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Strategies to improve the
corrosion resistance of
9
microarc oxidation coatings
on magnesium and its alloys:
Implications for biomedical
applications
T.S.N. Sankara Narayanan, Il-Song Park, Min-Ho Lee
Chonbuk National University, Jeonju, South Korea
9.1 Introduction
The development of biodegradable implants is one of the most interesting research
topics in the area of biomaterials (Brar, Platt, Sarntinoranont, Martin, & Manuel,
2009; Hermawan, Purnama, Dube, Couet, & Mantovani, 2010; Hermawan,
Dube, & Mantovani, 2010; Kirkland, Birbilis, & Staiger, 2012; Kirkland,
Lespagnol, Birbilis, & Staiger, 2010; Moravej, Purnama, Fiset, Couet, & Mantovani,
2010; Moravej, P ri ma, Fiset, & Mantovani, 2010 ; Shaw, Sikora, & Virtanen, 2008;
Song and Song, 2007; Staiger, Pietaket, Huadmai, & Dias, 2006; Virtanen, 2011;
Witte, 2010; Witte et al., 2008; Xin, Hu, & Chu, 2011; Zeng, Dietzel, Witte,
Hort, & Blawert, 2008). The idea that, once implanted, the biodegradable implant
wouldstayinthehumanbodyonlyforthetimeittakestofix the problem and sub sequently be gradually dissolved, absorbed, consumed, or excreted, without warranting a secondary surgery, is indeed fascinating (Shaw et al., 2008). This approach
considers corrosion to be a desirable material property ra ther than a limitation. Their
biodegradable properties in combination with a nontoxic nature means that Mg and
its alloys are potential candidate materials for biodegrada ble implants. However, the
rapid corrosion, g eneration of a large volume of hydrogen gas, accumulation of
hydrogen bubbles in gas pockets adjacent to the implant, and increase in the local
pH value of body fluid are the most critical limitations in using them as implant
materials. A variety of surface modification methods were explored to control the
rate of corrosion of Mg and it s alloys ( Hornberger, Virtanen, & Boccaccini, 2012;
Wang, Tang, et al., 2012; Yang, Cui, & Lee, 2011); amon g them, microarc oxidation
(MAO) has received considerable attention.
Surface Modification of Magnesium and its Alloys for Biomedical Applications. http://dx.doi.org/10.1016/B978-1-78242-078-1.00009-8
Copyright © 2015 Elsevier Ltd. All rights reserved.
236 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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9.2 Surface modification of Mg and its alloys by
microarc oxidation (MAO)
MAO, also referred to as plasma electrolytic oxidation or micro-plasma oxidation, is a
high-voltage plasma-assisted anodic oxidation process, widely employed for the
surface modification of magnesium and its alloys (Blawert, Dietzel, Ghali, & Song,
2006; Cai et al., 2011; Ghasemi, Raja, Blawert, Dietzel, & Kainer, 2008; Gu, Chen,
et al., 2012; Guo and An, 2005; Guo, An, Huo, Xu, & Wu, 2006; Han, Zhu, & Lei,
2011; Hussein, Zhang, Nei, Xia, & Northwood, 2011; Liang, Hu, & Hao, 2007;
Liu, Lu, Jing, Yuan, & Zhang, 2009; Malayoglu, Tekin, & Shrestha, 2010; Wu, Su,
Jiang, & Meng, 2010; Yao, Li, Liu, & Jiang, 2010). The plasma discharges generated
during the process cause partial short-term melting of the oxide layer and promote the
formation of a highly adherent ceramic oxide coating (Malayoglu et al., 2010). The
MAO coating possesses high hardness, good wear resistance, moderate corrosion
resistance, better thermal stability, and dielectric properties (Blawert et al., 2006;
Cai et al., 2011; Hussein et al., 2011; Yao et al., 2010).
9.2.1 Characteristics and architecture of MAO coatings
The mechanism of deposition of MAO coatings involves the following stages: (1) formation of an anodic barrier film during the first few seconds of oxidation; (2) dielectric
breakdown of the barrier film; (3) local thickening of the oxide film at locations of
sparking; (4) formation of pores in the oxide layer; (5) sustained growth of the oxide
layer in the presence of sparking; and (6) formation of discrete larger sparks and breakdown channels. The cavities/pores are formed when the molten oxide and gas bubbles
are thrown out of microarc discharge channels (Duan, Du, Yan, & Wang, 2006; Guo
and An, 2005). The rapid solidification of the molten oxide by the relatively cool elec-
trolyte imparts thermal stress in the oxide film; when the stress is relieved, microcracks
are formed (Duan et al., 2006; Han et al., 2011). The lower value of the PillingBedworth ratio is also considered as one of the reasons for the higher porosity of
MAO coatings on Mg and its alloys (Duan et al., 2006; Su, Wu, Guo, & Jiang,
2009). The pores are generally classified as open pores and closed pores. The open
pores are relatively larger and are concentrated on the surface of the MAO coatings,
while the closed pores are present in the middle region of the coating (Malayoglu
et al., 2010). In addition, numerous hairline crack-like networks formed by
the discharge paths that are extended almost down to the base metal are observed
(Malayoglu et al., 2010).
The architecture of MAO coatings on Mg and its alloys has a three-layered structure: a porous outer layer with several large-sized, deep pores/cavities; a middle layer
with less porosity; and a thin barrier layer (Han et al., 2011)(Figure 9.1). The barrier
layer, which is about one-third of the total thickness, is compact, free of pores and
cracks, and integrated firmly with the substrate by sintered interlocking (Duan et al.,
2006; Guo, Cao, Lu, Liu, & Xu, 2011). The absence of any apparent discontinuity
between the substrate and the barrier layer in the bonding zone confirms the occurrence
of an excellent metallurgical bonding between the ceramic coating and the substrate
Strategies to improve the corrosion resistance of MAO coatings on Mg and its alloys 237
Inner and outer layers with gradation
in pore size and distribution
Mg/Mg alloy
Figure 9.1 Schematic of the architecture of microarc oxidation (MAO) coatings on Mg and its
alloys.
Adapted from Sankara Narayanan et al. (2014) with permission from Elsevier.
Thin barrier layer
(Pan, Chen, Wang, Yu, & Lin, 2012). The micropores and microcracks are randomly
distributed on the surface of MAO coating, and they do not penetrate the entire coating
layer (Gu, Xiong, Ning, & Zhang, 2012). The evolution of pores with time is largely a
function of the applied potential/current.
9.2.2 Advantages and limitations of the porous structure
of MAO coatings
The pores and cracks formed in the MAO coatings on Mg and its alloys have both
beneficial and detrimental effects. The micropores and cracks generated during the
microarc discharges help to release the residual stress of the coating (Gu, Xiong,
et al., 2012). The presence of a porous outer layer in MAO coatings would signifi-
cantly improve the mechanical interlocking effect, the bonding area, and stress distribution across the adhesiveesubstrate interface of the joints, resulting in higher
bond strength (Tang, Zhao, Jiang, Chen, & Zuo, 2010). However, the presence of
a higher pore density on the surface of the MAO coatings of Mg and its alloys
increases the effective surface area and thus the tendency of the corrosive medium
to adsorb and concentrate into these pores. This would facilitate quicker infiltration
of the corrosive medium into the inner regions of the coating and subsequently down
to the substrate, thus deteriorating the corrosion resistance of the coating by changing
its local pH. The pore density, distribution of pores, and interconnectivity of the
pores with the substrate are the important factors that decide its corrosion protective
ability (Malayoglu et al., 2010). Hence, it is not just essential but mandatory to
increase the corrosion resistance of MAO coatings. In this perspective, this review
aims to address the various strategies explored in improving the corrosion resistance
of MAO coati ngs on Mg and its alloys.
9.3 Strategies to improve the corrosion resistance
of MAO-coated Mg and its alloys
9.3.1 Choice of electrolytes
The concentration and composition of the electrolytes used has a significant influence
on the morphological characteristics, porosity, thickness, and corrosion resistance of
MAO coatings (Liang et al., 2005). Hence, a proper choice of the electrolyte
238 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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composition and concentration is imperative to impart a rapid metal passivation and to
promote an earlier attainment of sparking voltage (Liang et al., 2005). An alkaline electrolyte is commonly used for the deposition of MAO coatings on Mg and its alloys to
prevent excessive metal dissolution from the anode. NaOH/KOH is used as the base
electrolyte while phosphate, silicate, fluoride, aluminate, zirconate, permanganate,
etc. are used as the primary electrolyte additives.
The concentration of the electrolyte is an important parameter in deciding the
discharge characteristics and quality of MAO coating. An increase in concentration
of KOH increased the conductivity of the electrolyte, decreased the ignition/breakdown voltage, enabled quicker passivation of the substrate, and promoted the formation of a less porous and more compact oxide layer (Cheng, Qin, Li, Wang, & Zhang,
2011; Ko, Namgung, & Shin, 2010). The increase in viscosity with the concentration
of the electrolyte has enabled an enrichment of Mg
2þ
ions at the interface, allowed the
formation of more concentrated plasma of Mg, and induced a more efficient deposition
of the MAO coating (Barchiche, Rocca, Juers, Hazan, & Steinmetz, 2007). In dilute
electrolytes, the sparking is relatively more intense (Ko et al., 2010) and the temperature increment is more than twice those observed with concentrated electrolytes
(Chang et al., 2011). The findings of Cheng et al. (2011), Ko et al. (2010), and
Barchiche et al. (2007) clearly revealed that the corrosion resistance of MAO coatings
prepared using concentrated electrolytes is much better than those obtained using
dilute electrolytes. The better corrosion resistance offered by the coatings prepared
using concentrated electrolytes is due to the decrease in porosity, increase in compactness of the coating, and increase in volume fraction of the MgO phase (Ko et al., 2010).
Blawert et al. (2007) have also recommended that one of the potential ways to reduce
the pore density and to improve the corrosion performance of MAO-coated Mg alloys
is to increase the concentration of the components in the electrolyte. According to
them, an increase in the concentration of KOH (from 3 to 10 g/l) and Na
2
SiO
(from 2 to 15 g/l) decreased the pore density from 136 to 24 pores/mm2. In addition,
an increase in the concentration of the electrolyte increased the reactivity and increased
the volume fraction of the Mg
SiO4(Forsterite) phase at the expense of the MgO
2
phase.
The addition of the phosphate, silicate, fluori de, aluminate, zirconate, permanganate, etc. to the base electrolyte (NaOH/KOH) increased the conductivity of the
electrolyte s oluti on , decreased the breakdown volt ag e, and offered an extended
window of opportunity for deposition of MAO coatings with desirable characteristics. Diffusion of the anions through the pores, formation of the corresponding
oxides, and their reaction with MgO at high temperatures, which prevai ls during
the MAO process, has led to the formation of additional phases other than MgO.
The formation of magnesium phosphate, magnesium silicate, magnesium fluoride,
etc. helped to increase the corrosion resistance of MAO coatings. The effect of
electrolytechemistryonthesizeanddensity of the pores, the various phases
formed, their volume fractions, and how they are going to influence corrosion resistance of the resultant MAO coating is quite complex and still not fully understood
(Rama Krishna, Poshal, & Sundararajan, 2010) because they are dependent on each
other.
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