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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5326_Библиотеки_им_академика_М_И_Перельмана
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Fluoride conversion coatings
for magnesium and its alloys
1
for the biological environment
Thiago F. da Conceiç~ao1, Nico Sch arnagl
1
Departamento de Química - CFM, UFSC, Campus Trindade, Florianopolis/SC, Brazil;
2
Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research, Geesthacht,
Germany
1.1 Introduction
In the last few years, magnesium and its alloys have been consid ered as biodegradable materials for applications such as orthopedic implants and stents for vessel
dilatation (Witte, 2010; Virtanen, 2011). Due to its biocompatibility (of magnesium
and of its c orr os ion products) and good mechanical propert ies, magnesium-based
implants may serve as load-bearing devices, contributing to the healing of the
organism and then gradually degrading without causing adverse effects. Among
the beneficial effects of such devices is that surgery for implant removal is not
required. However, magnesium alloys have low corrosion resistance in aqueous
environments containing ions such as Cl
fore, magnesium implants in such mediums may undergo earlier failure, excessive
hydrogen produc tion (which forms gas cavities and inflammation), and a high pH
increase in the neighborhood of the implant, causing postoperatory complications
(Virtanen, 2011).
The literature reports numerous surface treatments and coatings to control
magnesium corrosion in biological en vi ronm en ts (Hornberger, Virtanen, &
Boccaccini, 2 012). Among these, th e preparation of fluoride conversion coatings
has received special attention. The most studied fluoride conv e rs ion coating on
magnesium is magnesium fluoride (MgF
sion of magnesium in a solution containing fluoride anions, as hydrofluoric acid
(HF) solutions. Reports in the literature suggest that, besides increasing corrosion
resistance, this conversion coating has antibacterial properties (Lellouche, Friedman,
Lelouche, Gedanken, & Banin, 2012; Lellouche, Kahana, Elias, Gedanken, & Banin,
2009) and may i nduce bone healing due to beneficial effects of fluoride (Berglundh,
Abrahamsson, Albouy, & Lindhe, 2007). In this chapter, the formation of MgF
coating on magnesium alloys is discussed. The potential of this treatment to protect
magnesium from corrosion and to enhance its corrosion resistance in biological environments is considered in detail, as well as future trends and current challenges.
2
, like the biological environment. There-
), which can b e easily prepared by immer-
2
2
Surface Modification of Magnesium and its Alloys for Biomedical Applications. http://dx.doi.org/10.1016/B978-1-78242-078-1.00001-3
Copyright © 2015 Elsevier Ltd. All rights reserved.
4 Surface Modification of Magnesium and its Alloys for Biomedical Applications
1.2 Coating formation: Mechanism and characteristics
1.2.1 Hydrofluoric acid immersion
The traditional method for preparing fluoride coating on magnesium and its alloys is
to immerse the sample in an aqueous solution of hydrofluoric acid for a time period
that can vary from a few minutes to days. It is generally assumed that the coating is
formed by the reaction of magnesium with HF, as shown in Eqn (1.1). This reaction
has a negative change in the Gibbs free energy, indicating that this is a product favored reaction (the change in free energy was obtained using the data in chemical
thermodynamic tables, in the temperature of 298.15 K, reported by Wagman et al.,
1982). The literature reports different condit ions for this treatment in regard to treat-
ment time, acid concentration, and substrate pretreatment. Table 1.1 shows some
characteristics of HF treatment described in the literature for different magnesium
alloys. A systematic investigation on solution concentration and treatment time on
the coating properties is reported in detail by Conceiç~ao, Scharnagl, Blawert, Dietzel,
and Kainer (2010), Verdier, Laak, Delalande, Metson, and Dalard (2004),and
Bakhsheshi-Rad, Idris, Kadir, and Daroonpavar (2013). Coating properties such as
thickness, constitution, and porosity change significantly depending on these
parameters.
Mg
ðsÞ
þ 2HF
ðaqÞ
/MgF
2ðsÞ
þ H
; DrG¼476:6kJ=mol (1.1)
2ðgÞ
In the studies of Conceiç~ao et al. (2010) and Verdier et al. (2004) it was shown that
the coating can present a considerable amount of hydroxides and oxides, even when
ground samples are used. In general, the lower the acid concentration the higher the
amount of hydroxides/oxides formed on the metal surface. This observation is related
to the possible formation of magnesium hydroxide during the HF treatment in aqueous
solutions. Magnesium hydroxide can be formed by the reaction of the metal with water
(Eqn (1.2)), which is the main reaction in the aqueous corrosion of magnesium. By
comparing Eqns (1.1) and (1.2) it can be seen that both reactions have a similar thermodynamic tendency to take place. Therefore, from a thermodynamic point of view, it
is expected that these reactions occur simultaneously when metallic magnesium is in
contact with water and HF. The rate of each process will depend on the HF
concentration.
Mg
þ 2H2O
ðsÞ
/MgðOHÞ
ðlÞ
2ðsÞ
þ H
; DrG¼359:3kJ=mol (1.2)
2ðgÞ
Increasing the acid concentration assures a coating constituted basically of MgF
with small amounts of magnesium oxide. By using a high HF concentration, magnesium hydroxide, eventually formed during the treatment, can be converted into magnesium fluoride according to the reaction shown in Eqn (1.3). This process is
thermodynamically favored, and therefore it is expected to take place at high HF concentrations. In fact, studies in the literature report the preparation of MgF
first preparing a Mg(OH)
layer and then performing the conversion shown in
2
coatings by
2
Eqn (1.3) (see the last entries in Table 1.1).
2
Fluoride conversion coatings for magnesium and its alloys for the biological environment 5
Table 1.1 Characteristics of MgF
coating reported in the literature for different
2
alloys prepared with different parameters
Time/
Reference Alloy HFaconcentration
Ground or as-received substrate
Conceiç~ao et al.
AZ31 7e28 (mol/L) 1e24/R
(2010)
3
Verdier et al.
AM60 10
e101(mol/L) eee
temperature
(h/
C)
b
Surface
color
Brown and
black
(2004)
Chiu et al. (2007) Pure 48% 6e24/R Golden/
brown
Mao et al. (2013) JDBM 40% 12/R e 1.5
Yan et al. (2010) AZ31B 50% 3e168/30 e 0.5e2.75
Jian-Zhong et al.
AZ91D 10e70% 10 mine1/R90 Gray-black e
(2009)
Sun et al. (2013) Mg-3Zn-
20% 6/37 0.5
8Zr
Xin-kuan et al.
AZ91 15% ee1.6e3.2
(2010)
Li, Zhong, Hu, and
AZ91D 20% 20/R ee
Kang (2008)
Carboneras,
AZ31 48% 24/R ee
Garcia-Alonso,
and Escudero
(2011)
Substrate previously treated in alkaline solutions
Bakhsheshi-Rad
et al. (2013)
Mg-
05Ca
5e48% 6e24 Brown and
black
Witte et al. (2010) LAE442 40% 96 e 150e200
Ma, Li, Li, Zhang,
and Huang
Mg-Li-
Al-Ce
40% 12/R e 6.0
(2013)
Drynda et al.
MgCa 40% 96/R Black 10e20
(2010)
Thoman et al.
(2009)
MgCa
and
40% 96/R Gray-black 5e15
WE34
Coating
thickness
(mm)
1.0e2.0
1.5
4.0e12.6
a
The concentrations reported as % are weight percentage.
b
Room temperature.
6 Surface Modification of Magnesium and its Alloys for Biomedical Applications
MgðOHÞ
2ðsÞ
þ 2HF
ðaqÞ
/MgF
2ðsÞ
þ 2H2O
; DrG¼117:3kJ=mol (1.3)
ðlÞ
According to studies in the literature, the quantity of hydroxides in the coating have
considerable effect on its protective properties, and in general, the less the better
(Conceiç~ao et al., 2010). Considering the discussion above, one may conclude that
the higher the HF concentration, the better the protective properties of the formed
coating. Nevertheless, depending on the alloy being treated, an increase in the HF concentration may result in thin coatings. For instance, for sheets of the alloy AZ31 (the
AZ family of magnesium alloys is under certain controversy about its biological application due to concerns related to aluminum), the optimum treatment condition was
found to be a concentration of 14 mol/L for 24 h, at room temperature (Conceiç~ao
et al., 2010). In this case, a coating with w2.0 mm of thickness was built on the metal
surface. Treating the same alloy with 28 mol/L in the same conditions resulted in a
thinner coating (below 1.0 mm) and in higher corrosion current densities in electrochemical tests (19 mA/cm
2
for 14 mol/L and 62 mA/cm2for 28 mol/L) (Conceiç~ao
et al., 2010). Similar observations were made by Bakhsheshi-Rad et al. (2013) for
the alloy of pure magnesium with 0.5 wt% of Ca. In this study, it was reported that
increasing the HF concentration from 40% to 48% produced a decrease in coating
thickness from 12 to 9 mm. These results are probably related to a higher rate of metal
dissolution than of conversion coating formation as the solution pH decreases.
The thickness of the formed coating is very similar for different alloys and in
different treatment conditions. For instance, in the study of Yan et al. (2010) the alloy
AZ31B was treated with 50 wt% HF at 30
C for times varying from minutes to 168 h.
The thicker coating was obtained with 70 h of treatment (2.7 mm) and no further thickness increase was observed until 168 h. This treatment time is among the longest reported in the literature. Nevertheless, the reached thickness is in the same range as
the ones obtained in short treatments. For AZ31 alloy, for example, the literature reports MgF
coating with 2.0 mm of thickness formed by treating with 7 mol/L HF
2
for 24 h, at room temperature (Conceiç~ao et al., 2010). These and the results summa-
rized in the firsts entries of Table 1.1 show that by simply immersing the sample in an
HF solution, the formed layer will have a thickness ranging from 1.0 to 2.5 mm, even at
treatment times as long as 168 h. This is indicative that the film growth takes place at
the metal/solution interface. After the surface is completely covered by the MgF
layer,
2
the conversion coating process stops . This interpretation is in accordance with the
weight and thickness increase observed by different authors during HF treatment
(Yan et al., 2010; Bakhsheshi-Rad et al., 2013).
However, studies in the literature have shown that thicker MgF
layers can be pro-
2
duced by a previous alkalinization of the substrate. By this process, a thick Mg(OH)
layer is formed on the metal surface, which is converted to MgF2by immersing the
coated sample in an HF solution. For instance, Bakhsheshi-Rad et al. (2013) reports
the development of a MgF
layer of 12.6 mm of thickness on the alloy of magnesium
2
with 0.5 Ca. The alloy was previously immersed in a solution of sodium hydroxide for
2 h and then immersed in HF. This thickness was obtained with an HF concentration of
48 wt% HF for 24 h at room temperature. A study of Witte et al. (2010) reported MgF
layers of 150e200 mm of thickness on the alloy LEA442 by treating with 40% at room
2
2
Fluoride conversion coatings for magnesium and its alloys for the biological environment 7
temperature for 96 h. Prior to the immersion in HF, the alloy was placed in a boiling
solution of sodium hydroxide to create a magnesium hydroxide coating, which was
converted to MgF
by immersion in HF. A similar process was used by Thomann
2
et al. (2009) and Drynda et al. (2010) for different alloys of magnesium with ca lcium
(from 0.4 to 1.0 wt% of Ca). Thicknesses ranging from 5 to 20 mm were reported. In
these cases, the final thickness is defined on the alkalizations step and the conversion of
Mg(OH)
to MgF2takes place from the film solution interface toward the metallic
2
substrate.
The coating appearance also depends on the treatment conditions. A brown/gold
color is generally considered as the traditional aspect for MgF
coating (Chiu,
2
Wong, Cheng, & Man, 2007). Nevertheless, the literature also reports samples with
a black surface. Conceiç~ao et al. (2010) and Bakhsheshi-Rad et al. (2013) report
that the black surface is formed when the sample is treated with high HF concentrations, while the golden color is more common at milder treatment conditions. In
both cases, the authors studied alloys instead of pure magnesium (AZ31 and Mg0,5Ca, respectively). Gray to black surfaces on magnesium samples treated with HF
are also reported by Jian-Zhong, Jiu-gui, Yan-wen, and Chang-sheng (2009), Pereda
et al. (2010), and Thomann et al. (2009). As discussed in the prior section, lower
HF concentrations enhance the formation of hydroxides. Therefore, it is suggested
that the bronze color is related to the presence of hydroxides in the coating. This
conclusion is further corroborated by the studies mentioned. Jian-Zhong et al.
(2009) report EDS analysis of the prepared gray and black surfaces. They detected
the presence of magnesium, aluminum, and fluoride on the surface (the alloy under
investigation was AZ91D). The oxygen concentration was not reported. Pereda
et al. (2010) report black surfaces for pure magnesium prepared by powder metallurgy
and coated with MgF
and KMgF3. No hydroxides were detected on these surfaces
2
according to X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS)
analysis, although magnesium oxide could be detected. A similar result is reported
by Thomann et al. (2009) investigating the composition of the coating in depth by
energy-dispersive X-ray spectroscopy (EDX). On the other hand, in the study of
Chiu et al. (2007), the obtained surface had a golden appearance. According to the
results reported by Chiu, XPS analysis indicated the presence of small amounts of
magnesium hydroxide in the coating.
Figures 1.1 and 1.2 show a detailed investigation on the correlation between
appearance and chemical composition. Sheets of the alloy AZ31 were treated either
by 14 mol /L or 28 mol/L HF solution for different lengths of time (from 1 to 24 h).
Figures 1.1 and 1.2 show images of the aspect and the infrared spectra of these sam-
ples, respectively. By treating the samples for 1 h both solutions produced a gray to
black surface. The infrared spectra show signals related to oxides and fluorides for
the sample treated with 14 mol/L. The spectrum of the sample treated with
28 mol/L shows weak signals, indicating the dissolution of native magnesium oxide
film. After 5 h the treatment with 14 mol/L produced a golden color while the sample
treated with 28 mol/L is completely black. The surface of the sample treated with
14 mol/L is very heterogenic in appearance, however, and its infrared spectrum
does not show the signal above 3000/cm related to hydroxides. Nevertheless, the
8 Surface Modification of Magnesium and its Alloys for Biomedical Applications
(a)
(b)
Figure 1.1 Image of AZ31 alloy sheets treated with (a) 14 mol/L HF and (b) 28 mol/L HF for
1 h, 5 h, 15 h, and 24 h (from left to right).
band related to oxides and fluorides (from 500e900/cm) became larger. In this same
time, the spectrum of the sample treated with 28 mol/L showed an increase in the
presence of fluoride. By increasing the treatment time with 14 mol/L the surface starts
to be more homogeneous, resulting in the brown/golden color, while treatment with
28 mol/L results in a black appearance. It is clear from the infrared spectra that the
presence of hydroxides in the sample treated with 14 mol/L, and the amount of hydroxides, increase with treatment time. These results further corrob orate the assumption
that the bronze color indicates the presence of hydroxides in the coating.
It is important to notice that hydroxides were detected after 15 h of treatment, indicating that they were formed way after magnesium fluoride. It is unlikely that magnesium fluoride was converted into magnesium hydroxide at this point of the process,
since this conversion is not thermodynamically favored on the applied conditions.
Nevertheless, it is possible that a compound of general formula MgF
(OH)xis formed
2-x
on the surface, as shown in different studies in the literature (Booster, Sandwijk, &
Reuter, 2003; Prescott, Li, Kemnitz, Deutsch, & Lieske, 2005). The literature does
not provide thermodynamic information about such compounds; therefore, it is not
possible to conclude if this process is favored from a thermodynamic point of view.
The late formation of hydroxide may also be related to a slower rate of magnesium
Fluoride conversion coatings for magnesium and its alloys for the biological environment 9
O-H
(a)
Absorbance (arb. units)
4000 3500
(b)
3000
2500
Wavenumber (cm-1)
CO
CO
Oxides
hydroxides
H2O
2
2000
2
1500 1000
fluorides
24 h
15 h
5 h
1 h
Untreated
AZ31
500
Oxides
fluorides
24 h
15 h
5 h
Absorbance (arb. units)
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
1 h
Untreated
AZ31
Figure 1.2 FTIR spectra of Mg AZ31 alloy sheets treated with (a) 14 mol/L HF and
(b) 28 mol/L HF, at different treatment times.
Reprinted with permission from Conceiç~ao et al. (2010).
hydroxide formation at the beginning of immersion in HF, on the tested conditions.
However, as the reaction between magnesium and HF (Eqn (1.1)) proceeds, the concentration of HF will decrease in the solution. It is possible that this concentrations falls
enough to allow the reaction in Eqn (1.2) to occur at a higher rate. Nevertheless, a
10 Surface Modification of Magnesium and its Alloys for Biomedical Applications
complete understanding on the formation of hydroxides during the MgF2formation
process by immersion in HF solutions needs further investigation considering thermodynamics and kinetics aspects.
1.2.2 Alternative methods
Due to the high toxicity of hydrofluoric acid, different alternative methods have been
proposed for creating a MgF
that has been considered by different authors is the immersion of magnesium allo ys
in solutions of fluoride salts such as NaF, KF, and NH
Olsen, 1993; Verdier et al., 2004). In some studies, the fluoride salt is mixed with
an acid or a base to control the pH. It is shown that the corrosion current density
of magnesium and its alloys is considerably reduced in the presence of fluoride
salt, which is attributed to the formation of a protective film. Some studies also
investigated the aid of electrochemical methods to enhance the MgF
rate on magnesium alloys exposed to KF solutions. In the study of Wu, Dong,
and Ke (2013), for example, a potential difference of 1.4 V was applied on mag-
nesium AZ31 alloy, exposed to 0.1 M kF solution. A mixed layer consisting of
Mg(OH)
, MgF2, and KMgF3was formed with a total thickness of about 0.6 m m.
2
The corrosion protection provided by this treatment was not reported. The use of
fluoride-containing salts to prepare MgF
oxidation (MAO) processes. Mu and Han (2008) prepared and characterized MAO
coatings on pure magnesium disks using potassium fluorozirconate as electrolyte.
The authors report that the presence of fluoride enhances coating properties by the
formation of MgF
. Nevertheless, a mixed coating is formed with tetragonal and
2
monoclinic zirconium oxide. Liu, Shan, Song, and Han (2011) report similar improvements for this electrolyte in plasma electrolytic oxidation (PEO) coatings. In
fact, the use of fluoride salts in MAO and PEO processes for magnesium alloys is
well known in methods such as DOW-7 and HAE (Gray & Luan, 2002). In all cases,
MgF
is formed among other compounds.
2
Recently, studies in the literature report the use of vacuum depositio n techniques to
deposit MgF
on the surface of magnesium alloy. In fact, such techniques are well
2
known for the preparation of MgF
Perales, Herrero, Jaque, & Heras, 2007). In the study of Li et al. (2013) , a magnesium
alloy with 1 wt% Ca was coated with MgF
cracked surface and a thickness of 0.95 mm. The coating increased the alloy biocompatibility and provided corrosion protection to some degree. Another interesting
method described by Lellouche et al. (2009) resulted in nanoparticles of MgF
pared by sonochemical and microwave radiation of a mixture of magnesium acetate
and 1-butyl-3-metylimidazolium tetrafluorborate. Glass sheets were coated with these
nanoparticles by dipping into the reaction medium. It was shown that these nano-sized
MgF
particles have antibiofilm properties. Nevertheless, this method has not been
2
applied for coating metals with corrosion protection purposes. The synthesis of magnesium fluoride from solegel routes is also described in the literature (Prescott et al.,
2005), but the performance of the synthesized particles on corrosion protection is not
conversion layer on magnesium alloys. An approach
2
F(Gulbrandsen, Tafto, &
4
formation
2
layers was also investigated in micro-arc
2
thin coatings on optical materials (see, for example,
2
by vacuum deposition. The coating had a
2
2
pre-
Fluoride conversion coatings for magnesium and its alloys for the biological environment 11
described. In general, these alternative methods tested on magnesium create mixed
layers, with higher amounts of hydroxides and oxides in comparison to the traditional
HF treatment. As a consequence, the corrosion protection is usually inferior, as well as
the coating thickness. Therefore, more research is necessary to create protective fluoride coatings without using hydrofluoric acid.
1.3 Corrosion protection properties
1.3.1 General characteristics
The liter at ur e reports d iff er ent studies about the protectiveness of MgF2coatings on
magnesium and its alloys in corrosive environments. Table 1.2 shows a summary
of the results obtained in corrosion tests for magnesium and magnesium alloys
coated with MgF
Table 1.2 Results of electrochemical polarization tests of magnesium
and magnesium alloys, coated with MgF
Reference Alloy
. Direct polarization tests have shown that the conversion coating
2
, in different solutions
2
Lowest
MgF
2
thickness
(mm)
Test
solution
corrosion
current
density
(mA/cm
Highest
corrosion
potential
2
(mV)
)
Conceiç~ao
et al.
(2010)
Chiu et al.
(2007)
Mao et al.
(2013)
Yan et al.
(2010)
Li et al.
(2008)
Li et al.
(2013)
Bakhsheshi-
Rad et al.
(2013)
Drynda et al.
(2010)
AZ31 2.0 3.5 wt%
NaCl
Pure 1.5 Hank’s
solution
JDBM 1.5 Artificial
plasma
AZ31B 2.7 SBF 1.14 10
AZ91D e 3.5 wt%
NaCl
Mg-1Ca 0.9 Hank’s
solution
Mg-05Ca 12.6 Kokubo
solution
MgCa 10e20 2.5% NaCl 150 e
13.0 1445
10.0 1580
1.05 1590
2
1.17 1533
6.06 e
6.20 1658
1478
12 Surface Modification of Magnesium and its Alloys for Biomedical Applications
process generally moves the corrosion potential toward more positive values,
indicating a decrease in the thermodynamic tendency for oxidation. Generally,
the thicker the coating the nobler the corrosion potential. Therefore, the corrosion
potential tends to move toward the nobler direction by increasing the treatment time
at a fixed HF concentration. For instance, Yan et al. (2010) show that the corrosion
potential of the alloy AZ31B continuously moves toward the noble direction by
increasing the treatment time with 50% HF. After 168 h of treatment, the corrosion
potential was 200 mV nobler than that of the untreated metal. Similar results are
reported by Conceiç~ao et al. (2010) for different HF concentrations. The effect
of HF concentration on the corrosion potential, at a fixed time, does not follow a
regular trend.
The corrosion current density does not decrease continuously with treatment time
at a fixed HF concentration. In the study of Yan et al. (2010) the corrosion current
density reached a constant value after 72 h (decreased from 12.62 mA/cm
0.011 mA/cm
2
), and no further decrease was observed by longer treatment times.
2
As the coating thickness reaches a constant value at this same treatment time, the
result indicates that the corrosion current density is directly related to the coating
thickness. This conclusion is corroborate d by the results of Conceiç~ao et al.
(2010) and by analyzing the effect of different HF concentrations, at a fixed treat-
ment time, on the corrosion current density. When the HF treatment is applied on
the as-received sample, the decrease in corrosion current density is also related to
a decrease on the Fe/Mn ratio to below its critical value. The Fe/Mn ratio is an
important parameter for studying magnesium corrosion, as manganese can dissolve
iron particles forming a phase with lower cathodic activity, decreasing the microgalvanic corrosion (Song & Atrens 1999). The treatment of as-received AZ31 sheets
with HF decrease the Fe/Mn ratio from 0.035 to 0.024, below the tolerance limit
of 0.032 (Conceiç~ao et al., 2010).
Impedance tests have shown that the protectiveness of MgF
layers formed by HF
2
treatment does not hold for a long time. It was shown that the impedance of AZ31
sheets coated with MgF
(2.0 mm of thickness) falls by 4 decades after 20 h of exposure
2
to a 3.5 wt% NaCl solution (Conceiç~ao et al., 2010). Such a behavior was observed for
black and golden MgF
layers on AZ31, of similar thickness. This indicates that the
2
corrosive solution can easily penetrate in the coating, reaching the metallic substrate
in short time periods. Some studies discuss a possible instability of MgF
in solutions
2
containing chloride as the reason for its short-term protection (for example, see Cowan
& Harrison, 1979). It is shown in Eqns (1.4)e(1.6) that MgF
dissolution in water is
2
not thermodynamically favored, as well as its reaction with chlorine. Thus, from a thermodynamic point of view, MgF
is rather stable in chlorine-containing solutions. The
2
low long-term protection it provides is probably related to cracks and holes in the
coating, which allow the metal underneath the coating to corrode, producing an undermining effect. Therefore, in order to improve the long-term stability, the coating
defects must be either covered by another layer or suppressed by optimizing the coating
process. Nevertheless, further studies are required to elucidate the influence of chlorine
anions on the degradation mechanisms of MgF
layers.
2
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