Ni(OH)2-ceramic nanoparticle composite coatings

    Improvement of the erosion-corrosion resistance of magnesium by electroless Ni-P/Ni(OH)2 -ceramic nanoparticle composite coatings J.A. Calder´on, J.P. Jim´enez, A.A. Zuleta PII: DOI: Reference:

S0257-8972(16)30323-1 doi: 10.1016/j.surfcoat.2016.04.063 SCT 21138

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

14 December 2015 19 April 2016 27 April 2016

Please cite this article as: J.A. Calder´on, J.P. Jim´enez, A.A. Zuleta, Improvement of the erosion-corrosion resistance of magnesium by electroless Ni-P/Ni(OH)2 ceramic nanoparticle composite coatings, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.04.063

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ACCEPTED MANUSCRIPT Improvement of the Erosion-Corrosion Resistance of Magnesium by Electroless NiP/Ni(OH)2-Ceramic Nanoparticle Composite Coatings

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J. A. Calderón1*, J. P. Jiménez1, A. A. Zuleta2

Centro de Investigación, Innovación y Desarrollo de Materiales – CIDEMAT, Universidad de 2

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Antioquia, Carrera 53 N◦61-30, Medellín, Colombia Grupo de Investigación de Estudios en Diseño - GED, Facultad de Diseño Industrial,

Universidad Pontificia Bolivariana, Sede Medellín, Circular 1 Nº 70-01, Medellín, Colombia

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* Corresponding author: +574-2196616

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E-mail address: [email protected] (J. A. Calderón)

ABSTRACT

In this study, Ni-P/Ni(OH)2-Ceramic nanoparticle composite coatings were directly deposited

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onto commercially pure magnesium in order to improve its resistance to erosion-corrosion

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damage. The effect of three incorporated ceramic nanoparticles (TiO2, SiC and diamond) on the erosion-corrosion resistance of the composite coatings was also investigated. The composite

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coatings were obtained by an electroless process and were characterized using scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The erosion-corrosion behavior of fabricated composite coatings was elucidated using in situ techniques of potentiodynamic

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polarization and electrochemical impedance spectroscopy (EIS). It was found that the simultaneous formation of Ni-P and β-Ni(OH)2 phases in the coating and the improvement in the micro hardness of the coating were due to the incorporation of nanoparticles. According to the polarization curves and EIS spectra, the β-Ni(OH)2 compound behaves like a pre-passive film which is responsible for substantial improvement in the anticorrosion properties of the coating. Better erosion-corrosion resistance was obtained for the composite coatings than the neat Ni-P coating. This was a consequence of the β-Ni(OH)2 co-deposition. The formation of the βNi(OH)2 compound in the coating does not depend on the nature and concentration of the nanoparticles.

Keywords: Magnesium, Ni-P Electroless, composite coating, erosion-corrosion test, ceramic nanoparticle

ACCEPTED MANUSCRIPT 1. Introduction

Magnesium and its alloys are light materials with excellent physical and mechanical properties

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such as heat conductivity and electromagnetic shielding effectiveness. They have the advantages

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of high specific strength and anti-shock resistance. For these reasons, magnesium is a promising

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light metal for use in several industrial applications. It is indispensable for the aerospace, sports and automotive industries and also for manufacturing electrical equipment such as cellular phones and television sets [1]. In spite of these attractive properties, it has poor surface performance in terms of chemical reactivity and the mechanical properties affecting its resistance

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against erosion-corrosion. Therefore, its application is limited in some cases and it is necessary to provide surface treatment in order to improve wear and corrosion resistance.

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Weight reduction for some applications in erosion-corrosion environments is needed to enhance the functional performance of the product. Magnesium and its alloys are of enormous interest due to mass reduction (30 % higher than aluminum), impact resistance, fatigue resistance and

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shock and vibration absorbance. Therefore, magnesium alloys are a viable material to be used in

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fields such transportation and sports. However, magnesium and its alloys suffer from high chemical/electrochemical activity. Corrosion failures have been encountered in humid

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environments, fresh water, seawater, most organic acids and their salts and inorganic acids and their salts due to a high susceptibility to corrosion. In the automotive and sports industries, the existence of particulate material as well as tap water or seawater is common. These favor the

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presence of erosion-corrosion. Some other components that are subject to erosion-corrosion are: the chassis of in-line skates, bicycle motocross frames, pedals for bikes, tubing for the longitudinal pieces in some kayaks and frames of protection goggles.

There are many techniques for the surface treatment of light metals. The most common surface treatments performed on magnesium include electrochemical plating [2–4], conversion coatings [5–7], anodizing [8], gas-phase deposition processes [9,10], laser surface alloying [11] and organic coatings [8]. However, some of these processes [2,4–6] involve the use of hexavalent chrome. Considering the relevant environmental concerns associated with hexavalent chrome, protective coatings involving chromium-free processes are common in modern technology. Electroless plating is one of the most suitable methods for depositing metal since deposition is carried out uniformly, crack-free and with good adhesion onto magnesium and its alloys without applying an external electrical circuit, giving coatings with good wear and corrosion resistance

ACCEPTED MANUSCRIPT [12]. Nickel composite coatings have been developed because advances in technology require materials with high resistance to more aggressive environments and faster processing systems. The ability to co-deposit particles into the nickel matrix has created new interest in the research

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of composite coatings. The co-deposition of a second phase may improve surface properties such

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as uniformity, hardness, wear and corrosion resistance [13–17]. Even though electroless nickel

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(EN) coatings have been widely studied and applied to magnesium alloys, there are few authors that have studied pure magnesium [12,18–22]. In our lab much research using electroplating and electroless methods has focused on the development of composite coatings incorporating SiC and diamond nanoparticles in the metallic matrix, giving enhanced resistance to corrosion and

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wear in salty and erosive environments [13]. Furthermore, several researchers have found that enhancement in the corrosion resistance of nickel coatings increases with the particle content.

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This is because the incorporation of the nanoparticles produces grain refining, leading to further improvements in the corrosion resistance of the nickel coating [13,23–26].

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Novakovic and Vassiliou [27] studied the properties of Ni-P-TiO2 electroless coatings deposited

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on carbon steel. They found that although the incorporation of ceramic microparticles improves the micro-hardness of the coating, a decrease in the anti-corrosion performance of the coating

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was observed. Hui Xu et al. [25] found that the corrosion and wear resistances of a Ni-Pnanometer diamond composite coating obtained by the electroless technique was superior to that of the Ni-P coating. Similarly, tribological benefits (wear resistance, a high hardness and a low

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friction coefficient) have been reported by other authors in studies performed on electroless NiP-nanoceramic particle composite coatings [28–30]. Mazaheri et al. [31], found that the incorporation of diamond nanometer particles improved the corrosion resistance of the coatings more than the Ni-P matrix, but increasing the concentration of the nanoparticle suspension up to 1 g L-1 resulted in a decrease in polarization resistance. The higher content of nanometer particles induced particle agglomeration and increased the porosity of the coating. He et al. [32] found that mechanical attrition applied during Ni-P electroless plating on magnesium alloys could improve the anti-corrosion properties of the metal. This is because the Ni-P coating became smooth, compact, had refined grains and was free from cracks and pores. Similarly, Islama et al. [33], found that the incorporation of SiO2 nanoparticles into the Ni-P induced nodule refinement, reduced the surface roughness and diminished the surface porosity in the coatings. The kinetics of the Ni-P electroless deposition can be modified by the incorporation of ceramic nanoparticles. Sarret et al. [34], found that during the Ni-P electroless deposition the

ACCEPTED MANUSCRIPT presence of SiC nanoparticles reduced the rate of deposition and modified the coating morphology. Conversely, Gay et al. [35], found that the deposition rate of the Ni-P coating increased when particle concentration was increased by 30 g L-1. A further increment of particle

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concentration reduced the deposition rate.

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Although corrosion resistance has been widely studied, there is still little literature directly assessing the cavitation and erosion issues in coatings. The advantages of the EN technique with respect to conventional Ni electroplating in producing composite coatings are well known. However, there is not enough literature about the reactional mechanism that causes ceramic

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nanoparticle incorporation in the formation of the Ni coating. Kamath et al. [36], extended the electroless method to the preparation of nickel hydroxide, which is the subject of many studies

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because of its application as an electrode material in rechargeable battery electrodes. According to Salvago and Cavalloti [37], nickel hydroxide may be obtained by a product reaction of the

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electroless bath under specific or special conditions.

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This work aims to establish a technique to obtain a Ni-P-Ni(OH)2 autocatalytic coating on magnesium substrates. Ceramic nanoparticles are incorporated to enhance surface properties

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using electroless plating without including the use of chromium in the surface preparation process. A reactional mechanism for the simultaneous deposition of Ni-P and Ni(OH)2 is also proposed. The characterization of the coatings was made through SEM/EDX analysis, Raman

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spectroscopy and X-ray diffraction analysis. The performance of the coating against erosioncorrosion was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PP).

2. Experimental phase

2.1.Sample preparation

Electroless Ni-P solutions were prepared according to the methodology established in previous work [38]. The electroless bath contained nickel sulphate as a nickel source and sodium hypophosphite as a reducing agent. Also, propionic, lactic and succinic acids were used as complexing agents and stabilizers. All chemicals used were pro-analysis grade. 13.3 g L-1 of NH4HF2 was added to the alkaline electroless bath. The chemical composition and operating

ACCEPTED MANUSCRIPT conditions of the baths are given in Tables 1 and 2. The electroless Ni-P and Ni-P-ceramic nanoparticles were deposited onto commercially pure magnesium substrates (99.9%) that were embedded in a commercial unsaturated polyester resin with an exposed working area of 1 cm2.

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Before the deposition, substrates were mechanically polished with 100 grit SiC paper to reduce

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the roughness of the surfaces caused by cutting the specimens. In order to enhance the surface

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reactivity, the specimens were sandblasted with a RENFERT-basic classic 2945–4025 microsandblasting apparatus containing alumina particles (~150 µm) and with a pressure of 0.4 MPa. After that, the substrates were cleaned ultrasonically for 900 s in ethanol and dried in a stream of warm air. The specimens were then immediately etched in an alkaline solution of 37 g L-1 NaOH

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and 10 g L-1 Na3PO4 for 600 s at 60°C in order to remove the surface oxide. Finally they were rinsed with deionized water and dried in a stream of warm air. The procedures were carried out

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in a laboratory at room temperature (~20 °C), with a relative humidity of ~70%. The cleaned specimens were transferred immediately to the fresh alkaline electroless solutions, in accordance with the operational conditions given in Tables 1 and 2. The electroless process was carried out

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in four steps with different conditions for the electroless nickel bath in each step. This was done

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according to previous studies performed in our lab [21]. The first step was to use an optimized alkaline bath as pretreatment for obtaining a baseline to generate a Ni-P coating without

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excessive dissolution of the magnesium substrate. The second step dealt with obtaining a deposit with higher resistance to corrosion by means of a slightly acidic bath, which formed a high phosphorus content coating. The third step was performed to obtain a multilayer coating with a

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different phosphorus content, as this enhances the anti-corrosion behavior [39,40]. Finally, the last step aimed to incorporate the ceramic nanoparticles using the same composition as the third bath. Fresh specimens and electroless baths were used for each treatment. Ceramic nanoparticles were added in the last stage to the plating bath (bath 4) in concentrations of 0.2 and 2.0 g L-1, representing low and high particle concentrations respectively. The particles used were TiO2 anatase phase (25 nm), diamond (4 nm) and SiC (25-50 nm), which gave Ni-P-TiO2, Ni-P-D and Ni-P-SiC composite coatings. The ceramic nanoparticles were ultrasonically dispersed for 60 s using a SONICS - Vibra-Cell VCX130 high power ultrasonic probe apparatus in a fresh electroless solution (bath 4) before the deposition.

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Table 1. Bath compositions used in the electroless process. Concentration (g L-1) Name

Formula

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Nickel sulphate hexahydrate

NiSO4.6H2O

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Baths 1, 2 Baths 3, 4 30.0

Lactid acid

C3H6O3

26.5*

9.0*

Propionic acid

C3H6O2

2.2*

---

Succinic acid

C4H6O4

12.0

---

NH4HF2

13.3

---

24.2

51.0

*Concentration in mL L-1

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2.2.Characterization of the coatings

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Ammonium hydrogen bifluoride

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Sodium hypophospite monohydrate NaH2PO2.H2O

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The morphological and semi-quantitative chemical analyses of the coatings were performed by scanning electron microscopy and X-ray energy dispersive spectroscopy (SEM-EDS) with a

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JEOL JSM 2490 CV device. X-ray diffraction (XRD) analysis was performed using a Panalytical MDP Expert-pro instrument with the configuration θ = 2θ (Bragg Brentano) and monochromatic Cu Kα radiation. Raman spectroscopy analyses were performed in order to

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assess the composition of the coating. Raman spectra were determined using a HORIBA-JobinYvon spectrometer system with excitation from a He–Ne laser source. The Raman spectra were obtained over a range of 100–3000 cm-1. Hardness measurements were carried out on the cross section of the coatings, using micro Knoop diamond indentation under 25 gf loads. The average value taken from 10 different measurements was reported for each specimen.

2.3.Erosion-corrosion tests

The experimental procedure for the erosion-corrosion tests was based on previous work [13]. The erosion-corrosion tests were performed in a NaCl 0.5 mol L-1 solution with the addition of silica particles (SiO2) of 300 µm in size (silica particle concentration = 20% w/w) and in aerated conditions. A volume of 1.2 L of the solution was used in the tests. The abrasive solution of

ACCEPTED MANUSCRIPT NaCl + SiO2 was driven by a disk (7 cm diam.) acting as an agitator impeller, rotating at a constant rate of 1500 rpm. With this apparatus the SiO2 particles were ejected to the working electrode at a linear velocity of 0.9 m s−1. The working electrode was located 3 cm from the

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agitator impeller. The impact angle of the abrasive solution was set at 90° to the sample in order

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to achieve the highest abrasive action. The setup details of the erosion–corrosion test can be

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found at [13]. During the erosion–corrosion test, in situ Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PP) measurements were performed. EIS and PP measurements were made using coated magnesium samples as working electrodes, a saturated calomel electrode as the reference and a platinum plate as an auxiliary electrode. EIS

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measurements were performed at open circuit potential (OCP) using a perturbation amplitude of 10 mV (rms) and a frequency range of 100 kHz to 5 mHz. The potentiodynamic polarization

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curves were generated from 200 mV of cathodic overpotential to 600 mV of anodic overpotential with respect to the OCP, using a scan rate of 0.166 mV s-1. Before initiating the electrochemical measurements, the OCP was monitored for approximately 1 h to achieve a constant value.

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Electrochemical impedance measurements were then made at the OCP. After that, polarization

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was carried out at a low rate (0.166 mV s-1) in order to attain pseudo-stationary conditions. The electrochemical measurements were made in triplicate and their reproducibility was confirmed.

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EIS and PP were measured using a BAS-ZAHNER IM6e potentiostat–galvanostat.

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3. Results and discussion

3.1.Compositional, morphological and structural analysis

3.1.1. Surface analysis Figure 1 shows SEM micrographs and EDX analyses of electroless Ni-P and Ni-P-ceramic nanoparticle composite coatings on magnesium. The surface of the Ni-P coating shows a typical cauliflower-like structure with good uniformity and dense coverage. However, the Ni-P-ceramic nanoparticle coatings exhibit a refined nodular structure due to the co-deposition of nanoparticles. It can also be observed that the dimensions of these nodules in the composite coatings is smaller than in the Ni-P coating. According to the reports of other researchers, the incorporation of ceramic particles produces nodule refinement in the electroless coating [32– 35,41]. A Similar effect was found in the electroplating process of Ni-SiC composite coatings on steel substrate [13]. It can be said that the incorporation of ceramic nanoparticles increases the

ACCEPTED MANUSCRIPT catalytic active sites on the matrix surface. During the deposition process, nanoparticles that are adsorbed onto the coating surface act as an additional nucleation center in which the Ni-P deposits grow and nanoparticles are embedded. Numerous nucleation centers allow for a

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decrease in the size of the nodules of the Ni-P alloy. As indicated by the EDX results (Figure 1),

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the presence of Ni, P, Ti, O, Si and C elements validates the incorporation of TiO2, SiC and

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diamond ceramic particles into the fabricated composite coatings.

Because of the metallic nature of the Ni-P deposit, the surface does not exhibit the Raman effect

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[42]. However this technique also lets us see evidence of the presence of ceramic nanoparticles. Figure 2 shows the Raman spectra for Ni-P-TiO2 and Ni-P-D coatings. As can be seen in Figure

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2(a), the incorporation of TiO2 nanoparticles is confirmed by the presence of the characteristic bands of the anatase phase of TiO2 at 146, 392, 520 and 639 cm-1 [43,44]. Similarly, in Figure 2(b) the characteristic band for diamond appears at around 1400 cm-1 [45]. Raman spectra for the

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Ni-P-SiC coating were also produced but no bands related to SiC were seen. This situation may

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have happened due to the monochromatic light beam not having the necessary energy to break through the surface and detect the particles which were embedded in the metallic matrix of the

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coating. This could mean that the SiC nanoparticles are not necessarily located in the surface of the coating. The thickness of the Ni-P coating was about 26±1.8μm, while the thickness of the Ni-P-ceramic nanoparticle composite coating was about 35±3.9μm. This result was similar for

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all types of nanoparticle and concentrations in the bath.

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Figure 1. SEM images of the surface, cross section and EDX spectra for the coatings prepared by the electroless technique: (a) Ni-P; (b) Ni-P-TiO2; (c) Ni-P-SiC; (d) Ni-P-D

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Figure 2. Raman spectra of coatings (a) Ni-P-TiO2 and (b) Ni-P-D

3.1.2. X-ray diffraction analysis

Figure 3 shows the XRD analysis pattern of the Ni-P and Ni-P-ceramic nanoparticle composite coatings. The diffraction pattern of the Ni-P coating is shown in Figure 3(a). A broad peak centered at 2 = 45° is shown, which is characteristic of electroless Ni-P deposits. The broad shape of this diffraction peak is attributed to the relatively amorphous character of the coating

ACCEPTED MANUSCRIPT [17,25,27–31]. The small size of Ni-P crystals (in nanometers) obtained by the electroless process has also been reported as another reason for the broad shape of the diffraction peak [46]. Additionally, diffraction peaks located at 37°, 63°, 69° and 77° are related to the magnesium

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substrate [47]. Figures 3(b), (c) and (d) show the diffraction patterns of the Ni-P-ceramic

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nanoparticle composite coating at low and high concentrations of particles. It is interesting to

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observe that, conversely to the Ni-P coating, all of the diffraction patterns of the coatings containing ceramic nanoparticles exhibit a broad peak located at 18°. This peak is distinctive to electroless nickel hydroxide [36]. Since such diffraction peaks only appear in the composite coatings which have nanoparticles incorporated, it is logical to think that particle co-deposition

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induces an additional chemical pathway to that normally followed by the deposition of conventional Ni-P coatings. These results suggest the co-deposition of Ni-P and Ni(OH)2 by

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parallel reactions, which is caused by the incorporation of the ceramic nanoparticles. The broad diffraction peak corresponding to Ni(OH)2 separates the characteristic TiO2 peak from the anatase phase, which is normally seen at 25° [48]. Similarly, the characteristic peak of nano

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diamond, located at 44°, is not observable in Figure 3(d) because of the superposition of the

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broad Ni-P peak at 45° [24]. The diffraction peaks of SiC cannot be detected due to the small size of the SiC particles and their low volumetric fraction in relation to the Ni-P matrix [49,50].

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In spite of the fact that the diffraction patterns of the Ni-P-ceramic nanoparticles did not reveal the diffraction peaks, the incorporation of nanoparticles was confirmed by SEM/EDX chemical analysis and the Raman spectroscopy measurements. The existence of the nickel hydroxide

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compound simultaneously co-deposited together with the Ni-P-ceramic nanoparticles was also confirmed by the Raman spectroscopy measurements using a slow acquisition of the signal and a low power of the laser beam. Figure 4 shows the Raman spectrum of the Ni-P-D coating obtained under these conditions. The characteristic bands of the β-phase of nickel hydroxide located at 429, 488, 601 and 707 cm-1 can be observed. The bands at 429 and 488 cm-1 correspond to Ni–OH stretching vibrations of the Ni2+ species associated with OH- groups. The bands at 601 and 707 cm-1 correspond to Ni–O stretching vibrations of the Ni2+ species associated with O2- [51]. The group factor theory predicts four Raman transitions due to the symmetry of the β-Ni(OH)2 molecule and it has been established that three of the four vibrational modes are found in the ranges of 310−315 cm−1, 445−453 cm−1 and 3581 cm−1 [52]. It is important to take into account that the ceramic nanoparticles that induce the co-deposition of βNi(OH)2 physically get caught in the Ni-P matrix, which can generate strains that lead to the displacement of the diffraction peaks [43,44].

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Figure 3. XRD patterns for the electroless Ni-P composite coatings on magnesium: (a) Ni-P; (b)

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Ni-P-TiO2; (c) Ni-P-SiC; (d) Ni-P-D

Figure 4. Raman spectrum of Ni-P-D composite coating with β-Ni(OH)2 co-deposited.

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3.2

Reaction mechanism of the Ni-P-ceramic nanoparticle composite coatings

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The formation and subsequent co-deposition of Ni(OH)2 after adding nanoparticles can be

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explained through the reactional mechanism proposed by Cavalloti and Salvago [27,43-45]. The

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proposed mechanism initially involves water ionization on the catalytic surface of nickel ions and the coordination of hydroxyl ions with nickel ions. This is then followed by the reduction of the hydroxylated nickel ions which take the charge liberated in the oxidation of the hypophosphite ions. This occurs in a series of successive stages that cause the formation of the

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intermediary adsorbed species NiOHads. The proposed reaction scheme is described as follows

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[37]:

Water ionization on the catalytic nickel surface:

(1)

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2 H2O → 2 H+ + 2 OH-

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Coordination of the hydroxyl ions to the solvated nickel ions: Ni(H2O)6+2 + 2 OH- → [Ni2+(OH-)2](aq) + 6 H2O

(2)

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Reduction of the hydroxylated species by the hypophosphite: [Ni2+(OH-)2](aq) + H2PO2- → NiOHads + H2PO3- + H

(3)

NiOHads + H2PO2- → Ni0 + H2PO3- + H

(4)

Where the NiOHads species is an intermediate hydroxylated species of Ni+ adsorbed onto the catalytic surface. According to the mechanism proposed by Cavalloti and Salvago [54], the incorporation of phosphorus into the coating occurs by direct interaction between the catalytic nickel surface (Nicat) and the hypophosphite ion to give phosphorus reduction: Nicat + H2PO2- → P + NiOHads + OH-

(5)

ACCEPTED MANUSCRIPT At this point, the process for coating formation from electrolytes with and without ceramic nanoparticles is the same. However, according to the experimental results shown after the coating was characterized by XRD analysis and the Raman spectroscopy measurements, it was

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seen that when there are ceramic particles in the electrolyte a simultaneous co-deposition of Ni-P

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and Ni(OH)2 occurs with the incorporation of the nanoparticles into the coating. Ni-P and Ni(OH)2 formation is given by two parallel and competitive reactions. The first considers the

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formation of Ni0 due to the reduction of the intermediary species NiOHads and hypophosphite oxidation corresponding to reaction (4). The second considers the oxidation of the intermediary

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species NiOHads and water reduction, as follows: NiOHads + H2O → [Ni(OH)2]aq + H

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(6)

According to the SEM images shown in Figure 1, particle incorporation occurs at discrete points in the coating and the nanoparticles are enclosed by Ni atoms. Because of this, it can be

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supposed that the ceramic particles are solvated by water molecules and surrounded by Ni2+ ions,

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as is schematized in Figure 5. Once the solvated nanoparticles arrive at the catalytic sites of the

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metal surface, the formation of Ni(OH)2 occurs in accordance with reaction (6).

Figure 5. Diagram of the coating formation by electroless co-deposition of Ni-P-ceramic nanoparticles and Ni(OH)2.

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3.3 Hardness assessment

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The Knoop micro-hardness evaluation was performed on the coatings according to the ASTM

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E384 standard on cross-sectional samples. The standard deviations of the hardness

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measurements were less than 10% with respect to the average value. Figure 6 shows the Knoop hardness values of the samples with their respective standard deviations. The hardness of the NiP-nanoparticle coating is around 100 times higher than that exhibited by the magnesium substrate and it is 25% higher than that shown by the Ni-P electroless coating. Similar behavior

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is reported by other studies [25,27–33]. The hardness increment of the coatings is due to the nature of the ceramic nanoparticles and their incorporation induces the formation of a more

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compact Ni-P deposit with a smaller nodule size than the pure Ni-P coatings. A slight increment in the coating hardness was observed when a lower concentration of the nanoparticles in the electrolyte was used (0.2 g L-1). The highest hardness values of the coatings were obtained when

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the concentration of the nanoparticles in the electrolyte was set at 2.0 g L-1. No influence of the

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nature of the particles on the coating hardness was observed. This indicates that changes in the electroless Ni deposition, like high a deposition rate and nodule refinement induced by the

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incorporation of nanoparticles, are responsible for the improvement of hardness in the coatings. Similar results have been reported by Xu et al. [25], in their studies on Ni-P-nano diamond electroless coatings. Novakovic et al. [27], reported that when lower bath particle concentrations

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(0.5-2.0 g L-1) were used in Ni-P-TiO2 electroless coatings it usually resulted in in better particle dispersion and stronger particle/matrix adhesion, thereby giving a greater hardness. In the current study, SEM analysis (Figure 1) corroborated that the incorporation of the nanoparticles into the coating induced nodule refinement in the composite coating and the rate of the Ni deposition was incremented from 2.45 μm h-1 to 12 μm h-1 when the nanoparticles were dispersed in the bath and incorporated into the coating. This is in agreement with the proposed reaction (6), since the presence of adsorbed hydrogen enhances the deposition rate of the electroless Ni-P coating [37,56]. Similarly, in studies of Ni-P-ZrO2 electroless coatings, Gay et al. [35] reported an increment in the deposition rate from 7.5 to 12.5 µm h-1 when the particle concentration in the bath was incremented by 30 g L-1 at 80°C, which is consistent with the current results. Table 2 shows the evolution of the thickness of the coatings during the plating process.

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Figure 6. Knoop hardness evaluation of the Ni-P-ceramic nanoparticle coatings. A: High

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ceramic nanoparticle content in solution (2.0 g L-1). B: Low ceramic nanoparticle content in

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solution (0.2 g L-1). Load = 25 gf.

Table 2. Evolution of coating thickness during the plating process. Bath

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1 2 3 4 (neat Ni-P) 5 (Ni-P ceramic nanoparticles)

Tickness (μm) 5.5 ±0,7 13±0.7 21.5±1.4 26±1.8 35±3.9

3.4. Evaluation of erosion-corrosion resistance of the coatings by electrochemical methods

In general terms, electroless Ni-P is a barrier coating, protecting the substrate against the deleterious action of corrosive environments. Figure 7 shows the potentiodynamic polarization curves of the samples obtained while the erosion-corrosion test was performed. Table 3 shows the electrochemical parameters calculated from the polarization curves and the impedance diagrams during the erosion-corrosion tests. As expected, pure magnesium presented a high

ACCEPTED MANUSCRIPT negative (-1.623 V) corrosion potential, a high corrosion current density (0.00428 A cm-2) and a low charge transfer resistance (3.1 Ω cm2). This is because of its high reactivity in aqueous and chloride media [8]. Magnesium corrosion is characterized by an anomalous phenomenon called

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the negative difference effect (NDE) [57,58]. When the anodic overvoltage increases, the

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cathodic release of hydrogen increases rather than decreases. Mechanisms of magnesium

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corrosion and the NDE phenomenon were proposed several decades ago. According to Perrault [57] and Petty et al. [58], metastable monovalent ions are produced as intermediate species and react chemically with water to evolve hydrogen. In the current study, it can be confirmed that at potentials close to the Open Circuit Potential (OCP), the cathodic currents are smaller than the

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anodic currents, indicating that there is cathodic control of magnesium corrosion in chloride and aerated solutions. This has also been reported by Curioni et al. [59]. Additionally, due to the

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NDE phenomenon and the instantaneous formation of magnesium hydroxide Mg(OH)2, and/or MgO [51-55], the corrosion current and corrosion rate of magnesium cannot be obtained by the direct extrapolation of the Tafel slopes [59,64,65]. This is better done by using the resistance of

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the high frequency capacitive loop (RHF) observed in the electrochemical impedance diagram at

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the OCP [50,56].

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Figure 8 shows the electrochemical impedance (Nyquist Plot) of pure magnesium recorded during the erosion-corrosion test in the NaCl (0.6 mol L-1) + SiO2 (20%w/w) particle solution. The impedance diagram exhibits two capacitive loops at high and intermediate frequencies.

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These capacitive loops are related to the charge transfer resistance/double-layer capacitance and the corrosion product formation, respectively. Additionally, two inductive loops at low frequencies which relate to the relaxation of the adsorbed species are also observed. Similar impedance diagrams of pure magnesium immersed in chloride are reported by Song et al. [66]. Small values of charge transfer resistance (3.1 ohm cm2) and polarization resistance (15.5 ohm cm2) can be observed located close to each other. The double-layer capacitance and the capacitance of the corrosion products of the magnesium substrate can be calculated from the values of the resistance and characteristic frequencies of the first and second capacitive loops, observed in the experimental impedance diagrams. The capacitance values were 10 µF and 253 µF, which correspond to a free exposed metallic surface and to non-dense corrosion products, respectively. The corrosion current density and the corrosion rate of pure magnesium calculated using the RHF were 4.28 mA cm-2 and 97.81 mm y-1, respectively. These are in agreement with the values reported in the literature [59]. The high corrosion rate of the magnesium obtained in

ACCEPTED MANUSCRIPT the current experiments was a consequence of the synergistic deleterious effects of the erosion and corrosion processes.

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As can be seen in Figure 7, the covering of pure magnesium with Ni-P and Ni-P-ceramic

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nanoparticle coatings leads to a displacement of the corrosion potential of the samples to more

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positive values (-0.32 V vs. SCE for all the coated specimens), thereby generating a less reactive surface in the electrolyte. Also, the corrosion current densities decrease by two orders of magnitude compared to pure magnesium, as can be seen in Table 3. These results provide evidence that the coatings obtained effectively protect the surface of the substrate against the

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erosion-corrosion phenomenon. The higher corrosion resistance of the Ni-P coatings is a result of their amorphous nature. Amorphous alloys offer better resistance to corrosion attack than the

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equivalent polycrystalline materials because they are free of the crystalline grain and grain

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boundaries at which the corrosion process can start.

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Figure 7. Polarization curves registered during erosion-corrosion tests for samples of Ni-Pceramic nanoparticle composite coatings at different concentrations of nanoparticles. (a) low

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concentration (0.2 g L-1) and (b) high concentration (2.0 g L-1). For corrosion-erosion tests the

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samples were immersed in a solution of NaCl (0.6 mol L-1) + SiO2 particles (20%w/w).

The Ecorr of the samples was obtained directly from the polarization curves. The Icorr value for magnesium substrate was achieved by using the resistance of the high frequency capacitive loop

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(RHF) observed in the electrochemical impedance diagram at the OCP, as explained previously. The Icorr value for the Ni-P coating was achieved by the Tafel extrapolation method, while for Ni-P-nanoparticle coatings the Icorr values were assumed to be equal to the passivation current and were read directly from the polarization curves. This because the passivation process of composite coatings occurs at just 50 mV of anodic polarization. In these conditions the Tafel extrapolation procedure does not apply. The corrosion rates of the samples were calculated from the Icorr values using the Faraday relationship.

Table 3. Electrochemical parameters calculated from the polarization curves and impedance diagrams during the erosion-corrosion tests. System

Particle Conc. ECORR vs. SCE (g L-1)

(V)

JCORR (A cm-2)

RP

VCORR

(Ω cm2) (mm y-1)

ACCEPTED MANUSCRIPT Pure magnesium

--

-1.623

4.28E-3**

3.1*

97.81**

Ni-P

--

-0.344

1.82E-04

650

4.20

0.2

-0.302

5.25E-05

1650

1.20

2.0

-0.327

5.29E-05

1750

1.21

0.2

-0.333

4.06E-05

1900

0.93

2.0

-0.316

5.03E-05

2200

1.15

0.2

-0.319

5.91E-05

1600

1.35

2.0

-0.338

6.05E-05

1250

1.38

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Ni-P-TiO2

Ni-P-D

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Ni-P-SiC

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* is the resistance of the high frequency capacitive loop (RHF) exhibited by electrochemical impedance

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** Calculated using the (RHF)

Figure 8. Nyquist plot of electrochemical impedance registered during the erosion-corrosion test for pure magnesium immersed in NaCl (0.6 mol L-1) + SiO2 (20%w/w) particle solution. For corrosion-erosion tests the samples were immersed in a solution of NaCl (0.6 mol L-1) + SiO2 particles (20%w/w).

There is a passivation phenomenon at low anodic overpotentials (50 mV) that only occurs in the polarization curves of the Ni-P-nanoparticle coatings. The Ni-P coating does not show any passivation process. This means that the active-passive transition of the Ni-P-nanoparticle coatings is promoted by nanoparticle incorporation. The active-passive behavior of nickel in aqueous media has been explained by the formation of a duplex layer passive film which consists

ACCEPTED MANUSCRIPT of a NiO inner layer and Ni(OH)2 outer layer [67]. The formation of this duplex layer follows the

Ni(I)[Ni(OH)] → Ni(I)[NiOH+] + e-

(8)

Ni(I)[NiOH+] + H2O → Ni(I) + Ni(OH)2 +H+

(9)

Ni(I)[Ni(OH)2] → Ni(I)[NiO] + H2O

(10)

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(7)

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Ni + H2O → Ni(I)[Ni(OH)] + H+ + e-

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reaction mechanism [68]:

Consistent with the reaction mechanism above, the complete passivation of the surface observed

previously formed Ni(OH)2 pre-passive film.

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in the Ni-P-nanoparticle coatings occurs by the formation of the NiO inner layer from the

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There is great discrepancy in the literature with respect to the corrosion resistance of the Ni-Pceramic particle electroless coatings. Many researchers have reported that the incorporated particles improve the corrosion resistance [13-16,18–20], whereas other studies contradict such

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observations [27,69]. The results of the current study show that electroless Ni-P-ceramic

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nanoparticle composite coatings have superior corrosion resistance when compared to Ni-P coatings. It is possible that the unsuccessful corrosion resistance results reported by some

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researchers are related to problems in the dispersion of the ceramic particles, excessive concentration of the particles in the bath or the large particle size. These experimental problems can produce particle agglomeration and microcracks and increment the porosity of the coating,

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establishing conditions which create pathways for the corrosive solution to enter and attack the substrate. On the contrary, in the current study it is evident that the resistance of the Ni-Pceramic nanoparticles to erosion-corrosion is almost 4 times superior to that of the Ni-P coating. As can be seen in Figure 7 and Table 3, the anodic corrosion current densities of the composite coatings are lower than those exhibited by the Ni-P coating. Similarly, the impedance diagrams of the Ni-P-ceramic nanoparticles show larger polarization resistance than that exhibited by the Ni-P coating. Additionally, the composite coatings developed in this work as anti-corrosion systems for magnesium can behave better than other systems for corrosion protection reported in the literature [6,70,71].

Figure 9 shows the impedance diagrams obtained at open circuit potential for the Ni-P coating and Ni-P-ceramic nanoparticle composite coatings at low and high concentrations of nanoparticles. In general terms, the impedance diagrams exhibit similar characteristics,

ACCEPTED MANUSCRIPT displaying a depressed capacitive loop at high and intermediate frequencies followed by an inductive loop at low frequencies. The depressed capacitive loop combines the charge processes of the electric-double layer and the corrosion product formation processes. Impedance diagrams

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registered for the Ni-P and Ni-P-ceramic nanoparticle coatings during the erosion-corrosion test

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do not exhibit loops associated with the dissolution of the magnesium substrate, which is an

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indication of the protection characteristics and the homogeneity of the coating. Regarding the Rp values of the Ni-P-ceramic nanoparticle composite coatings shown in Table 3, it can be verified that the composite coatings obtained in both low and high nanoparticle concentrations develop higher polarization resistance (Rp) than the Ni-P coatings. These results are coherent with those

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obtained from the polarization curves. The excellent anti-corrosion performance of the

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composite coatings is maintained even 24 h after the erosion-corrosion test.

Figure 9. Nyquist plots of electrochemical impedance registered during erosion–corrosion tests for Ni-P and Ni-P-ceramic nanoparticle composite coatings at different concentrations of nanoparticles. (a) low concentration (0.2 g L-1) and (b) high concentration (2.0 g L-1). For the

ACCEPTED MANUSCRIPT corrosion-erosion test the samples were immersed in a in a solution of NaCl (0.6 mol L-1) + SiO2 particles (20%w/w).

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The depressed capacitive loop observed in the impedance diagrams of coated magnesium

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samples can be attributed to the distribution of time constants over the area of the samples. The

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distribution of time constants could arise from surface heterogeneities such grain boundaries and different crystal faces on a polycrystalline sample [72], like Ni-P or Ni-P-nanoparticle coatings. The depressed capacitive loop observed in the impedance diagrams of the Ni-P and Ni-Pnanoparticle samples was fitted using a cascade electrical circuit with two time constants

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[R1Q1(R2Q2)], which is normally used to describe the reaction mechanism of a coated metallic surface. R1 and Q1 represent the resistance and Constant Phase Element (CPE) respectively of

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the oxide layer, while R2 and Q2 represent the resistance and CPE of the substrate. The CPE elements take into account the distribution of the time constant arising from surface heterogeneities and comprise the effective capacitance of the oxide layer (Ceff-1) and double-layer

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capacitance of the substrate (Ceff-2). The parameters a1 and a2 are the exponential term of CPE1

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and CPE2, respectively. When the parameters a1 and a2  1, the system shows behavior attributed to the surface heterogeneities. The inductive loop observed in the experimental

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impedance diagrams was not considered when fitting with the electrical circuit because it does not represent any physical meaning in the process.

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The (Ceff-1) and (Ceff-2) values were calculated from the parameters of the CPE, as reported by Hirschorn et al. [72]. Given the heterogeneous distribution of the oxide layer, to calculate the effective capacitance (Ceff-1) the normal distribution equation was used. To calculate the effective capacitance of the substrate, the surface distribution was used, given that the electrode surface (heterogeneities of the metal/oxide interface) influences the admittance.

Figure 10 shows the electrical circuit used to fit the depressed capacitive loop observed in the impedance diagrams of the Ni-P and Ni-P-nanoparticle samples, while Table 4 shows the values of the fitting parameters. The good fit indicates good correlation between experimental and theoretical values of electrochemical impedance for the depressed capacitive loop. Furthermore, errors of the values were lower than 10%. The Rp was calculated as Rp=R1+R2-Rinduct., and the obtained values of Rp fit adequately to those observed in the experimental impedance diagrams shown in Table 3.

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ACCEPTED MANUSCRIPT

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Figure 10. Electrical circuit model used to fit the depressed capacitive loop observed in the

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impedance diagrams of the Ni-P and Ni-P-nanoparticle samples.

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Table 4. Values of the electrical parameters used to fit the depressed capacitive loop observed in

Sample

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the impedance diagrams of the Ni-P and Ni-P-nanoparticle samples.

R1

(Ceff-1)

2

-2

µF cm

Ni-P

587

61.5

Ni-P-TiO2 (a)

1420

R2 a1

(Ceff-2) 2

-2

a2

Quality of fit x10-3

(ohm cm )

µF cm

0.87

469

300.0

0.72

3.45

56.1

0.91

780

412.6

0.69

0.56

1240

59.7

0.92

863

171.6

0.67

1.28

1710

50.8

0.90

540

634.8

0.79

0.60

Ni-P-SiC (b)

1390

73.9

0.85

837

601.6

0.55

4.76

Ni-P-D (a)

1730

55.6

0.90

553

248.8

0.81

1.54

Ni-P-D (b)

1530

55.5

0.86

470

81.6

0.65

3.67

Ni-P-TiO2 (b) Ni-P-SiC (a)

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(ohm cm )

Electrical Parameter

The outstanding resistance of the composite coatings against the deleterious action of erosioncorrosion is due to two special characteristics developed by the incorporation of the ceramic nanoparticles into the Ni-P coatings. The first is related to the nodule refinement and compactness of the Ni-P electroless coating produced by the incorporation of the nanoparticles. The second is related to the simultaneous co-deposition of the β-Ni(OH)2 that is also induced by

ACCEPTED MANUSCRIPT the nanoparticle incorporation. The β-Ni(OH)2 compound is responsible for the passivation phenomenon observed in the anodic branch of the polarization curves of the Ni-P-ceramic nanoparticle coatings at only 50 mV of anodic overpotential (see Figure 7). Conversely, samples

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of Ni-P coatings do not exhibit passivation. The drop in corrosion current density at low anodic

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overpotentials exhibited by Ni-P-nanoparticle composite coatings during erosion-corrosion is

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characteristic of the passivation process. This was also corroborated by the electrochemical impedance performed at 50 mV of anodic overpotential. Figure 11(a) shows the electrochemical impedance of the Ni-P-D composite coating at its passivation potential (50 mV) registered during erosion-corrosion tests. It can be seen that the impedance diagram exhibits large

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impedance values at low frequencies and a negative resistance at the zero-frequency limit. This is characteristic of a very passive surface as a consequence of the formation of a barrier film.

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Similar behavior was observed for all composite coatings at low and high ceramic nanoparticle concentrations in the electroless bath. The formation of the film of β-Ni(OH)2 does not depend on the nature or the concentration of the nanoparticles used in the electroless coating. The

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formation of β-Ni(OH)2 films by the electroless technique has already been reported by

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Cavallotti [53,54] and Kamat [36]. However, to the knowledge of the authors of this article, the simultaneous co-deposition of Ni-P and β-Ni(OH)2 induced by ceramic nanoparticle

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incorporation has not been reported so far. The passive behavior of the β-Ni(OH)2 compound has been described by several researchers [73–77]. Figure 11(b) shows the Raman spectra of the NiP-D composite coating after the erosion-corrosion test. The two main bands observed at 551 cm-1

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and 1091 cm-1 are related to the NiO compound [78,79], while small bands at 380 cm-1 and 450 cm-1 are associated to Ni(OH)2. These bands are slightly displaced because of the presence of NiO. Figure 11(c) shows the SEM image of the surface sample after the erosion-corrosion test. It can be seen that the surface is homogeneous and there is no evidence of the deleterious action of the erosion-corrosion process. According to surface characterization performed by X-ray diffraction and Raman spectroscopy and the electrochemical behavior (polarization curves and electrochemical impedance) of the NiP-ceramic nanoparticle coatings, the β-Ni(OH)2 film gave protection against erosion-corrosion at the OCP. The existence of the β-Ni(OH)2 film makes complete passivation of the surface possible at a very low anodic overpotential (50 mV), as can be seen in the polarization curves. The good anticorrosive performance of the Ni-P-nanoparticle composite coatings does not depend on the type or concentration of the nanoparticle used in the electroless bath. This

ACCEPTED MANUSCRIPT indicates that the passivation phenomenon observed during anodic polarization is independent of the nature of the particles and is directly related to the formation of the β-Ni(OH)2 species, which was formed in a similar way to TiO2, SiC and Diamond particles at low and high concentrations

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in the bath.

Figure 11. Characterization of Ni-P-D composite coating. (a) EIS Nyquist plot at passivation potential (50 mV). (b) Raman spectrum and (c) SEM image of the sample after the erosioncorrosion test.

In this study, improvement in the erosion-corrosion resistance and mechanical properties of the Ni-P composite coatings has been achieved by the incorporation of ceramic nanoparticles. Also, the chemical changes of the coating surface because of the formation of β-Ni(OH)2 as a prepassive film are reflected in the remarkable improvement in erosion-corrosion resistance. The enhancement in the anti-corrosion performance of the composite coatings was confirmed by comparative visual inspection of the pure magnesium substrate, the Ni-P coating and the Ni-Pceramic nanoparticle composite coatings obtained after exposure to the erosion-corrosion tests.

ACCEPTED MANUSCRIPT Regarding Figure 12(a), it can be seen that corrosion is widespread over the entire exposed surface of pure magnesium, while pitting corrosion can be observed on the Ni-P coating sample (Figure 12(b)). On the other hand, in spite of the severity of the test, there is no indication of

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corrosion on the surface for any of the Ni-P-ceramic nanoparticle composite coating samples

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(see Figures 12(c)-(e)). Visual inspection of the samples after the erosion-corrosion test confirms

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the electrochemical results.

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Figure 12. Images of (a) the magnesium substrate, (b) Ni-P, (c) Ni-P -TiO2, (d) NiP-SiC and (e) NiP-D coating samples after EIS and polarization measurements during the erosion-corrosion test in a solution of NaCl (0.6 mol L-1) + SiO2 particles (20%w/w).

4. Conclusions

The deposition of Ni-P autocatalytic coatings modified with ceramic nanoparticles on commercially pure magnesium through an electroless process with chromium-free surface preparation was developed. The excellent performance of the Ni-P-ceramic nanoparticle composite coatings against the deleterious action of erosion-corrosion was demonstrated. Such behavior is due to two special characteristics developed by the incorporation of the ceramic nanoparticles into the Ni-P coatings. The first is related to nodule refinement and the compactness of the Ni-P electroless coating produced by the nanoparticle incorporation. The

ACCEPTED MANUSCRIPT second is related to the simultaneous co-deposition of a passive film of β-Ni(OH)2, also induced by the nanoparticle incorporation. The formation of a passive film of β-Ni(OH)2 was verified by surface characterization and electrochemical techniques. The incorporation of the ceramic

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nanoparticles into the Ni-P coating reduces the corrosion rate of the coating by 4 times with

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respect to the neat Ni-P coating and increases the deposition rate by around 5 times. The

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formation of the passive film of β-Ni(OH)2 does not depend on the nature and the concentration of the nanoparticles used in the electroless coating. The incorporation of the ceramic nanoparticles into the Ni-P coating also increases the microhardness of the coating by at least 25%. A reactional mechanism has been suggested to explain the formation and simultaneous co-

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deposition of Ni-P and β-Ni(OH)2 in the presence of ceramic nanoparticles during electroless Ni-

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P coating formation.

Acknowledgements

The authors are pleased to acknowledge the financial assistance of the Universidad de Antioquia,

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the Universidad Pontificia Bolivariana, CIDI and Colciencias.

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ACCEPTED MANUSCRIPT Highlights

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- A chromium-free composite electroless nickel coating was developed for magnesium - Simultaneous co-deposition of NiP and β-Ni(OH)2 was successful achieved - Incorporation of TiO2, diamond or SIC particles promotes the β-Ni(OH)2 formation - The surface passivation was achieved by β-Ni(OH)2 formation - The corrosion rates were reduced 4 times with respect to the NiP without particles