Surface & Coatings Technology 198 (2005) 454 – 458
www.elsevier.com/locate/surfcoat
Corrosion resistance improvement of magnesium alloy using nitrogen
plasma ion implantation
X.B. Tiana,*, C.B. Weia, S.Q. Yanga, Ricky K.Y. Fub, Paul K. Chub
b
a
State Key Laboratory of Welding Production Technology, Harbin Institute of Technology, Harbin, China
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Available online 21 January 2005
Abstract
Magnesium and its alloys are receiving more attention due to their low mass density as construction materials. However, their poor
corrosion resistance has hampered their wide application in industries. In this work, we conduct plasma ion implantation on magnesium
alloys. This technique possesses the capability of treating objects with irregular shape, and it is also a clean process. During plasma ion
implantation, nitrogen plasmas were employed. After treatment, Rutherford backscattering spectrometry (RBS) was used to evaluate the
elemental depth profiles, and N1s spectra along the depth were acquired by X-ray photoelectron spectroscopy (XPS). The film structural
identification of phases was performed by X-ray diffractometry (XRD). The corrosion behavior of the samples was evaluated using
immersion test and potentiodynamical polarization test. We have found that the processes can effectively improve corrosion resistance of
magnesium alloys. However, the implantation parameters have to be chosen closely and the improvement of corrosion resistance is
mostly dependent on implantation voltage and incident dose. The results have demonstrated that the corrosion potential can be improved
by 200 mV and the corroded area of the samples properly treated is substantially reduced compared to that of untreated ones after
immersion tests.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Magnesium; Corrosion resistance; Nitrogen plasma ion implantation
1. Introduction
Magnesium and its alloys are receiving more and more
attention because of their light weight and superior specific
strength. They have been used to fabricate, for example,
automobiles, computer parts, aerospace components, mobile
phones, sporting goods, handheld tools, household equipment, etc. [1]. However, their wide application is still limited
due to their low corrosion resistance. Their low potential
(E8= 2.34 V vs. normal hydrogen electrode) leads easily to
reactivation even in the atmosphere. Unfortunately, the
natural oxide layer on magnesium surfaces is very loose
and cannot offer an effective resistance to corrosion. So,
many techniques such as ion implantation [2,3], laser surface
* Corresponding author. Tel.: +86 451 86418695; fax: +86 451
86416186.
E-mail addresses: [email protected], [email protected]
(X.B. Tian).
0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2004.10.117
treatment [4–6], microarc oxidation [7], physical vapor
deposition [8,9], etc. have been utilized to improve the
corrosion resistance of magnesium alloys. As a novel surface
treatment technology, plasma immersion ion implantation
(PIII) has received much interest due to its high efficiency,
simple instrumentation, and small equipment footprint.
Moreover, being a non line-of-sight process, PIII may treat
large and irregularly shaped specimens, and multiple samples
in a single batch without complex manipulation of target
holder are possible [10,11].
Nitrogen ion implantation to improve corrosion resistance of materials such as austenitic stainless steels [12] and
titanium alloys [13], etc., has proved to be feasible. As
investigated [3], nitrogen ion implantation may be utilized
to improve the corrosion resistance of magnesium alloy,
which is speculated to be related to the formation of
bimplantation-affected zoneQ. It was also reported [14] that
under nitrogen atmospheric condition, corrosion resistance
of magnesium alloys was improved significantly after laser
X.B. Tian et al. / Surface & Coatings Technology 198 (2005) 454–458
455
cladding. It was considered to be due to the formation of
magnesium nitrides in the laser-modified layer.
In this paper, magnesium alloys AZ31B (Al: 3.0–4.0
wt.%, Mn: 0.15–0.50 wt.%, Zn: 0.20–0.80 wt.%, Cu: 0.05
wt.%, Si: 0.1 wt.%, Fe: 0.05 wt.%, Be: 0.01 wt.%, Ni: 0.05
wt.%, impurity: 0.30 wt.%, Mg: balance) were treated using
nitrogen plasma ion implantation. The process may be
characterized by ion implantation and excited nitrogen
atmosphere. It is speculated to be effective in improving
the corrosion resistance of magnesium alloys.
2. Experimental
The samples were AZ31B magnesium alloy sheet
(12125 mm). One side of each sample was ground
with 800-grit diamond paper. Then the samples were
chemically polished with 10% nitric acid–methanol solution
before loading into the plasma immersion ion implanter
[11]. The facility is equipped with RF plasma source, hot
filament glow discharge source, vacuum arc source, etc.
The chamber is 1200 mm in height and 1000 mm in
diameter. Before PIII, the samples were sputter-cleaned with
argon plasma ion bombardment. The pretreatment instrumental parameters were: RF inforward energy=1000 W
with reflected power of around 20 W, bias voltage=500 V,
gas flow=10 sccm, and clean time=20 min. Afterwards,
nitrogen was bled into the vacuum chamber and nitrogen
plasma was sustained by RF power supply with a power of
1000 W, work pressure of 5.510 4 Torr, and gas flow of
20 sccm. Instrumental parameters during experiments are
shown in Table 1.
To evaluate corrosion behavior, potentiodynamic polarization tests were conducted using a model 342 softcorrvk
corrosion measurement system. The tests were carried out in
a 3 wt.% NaCl solution and the scanning rate was 0.5 mV/s.
Rutherford backscattering spectrometry (RBS) was used to
measure elemental depth profiles, and X-ray photoelectron
spectroscopy (XPS) was utilized to detect depth profiles of
N1s spectra. The structural identification of phases was
performed by a grazing incidence X-ray diffractometry
(GXRD) with the incident beam angle (a) of 28. The
samples were also immersed in 3.5% NaCl solution
saturated with Mg(OH)2 to evaluate the effectiveness of
the process. All the immersion essays were performed in
open vessels without agitation. The edges of samples were
Table 1
Instrumental parameters during nitrogen plasma ion implantation
Sample no.
Pulse
voltage
(kV)
Pulse
frequency
(Hz)
Pulse
duration
(As)
Implantation
time (h)
Mg-00
Mg-22
Mg-42
Mg-44
Unimplanted
20
40
40
0
200
200
200
0
30
30
30
0
2
2
4
Fig. 1. RBS spectra of unimplanted and implanted magnesium alloy.
not sealed. After immersion tests, optical microscope was
used to observe the morphology of the surfaces. Afterwards,
the corrosion area was evaluated using a software according
to the colour difference of the digital pictures of the
corroded sample surfaces.
3. Results
Fig. 1 depicts the RBS results showing the elemental
depth profiles. Apparently, the nitrogen signal is
observed, which indicates that the nitrogen has been
implanted into samples. It also shows that oxygen
concentration of implanted samples decreases compared
to that of the unimplanted sample. Higher Al concentration is found on the top surface of implanted samples
and a trace of Zinc is also detected. These may be related
to the sputter effect of ion implantation. It was reported
that the sputtering effect is sometimes severe during
plasma ion implantation [15]. It is confirmed by the
morphology of the surfaces of treated samples shown in
Fig. 2. The boundaries and groves can be clearly seen
after plasma ion implantation, while the surface of the
unimplanted sample is very smooth.
Fig. 3 depicts depth profiles of N1s spectra acquired from
the sample Mg-42. Interestingly, nitrogen is not detected at
the top surface in spite of nitrogen implantation and excited
nitrogen environment. In fact, the top surface is rich in
oxygen. Only in the deeper surface region does nitrogen
concentration show a Gaussian distribution. The peaks of
nitrogen appear at 397.4 eV, which is related to nitrogen in
AlN [16].
Fig. 4 shows the XRD patterns of all the samples.
Unfortunately, the difference of all patterns is not substantial. The diffraction peaks are mainly composed of those
of MgO and Mg. The new diffraction peaks are very weak,
except the original peaks of magnesium alloys. It may be
due to the thin implanted layer in our experiments. AlN
peaks seem to be observed with increasing implantation
time and confirmed by XPS results.
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X.B. Tian et al. / Surface & Coatings Technology 198 (2005) 454–458
Fig. 2. Optical images showing surface morphology: (a) Mg-00; (b) Mg-44.
The results of polarization tests are indicated in Fig. 5.
The polarization tests demonstrate that with proper implantation parameters, corrosion resistance of magnesium alloy
can be effectively improved by nitrogen plasma ion
implantation. The sample Mg-22 shows no enhancement
of the corrosion resistance and the corrosion resistance even
decreases. In contrast with increasing implantation energy,
the corrosion potential of Mg-42 is improved by about 200
mV, indicating better corrosion resistance, which is confirmed by immersion test discussed later. However, with
increasing implantation time accompanied by higher energy,
the corrosion potential of the treated sample contrarily
decreases, although it still is higher than that of the
unimplanted one.
Fig. 6 shows corrosion appearance on the surfaces of the
samples immersed in 3.5% NaCl solution saturated with
Mg(OH)2. Generally, the corrosion primarily initiates at the
defects of edges and, usually, strip-like pit trace is
observed. The immersion test agrees well with the polarization tests. After 8 h of immersion test, 40% of the
surface area of the sample Mg-00 has been eroded. In
contrast, the sample Mg-42 is only corroded by 8.4% even
after 11 h of immersion test. In addition, the corrosion area
of the sample Mg-22 is 52% that of the whole sample,
larger than that of untreated sample Mg-00. Again, the
results in the immersion test are consistent with potentiodynamic polarization test results.
Fig. 3. Depth profiles of XPS N1s spectra acquired from the sample Mg-42.
Time interval: 2 min.
Fig. 4. Grazing incidence XRD patterns of unimplanted and ion-implanted
magnesium alloy.
4. Discussion
It has been shown that nitrogen plasma ion implantation
can effectively improve the corrosion resistance of magnesium alloys. On one hand, as proposed by Nakatsugawa
et al. [3], the main factor that contributes to improving the
corrosion resistance of magnesium alloy by ion implantation was not the ion species of ion sources, but the amount
of ions or the degree of the bimplantation-affected zone.Q
Ion bombardment would create a deep zone characterized
as the nodal and line dislocations that may improve surface
properties. On the other hand, MgO on magnesium alloys
has a positive influence on corrosion resistance and the
dense coatings are of great importance in preventing
X.B. Tian et al. / Surface & Coatings Technology 198 (2005) 454–458
457
surface corrosion [17]. In fact, protective coatings must
primarily have low porosity and high compactness. From
the standpoint of the above facts, plasma ion implantation
produces hybrid effects of ion irradiation and surface oxide
layer indicated by RBS and XPS. The irradiation effect is
easy to understand during plasma ion implantation. As for
oxide layer, it may stem from two sources: one is
attributed to the chemical polishing in 10% nitric acid–
methanol solution; the other is the oxidation effect during
plasma ion implantation. This is commonly encountered
due to the excited state and reactivity of residual oxygen
[18,19].
As seen from the above tests, the corrosion resistance is
much dependent on the implantation energy and incident
dose. A lower implantation energy does not lead to the
improvement and even has an adverse effect on corrosion
resistance. This is sharply in contrast with the results when
boron was implanted [20], where the low energy was more
effective. This adverse effect may be due to sputtering
during plasma implantation as shown in RBS and Fig. 2.
The original oxide layer has been eroded by ion bombardment, which reduces the natural block capability and,
consequently, the sample is apt to corrosion. When the
implantation energy increases, the sputtering effect
decreases and most of the primary oxide layer still exists.
More importantly, the bombardment effect increases, which
may compact the original layers. In other words, the higher
energy produces two effects (i.e., more irradiation effect and
much denser oxide layer). Therefore, it is not strange that
sample Mg-42 possesses the best corrosion resistance. In
contrast, a much higher dose with higher energy may
weaken the improvement of corrosion resistance. This
phenomenon is also reported elsewhere [3,21]. Fukumoto
et al. [21] thought that it is due to the formation of Mg3N2
phase with the higher dose. Mg3N2 is sensitive to the
atmospheric humidity; leading to that higher dose may have
probably negative influence on corrosion resistance. It was
also thought to be related to the possibility of excessive ion
Fig. 6. Morphology of the surface after immersed in 3.5% NaCl solution
saturated with Mg(OH)2: (a) Mg-00 after 8 h; (b) Mg-22 after 8 h; (c) Mg-42
after 11 h.
agglomerating and moving with the interface, which makes
the surface reactive [3].
5. Conclusion
Fig. 5. Potentiodynamic polarization curves exposed to 3% NaCl solution.
Nitrogen plasma ion implantation can effectively
improve the corrosion resistance of magnesium alloy under
proper conditions. The implantation time and energy should
be closely optimized to achieve better surface properties.
Severe surface sputtering and possible formation of a small
amount of Mg3N2 phase [21] may have an adverse effect. In
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X.B. Tian et al. / Surface & Coatings Technology 198 (2005) 454–458
our case, sample Mg-42 (bias voltage of 40 kV, treatment
time of 120 min) possesses the highest corrosion resistance,
confirmed by both polarization and immersion tests. The
improvement of corrosion resistance after plasma implantation may be attributed to the compactness of the loose
natural oxide layer and ion irradiation effect.
Acknowledgments
The work was jointly supported by the Natural Science
Foundation of China (nos. 10345003 and 50373007), Hong
Kong RGC CERG (no. CityU1137/03E) and City University of Hong Kong SRG (no. 7001642).
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