Improving surface characteristic and corrosion inhibition of coating on Mg alloy by trace stannous (II) chloride

Accepted Manuscript Title: Improving Surface Characteristic and Corrosion Inhibition of Coating on Mg Alloy by Trace Stannous (II) Chloride Authors: Rong Gan, Dongmei Wang, Zhi-Hui Xie, Ling He PII: DOI: Reference:

S0010-938X(16)30647-3 http://dx.doi.org/doi:10.1016/j.corsci.2017.04.018 CS 7065

To appear in: Received date: Revised date: Accepted date:

27-8-2016 20-4-2017 22-4-2017

Please cite this article as: Rong Gan, Dongmei Wang, Zhi-Hui Xie, Ling He, Improving Surface Characteristic and Corrosion Inhibition of Coating on Mg Alloy by Trace Stannous (II) Chloride, Corrosion Sciencehttp://dx.doi.org/10.1016/j.corsci.2017.04.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

7Improving Surface Characteristic and Corrosion Inhibition of Coating on Mg Alloy by Trace Stannous (II) Chloride

Rong Gana, Dongmei Wanga, Zhi-Hui Xiea, b,*, Ling Hea

a)

Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, Sichuan, China b) Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, USA *Corresponding

author. Tel./Fax: +86 817 256 8081. Email address: [email protected] (Z.-H.

Xie).

Highlights 

An effective additive is demonstrated for preparing Ni coating on Mg alloy.



Trace stannous chloride (SnCl2) is helpful to obtain smoother coating surface.



Trace SnCl2 is powerful in enhancing coating corrosion resistance.



The coating is Sn-free but properties improved.

ABSTRACT Duplex Ni coatings consisting of Ni–P deposits as primer layer and Ni–Sn–P deposits as outer layer were prepared on Mg alloy. These coatings exhibit uniform surface and improved corrosion resistance after adding trace stannous (II) chloride (SnCl2) into the outer layer plating bath. X-ray photoelectron spectroscopy proved the non-existence of Sn in the coating. The coating surface was characterized by scanning electron microscopy and atomic force microscopy. X-ray diffractometer patterns show that SnCl2 is favourable for amorphisation of the coating. The corrosion protection of

1

the coating was confirmed by potentiodynamic polarisation and electrochemical impedance spectroscopy measurement.

Keywords：A. Alloy; A. Metal coatings; B. XPS; B. EIS; B. AFM; C. Neutral inhibition

1. Introduction The inferior corrosion and wear resistance as well as poor creep resistance of magnesium (Mg) alloy have greatly restricted its application in many industry fields[1]. Electroless Ni–P plating is considered as one of the most effective methods to modify those chemical and physical defects of Mg alloy due to its outstanding properties in hardness as well as wear and corrosion resistance [2,3]. One way to further improve the characteristics of electroless Ni–P plating coatings and satisfy special demands is alloying a third element such as Sn, Co, W and Cu, to form a composite layer [1, 4-14]. Ni–Sn–P alloy is a promising composite coating used in many electronic components, such as leading wire, magnetic recording medium, printed circuit board and anode for lithium-ion batteries, because this alloy exhibits exceptional corrosion protection and weldability [1, 4-14]. With the introduction of Sn, the film surface on a Mg alloy becomes more compact, smoother, and nonporous than the surface without Sn. Sn-alloyed deposit can also be used as an under layer in an electroless Ni–P plating coating to strengthen the adhesion between the coating and substrate effectively [1]. The porosity of coating deposited from an alkaline sodium citrate is lower than that of Sn-free coating, and the former can withstand approximately 10 h of erosion in 10% HCl solution[7].

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The inhibition capability of Sn-alloyed coating depends largely on tin content [14, 15]. At > 14 at.% Sn, Sn may produce a crystalline coating structure transformed from an amorphous structure, which provides a more efficient inhibition ability than its equivalent polycrystalline materials do [16]. The crystallinity of coatings also increases as Sn content further increases [15]. Weight loss tests in HCl, H2SO4, NaCl and NaOH media indicate that adding Sn to Ni–P deposits on a copper matrix from an alkaline plating solution improves corrosion resistance in some instances[14]. Polarisation curve experiments demonstrate that the incorporation of Sn into a coating on low-carbon steel or copper may reduce the inhibition ability of the coating in inorganic acid solutions [5, 15]. An abnormally high concentration of stannous (II) chloride (SnCl2) in a plating bath triggers the suppression or termination of electroless Ni–P plating processes [1]. A high concentration of SnCl2 in a plating bath and the incorporation of Sn into a coating lead to favourable results in some instances, although corrosion resistance of the coating is improved. An electroless Ni–P plating bilayer coating is also proposed to improve the corrosion resistance of Mg alloy [17, 18]. The different structural and mechanical natures of the inner and outer layers of a duplex coating possess different defects that are usually mismatched. This difference in defects presents a smaller chance of the corrosive media to reach a substrate than that in a single-layer coating or duplex coating with the same defect. Thus, corrosion resistance is enhanced [17]. The addition of trace stannous ions to an acidic electroless Ni–P plating bath of Mg alloys has yet to be performed to prepare and improve the properties of a double-layer Ni coating. In this work, a double-film coating with a high P content in inner and outer layers was constructed. Through an environmentally-friendly pre-treatment, which was described in our previous work [3, 19], a film with high

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corrosion protection was used to prepare the inner layer, and SnCl2, which is likely more promising than stannous (IV) chloride in future applications, was added to the plating bath to produce the outer layer [8]. To avoid unpredictable detrimental effects on the coating in a high SnCl2 concentration, we introduced trace SnCl2 to the plating bath and improved the corrosion resistance with a Sn-free coating.

2. Experimental 2.1 Material and electroless Ni–P plating procedure Palates made of AZ31 Mg alloy (2.75 wt.% Al; 1.15 wt.% Zn; 0.16 wt.% Mn; balance–Mg) were cut into 30 × 20 × 2.0 mm3 rectangular shape and were used as samples for study. All specimens were first ground mechanically with 800 and 1200 grit SiC waterproof abrasive papers, rinsed with distilled water, and dried in air. Then, the Mg alloy samples were pre-treated sequentially in acetone, alkaline and acidic solution, and activation solution. Finally, these samples were deposited in a plating bath A followed by a bath B with different concentrations of SnCl2. The compositions of plating bath solution and the operating conditions are given in Table 1. During plating, the samples were held by a Nylon wire and were suspended in a plastic beaker containing 200 mL of plating bath. The temperature of the plating bath in the beaker was controlled by a digitally controlled thermostat. All solutions used were freshly prepared from analytical grade reagents and deionized water. 2.2 Property evaluation and electrochemical measurements The corrosion resistance was evaluated by dipping the coating in 3.5 wt.% NaCl solution and collecting the volume of the hydrogen gas during corrosion process. The high reliability of this measurement has been proved in many literature [20-23]. The schematic diagram to measure the corrosion rate was shown in Fig. 1, which was

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derived from Song et al [20, 24, 25] but modified in the present work for obtaining more accurate gas volume. The hydrogen volume read by the modified apparatus is not affected by the height of the water in the burette which may make many hydrogen bubbles trapped in the funnel and lead to deviation volumes. Thus, this apparatus is more sensitive to the volume change of the hydrogen. The thickness of the coating was determined by scanning electron microscopy (SEM, JSM-6510, Japan). The calculation of the coating thickness is as follows: Three samples at least were deposited at each concentration of SnCl2. The thickness of the coating for a certain concentration of SnCl2 is the average value of these samples above (Fig. 2). The thickness of each sample is the average value based on the thickness of five random selected areas from the cross-sectional SEM image. Fig. 2B(C) shows the typical sample which has the closest value of thickness to the average value in Fig. 3A. Adhesion of Ni–P coating was estimated by scribe and grid test according to the Chinese national standard (GB/T5270-2005) and the international organization for standardization (ISO 2819) [3]. Electrochemical properties of the samples were investigated by means of potentiodynamic polarisation (PDP) and electrochemical impedance spectroscopy (EIS) measurement in 3.5% NaCl aqueous solution at 25 ◦C [26]. The tests were carried out by SP150 (Bio-Logic, France) and CHI618B (Chenhua, China) electrochemical workstations. A classical three-electrode system including a reference electrode (saturated calomel electrode, SCE), a as counter electrode (Pt foil) and a working electrode that has been electroless Ni–P plating coated freshly and sealed by epoxy resin with an exposed area of 1 cm2, was used for PDP and EIS examination. The electrochemical tests of these specimens were performed after immersion of 10 minutes in NaCl solution. The scan rate of polarisation is 5 mV/s in the range of −0.5

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to 0.5 VSCE in respect to open circuit potential (OCP). A sinusoidal signal with 5 mV amplitude was used to perform EIS tests. The spectra were recorded at an open circuit potential in a frequency range of 0.1 Hz ~ 100 kHz. The analysis of the PDP and EIS data were carried out using ZSimpWin software and the attached program in the electrochemical workstation [27]. The effectiveness of the corrosion resistance of the coating (i) was evaluated by corrosion current as follows [28, 29]:

i 

ic, sub  ic, coat ic, sub

100% (1)

where ic, sub and ic, coat are the corrosion current densities fitted from the polarisation curves of matrix and coatings, respectively. The corrosion current densities were obtained by extrapolation of the linear part of the cathodic branch of the polarisation to the corrosion potential (logarithmic current scale).

2.3 Characterization The morphologies of the coating surface and cross section were observed by SEM and atomic force microscopy (AFM, Dimension Icon, Germany). The elemental composition of the coating was inspected by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Scientific) and energy-dispersive X-ray (EDX) analysis facility affiliated to the SEM. XPS measurements were performed with an X-ray photoelectron spectrometer using Al Kα radiation (1486.6 eV, 150 W) for excitation. The contaminant carbon signal (C 1s = 284.8 eV) was used to calibrate the spectra for charging. The background was subtracted by Shirley method. Microstructure of the coating was characterized by X-ray diffractometer (XRD, D8 Advance, Germany) using Cu Kα radiation (2θ: 10° ~ 90°, λ= 1.54 Å.). The acquisition was operated at 40 kV and 40 mA with a step size of 0.02°.

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3. Results 3.1 Deposition thickness and adhesion Fig. 2 shows the thickness of electroless Ni–P plating coating from a bath in different SnCl2 concentrations. The coating thickness increased from appropriately 15.8 µm to appropriately 18.5 µm when the SnCl2 concentration varied from 0 mg/L to 30 mg/L. Xie et al indicate that the deposition rate of Ni–Sn–P on copper sheet decreases with increasing concentration of sodium stannate [6]. However, this work found trace SnCl2 slightly increasing the deposition rate instead of decreasing. Results of scribe and grid test indicate that a higher SnCl2 concentration has bad influence on the adhesion (Insets in Fig. 2A and Table 2). After SnCl2 (30 mg/L) was incorporated into the plating bath, the coating becomes swollen and peels off (see white arrows in inset II in Fig. 2A). This finding suggests that the adhesion of the Sn-free coating is more efficient than that of the Sn-containing coating. 3.2 Structural characteristics The influence of SnCl2 concentration on the surface morphological characteristics of the coating is presented in Fig. 3. As shown in Fig. 3A, an undulated surface with some abnormal big nodules was obtained when SnCl2 was not added into the plating bath. The inset in Fig. 3A shows that the nodules are uneven and the diameters range from 1 m to 15 m. Furthermore, some obvious cavities are observed on the coating surface (white circles in Fig. 3A). These defects are improved slightly, and the size distribution of the nodules is more even after 5 mg/L SnCl2 introduction in the bath (Fig. 3B). This levelling effect on the coating surface is more evident when the SnCl2 concentration was increased to 10 mg/L, leading to less cavities on the coating surface (Fig. 3C). Further SnCl2 concentration increase to 20 mg/L, leads to homogeneous and smooth coating surface, and the local inward hollow are inconspicuous though

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some irregular nodules could be still observed (Fig. 3D). The close to normal distribution of nodule diameter (inset in Fig. 3D) and AFM results from Fig. 4, indicates that the coating surface is more even and more uniform than that in coating without SnCl2 in the plating bath. The EDX result in Fig. 3E and F shows that the SnCl2 did not evidently affect the Ni and P contents. AFM test confirmed the smoother surface after SnCl2 incorporation in bath in comparison with that without SnCl2 (Fig. 4). From the AFM results, the average surface roughness values decreased from 2.73 m to 1.49 m for the Ni coating before and after SnCl2 addition in the bath. The XRD patterns of all the coatings consist of a broad peak at 2θ ≈ 45o and a series of sharp peaks (Fig. 5). The broad peaks are attributed to the formation of an amorphous–crystalline structure because of the high P content and the preferred orientation of Ni (111), whereas the sharp peaks are due to the crystalline Mg on the matrix, i.e., the strong X-ray penetrated deep into substrate from outside coating surface.

3.3 Electrochemical properties Fig. 6 shows the OCP vs time curves of the substrate and different coatings in NaCl solution. The OCP for bare Mg alloy increased rapidly and then gradually reached to a steady value at around 1.46 VSCE in less than 10 minutes. The increase and steady OCP with time is attributed to the accumulation of corrosion products and hydrogen bubbles at the surface of the substrate [19]. All OCP vs. time curves for coatings exhibit similar trend of variation. The OCP of the coatings slight increased initially and then decreased continuously with a very slow rate with increasing immersion time. The coated sample shows more noble OCP than that of the bare Mg alloy during the one hour of immersion. The electrochemical tests were performed

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after 10 minutes of immersion in NaCl solution at room temperature as the OCP of all the samples was levelled off before this moment. Fig. 7A shows that all the coatings exhibit similar behaviour in both cathodic and anodic polarisations and evident passive behaviour in the anodic branches, but slight positive shifts in corrosion potential are observed with increasing SnCl2 concentration. The electrochemical parameters, such as anodic and cathodic Tafel slope of the polarisation curves, are summarised in Table 3. The corrosion current densities of the coatings obtained through two different mechanisms are almost lower than 10 µA/cm2, which are by two orders of magnitude lower than that of Mg alloy (870 or 1012 µA/cm2) (Table 3). The corrosion current density of the coatings is decreased by about 23.5% when the SnCl2 concentration in the bath increases from 0 mg/L to 20 mg/L. The SnCl2 effect on the corrosion protection effectiveness of the coatings is also estimated by inhibition efficiencies, which was measured by corrosion current density derived from the polarisation curves (Fig. 7B). Fig. 7B indicates that the coatings provide good protection for the Mg alloy, and the SnCl2 introduction in the plating bath is favourable for the enhancement of protection according to corrosion current density. Moreover, the inhibition efficiencies of the coatings are higher than those reported for plasma electrolytic oxide coatings (75%) in a previous study [28]. To validate further the favourable effect of the SnCl2 on the corrosion resistance of the coating, we present the EIS results of the coating in NaCl solution and the corresponding equivalent electrical circuits in Fig. 8. Fig. 8A and B–C are the Nyquist and Bode plots, respectively. Fig. 8D are the EIS fitting equivalent electrical circuits for Mg alloy and coatings. The fitting results of the experimental EIS data by these two electrical circuits are presented inTable 4. The Fig. 8A inset shows the Nyquist plot of Mg alloy. The Nyquist plots consists of one capacitive loop at high frequency

9

and two inductive loops at intermediate and low frequencies. The capacitive loop is related to the characteristic of the electric double layer at the interface of electrode/electrolyte [30], while the inductive loops at intermediate and low frequencies in the fourth quadrant are the respective results of the chemical reaction between Mg+ and H2O at the breaking area of the corrosion product layer and the desorption of the corrosion products [31, 32]. The solid in Fig. 8A inset was plotted according to the calculated values by using equivalent circuit of Fig. 8D(a). In Fig. 8D(a), Rs is the solution resistance, Qdl is the constant phase element of the double layer used to substitute a pure capacitance accounting for non-ideal capacitive response of the interface, Rct is the charge transfer resistance [33, 34], RLMg+ and LMg+ in series connection is related to the chemical reaction of Mg+ [32], and the RL and L in series connection is because of the desorption of corrosion products [35]. The parallel connection of the components shown in Fig. 8D(a) is due to these physical/chemical processes taking place on the surface simultaneously [36]. Fig. 8D(b) is usually used for fitting a EIS of protection coating on Mg or Mg alloy [28, 37]. Apart from the Rs, Qdl and Rct, two other elements are included in the model: constant-phase element (Qc) and resistor (Rc) of the Ni–P coating. All the fitted values according to this model were plotted and presented in black solid lines in Figs. 8A–C.

3.4 Immersion tests The hydrogen evolution and surface morphological characteristics of the coating after immersion in 3.5 wt.% NaCl solutions are shown in Fig. 9. In the first 2 days of immersion, no hydrogen evolved for both kinds of coating. After 2 days of immersion, hydrogen gas was collected from the coating prepared from a bath without SnCl2 (Ni– P/Ni–P). The volume of hydrogen gas increased to 1.225 ml/cm2 and the slope of the

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line was approximately 0.235 after 7 days of immersion. Nevertheless, hydrogen evolution was also not detected after 7 days of immersion for the coating deposited from a bath with SnCl2 (Ni–P/Ni–Sn–P). Many white corrosion pits were observed in almost all of the nodules of the Ni–P/Ni–P coating surface (SEM and optical images in Fig. 9I). By comparison, most nodules of Ni–P/Ni–Sn–P coating remained smooth and uncorroded, although some white corrosion pits could be found on some nodules (SEM and optical images in Fig. 9II).

4. Discussion In general, the corrosion resistance of a nickel-based coating should be related to five factors at least: i) thickness, ii) porosity, iii) composition, iv) surface morphology, and v) microstructure (amorphous and crystalline) of the coating. The thickness in different areas of the coating is uneven (Fig. 2C). This thickness difference in different areas of a coating is even higher than the increased average thickness difference which results from the increase of the SnCl2 concentration. For example, the coating thickness difference in the left and right hand (4.8 µm = 23.2  18.4) in Fig. 2C is even higher than the increased average thickness difference (2.6 µm = 18.4

 15.8) when the SnCl2 concentration increased from 0 mg/L to 30 mg/L. In this situation, the XRD peak intensity of the substrate element in the left-hand will be stronger than the value in the right-hand. In other words, although the average thickness of the coating obtained from a bath with higher concentration of SnCl2 is higher than that of the coating obtained from a lower SnCl2 concentration, the thickness of regional area of the coating selected for data acquisition during XRD diffraction is difficult to be identified. The regional thickness of the coating deposited from a bath with higher SnCl2 concentration may even lower than the one with lower

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SnCl2 concentration. Furthermore, AZ31 Mg alloy has a typical Mg–Al–Zn alloy microstructure consisting of primary α-Mg, eutectic α-Mg and intermetallic compound β-Mg17Al12 which mainly exists along the grain boundaries [38, 39]. This microstructure of the substrate results in non-uniform distribution of phases and elements, such as Mg, Al, and Zn. Thus, the coating obtained from a bath with lower SnCl2 concentration and lower average thickness may also has stronger intensity in the XRD spectra. For instance, although the coating obtained from a bath with 20 mg/L SnCl2 has the strongest peak at 63.0°, 47.8°and 34.4°, the one with 10 mg/L SnCl2 has the strongest peak at 70°, 36.7° and 32°, and the one with 5 mg/L SnCl2 has the strongest peak at 72.8° (Fig. 5). Nodular plating in a coating is related to the nucleation rate and growth of the deposit [11]. Because a more uniform microstructure is observed with increasing SnCl2 concentration (Fig. 3 and Fig. 4), it can be concluded that the introduction of SnCl2 in the plating bath affects the nucleation rate or growth of the deposit. More specifically, the nucleation rate was enhanced and/or the growth rate was retarded although it is difficult to substantiate which case it should be here. The more uniform in size distribution of the nodules is responsible for the corrosion inhibition improvement of the coating after introduction of SnCl2. The inset in Fig. 5 indicates that the degree of amorphisation of the coating enlarged with increasing SnCl2 concentration in plating bath, which was also report by Zou et al [5]. Usually, amorphous materials have better corrosion inhibition than that of their crystalline equivalents due to less defects, such as grain boundaries, dislocations, and absence of surface inhomogeneities which are active sites for corrosion attack [19, 40, 41]. This should be the second reason why trace introduction of SnCl2 in the plating bath improves the corrosion resistance of the coating. The improvement of the corrosion

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inhibition is confirmed by the OCP vs. time, PDP and EIS results. Generally, the coatings with higher OCP value would be more difficult to be degraded [19]. When the SnCl2 concentration varied from 0 mg/L to 20 mg/L, the OCP of the coating shifted positively from about 0.50 VSCE to 0.40 VSCE and the corrosion current density decreased to 5.2 µA/cm2 from 6.8 µA/cm2, suggesting a favourable impact on the corrosion resistance. The low and approximately same decrease rates in OCP for different coatings indicate that the main degradation reaction at electrode in this period is dissolution of deposited nickel, and the degradation proceeds very slowly. An electroless Ni–P plating coating on Mg alloy is cathodic coating. When the coating is immersed in NaCl electrolyte, the coating degrades rapidly and the OCP drops to a more negative value [42] within several minutes due to galvanic corrosion if the surfaces of the samples are not covered completely or once a channel is formed between the substrate and the corrosive media. The relatively steady in OCP suggests that the surfaces of the Mg alloy substrate were fully covered by coatings during deposition and corrosion channel was not formed between coating and substrate in this stage. In fact, hydrogen was not observed until 2 days of immersion for both coatings. But after that, hydrogen was collected for the coating obtained from a bath without SnCl2. Under the same condition, hydrogen was not observed for the coating deposited from a bath with SnCl2 even after 7 days of immersion. The long-term immersion test as well as the SEM images in Fig. 9 further demonstrate that the substrate was covered completely by the coating and pinhole was not formed in the coating. The improved corrosion inhibition of the coating after introduction of SnCl2 cannot be attributed to the increase of the thickness because the corrosion proceeds mainly at the surface of the coating and channel between substrate and corrosive media was not formed for both coatings.

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EIS results quantitatively supported the increase of the corrosion resistance of the coating with increasing SnCl2 concentration. The diameter of the capacitive loop in Nyquist diagrams is usually inversely proportional to corrosion rate, i.e., the greater the diameter, the lower the corrosion rate [43]. Fig. 8A clearly shows that the diameter of the capacitive loop of the Ni–P coating increases with increasing SnCl2 concentration in a 3.5 wt% NaCl solution. All the coatings have much greater corrosion inhibition than that of the bare AZ31 Mg alloy. All the phase diagrams of the coatings as shown in Fig. 8B represent a broad ware crest close to 90o at the middle frequency. The high Rct and/or Qdl are favourable for the improving of corrosion resistance of Ni coating [43, 44]. The significantly enhanced corrosion protection can be confirmed by the impedance modulus values measured at low frequency (|Z|f=0.1 Hz) as illustrated in Fig. 8C. The value for the coating from a bath containing 20 mg/L SnCl2 is 40 kΩcm2, which is 1.5-fold higher than value of the coating without SnCl2 (26 kΩcm2) and by three orders of magnitude higher than that value of the bare substrate (28.4 Ωcm2). The Rct is another parameter directly dependent on the active protection provided by the coating [45]. Table 4indicates that the Rct for Mg alloy is only 45.3 Ωcm2, whereas the value for coating increases from 28 kΩcm2 to 47 kΩcm2 when the SnCl2 increased from 0 mg/L to 20 mg/L. These EIS data are well consistent with PDP test results. Both EIS and PDP results indicate that the Ni–P coatings provide wonderful protection for Mg alloy and the addition of a certain SnCl2 concentration in the plating bath is efficient to enhance the corrosion resistance of the coating. Considering that there are almost the same contents in Ni and P in the two coatings before immersion measurement (see EDX results in Fig. 3) and the corrosion proceeds mainly at the surface layer of the coating during immersion in NaCl solution, 14

analysis of the composition of the corrosion products at the coating surface becomes crucial important to understand the reason why a coating obtained from a bath with SnCl2 exhibits better corrosion inhibition than that without SnCl2. XPS is a good option because it provides a typical test depth of less than 5 nm, which enables the test results cannot be interfered too much by the elements inside the coating. The XPS results of the two coatings after 7 days of immersion in NaCl solution are shown in Fig. 10. In Fig. 10A, the full spectra show no obvious differences in the two coatings, which mainly contain Ni, P, O and C elements. The C peak at ~284.6 eV is attributed to the ubiquitous carbon detected on most air-exposed metal surfaces or mainly from pump oil in the vacuum system of the XPS equipment [46, 47]. The high-resolution spectra of Ni 2p for Ni–P/Ni–P and Ni–P/Ni–Sn–P coatings are shown in Fig. 10B and C, respectively. The Ni 2p main peak is fitted by two peaks at 855.3 and 856.5 eV, and the relating satellite peak was also fitted by two peaks at 861.2 and 862.6 eV (863.3 eV for Ni–P/Ni–P coating), which correspond to Ni(OH)2 and NiO, respectively [47, 48]. The involved chemical and electrochemical steps for the formation of Ni(OH)2 and NiO passive films in this work are proposed as follows: i) oxidation of Ni to form Ni2+ (reaction (2)) [41], ii) hydrolysis of Ni2+ to produce Ni(OH)+ (reactions (3)) [49], iii) the intermediate Ni(OH)+ could be further transformed into more stable Ni(OH)2 by combining with H2O (reaction (4)) [49], or NiO via dehydrogenation (reaction (5)) [50]. The NiO may be also formed via dehydration of Ni(OH)2 especially when the coatings were taken out from the aqueous solution and exposed to dried environment (reaction (6)) [49]. Some research indicated that there is a higher oxide Ni2O3 at the surface of the coating based on XPS [51] and XRD results [52]. Because the binding energies of Ni(OH)2 and Ni3+ are respectively 856.6 eV and 856.5 eV [53], and the binding energy difference is only

15

0.1 eV, making the quantitatively distinction of these peaks impossible. Thus, it is difficult to ascertain whether there is really any Ni2O3 oxide in the corrosion products. The O peak at ~531.1 eV further confirms the oxidation and corrosion of the coating surface. The full spectrum of Ni–P/Ni–Sn–P coating and the high-resolution spectra of Sn 3d demonstrate that the coating does not contain Sn. Ni → Ni2+ + 2e

(2)

Ni2+ + H2O → Ni(OH)+ + H+

(3)

Ni2+ + 2H2O → Ni(OH)2 + 2H+

(4)

Ni(OH)+ → NiO + H+

(5)

Ni(OH)2→ NiO + H2O

(6)

Pure nickel is unable to passivate and corrodes rapidly in solution [54]. Lo et al claimed that, in addition to the formation of nickel oxide and hydroxide, one more important reason that results in Ni-P deposits with better corrosion inhibition than pure nickel is the formation of a phosphorus-enriched layer at the coating surface [40]. The formation of phosphorus-enriched layer occurred according to the following reaction [40]: P + 2H2O → H2PO2 + 2H+ + e

(7)

The reaction is confirmed by the high-resolution XPS spectra of P 2p for the both nickel-based coatings after a long-term of immersion in the NaCl solution, as shown in Fig. 10D. Thus, the surfaces of both coatings after immersion have the same chemical composition consisting of Ni(OH)2, NiO and H2PO2. Fig. 10D shows only one peak for P+ at a binding energy of 133.2 eV, whereas the peak for elemental phosphorus at a binding energy of 130.0 eV is not observed. This finding means most of the phosphorus was oxidized to form P+ during corrosion. Lo et al and Zeller III stated that the formation and adsorption of hypophosphite on the surface of Ni-P

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alloys could provide passivity and reduce the corrosion rate of the deposits in a aqueous environments [40, 55]. However, note that it does not mean the more content in nickel hydroxide or hypophosphite, the better in corrosion resistance. Figs. 10B–D indicate that the Ni–P/Ni–P coating with more severe corrosion exhibit stronger peaks of Ni(OH)2 and P+ in comparison with that of Ni–P/Ni–Sn–P coating. We can conclude that the introduction of SnCl2 into the plating bath primary affects the thickness, microstructure, surface morphologies, and corrosion inhibition of the nickel coatings. The enhanced corrosion inhibition is mainly attributed to the amorphisation and more uniform size distribution of nodules at the surface of the coating.

5. Conclusions 1. The effects of trace SnCl2 on the properties of electroless Ni–P plating coating on Mg alloy were investigated. Our results showed that the deposition rate slightly increased as SnCl2 concentration increased. Nevertheless, a higher SnCl2 concentration might result in poor coating adhesion. 2. Trace SnCl2 could improve the surface uniformity and greatly increase the corrosion resistance of electroless Ni–P plating coating, although Sn was not detected in the coating. 3. The average surface roughness and corrosion current density of the electroless Ni– P plating coatings decreased obviously after SnCl2 was incorporated into the plating bath. Impedance modulus value at low frequency (|Z|f = 0.1 Hz) of the coating increased by a factor of 1.5 compared with that obtained from a bath without SnCl2. 4. Hydrogen evolution tests demonstrated that the newly prepared coating could

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tolerate at least 7 days of immersion without evident corrosion in 3.5 wt.% NaCl solution, and no hydrogen was collected. 5. The improved corrosion resistance might be attributed to the more uniform size distribution of nodules at the surface and the amorphisation of the coating.

Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, 51501157); the China Scholarship Council (CSC, 201508510046); the Innovative Research Team Program of the Education Department of Sichuan Province (15TD0018); the Innovative Team Project of CWNU (CXTD2015-1); and the Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province (CSPC2014-4-2).

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Figure Captions:

Fig. 1 Schematic diagram of measurement to monitor the volume of hydrogen evolved. The burette (closed) and bottle are firstly and respectively filled with water and NaCl solution. Then, the burette and bottle are connected by a tube after the sample was immersed into the NaCl solution. Finally, open the switch on the burette, the water will be quickly flow into the beaker at the beginning, and then the scale will be stabled unless some gases are released in the bottle. The hydrogen volume can be determined based on the scale variation on the burette. Fig. 2 (A) Dependence of coating thickness on SnCl2 concentration (Insets are optical pictures of coatings from different baths after scribe and grid test: (I) without SnCl2 in bath B, (II) with 30 mg/L SnCl2 in bath B). (B) and (C) are the cross-sectional morphologies of coatings deposited from bath A followed by bath B without and with 20 mg/L SnCl2, respectively. Fig. 3 A–D: SEM micrographs of coatings from a bath A followed by bath B with different concentration of SnCl2 (Insets are the size distribution of the nodules. The abnormal nodules are not counted) (A) 0, (B) 5, (C) 10, and (D) 20 mg/L; E and F are the EDX spectra for A and D, respectively. Fig. 4 AFM images of coatings (A) without and (B) with 20 mg/L SnCl2 in the plating bath and (C) the corresponding profile extractions. Fig. 5 XRD patterns of coatings obtained from a bath A followed by bath B with different concentration of SnCl2 (A) 0, (B) 5, (C) 10, and (D) 20 mg/L. Fig. 6 OCP–time curves recorded for substrate and different nickel coating in 3.5% NaCl solution. (A) AZ31 Mg alloy, (B)–(E) are the coatings obtained from a bath A followed by bath B with different concentration of SnCl2 (B) 0, (C) 5, (D) 10, and (E) 20 mg/L, respectively. Fig. 7 (A) PDP curves of Mg alloy and coatings obtained from bath A followed by bath B with different concentration of SnCl2 in 3.5% NaCl solution. (B) Inhibition efficiency of coating calculated by corrosion current density based on equation (1). Fig. 8 Nyquist plots (A) and Bode plots (B and C) of substrate and coatings in 3.5% NaCl solution and (D) equivalent circuits used for EIS fitting. Schematics a and b in D are respectively the equivalent circuits for substrate (Rs(QdlRct(RLMg+LMg+(RLL)))) and coatings (Rs(Qc(Rc(QdlRct)))). Insets are partly enlarged portion. The EIS spectra were recorded at the OCP which were obtained after immersion of 10 minutes in corrosive media. The OCP are respectively around 1.45 VSCE for substrate and 0.50, 0.50, 0.43, and 0.40 VSCE for coatings when the SnCl2 concentration ranged from zero to 20 mg/L. Fig. 9 Hydrogen evolution of Ni coating in 3.5% NaCl solution and SEM images after 7 days of immersion (I: without SnCl2 in plating bath B; II: with SnCl2 in plating bath B. Insets are the corresponding optical pictures of the coatings). The Y axis is the specific volume (ml·cm2) which equals to the ratio of the total volume of hydrogen (ml) to the surface area of the Mg alloy (cm2). Fig. 10 XPS analysis of Ni coatings after immersion in NaCl solutions: (A) Full spectra of (a) Ni– P/Ni–Sn–P and (b) Ni–P/Ni–P coatings (Inset is the high-resolution spectrum for detecting Sn). (B)

22

and (C) are the high-resolution spectra of Ni 2p for Ni–P/Ni–Sn–P and Ni–P/Ni–P coatings, respectively. (D) is the high-resolution spectrum of P 2p for both coatings.

Fig. 1

Fig. 2

23

Fig. 3

Fig 4

24

Fig 5

Fig 6

25

Fig. 7

Fig 8

26

Fig 9

27

Fig. 10

28

Table 1: Pre-treatment solutions for degreasing and activation of Mg alloy, plating baths with and without fluoride, and their operation conditions (C6H8O7·H2O is citric acid). Processes*

Bath compositions

Conditions

1. Ultrasonically degreased

Acetone

25 oC, 10 min

2. Alkaline cleaning 3. Pickling

NaOH Na3PO4·12H2O H3PO4

50 g/L 10 g/L 400 g/L

60  5 oC 10 min 25 oC, 1 min

4. Activation

HF

350 mL/L

25 oC, 8 min

20 g/L 20 g/L 5 g/L 12 mL/L 10 g/L 1 mg/L App. 30 mL/L 20 g/L 20 g/L 5 g/L 1 mg/L 0-50 mg/L App. 30 mL/L

85 oC pH=5.5~ 6.0 1h

5. Electroless Ni plating bath 5.1 Bath with fluoride (Bath A)

NiSO4·6H2O NaH2PO2·H2O C6H8O7·H2O HF (40%) NH4HF2 CS(NH2)2 NH3·H2O (25 %) 5.2 Bath without fluoride NiSO4·6H2O (Bath B) NaH2PO2·H2O C6H8O7·H2O CS(NH2)2 SnCl2 NH3·H2O (25 %) * Rinse with distilled water in each step.

pH adjustor 80 oC pH=5.5~ 6.0 1h

pH adjustor

Table 2: The adhesion of Ni–P/Ni–Sn–P coatings obtained under different concentration of SnCl2. Concentration/(mg/L) ※

0 O

10 O

20 O

30 Δ

40

50

  The symbol “O” stands for excellent adhesion which no coating could be exfoliated from the sample surface, “Δ” indicates a small coating swell occurring occasionally in some samples but no Adhesion ※

peeling off and “” represents a coating with very poor adhesion and continuously sloughed-off.

29

Table 3: The results of PDP test of AZ31 Mg alloy and different coatings in 3.5% NaCl solution. The symbol S stands for Mg alloy substrate, C0–C20 are the Ni–P/Ni–Sn–P coatings. These coatings were deposited in bath A followed by bath B with different concentration of SnCl2 (C0: 0, C5: 5, C10: 10, and C20: 20 mg/L).

Samples

corrosion potential (VSCE)

corrosion density (µA/cm2)

S

1.46

870

68

406

C0

0.50

6.8

571

516

C5

0.50

5.7

548

430

C10

0.43

5.5

519

366

C20

0.40

5.2

505

453

current anodic slope (mV/dec)

Tafel

cathodic slope (mV/dec)

Tafel

Table 4: Calculated parameters of the elements of the equivalent electrical circuit for the AZ31 Mg alloy and Ni–P/Ni–Sn–P coatings in 3.5% NaCl solution χ2 Samples* Rs Qdl Rct RLMg+ LMg+ RL L Qc Rc |Z|f=0.1HZ (×103) S 9.7 12.5 45.3 63.5 6.4 18.3 54.2 N/A N/A 28.4 4.5 C0

5.4

8.1

2.8×104

N/A

N/A

N/A

N/A

4.7

10.7

2.6×104

0.3

C5

3.1

7.4

3.2×104

N/A

N/A

N/A

N/A

4.3

9.4

2.9×104

0.3

C10

2.8

9.8

3.5×104

N/A

N/A

N/A

N/A

3.9

10.4

3.1×104

0.4

10.3

4.7×104

11.7

4.0×104

0.4

C20 *

2.0

The unit for R is

Ωcm2,

for Q are

N/A

N/A

Ssn/cm2×106,

30

N/A L are

N/A

Hcm2.

3.0