Use of Magnesium in Orthopedics

Use of Magnesium in Orthopedics
41661 Metals Technology
Filip Jakub Bedka (s151842)
Georgios Pitsilis (s152087)
November 24, 2016

41661 Metals Technology Use of Magnesium in Orthopedics
Abstract
This report covers the research and literature study regarding the use of magnesium in orthopedics with
an emphasis put on the WE43 alloy. Manufacturing processes are described with possible surface and heat
treatments. An extensive description of microsturcture is presented, as well as dislocation mechanisms
in magnesium. Strengthening methods are presented: grain reﬁnement, precipitation hardening, work
hardening and solid solution hardening. Biodegradability is described. Corrosion problem and its beneﬁts
are thoroughly deliberated, with possible use of coating or surface treatment.
i

Contents
1 Introduction 1
1.1 Magnesium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Medical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Alloying elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Manufacturing 3
2.1 Magnesium Alloys Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 WE43 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Joining Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5 Heat Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.6 Surface Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.7 Thermal Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Microstructure 11
3.1 Magnesium and WE43 Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3 WE43 Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.4 Ultra Fine Grain Reﬁnement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4 Biodegradability 17
4.1 Biodegradable properties of WE43 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5 Corrosion and Surface Treatment 18
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2 Corrosion in Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.3 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.4 Galvanic Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.5 Stress Corrosion and Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.6 Pitting Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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5.7 Intergranular Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.8 Coating and Surface Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6 Conclusion 21
A Appendix 24
A.1 Mg-Y Phase diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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1 Introduction
1.1 Magnesium
Magnesium is a very light metal in great quantities in earth’s oceans and subsoil. The density of pure
Mg is 1.73 g/cm3
and the average density of its alloys varies around 1.8 g/cm3
. Due to its high ratio of
strength to density it shows a highly increasing demand over the second half of the last century, pushing
away the aluminum which used to be the dominant material for these applications, such as car engines
components and aircraft propulsion system (aerospace). [1] Moreover due to its high specific strength
further fuel consumption and components of increased reliability can be achieved. The unalloyed Mg has
the same high damping capacity but lower strength due to the dislocation motion [2].
The magnesium has hexagonal lattice structure with atomic diameter at 3.2 A (Armstrong). On mag-
nesium phase diagrams it is pretty common to see peritectic and eutectic systems, having intermetallic
particles such as Al, Mn, Ag, Zr, Ce etc. The most frequent alloys types are WE43, WE54, A356, AZ91
and EZ54. The alloys’ types can also be categorized on wrought and cast products. The cast products
has as negative point its low corrosion resistant. This situation stated until 1925 when Manganese was
firstly added at low portion, only 0.2 wt%. Moreover a second disadvantage is that the grains of mg
alloys have a tendency to enlarge and to vary on size. This tendency degrades the microstructure of the
alloy creating microporosity. By adding Zr as alloying element this tendency is limited because the ZR
particles have grain refining effect over the Mg ones. But the Zr particles tend to create compounds with
the AL and Mn elements. In order to counter that and to increase the mechanical properties of the alloy
precipitation hardening and ageing has become essential for every Mg alloy.
The average Mg production per year in the 90s in the western world was 250,000 tones and until now is
increasing rapidly reaching over 900,000 tones on world scale. The demand over the past decades increased
due to its applications on automotive and aeronautic industries. Due to its continuously improving
properties and its lower price the mg alloys will be able to compared with the stainless steel etc. In 1808,
Humphry Davy successfully extracted Mg in its pure form. The most typical form of unprocessed Mg is
called dolomite. The most common forms are the arbonates dolomite (MgCO3 CaCO3), oxide mineral
brucite (MgO +H2O) and carnalite chloride (MgCl2 + KCl +6H2 2O) and magnesite (MgCO3). The
most common production method is the electrolytic reduction of magnesium chloride.
Figure 1: Graph of creep resistance as a function of operational temperatures for automotive components
[3].
1

Table 1: Mechanical Properties of the WE43 [8].
Tensile Strength [MPa] Yield Strength [MPa] Elongation [%] HV3
230 178 7 85
1.2 Medical Applications
The most interesting part on magnesium is that it can be used in medical devices due to the biocompat-
ibility and if it is tailored enough it can become biodegradable in the human body and eventually can be
replaced by natural tissue or by healed bone . One of its applications in medicine is as orthopedic implant
due to its load baring properties. Other applications on the human body is as compression screws, as
vascular stents and as bone fixation devices [4, 5]. Metals are better candidate materials for load bearing
applications combined with biocompatibility compared to the ceramics and polymers due not only to
mechanical properties but to their superior fracture toughness [5] . Moreover its ability to sustain with
precision, initially low and afterward higher dissolution rate in the human body [6] offer to the patients
a higher standard of living because it relieves them from the risk of complications form an extra surgery.
The most common alloy for biomedical uses, especially for osteosynthesis, is the WE43(Electron WE43
Castings) Mg alloy [7] The need of a light, relative stiff and creep resistant materials in auto industries
lead to the creation of the alloys Mg-Y-Re-Zr series [8]. At elevated temperatures, up to 250 ◦
C presents
pretty adequate mechanical properties and an increase in ductility and creep resistance is also noticed
[8].
1.3 Alloying elements
Mg- RE
The magnesium can form solid solutions with a variety of raw material elements. Furthermore the
microporosity can be suppressed by the grain boundaries. Moreover, after ageing, precipitation can be
performed inside of the grains leading also to improved mechanical properties even further. The zirconium
combined with RE decreases the size of the grains , increasing the strength but limiting the elongation [9].
Mg- Yttrium
Alloys like WE43 which includes Y, Nb and Zr present good solubility with the Mg. the drawback of
using Y is its high cost and that Y forms compounds with MgCl2. In order to hinter the latter formation
the alloys are treated in Ar and SF6 at high temperatures. Moreover alloying elements like Gadolinium
and Erbium are added in order to decrease the need Y, sustaining Y’s impact on mechanical properties.
2

2 Manufacturing
2.1 Magnesium Alloys Manufacturing
To fabricate any magnesium alloy, first the pure magnesium has to be extracted from the raw form it
exists in nature. There are several methods of extraction:
• Calcination
• Pidgeon Process
• Dow Process
Calcination is based on heating up the raw MgCO3 to produce MgO. After that the product is mixed
with petroleum coke to reduce it to magnesium. Pidgeon process, on the other hand, uses siliconthermic
reduction. A ferrosilicon is mixed with MgO to be heated up under vacuum to obtain magnesium
vapour. Which condenses into magnesium. The last process, Dow process, is an electrolysis process. The
magnesium in raw form is treated with HCl to produce MgCl2. MgCl2 is later electrolysed to obtain, by
reduction, magnesium at the cathode of the setup. [10]
Alloys are produced by melting the magnesium and adding alloying elements to the molten magnesium.
Everything is stirred and given time for the dross and impurities to settle at the bottom. The samples
are next taken to check the proper compostition of the alloy and ingots are made. [11]
Regarding manufacturing processes, magnesium can be fabricated by casting or by metal working. Three
methods of casting are available:
• Sand Casting
• Die Casting
• Thixo-casting
For metal working magnesium can be fabricated by:
• Rolling
• Extrusion
• Forging
The typical microstructures after different manufacturing processes can be seen in the following figures.
[11]
In the Figure 2, a casted billets are seen. The upper left microstructure shows very coarse grains, due
to the slow cooling rate. Sometimes, if the alloy contains larger amounts of alloying elements, a big
intermetallic phases can form, as seen in the upper right microsturcture, in this case a magnesium-
aluminum compound. If direct chill casting is used, the grains are becoming finer, as seen in the lower
left microstructure. If heat treatment is applied, alloying elements can precipitate and increase the
mechanical properties on the alloy, as seen in the lower right microstructure.
In the Figure 3 microstructures after sand casting are shown. Upper left microstructure is after typical
sand casting, upper right after additional solution heat treatment, lower left - solution heat treatment
and aging.
3

Figure 2: Microstructures after casting. [11]
4

Figure 3: Microstructures after sand casting. [11]
5

Figure 4: Microstructures after metal working. [11]
6

Figure 4 shows microstructures after metal working. Upper left is after extruding and ageing, upper right
after rolling and annealing, lower left after hard rolling, and lower right after rolling and annealing a
different alloy.
2.2 WE43 Fabrication
The WE43 alloy is commercially available for sale in a wrought form as extruded or forged parts. The
schematic of fabrication of a WE43 part can be seen in the Figure 5. Those steps consists of, first of
all, melting the magnesium and alloying with the alloying elements, and next direct chill casting. To get
rid of the residual stresses after casting and to prevent crack formation, the part is annealed. Then the
surface is scalped as a preparation for hot rolling. The part is preheated and rolled. The final step is
heat treatment. Typical heat treatments of WE43 alloy are T6 - solutionized and artificially aged or T5
- artificially aged. This gives the optimal mechanical properties and the part is ready to be cut for the
final form. [12]
Figure 5: Fabrication of WE43 alloy process chain. [12]
2.3 Joining Processes
There are several ways of magnesium joining, like welding, riveting or other mechanical fasteners, or
adhesive bonding. Regarding welding, it can be done by arc welding, resistance welding or friction stir
welding. The most common methods are gas tungsten arc welding (GTAW) and gas metal arc welding
(GMAW). They can be used also for repair welding and even defect removal. Magnesium alloys show
good weldability by friction stir welding, a solid state joining technique, resulting in low porosity, good
mechanical properties and minimal distortion. [12]
Some researches were conducted on GTAW welding of WE43 alloy, e.g. by A. Turowska and J. Adamiec.
[3] The resulting microstructure of the weld can be seen in the Figure 6. In the picture (c) we can observe
new grains strat to grow, and in the picture (d) a refined grains. As concluded in this research paper
the welds are sufficient to fulfill the requirements for the part’s operating conditions. However, if the
fracture may occur it would happen in the intermetallic phase as many voids form there and on the grain
boundaries. [3]
2.4 Creep
Kierzek and Adamiec (2012) performed creep test for weld joint of WE43 comparing the surface quality
of samples that had been heat treated and not. On the weld non heat treatable samples it can be seen
their poor surface quality (Figure 7) . Many cracks are located in grain boundaries, presenting high
creep rates. On contrary the samples that were heat treated present the opposite result. Due to the fine
dispersion of the Mg12NdY in the Mg matrix, the dislocation are hindered, so the creep resistance is
risen. According to the Table 2 the samples that heat-treated by the T6 process have deformed less. As
expected, the strain values are higher on elevated temperatures while on the HT samples the strain can
be four times smaller.
7

Figure 6: Welded joint of WE43 alloy: (a) macrostructure, (b) microstructure of the base material, (c)
microstructure in the heat aﬀected zone, (d) grain reﬁnement in the weld area. [3]
Table 2: Creep test with and without T6 heat treatment [8]
Figure 7: Microstructure of weld σ=90 MPa ,T= 250o
C a) cracks in weld , b) no cracks in heat treated
sample [8]
8

2.5 Heat Treatment
In order to increase the creep resistance, heat treatments can occur. According to the manufacturer’s
recommendation [13] the T6 HT is the one appropriate. T6 includes the process of solution heat treatment
for 8 hours at 525o
C and then air ageing at 250o
C for 16 hours. The results are grain growth and
dissolution of the precipitation phases as it is clear from the Figure 8.
Figure 8: a) Structure of WE43 without heat treatment b) Structure of WE43 with T6 heat treatment
[8]
2.6 Surface Treatments
The surface treatment techniques aim to protect the material form the environment and to decrease the
corrosion rate. In order to fulﬁll the biodegradable role the WE43 the formed coating must be has slow
corrosion rate at early stage which will be increasing progressively by the time in order to be absorbed
by the human body.
The oxide layers which protect the surface from the corrosion are MgO and crystalline Y2O3 [7]. The
initial low corrosion rate is resulted due to the progressively penetration of the protective layer. The
corrosion rate increases gradually where more and more parts of the surface are removed. The corrosion
resistance is depended from the layer thickness. The yttrium is depleted below the oxide layer and
improves the corrosion resistance of the material. The negative of point of the alloying is the decrease of
the ductility which can be countered by the strengthening techniques.
2.7 Thermal Treatments
Thermal treatments like recrystallization and ageing h.t. can increase the ductility in terms of elongation
to fracture up to 25%, but the yield stress will be minor decreased. The latters are result of the small
grain growth and to the easier dislocation propagation under the recrystallization process.
The high strength of the alloy WE43 is owed to the precipitation hardening and the T6 heat treatment.
The precipitation hardening happens at 210o
C during the artiﬁcial ageing and express the precipitation
of β” to β’.Worth noticing is that there are 4 precipitates starting from the solid solution [14]. The giving
phases are β”, β’, β1 and β. β” is a metastable phase, Hexagonal structure and coherent with the -Mg
matrix. β’ is also a metastable phase but semi coherent with the matrix, having chemical composition
Mg12Nd. β1 is an intermediate phase having face centered structure and chemical composition Mg3Y.
Finally the last precipitate is Mg14Nd2Y and has FCC structure. I.Peter and C.Castella et al. [14] carried
out additional heat treatments with varying ageing temperature and ageing time, keeping the 8h/5250C
9

from T6 HT, adding a quenching stage (water ,60o
C) among the two stages (solution treatment and
artiﬁcial ageing) and discovered that the highest hardening values can be obtained for the 210o
C ageing.
10

3 Microstructure
3.1 Magnesium and WE43 Alloy
In this chapter mechanical properties of Magnesium and more specifically WE43 alloy based on the
microstructure are considered. The structure of magnesium is described as well as mechanisms of defor-
mation and methods how to increase the strength of the alloys.
3.2 Mechanical Properties
Magnesium and its alloy have rather high specific strength, while at the same time the weight is relatively
low. The density of magnesium is 1.7 g/cm3
and elastic modulus is 45 GPa. [15] This makes the
magnesium alloys difficult to deform at low temperatures. Magnesium has a hexagonal closed-packed
(HCP) structure with low number of independent slip systems. A slip system is a combination of slip
plane and a slip direction. In HCP there are only basal slips available in the (0001) and {10¯10} plane
in 11¯20 direction, and the total number of slip systems is 3. [15, 16] According to the Taylor criterion,
which requires at least five independent slip system for homogeneous plastic flow, the HCP structure
does not comply. [17] At elevated temperatures, additional dislocation planes are thermally activated -
the pyramidal plane {10¯11} in the 11¯20 direction. The hexagonal structure with indicated planes and
directions of dislocations can be seen in the Figure 9. [12] All this properties makes the manufacturing
of magnesium and its alloys rather difficult. The best way to increase the strength and ductility of
magnesium alloys is to achieve a fine microstructure. The tensile ductility increases with smaller sizes
of grains. [18] As can be observed in the Figure 10, tensile ductility drastically increases with really fine
grains of a size smaller than 10 µm. [16, 18]
Figure 9: HCP planes and direction of dislocation slips. [12]
While decreasing the size of grains, the yield strength increases, because of the grain boundary strength-
ening. The effect was discovered by Hall and Petch in 1950s. The grain boundary strengthening or
Hall-Petch strengthening is based on the dislocation movement obstruction by the grain boundaries. The
grain boundaries pin the dislocations, which accumulate causing further dislocations in neighbouring
grains. With small sizes of grains the dislocations are not being piled-up in one point, but disperse them
at multiple boundaries. This requires more stress levels to cause deformation. [16] It is worth remember-
ing that the inverse Hall-Petch relation exists for very small grain sizes. [19] The Figure 11 shows values
for yield strength for different grain sizes in magnesium alloys.
The mechanism for deformation in HCP materials, with limited slip systems, like magnesium is twinning.
This can be observed in the Figure 12. [20] The twinning is indicated with blue lines. For pure magnesium
if the stress is increased the number of twinning also increases. According to Stanford et al. [21] the
precipitate particles in magnesium alloys prevent the formation of twinning as the twin growth requires
11

Figure 10: Grain size relation to elongation in a pure Mg at room temperature. [16]
(a) From [16] (b) From [20]
Figure 11: Hall-Petch relation for diﬀerent grain sizes.
12

the twinning dislocation to propagate along the twin interface along the twinning direction. The present
precipitate particles obstruct this movement.
Figure 12: Deformed pure magnesium samples with an applied strain of (a) 1% and (b) 4%. [20]
Another way to strengthen magnesium is through work hardening. This process increases the density
of the dislocations inside the magnesium structure. Multiple dislocations can act on each other and pin
themselves and obstruct the dislocation movement, which in result increase the strength. [16] One way to
accomplish that it to be done by equal-channel angular pressing. [22] The idea of this process is to press a
magnesium sample through an L-shaped die, while the strain is applied with no change in cross-sectional
dimensions. This method also reduces the size of the grains through recrystallization during pressing.
The results of the experiments with equal-channel angular pressing are shown in the Figure 13. [22] Work
hardening both reduces the size of the grains in magnesium and increases the ductility as well as the
strength.
(a) Grain size reduction after equal-channel angular
pressing.
(b) Stress - strain curve for diﬀerent number of press-
ings.
Figure 13: Results of equal-channel angular pressing. [22]
The last way to strengthen magnesium and its alloy is through solid solution strengthening and precip-
itation hardening. Both methods works on similar principle of introducing internal stress hindering the
13

movement of the dislocations. In solid solution strengthening a stress is generated by a size mismatch
compared to the regular crystal structure of interstitial or substitutional point defects. In precipitation
hardening, the precipitates (e.g. alloying atoms) act as a pinning points. [16]
3.3 WE43 Alloy
The WE43 magnesium alloy is a suitable alloy to use as a material for implants used in orthopedics.
The main area of use is osteosynthesis, which is stabilization of a fracture of a bone with the implants.
WE 43 is characterized by strength, corrosion resistance and non-toxicity. [16] The alloy consists of
magnesium, 3.7 - 4.3 wt.% of yttrium, 2.4 - 4.4 wt.% of rare earth elements like neodymium, ytterbium,
erbium, dysprosium, and ∼0.4 wt.% of zirconium. [23] What makes this alloy ideal for implant use is the
corrosion properties that will be explained in detail in further chapter. Overall, WE34 corrodes relatively
fast at the beginning until a protective layer forms on the surface. [16] The high strength of WE43 alloy
and similarity of the value of the Young’s modulus to the one of human bone is what makes this alloy
a perfect fit for osteosynthesis. The comparison of mechanical properties of the WE43 alloy to human
tissues can be seen in the Figure 14. After the bone is healed magnesium dissolves in the body and there
is no need for another operation to remove the fixture. [24]
Figure 14: Mechanical properties of different tissues compared to WE43 alloy. [10]
According to K. Kubota et al., the properties of thermo-mechanical treated WE43 are as seen in the
Figure ??. The tensile strength and elongation to failure is shown in regard to the temperature. WE43
possesses high strength, high ductility, high creep resistance and high strain rate superplasticity, thanks
to the fine precipitates and small grain size of about 1 µm. [25]
Regarding the phases of the WE43 alloy, the manufacturing method matter. Considering casting the
alloy consists of a solid solution α-Mg matrix with percipitates of intermetallic phases located both at
grain boundaries and grain interiors. [26] The X-ray diffraction shows α-Mg matrix, toghther with two
other phases: Mg24Y5 and Mg41Nd5. The micrograph and diffraction profile can be seen in the Figure
16. The binary phase diagram of magnesium and yttrium can be found in the Appendix A.1.
As concluded by T. Rzychoń and A. Kiełbus the microstructure of a casted WE43 alloy, beside the
aforementioned α-Mg matrix, consists of irregular shaped precipitates of Mg41Nd5, rectangular particles
of MgY, particles of Mg24Y5 and longitudinal percipitates of β (Mg14Nd2Y). [26]
3.4 Ultra Fine Grain Refinement
The Mg alloy WE43 has higher mechanical properties compared to the pure Mg but with cyclic defor-
mation by equal channel pressing followed by annealing at 360o
C for one hour ultra-fine grains with size
smaller than 1 µm can be produced, as seen in the Figure 17.
Fine grain structures can offer smaller grain sizes which increase the strength by 30% and the elongation
by 40% [5]. According to the Table 2 the mechanical properties of the alloy are increased. Worth noticing
14

Figure 15: Tensile strength and elongation to failure of WE43 as a function of temperature. [25]
(a) Micrograph of a casted WE43 alloy. (b) Diﬀraction proﬁle of a casted WE43 alloy.
Figure 16: Phases of a WE43 alloy. [26]
15

Figure 17: Microstructure of ultra-grained WE43 (grain size < 1µm)
is that the refined samples is that the corrosion rate is increased. The ultra-fine grained alloys is more
vulnerable to corrosion by 33% material loss in a period of 7 days compared to the industrial alloy which
presented only 17%. faster than the coarse grained due to the larger area of the grain and the interphases
boundaries in the ultra-fined samples. [27]
Table 3: Comparison of industrial WE43 and WE43 which has ultra-fine grain size below 1 µm [5]
16

4 Biodegradability
4.1 Biodegradable properties of WE43
Mg alloys is one of the most common choice for the osteosynthesis as implant material. This selection is
down to three main abilities that characterize this alloy. Firstly, it is compatible to the tissue and to the
bone, characterized by similar mechanical properties (tensile strength) (Table 4), secondly by tailoring
its corrosion rate can be fully biodegradable to the human body (7.8pH), taking into consideration that
it is not toxic or harmful in any way. The last ability of the alloy, which makes it a very favorable choice
is its ability to stimulate the bone formation [28].
Table 4: Young’s modulus for the most common orthopedic implants, worth noticing is that the Mg
alloys are closer to the bone’s modulus [16]
Implant Material Young’s Modulus [MPa]
Mg alloy 41-45
Ti 110-120
Co-Cr alloy 230
Human bone 3-20
In order to achieve a material loss for biomedical application , a control degradation of the surface
is needed. At early stages the degradation rate must be slow in order to assist the damaged bond and
support it; and afterwards to be higher in order to be vanished from the body. This can be accomplished by
applying surface coating techniques [29]. Moreover for the fully bone’s recovery the mechanical properties
of the alloy must be taken into account beside the corrosion ones. It is clearly understandable that high
corrosion rate equals to lower mechanical properties.
17

5 Corrosion and Surface Treatment
5.1 Introduction
At room temperature the electrode potential for the Mg is – 2.30V. According to the electrochemical
series its position is on the highest series making it one of least noble metals. Worth noticing is that
the mg solution potential in dilute chloride solution is less than -1.7V thanks to the polarization that
happens in the surface with a Mg(OH)2 film. The manufacture method plays a heavily role because during
the casting a lot of impurities can be casted which act as cathodes and will be able to create galvanic
corrosion. The impurities act as cathodic cells on the anodic magnesium matrix leading to a rise of the
corrosion rate. The Ni, Cu and the iron particles are the most common impurities in casting Mg alloys.
In order to eliminate the latter problem, one solution is the addition of Manganese particles which create
intermetallic compounds with the iron particles positioning them in the bottom of the melt. Moreover,
by adding zirconium particles the results are similar. One other approach to the corrosion problem is to
age threaten the alloy according to T6 process. Furthermore the creation of Zr-alloys and casting instead
of wrought products can also minimize the corrosion development. The corrosion is an electrochemical
process which is characterized by the loss of material usually starting from the surface. An electrolyte
in liquid form, specifically the blood consist the environment of the material. Corrosion tends to happen
when the surrounding environment is too aggressive for the Mg alloy. Then the elements of the material
which form a protective –passive layer, are not in sufficiently levels in order to hinder the corrosion rate.
As a result, the passive film cracks and a material loss is happening. In order to characterize a material
as corrosion resistant must has a corrosion rate less than 0.1 mm/year. Concerning the We43 which
is biodegradable the corrosion rate must be tailored with high precision, offering low corrosion rate at
the initial stage in order to assist the healing and strengthening phase of the bone and afterwards at
higher rates in order to be displaced from the body. The problem emanates from the difficulty of the
manipulation process because the corrosion rate can be increased dramatically during a wrong period.
The most common types of corrosion that affects the WE43 are the pitting and crevice corrosion, the
stress corrosion cracking and the intergranular one.
5.2 Corrosion in Biodegradability
On specific pH level, the corrosion rate of the alloy must be on a certain level in order to be degraded
fully on the pH level of the human body. This can be achieved by alloying with various elements or by
coating the mg surfaces. Magnesium can be dissolved in the human body with no complications because
on average the body contains up to 28 grams of Mg. In case of high corrosion rate the implant can
be degraded without fulfilling its purpose which is to mechanically support the broken bone. Moreover
during the dissolution of Mg, hydrogen is released and can create frustration on the damaged tissue.
The most significant application of a degradable implant is the non-required removal operation, which
by its avoidance can have numerous advantages in a patient’s life. Concerning the competitive implant’s
materials, the Mg alloys present higher strengths and creep resistance compared to the polymers and
extremely closer values to the human bone’s Young modulus compared with other biocompatible metals
lie Titanium, stainless Steel and d Co-Cr alloys. Moreover the latter materials, which are considered as
‘permanent implants’, after a period of time can release toxic debris due to the wear.
5.3 Corrosion
The corrosion type that is subjected the Mg alloys are mainly galvanic. The corrosion in aqueous solution
is happening according to:
Mg(s) + 2H2O(aq) → Mg(OH)2(s) + H2(g)
18

Mg → Mg2+
+ 2e−
2H2O + 2e−
→ H2 + 2OH−
Mg2+
+ 2OH−
→ Mg(OH)2
The formation of the magnesium hydroxide is key factor for the corrosion protection because it is the
main component of the protective film. In the pH of the human body which ranges around 7.35, the Mg
is actively dissolved so any formation is easy to happen. For pH values lower than 11.5, the Mg formats
Mg(OH)2 a passive film which protects the bulk part (Figure 18).
Figure 18: Pourbaix diagram of Mg. Worth noticing is the passivation starts at 11.5 [16].
5.4 Galvanic Corrosion
Low solubility of Mg- galvanic cells This type of corrosion is happening when the Mg is in contact with
other, less noble metal or due the existence of matrix’s impurities. On pure Mg impurities such as Fe,Ni
etc. can create corrosion even in extremely small portions. Moreover the low solubility of the Mg favors
the formation of micro galvanic cells causing internal galvanic corrosion. The impurities act as cathodes
at small areas, leading to dissolution of Mg [16]. One other influential factor is the grain boundaries,
where acting as cathodic regions compared to the grain interior. Moreover the dislocations that exist can
drop the equilibrium potential, accelerating the dissolution of Mg [16].
5.5 Stress Corrosion and Cracking
The SCC is usually affected and resulted by the hydrogen embrittlement, which tends to infiltrate to
the Mg matrix, degrading it from the inside. Hydrogen can also be diffused to the crack tips and create
19

magnesium hydride leading to brittle fracture due to hydrogen embrittlement [16].Due to the load bearing
application of the implants, stress corrosion cracking (SCC) consist one more corrosion type that can be
hazardous for the structure. For that reason the Zr and other RE are used as alloying elements in order
to limit the SCC effect [16].
5.6 Pitting Corrosion
The pitting corrosion is a highly localized type of corrosion that is resulted by the existence of halogenides
in the blood like chlorides which can break down the passive film. The pitting corrosion can characterize
the degradation process where the attack happens at a significant depth on the bulk material (high aspect
ratio). On the other hand, the crevice is defined by a narrow area where within it the corrosion happens.
5.7 Intergranular Cracking
The main cause of intergranular cracking is due to precipitation of chlorium carbides in the grain bound-
aries. The most common corrosion attacked areas are the stress concentration points and regions close to
the cathodic precipitates. Also crack propagation on the film surface is able to create transgranular stress
corrosion cracking. Brittle fracture can occur due to electrochemical attacks which form pits and notches,
which by their turn will assist the further dissolution of the Mg alloy [16]. In order to minimize the risk
of this corrosion type a stabilizing element like Nb must be added (niobium carbide does not precipitate).
For the above reason the WE43 has extremely low possibility to get degraded by intergranular corrosion.
5.8 Coating and Surface Treatment
The corrosion resistance of pure Mg can be improved at high temperatures by adding alloying elements
like Zr, like the WE43 alloy. Further improvement of corrosion resistance can be achieved by coating-
surface treatment techniques [6]. Coating techniques on the Mg alloy are used broadly in order to tailor
the corrosion rate of the alloy due the formation of a protective surface oxide layer. One of them is to
cover with Parylene film the mg alloy. One other and more typical approach is to immerse the alloy
to hydrofluoric acid. The reaction will produce MgF2 which form a protective layer. By applying high
voltage in range of 200 to 400 Volts, a anodic film containing Mg oxide and hydroxide will be formed.
The minimum corrosion rate (higher corrosion resistance) is achieved by the creation of an outer layer by
oxide fusion. The process parameters for the number and the size of the pores are the anodizing current,
the electrochemical potential and the immersion time. V.Birs et al. (2004) applying 10 mA/cm2
achieved
to create pores with diameter 0.5 µm. At high voltages the size of the pores become too large which
result to failure because the thin film cracked making it vulnerable to corrosion attacks.t Thus the highest
resistance emanates from the small sizes and numbers of the pores. In case of cracks appeared, localized
attacks will occur, they will decrease the polarization resistance (Polarization resistance: Rp behaves
as a resistor, meaning that high Rp equlas to high corrosion resistance of a metal) and undermine the
oxide layer over time [7]. Moreover on WE43 particles of Y2O3 will appear and they will try to slow the
degradation (increase corrosion resistance) by increasing the polarization resistance. Coating techniques
like covers by siloxane, phosphorylcholine (PC) and sulfobetaine (SB) are developed in order to manipulate
the corrosion rate. These macromolecules act as surface agents and can increase the biocompatibility or
decrease the corrosion rate [30].
20

6 Conclusion
Summarizing, properties of magnesium alloys with a focus put on WE43 alloy were studied and deemed
as suitable for medical use in orthopedics and osteosynthesis. Hexagonal closed-packed structure of
magnesium and its microstructure were researched based on the literature review. The state-of-the-art
research papers were studied to determine mechanical properties and strengthening methods of WE43
alloy. The most important feature of WE43 alloy was investigated - corrosion. Mechanisms of corrosion
and its inﬂuence on medical use were described. All in all, magnesium alloy WE43 was considered as
a ideal representative of the magnesium alloys to be used in medicine, and especially in orthopedics
as an implant ore bone fracture ﬁxture, as the second operation is eliminated because of the non-toxic
dissolution in the human body.
21

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23

A Appendix
A.1 Mg-Y Phase diagram
24

Use of Magnesium in Orthopedics

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