Microstructural characteristics and corrosion behavior
of cerium oxide conversion coatings on AA6063
M. Kanani, I. Danaee* and M. H. Maddahy
Cerium‐based conversion coating was deposited on aluminum 6063 and its
corrosion behavior was studied. Corrosion resistance was investigated at
different pH in 3.5 wt% NaCl solution by means of electrochemical impedance
spectroscopy, potentiodynamic polarization, and surface techniques. The coated
samples showed a signi ficant decrease in corrosion rate. The coating obtained by
600 s immersion covers uniformly the alloy surface. Electrochemical impedance data
shows that with increasing immersion time, the charge transfer resistance of
aluminum increased. Surface morphology and its chemical composition were
analyzed by means of scanning electron microscopy (SEM) and energy dispersive
spectroscopy (EDS). SEM images in agreement with the electrochemical results
indicated that aluminum surface was not completely covered after short immersion
time. The EDS analysis confirmed the presence of cerium oxide on the aluminum
surface and the cerium content was increased with increasing immersion time.
1 Introduction
Aluminum is widely used as a structural material because of its
good properties such as good strength to weight ratio, corrosion
resistance, and low cost [1,2]. Also owing to its low density [3,4]
aluminum is used in military and aerospace industry [1–3].
Aluminum needs alloying in order to develop high strength [1–3]
and thus decrease the corrosion properties of aluminum. Due to
the presence of the second phase particles in alloys, a potential
difference between aluminum matrix and alloy element
appeared. So a galvanic cell is formed, followed by a decrease
in aluminum corrosion resistance particularly against halide
ions [3–6]. AA6xxx aluminum alloys containing Al–Mg–Si,
attract significant attention nowadays. Presence of Mg
2
Si, give
them medium strength, formability, weld ability, and low cost.
These properties make this series of aluminum alloys more
applicable instead of more expensive AA2xxx and AA7xxx
alloys [7].
Conventionally chromate conversion coatings have been
largely used to prevent aluminum alloys from corrosion [8–12].
However, hexavalent chromium compounds are highly carci-
nogenic and toxic. Amongst the alternatives, rare‐earth coat-
ings particularly cerium have attracted significant attention
[13–21]. Cerium forms a stable oxide and hydroxide film, is
non‐toxic, and inexpensive. The deposition mechanism
involves both the oxidation of aluminum and the reduction
of H
2
O
2
in the reaction [13–17]:
Al ! Al
3þ
þ 3e ð1Þ
H
2
O
2
þ 2e ! 2OH
ð2Þ
Ce
3þ
þ OH
þ
1
2
H
2
O
2
! CeðOHÞ
2þ
2
ð3Þ
CeðOHÞ
2þ
2
þ 2OH
! CeðOHÞ
4
ð4Þ
CeðOHÞ
4
! CeO
2
þ 2H
2
O ð5Þ
H
2
O
2
is added to the coating bath as an effective accelerator
additive. The acceleration provided by the H
2
O
2
may be due to the
rapid increase in pH caused by H
2
O
2
reduction which would
improve the deposition of cerium oxide and cerium hydrox-
ide [6,22]. At lower pH according to the Pourbaix diagram Ce(III)
is more stable, so Ce(IV) reduces to Ce(III). However, in the
higher pH range, Ce(IV) is more stable particularly when
oxidizing agents like O
2
or H
2
O
2
are available [7]. Johansen and his
co‐workers report that the addition of glycerol results in finer
particles of cerium based conversion coating and acts as a
plasticizer [7]. Johansen and his co‐workers investigated the
corrosion protection of AA6063 aluminum alloy by cerium
conversion and conducting polymer duplex coating. The cerium
conversion, polyaniline, and duplex protective coatings, all shift
the values of corrosion and pitting potentials to more positive
values indicating corrosion protection [7].
The present work deals with the electrochemical properties of
6063 aluminum alloy covered with cerium obtained after different
M. Kanani, I. Danaee, M. H. Maddahy
Abadan Faculty of Petroleum Engineering, Petroleum University of
Technology, Abadan, (Iran)
E-mail: danaee@put.ac.ir
Materials and Corrosion 2014, 65, No. 11 DOI: 10.1002/maco.201307539 1073
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