2417 4.8208 4.7125 4.3792 4.1500

� 0.2526 � 0.5250 � 0.5236 � 0.5203 � 0.2267

4.0792 4.0792 4.0792 4.0792 4.0792

� 0.5510 � 0.5510 � 0.5510 � 0.5510 � 0.5510

0.0061 0.0081 0.0043 0.0036 0.0039

� � � � �

0.0001 0.0009 0.0006 0.0016 0.0001

0.1408 0.7119 0.6384 0.3412 0.1157

0.0656 0.0716 0.0406 0.0052 0.0012

0.2064 0.7835 0.6790 0.3464 0.1169

T

E

C

Fig. 6. Fractions of different components to the cumulative CE mass loss obtained from Tables 4 and 5 for (a) cast NAB, (b) FSPed NAB, (c) cast MAB and (d) cast MB in 3.5% NaCl solutions with different sulphide concentrations. 6

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P EW ¼ [ (nifi⋅Wi 1)] 1,

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(4)

Cracks are found at both the α and β phases in the solutions whether sulphide is present or not (Fig. 10), demonstrating that the sulphide addition in 3.5% NaCl solution has a slight influence on the CE-C behaviour of the cast MAB and MB.

where ni, fi and Wi are the valence, mass fraction and atomic weight of the ith element of the alloy, respectively [38]. In the present study, n is þ2 for Fe, Mn and Zn, þ3 for Al and þ1 for Cu. Fig. 6 presents the fractions of different components to the total CE mass loss obtained from Tables 4 and 5. E/T reaches 67.80% when sulphide is absent and 55.43% in the presence of 20 ppm sulphide for the cast NAB. It also reaches 50.84% for the FSPed NAB in the absence of sulphide. This indicates that the mechanical attack induced by CE is the dominated factor for the CE-C damage. In the other sulphide solutions, S/T is the highest and the CE-corrosion synergy mainly contributes to the remarkable mass loss. S/T increases with sulphide concentration and reaches approximately 90% in the 200 ppm sulphide solution. For the cast MAB and MB, E/T is the highest in all solutions, revealing a me­ chanical attack-dominated CE-C degradation mechanism.

4. Discussion 4.1. Effect of CE on the corrosion behaviour As shown in Figs. 4 and 5, CE has different effects on the OCP and icorr of the four copper alloys. The OCP, Ecorr and icorr are determined by the cathodic and anodic processes [39]. The effect of CE on the Ecorr (or OCP) of materials depends on the following two competing factors [34, 40,41]. First, the corrosion product film, which is formed on the material under the quiescence condition, is destroyed by the cavitation impact. Second, CE accelerates the diffusion process in the solution. If the ma­ terial is covered by a protective corrosion product film in a passivation state under the quiescence condition, a negative corrosion potential shift is induced by CE because of the remarkably accelerated anodic process. If the material undergoes anodic dissolution or the film possesses infe­ rior protectiveness, CE primarily promotes the cathodic process by accelerating the diffusion and mass transfer in the solution (mainly the oxygen diffusion in the absence of sulphide and the HS diffusion in the presence of sulphide), thereby resulting in a positive corrosion potential shift. In either case, the current density increases because of the accel­ erated anodic and cathodic processes by CE. In the absence of sulphide, the main cathodic process of copper and its alloys is the oxygen reduction by Ref. [42,43]:

3.5. Surface and cross-sectional morphologies of the eroded copper alloys The surface and cross-sectional morphologies after CE for 5 h in different solutions are presented in Figs. 7 and 8 for the cast and FSPed NAB. It can be seen that some of the eutectoid microstructure with high hardness still exists on the eroded surface of the cast NAB when the sulphide concentration is less than 20 ppm (Figs. 7a and b). However, the eutectoid microstructure and β0 phase undergo preferential corro­ sion, and they are severely eroded under the CE impact at high sulphide concentration (200 ppm), as presented in Fig. 7c. The FSPed NAB is evenly eroded in the absence of sulphide (Fig. 7d), whereas large cav­ ities appear on the surface in the presence of sulphide (Figs. 7e and f). Compared with the result when sulphide is absent (Fig. 8a), more and much longer cracks are found at the eutectoid microstructure and β0 phases in the sulphide solutions for the cast NAB (Figs. 8b and c). For the FSPed NAB, cavities mainly occur at the soft α phases in the absence of sulphide (Fig. 8d), while long cracks are found at the β’ phases when sulphide is present (Figs. 8e and f). The coalescence of these cracks gives rise to great mass loss in the sulphide solutions for these two NABs. Figs. 9 and 10 show the surface and cross-sectional morphologies of the cast MAB and MB after CE for 5 h in different solutions. For the cast MAB and MB, the β phase suffers severe cleavage fracture, and the α phase undergoes plastic deformation under the CE impact (Fig. 9).

(5)

O2 þ 2H2O þ 4e → 4OH .

The anodic process is copper dissolution and formation of copper oxide film by: (6)

Cu þ 2Cl- → CuCl-2 þ e . 2CuCl-2

-

þ 2OH → Cu2O þ H2O þ 4Cl .

(7)

In the sulphide solutions, hydrogen evolution occurs in the cathodic process by Ref. [44,45]: 2HS- þ 2e → H2 þ 2S2-.

(8)

Fig. 7. Surface morphologies of the cast and FSPed NAB after CE in 3.5% NaCl solutions with different sulphide concentrations for 5 h. Cast NAB: (a) 0 ppm, (b) 20 ppm, (c) 200 ppm; FSPed NAB: (d) 0 ppm, (e) 20 ppm, (f) 200 ppm. 7

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Fig. 8. Cross-sectional morphologies of the cast and FSPed NAB after CE in 3.5% NaCl solutions with different sulphide concentrations for 5 h. Cast NAB: (a) 0 ppm, (b) 20 ppm, (c) 200 ppm; FSPed NAB: (d) 0 ppm, (e) 20 ppm, (f) 200 ppm.

Fig. 9. Surface morphologies of the cast MAB and MB after CE in 3.5% NaCl solutions with different sulphide concentrations for 5 h. Cast MAB: (a) 0 ppm, (b) 20 ppm, (c) 200 ppm; Cast MB: (d) 0 ppm, (e) 20 ppm, (f) 200 ppm.

Fig. 10. Cross-sectional morphologies of the cast MAB and MB after CE in 3.5% NaCl solutions with different sulphide concentrations for 5 h. Cast MAB: (a) 0 ppm, (b) 20 ppm, (c) 200 ppm; Cast MB: (d) 0 ppm, (e) 20 ppm, (f) 200 ppm.

The anodic process is copper dissolution and formation of copper sulphide film by: Cu þ HS → Cu(HS)ads þ e ,

(9)

Cu(HS)-2,

(10)

Cu(HS)ads þ HS →

Cu þ Cu(HS)-2→Cu2S þ H2S þ e .

(11)

In chloride-containing solutions, a protective film with Cu2O (Eqs. (6) and (7)) in the outer layer and Al2O3 in the inner layer is quickly 8

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formed on the NAB surface and enables high corrosion resistance for NAB [9,46]. CE destroys the film and remarkably expedites the anodic process, thereby giving rise to a decreased OCP. Once CE is terminated, the film can be rebuilt rapidly because of the high affinity between Al and O, and the OCP increases accordingly. Similar results can be found in the research of Al-Hashem et al. [47]. Notably, Al2O3 is unstable and absent inside the film in the alkaline sulphide solutions [48]. Moreover, the copper sulphide film, which is formed under the quiescence condi­ tion (Eqs. (9)–(11)), possesses inferior protectiveness [31]. Thus, CE primarily accelerates the HS diffusion of the cathodic process (Eq. (8)) and increases the OCP when the sulphide concentration exceeds 50 ppm. The OCP result in the 20 ppm sulphide solution is more complex. As shown in Figs. 5a and b, the polarization curve in the 20 ppm sulphide solution is very close to that in the absence of sulphide for both the cast and FSPed NABs, evidencing similarity of the electrochemical processes in these two solutions. In the presence of 20 ppm sulphide, the film consists of oxides and a small percentage of sulphides. It has a loose structure and reduced protectiveness [30,31]. It is still protective for the NAB substrate, considering that the low sulphide concentration only results in a small percentage of sulphides inside the film. CE decreases the OCP because of the film destruction. Once CE is terminated, the fresh material surface is exposed to the sulphide solution. The OCP continues to decrease due to the HS adsorption on the NAB surface [44,45] and the slow-down of mass transfer. Compared with the film which is formed under the first quiescence condition, the rebuilt film is considered to possess inferior protectiveness, since it is formed at a more negative potential (approximately 0.6 V) and probably contains a large per­ centage of sulphides. Thus, the next CE condition shifts the OCP in the positive direction as a result of the more distinctly accelerated cathodic process. It is also noted that the icorr increase induced by CE in the sulphide solutions is larger than that in 3.5% NaCl solution. This phenomenon can be explained as follows. In an aqueous 3.5% NaCl solution, the dissolved oxygen content is very limited. The maximum icorr under the CE condition is equal to the limiting current density (iL), supposing that the oxygen content at the solution/metal surface is equal to that in the bulk solution and the oxygen is consumed very rapidly. Therefore, the icorr values of the four copper alloys are very close under the CE condi­ tion. HS is not transport limited under the CE condition, and the HS content in the sulphide solution is much higher than the dissolved ox­ ygen content in 3.5% NaCl solution. Moreover, sulphide worsens the SPC damage of the cast and FSPed NABs, and thereby further increases the icorr. Therefore, the icorr value in the presence of sulphide is much larger than that in 3.5% NaCl solution under the CE condition. FSP does not change the chemical composition of the cast NAB. Therefore, the electrochemical processes, the OCP and icorr results of the cast and FSPed NABs are very similar. For the cast MAB and MB, the OCP results in Figs. 4c and d indicate that the films formed under the quiescence condition exhibit low corrosion resistance, or intact films cannot form rapidly in a short period (i.e. 20 min). In 3.5% NaCl solution, the large dendritic-shaped κ phases of the cast MAB, which are rich in Fe and Mn, suffer preferentially active dissolution [49]. Moreover, the mismatch between Fe and Cu oxides would also reduce the film compactness and protectiveness [32]. The cast MB undergoes active dissolution by means of dezincification corrosion [50,51]. It is found that the β matrix of the cast MB is pref­ erentially corroded due to its high Zn content (41.4 � 0.8 wt%) after immersion in 3.5% NaCl solution for 7 days, and its Zn content decreases to (33.8 � 6.1) wt.% with the corrosion products wiped off, based on the energy-dispersive spectroscopy (EDS) analysis result. Therefore, the oxygen diffusion in the cathodic process (Eq. (5)) determines the corrosion rate of these two copper alloys. CE accelerates the diffusion of the dissolved oxygen, and thereby causes a positive potential shift. Trethewey et al. [52] also reported that CE induced positive potential shift for MAB in seawater. However, the Mn and Zn elements are beneficial for copper alloys to resist corrosion when sulphide is present

[53,54]. The formation of ZnS film is reported to act as an insulating layer and thereby decreases the dissolution rate of copper alloys [54]. Additionally, no deteriorated SPC damage is found on either the cast MAB or MB in sulphide solutions. Therefore, the CE induced-icorr in­ crease of the cast MAB and MB is smaller than that of the two NABs. 4.2. CE-corrosion synergy and degradation mechanism The cast NAB exhibits mechanical attack-dominated degradation mechanism when the sulphide concentration is lower than 20 ppm, whereas the CE-corrosion synergy dominates the CE damage in the other sulphide solutions. For the FSPed NAB, it is the mechanical attack and the CE-corrosion synergy that largely contributes to the CE-C damage in the absence and presence of sulphide, respectively. △C/T of these two NABs in the sulphide solutions is much higher than that in 3.5% NaCl solution (Fig. 6), because of the remarkable icorr increase under the CE condition when sulphide is present. Corrosion weakens the phase boundary adhesion, increases the roughness and decreases the me­ chanical properties of the metal surface. For the cast NAB, the lamellar α and κIII (NiAl-based) phases are alternatively arranged inside the eutectoid microstructure. There is also a high density of small inter­ metallic particles (Fe3Al or NiAl-based) precipitated inside the β0 phase of the FSPed NAB [20]. The chemical composition and structure het­ erogeneity of different phases lead to severe galvanic corrosion. In 3.5% NaCl solution, the SPC occurs at the lamellar α phase of the cast NAB [20,42,55,56] and the β’ phase of the FSPed NAB [20], since the Al-rich intermetallic phases are covered by an Al2O3 film and act as cathodes. However, Al2O3 is not stable in the strongly alkaline sulphide solutions [48]. Therefore, the Al-rich intermetallic phases in the cast and FSPed NABs act as the anodes and undergo SPC. The SPC damage is aggravated by sulphide for the two NABs [57], as also indicated in Figs. 7 and 8, and thereby results in decreased mechanical properties and high △E. In the 200 ppm sulphide solution, △E/T reaches 73.91% for the cast NAB and 78.45% for the FSPed NAB. As a result, the CE-C resistance of the two NABs decreases with sulphide concentration. FSP reduces the content of the lamellar eutectoid microstructure, homogenizes the cast micro­ structure and consequently decreases the galvanic corrosion tendency. It also remarkably improves the tensile strength and elongation of the cast NAB [16], and thereby decreases the mechanical damage under CE. Therefore, the FSPed NAB exhibits higher CE-C resistance than the cast NAB in all tested solutions. For the cast MAB and MB, the mechanical attack caused by the cavitation impact is mainly responsible for the CE-C damage in all so­ lutions. In the sulphide solutions, the icorr is slightly increased by CE, and the △C value is much smaller than that of the cast and FSPed NABs. Meanwhile, the low △E value implies that the corrosion effect does not give rise to distinct CE mass loss, especially in the solutions with high sulphide concentrations (Tables 4 and 5). As indicated in Figs. 9 and 10, the presence of sulphide has a slight effect on the CE damage mode of these two copper alloys. For the cast MAB, as presented in Fig. 6c, △E/T is much larger than △C/T in the 20 and 50 ppm sulphide solutions, indicating that the synergistic effect between corrosion and CE is largely attributed to the corrosion-enhanced CE (△E). Whether the sulphide is present or not, preferential corrosion occurs at the β phase with a continuous structure and the large Fe, Mn-rich κ phases of the cast MAB [32], and it consequently causes high △E under the cavitation impact. However, in the solutions with higher sulphide concentrations (100 and 200 ppm), corrosion products, which are formed on the rough eroded surface of the cast MAB, can act as a barrier to buffer the cavitation attack, thereby resulting in relatively low S/T and probably negative △E/T ( 2.26% in the 100 ppm sulphide solution) [32]. For the cast MB, the E/T value is very large (higher than 80%), indicating that the inferior CE resistance is mainly attributed to the relatively low mechanical property. The synergistic effect between CE and corrosion is relatively high in the solutions with low sulphide concentrations (20 and 50 ppm), and it is mainly caused by the corrosion enhanced CE (△E). However, it 9

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is negligible for the cumulative CE damage when the sulphide concen­ tration is higher than 100 ppm (Fig. 6d). Under the cavitation impact, cracks initiate at the α/κ and β/κ phase boundaries due to the crystal structure difference, resulting in the detachment of the large κ phases [10]. The β matrix of the cast MB suffers a severe cleavage fracture [2], resulting in large grooves and cavities (Figs. 9d–f and Fig. 10d–f). In contrast, the α matrix of the cast MAB undergoes plastic deformation under CE (Figs. 9a–c). It was also reported that the α phase in the cast MB, which had a much higher Zn and lower Al contents than that in the cast MAB, possessed a lower work-hardening capacity under the cavi­ tation impact. The α phase was severely attacked and even removed in a brittle manner [2], as also evidenced in Figs. 10d–f. Therefore, the cast MAB is more CE-C resistant than the cast MB. From the above, special care should be taken to prevent the CE-C damage of NAB ship propellers in sulphide-polluted seawater, since the sulphide remarkably aggravates the SPC damage and consequently reduces the mechanical property to resist the CE damage. By compari­ son, the cast MAB exhibits the best CE-C resistance in the sulphide so­ lutions when the sulphide concentration exceeds 50 ppm, due to its high corrosion resistance and good mechanical property. Therefore, the MAB propeller is recommended for use in sulphide-polluted seawater.

Wang: Formal analysis. Y.X. Qiao: Resources. Acknowledgements This research was financially supported by National Natural Science Foundation of China (No. 51601058 and 51879089), Natural Science Foundation of Jiangsu Province (BK20191161), Changzhou Sci & Tech Program (Grant No. CJ 20180045) and Fundamental Research Funds for the Central Universities (No. 2018B48614). References [1] A.H. Tuthill, Guidelines for the use of copper alloys in seawater, Mater. Perform. 26 (1987) 12–22. [2] J. Huci� nska, M. Głowack, Cavitation erosion of copper and copper-based alloys, Metall. Mater. Trans. 32 (2001) 1325–1333. [3] A. Karim, J.L. Martin, Cavitation erosion of materials, Int. Met. Rev. 31 (1986) 1–26. [4] Z.P. Shi, Z.B. Wang, J.Q. Wang, Y.X. Qiao, H.N. Chen, T.Y. Xiong, Y.G. Zheng, Effect of Ni interlayer on cavitation erosion resistance of NiTi cladding by tungsten inert gas (TIG) surfacing process, Acta Metall. Sin. (2019), https://doi.org/ 10.1007/s40195-019-00947-7. [5] L.M. Zhang, A.L. Ma, H. Yu, A.J. Umoh, Y.G. Zheng, Correlation of microstructure with cavitation erosion behaviour of a nickel-aluminum bronze in simulated seawater, Tribol. Int. 136 (2019) 250–258. [6] R.J.K. Wood, Marine wear and tribocorrosion, Wear 376-377 (2017) 893–910. [7] X.Y. Li, Y.G. Yan, L. Ma, Z.M. Xu, J.G. Li, Cavitation erosion and corrosion behavior of copper–manganese–aluminum alloy weldment, Mater. Sci. Eng. A 382 (2004) 82–89. [8] C.H. Tang, F.T. Cheng, H.C. Man, Effect of laser surface melting on the corrosion and cavitation erosion behaviors of a manganese-nickel-aluminium bronze, Mater. Sci. Eng. A 373 (2004) 195–203. [9] J.A. Wharton, R.C. Barik, G. Kear, R.J.K. Wood, K.R. Stokes, F.C. Walsh, The corrosion of nickel-aluminium bronze in seawater, Corrosion Sci. 47 (2005) 3336–3367. [10] H. Yu, Y.G. Zheng, Z.M. Yao, Cavitation erosion corrosion behaviour of manganesenickel-aluminum bronze in comparison with manganese-brass, J. Mater. Sci. Technol. 25 (2009) 758–766. [11] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Temple–Smith, C. J. Dawes, Friction Stir Butt Welding, 1991. GB Patent Application, No. 91259788. [12] R.S. Mishra, Z.Y. Ma, Friction stir welding and processing, Mater. Sci. Eng. R (2005) 1–78. [13] K. Oh-Ishi, T.R. McNelley, Microstructural modification of as-cast NiAl bronze by friction stir processing, Metall. Mater. Trans. 35A (2004) 2951–2961. [14] K. Oh-Ishi, A.P. Zhilyaev, T.R. McNelley, A microtexture investigation of recrystallization during friction stir processing of as-cast NiAl bronze, Metall. Mater. Trans. 37A (2006) 2239–2251. [15] D.R. Ni, B.L. Xiao, Z.Y. Ma, Y.X. Qiao, Y.G. Zheng, Corrosion properties of frictionstir processed cast NiAl bronze, Corrosion Sci. 52 (2010) 1610–1617. [16] D.R. Ni, P. Xue, D. Wang, B.L. Xiao, Z.Y. Ma, Inhomogeneous microstructure and mechanical properties of friction stir processed NiAl bronze, Mater. Sci. Eng. A 524 (2009) 119–128. [17] Y.T. Lv, Y. Ding, Y.F. Han, L.C. Zhang, L.Q. Wang, W.J. Lu, Strengthening mechanism of friction stir processed and post heat treated NiAl bronze alloy: effect of rotation rates, Mater. Sci. Eng. A 685 (2017) 439–446. [18] S. Thapliyal, D.K. Dwivedi, Study of the effect of friction stir processing of the sliding wear behavior of cast NiAl bronze: a statistical analysis, Tribol. Int. 97 (2016) 124–135. [19] Q.N. Song, Y.G. Zheng, S.L. Jiang, D.R. Ni, Z.Y. Ma, Comparison of corrosion and cavitation erosion behaviours between the as-cast and friction-stir-processed nickel aluminum bronze, Corrosion 69 (2013) 1111–1121. [20] Q.N. Song, Y.G. Zheng, D.R. Ni, Z.Y. Ma, Studies of the nobility of phases using scanning Kelvin probe microscopy and its relationship to corrosion behaviour of Ni–Al bronze in chloride media, Corrosion Sci. 92 (2015) 95–103. � [21] G. Sekularac, I. Milo�sev, Corrosion of aluminium alloy AlSi7Mg0.3 in artificial sea water with added sodium sulphide, Corrosion Sci. 144 (2018) 54–73. [22] S.J. Yuan, S.O. Pehkonen, Surface characterization and corrosion behavior of 70/ 30 Cu–Ni alloy in pristine and sulfide-containing simulated seawater, Corrosion Sci. 49 (2007) 1276–1304. [23] J.N. Al-Hajji, M.R. Reda, The corrosion of copper-nickel alloys in sulfide-polluted seawater: the effect of sulfide concentration, Corrosion Sci. 34 (1993) 163–177. [24] L.E. Eiselstein, B.C. Syrett, S.S. Wing, R.D. Caligiuri, The accelerated corrosion of Cu-Ni alloys in sulphide-polluted seawater: mechanism No. 2, Corrosion Sci. 23 (1983) 223–239. [25] E.A. Ashour, L.A. Khorshed, G.I. Youssef, H.M. Zakria, T.A. Khalifa, Electrochemical and stress corrosion cracking behaviour of alpha-Al bronze in sulphide-polluted salt water: effect of environmentally-friendly additives, Mater. Sci. Appl. 5 (2014) 10–19. [26] S.M. Sayed, E.A. Ashour, G.I. Youssef, Effect of sulfide ions on the corrosion behaviour of Al–brass and Cu10Ni alloys in salt water, Mater. Chem. Phys. 78 (2003) 825–834.

5. Conclusions 1. The CE-C mass loss of the cast and FSPed NABs remarkably increases as the sulphide concentration increases in 3.5% NaCl solution. However, the CE-C behaviours of the cast MAB and MB are relatively insensitive to the sulphide addition. When the sulphide concentra­ tion is less than 20 ppm, the CE-C resistance order is: FSPed NAB > cast NAB > cast MAB > cast MB. However, the CE-C resistance order is: cast MAB > cast MB > FSPed NAB > cast NAB in the 200 ppm sulphide solution. 2. In 3.5% NaCl solution, CE causes a negative potential shift of the cast and FSPed NABs because of the film destruction, and increases the current density by approximately one order of magnitude. Mechan­ ical attack is the dominated factor for the CE-C damage. In the sul­ phide solutions, CE increases the current density by two orders of magnitude, and causes a positive potential shift due to the acceler­ ated the HS diffusion when the sulphide concentration exceeds 50 ppm. The severe SPC at the eutectoid α þ κIII of the cast NAB and the β0 phases of the FSPed NAB results in long cracks and great CE mass loss. The synergistic effect between CE and corrosion contributes largely to the CE damage for the cast NAB when the sulphide con­ centration exceeds 50 ppm and for the FSPed NAB in all the sulphide solutions. 3. In all solutions, CE causes a positive potential shift and increases the current density by less than one order of magnitude for the cast MAB and MB. Both the cast MAB and MB exhibit a mechanical attackdominated CE-C degradation mechanism. 4. In sulphide-polluted seawater, special care should be taken for the NAB ship propellers to prevent the CE-C damage, and a MAB pro­ peller is recommended for use. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Q.N. Song: Conceptualization, Methodology, Formal analysis, Writing - original draft. Y. Tong: Validation, Formal analysis, Investi­ gation. N. Xu: Formal analysis, Writing - review & editing. S.Y. Sun: Investigation. H.L. Li: Investigation. Y.F. Bao: Funding acquisition, Project administration. Y.F. Jiang: Writing - review & editing. Z.B. 10

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