Study of the formation of alkaline electroless Ni-P coating on magnesium and AZ31B

magnesium alloy

A.A. Zuleta1,*, E. Correa2, J.G. Castaño3, F. Echeverría3, A. Baron-Wiechec4

P. Skeldon5 and G.E Thompson5

1Grupo de Investigación de Estudios en Diseño - GED, Facultad de Diseño Industrial,

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

2Grupo de Investigación Materiales con Impacto – MAT&MPAC, Facultad de Ingenierías,

Universidad de Medellín, Carrera 87 N° 30 – 65, Medellín, Colombia

3Centro de Investigación, Innovación y Desarrollo de Materiales – CIDEMAT, Universidad

de Antioquia, Carrera 53 N◦61-30, Medellín, Colombia

4UK Atomic Energy Authority, Culham Centre for Fusion Energy, OX11 3DB, Abingdon,

UK

5Corrosion and Protection Centre, School of Materials, The University of Manchester, Oxford

Rd., Manchester M13 9PL, UK

Abstract

In this work, alkaline electroless Ni-P coatings were directly formed on commercial purity

magnesium and AZ31B magnesium alloy substrates using a process that avoided the use of

Cr(VI) compounds. The study focused on two aspects of coating formation: (i) the effect of

the substrate roughness on the kinetics of the electroless Ni-P deposition process on

magnesium; (ii) the morphological and chemical evolution of the coating on both magnesium

*corresponding authorTel.: +57 4 448 83 88 Ext. 13642Fax : +57 4 448 83 88 E-mail: alejandro.zuleta@upb.edu.co

1

and the AZ31B alloy. For these purposes, gravimetric measurements, scanning electron

microscopy (SEM), X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS)

and open-circuit potential (OCP) measurements were employed. It is shown that a relatively

rough substrate promotes the rapid formation of the Ni-P coating on the substrate surface in

comparison with smoother substrates. Furthermore, the presence of fluoride ions derived from

the NH4HF2 reagent in the electroless Ni-P plating bath leads to formation of MgF2 a few

seconds after immersion in the bath; the MgF2. Subsequently, crystals of NaMgF3, with a

cubic morphology, are developed, which later become embedded in the Ni-P matrix. The

presence of fluorine species passivates the substrate during coating formation and hence

restricts the decomposition of the electroless Ni-P plating bath, which can occur due to release

of Mg2+ ions. Finally, according to gravimetric measurements, SEM and XRD, the plating

process is initially faster on magnesium than on the alloy.

Keywords: Magnesium, electroless coatings, surface morphology, coatings grown.

2

1. Introduction

In the last few years, the automotive, aerospace and electronic products industries, among

others, have triggered renewed interest in magnesium alloys as structural materials due to

properties such as low density (1.74 g/cm3), high specific strength and stiffness,

electromagnetic shielding capacity, excellent machinability and good castability [1,2].

However, the range of applications has been limited by poor wear and corrosion resistance

[1,3,4]. For this reason, surface tratments are needed in order to improve the surface

properties and thereby extend the use of magnesium and its alloys. Various processes, such as

electrodepostion [5–7], electroless coating [8–12], conversion coating [13–15],

microarcoxidation [16,17], gas-phase deposition [18,19], surface laser modification [20], and

the combination of two or more of the mentioned approaches, have been used to improve the

surface performance of the alloys; among these, electroless Ni-P plating offers the possibility

of generating uniform, crack-free, and well-adhered coatings, without the need of an external

power source [21,22].

It is well known that magnesium and its alloys are highly susceptible to corrosion during the

growth of electroless coatings [10,23], which are commonly formed in slightly acidic baths at

temperatures around 80 °C. Therefore, it is common practice to apply a pre-treatment to the

alloys before the immersion in the electroless Ni-P plating bath to avoid the corrosion during

the deposition process. One of the most common pre-treatments of the alloys involves acid

attack using a Cr(VI) containing solution [5,7,13,14] followed by passivation in a HF solution

[24] in order to form a protective film, which allows the deposition of the electroless Ni-P

coating without the corrosion of the substrate. However, it is important to reduce the use of

carcinogenic reagents, such as a hexavalent chromium, and it is desirable that hydrofluoric

acid be replaced by a less hazardous and lower volatility solution.

3

In this work, a new alkaline electroless Ni-P coating was deposited on the magnesium and

AZ31B magnesium alloy, using an optimized bath composition that provides enhanced

corrosion resistance. The selected process avoids the use of Cr(VI) species or hydrofluoric

acid, as demonstrated in our previous works [8,10]. The bath contains NH4HF2 as a source of

fluoride ions that enables passivation of the substrate. Furthermore, the reaction between

magnesium and fluoride ions in the bath leads to formation of fluorinated reaction products in

the coating [25,26]. However, little attention has been paid to the study of such species. Liu et

al. studied [26] the physical characteristics and microstructure of the fluoride film formed

during activation, finding the presence of inclusion-like products near the coating base,

although these were not examined in detail. Furthermore, Lui and Gao [25], who investigated

the electroless nickel plating on several magnesium alloys, observed the presence of “three-

dimensional crystallites” using scanning electron microscopy. Similarly, Qin et al. [27]

identified cubic crystals in the initial stages of formation of a Ni−P coating.

They are many publications that describe the optimization of electroless coating processes, the

coating properties and the effects of heat treatments on the coatings. However, the literature

on the initiation and growth of coatings on magnesium alloys is comparatively limited. In

addition, it is well known that the surface condition of the substrate has an important role on

both the plating process and the coating properties [28–31]. However, little importance has

been given to the study of the effect of surface finish on the Ni-P deposition process on

magnesium and magnesium alloys.

The aim of this work is to study the initiation and growth of electroless Ni-P coatings directly

on magnesium and AZ31B magnesium alloy substrates using an alkaline electroless Ni-P

plating bath, with a Cr(VI)-free substrate pre-treatment. The morphological and chemical

evolution of the coating during the plating process was studied, with a particular interest in

4

the development of fluorinated reaction products. In addition, an evaluation of the effect of

substrate roughness on the deposition process was assessed.

2. Experimental details

2.1 Specimen preparation

The study of the effect of surface roughness on the deposition process was made on

magnesium substrates only in order to avoid the possible effect of alloying elements on the

process. Specimens of commercial purity magnesium (99.9%), with dimensions of 10 x 10 x

2 mm, were ground with SiC paper of different grit sizes (#120, #600 or #1200). They were

then cleaned with ethanol, followed by removal of surface impurities left by the grinding

process, in an alkaline cleaning solution of 37 g/L NaOH and 10 g/L Na3PO4 for 600 s at 60

oC, with final rinsing in deionized water and drying in a hot air stream. The average roughness

(Ra) of the substrates was assessed on selected specimens by profilometry using a Veeco 3D

ContourGT optical surface profiler. The area of analysis was ~ 90 × ~ 70 μm. Specimens were

illuminated through a 2.5x objective. The remaining specimens were immersed immediately

in 50 cm3 of electroless Ni-P plating solution in order to form the coating on freshly ground

surfaces. The electroless Ni-P plating solution was contained in a polypropylene beaker

located in a thermostatically controlled bath. Two specimens were immersed simultaneously

for each test condition. In order to check the reproducibility of the coating process, tests were

performed in triplicate, using a fresh electroless Ni-P plating bath for each test. After the

immersion in the electroless Ni-P plating solution, the specimens were rinsed with deionized

water and dried in a warm air stream. The composition of the electroless Ni-P plating bath and

the plating conditions are given in Table 1. Additional details of the optimization of the

formulation of the electroless solution can be found elsewhere [10].

5

Table 1. Chemical composition and operating conditions of the electroless Ni-P plating bath.

Bath composition

Name Formula Concentration

Nickel sulphate hexahydrate NiSO4.6H2O 21.2 g/L

Sodium hypophosphite monohydrate NaH2PO2. H2O 24.2 g/L

Lactic acid C3H6O3 26.5 mL/L

Propionic acid C3H6O2 2.2 mL/L

Succinic acid C4H6O4 12.0 g/L

Ammonium hydrogen bifluoride NH4HF2 13..3 g/L

Operating conditions

pH* 10.5 ± 0.1

Temperature 80 oC ± 1 °C

Time 1800 s

*Adjusted with the addition of ammonium hydroxide (NH4OH)

The development of the electroless Ni-P coatings at different times of immersion in the

electroless Ni-P plating solution was examined on commercial purity magnesium (99.9%) and

AZ31B magnesium alloy (Al 2.9 %, Zn 0.82 %, Mn 0.34 % bal. Mg (wt.%)). Specimens of

the respective materials, with dimensions of 10 x 10 x 2 mm, were ground to a #100 grit SiC

finish and then grit-blasted for 5 min using a micro-grit-blasting apparatus, RENFERT- basic

classic 2945-4025, containing alumina grains (150 m), with a pressure of 60 psi, before

applying the pre-treatment described above. The coating deposition on the grit-blasted

specimens was performed as described previously, but with removal of specimens from the

electroless Ni-P plating bath after immersion for 15, 60, 120, 300, 600, 1200 and 1800 s.

6

2.2 Coating characterization

The kinetics of the coating deposition in the alkaline electroless Ni-P plating solution were

determined gravimetrically using a Metter Toledo AB 204 analytical balance. Before the

immersion, the specimens were measured and weighed. After immersion, the specimens were

extracted from the electroless Ni-P plating bath, rinsed with deionized water and dried in a

warm air stream, and then weighed again in order to calculate the weight change per unit

surface area for the corresponding surface condition. Fresh specimens and electroless Ni-P

plating solutions were used for each treatment time. Experiments were conducted in triplicate,

with the average values of the individual measurements presented in the results.

Morphological and microchemical examinations of the coatings were carried out by scanning

electron microscopy (SEM) using Carl Zeiss AG - EVO® 50 and ZEISS Ultra 55 scanning

electron microscopes (the latter for high-resolution images), both equipped with energy

dispersive X-ray (EDX) spectroscopy (OXFORD INCAPentaFET-x3). Cross-sections of

coatings were prepared metallographically, with polishing down to a 0.3 μm alumina finish;

no chemical etching was used. The coating structure was studied by X-ray diffraction (XRD)

using an X’Pert APD X-ray diffractometer with Cu Kα radiation. The data were recorded over

the 2θ range of 15–95o, with a step size of 0.05o and a step interval of 10 s.

The grit-blasted magnesium and AZ31B substrates after immersion in the alkaline cleaning

solution were analysed by Rutherford backscattering spectroscopy (RBS) using the Van de

Graaff accelerator of the Institut des NanoSciences de Paris. The area of analysis was ∼ 1

mm2. RBS was carried out using 1.75 MeV He+ ions, with the ion beam at normal incidence

to the specimen surface and with a scattering angle of 165°. The data were interpreted using

the RUMP program [32]. 

7

Finally, open circuit potential (OCP) measurements during the deposition process were

recorded as a function of time in a two-electrode Pyrex cell using an μAutolab Type III

electrochemical workstation. Either pure magnesium or AZ31B alloy was used as the working

electrode, with a saturated calomel electrode (SCE) as the reference electrode. The

measurements were repeated three times for both substrates; a typical example of the

measurements is shown in the results section.

3. Results and discussion

3.1 Effect of the substrate roughness on the formation of Ni-P coatings on magnesium

The surface roughness (Ra) values of magnesium specimens with either ground or grit-blasted

surfaces prior to immersion in the electroless Ni-P plating bath are listed in Table 2, together

with the weight gains following formation of electroless coatings for 1800 s. The mass gain of

the grit-blasted surface, which had the highest Ra of 2.1 μm, was 6.5, 22.3 and 52.2 times

higher than that of the surfaces ground to # 100, # 600 and # 1200 grit SiC, which had

progressively reducing Ra values of 1.33, 0.62 and 0.30 μm respectively.

Table 2. Surface condition and initial roughness of magnesium and mass gain after subsequent immersion in the

electroless Ni-P plating bath for 1800 s.

Surface condition Ra (µm) Mass change (mg/cm2)

Blasted with Al2O3 (150 µm) 2.10 ± 0.22 4.70 ± 0.42

# 120 SiC grit 1.33 ± 0.04 0.73 ± 0.09

# 600 SiC grit 0.62 ± 0.10 0.21 ± 0.04

# 1200 SiC grit 0.30 ± 0.01 0.09 ± 0.01

Figure 1 shows optical images of the magnesium substrates with the different Ra values before

and after immersion in the electroless Ni-P plating bath. The alumina blasted surface was

8

uniformly covered by the Ni-P coating. In contrast, the ground substrates exhibited dark and

light regions, labelled “1” and “2”, respectively, due to non-uniform coating coverage on the

surfaces of reduced roughness. A similar finding was made by Vitry et al. [33] for steel

substrates.

Figure 1. Optical images of magnesium with different surface finishes before and after immersion for 1800 s in

the alkaline electroless Ni-P plating bath.

A closer view of the specimens by SEM (Figure 2) revealed that the deposit formed on the

grit-blasted surface was of uniform appearance across the magnesium surface, consisting of

fine nodules, with a cauliflower-like surface morphology, which is typical of an electroless

nickel coating. In contrast, the ground surfaces showed distinct regions where either nodular

or cubic morphologies dominated.

9

Figure 2. Scanning electron micrographs (secondary electrons) of the magnesium surfaces with varying

roughness after immersion in the alkaline Ni-P plating bath for 1800 s. The top and bottom rows present details

of the dark and light regions respectively that are evident in the optical images of Figure 1.

According to the EDX analysis the nodules on the surface ground to a # 1200 SiC finish

(Figure 3 (a)) contained Ni and P indicating that they consisted of Ni-P coating material. For

the areas of predominantly cubic crystals (Figure 3 (b)), detection of Mg, Na, F and O

suggested the presence of a fluorinated compound and possibly magnesium oxide and/or

hydroxide. The crystals were of size up to about 2 μm,

10

Figure 3. Scanning electron micrographs (secondary electrons) and EDX spectra of the magnesium surface (#

1200 SiC grit finish) after immersion for 1800 s in the alkaline electroless Ni-P plating bath. (a) region 1 and (b)

region 2.

For the same specimen, XRD analysis (Figure 4) at the region dominated by cubic features

(region 2) revealed sharp peaks due to (i) the magnesium substrate, (ii) Mg(OH)2 (at 2Ɵ =

18.5o and 2Ɵ = 38o) and NaMgF3; the latter were products formed in the electroless Ni-P

plating bath. Furthermore, a broad peak centered at 2Ɵ = 45o was also present, corresponding

to Ni (111), which is attributed to either a nanocrystaline or amorphous phase. The results

support the analysis of the cubic crystals by EDX spectroscopy shown in Figure 3.

The relatively low rates of coating deposition on the less rough, ground surfaces expose

magnesium to the electroless Ni-P plating solution for a longer time than the rough surface

11

generated by alumina grit blasting, which, as will be shown later, is fully covered by the Ni-P

coating after a few minutes of immersion.

Figure 4. XRD pattern of the magnesium (#1200 SiC grit finish) after immersion for 1800 s in the alkaline

electroless Ni-P plating bath.

The prolonged contact of the magnesium with the electroless solution allows the Mg2+ ions to

react with the fluorine species in the bath [26,34] forming a passive layer that protects the

substrate against dissolution [35]. The fluorinated species form before the deposition of the

Ni-P coating , which correlates with the more negative values of the standard enthalpy of

formation of MgF2 (ΔHf = −1124 kJ/mol [36]) and NaMgF3 (ΔHf = −1716 kJ/mol [36]) in

comparison with Ni-P (ΔHf = -49.91 kJ/mol [37]). In addition, it is well known that ground

surfaces are more homogeneous when a finer abrasive paper is used, which reduces the

number of catalytic sites on the surface and hinders the nucleation of the coating [33].

Although the mechanism whereby the alumina grit-blasting ensures a complete covering of

the surface within the selected coating time is uncertain, it is well known that the amount of

energy stored in the magnesium surface by the grit-blasting is increased due to the severe

plastic deformation conferred by the impact of the Al2O3 particles with the substrate surface

and the associated generation of a high density of non-equilibrium defects (i.e vacancies,

dislocations) within the grains [9,38,39]. In addition, the alumina grit-blasted surface exhibits

a higher number of surface peaks and creates a large surface area [40], which results in a

12

higher chemical reactivity in comparison with a ground specimen [41]. Hence, for the grit-

blasted surface, the high chemical reactivity enables faster deposition of the Ni-P coating

material (see Table 2), such that the coating soon extends across the magnesium surface and

covers the fluorinated compounds formed in the early stages of the process.

3.2 Initiation and growth of alkaline Ni-P coatings on alumina grit-blasted magnesium

and AZ31B alloy substrates.

Figure 5 presents scanning electron micrographs that show the morphological changes of the

commercial purity magnesium surface, with prior alumina grit blasting, during the first 60 s of

the electroless Ni-P treatment. Before immersion, the substrate reveals a lamellar morphology

(Figure 5 (a)); this is typical of Mg(OH)2, which was identified to be present by XRD and

consistent with the presence of significant amounts of oxygen on the surface identified by the

results of RBS that are shown later. The Mg(OH)2 is formed during the cleaning process,

which was performed in a solution of pH ~ 10.5. After 15 s of immersion (Figure 5 (b)), the

surface was slightly corroded in the electroless Ni-P plating solution. A few roughly spherical

particles, of about 0.5 µm diameter, with a fine surface texture (indicated by arrows) were

distributed apparently randomly on the surface. After 30 s of immersion (Figure 5 (c)), two

different regions were evident, one with the typical morphology of Mg(OH)2 mainly

consisting of lamellae of a few nanometres in thickness that projected from the substrate

surface (labeled 2) and the other where the substrate appeared to have been passivated by

formation of a film with a nanometric texture (labelled 1). In the latter regions, the roughly

spherical particles observed at 15 s of immersion had increased in size to about 1 μm and

exhibited a pseudocubic shape. The presence of the different regions suggests a reaction had

occurred similar to that observed by Bradford et al. [42] and Turhan [43], who found that the

MgF2 was formed from Mg(OH)2 in solutions containing fluorides. The reaction involved

either the dissolution of Mg(OH)2 and reaction of the dissolved Mg2+ ions and F-ions or the

replacement or exchange of the hydroxyl ions by fluoride ions, resulting in the formation 13

MgF2. The Mg(OH)2 can be transformed during the coating formation due to an decrease in the

local pH on the magnesium surface during the deposition process as result of the oxidation of

the hypophosphite in the substrate surface [21]. Furthermore, NH4+ ions derived from the

NH4HF2 can combine with OH- ions [44] from the Mg(OH)2, according to the following

reactions:

Mg (OH )2(s )❑→ Mg (II)(ac)+2OH (ac )−¿ ¿ (1)

2 NH¿+¿+2OH (aq)

−¿ ❑→

2 NH 3(aq )+2 H 2 O( l)¿¿ (2)

___________________________________________

Mg (OH )2(s )+2 NH ¿+¿❑

→Mg ( II )(aq)+2 NH 3(aq)+2 H 2O(l)¿ (3)

Thus, Mg2+ ions provided by the Mg(OH)2 are available to react with fluoride ions from

NH4HF2 in the solution (33), forming MgF2 (34):

Mg (OH )2(s )+NH 4 HF2(ac)❑→ MgF2+NH 3(ac )+2 H 2O(l ) (4)

14

Figure 5. Scanning electron micrographs (secondary electrons) of the magnesium surface after (a) 0, (b) 15, (c)

30 and (d) 60 s of immersion in the alkaline electroless Ni-P plating bath

After 60 s of immersion, the transformation process of the Mg(OH)2 had progressed and

lamellar Mg(OH)2 was no longer observed (Figure 5 (d) and Figure 6). The backscattered

electron image of Figure 6 (b) reveals particles of light appearance, which correspond to the

Ni-P coating. The pseudocubic crystals that were present after 30 s have been transformed

into well-defined cubic crystals, with smooth facets with sizes typically in the range from

about 0.5 to 1 μm. The crystals are later identified as NaMgF3. Similar morphological changes

of MgF2 to NaMgF3 were observed by Sevonkaev et al. from mixing of a solution of MgCl2

and NaF at 80 °C [36]. They observed the formation of spherical particles of MgF2 that were

produced by aggregation of nanosize precursors. The particles were subsequently transformed

to cubic NaMgF3, with the rate of formation of the latter depending on the concentration of

NaF. Our findings of cubic crystals of NaMgF3 are consistent with the morphological

observations of these authors [36].

Figure 6. Scanning electron micrographs ((a) secondary electrons and (b) backscattered electrons) of the

magnesium surface after 60 s of immersion in the alkaline electroless Ni-P plating bath.

15

Figure 7 presents scanning electron micrographs that show the details of the transformation

during immersion of the magnesium in the electroless Ni-P plating solution for times from 15

to 60 s. The micrograph of Figure 7 (a) shows a spherical particle typical of those present on

the surface after immersion in the plating bath for 15 s. Figure 7 (b) reveals an intermediate

stage of the transition following immersion in the plating bath for 30 s; the particles display

the development of a cubic appearance. Figure 7 (c) shows that after immersion for 60 s

smooth-sided cubic crystals were fully developed. Figure 7 (d) shows the results of EDX

analysis of a region on the magnesium surface remote from the pseudocubic crystals formed

following immersion for 30 s in the alkaline electroless Ni-P plating bath. Figure 7 (e)

presents the analysis of a cubic crystal formed after a longer period of immersion of 60 s. The

analyses at both locations revealed the presence of oxygen, fluorine, magnesium and sodium.

The high concentration of magnesium that was detected at both locations (see Table 3)

resulted from detection of X-rays generated from the magnesium substrate in addition to those

generated in the reaction products. The presence of sodium and fluorine with an atomic ratio

close to 1:3 at the site of the cubic crystal (Figure 7 (e)) is consistent with the formation of

NaMgF3. The oxygen that was also detected possibly originates from Mg(OH)2, which may be

present along with MgF2 beneath the crystal. The significantly lower Na:F ratio of about 1:6

for the surface layer analysed in Figure 7 (d) suggests that both MgF2 and NaMgF3 are

present, in addition to Mg(OH)2. The localized formation of the MgF2 and NaMgF3 particles

is possibly due to the availability of a sufficient concentration Mg2+ ions to nucleate precursor

MgF2 particles; this may be preferentially achieved at defect sites in the passive film where

the film is less protective.

16

Figure 7. Scanning electron micrographs (secondary electrons) of the transformation process of MgF2 into

NaMgF3 after (a) 15 s, (b) 30 s and (c) 60 s of immersion in the alkaline electroless Ni-P plating bath. (d,e) EDX

spectra of the magnesium surface after 30 s of immersion at locations indicated in (b) and (c), respectively.

Previous studies [45,46] have shown that the Mg(OH)2/MgF2 film formed on magnesium

substrates in solutions containing fluorides is porous. In addition, the high reactivity of the

magnesium that is accessible to the electroless plating solution favours the reduction of nickel

on the Mg(OH)2/MgF2 layer, creating nucleation points for the growth of the Ni-P coating.

Hence, two morphologies are evident after 60 s, corresponding to the nickel and NaMgF3.

Table 3. Compositions (at.%) of the regions shown in Figure 7 after 30 s and 60 s of immersion in the alkaline

electroless Ni-P plating bath, determined by EDX analysis.

O F Na Mgfilm region (30 s) 3.5 7.3 1.2 88.0cubic crystal (60 s) 6.8 33.8 9.0 50.4

17

Figure 8 (a) shows that after 120 s of immersion, more Ni-P nodules had formed around the

pre-existing nodules. The Ni-P deposition then continued in an autocatalytic manner (Figure 8

(b)). After 600 s, much of the magnesium surface was covered by Ni-P nodules and NaMgF3

crystals were embedded in the coating (Figure 8 (c)). After 1800 s of immersion, the surface

was fully covered by the coating (Figure 8 (d)).

Figure 8. Scanning electron micrographs (secondary electrons) of the magnesium surface after (a)

120, (b) 300, (c) 600 and (d) 1800 s of immersion in the alkaline electroless Ni-P plating bath

Figure 9 shows the morphological evolution of the AZ31B alloy surface after (a) 0, (b) 15, (c)

60, (d) 120, (e) 300, (f) 600, (g) 1200 and (h) 1800 s of electroless Ni–P treatment. After 15 s

of immersion, a change of the surface morphology and the presence of some nickel nodules

were evident. The morphological change is similar to that observed on magnesium, indicating

the transformation of Mg(OH)2/MgF2. After 60 s of immersion, the hydroxide/oxide layer

appeared to have been dissolved or transformed, with NaMgF3 particles (see arrows in Figure)

being present on the surface. After 120 s of immersion the number of nickel nodules was

18

increased. By 1200 s of immersion, the nodules had increased in size, coalesced and covered

completely the surface. The coverage of the surface occurred at a faster rate than on pure

magnesium.

The coating development on the magnesium and AZ31B alloy was also investigated by XRD;

the results are presented in Figure 10. In the XRD pattern for magnesium (Figure 10 (a)),

peaks of Mg, Mg(OH)2, Al2O3 are present. The presence of Mg(OH)2 is due to the alkaline

cleaning process, as mentioned above. Since the surface was pre-treated using alumina

blasting, alumina particles were embedded in the soft magnesium alloy, which explains the

presence of characteristic peaks of Al2O3 in the XRD patterns. Additional peaks of NaMgF3

are observed at 60, 120 and 300 s of immersion in the electroless Ni-P plating bath, which

corresponds to the cubic species present in the SEM micrographs at the same times. The

presence of a peak centered at 2Ɵ = 45° for Ni (111) is observed after 300 s of immersion,

Finally, after 1800 s immersion, in addition to the substrate peaks, three broad peaks at 2θ =

44.48, 77.02 and 93.24 are evident, attributed to Ni (111) or (200), Ni (220) and Ni (311),

which correspond to the cubic phase of nickel and indicate the crystalline nature of the

coating due to the low concentration of phosphorus in the coating [47]. The peak broadening

at 1800 s is related to a slight distortion of the crystal structure of nickel caused by

phosphorus atoms in the coating [48].

19

Figure 9. Scanning electron micrographs (secondary electrons) of the surface of the grit-blasted AZ31B alloy

after (a) 0, (b) 15 (c) 60 (d) 120, (e) 300 (f) 600, (g) 1200 and (h) 1800 s of immersion in the alkaline electroless

Ni-P plating bath.

20

The results of the XRD characterization of the coating development on AZ31B substrate are

similar to that observed for magnesium (Figure 10 (b)). The main difference, additional to the

presence of Mg17Al12 phase due the β-phase of the substrate, lies in the rate of formation of

coating, which was higher for the initial deposition times on the AZ31B substrate. The

increased rate of deposition is indicated by broad peaks around 2Ɵ = 45° in the XRD patterns

from 60 s (Figure 10 (b)) due to the presence of nickel in the coating, compared with after 300

s for the magnesium substrate. This can be explained by taking into account that, as for the

magnesium, the α phase (Mg) of AZ31B alloy tends to form MgF2 on its surface, while the β

phase (Mg17Al12) tends to form soluble compounds, such as (AlF6)3− [49]. The electroless bath

provides a high pH environment which solubilizes (AlF6)3−. Such a dissolution generates

anodic areas on the alloy surface that generate electrons that catalyze the reduction of nickel

ions and increase the disposition rate [49,50]. The increased initial deposition rate on the alloy

compared with magnesium is evident in the weight gain measurements shown in Figure 12.

Figure 10. XRD patterns of (a) magnesium and (b) AZ31B after different times of immersion in the alkaline

electroless Ni-P plating bath.

21

However, for immersion times greater than about 1000 s, faster growth occurred on the

magnesium than on the alloy. It is proposed that this may be due to the presence of a zinc-

enriched layer in the alloy (confirmed later by the results of RBS). The zinc-enriched layer

may promote the formation of a coating with reduced porosity [51–53] and hence limit the

access of the plating solution to the substrate. Accordingly, less magnesium is released to the

plating bath by corrosion of the substrate. This minimizes the reduction of Ni2+ ions within the

Ni-P plating solution by released magnesium, which can cause a reduced deposition rate of

nickel on the substrate.

Figure 11 presents the results of RBS analyses of the grit-blasted magnesium and AZ31B

alloy after the cleaning pre-treatment that involved immersion in an alkaline solution of 37

g/L NaOH and 10 g/L Na3PO4 for 600 s at 60 oC. Both substrates revealed yields from oxygen

and magnesium. The yields are consistent with a observations of Mg(OH)2 lamellae on the

surface by SEM. In addition to Mg(OH)2, the film may also contain hydrated MgO. The layer

on the alloy probably also contains aluminium oxide/hydroxide. However, the yields from

aluminium in the spectra are similar for both substrates, which indicates that they originate

mainly from particles of alumina that were embedded in the substrates during grit-blasting.

The presence of embedded alumina was also identified in the XRD results of Figure 10. The

edge due to scattering from magnesium beneath the oxide/hydroxide layer was not sharp for

the magnesium substrate compared with that for the alloy, which may be due to a greater

roughness of the substrate and less uniform thickness of the oxide/hydroxide layer at the

location of the analysis on the magnesium compared with the alloy. The fitting of the data for

the AZ31B alloy indicated that ~ 5 ± 1 x 1017 oxygen atoms cm−2 were associated with the

film. From the atomic densities of oxygen in MgO and Mg(OH)2 (5.32 x 1022 and 5.12 x 1022

atoms cm-3), a film of about 100 nm thickness can be estimated. The roughness of the

substrate precluded estimating the film thickness. Between channels 300 and 400 the

spectrum for the AZ31B alloy shows a yield due to the presence of zinc, with a peak near 22

channel 390. The peak is due to enrichment of zinc at the in the alloy at the interface between

the alloy and the MgO/Mg(OH)2 layer. The enrichment corresponds to ~ 1.6 x 1015 zinc atoms

cm−2. Enrichments of alloying elements that are more noble than magnesium have been

reported previously to form beneath both oxide- and fluoride-containing films on magnesium

alloys [54]. Usually, these enriched alloy layers are only a few nanometers in thick [9]. In the

case of an anodized AZ31 alloy, an enrichment of 6.2 x 1015 zinc atoms cm-2 was measured,

which was estimated to be equivalent to a concentration of 29 at.% zinc in an enriched alloy

layer of 5 nm thickness [55]. Therefore, the present enrichment suggests that a relatively high

average concentration of zinc has been developed by the alkaline cleaning process. The

spectrum for the magnesium also shows the presence of relatively heavy elements in the range

of channel numbers from 300 to 400; the yield in the range is probably due mainly iron,

nickel and copper impurities in the magnesium.

Figure 11. RBS spectra for the magnesium and AZ31B alloy after the cleaning process in an alkaline solution of

37 g/L NaOH and 10 g/L Na3PO4 for 600 s at 60 oC.

23

The above findings with regard to the development of the coating, as well as earlier

observations by SEM, are supported by the measurements of the OCP during the deposition

over a period of 1800 s, which are shown in Figure 12. For both substrates, three stages are

distinguished: (i) an initial drop in potential to a minimum value, which is associated with the

predominance of oxidation of the substrate and formation of Mg(OH)2, MgF2 and NaMgF3

(Figures 5 and 9 (b,c)) (ii) a rise in potential as nucleation of the Ni-P coating becomes

increasingly significant and reduction of both nickel and hydrogen ions occurs [21]. (Figures

8 (a) and 9 (d)), and (iii) a slowly rising potential that occurs when the coating has covered the

substrate and increases in thickness (Figure 8 (c) and 9 (f)). The first period is more prolonged

for magnesium (180 s) than for AZ31B alloy (100 s).

Figure 12. Plots of (a) weight gain as a function of time following immersion and (b) variation of the OCP with

time during the immersion, of both pure magnesium and AZ31B alloy in an alkaline electroless Ni-P plating

bath.

24

A rough surface is created by alumina grit blasting. During alkaline cleaning, a layer

Mg(OH)2 is formed, which is subsequently transformed on immersion in the electroless Ni-P

plating bath into MgF2 that passivates the surface. Localized reaction between the MgF2 and

sodium and fluoride ions in the bath results in formation of cubic NaMgF3 crystals above the

passive layer. Figure 13 shows cross-sections of the coatings formed on the magnesium the

AZ31B alloy after immersion for 1800 s in the electroless Ni-P plating bath. The cross-

sections reveal coatings of total thickness 6.5 and 6.0 μm on the respective substrates. Cubic

NaMgF3 crystals are located in a layer of thickness up to ~ 2.5 μm at the base of each coating.

The crystals are partially embedded in the electroless nickel deposit. In a previous study [8], it

was shown that the coatings on magnesium adhered well to the substrate, with a bond strength

of ˃ 9.3 MPa. However, further investigations are necessary to determine the role of the

cubic NaMgF3 crystals on the mechanism of bonding, and whether they can reduce the bond

strength or enhance it, for instance by mechanical keying of the coating to the substrate.

According to the previous discussion of the mechanism of growth of the coatings, a schematic

overview of the Ni-P formation process on the magnesium and the AZ31B alloy is presented

in Figure 14.

Figure 13. Cross-sections of (a) magnesium and (b) AZ31B alloy following immersion for 1800 s in an alkaline

electroless Ni-P plating bath.

25

Figure 14. Schematic overview of the process of formation of Ni-P coating on magnesium and AZ31B alloy.

Corrosion and wear tests that were carried out in previous work [8,10] indicated that the

presence of NaMgF3 particles within coatings of much greater thickness (about 6.0 to 6.5 μm) 26

than the size of the NaMgF3 particles (about 1-2 μm) have no major impact on the corrosion

protection of the magnesium substrate by the coatings. Whether there is an influence of the

particles on coatings of a few microns thickness, where particles are either located close to the

coating surface or exposed at the surface, remains to be investigated.

4. Conclusions

1. The dependence of the surface roughness produced by either grinding or grit-blasting on

the rate of formation of an alkaline Ni-P electroless coating on magnesium has been

determined. Alumina grit blasting produced the roughest surface and the highest rate of

coating formation. In contrast, grinding with SiC papers resulted in progressively reducing

rates of coating growth as the grit size was reduced. The results are explained by effect of the

mechanical treatments on topography of the magnesium surface and the amount of plastic

deformation of the substrate, with the rougher surface increasing the number of favorable sites

for nucleation of the Ni-P coating.

2. The study shows in greater detail than previously available the development of the Ni-P

coating from early times of immersion in the electroless Ni-P plating bath. The presence of

fluoride ions from NH4HF2 reagent and sodium ions from NaH2PO2.H2O reagent in the

electroless solution promotes the formation of fluorinated species, including MgF2 and

NaMgF3, on the magnesium and AZ31 alloy surfaces. MgF2 is formed after a few seconds of

immersion in the electroless solution and passivates the surface. At local sites, a relatively low

population density of cubic NaMgF3 particles, with a size of about 1 μm, are developed from

spherical MgF2 particles. Such a morphological transformation has only been reported

previously from studies of mixed solutions of MgCl2 and NaF, in contrast to the present

observations of the transformation on magnesium surfaces during electroless plating.

27

3. At about the same time as particles of MgF2 and NaMgF3 are forming, spherical particles of

Ni-P are deposited; these particles subsequently increase in number and size and spread across

the substrate surface to eventually envelop the NaMgF3 crystals. The crystals then become

covered by further growth of the Ni-P coating and remain at the coating base. The presence of

the fluoride species in the electroless Ni-P plating bath allows the Ni-P coating to form

without significant corrosion of the magnesium substrate and avoids the decomposition of the

electroless solution due to the presence of Mg2+ ions released by corrosion of the substrate.

4. Finally, according to gravimetric measurements, SEM and XRD results, the plating process

is initially faster on the surface of the AZ31 alloy than on the pure magnesium, which is

suggested to be related to an enrichment of zinc in the alloy substrate.

Acknowledgements

The authors are grateful to “Departamento Administrativo de Ciencia, Tecnología e

Innovación–COLCIENCIAS (project 111545221131)”, Universidad Pontificia Bolivariana,

University of Antioquia and University of Manchester. The authors also thank the

Engineering and Physical Sciences Research Council (U.K.) for financial support

(Programme Grant: LATEST2) and Professor Jun Yen Uan, Department of Materials Science

and Engineering, National Chung Hsing University (Taiwan) for supply of the AZ31B

magnesium alloy. They also thank the European Community for financial assistance within

the Integrating Activity “Support of Public and Industrial Research Using Ion Beam

Technology (SPIRIT)“, under EC contract no. 227012.

28

References

[1] F. Cao, G. L. Song, A. Atrens, Corrosion and passivation of magnesium alloys, Corros.

Sci. In Press (2016).

[2] J. Li, Q. Jiang, H. Sun, Y. Li, Effect of heat treatment on corrosion behavior of AZ63

magnesium alloy in 3.5 wt.% sodium chloride solution, Corros. Sci. In Press (2016).

[3] A. Atrens, G.L. Song, M. Liu, Z. Shi, F. Cao, M.S. Dargusch, Review of recent

developments in the field of magnesium corrosion, Adv. Eng. Mater. 17 (2015) 400–

453.

[4] M. Esmaily, D.B. Blücher, J.E. Svensson, M. Halvarsson, L.G. Johansson, New

insights into the corrosion of magnesium alloys — The role of aluminum, Scr. Mater.

115 (2016) 91–95.

[5] X. Shu, Y. Wang, C. Liu, A. Aljaafari, W. Gao, Double-layered Ni-P/Ni-P-ZrO2

electroless coatings on AZ31 magnesium alloy with improved corrosion resistance,

Surf. Coatings Technol. 261 (2015) 161–166.

[6] J. Lee, W. Chung, U. Jung, Y. Kim, Direct nickel electrodeposition on magnesium

alloy in pyrophosphate electrolyte, Surf. Coatings Technol. 205 (2011) 4018–4023.

[7] V.E. Selvi, P. Chatterji, S. Subramanian, J.N. Balaraju, Autocatalytic duplex Ni–P/Ni–

W–P coatings on AZ31B magnesium alloy, Surf. Coatings Technol. 240 (2014) 103–

109.

[8] J.A. Calderón, J.P. Jiménez, A.A. Zuleta, Improvement of the erosion-corrosion

resistance of magnesium by electroless Ni-P/Ni(OH)2-ceramic nanoparticle composite

coatings, Surf. Coatings Technol. 304 (2016) 167-178.

[9] E. Correa, A.A. Zuleta, L. Guerra, M.A. Gómez, J.G. Castaño, F. Echeverría, H. Liu,

A. Baron-Wiecheć, T. Hashimoto, P. Skeldon, G.E. Thompson, Coating development

during electroless Ni-B plating on magnesium and AZ91D alloy, Surf. Coatings

Technol. 232 (2013) 784–794.

[10] A.A. Zuleta, E. Correa, M. Sepúlveda, L. Guerra, J.G. Castaño, F. Echeverría, P. 29

Skeldon, G.E. Thompson, Effect of NH4HF2 on deposition of alkaline electroless Ni-P

coatings as a chromium-free pre-treatment for magnesium, Corros. Sci. 55 (2012) 194–

200.

[11] Z. Liu, Electroless nickel-phosphorus (EN) coatings on magnesium and magnesium

alloys, (Doctoral thesis), University of Auckland, (2006).

[12] R. Hu, Y. Su, H. Liu, Deposition behaviour of nickel phosphorus coating on

magnesium alloy in a weak corrosive electroless nickel plating bath, J. Alloys Compd.

658 (2016) 555–560.

[13] X. Dong, Handbook of Manufacturing Engineering and Technology, in: A. Nee (Ed.),

Springer London, London, 2013: pp. 1–21.

[14] X. Yang, G. Wang, G. Dong, F. Gong, M. Zhang, Rare earth conversion coating on

Mg–8.5Li alloys, J. Alloys Compd. 487 (2009) 64–68.

[15] G. Wang, M. Zhang, R. Wu, Molybdate and molybdate/permanganate conversion

coatings on Mg–8.5Li alloy, Appl. Surf. Sci. 258 (2012) 2648–2654.

[16] B.L. Jiang and Y.F. Ge, 7 - Micro-arc oxidation (MAO) to improve the corrosion

resistance of magnesium (Mg) alloys, In Woodhead Publishing Series in Metals and

Surface Engineering, edited by Guang-Ling Song, Woodhead Publishing Series in

Metals and Surface Engineering, Corrosion Prevention of Magnesium Alloys, 2013,

Pages 163-196,

[17] L. White, Y. Koo, S. Neralla, J. Sankar, Y. Yun, Enhanced mechanical properties and

increased corrosion resistance of a biodegradable magnesium alloy by plasma

electrolytic oxidation (PEO), Mater. Sci. Eng. B. 208 (2016) 39–46.

[18] H. Hoche, S. Groß, M. Oechsner, Development of new PVD coatings for magnesium

alloys with improved corrosion properties, Surf. Coatings Technol. 259, Part A (2014)

102–108.

[19] M. Sivapragash, P. Kumaradhas, B. Stanly Jones Retnam, X. Felix Joseph, U.T.S.

Pillai, Taguchi based genetic approach for optimizing the PVD process parameter for 30

coating ZrN on AZ91D magnesium alloy, Mater. Des. 90 (2016) 713–722.

[20] Z. Cui, H. Shi, W. Wang, B. Xu, Laser surface melting AZ31B magnesium alloy with

liquid nitrogen-assisted cooling, Trans. Nonferrous Met. Soc. China. 25 (2015) 1446–

1453.

[21] G.O. Mallory, J.B. Hajdu, Electroless Plating -Fundamentals and Applications,

William Andrew Publishing/Noyes, 1990.

[22] A.A. Zuleta, E. Correa, C. Villada, M. Sepúlveda, J.G. Castaño, F. Echeverría,

Comparative study of different environmentally friendly (Chromium-free) methods for

surface modification of pure magnesium, Surf. Coatings Technol. 205 (2011) 5254–

5259.

[23] L. Wu, J. Zhao, Y. Xie, Z. Yang, Progress of electroplating and electroless plating on

magnesium alloy, Trans. Nonferrous Met. Soc. China. 20 (2010) s630–s637.

[24] X. Shu, Y. Wang, J. Peng, P. Yan, B. Yan, X. Fang,Y. Xu, Recent progress in

electroless ni coatings for magnesium alloys, Int. J. Electrochem. Sci. 10 (2015) 1261–

1273.

[25] Z. Liu, W. Gao, The effect of substrate on the electroless nickel plating of Mg and Mg

alloys, Surf. Coatings Technol. 200 (2006) 3553–3560.

[26] X. Liu, Z. Liu, P. Liu, Y. Xiang, W. Hu, W. Ding, Properties of fluoride film and its

effect on electroless nickel deposition on magnesium alloys, Trans. Nonferrous Met.

Soc. China. 20 (2010) 2185–2191.

[27] T. Qin, L. Ma, Y. Yao, C. Ni, X. Zhao, Y. Ding, An in situ measure method to study

deposition mechanism of electroless Ni-P plating on AZ31 magnesium alloy, Trans.

Nonferrous Met. Soc. China. 21 (2011) 2790–2797.

[28] R. Dhinakaran, R. Elansezhian, A.A. Lalitha, Effect of nanoadditives with surfactant

on the surface characteristics of electroless nickel coating on magnesium-based

composites reinforced with MWCNT, Adv. Tribol. 2013 (2013).

[29] P. Sahoo, Optimization of electroless Ni-P coatings based on multiple roughness 31

characteristics, Surf. Interface Anal. 40 (2008) 1552–1561.

[30] V. Vitry, A. Sens, A.F. Kanta, F. Delaunois, Experimental study on the formation and

growth of electroless nickel-boron coatings from borohydride-reduced bath on mild

steel, Appl. Surf. Sci. 263 (2012) 640–647.

[31] M.A. Xavior, P.K.D. V Yarlagadda, P. Gadhari, P. Sahoo, influence of process

parameters on multiple roughness characteristics of Ni–P–TiO2 composite coatings,

Procedia Eng. 97 (2014) 439–448.

[32] L.R. Doolittle, Algorithms for the rapid simulation of Rutherford backscattering

spectra, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater.

Atoms. 9 (1985) 344–351.

[33] V. Vitry, A.-F. Kanta, F. Delaunois, Initiation and formation of electroless nickel–

boron coatings on mild steel: Effect of substrate roughness, Mater. Sci. Eng. B. 175

(2010) 266–273.

[34] R. Ambat, W. Zhou, Electroless nickel-plating on AZ91D magnesium alloy: effect of

substrate microstructure and plating parameters, Surf. Coatings Technol. 179 (2004)

124–134.

[35] G. Song, A. Atrens, Understanding Magnesium Corrosion—A Framework for

Improved Alloy Performance, Adv. Eng. Mater. 5 (2003) 837–858.

[36] I. Sevonkaev, D. V Goia, E. Matijević, Formation and structure of cubic particles of

sodium magnesium fluoride (neighborite), J. Colloid Interface Sci. 317 (2008) 130–

136.

[37] D. Zhao, L. Zhou, Y. Du, A. Wang, Y. Peng, Y. Kong, C. Sha, Y. Ouyang, W. Zhang,

Structure, elastic and thermodynamic properties of the Ni–P system from first-

principles calculations, Calphad. 35 (2011) 284–291.

[38] S.B. Lee, Y.M. Kim, Signature of surface energy dependence of partial dislocation slip

in a gold nanometer-sized protrusion, Scr. Mater. 64 (2011) 1125–1128.

[39] S. Bagheri, M. Guagliano, Review of shot peening processes to obtain nanocrystalline 32

surfaces in metal alloys, Surf. Eng. 25 (2009) 3–14.

[40] Deendarlianto, Y. Takata, M. Kohno, S. Hidaka, T. Wakui, A.I. Majid, H. Kuntoro,

Indarto, A. Widyaparaga, The effects of the surface roughness on the dynamic behavior

of the successive micrometric droplets impacting onto inclined hot surfaces, Int. J. Heat

Mass Transf. 101 (2016) 1217–1226.

[41] C. Zhou, Y. Yang, J. Zhang, S. Xu, S. Wu, H. Hu, B. Chen, Q. Tai, Z. Sun, X. Zhao,

Enhanced electrochemical performance of the counterelectrode of dye sensitized solar

cells by sandblasting, Electrochim. Acta. 54 (2009) 5320–5325.

[42] P.M. Bradford, B. Case, G. Dearnaley, J.F. Turner, I.S. Woolsey, Papers presented at a

Conference on Ion Implantation and Ion Beam Analysis Techniques in Corrosion

Studies: Ion beam analysis of corrosion films on a high magnesium alloy (Magnox Al

80), Corros. Sci. 16 (1976) 747–766.

[43] C.M. Turhan, Surface Modification of Mg and Mg Alloys, Universität Erlangen-

Nürnberg: Erlangen-Nürnberg, 2011.

https://opus4.kobv.de/opus4-fau/frontdoor/index/index/year/2012/docId/2106.

[44] K. Whitten, R. Davis, L. Peck, G. Stanley, Chemistry, 10th editi, Brooks Cole, Boston,

2014.

[45] Z. Zhang, G. Yu, Y. Ouyang, X. He, B. Hu, J. Zhang, Z. Wu, Studies on influence of

zinc immersion and fluoride on nickel electroplating on magnesium alloy AZ91D,

Appl. Surf. Sci. 255 (2009) 7773–7779.

[46] G.L. Makar, J. Kruger, Corrosion of magnesium, Int. Mater. Rev. 38 (1993) 138–

[47] Y.Z. Zhang, Y.Y. Wu, M. Yao, Characterization of electroless nickel with low

phosphorus, J. Mater. Sci. Lett. 17 (1998) 37–40.

[48] L. Wang, J. Li, H. Liu, A simple process for electroless plating nickel--phosphorus film

on wood veneer, Wood Sci. Technol. 45 (2011) 161–167. doi:10.1007/s00226-010-

0303-0.

[49] F. El-Taib Heakal, A.M. Fekry, M.Z. Fatayerji, Influence of halides on the dissolution 33

and passivation behavior of AZ91D magnesium alloy in aqueous solutions,

Electrochim. Acta. 54 (2009) 1545–1557.

[50] S. Verdier, N. van der Laak, S. Delalande, J. Metson, F. Dalard, The surface reactivity

of a magnesium–aluminium alloy in acidic fluoride solutions studied by

electrochemical techniques and XPS, Appl. Surf. Sci. 235 (2004) 513–524.

[51] M. Schlesinger, X. Meng, D.D. Snyder, The microstructure and electrochemical

properties of electroless zinc‐nickel‐phosphorus alloy, J. Electrochem. Soc. . 138

(1991) 406–410.

[52] J.S. Lian, G.Y. Li, L.Y. Niu, C.D. Gu, Z.H. Jiang, Q. Jiang, Electroless Ni–P

deposition plus zinc phosphate coating on AZ91D magnesium alloy, Surf. Coatings

Technol. 200 (2006) 5956–5962.

[53] J. Chen, G. Yu, B. Hu, Z. Liu, L. Ye, Z. Wang, A zinc transition layer in electroless

nickel plating, Surf. Coatings Technol. 201 (2006) 686–690.

[54] M. Abulsain, A. Berkani, F.A. Bonilla, Y. Liu, M.A. Arenas, P. Skeldon, et al., Anodic

oxidation of Mg–Cu and Mg–Zn alloys, Electrochim. Acta. 49 (2004) 899–904.

[55] A. Němcová, O. Galal, P. Skeldon, I. Kuběna, M. Šmíd, E. Briand, I. Vickridge, J-J.

Ganem, H. Habazaki, Film growth and alloy enrichment during anodizing AZ31

magnesium alloy in fluoride/glycerol electrolytes of a range of water contents,

Electrochim. Acta. 219 (2016) 28–37.

34