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Strategies to improve the corrosion resistance of MAO coatings on Mg and its alloys 239
Among the various types of electrolytes used for preparing MAO coatings, the use
of calcium- and phosphorus-containing compounds, such as Ca(OH)
and Na3PO4,as
2
electrolytes assumes significance in terms of biomedical applications because these
coatings could offer better biocompatibility and biological activity than those prepared
using the conventional alkaline electrolytes (Bala Srinivasan, Liang, & Balajee, 2010;
Bala Srinivasan, Liang, Blawert, Stormer, & Dietzel, 2010). The pore diameter and
roughness of the coatings obtained from Ca(OH)
-based electrolytes are relatively
2
less when compared to those prepared using the conventional KOH-based electrolyte.
In spite of its lower thickness, the compactness and presence of calcium-based compounds offered a better corrosion resistance for MAO coatings obtained from a
Ca(OH)
-based electrolyte. Because calcium-based compounds are biocompatible as
2
well as bioactive, it appears that it would be a better strategy to prepare MAO coatings
using electrolytes having calcium- and phosphorus-based compounds.
9.3.2 Process parameters and conditions
The process parameters, such as applied voltage/current density, frequency, and duty
cycle, have a significant influence on the surface morphology, porosity, and thickness
of the MAO coatings. In general, the MAO coating is thin, smooth, and uniform when
the applied voltage/current density is low. An increase in voltage/current density
increases the potential of oxidation and increases the thickness of the oxide coating.
In addition, it also increases the reactivity between the Mg/Mg alloy and oxygen, leading to a better bonding at the interface, and increases the bond strength. However, an
increase in spark size with voltage increases the porosity of the resultant coatings. An
increase in voltage/current density beyond a critical level would increase the porosity
of the outer layer and make the resultant coating rough and nonuniform (Chang et al.,
2011; Zhang, Shan, Chen, & Han, 2008).
It is a common practice to carry out MAO of Mg/Mg alloys at a constant current
density mode because it is generally believed that this methodology offers better process control and considerable savings in treatment time (Liang et al., 2007). With the
application of a constant current density, however, the intensity of the spark discharge
is increased and the number of spark discharges is decreased with an increase in process time. Hence, the resultant coating has a coarse and porous microstructure with
large micropores. Instead, if the current density were allowed to decay freely at the
later stage of the MAO process, then it would help reduce intensity of spark discharges
and enable the formation of smaller sparks distributed over the entire surface of the
oxide coating. Coatings obtained under such conditions would be intact, relatively
smooth, possess a homogeneous microstructure, and be less porous (Liang et al.,
2007). Obviously, the decrease in porosity, heterogeneity, and roughness would offer
a better corrosion resistance. Hence, adopting suitable current density waveforms, such
as freely decaying current density at the later stages or a step down in current density at
a programmed time interval, appears to be one of the useful strategies for improving
the corrosion resistance of MAO coatings on Mg and Mg alloys (Liang et al., 2007).
Different current modes such as direct current (DC), alternating current (AC), and
pulse current (unipolar and bipolar modes) are used for MAO of Mg/Mg alloys
240 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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(Arrabal, Matykina, Hashimoto, Skeldon, & Thompson, 2009; Hussein et al., 2011).
The characteristics of spark discharge vary considerably with the type of current
mode employed. When the MAO coatings are prepared under unipolar current
mode, the strong spark discharges increase the porosity and microcracks and an
increase in the T
period provides little help to resolve the defects. For coatings pre-
off
pared under the bipolar mode, the cathodic component helps to reduce the strong spark
discharges between the anodic periods. In addition, this current mode provides sufficient time for the oxide to cool down before the initiation of subsequent anodic pulses
(long T
). The increase in sintering time (Ton) would enable the formation of a dense
off
oxide coating with minimum porosity. Hence, by suitably combining the cathodic current pulses with the T
on
and T
periods, it will be possible to control the porosity and
off
other structural defects. Hussein et al. (2011) have studied the influence of direct current and pulse unipolar and bipolar current modes on the characteristics and corrosion
resistance of MAO coatings on AJ62 Mg alloy. MAO coatings prepared under DC
mode offered an i
unipolar mode instead of DC decreased the i
whereas bipolar mode leads to a further decrease in i
of 2.24 105A/cm2in 3.5% NaCl. The use of pulsed current
corr
from 2.24 105to 5.39 107A/cm2,
corr
to 4.30 108A/cm2.The
corr
corrosion behavior of coatings prepared under diff eren t current modes is correlated
to the porosity and defects of the coati ng.
Su et al. (2009) have also recommended the use of cathodic current pulses to
improve the characteristics of the MAO coatings deposited on ZK60 Mg alloy. An
increase in cathodic current pulse leads to dissolution of the oxide coatings and a
decrease in coating thickness. However, it enables the formation of a compact
coating with reduced porosity. An increase in the anodic/cathodic (j
ratios from 100/50 (2/1) to 100/150 (2/3) mA/cm
2
has lead to a decrease in the number and size of the pores, a decrease in the extent of penetration of the electrolyte
through to the coating-metal interface, and a decrease in i
from 9.246 10
corr
) current
a/jc
7
to 1.098 108A/cm2.
Hussein, Northwood, and Nei (2012) studied the effect of unipolar, bipolar, and
hybrid (a combination of both) current modes on the properties and corrosion resistance
of MAO coatings deposited on AM60 Mg alloy. MAO coatings fabricated using unipolar current mode contain relatively larger pores due to the strong spark discharges
(type B) (Hussein, Nei, Northwood, Yerokhin, & Matthews, 2010). Unlike the unipolar
mode, the cathodic component and the longer T
duration provide a balance of the
off
discharge effect under bipolar current mode (Hussein et al., 2011) that enables the formation of MAO coating with minimum porosity. However, the coating growth rate is
low under the bipolar current mode. Coatings prepared using a hybrid current mode
enable a significant reduction in pore density and size due to the ability of the bipolar
mode to decrease both the number and intensity of strong B-type discharges. MAO
coatings obtained under the hybrid current mode are dense with a minimal amount of
pores, through-coating cracks, and other defects. MAO coatings prepared using the
hybrid current mode that consists of unipolar (for 15 min) followed by bipolar (for
15 min) are more compact, uniform, and offer a better corrosion resistance. The ability
of the bipolar mode to repair the damages caused by the unipolar mode and to produce a
dense coating with minimum defects combined with the ability of the unipolar mode to
Strategies to improve the corrosion resistance of MAO coatings on Mg and its alloys 241
provide chemically stable phases are considered to be responsible for the improved
corrosion resistance of coatings obtained under this hybrid current mode.
Pulse frequency is another important electrical parameter that could influence the
characteristics of MAO coatings deposited on Mg and its alloys. The effect of frequency on the structural characteristics and corrosion resistance of MAO coatings
deposited on ZM5 Mg alloy using 0.018 M NaOH, 0.016 M (NaPO
0.190 M NaF at 2 A/dm
2
with a frequency of 800 Hz for 60 min and 100 Hz for
3)6
,and
110 min was studied by Lv et al. (2008). The thicknesses of the MAO coating prepared
at 100 Hz for 60 and 110 min were 8 and 26 mm, respectively, whereas those obtained
at 800 Hz were only 2.7 and 4 mm, respectively, at similar time intervals. A lower frequency provides a longer time for one single pulse, which induced continuous
discharge and enabled faster growth of the coating. For MAO coatings prepared at
100 Hz, the pore size is much larger while the density is lower compared with those
prepared at 800 Hz (Figure 9.2). With an increase in treatment time, the pore size of
the coatings prepared at 100 Hz is increased along with the formation of some
microcracks, which make the coating rougher. For MAO coatings obtained at
800 Hz, an increase in treatment time has resulted in a slight increase in pore size,
while no distinct cracks were evident.
Figure 9.2 Surface morphology of microarc oxidation (MAO) coatings deposited on ZM5 Mg
alloy using 0.018 M NaOH þ 0.016 M (NaPO
frequencies and time: (a, c) 100 Hz and (b, d) 800 Hz; (a, b) 60 min and (c, d) 110 min.
Adapted from Lv et al. (2008) with permission from Elsevier.
þ 0.190 M NaF at 2 A/dm2at various pulse
3)6
242 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Van, Brown, and Wirtz (1977) reported that the lifetime of a single spark during
MAO is about 0.17 ms. The pulse width at 800 Hz is 0.6 ms, which is comparable
with the spark lifetime. Hence, during one pulse, either one or two spark discharges
occur at the same place; during the next pulse, the discharge would occur at a
different place. The continuous movement of spark discharges during deposition enables the formation of MAO coatings with a small pore size and higher density at
800 Hz. In contrast, at 100 Hz, the pulse width is 5 ms, which is much longer than
the lifetime of the spark. Hence, the continuous breakdown of a large period leads
to the formation of MAO coatings with larger pore sizes. The higher temperature
generated during spark discharges and subsequent quenching by the surrounding
electrolyte induces the formation of microcracks on coatings prepared at 100 Hz.
The higher energy per pulse and higher degree of plasma chemical reactions enables
a higher coating thickness at low frequencies, whereas the coating thickness is
decreased with an increase in frequency. Moreover, at higher frequencies, with
shorter pulses on time and lower energy per pulse, the movement of cations and
anions towards the growth front and the incorporation of the corresponding elements
in the resultant coating are decreased.
Duty cycle is an important electrical parameter that influences the characteristics
of MAO coatings deposited on Mg/Mg alloys. An increase in duty cycle refers to
an increase in the time of application of current/voltage during each cycle, and it
has a similar effect as the applied voltage on the characteristics of MAO coatings.
An increase in duty cycle increases the energy density of the sparks, the amount of
heat released during the process, and the amount of molten oxide that is thrown out
to the electrolyte (Wang, Zhu, et al., 2009; Tang et al., 2010). Hence, an increase in
duty cycle increases the porosity and decreases the coating thickness. For a duty
cycle of 0.1, the coating is thin, smooth, and uniform, whereas it becomes very
rough with large pores and cracks for a duty cycle of 0.9 (Chang et al., 2011).
The duty cycle has to be carefully optimized to deposit MAO coatings with a
reasonable thickness and relatively smooth coating, with a lesser number of pores
and better corrosion resistance.
The effect of treatment time is also an important parameter that influences the
thickness, morphological features, size, and distribution of pores and corrosion
resistance of MAO coatings. An increase in treatment time provides enough opportunity for the buildup of the coating, resulting in an increase in coating thickness.
However, with an increase in time, the spark discharges become more energetic,
more violent, and move slowly over the sample surface, thus resulting in the formation of nonuniform coatings with higher roughness and larger pores (Figure 9.3).
With an increase in treatment time, t he accumulation of liquid melt around the
micropores could cover the small micropores, which would help to increase the
corrosion resistance. However, an increase in surface roughness could cause a deleterious influence on corrosion resistance. According to Lv et al. (2009),MAO
coatings deposited on AZ91D Mg alloy using 10 g/l Na
2 g/l NaOH at 2 A/dm
and 30 min offered an i
and 1.4768 10
9
2
with a pulse frequency of 800 Hz for 2.5, 5, 20,
of 1.9687 109, 8.3312 108, 3.1357 10
corr
A/cm2, respectively.
SiO3, 8 g/l NaF, and
2
10
,
Strategies to improve the corrosion resistance of MAO coatings on Mg and its alloys 243
(a)
(c)
(b)
(d)
(e)
Figure 9.3 Surface morphology of microarc oxidation (MAO) coatings deposited on AZ91D
Mg alloy using 10 g/l Na
time: (a) 2.5 min; (b) 5 min; (c) 15 min; (d) 20 min; (e) 30 min.
Adapted from Lv et al. (2009) with permission from Elsevier.
SiO3þ 8 g/l NaF þ 2 g/l NaOH at 2 A/dm2for various durations of
2
9.3.3 Pretreatment
Wang, Zhou, Liang, and Chen (2012) explored the use of laser surface melting (LSM)
as a pretreatment of AZ91D Mg alloy for MAO coating. The LSM was performed
using a 10 kW transverse-flow, continuous-wave CO
gas at a pressure of 0.2 MPa. A constant beam diameter of 3 mm with 50% track overlapping was used while the energy density was varied between 32 and 89 J/mm
to achieve a melting depth of 969.5e1503 mm. LSM enabled refinement of
laser using Ar as shielding
2
2
so as
244 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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microstructure, altered the crystal orientation of Mg from (101) to (112), enriched Al at
the surface, and allowed redistribution of b-phase (Mg
) on the AZ91 Mg alloy
17Al12
matrix, all of which promoted a better reactivity of the alloy surface during MAO treatment. The resultant coatings are more compact, have fewer cracks on the surface,
possess higher volume fraction of MgAl
6.40 10
8
to 5.00 109A/cm2, when compared to those obtained on the alloy
, and offered a decrease in i
2O4
corr
from
not pretreated with LSM.
Jiang et al. (2013) have shown that equal-channel angular pressing of AZ91D Mg
alloy leads to a change in microstructure and a homogeneous distribution of the bphase (Mg
) in the matrix. This increased the reactivity of the alloy during
17Al12
MAO treatment, resulting in the formation of a compact coating with uniform
micropores and better corrosion resistance. The findings of Jiang et al. (2013) suggest
that other deformation processes could also be explored as a pretreatment of Mg alloys
for MAO coating.
Cerium conversion coating has been explored as a pretreatment for MAO coatings
on AZ91D Mg alloy (Cai et al., 2011). During the initial periods of MAO treatment,
which is similar to conventional anodizing, the cerium from the conversion coating
becomes available at the growth front and is likely to be incorporated in the passive
oxide film as it grows. The barrier layer of MAO coating formed on AZ91D Mg alloy
pretreated with cerium conversion coating is more compact and exhibits better bonding
with the matrix than those obtained without pretreatment. Unlike those formed on
untreated AZ91D Mg alloy, MAO coatings deposited on the Mg alloy subjected to
cerium conversion coating pretreatment are uniform, have a fewer number of smaller
size pores, and lead to a decrease in i
The role of immersion pretreatment in Al(NO
from 1.3 106to 1.20 107A/cm2.
corr
with or without ultrasound (40 kHz)
3)3
has been explored as a pretreatment for MAO coating of AZ91D Mg alloy (Ximei,
Liqun, Huicong, & Weiping, 2008). Pretreatment in 0.1 M Al(NO
(pH: 4.0) for
3)3
30 min promoted the formation of oxides and hydroxides of magnesium and aluminum,
decreased the breakdown voltage, decreased the thickness, and enabled the formation of
a more uniform oxide layer. Ultrasonic treatment increased the uniformity and decreased
the diameter of the micropores of the oxide layer. Immersion pretreatment in Al(NO
without ultrasonic treatment offered an i
ment, the i
is decreased to 180 106A/cm2.
corr
of 1540 106A/cm2; with ultrasonic treat-
corr
3)3
9.3.4 Additives
A variety of organic and inorganic compounds and other special additives, in addition
to the primary electrolyte additives, have been used to modify the electrolyte solution
to impart desirable qualities in MAO coatings, particularly a better corrosion resistance. The effect of various additives on the characteristics of MAO coatings deposited
on Mg alloy and the extent of improvement in corrosion resistance are compiled in
Table 9.1. Among the various types of additives studied, surfactants have the ability
to modify the interfacial tension, decrease the contact angle and diameter of the oxygen
bubbles, and enable an easy release of oxygen so that the resultant coatings have a
lower porosity and better quality (Guo & An, 2006). The addition of glycerol
Table 9.1 Effect of various additives on the characteristics of microarc oxidation (MAO) coatings deposited
on Mg and its alloys and the extent of improvement in corrosion resistance
Strategies to improve the corrosion resistance of MAO coatings on Mg and its alloys 245
Composition of the base
electrolyte and operating
Number
1 6 g/l Na2SiO3; 3 g/l of Na3PO4;
2 6 g/l Na
3 15 g/l NaOH; 12 g/l phytic acid
4 10 g/l NaOH
conditions
1.5 g/l NaF; 1.5 g/l NaOH
Current density: 2 A/dm
Pulse frequency: 300 Hz
Duty cycle: 45%; time: 15 min
SiO3;
2
2 g/l KF;
2 g/l KOH;
10 ml/l glycerol
Current density: 100 mA/cm
Time: 10 min
Current density: 40 mA/cm
Pulse frequency: 2000 Hz
Duty cycle: 20%; time: 3 min
Current density: 40 mA/cm
Pulse frequency: 2000 Hz
Duty cycle: 20%; time: 3 min
2
Effect of additives, coating characteristics,
Additives used and
their concentration
Na
(1 g/l); EDTA
2B4O7
(0.5 g/l)
Dodecyl sodium sulfate
(0.25 g/l);
diphenylamine-
2
2
2
4-sulfonic acid sodium
(0.25 g/l); Dodecyl
phenyl sodium sulfonate
(0.25 g/l)
Sodium borate (10, 20, 30,
and 40 g/l)
Phytic acid
(4, 8, and 12 g/l)
and extent of improvement in corrosion
resistance of the Mg alloy Reference
7
2
Shi et al. (2010)
Guo and An
(2006)
Zhang, Zhang,
Shen, et al.
(2012)
Zhang, Zhang,
and Duo.
(2009)
Continued
Addition of Na2B4O7and EDTA prevented the
generation of nodules, altered the topography of the
resultant coatings, and decreased the i
coatings deposited on Mg-5Lie3.5Ale 1.2Zne1.2Ce
alloy in 3.5% NaCl from 1.37 10
corr
6
to 5.92 10
of MAO
A/cm2and 3.76 108A/cm2, respectively
Addition of these surfactants (0.25 g/l) modified the
interfacial tension, decreased the contact angle and
diameter of the oxygen bubbles, and enabled an easy
release of oxygen so that the resultant MAO coatings
deposited on AZ31B Mg alloy possess a lower
porosity and better quality
Addition of sodium borate (10e40 g/l) increased the
coating thickness (from 3 to 8.5 mm), promoted the
formation of Mg
, increased the porosity and roughness, and
a-Al
2O3
increased the i
alloy in 3.5% NaCl from 0.49 to 11.9 mA/cm
phase besides MgO and
3B2O6
of MAO coatings deposited on Mg
corr
2
Addition of phytic acid increased the coating thickness
from 3 to 7 mm. MAO coatings deposited using 4 g/l
phytic acid offered an i
while an increase in its concentration from 4 to 8 g/l
decreased the i
corr
of 7.696 106A/cm
corr
to 2.559 106A/cm
2
Table 9.1 Continued
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246 Surface Modification of Magnesium and its Alloys for Biomedical Applications
Composition of the base
electrolyte and operating
Number
5 10 g/l Na2SiO3; 2 g/l NaOH;
6 15 g/l Na
7 10 g/l NaSiO
conditions
2 g/l Na
EDTA
2
Pulse-reverse voltage: 400/120 V
Pulse frequency: 100 Hz
Time: 15 min
SiO3;
2
10 g/l NaOH;
8 g/l KF;
5 ml/l glycerin
Voltage: 500 V
Pulse frequency: 600 Hz
Duty cycle: 8%
Time: 5 min
; 3 g/l NaOH;
3
10 ml/l triethanolamine
Current density: 5 A/dm
Pulse frequency: 2000 Hz
Duty cycle: 15%
Time: 10 min
Effect of additives, coating characteristics,
Additives used and
their concentration
Glycerol
(2, 4, and 6 ml/l)
TiO
sol
2
(5 and 10 vol.%)
sol
SiO
2
2
(1 vol. %)
and extent of improvement in corrosion
resistance of the Mg alloy Reference
corr
of
8
Wu et al. (2009)
Wang, Wang,
et al. (2009)
Liu et al. (2009)
Addition of glycerol decreased the interfacial tension,
changed the anode-electrolyte interface, increased the
adsorptive capacity of negative ions, and promoted the
formation of numerous intensive discharge sparks with
small size, a smooth coating with reduced pore size,
and cracks with higher volume fraction of MgO.
Addition of 4 ml/l of glycerol has decreased the i
MAO coatings deposited on AZ91D Mg alloy in 3.5%
NaCl from 6.16 10
5
to 5.18 107A/cm2.
Addition of 6 ml/l of glycerol limited the diffusion of
anions, decreased the coating thickness from 111 to
64 mm, and restricted the improvement in corrosion
resistance
Addition of TiO2sol has decreased the coating thickness
from 22 to 18 mm and reduced the pore size.
Nevertheless, the i
AZ91D Mg alloy in SBF is increased from 3.16 10
A/cm2to 7.76 108A/cm2(for 5 vol. % TiO2sol)
and 1.51 10
of MAO coating deposited
corr
7
A/cm2(for 10 vol. % TiO2sol). Extent
of corrosion damage after 312 h of immersion in SBF is
also relatively higher for MAO coatings prepared using
sol
TiO
2
Addition of SiO2sol decreased the coating thickness from
17 to 14 mm, decreased the micropores and structural
imperfections, enabled the formation of a uniform
coating, promoted the formation of Mg
expense of MgO, and decreased the i
coatings deposited on Mg-Li alloy in 3.5% NaCl from
7
6.29 10
A/cm2to 1.00 107A/cm
SiO4at the
2
of MAO
corr
2
8 10 g/l Na2SiO3; 10 g/l KOH
Current density: 10 A/dm
Pulse frequency: 200 Hz
Duty cycle: 15%
Time: 20 min
9 15 g/l NaAlO
25 mA/cm
and 1.2 g/l KOH at
2
2
for 25 min
10 40 g/l NaOH;
50 g/l NaSiO
30 g/l Na
10 g/l C
Current density: 10 mA/cm
;
3
2B4O7
6H5O7Na3
;
Time: 10 min
11 1.5 M KOH
0.04 M Sodium citrate
0.1 M H
Current density: 0.02 A/cm
; 0.08 M NaSiO
3PO4
Time: 2 h
12 15.0 g/l Na
SiO3; 2 g/l NaOH
2
5 ml/l triethanolamine
Current density: 5 A/dm
Pulse frequency: 2000 Hz
Duty cycle: 15%
Time: 10 min
ZrO
sol (5 vol. %) Addition of ZrO2sol lead a marginal increase in thickness
2
2
from 36 to 40 mm, altered the morphological features,
Tang, Liu, Li, and
Zhu (2011)
Strategies to improve the corrosion resistance of MAO coatings on Mg and its alloys 247
enabled the formation of a uniform coating with less
micropores and cracks, promoted the formation of
Mg
and decreased the i
AZ91D Mg alloy in 3.5% NaCl from 5.38 10
A/cm2to 1.436 108A/cm
CeCl
(0.08 g/l) Addition of CeCl3enabled the formation of a uniform
3
coating, decreased the porosity from 27% to 1.7%, and
decreased the i
AZ91D Mg alloy from 1.607 10
0.179 10
Ethylene glycol (EG) (10 g/l);
Polyethylene glycol
(PEG 400, PEG 1000,
2
and PEG 4000) (10 g/ l)
Addition of EG, PEG 400, and PEG 1000 decreased the
pore size, nodule size, and average surface roughness.
PEG 4000 exhibits poor solubility in the electrolyte.
Addition of 10 g/l of EG, PEG 400, PEG 1000, and
PEG 4000 decreased the i
deposited on AZ31B Mg alloys in 3.5% NaCl from
3.67 10
1.23 10
Hexamethylenetetramine
(HMTA) (0.1 M);
3
2
sodium borate (0.1 M)
Addition of HMTA decreased the average surface
roughness (R
pore size, reduced the thermal stress and cracks,
enabled the formation of a homogenous coating, and
phase besides MgO and Mg2SiO4phases,
2Zr5O12
6
6
7
, and 3.83 107A/cm2, respectively
of MAO coatings deposited on
corr
2
of MAO coatings deposited on
corr
2
A/cm
of MAO coatings
corr
A/cm2to 1.97 106A/cm2,1.54 107,
) from 10.12 to 1.22 mm, decreased the
a
6
A/cm2to
7
Laleh, Kargar, and
Sabour
Rouhaghdam
(2011a)
Zhu, Wang, Li,
and Zhang
(2012)
Bai and Chen
(2009)
increased the corrosion resistance of the MAO coatings
deposited on AZ91D Mg alloy in 5% NaCl. Addition
of sodium borate changed the composition but was not
able to influence the morphology and roughness. MAO
coatings obtained with the addition of HMTA and
borate offered an i
(0.6 g/l) Addition of tungstate increased the coating thickness,
Na
2WO4
2
decreased the size and number of pores and
microcracks, provoked the formation of a dense
of 4.2 107A/cm
corr
2
Li, Yuan, Sun,
and Jing
(2011)
coating with less structural imperfections, and
decreased the i
Mg-Li alloy from 2.0 10
of MAO coatings deposited on
corr
5
to 8.8 106A/cm
2
Continued
Table 9.1 Continued
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248 Surface Modification of Magnesium and its Alloys for Biomedical Applications
Composition of the base
electrolyte and operating
Number
13 10 g/l NaOH; 18 g/l Na2SiO
14 10 g/l NaOH;
15 0.18 M KOH
16 45 g/l NaOH;
conditions
Current density: 40 mA/cm
Pulse frequency: 2000 Hz
Duty cycle: 20%; time: 3 min
18 g/l Na
SiO3.9H2O
2
Current density: 40 mA/cm
Pulse frequency: 2000 Hz
Duty cycle: 20%; time: 3 min
0.09 M KF
0.08 Na
SiO
2
2
SiO
4
3
Current density: 100 mA/dm
Time: 300 s
100 g/l Na
Current density: 20 mA/cm
Time: 30 min
Effect of additives, coating characteristics,
Additives used and
their concentration
3
2
2
2
2
8-hydroxyquinoline (8-HQ)
(2, 5, and 8 g/l)
Tannic acid (4 g/l) Tannic acid decreased the interfacial tension of
KMnO
(0.07 M) MAO coatings obtained in the presence of KMnO4lead to
4
Polyaspartic acid (PASP)
(9.6, 19.2, 28.8, and
38.4 g/l)
and extent of improvement in corrosion
resistance of the Mg alloy Reference
of
7
from
Zhang, Zhang,
Yang, et al.
(2012)
Zhang, Zhang, Li,
et al. (2012)
Hwang, Kim,
Park, Yoo, and
Shin (2009)
Liu, Zhag, et al.
(2011)
Addition of 2 g/l of 8-HQ decreased the number of
micropores from 0.17 to 0.12/mm
uniformity of the coating, and decreased the i
MAO coatings from 40 to 2.2 mA/cm
2
, increased the
2
, whereas
corr
addition of 5 and 8 g/l of 8-HQ increased the
heterogeneity of the coating and decreased the
corrosion resistance
gaseliquid and solideliquid interface, promoted the
formation of a uniform coating with higher thickness,
and decreased the i
AZ91 Mg alloy in 3.5% NaCl from 6.125 10
A/cm2to 1.385 107A/cm
of MAO coatings deposited on
corr
2
closure of many of the pores, a decrease in thickness
from 11 to 3 mm, formation of Mn
with MgO, MgF
coating density, and a significant decrease in i
1.78 10
, and Mg2SiO4, an increase in
2
7
A/cm2to 6.95 109A/cm
phase along
2O3
corr
2
Addition of 9.6, 19.2 and 29.8 g/l of PASP increased the
coating thickness from 6.4 to 7.5, 14.6, and 23.5 mm
while at 38.4 g/l the increase in coating thickness is
limited to 16.2 mm. Addition of 9.6, 19.2, 28.8, and
38.4 g/l of PASP decreased the i
deposited on AZ31 Mg alloy in 3.5% NaCl from
1.831 10
2.204 10
5
A/cm2to 1.071 106, 2.229 107,
7
, and 1.388 107A/cm2, respectively
of MAO coatings
corr
Data compiled from Sankara Narayanan et al. (2014) with permission from Elsevier.
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