Materials Chemistry and Physics 179 (2016) 5e9

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Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys

Materials science communication

Surface potential analysis on initial galvanic corrosion of Ti/Mg-Al
dissimilar material
Junko Umeda, Nozomi Nakanishi, Katsuyoshi Kondoh*, Hisashi Imai
Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaragi, Osaka 567-0047, Japanh i g h l i g h t sg r a p h i c a l a b s t r a c t

 Pure Ti/AZ80 Mg alloy dissimilarmaterial was prepared by spark
plasm sintering.
 Additional heat treatment caused aformation of Ti-Al interlayer.
 Signiﬁcant decrease of potential difference of 0.24e0.33 V was found atinterface.
 Heat treated dissimilar material
showed very few Mg(OH)2 debris after salt water immersion test.

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 21 December 2015
Received in revised form
6 May 2016
Accepted 11 May 2016
Available online 24 May 2016

A surface potential analysis was carried out to establish a new materials design of pure Ti/Mg-Al alloy
dissimilar materials with an excellent galvanic corrosion resistance. The formation of Ti-Al intermetallic
interlayer and Al diffusion layer at interface effectively reduced the surface potential difference at theinterface. As a result, a formation of local cells, causing a galvanic corrosion phenomenon of Ti/Mg alloy
dissimilar material, was prevented because the potential difference of Mg alloy with Ti-Al interlayercompletely decreased. A salt water immersion test obviously indicated no Mg(OH)2 corrosion product on
Mg alloy surface due to Ti-Al interlayer formation, and the initial galvanic corrosion phenomenon waseffectively obstructed.
© 2016 Elsevier B.V. All rights reserved.

Keywords:
Intermetallic compounds
Interfaces
Corrosion test
Diffusion

1. Introduction
A multi-materials design is widely applied to automotive andaerospace components to realize both signiﬁcant weight reduction
and strength improvement at the same time. The dissimilar material by joining the different materials plays an important role toobtain many advantages such as downsizing, cost reduction, andhigh-performance of the components. Al/steel [1], Mg/steel [2], andTi/steel [3] dissimilar jointed materials are representative* Corresponding author.
E-mail address: kondoh@jwri.osaka-u.ac.jp (K. Kondoh).
http://dx.doi.org/10.1016/j.matchemphys.2016.05.031
0254-0584/© 2016 Elsevier B.V. All rights reserved.

combinations in the industrial application. A dissimilar material
consisting of Ti and Mg alloys has a large potential for weight
reduction because both metals show high speciﬁc strength andspeciﬁc Young’s modulus compared to the other industrial metals.
This also has a possibility to be employed as lightweight reinforcements of aircrafts. With regard to the bonding ability of Ti toMg, the previous study shows a good wettability of pure Ti bymolten pure Mg because of Ti plate surface modiﬁcation after TiO2
layer chemically reduced by contact with molten Mg [4]. A ﬁbre
laser welding process was one of the suitable joining techniques toprepare this unique dissimilar material [5]. In this combination,
however, there are two important problems in joining; a small solid

6J. Umeda et al. / Materials Chemistry and Physics 179 (2016) 5e9Fig. 1. SEM-EDS analysis results (a) and XRD proﬁle (b) near interface of pure Ti/AZ80 dissimilar material after heat treatment at 1073 K for 1.8 ks in vacuum (~6 Pa). TiAl3
intermetallic compound inter layer and Al concentrated area are formed via reaction between Ti and Al elements from molten AZ80 alloy.

solubility of Ti in Mg and that of Mg in Ti, and a large melting point
difference between pure Mg and pure Ti (1019 K) [6]. In addition,
the Al content contained as a strengthening alloying element in Ti
and Mg alloys has a signiﬁcant effect on the interfacial properties ofthe dissimilar material. For example, microstructures and mechanical strength depend on the formation of Mg-Al eutectic phases [7] and Ti-Al intermetallic compounds [8] near the interface of
the Ti/Mg dissimilar material. From a viewpoint of galvanic corrosion originated in the local cells at interfaces of the dissimilarmaterial, a large difference of the standard electrode potential between Ti ( 1.63 V) and Mg ( 2.35 V) possibly causes a galvanic

corrosion phenomenon [9]. The galvanic corrosion is also inﬂuenced by the Al concentration near the interface because of Mg-Al
phase formation and Al solid-solution into a-Mg matrix [10].
In this study, the initial galvanic corrosion behavior at theinterface of pure Ti/AZ80 (Mg-Al) alloy dissimilar material preparedby spark plasm sintering (SPS) in solid-state and the following heat
treatment was investigated by surface potential difference measurement using scanning Kelvin prove force microscopy (SKPFM)
system [11e13]. The previous study showed that SKPFM measurement results were useful to estimate the corrosion potentials ofmaterials because a linear relationship between the work functions

J. Umeda et al. / Materials Chemistry and Physics 179 (2016) 5e9

7Fig. 2. Mg, Ti and Al element distribution measured by EDS line-analysis and surface potential distribution mapping analyzed by SKPFM of Sample I (a) and Sample II (b). Sample Ishows never Al diffused and concentrated area at Ti/AZ80 interface, and a large potential difference of 0.52e0.71 V at the interface. Sample II obviously reveals Al diffusion fromAZ80 into pure Ti, and a stepwise change in surface potential at the interface. In particular, AZ80 alloy shows a small potential drop of 0.24e0.33 V due to TiAl3 compound interlayer
formation.

Originalinterface
(b)

(a)

AZ80

AZ80

Ti

No reaction layer

V1

Ti

Diffusion layer
Potential

Potential

Interlayer

V2Fig. 3. Schematic illustration of surface potential changes near Ti/AZ80 interface without reaction layer, Sample I (a) and with interlayer and diffusion area, Sample II (b) of pure Ti/
AZ80 dissimilar material. A large surface potential difference (V1) in Sample I is certainly reduced due to formation of interlayer and diffusion layer by Al elements originated from
AZ80 alloy, and results in a small potential drop (V2) in Sample II.

by SKPFM and corrosion potentials of pure metals and intermetallic
compounds was clearly found [14]. In addition, the potential difference measured by SKPFM was applied to evaluate the local
corrosion phenomenon of cold-rolled stainless steels [15]. The effect of Al elements originated from AZ80 alloy on the interlayer
formation and surface potential distribution at the interface, which
affected galvanic corrosion phenomena, was discussed. In addition,
the morphology observation on the dissimilar specimen surface
after salt water immersion test was also carried out in this study.
2. Experimental procedure
Disk-shaped specimens of Mg-7.8Al-0.68Zn-0.18Mn (AZ80)
alloy (in mass%) after homogenization at 688 K for 24 h and pure Ti(purity; 99.5%) were put in a carbon die (inner diameter of 41 mm)

installed in the SPS equipment. 40 MPa pressure was applied tobond them at 673 K for 2 h in vacuum (~6 Pa). As-joined Ti/AZ80
dissimilar material (Sample I) was served to the following heat
treatment at 1073 K for 0.5 h in argon gas atmosphere to synthesize
Ti-Al intermetallic layers at the bonding interface. This is because
the heating temperature is much higher than the melting point ofAZ80 alloy and effective enough to their compounds formation bythe active liquid state diffusion of Al elements originated in AZ80
alloy [8]. Scanning electron microscope (SEM) with energydispersive X-ray spectroscopy (EDS) analysis was carried out toinvestigate the interfacial microstructures of the dissimilar material, in particular, the formation of Ti-Al interlayer and Al diffusion
layer in pure Ti. Scanning Kelvin probe force microscope (SKPFM)
analysis was conducted to directly measure the surface potential
drop at the interface causing the initial galvanic corrosion. In8J. Umeda et al. / Materials Chemistry and Physics 179 (2016) 5e9addition, a salt water immersion test was performed to analyze aformation of surface corrosion products of AZ80 alloy by using 5 wt
% NaCl solution, where the solution temperature was 303 K andsoaking time was 3 h. SEM-EDS analysis was conducted to examine
the surface changes and corrosion damages near the joined interface after the above corrosion immersion test.
3. Results and discussion
Fig. 1 (a) reveals an interlayer formation with about 2 mm
thickness at the interface between AZ80 and pure Ti of the dissimilar material after the above heat treatment (Sample II) by
scanning electron microscopy (SEM). Al elements concentrated at
the layer are also detected by energy-dispersive X-ray spectroscopy(EDS) analysis. In addition, the diffused Al elements area close to
the interface, but no Ti diffusion into AZ80 side is found after heattreatment. X-ray diffraction (XRD) analysis in (b) obviously showssome diffraction peaks of not only Ti, Mg, MgO, Mg17Al12 (eutectic),
Al6Mn originated from the raw materials, but also in-situ formed
TiAl3 compounds during heat treatment, which are most thermallystable at 1073 K in Ti-Al phases [16], near the interface.
Fig. 2 indicates SEM-EDS and SKPFM analysis on Samples I andII. In Sample I, neither Al diffusion nor interlayer formation at thebonding interface is observed in SEM photo with EDS-line analysis
result (a-1). This is because SPS temperature of 673 K, which islower than that in the homogenization treatment (688 K), is noteffective to Al diffusion of AZ80 alloy. In addition, the bonding
interface is very ﬂat and smooth compared to Sample II due to no
metallurgical reaction between these metals in Sample I. SKPFM

was conducted at the same area in (a-1) to measure the surface
potential distribution near the bonding interface. As shown in (a-2)
and (a-3), the potential at AZ80 specimen is much higher than thatof pure Ti, and drastically deceases at the interface, where the potential drop is 0.52e0.71 V (average at 4 measurement; 0.62 V). On
the other hand, as shown in Fig. 2 (b-1) of Sample II, it evidently
reveals not only the concentrated Al elements at the interface but
also the uneven interface after heat treatment because of themetallurgical bonding after the chemical reduction of TiO2 surface
ﬁlm of pure Ti specimen by liquid Mg elements [4]. Regarding thesurface potential distribution mapping, a stepwise change in thepotential near the interface was detected in (b-3). It means thegradual slopes from AZ80 to the interface and from the interface topure Ti specimen were clearly observed. The formation of TiAl3
reaction layer, having a constant potential of 0.79e0.84 V, interrupts the direct bonding of AZ80 alloy to pure Ti, and results in asmall potential drop (0.24e0.33 V) of AZ80 specimen near theinterface. Fig. 3 schematically illustrates the potential changes near
the interface of pure Ti/AZ80 dissimilar materials with no reaction
layer, Sample I (a) and with Ti-Al interlayer and Al diffusion layer,Sample II (b). It is concluded that the interlayer formation between
AZ80 and pure Ti by heat treatment in Sample II effectively decreases a surface potential difference at the interface, which causes
local cells to accelerate the galvanic corrosion phenomenon.
Fig. 4 presents the surface morphology changes of each Ti/AZ80
dissimilar material after the salt water immersion corrosion test
observed by SEM-EDS analysis. In general, Mg easily resolves into
an aqueous solution, and magnesium hydroxide (Mg(OH)2) ﬁlms
are formed as corrosion products on Mg specimen surface via

Fig. 4. Surface morphology changes near pure Ti/AZ80 interface of Sample I (a) and Sample II (b) by salt water immersion test, and SEM-EDS analysis results of each specimen.
Mg(OH)2 corrosion products and oxygen elements concentration are obviously observed on AZ80 alloy surface of Sample I. In Sample II, very few corrosion products are detected
near the interface after immersion corrosion test.

J. Umeda et al. / Materials Chemistry and Physics 179 (2016) 5e9

general whole surface corrosion phenomena [17]. Additionally,
when a large potential difference at the interface between intermetallic compounds and a-Mg matrix occurs, it also causes agalvanic corrosion phenomenon with Mg(OH)2 formation [18e20].
Fig. 4 (a-1) and (b-1) indicate no Mg(OH)2 formation on AZ80 alloy
in both samples before the corrosion test. As shown in (a-2),
however, Mg(OH)2 corrosion products are obviously detected onAZ80 surface near the interface of Sample I after corrosion test. EDS
analysis result also reveals the concentrated oxygen elements
originated from Mg(OH)2 at AZ80 surface. The potential differenceof 0.52e0.71 V at the interface shown in Fig. 2 (a-3) is almost thesame as that in a combination of Mg with pure Cu (0.33e0.68 V)and pure Fe (0.80e0.87 V) measured by SKPFM, It is well known asmall addition of Cu and Fe elements as impurities in Mg materials
causes a severe galvanic local corrosion [21]. Then, it is concluded
that such a large potential difference in Sample I is enough for an
acceleration of the galvanic phenomenon and results in formationof many corrosion products on AZ80 surface near the interface
shown in Fig. 4 (a-2). On the other hand, (b-2) indicates very few
corrosion products in AZ80 alloy near the interface of Sample IIafter corrosion test, and no concentration of oxygen is detected atthe interface. In the previous studies on the surface potential difference measurement between intermetallic compounds and Mg
matrix by using SKPFM system, the average of Mg with Mg17Al12and Al8Mn5, which are commonly contained in the commercial Mg
alloys, are 0.15 V and 0.26 V, respectively [22,23]. It is also wellopened that these compounds have no effect on the galvanic acceleration of the conventional Mg alloys. It means the potential
difference in the above range hardly causes a galvanic corrosion ofMg alloys. A small potential drop (0.24e0.33 V) at the interface byboth TiAl3 interlayer formation and Al concentration in Sample IIshown in Fig. 2 (b-3) is not effective in inducing the galvanic
corrosion, and results in very few Mg(OH)2 corrosion products
formation as shown in Fig. 4 (b-2).
4. Conclusions
The pure Ti/AZ80 bonded dissimilar material prepared by SPS in
solid-state had no formation of Ti-Al intermetallic compound layer

9at the interface. The additional heat treatment at 1073 K to thedissimilar material affected the interlayer formation and Al diffusion near the bonding interface. In the former material, a large
potential drop of 0.52e0.71 V at the interface was measured by
SKPFM, and caused many corrosion products of Mg(OH)2 debris bygalvanic local corrosion phenomenon. The latter with the interlayers showed a signiﬁcant decrease of the potential differencewith 0.24e0.33 V at the interface, and resulted in preventing thegalvanic corrosion of AZ80 specimen.

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