10 Cu-Ni alloy coupled to Ti6Al4V alloy

Journal Pre-proof Effect of hydrostatic pressure on the galvanic corrosion of 90/10 Cu-Ni alloy coupled to Ti6Al4V alloy Shengbo Hu, Rui Liu, Li Liu, Yu Cui, Emeka E. Oguzie, Fuhui Wang

PII:

S0010-938X(18)32179-6

DOI:

https://doi.org/10.1016/j.corsci.2019.108242

Reference:

CS 108242

To appear in:

Corrosion Science

Received Date:

25 November 2018

Revised Date:

13 September 2019

Accepted Date:

23 September 2019

Please cite this article as: Hu S, Liu R, Liu L, Cui Y, Oguzie EE, Wang F, Effect of hydrostatic pressure on the galvanic corrosion of 90/10 Cu-Ni alloy coupled to Ti6Al4V alloy, Corrosion Science (2019), doi: https://doi.org/10.1016/j.corsci.2019.108242

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Effect of hydrostatic pressure on the galvanic corrosion of 90/10 Cu-Ni alloy coupled to Ti6Al4V alloy

Shengbo Hu1,2, Rui Liu1,2, Li Liu1,3*, Yu Cui1, Emeka E. Oguzie4, and Fuhui Wang1,3

1

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

2

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Academy of Sciences, Wencui Road 62, Shenyang 110016, China.

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

Wencui Road 62, Shenyang 110016, China.

Key Laboratory for Anisotropy and Texture of Materials (MoE), School of Materials Science

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3

Shenyang 110819, Liaoning, China.

Electrochemistry and Materials Science Research Laboratory, Department of Chemistry,

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4

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and Engineering, Northeastern University, NO.3-11, Wenhua Road, Heping District,

*

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Federal University of Technology Owerri, PMB 1526, Owerri. Nigeria.

Corresponding author: Fax: +86-24-2392-5323; Tel: +86-24-8108-3918

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E-mail: [email protected]

Highlights 

The galvanic corrosion of 90/10 Cu-Ni alloy coupled to Ti6Al4V under high pressure was first investigated.



High pressure sustains a promoting effect on both the galvanic current and natural corrosion of 90/10 Cu-Ni during coupling, which obviously increases the galvanic corrosion rate of the alloy.

Abstract The corrosion of 90/10 Cu-Ni coupled to Ti6Al4V in 3.5% NaCl solution at different hydrostatic pressures was investigated by means of electrochemical methods, weight loss experiment, SEM/EDS, finite element analysis, etc. The 90/10 Cu-Ni was always found to be the anode part in the galvanic cell at all pressures studied. The galvanic current density and

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natural corrosion current density of 90/10 Cu-Ni during coupling to Ti6Al4V were both increased with increasing hydrostatic pressure, which resulted in an obvious increase of the

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total corrosion rate of 90/10 Cu-Ni.

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Keywords: A. Alloy; B. Weight loss; B. SEM; B. EIS;

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1. Introduction

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For metallic materials constituting a galvanic couple, the one with more negative corrosion potential will act as the anode and undergo increased corrosion; while the one with

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more positive corrosion potential hence becomes the cathode and manifest a decreased corrosion rate in the test media compared to corrosion rates in same media without galvanic

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coupling [1]. The extent of galvanic corrosion between two or more coupled metals depends on many factors such as the effective area ratio of the anodic vs. cathodic members, solution conductivity, temperature, the magnitude of the potential between dissimilar alloys [2]. Some specific deep-sea submergence vehicles like underwater robots and submerged oil production system, will often require electrical connection of different kind of alloys in order

to meet the mechanical and electrical demand. Such systems are inevitably prone to galvanic corrosion problems [3, 4]. Hydrostatic pressure which is one of the important characteristics of deep sea environment has been found to influence the galvanic corrosion between some alloys [5-7]. Xing et al. [6] found that high hydrostatic pressure increased the galvanic corrosion rate of Cr-Ni low alloy steel coupled to 90/10 copper-nickel. Sun et al. [7] found that the discharge efficiency of Al-Zn-In-Mg-Ti sacrificial anode dropped dramatically at

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high hydrostatic pressure, due to increased loss in weight and decreased discharge capacity. Despite such observations, the challenges of the galvanic corrosion in deep sea environment are still not attracting sufficient interests, notwithstanding the huge influx of deep sea

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investments and assets.

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Copper and titanium alloys are materials of choice for deep sea structures. The former is used preferably because of its good mechanical workability, resistance to chloride erosion,

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high thermal conductivity and useful resistance to fouling [8, 9], whereas the latter is mainly

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used due to its excellent tensile strength and fatigue strength, adequate ductility, acceptable fracture toughness and overall good resistance to corrosion [10]. When these two metals are

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used in fabricating the same equipment, the galvanic couple created may contribute to corrosion damage. Interestingly, some studies have focused on the galvanic corrosion

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behavior of Ti/Cu couples in both NaCl solution and sea water. Wang [11] found that naval brass can anodically polarize titanium in 6% NaCl solution, while copper-nickel alloy can polarize titanium both anodically and cathodically, depending upon the testing temperature and pH. Cheng [12] studied the corrosion of aluminum bronze coupled to titanium in artificial sea water and found that the corrosion rate of aluminum bronze increased with

increasing titanium/aluminum bronze area ratio. Unfortunately, not much has been done to explore the effect of hydrostatic pressure on the galvanic corrosion of Cu/Ti alloys, which is possible in deep-sea environment. The objective of this work is to study the galvanic corrosion behavior of 90/10 Cu-Ni alloy coupled to Ti6Al4V under different hydrostatic pressure conditions, using a combination of polarization tests, EIS, SEM/EDS, galvanic corrosion tests and the finite

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element analysis techniques.

2. Materials and experiments

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2.1. Materials and specimens

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The test metal specimens were made from Ti6Al4V alloy with a chemical composition (wt.%) of Al 6.03, V 4.08, Fe 0.15, C 0.013, Ti balance and 90/10 Cu-Ni alloy with a

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chemical composition (wt.%) of Ni 10.3, Fe 1.51, Mn 0.75, Zn 0.024, S 0.004, Cu balance.

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The specimens with a cylinder shape (ϕ12 mm × 5 mm) were cut from the materials to make electrodes. After a copper wire connected to the specimen by soldering, electrodes were

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mounted in epoxy resin with one of the subface exposed. Meanwhile, to make the specimens for weight loss experiments, bricks with dimension 20 mm × 10 mm × 6 mm were obtained

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and processed as appropriate. The surface areas of the 90/10 Cu-Ni and Ti6Al4V specimens were made to be the same (for electrochemical experiments, the tested area was 1.13 cm2; for weight loss experiments, the tested area was 7.0 cm2) in order to avoid the effects of cathode/anode area ratio in the galvanic study.

2.2. Galvanic corrosion tests The galvanic corrosion tests were carried out in a high pressure vessel as shown in Fig. 1. The high hydrostatic pressure was attained by injecting the salt water into the pressure vessel via the pressurization equipment (2). The reference electrode (T310 P3000-Re) used in the pressure vessel was made by Corr-instruments company and can withstand pressure up to 25 MPa. The electrolyte used in the vessel was neutral 3.5% NaCl (wt. %) solution, which was

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prepared using analytical-grade reagent and deionized water. Experiments were run at hydrostatic pressures of 3.5 MPa and 0.1 MPa for comparison and the temperature was kept at

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2.2.1. Zero-resistance ammeter measurements

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25 ± 1 oC.

The galvanic corrosion tests were designed according to ASTM G71-81(2014) and

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involved two different kinds of measurements. Zero-resistance ammeter (ZRA) tests were

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carried out using an Autolab PGSTAT302 electrochemical measurement system to monitor the galvanic current and potential. Both 90/10 Cu-Ni electrode and Ti6Al4V electrode were used.

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Before the tests, the electrodes were wet ground to 2000 grit, cleaned with alcohol and dried in cold air. Considering the damage of the passive film on Ti6Al4V by the preparing process

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of electrode samples, the ZRA tests began after the repairing of the passive film finished (acquiring a relatively stable OCP) in the tested solution at different pressures. The current flowing between the two electrodes as well as the galvanic potential were measured with respect to the Ag/AgCl electrode (0.1 mol/L KCl ). The 90/10 Cu-Ni electrode was connected as the working electrode (WE) and Ti6Al4V electrode was grounded. As the potentiostat

measured the current coming from the WE terminal, current values were positive when electrons flowed from the 90/10 Cu-Ni to the WE terminal indicating that 90/10 Cu-Ni alloy was the anode of the galvanic couple. However, current values were negative when electrons flowed in the opposite direction, that is, Ti6Al4V alloy was the anode of the galvanic couple. The galvanic current and potential were continuously monitored for 168 h.

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2.2.2. Galvanic weight loss measurements

Galvanic weight loss measurements were specially designed as follows: specimens were wet ground to a 2000 grit-finish, cleaned with acetone, washed with deionized water, dried in

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warm air and immediately stored in a desiccator; the 90/10 Cu-Ni specimens were weighed by

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means of an analytical balance (Sartorius CF225D) with a precision of 0.0001 g for the original weight; due to the unstable nature of the freshly-prepared Ti6Al4V, the specimens

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were first immersed in the high pressure oven for 40 h to obtain a relative steady OCP prior to

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the weight loss experiments; the specimens of the two materials were subsequently connected electrically by a specially made brass support and sealed with silicone rubber, as shown in Fig.

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2; the sealed specimens were stored in a desiccator for 24 h to wait for the silicone rubber to solidify; after immersing for 168 h in the high pressure oven under different conditions, the

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silicone rubber was wiped off and cleaned with paint remover (wt. %: dichloromethane 50, alcohol 20, acetone 16, formic acid 8, liquid paraffin 5, ethanol amine 0.97, cupferron 0.05); the corrosion products on the specimens of 90/10 Cu-Ni then were removed by ultrasonic cleaning in 6 mol/L hydrochloric acid solution; afterwards, the 90/10 Cu-Ni specimens were washed with deionized water, dried in air and weighed again to obtain the final weight. Six

different specimens were used in total for each weight loss data to ensure the results are reproducible and reliable.

2.3. Electrochemical measurements Before the electrochemical tests, the electrodes were ground to 2000 grit, cleaned with ethanol and dried in cold air. All electrochemical measurements were carried out using the

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same Autolab PGSTAT302 electrochemical measurement system mentioned earlier, in a conventional three-electrode cell with a large platinum plate as the counter electrode and the Ag/AgCl (0.1 mol/L KCl ) electrode as reference electrode.

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The OCP of the two alloys in the test solution were continuously monitored for 96 h. After

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the OCPs of two alloys reach relatively stable, potentiodynamic polarization experiments for the two alloys under different pressures were carried out by sweeping the potential from

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corrosion potential to anodic side and cathodic side at a scan rate of 0.5 mV s-1 respectively.

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To study the electrochemical dynamic conditions of the anode in a coupling cell, common electrochemical methods have been used [13-15]. Ai et al. [13] used the potentiodynamic

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polarization method to study the adsorption behavior and corrosion inhibition mechanism of anionic inhibitor on galvanic electrode (N80 steel/ S31803 stainless steel). As the cathode side

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(S31803 stainless steel) has a higher corrosion-resistance comparing with the anode side (N80 steel), Ai et al thought that the polarization curve for the galvanic couple mainly reflected the electrochemical corrosion character of the anode side and utilized the polarization result to compare the corrosion inhibition efficiency of the studied inhibitor on the anode alone with that in a couple. Yin et al. [14] carried out the EIS tests only on the surface of the anode in a

galvanic couple and utilized the result to discuss the influence of temperature on the corrosion behavior of the anode material during coupling. Du et al. [15] carried out the EIS tests as well as electrochemical noise tests on the Cu/Ti galvanic couple and thought that the experimental results mainly reflected the corrosion character of the anode part (Cu) as the cathode part (Ti) was in good passive state, which was also the case of Ai [13]. However, no matter how small the current from the cathode side accounts for the total

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current during electrochemical tests, it should not be ignored when utilizing the signal (potential-current) to analyze the corrosion behavior of the anode in a galvanic cell. During a potentiodynamic polarization test, the anode and the cathode in a galvanic pair can be taken as

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two pure resistances in parallel for a simplified model. Under this consideration, the

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potential-current relation of the couple can be expressed as:

Itotal = I(anode)c + I(cathode)c = ftotal(V)

(1)

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where Itotal means the total current density detected from the couple; I(anode)c and I(cathode)c are

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the induced current density coming from the anode side and cathode side in the galvanic couple respectively. When the state of cathode is in high stability (such as good passivation),

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the induced current density from cathode alone or in the galvanic couple can be taken as unchanged under the same applied potential during the same polarization process. Hence,

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I(cathode)c also can be expressed as: I(cathode)c = I(cathode)self = f(anode)self (V)

(2)

where I(cathode)self means the current density induced from cathode alone under the applied potential. Combining equation (1) and (2), the (current-potential) relationship of the anode in a galvanic couple during polarization can be deduced as:

I(anode)c = ftotal(V) - f(anode)self (V)

(3)

Therefore, to study the electrochemical dynamic features of 90/10 Cu-Ni during coupling to Ti6Al4V at different pressures, the potentiodynamic polarization tests were carried out by sweeping the same potential range of 90/10 Cu-Ni / Ti6Al4V couple and Ti6Al4V electrode alone at the same scan rate of 0.5 mV s-1 respectively; the polarization curves of 90/10 Cu-Ni coupled to Ti6Al4V were then obtained by the difference of the two polarization curves.

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Impedance measurements were carried out with a frequency range from 100 kHz to 10 mHz using a 5 mV amplitude sinusoidal voltage at OCP. The EIS data were analyzed by using the commercial software ZsimpWin.

2.4. Corrosion morphology observation

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All the electrochemical experiments were repeated over three times.

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Morphologies of the corrosion products on specimens formed in the galvanic corrosion

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test under different pressures were examined by scanning electron microscopy. The 2D size (Diameter and Depth) of a number of random selected corrosion pits on the

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surface of 90/10 Cu-Ni generated after 5 h galvanic corrosion at different hydrostatic pressures in 3.5% NaCl solution were measured using a confocal scanning laser microscope

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(OLYMPUS OLS3100).

2.5. Finite element analysis In this work, the localized electric potential and the dissolution current density around pits formed at the 90/10 Cu-Ni alloy surface under different pressures were calculated using the COMSOL multiphysics software. The physical model was built according to the measured

sizes of pits under different hydrostatic pressures. At the bottom boundary, the pitting area on the alloy was considered as the anode area, and the counterpart of the alloy surface was the cathode area. The ratio of the anode/cathode area was determined as follows: For each given area with dimensions 10 mm × 10 mm, the number of pits and their 2D sizes (depth and diameter) were measured; afterwards, the ratio of the total area of the pits to the total area was calculated. The anode area in the model then was determined by the statistical mean value of

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the measured pits and the model size was determined by the calculated ratio. At the top boundary, the electrolyte current density was set as the galvanic current density at time t = 1 h, which flowed to the Ti6Al4V through the electrolyte. The side boundaries were set as periodic

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boundaries.

3.1. OCP and Polarization tests

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3. Results

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The OCP variation with time for the two alloys in 3.5% NaCl solution is shown in Fig. 3. The OCP of 90/10 Cu-Ni alloy maintained relatively stable values with time. Conversely, the

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OCP of Ti6Al4V initiates at a very cathodic value of -532.3 mV (vs. Ag/AgCl) and then quickly moved to the anodic direction, became more anodic than that of the 90/10 Cu-Ni alloy

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after 5.8 h and finally achieved relatively stable values after 40 h. The trend of OCP evolution implies that the 90/10 Cu-Ni alloy was the anode part and had an accelerated corrosion rate when coupled with Ti6Al4V in long periods. The OCP trend for Ti6Al4V can be attributed to re-growth or repair of the air-formed oxide film that was extensively damaged during surface preparation [4].

Fig. 4 shows the polarization curves obtained for the constituent alloys of the galvanic couple when the OCPs of the two alloys reached relatively stable at different pressures. Polarization curves of the individual constituent alloys are overlaid on each other to estimate the corrosion behavior of the couple from that of individual constituent alloys using the mixed potential theory. The electric potential, Ecouple, and the current density, Icouple, of the galvanic couple are estimated from the intersection of the polarization curves of the individual

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constituent alloys [16, 17]. Additionally, Table 1 gives the typical galvanic corrosion parameters obtained from the polarization curves. For the 90/10 Cu-Ni alloy, an increase in the hydrostatic pressure decreased the corrosion potential from -262 mV (vs. Ag/AgCl) to

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-286 mV (vs. Ag/AgCl); meanwhile, hydrostatic pressure accelerated the anodic dissolution

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rate and made no difference on the cathodic reaction of this alloy. Such kind of influence on the corrosion behavior of alloys made by the hydrostatic pressure has also been found by

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other author [18], and attributed to enhanced adsorption of chloride ion on the surface of

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90/10 Cu-Ni under high pressure [19]. For the Ti6Al4V alloy, the corrosion potential of Ti6Al4V increased from 60.0 mV (vs. Ag/AgCl) to 149.5 mV (vs. Ag/AgCl) with increasing

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pressure. The corrosion of Ti6Al4V could seem to be greatly limited at both pressures for good passivation performance. For both pressures, the 90/10 Cu-Ni was the anodic element of

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the pair, in accordance with the OCP result shown in Fig. 3. The difference between the corrosion potential of the cathode (Ti6Al4V) and the anode (90/10 Cu-Ni) of the couple is an important determinant of the galvanic effect [20]. Table 1, which includes these differences, shows that the galvanic effect at 3.5 MPa (435.5 mV) is more significant than that at 0.1 MPa (322.0 mV). As the cathodic process on the Ti6Al4V was also highly limited, the values of

predicted Ecouple were almost the same with the Ecorr of the 90/10 Cu-Ni, indicating that coupling effect on the corrosion of 90/10 Cu-Ni is quite limited. In addition, a bigger galvanic current density was obtained at 3.5 MPa mainly due to accelerated cathodic process on the Ti6Al4V.

3.2. Galvanic corrosion tests

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3.2.1. Zero-resistance ammeter measurements

In order to study the galvanic corrosion of the 90/10 Cu-Ni / Ti6Al4V pair by another electrochemical technique and compare the results with those obtained by means of the mixed

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potential theory, zero-resistance ammeter measurements (ZRA) were carried out and the

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obtained galvanic potential and current densities are given in Fig. 5. As the ZRA tests commenced after the OCP of the two alloys had attained relative stability, the positive current

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density values recorded imply that the 90/10 Cu-Ni was the anodic member of the pair under

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different pressures, in accordance with the OCP and polarization results. A general tendency of decrease of the galvanic current density (Icouple) with time (Fig. 5 (a)) was observed at both

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pressures. The decrease was sharp during the initial 20 hours, after which the current density stabilized. Icouple was higher at 3.5 MPa (0.389 μA/cm2 ~ 1.93 × 10-2 μA/cm2) than at 0.1MPa

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(0.226 μA/cm2 ~ 9.39 × 10-4 μA/cm2) through the whole monitoring period. The galvanic current densities were transferred to mass loss of 90/10 Cu-Ni by Faraday’s equation [7]:

(4) where w is the mass loss rate, M is molar weight of Cu, z is the number of electrons released by one Cu atom converted to Cu+ ion [19], F is Faraday’s constant and I(t) is the galvanic

current density. The calculated results (MI

couple

) are listed in Table 2 and show that the weight

losss caused by galvanic current density is very small, which is in accordance with the polarization results. The galvanic potential (Fig. 5 (b)) tended towards more negative (cathodic) values, which is opposed to the observed anodic shift of OCP (Fig. 3). The polarization results imply that the galvanic potential was about the same as the corrosion potential of 90/10 Cu-Ni, which means

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that the corrosion potential of the alloy must also decrease during coupling. During the test time, the galvanic potentials drop by 29.8 mV at 0.1 MPa and 36.1 mV at 3.5 MPa, showing

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3.2.2. Galvanic weight loss measurements

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that the high pressure resulted in a more negative value.

The galvanic weight loss results at different pressures are also given in Table 2. In the

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Table, the Mcouple is the total galvanic weight loss of the 90/10 Cu-Ni alloys; Mself is the self-corrosion weight loss of the alloy. Table 2 shows that the corrosion caused by the ) only takes a small part of the total galvanic corrosion

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galvanic current density (MI

couple

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(Mcouple) and does not account for the natural corrosion of 90/10 Cu-Ni. In spite of this, it should be pointed out that the corrosion rate induced by galvanic current should not be

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ignored for it is persistent for long periods. Both the coupling effect and hydrostatic pressure have an accelerating effect on the total corrosion rate of 90/10 Cu-Ni. Considering that the corrosion rate induced by galvanic current only takes small part of the total corrosion rate at different pressures, the accelerating effect mainly reflect the influence of each factor on natural corrosion rate. The accelerating effect (ξ) of different factors on the weight loss of

90/10 Cu-Ni alloy was valued by equations (5) ~ (7): Couple:

ξ% =

M couple( 0.1MPa ) - M self ( 0.1MPa ) M self ( 0.1MPa )

Pressure:

ξ% =

Couple + Pressure:

ξ% =

(5)

× 100%

M self (3.5MPa ) - M self ( 0.1MPa ) M self ( 0.1MPa )

M couple(3.5MPa ) - M self ( 0.1MPa ) M self ( 0.1MPa )

× 100% × 100%

(6) (7)

The obtained results are shown in Fig. 6. The accelerated weight loss caused by coupling with Ti6Al4V at 0.1 MPa was low (6.89% ± 10.8%). The high pressure (3.5 MPa) alone also had a

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promoting effect on the self-corrosion of the 90/10 Cu-Ni alloy (8.20% ± 1.50%). The accelerating effect of hydrostatic pressure on self-corrosion of alloys undergoing uniform corrosion has been investigated by several authors [18, 21]. Yang et al. [18] reported that

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increasing hydrostatic pressure accelerates the initiation rate of metastable pitting and

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decreases pit growth probability, which increases the uniform corrosion susceptibility of alloys. Meanwhile, according to Sun et al. [21] the enhanced Cl- adsorption on the surface of

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alloys should also be responsible for the accelerating corrosion rate under high hydrostatic

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pressure. When coupled to Ti6Al4V at 3.5MPa, the weight loss of 90/10 Cu-Ni alloy was significantly increased by 58.9% ± 5.38%, which is much more pronounced than what was

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obtained for either of the isolated effects (coupling or pressure).

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3.3. Corrosion morphologies of the 90/10 Cu-Ni alloy 3.3.1. Pitting morphology and sizes The typical galvanic corrosion morphologies without corrosion products under different pressure are given in Fig. 7. The images show obvious big pits generated on the surface of the 90/10 Cu-Ni alloy. Typically, the pitted region (circled) appears jagged and steep boundaries

could be seen. At 3.5 MPa, the corroded surface was much craggier, which implies that the total surface of 90/10 Cu-Ni going through corroding attack was enlarged comparing with that at 0.1 MPa; meanwhile, larger pits in a much wider area exist on the corroded surface at 3.5 MPa, indicating that the extent of both uniform and pitting corrosion of the alloy was much more severe under high hydrostatic pressure. This behavior is suggestive of greater galvanic weight loss of the 90/10 Cu-Ni alloy at 3.5 MPa, which is consistent with the results in Table

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2.

In order to further examine the evolution of pits under different pressures, statistical distributions of the 2D sizes of pits are plotted in Fig. 8. In this figure, the cumulative

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probability of geometry parameters (diameter, depth) were calculated as n/(N+1) by a mean

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rank method [22, 23], where N is the total number of pits and n is the order in total number. Typical depth, diameter (with cumulative probability P=0.5) of corrosion pits of 90/10 Cu-Ni

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alloy at different conditions, as obtained from the statistical results are also shown in Fig. 8. With the increase of hydrostatic pressure, the average depth of corrosion pits increased from

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0.640 μm to 0.913 μm and diameter from 6.579 μm to 8.097 μm, which means that the

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hydrostatic pressure accelerated the pit growth rate both in parallel and vertical orientations. Pit geometry can be described according to the following equation:

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D/2h>1, pit is wide-shallow shape [24-26]; D/2h=1, pit shows semi-spherical shape; D/2h<1, pit exhibits narrow-shallow shape; where D is the diameter and h is the depth of pits respectively. D/2h values of 5.14 and 4.43 were obtained at 0.1 MPa and at 3.5 MPa respectively, implying that pits formed at both

pressures were wide-shallow shape.

3.3.2. The morphologies of corrosion products Fig. 9 shows the SEM images of corrosion products on the 90/10 Cu-Ni alloy with different magnifications after galvanic corrosion for 168 h at different pressures. The corresponding EDS analysis of the marked sites on the corroded surface is presented in Table

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3 - 4. Fig. 9 (a) shows a layer of corrosion products with many pores on the surface of the corroded sample at 0.1 MPa; the magnified image (Fig. 9 (a1)) indicates another layer of corrosion products exists beneath the surface layer. The chemical composition analysis of the

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corrosion products in Table 3 shows an obvious increase on the contents of O and Cl in the

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outer corrosion layer when compared with the inner layer. At the pressure of 3.5 MPa, the surface corrosion products experience heavy exfoliation and many dark points exist at the

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exposed surface (Fig. 9 (b)). The EDS results (Table 4) show that the content of Cl- at the

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dark points is higher than other parts of the exposed surface. Notice that the particles of the

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surface corrosion products at 3.5 MPa are bigger than that at 0.1 MPa (Fig. 9 (a1, b2)).

3.4. The EIS measurement of the corrosion products

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Electrochemical impedance spectra were recorded at the open circuit potential for the

corroded samples after galvanic corrosion for 168 h, in order to interpret the structure and resistance of the corrosion products formed at different pressures. The obtained results are displayed in both Nyquist and Bode plots in Fig. 10. It is remarkable that the diameter of the semicircle decreases dramatically after galvanic corrosion at 3.5 MPa compared with that at

0.1 MPa (Fig. 10 (a)), which suggests the decrease in the charge transfer resistance at high hydrostatic pressure. The Bode plots in Fig. 10 (b) show that the impedance response of samples with corrosion products formed at 3.5 MPa is smaller than that formed at 0.1 MPa. The phase angle plots show two peaks at high and low frequencies after galvanic corrosion at 0.1 MPa, whereas a single peak at intermediate frequencies was obtained at 3.5 MPa. The impedance spectra were fitted to equivalent circuit parameters. Considering the

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obtained phase angle plots and the morphologies of the corrosion products, two time constants were used. As the corrosion products formed at different pressures are quite defective, the fitting model shown in Fig. 11 was used. The equivalent circuit assumes that the corrosion

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products do not totally cover the metal and cannot be considered as a homogeneous layer but

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rather as a defective layer [27, 28]. Due to heterogeneity of the electrode surface, in the model, constant phase element (Q) was used to replace an ideal capacitance element because the

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latter hardly applies to real electrochemical processes [29]. Therefore, in the equivalent circuit,

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a pair of elements in parallel, namely, Qdl (the double electric layer) and Rct (charge transfer resistance) represent the dielectric properties of the double electric layer at the corrosion

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products layer/solution interface, and another pair of Q1 and R1 in parallel describes the charge transfer process through the corrosion products respectively. The calculated curve

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fitting results are shown with solid lines (Fig. 10) and are in good agreement with the experimental data. The model parameter values extracted from the EIS data under both environments are listed in Table 5, in which the value of constant phase element has been converted to capacitance using the conversion equation C=Q[2πfm]n-1 [30,31], where Q is the fitting value of constant phase element and fm is the frequency at which the imaginary part of

the impedance reaches a maxima for each (RQ) pair. The chosen conversion equation takes into consideration of the layered structure of electrodes, whose impedance response includes additive contributions from each part of the layers [32]. The Chi-square (χ2) value refers to the precision of the simulated data. It can be seen that the χ2 values are small, reflecting the fact that the simulated data is in good agreement with the experimental data. In general, the higher Rct reflects a lower corrosion rate of the process since the exchange current is directly

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associated with the electrochemical process of corrosion [33, 34]. It can be seen that lower Rct as well as lower R1 was obtained at 3.5 MPa compared with that at 0.1 MPa. This indicates that the charge transfer process between the metal surface and the bulk electrolyte is highly

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accelerated at high hydrostatic pressure. To compare the results obtained with EIS and the

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results obtained with polarization tests, the Stern-Geary’s equation can be used. For the EIS results, Zω→0 can represent the polarization resistance (Rp), including the resistance to both

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charge and mass across the double electrolyte (Rct) and the corrosion products (R1) [33].

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According to the fitted equivalent, when ω levels off to 0 Hz, Rp should equal to ( Rct + R1). At the same time, the tafel slopes can be obtained through the polarization curves, which are

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also shown in Table 1. Through the Stern-Geary’s equation, the Icorr can evaluate to be 2.43 μA.cm-2 at 0.1 MPa and 5.88 μA.cm-2 at 3.5 MPa, which can be thought to be in accordance

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with the results obtained by polarization tests (Table 1).

4. Discussion 4.1. The influence of hydrostatic pressure on the galvanic current density

The galvanic current flowing in a galvanic cell can be theoretically expressed by equation (8) [35, 36]:

Ig = (Φc - Φa ) /(R a + R c + R s + R m )

(8)

where Ig is the galvanic current between the anode and cathode, Φc and Φa are the open circuit potentials of the cathode and anode, Rc and Ra are the cathode and anode resistances, respectively, Rs is the resistance of the solution between the anode and cathode, and Rm is the

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metal resistance from the anode surface to the cathode surface through a metallic path. In this paper, Rm is negligible since the two electrode metals are connected by copper wire directly. Any factor that can affect these parameters will influence the galvanic current. Fig. 5 (a)

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shows that the galvanic current densities go through a rapid drop and then become relatively

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stable at both pressures.

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4.1.1. At the rapid dropping of the galvanic current density

During the early stages of galvanic corrosion (0 h~20 h), the value of the galvanic current

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density can be predicted by the mixed potential theory, as the corrosion potentials of the two alloys are more than 120 mV apart [37]. From the polarization results (Table 1), the galvanic

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currents are 0.108 μA/cm2 and 0.165 μA/cm2 at 0.1 MPa and 3.5 MPa respectively. At the

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same time, the ZRA tests show that the galvanic current densities are 0.23 μA/cm2 and 0.39 μA/cm2 at 0.1 MPa and 3.5 MPa respectively during the first monitoring hour. The predicted results of polarization curves can correspond to the ZRA test, indicating that the polarization curves can be used to analyze the variation of the galvanic current during the rapid dropping period. The polarization curves (Fig. 4) show that the 90/10 Cu-Ni alloy can undergo active dissolution in the 3.5% NaCl solution at different pressures, whereas the Ti6Al4V is well

passivated, with the cathodic process limited by oxygen diffusion process. As the galvanic potential of the couple predicted by the polarization curves lies within the range of the oxygen diffusion potential of Ti6Al4V at different pressures, the galvanic current density then should be determined by the limited oxygen diffusion current density (IL). The schematic shown in Fig. 12 describes the rapid drop of the galvanic current density. When the 90/10 Cu-Ni alloy is initially coupled to Ti6Al4V, the galvanic current density

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corresponds to the IL of the Ti6Al4V alloy. According to Fig. 5 (b), the Ecouple decreases with time. If it is assumed that the Ti6Al4V stays unchanged during coupling, then Icouple will stay unchanged when Ecouple decreases, which is contrary to the experimental results. The rapid

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decrease of Icouple can only happen when the corrosion potential (Ecorr) of the Ti6Al4V moves

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towards more cathodic values during the coupling. As shown in [11], the oxide film of titanium can get dissolved in aerated salt water. At all times, the current flowing to the

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Ti6Al4V cathode should be equal to the oxygen reduction current on its surface. As the

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oxygen diffusion current density is greatly limited, the dissolution of the oxide film would be accelerated, leading to a decrease of the corrosion potential of Ti6Al4V, which leads to the

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rapid drop in galvanic current density.

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4.1.2. At the relatively stable time of the galvanic current density The corrosion morphologies given in Fig. 9 show that the 90/10 Cu-Ni alloy suffers

uniform corrosion, indicating that the anodic and cathodic reaction are in dynamic equilibrium during coupling [38]. In order to identify the main factor determining the galvanic current density, some deductions need to be made. If the circuit between the 90/10 Cu-Ni and Ti6Al4V is disconnected, the electrochemical

reactions on the 90/10 Cu-Ni and Ti6Al4V can be considered separately. According to the polarization test of the 90/10 Cu-Ni, the corrosion current density (Icorr,Cu-Ni) of the alloy can be expressed by simplified Butler-Volmer equations (9): I corr ,Cu-Ni = I a ,Cu-Ni = i 0,Cu-Ni × exp = i 0,C × exp（ -

Vcorr - Ve,C βa

Vcorr - Ve,Cu Ni βa

= -I c,Cu-Ni

(9)

）

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where i0,Cu-Ni and i0,C are the exchange current densities of the anodic and cathodic reactions respectively; βa and βc are the anode tafel slope and cathode tafel slope respectively; Ve,Cu-Ni and Ve,C are the equilibrium potentials of the anodic and cathodic reactions respectively; Vcorr

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is the corrosion potential of the 90/10 Cu-Ni alloy.

When the circuit between the 90/10 Cu-Ni and the Ti6Al4V is connected, the potential of

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the system (Vcouple) will lie between the corrosion potential of the two alloys. The galvanic

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current density (Icouple) can be calculated from the 90/10 Cu-Ni side, as shown in equation (10):

Vcouple - Ve,Cu Ni

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I couple = I a ,Cu-Ni + I c,Cu-Ni = i 0,Cu-Ni × exp

βa

- i 0,C × exp（ -

Vcouple - Ve,C βa

）

(10)

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The polarized potential (∆E) is defined as the difference between the Vcouple and Vcorr of the 90/10 Cu-Ni alloy, as expressed in equation (11):

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ΔE = Vc o u p- lVe c o r r

(11)

From equations (9) ~ (11), the Icouple can be calculated by equation (12):

ΔE ΔE I couple = I corr ,Cu-Ni • e x p ( ) - I corr ,Cu-Ni • e x p- ( ) βa βc

(12)

The polarization curves (Fig. 4) show that galvanic potential is almost same with the corrosion potential of 90/10 Cu-Ni alloy, which indicates that the polarized potential (∆E) of

the alloy is very small. During the relatively stable periods of the galvanic current density, the value of ∆E may further decrease, leading to a smaller value of Icouple. Therefore, equation (12) can be simplified further by expanding the Taylor series to the first order and rearranging:

I couple= I corr,Cu-Ni • ΔE • (

1 1 + ) βa βc

(13)

At the same time, the Icorr,Cu-Ni can be obtained by using the Stern-Geary equation (14) [39]:

βa • βc 1 × βa + βc R p

(14)

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I corr,Cu-Ni =

where Rp is the polarization resistance of the 90/10 Cu-Ni alloy. Substituting equation (14)

I couple ΔE

=

1 Rp

(15)

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into equation (13) can result in:

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Equation (15) shows that during the relatively stable periods, the value of Icouple is

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determined by the polarized potential and the polarization resistance of the 90/10 Cu-Ni alloy at the different conditions. The value of ∆E is however too low to measure for this

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studied system. On the other hand, the EIS fitting results in Table 5 show that the charge transfer resistance (9791.16 Ω.cm2) at 0.1 MPa is about 3 times bigger than that

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(3951.89 Ω.cm2) at 3.5 MPa, which may be the main reason for the bigger galvanic

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current density at high hydrostatic pressure.

4.2. The influence of hydrostatic pressure on natural corrosion during coupling The polarization curves of 90/10 Cu-Ni when coupled with Ti6Al4V at different pressures are shown in Fig. 13. The influence of hydrostatic pressure on the anodic dissolution of 90/10 Cu-Ni during coupling is the same with that on the anodic dissolution of self-corrosion (Fig.

4), which shows an accelerating effect. Meanwhile, the cathodic polarization curves displayed in Fig. 13 show that at the same cathode polarization potential (ΔEc) the cathodic polarization current density (Ic) at 3.5 MPa is greater than that at 0.1 MPa, which implies that the cathodic process of 90/10 Cu-Ni in the galvanic couple is also accelerated by increasing hydrostatic pressure. With the work of high pressure, the natural current density of 90/10 Cu-Ni during coupling doubles from 8.07 μA/cm2 to 16.3 μA/cm2. Notice that the coupling effect also has a

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promoting effect on the natural corrosion of 90/10 Cu-Ni alloy.

Surface morphologies without corrosion products (Fig. 7) indicate that galvanic corrosion of 90/10 Cu-Ni alloy initiates with the formation of local pits and ends up in uniform

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corrosion (Fig. 9) following growth and coalescence of the pits. Fig. 8 also possibly implies

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that the growth speed of the pits is accelerated by high hydrostatic pressure. In order to further analyze the influence of hydrostatic pressure on the pit growth of 90/10 Cu-Ni alloy during

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coupling with Ti6Al4V, the electric potential and dissolution current density of single pit at

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different pressures were calculated by finite element analysis. In the finite element model, typical 2D sizes of corrosion pits in Fig. 8 were used; typical Butler-Volmer equations

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deduced from anodic polarization curves in Fig. 13 were applied to the anodic part of the physical model (pit area); typical Butler-Volmer equations deduced from cathodic polarization

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curves in Fig. 13 were applied to the cathodic part of the physical model (counterpart of pits) [16].

The electric potential on the metal surface of 90/10 Cu-Ni alloy at different pressures are shown in Fig. 14 (a) and (b). It can be seen that the mean electric potential of the metal surface is about -265.0 mV (vs. Ag/AgCl) at 0.1 MPa and -281.0 mV (vs. Ag/AgCl) at 3.5

MPa respectively, which is in accordance with the results predicted by the polarization curves (Table 1). As the counterpart of pit possesses higher potential than the pit area, the current in the metal should flow to the pit; accordingly, the dissolution current of pit should flow outside the pit through the electrolyte. This electric field in the electrolyte drives the dissolution of Cu ions into the solution and the migration of Cl- into the pits [16]. As the boundary conditions considered both the galvanic effect and the local dissolution of 90/10 Cu-Ni, the calculated

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dissolution current density around pits (Fig. 14 (c) (d)) can reflect the total dissolution rate of the alloy under both conditions. The dissolution current densities at the edge of pits are about 12 μA/cm2 under 0.1 MPa and 40 μA/cm2 under 3.5 MPa. At the bottom of pits, the

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dissolution current densities are about 2.0 μA/cm2 under 0.1 MPa and 5.0 μA/cm2 under 3.5

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MPa. Under both pressures, the dissolution current density at the edge remains bigger than that at the bottom, which indicates that the growth of pits can finally lead to uniform

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corrosion, in accordance with the experimental observations (Fig. 9). Also, higher dissolution

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current density at the edge and the bottom of pits further explains the bigger average pit sizes

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and faster galvanic corrosion rate at 3.5 MPa.

5. Conclusion

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The galvanic corrosion of 90/10 Cu-Ni/Ti6Al4V couple in 3.5% NaCl solution was

investigated at different hydrostatic pressures. Our findings revealed that: (1) At different hydrostatic pressures, the 90/10 Cu-Ni is always the anode part when coupled to Ti6Al4V in 3.5% NaCl solution. The corrosion potential of Ti6Al4V becomes more cathodic for the dissolution of the oxide film under cathodic polarization during

coupling, leading to a rapid decrease in the galvanic current density. (2) Hydrostatic pressure has an accelerating effect on the galvanic current density during coupling. At the initial periods (0~20 h), a higher galvanic current density is obtained mainly for the bigger limited oxygen diffusion current density (IL) at 3.5 MPa; at the later stages (20 h ~ 168 h), the hydrostatic pressure decreases the polarization resistance of 90/10 Cu-Ni alloy and thus increases the galvanic current density.

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(3) The natural corrosion rate of the 90/10 Cu-Ni alloy during coupling increases with increasing hydrostatic pressure, caused by an increased anodic dissolution rate as well as an increased cathodic reaction rate at high pressure. With the effect of hydrostatic pressure and

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galvanic effect with Ti6Al4V, the dissolution current density around pits at 3.5 MPa increases

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significantly compared with that comparing with that at 0.1 MPa, which account for a much

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Declaration of interests

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higher galvanic weight loss rate at high pressure.’

The authors declare that they have no known competing financial interests or

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personal relationships that could have appeared to influence the work reported in

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this paper.

Acknowledgements The investigation was supported by the National Natural Science Foundation of China under the Contract No.51622106 and No.51871049 and by the A-class pilot of the Chinese Academy of Sciences under the Contract No.XDA22010303.

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of effective capacitance and film thickness from constant-phase-element parameters, Electrochimica Acta, 55 (2010) 6218-6227. [33] A.V.C. Sobral, W. Ristow, D.S. Azambuja, I. Costa, C.V. Franco, Potentiodynamic tests and electrochemical impedance spectroscopy of injection molded 316L steel in NaCl solution, Corrosion Science, 43 (2001) 1019-1030.

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[37] H.P. Hack, J.R. Scully, H.P. Hack, C.L. Meade, H.P. Hack, R.A. Corbett, H.P. Hack, R.D. Kane, H.P. Hack, S.W. Dean, Corrosion Tests and Standards: Application and Interpretation Second Edition, (2009).

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[38] C.N. Cao, Principles of Electrochemistry of Corrosion, Chemical Industrial Press,

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Beijing, (2008).

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Cu-Ni Alloy in Chloride-Based Electrolytes, Journal of Applied Electrochemistry, 34 (2004)

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659-669.

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Fig. 1 submitted to Corrosion Science by Shengbo Hu et al.

Fig. 2 submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 3 submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 4 submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 5 (a) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 5 (b) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 6 submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 7 (a) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 7 (b) submitted to Corrosion Science by Shengbo Hu et al

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Fig. 8 (a) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 8 (b) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 9 (a) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 9 (a1) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 9 (b) submitted to Corrosion Science by Shengbo Hu et al

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Fig. 9 (b1) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 9 (b2) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 10 (a) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 10 (b) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 11 submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 12 submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 13 submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 14 (a) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 14 (b) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 14 (c) submitted to Corrosion Science by Shengbo Hu et al.

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Fig. 14 (d) submitted to Corrosion Science by Shengbo Hu et al.

Table. 1 submitted to Corrosion Science by Shengbo Hu et al 0.1MPa

3.5MPa

90/10 Cu-Ni Ecorr (mVAg/AgCl)

Ti6Al4V

-262

60.0

90/10 Cu-Ni -286

149.5

≈ -262

≈ -286

Ecorr-c-Ecorr-a (mVAg/AgCl)

322.0

435.5

99.61

βc (mV/dec)

29.09

-

Icorr (μA/cm2)

2.72

0.0301

Icouple (μA/cm2)

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0.108

171.5

103.9

26.79

-

3.81

0.0258

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131.7

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Ecouple (mVAg/AgCl)

βa (mV/dec)

Ti6Al4V

0.165

Table. 2 submitted to Corrosion Science by Shengbo Hu et al -2

MIcouple（g.m ）

Mcouple（g.m-2）

Mself（g.m-2）

0.1 MPa

0.040

6.52 ± 0.60

6.10 ± 0.068

3.5 MPa

0.13

9.69 ± 0.30

6.90 ± 0.029

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Condition

Table. 3 submitted to Corrosion Science by Shengbo Hu et al Cl

Mn

Fe

Ni

Cu

1

36.9

2.51

0.83

1.48

9.21

49.06

2

35.29

2.27

0.69

1.65

9.27

50.83

3

46.35

7.35

0.75

1.14

7.83

36.58

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Table. 4 submitted to Corrosion Science by Shengbo Hu et al Cl

Mn

Fe

Ni

Cu

1

43.56

3.79

1.20

2.02

13.02

36.41

2

49.09

2.96

1.20

1.54

10.35

34.87

3

31.15

-

0.87

1.25

6.85

59.88

4

34.73

1.22

0.93

1.13

6.50

55.50

5

52.51

8.35

0.74

1.50

8.69

28.20

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Table. 5 submitted to Corrosion Science by Shengbo Hu et al Q1/uF.cmConditions Rs/

Ω.cm2

Qdl/uF.cm-2

ndl

Rct/Ω.cm2

R1/Ω.cm

2

29.09

1

26.16

1180

3.5 MPa

10.95

54.07

1

9.894

660.6

0.6245 9765

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0.445

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11.39

2

0.6521

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0.1 MPa

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Σχ2 × 10-3

n1

3942

1.22