31_2012_Ping Qiu

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Materials Chemistry and Physics 133 (2012) 419– 428
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics
j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys
volution of corrosion products and metal release from Galvalume coatings on
teel during short and long-term atmospheric exposures
ing Qiu ∗, Christofer Leygraf, Inger Odnevall Wallinder
TH Royal Institute of Technology, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden
 r t i c l e i n f o
rticle history:
eceived 17 May 2011
eceived in revised form 14 January 2012
ccepted 18 January 2012
eywords:
alvalume coating
tmospheric corrosion
unoff
a b s t r a c t
Non-treated Galvalume (55% Al, 43.4% Zn and 1.6% Si by weight) coatings have been studied through a
combination of surface, near surface and bulk analysis after exposure at marine conditions, and for com-
parison also in an urban test site and in successively more complex short-term laboratory exposures.
Slightly polished Galvalume surfaces exhibit dendritic aluminum-rich areas with higher Volta potential
compared with interdendritic zinc-rich areas. These effects were not observed on bare as-received sur-
faces due to the overall presence of aluminum oxide. As a result, preferential corrosion occurred initially
in interdendritic areas. The zinc release rate followed the same time-dependence as the surface coverage
of zinc-containing phases at the marine exposure condition with zinc predominantly released compared
orrosion product evolution to aluminum. Short term laboratory exposures generated the same main phases as formed at marine
conditions. This confirms that the evolution of corrosion products and time dependence of zinc release
rates can be explained by the uniform formation of less soluble Al2O3, AlOOH and Al(OH)3 compared to
observed zinc-containing phases, e.g. ZnO, zinc hydroxycarbonate and zinc hydroxychloride. The same
underlying mechanism is believed to operate also during exposure of Galvalume in the urban site studied.
© 2012 Elsevier B.V. All rights reserved.
. Introduction
Different zinc-based coating systems exist on the market pro-
iding sacrificial protection of steel substrates used in automotive,
onstruction and industrial applications. Depending on application,
he coating is often pre-treated in different ways. Hot-dipped gal-
anized coatings with small amounts of aluminum on steel are
redominantly used due to their beneficial corrosion resistance
roperties, in particular at atmospheric conditions where the sur-
ace is exposed bare or with temporary surface treatments [1,2]. To
ptimize the corrosion properties even further, the Galvalume coat-
ng (Al–43.4% Zn–1.6% Si by weight) was developed in the 1970s by
ethlehem Steel Co., US. The combination of the beneficial proper-
ies of aluminum and zinc allows an improved galvanic protection
f steel compared to galvanized steel at equivalent coating thick-
esses, and an efficient ability to galvanically protect the substrate
t scratches and cut-edges [3–5]. Its durability and corrosion resis-
ance at atmospheric conditions are typically improved two to four
imes compared to conventional galvanized steel coatings [6–12].
he coating is applied on steel strip in a continuous hot-dip process
∗ Corresponding author. Tel.: +46 08 7909925; fax: +46 08 208284.
E-mail address: pingq@kth.se (P. Qiu).
254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
oi:10.1016/j.matchemphys.2012.01.054
and exists with different trade names (e.g. Galvalume, Zincalume,
Aluzink, Aluzinc, Zalutite) depending on producers and part of the
world. Its microstructural characteristics with aluminum-rich den-
drites and zinc-rich interdendritic areas have been characterized
to some extent [13,14]. Formation and growth of corrosion prod-
ucts have primarily been reported to occur in interdendritic regions
[9,14–18].
Due to an increased environmental concern related to the
diffuse dispersion of metals from different sources in the soci-
ety, metal release induced by atmospheric corrosion of outdoor
constructions has during the last decades been investigated for
zinc-based materials, e.g. [19–28]. Scarce information is though
available for Galvalume [15,19,29,30].
The aim of this paper is to investigate the initiation and evolution
of corrosion products and their lateral distribution on Galval-
ume at atmospheric conditions. This has been accomplished by
using a combination of surface, near-surface and bulk analytical
tools on non-treated Galvalume surfaces exposed at short (weeks)
and long-term (years) marine and urban non-sheltered condi-
tions, and through analysis of coatings exposed to successively
more complex short-term laboratory conditions of relevance for
the field conditions. The stability of corrosion products formed at
outdoor conditions was quantitatively assessed by metal release
measurements.
dx.doi.org/10.1016/j.matchemphys.2012.01.054
http://www.sciencedirect.com/science/journal/02540584
http://www.elsevier.com/locate/matchemphys
mailto:pingq@kth.se
dx.doi.org/10.1016/j.matchemphys.2012.01.054
4 try an
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20 P. Qiu et al. / Materials Chemis
. Material and methods
.1. Materials and surface preparation
Non-treated (without any surface treatment or conversion
oating) 20 �m thick GalvalumeTM coatings (55Al–43.4Zn–1.6Si,
t%) applied on steel in a continuous hot-dip process were sub-
ect for parallel field and laboratory testing. Bare surfaces were
nvestigated to enable studies between corrosion initiation and
icrostructural differences, processes difficult to follow for treated
urfaces. The lifetime of such surface treatments are furthermore
imited after which the material will corrode in a similar way as the
ntreated material [26]. SSAB, Sweden, supplied all samples.
Prior to the laboratory investigation, all surfaces (sized
0 mm × 20 mm × 1 mm) were gently polished with 0.25 �m dia-
ond paste. Pure aluminum metal (99.9 wt%) and zinc metal
99.7 wt%) surfaces exposed in parallel studies were abraded and
olished to the same surface finish. Prior to exposure, each sample
as sonicated for 5 min in 99.5% ethanol and dried with dry N2 gas.
Samples of Galvalume and zinc sheet exposed at field condi-
ions were exposed as-received, i.e. non-treated and non-polished.
ingle-sided panels (300 cm2) were used to continuously moni-
or metal runoff (marine site: 5 years; urban site: 10 years) and
maller coupons (40 cm2) exposed for surface analysis (marine site:
, 4, 12, 26 weeks, 1 and 5 years, urban site: 5 and 10 years).
ll cut-edges of the Galvalume coating were sealed with a non-
inc-containing polyurethane paint. Further detailed information
s given elsewhere [19,29].
.2. Laboratory and field exposure conditions
Measurements in CO2-reduced and non-CO2 reduced humid air
ere conducted in an in situ exposure chamber integrated with
he IRAS spectrometer. Detailed description of the preparation of
ynthetic air has been presented elsewhere [31,32]. The air flow
ate along the sample surface was 3.5 cm s−1. All exposures were
onducted at a relative humidity (RH) of 90 ± 3%, and a temper-
ture of 19.5 ± 0.5 ◦C. The CO2-concentration was set to 350 ppm
approximately ambient air concentration) [33]. Pre-deposition
f Galvalume surfaces was conducted at a NaCl concentration of
5 �g cm−2 according to a procedure described elsewhere [34].
Long term field exposures were performed on surfaces exposed
t non-sheltered conditions at the marine site of Brest, France
December 2004–December 2009) and at the urban site of Stock-
olm, Sweden (June 1998–June2008). All surfaces were exposed
t 45◦ from the horizontal, facing south, in agreement with the ISO
226 standardized exposure condition for corrosion and runoff rate
easurements [35,36]. All samples exposed at the marine site were
ositioned 5–10 m from the waterline [26]. Samples exposed at the
rban site were located on a roof of an 8-storey building within
he city center [26]. Both sites are characterized by low pollutant
evels of gaseous SO2. The main differences between the sites are
ignificantly higher deposition rates of chlorides, annual rainfall
uantities, and longer wet conditions at the marine site compared
o the urban site. Details are given elsewhere [19,26,29].
.3. Metal runoff measurements
All samples were mounted on Plexiglas fixtures equipped with
nclined gutters into which impinging rainwater continuously was
ollected and transported to polyethylene collecting vessels. All
essels were acid cleaned to avoid contamination [26]. Runoff
ater from a bare Plexiglas fixture exposed in parallel was col-
ected to determine background levels of zinc and aluminum. Total
eleased concentrations of zinc and aluminum were determined
y means of atomic absorption spectroscopy (Perkin Elmer Analyst
d Physics 133 (2012) 419– 428
800). Five replica readings were performed for each sample with the
flame technique (AAS-F) and triplicate readings were performed
during the graphite furnace (AAS-GF) analysis. Limits of detection
were determined to 10 �g L−1 for zinc (AAS-F) and 20 �g L−1 for
aluminum (AAS-GF). Quality control samples were analyzed every
10th sample for both methods to verify the calibration curves and
to document the internal drift of the instrument. All samples were
acidified with 65% HNO3 analytical grade to a pH less than 2 before
analysis.
2.4. Surface analysis
2.4.1. In situ infrared reflection absorption spectroscopy (IRAS)
The IRAS setup has thoroughly been described elsewhere [37].
The IRAS spectra were recorded with 1024 scans at a resolution of
4 cm−1 in absorbance units (−log(R/R0)), where R is the reflectance
of the exposed sample and R0 the reflectance of the sample before
exposure (background spectrum). Prior to continuous in situ IRAS
spectra recording, the samples were kept in humidified air for
30 min to eliminate various forms of water contribution [38].
2.4.2. Confocal Raman microspectroscopy (CRM)
The confocal Raman measurements were performed with a
WITec alpha 300 system equipped with a laser source of wave-
length 532 nm. The integration time per Raman spectrum was in the
order of 50 ms. A Nikon objective, Nikon NA0.9 NGC, was used for
the measurements together with a pinhole with 100 �m diameter.
The resulting stack Raman spectra were produced in the scanning
area with a lateral resolution around 300 nm and a vertical resolu-
tion around 2 �m.
2.4.3. Scanning electron microscopy with energy dispersive X-ray
analysis (SEM/EDS) mapping
SEM images of unexposed Galvalume were generated using a
digital HKL Nordlys II F+ camera attached to a FEG-SEM Leo1530
upgraded to equivalent with Zeiss Supra 55. The software used was
the Channel 5 suite from HKL. EDS analysis was conducted using a
50 mm2 X-Max SDD (Silicon Drift Detector) and the INCA software
from Oxford Instruments. Back-scatter images were acquired for
corroded surfaces using a table-top SEM (Hitachi TM-1000) with an
accelerating voltage of 15 kV, equipped with Hitachi EDS facility.
2.4.4. Scanning Kelvin probe force microscopy (SKPFM)
SKPFM measurements were conducted on polished samples
using a Nanoscope IV with facilities for Volta potential measure-
ments. Surface topography and Volta potential mapping were
determined simultaneously on the very same surface area [39]. The
probe, antimony-doped Si, supplied by Veeco, was lifted up to a cer-
tain distance to collect Volta potential data at a constant distance
from the surface (80 nm in this work).
2.4.5. Grazing incident X-ray diffraction (GIXRD)
GIXRD was carried out on an X’pert PRO PANALYTICAL system,
equipped with an X-ray mirror (CuK ̨ radiation) and a 0.27◦ parallel
plate collimator on the diffracted side. Scanning was conducted on
a 1 cm × 1 cm large surface area at a grazing angle of 88◦ versus the
surface normal.
2.4.6. X-ray photoelectron spectroscopy (XPS)
Analysis of chemical composition of the outermost surface layer
of sample surface was performed with a XPS (Kratos AXIS HS)
system. Wide scans and detailed scans (pass energy 20 eV) of Al
2p, Zn 2p, ZnLMM, Cl 2p, S 2p, C 1s and O 1s were accomplished
using a monochromatic Al K� X-ray source (1486.6 eV) operated
try an
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P. Qiu et al. / Materials Chemis
t 300 W (15 kV/20 mA). Measurements were conducted at two
ifferent areas of analysis, each approximately sized 0.4 mm2.
. Results and discussion
.1. Bulk and surface characterization before exposure
The commercial Galvalume coating (55% Al, 43.4% Zn and 1.6%
i) consists of a layer of aluminum-rich dendritic areas with a typ-
cal dendrite arm spacing of the order of 10 �m (related to coating
hickness) that cover approximately 80% of the surface, and zinc-
ich interdendritic areas with locally distributed silicon particles.
he thin intermetallic layer, present at the steel/coating inter-
ace, is not investigated in this study. The surface distribution of
luminum- and zinc rich phases is illustrated for the as-received
urface condition in Fig. 1 by means of SEM and corresponding
DS-elemental mapping of zinc, aluminum and silicon. The local
istribution of silicon primarily in interdendritic regions is evident.
Microstructural information of the Galvalume coating is
ocumented in the literature [13,40,41] with reported mass
ompositional ratios between 0.6 and 0.8 (Al/(Al + Zn)) for the alu-
inum rich dendritic areas (often denoted �-phase), and 0.2 for the
inc-rich interdendritic areas (denoted �-phase) containing differ-
nt zinc-rich precipitates and meta-stable phases. Recent studies
ave however shown that the zinc-rich interdendritic areas contain
oth zinc-rich and aluminum-rich phases [14].
Average compositional analysis of the unexposed as-received
alvalume coating by means of XPS revealed a dominance of alu-
inum in the outermost surface layer, with a mass ratio between
luminum and zinc (Al/(Al + Zn)) between 0.85 and 0.99. A peak
osition at 74.2 ± 0.1 eV (Al 2p3/2) implied predominance of Al2O3.
stimations of the oxide layer thickness [42], based on the ratio of
he observed aluminum metal and aluminum oxide peak, suggested
n oxide thickness of 4–6 nm. The Auger parameter can be used
or chemical state identification and is calculated from the energy
ifference of the photoelectron peak and the Auger peak, to which
he energy of incident photons is added [43]. A weak photoelectron
eak of zinc and of the Zn LMM Auger peak located at 497.2 eV, cor-
esponding to a calculated Auger parameter of 2011.4 eV, implied
inc in an oxidized state, most probably as ZnO [41,44].
The surface dominance of aluminum oxide was also supported
y qualitative AES findings, showing no significant compositional
ariations in the aluminum to zinc signal when probing different
endritic and interdendritic areas, c.f. Fig. 1. However, composi-
ional differences were observed when probing similar areas with
DS, a technique that provides bulk information compared with
utermost surface information gained with both XPS and AES, and
ith a much better surface lateral resolution. These findings imply
hat even though large compositional differences are evident in the
ulk of the coating, the surface composition is similar and mainly
onsists of an aluminum oxide on both dendritic and interdendritic
reas, caused by the high affinity of aluminum to oxygen.
The large difference in topography between dendritic and inter-
endritic regions for the as-received surface, c.f. Fig. 2(a), disabled
he possibility to conduct AFM-based Volta-potentialmeasure-
ents to assess possible differences in nobility between different
arts of the unexposed as-received Galvalume surface. In order to
mprove the conditions for performing AFM-studies, the surface
as slightly polished to reduce the topography difference, Fig. 2(b).
As revealed by XPS, the removal of the outmost aluminum rich
xide disclosed zinc-rich areas, with a mass ratio (Al/(Al + Zn)) that
hanged from 0.85–0.99 to 0.17–0.23 after polishing. This treat-
ent resulted in the disclosure of three zinc Auger peaks (Zn LMM)
t 493.8, 497.2 and 500.0 eV and corresponding calculated Auger
arameters of 2014.5, 2011.1 and 2008.3 eV respectively. The first
d Physics 133 (2012) 419– 428 421
peak was assigned to metallic zinc and the other two to oxidized
forms of zinc. The Auger parameter at 2011.1 eV is assigned to ZnO
[44,45] and the Auger parameter at 2008.3 eV may be related to
Zn(OH)2. However, this could not be confirmed by independent
measurements.
From observed photoelectron binding energies, aluminum was
assigned to Al2O3 without any metal contribution, and hence the
aluminum oxide exhibited a larger thickness compared to the oxide
formed on as-received Galvalume where the aluminum metal peak
was detected. The XPS findings may imply a fast oxidation of both
zinc and aluminum phases disclosed after polishing and removal
of the relatively protective aluminum-rich surface oxide formed
during coating manufacture at high temperatures.
Scanning Kelvin probe force microscopy measurements clearly
revealed a higher relative nobility (higher Volta potential) of the
aluminum-rich dendritic branches compared with the zinc-rich
interdendritic areas, Fig. 2(c), and hence the latter area may be
expected to be more susceptible to corrosion than the former. How-
ever, even though average compositional measurements reveal
the interdendritic areas to be zinc-rich, literature findings have
shown the presence of both aluminum-rich and zinc-rich phases
to exist within the interdendritic areas. Preferential corrosion of
the aluminum-rich phase of the interdendritic area has also been
reported by means of detailed cross-sectional studies of the coating
[14].
To summarize, the non-treated as-received unexposed Galval-
ume surface is covered by an aluminum oxide on all parts of the
surface. Upon slight diamond polishing, the aluminum oxide is
removed and zinc rich phases dominate. The Volta potential on den-
dritic parts of the surface is higher than on interdendritic parts, from
which is expected that interdendritic areas are more susceptible to
corrosion than dendritic areas.
In order to investigate the initial corrosion behavior of Galval-
ume, polished surfaces were exposed to humidified air in controlled
laboratory exposures.
3.2. Initial corrosion of Galvalume in controlled laboratory
exposures – CO2-reduced humidified air
In situ studies by IRAS of the kinetics of surface oxidation of the
slightly polished Galvalume surface were conducted during expo-
sure up to 3 days in 90% humidified CO2-reduced air. CO2-reduction
was performed in order to study the effect of humidified air first
without and then with a natural concentration of CO2. For compar-
ison, identical experiments were conducted for polished bare zinc
and bare aluminum metal surfaces. A ZnO peak at 570 cm−1 was
identified on exposed zinc, Fig. 3(a). The exposure of aluminum
resulted in band domains centered at 755 and 958 cm−1, assigned
to crystalline and amorphous Al2O3 [46], respectively, Fig. 3(b).
Contradictory to findings for aluminum, exposure of Galvalume to
humidified air resulted only in the formation of amorphous Al2O3,
Fig. 3(c). The presence of Al2O3 was also confirmed with XPS (Al 2p
74.4 eV). No formation of ZnO on Galvalume was observed by means
of IRAS during the exposure period. Ex situ measurements using the
more surface sensitive technique XPS, revealed though the pres-
ence of a distinct Zn LMM Auger peak at 493.9 eV (calculated Auger
parameter 2011.2 eV) assigned to ZnO [40,44].
Previous studies by the authors have shown that the absorbance
of IRAS-peaks of any well-defined compound can be used to deduce
the absolute amounts of that compound. Under optimized con-
ditions the relative accuracy of the estimate can be 10% [32].
Time-dependent changes in intensity of the amorphous Al2O3
peak formed on Galvalume are presented in Fig. 4 together with
corresponding data for zinc (formation of ZnO) and aluminum (for-
mation of amorphous and crystalline Al2O3). The results clearly
show a significant formation of ZnO on zinc, whereas no formation
422 P. Qiu et al. / Materials Chemistry and Physics 133 (2012) 419– 428
F -receiv
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ig. 1. Backscatter electron images (40 �m × 40 �m) of non-treated unexposed as
reas) of aluminum (b), zinc (c) and silicon (d).
r growth of ZnO was observed on Galvalume during the same time
eriod. A very slow, but significant growth of amorphous Al2O3 was
bserved on Galvalume, at lower rates compared with aluminum.
hese findings imply that aluminum rich phases are preferentially
xidized in humid air. Similar observations with the formation of
l2O3 and the absence of ZnO have been reported for Galvalume
xposed in a pure oxygen environment [45]. This dominance of alu-
inum oxide in the surface layer upon exposure to CO2-reduced
umidified air was also confirmed by XPS showing an increased
elative mass ratio Al/(Al + Zn) from 0.17 to 0.23 for the polished
on-exposed surface to 0.62–0.77 after 3 days of exposure. The
ig. 2. AFM topography image (40 �m × 40 �m) of a non-exposed as-received bare Galva
n Volta potential (c). Lighter areas correspond to aluminum-rich dendritic areas.
ed Galvalume coating on steel (a) and corresponding elemental mapping (lighter
fact that both Al2O3 and ZnO are formed at lower formation rates
on Galvalume than on the pure metals shows that the formation
rates are governed by kinetic rather than galvanic effects, due to
the formation of the highly protective Al2O3 layer.
Since IRAS data reflect average values for relatively large sur-
face areas (4 cm2), any distinction between oxidation in more
aluminum-rich dendritic areas or in more zinc-rich interdendritic
areas is impossible. Ex situ measurements were therefore con-
ducted by means of CRM, with a lateral resolution of 0.3 �m
under optimized conditions. This allows information to be collected
within different surface domains. Since silicon is very Raman active
lume (a), non-exposed slightly polished surface (b), and corresponding differences
P. Qiu et al. / Materials Chemistry an
Fig. 3. In situ IRAS spectra of zinc (a), aluminum (b) and Galvalume(c) exposed for
72 h in 90% humidified air.
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ig. 4. Intensity of zinc oxide and aluminum oxides as a function of the expo-
ure time of polished zinc metal, aluminum metal, and Galvalume exposed in 90%
umidified air. No substantial amount of ZnO was observed on Galvalume.
47] and preferentially distributed in the interdendritic areas, c.f.
ig. 1 and displayed in the optical microscopy image in Fig. 5(a),
his band (470–550 cm−1) was integrated and mapped to indicate
he location of interdendritic areas, Fig. 5(b).
The non-exposed surface also revealed a weak Raman band at
23–1008 cm−1. This mode is believed to be associated to Al O Si
ntisymmetric vibrations [48]. As Raman bands for Al2O3 have been
eported between 980 and 995 cm−1 [49], this band was used to
ocate surface regions dominated by aluminum-oxide phases. As
vident from Fig. 5(c) and literature findings, the even distribution
f the Raman band at 923–1008 cm−1 may suggest an even dis-
ribution of aluminum oxide on both dendritic and interdendritic
reas, in good agreement with results from the previous section.
ig. 5. Optical microscopy image of a non-exposed polished Galvalume surface (a) and
lighter areas) integrated between 470 and 550 cm−1 (b) and of the Al O Si band (lighte
d Physics 133 (2012) 419– 428 423
By means of Ramanmapping, a preferential oxidation of
aluminum-rich phases in the interdendritic areas, visualized in
Fig. 6, was evident after 72 h of exposure to humid air. This obser-
vation is in concordance with previous literature findings [14].
3.3. Initial corrosion of Galvalume in controlled laboratory
exposures – humidified CO2-containing air and presence of
sodium chloride
The formation and dominance of Al2O3 on the surface of Galval-
ume was also evident from XPS analysis and IRAS upon exposure
to CO2-containing (350 ppm) humid air (90%). Very similar mass
compositional ratios (Al/Al + Zn) as measured for the CO2-reduced
humid air exposure were determined for the outermost surface
oxide (0.62–0.72). This illustrates that the presence of CO2 does
not largely influence the formation of aluminum oxide. In addition
to the presence and growth of amorphous Al2O3, the IRAS spec-
trum presented two relatively weak bands in the region of 3488
and 3605 cm−1 and between 1380 and 1550 cm−1, not observed
in CO2-reduced humidified air. These bands were allocated to
water/hydroxyl and carbonate vibrations, respectively, and the lat-
ter possibly assigned to a zinc and/or aluminum hydroxycarbonate.
However no unambigous phase assignment was possible by means
of IRAS. A shoulder of the Zn2p3/2 photoelectron peak at higher
energies was observed that was not seen in the CO2-reduced envi-
ronment by XPS. This suggests an association of carbonate to zinc
rather than to aluminum. The initial formation of a basic zinc
carbonate (Hydrozincite, Zn5(CO3)2(OH)6) in interdendritic areas
has previously been reported for Galvalume exposed to outdoor
conditions [9,50]. This phase is typically formed on bare zinc or
galvanized surfaces exposed in non-polluted environments [16]. A
mixed aluminum/zinc hydroxycarbonate formed in interdendritic
areas has been reported during wet-storage of Galvalume [18].
The probable formation of zinc-rich phases, in addition to
aluminum oxide formation, was further observed for the Galval-
ume coating when pre-deposited with sodium chloride (15 �g
NaCl cm−2) and subsequently exposed to humid CO2-containing
air for 72 h. According to IRAS, both amorphous and crystalline
Al2O3 were identified. According to literature findings, crystalline
Al2O3 is favored by the presence of chlorides [51,52]. A chemical
shift of the binding energy of aluminum to slightly higher binding
energies (75.0–75.3 eV), compared to conditions without chlorides
(74.5–74.7 eV), may imply the presence of AlOOH and/or Al(OH)3
[53]. The relative amount of aluminum compared to zinc as deter-
mined by XPS was slightly higher in CO2- and NaCl-containing
environment (0.75–0.80) than in CO2-reduced humid environment
(0.62–0.77).
In addition to the presence of bands assigned to carbonate,
the coating exposed to chlorides revealed a splitting of the IRAS
band around 3640 cm−1, an effect previously observed for basic
zinc chlorides (Simonkolleite, Zn5(OH)8Cl2·H2O) [54]. This phase
 corresponding Raman mapping image (scan size: 30 �m × 30 �m) of the Si band
r areas) integrated between 923 and 1008 cm−1 (c).
424 P. Qiu et al. / Materials Chemistry and Physics 133 (2012) 419– 428
Fig. 6. Raman mapping images of the Al O Si band (lighter areas) integrated between 923 and 1008 cm−1 (a) and the Si band (lighter areas) integrated between 470 and
550 cm−1 (b) for Galvalume after 72 h exposure in humidified air.
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after 12 weeks and to 0.80 after 26 weeks of exposure. After 1 year
of exposure were aluminum-rich phases instead dominating the
surface (Zn/(Zn + Al): 0.30), and dominates almost completely the
ig. 7. SEM backscatter images of corrosion products formed primarily in interdendr
onditions in the marine environment of Brest, France.
ypically forms on bare zinc and galvanized steel surfaces exposed
n marine as well as in laboratory studies [21,55–57] and has also
een identified on Galvalume coatings exposed in similar environ-
ents [58]. Because other specific bands of Simonkolleite at 910
nd 730 cm−1 [59], and bands of amorphous and crystalline Al2O3
s well as specific bands related to Hydrozincite, all fall within the
ame band region (1000 and 750 cm−1) that dominates the IRAS-
pectrum, no unambiguous phase assignment was possible. The
ormation of zinc-rich phases and the presence of chloride were
lso supported by XPS findings. An Auger parameter of 2010.8 eV,
alculated from the ZnLMM and the main photoelectron peak of
n2p3/2 suggested the presence of ZnO [44,45]. Since the Na KLL
eak overlaps the Zn LMM peak at higher binding energies, no cal-
ulations of the Auger parameter were possible for other potentially
xisting oxidized phases of zinc. A shoulder of the photoelectron
eak of zinc at higher binding energies indicated the presence of
nother zinc-rich phase, as suggested by IRAS findings.
In all, the exposure of Galvalume to CO2-containing humidified
ir suggests the overall formation of amorphous Al2O3 and the local
ormation of zinc hydroxycarbonate, presumably as Hydrozincite,
n interdendritic areas. Additional predeposition of NaCl manifests
he overall formation of aluminum-containing species, this time
ore likely as AlOOH and/or Al(OH)3, and the local formation of
inc-containing phases on interdendritic areas, possibly as Hydroz-
ncite, Simonkolleite and ZnO.
The formation of corrosion products on Galvalume was further
xplored for both short (weeks) and long term (years) exposure
eriods at non-sheltered marine and urban conditions, discussed
n the next section.
.4. Formation of corrosion products on Galvalume coatings at
on-sheltered field conditions
Non-treated surfaces of as-received Galvalume coatings and
are zinc sheet were exposed for 2, 4, 12, 26 weeks, 1 and 5 years
eas of Galvalume exposed for 12 weeks (a), 1 year (b) and 5 years (c) at non-sheltered
at the marine site of Brest, France. In agreement with the lab-
oratory investigation and with literature findings, the SEM/EDS
investigation revealed zinc-rich corrosion products locally formed
that gradually developed with time on the interdendritic areas
[10], Fig. 7. No crystalline zinc-rich phases were possible to detect
throughout the whole exposure period by GIXRD.
The gradual formation of zinc-rich phases, observed as increased
surface coverage compared to aluminum-rich phases, during the
first half year of exposure was clearly demonstrated by XPS, Fig. 8.
The surface fraction by mass (Zn/(Zn + Al)) increased from 0.01
to 0.15 for the as-received non-exposed Galvalume surface to
0.22–0.32 after 2 weeks, to 0.55–0.62 after 4 weeks, to 0.40–0.67
Fig. 8. The mass fraction (Zn/(Zn + Al)) determined by means of XPS in the outermost
surface layer at two randomly selected surface areas (each sized 0.4 mm2, area 1 in
gray color, area 2 in black color) of as-received Galvalume coatings prior to (non-
exposed) and after 2, 4, 12, 26 weeks, 1 and 5 years of non-sheltered exposure to a
marine environment.
P. Qiu et al. / Materials Chemistry and Physics 133 (2012) 419– 428 425
gs exp
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Fig. 9. Scanning electron microscopy backscatter images of Galvalume coatin
utermost surface (Zn/(Zn + Al): 0.01–0.10) after 5 years of non-
heltered exposure. According to GIXRD, was Al(OH)3 the main
rystalline phase observed after this time period, a phase previ-
usly identified in marine environments [9]. IRAS bands located
t 1100 and 700 cm−1 were observed after all exposure periods,
nd may be associated with Al(OH)3 or AlOOH [60]. However,
s also the basic zinc carbonate, Zn5(CO3)2(OH)6 (Hydrozincite),
resents a strong peak located at 1060 cm−1 and a weak band at
33 cm−1 [61], any conclusive phase assignments were not possi-
le. Similar to the laboratory findings with pre-deposited NaCl on
alvalume in humid air, IRAS revealed a possible split of the band at
500 cm−1 after 5 years of exposure. This suggests the local forma-ion of a basic zinc chloride, presumably Simonkolleite, previously
nown to form on Galvalume [9,62]. The broad band centered at
pproximately 1100 cm−1, may also reflect the presence of sulfate-
ontaining phases, as also XPS indicated the presence of sulfate.
ulfate-rich aluminum phases have previously been reported for
alvalume exposed to marine conditions [9]. Identification of cor-
osion products by CRM, IRAS and GIXRD on bare zinc sheet exposed
n parallel on the marine test site revealed Hydrozincite, and ZnO
crystalline and amorphous) to be the main components [63].
As a result of very complex exposure conditions and the forma-
ion and presence of a large variety of different corrosion products
t the marine site, confocal Raman spectroscopy mapping did not
esult in any conclusive interpretations of the location of corrosion
roducts, in contrast to the experience in the laboratory investiga-
ion.
Investigations on the lateral corrosion product distribution were
herefore conducted on Galvalume exposed to a significantly less
olluted urban site (5 years in Stockholm) by means of XRD,
EM (Fig. 9) and confocal Raman spectroscopy mapping (Fig. 10).
inc-rich phases (possibly Hydrozincite and zinc hydroxyl sul-
ates indicated by XRD) and aluminum-rich phases (sulfate-rich
ig. 10. Raman mapping images (scan size: 30 �m × 30 �m) of the Si band, indicating th
l O Si band (integrated between 923 and 1008 cm−1) (b), and SO42− band (970–1200 
osed for 5 years at non-sheltered urban conditions in Stockholm, (a) and (b).
and Al2O3, identified by means of XRD and indirectly evidenced
by the Al O Si band integrated between 923 and 1008 cm−1 by
CRM), were primarily formed in the interdendritic areas, Fig. 10.
These indications of corrosion product location were in accordance
with previous studies by the authors on Galvalume exposed to
rain-sheltered conditions at the same site [50]. The formation of
sulfate-rich aluminum corrosion products in interdendritic areas
has also been reported elsewhere [9].
The fact that zinc-containing corrosion products primarily form
on the interdendritic areas in the urban site of Stockholm as well
as in all laboratory exposures presented before and also in Brest,
suggests that zinc-containing corrosion products in general start to
form on interdendritic areas, Fig. 9(a), and then eventually spread
to other areas, Fig. 9(b).
In all, the exposure of Galvalume in the marine test site exhibits
a remarkable variation in corrosion product composition. Dur-
ing shorter exposures up to 26 weeks Al(OH)3 and AlOOH were
detected in addition to the possible presence of aluminum sulfate
of unknown identity, with the parallel gradual formation of zinc-
containing phases primarily ZnO, Hydrozincite and Simonkolleite.
With exception of the aluminum sulfate, the phases detected are
within the accuracy of the analysis the same as in the laboratory
exposure containing humidified air, CO2 and predeposited NaCl.
The fraction of zinc-containing phases reached a maximum during
the first year and then declined. After 5 years the Galvalume surface
was mainly covered by aluminum-containing corrosion products,
dominated by crystalline Al(OH)3.
3.5. Release of zinc and aluminum from Galvalume coatings at
non-sheltered field conditions
The long-term release of the main metal constituents from Gal-
valume, exposed both at the marine test site in Brest and, for
e location of interdendritic areas (integrated between 470 and 550 cm−1) (a), the
cm−1) (c).
426 P. Qiu et al. / Materials Chemistry and Physics 133 (2012) 419– 428
F c and 
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and Al4SO4(OH)10·2H2O were identified by GIXRD after 10 years
of exposure. Similar to the results from Brest, measurements of
the release rate of aluminum from selected samples during the
10-year exposure period showed very low values, again explained
ig. 11. Average release rates of zinc and aluminum (a), annual release rates of zin
ainfall quantity (c) for Galvalume exposed to non-sheltered conditions at the mari
alvalume compared with zinc released from zinc sheet exposed in parallel (d). A r
omparison, also in the urban test site of Stockholm, is displayed in
ig. 11(a) as average release rates of aluminum and of zinc versus
ime for up to 5 years of exposure. The zinc release rate gradu-
lly increases during the first year, reaches a maximum after about
 year and then gradually declines due to the parallel growth of
luminum-rich phases and partial dissolution of zinc-rich phases.
ence, a strong correlation exists between the formation and sur-
ace coverage of zinc-rich corrosion products, c.f. Fig. 8, and the
artial dissolution and release of zinc from these phases via the
ction of rainwater, c.f. Fig. 11.
The same trend was observed when displaying the zinc release
ate as annual release rates rather than average release rates
Fig. 11(b)) exhibiting a maximum after year one and a continu-
us reduction with time during the subsequent 4-year exposure
eriod. This reduction can be explained by the gradual forma-
ion and dominance of aluminum-rich compounds such as AlOOH
nd Al(OH)3 over zinc-rich phases in the outermost surface layer,
ig. 8, which efficiently hinder the release of zinc. These conclu-
ions are supported by the laboratory exposures, c.f. Fig. 6, showing
 preferential growth of aluminum-rich phases in parallel with
inc-rich phases in mainly interdendritic areas. Aluminum-rich
endritic areas have been proposed to mechanically anchor zinc-
ich corrosion products formed in zinc-rich interdendritic regions
33,63]. The annual release of zinc from Galvalume was 5–7.5 times
ower compared to that of bare zinc sheet (Fig. 11(d)), as a con-
equence of the surface dominance of aluminum-rich phases in
ainly dendritic areas. A reduced release rate of zinc from Gal-
alume compared to galvanized steel has also been reported in the
iterature [15].
The release rate of zinc was 20–30 times higher than of alu-
inum (Fig. 11(a) and, hence, not at all related to its nominal bulk
lloy content). The extent of metal release from alloys has in several
revious studies been proven to largely differ from their pure metal
onstituents, and not be possible to estimate based on bulk compo-
ition [64,65]. Calculations based on the nominal bulk composition
nd release data for zinc sheet in this study confirms this state-
ent also for Galvalume with largely overestimated rates (at least
aluminum (b) and released annual amount of aluminum normalized to the annual
 of Brest, France, as a function of exposure time. Annual ratio of zinc released from
f one corresponds to identical release rates.
with a factor of two) compared with real observations. Significantly
lower release rates of aluminum compared with zinc throughout
the entire 5-year exposure period suggests that the aluminum-
containing corrosion products on average are much less soluble
than those for zinc under current conditions. When normalizing
the annual average aluminum release rate to the annual rainfall
quantity, a slight reduction can be seen after 3 years of exposure,
possibly related to the formation of Al(OH)3, Fig. 11(c).
Similar trends, with significantly lower release rates of zinc from
Galvalume compared to the bare zinc sheet, was also observed
for Galvalume exposed at non-sheltered conditions in the low-
polluted urban environment of Stockholm for 10 years, Fig. 12.
Throughout the whole 10-year period the zinc release rate grad-
ually declined. Based on our findings from the marine test site, the
explanation is most likely due to the gradual built-up of aluminum-
rich phases in the surface layer of Galvalume, since both Al2O3
Fig. 12. Annual ratio of zinc released from Galvalume compared with zinc released
from zinc sheet during 10 years of non-sheltered urban exposure conditions in
Stockholm, Sweden. A ratio of one corresponds to identical release rates.
try an
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P. Qiu et al. / Materials Chemis
y the low chemical solubility of aluminum phases under present
xposure conditions.
. Conclusions
The non-treated as-received and unexposed Galvalume surface
onsists of dendritic aluminum-rich areas and interdendritic zinc-
ich areas, both covered by an aluminum oxide. A higher Volta
otential was determined for dendritic areas compared with inter-
endritic regions on a slightly polished Galvalume surface, an effect
ot observed for the as-received condition.
Upon exposure to humidified air with reduced CO2-
oncentration both Al2O3 and ZnO are formed, preferentially
n the interdendritic areas. The formation rates of both phases
re lower than on the pure metals showing that the protective
roperties of the Al2O3 layer are crucial.
CO2-containing humidified air results in an overall formation of
morphous Al2O3 and local formation of zinc hydroxycarbonate,
resumably Hydrozincite, on interdendritic areas. Predeposition
f NaCl triggers the overall formation of AlOOH and/or Al(OH)3,
nd the local formation of zinc-containing phases on interdendritic
reas, possibly Hydrozincite, Simonkolleite and ZnO.
The same phases are observed on Galvalume upon long-term
nsheltered marine exposure. During the first half year a prefer-
ntial corrosion of interdendritic areas occurs observed through
he evolution of AlOOH, Al(OH)3, an aluminum sulfate of unknown
dentity, and the parallel formation of primarily ZnO, Hydrozincite
nd Simonkolleite. Due to a generally higher solubility of the zinc-
han of the aluminum-containing phases the surface coverage of
he former reaches a maximum during the first year and then
eclines. After 5 years the Galvalume surface is mainly covered by
rystalline Al(OH)3.
The annual release rate of zinc in the marine site also exhibits
 maximum after year one and a continuous reduction with
ime during the subsequent 4-year exposure period. This behav-
or follows the same time-dependence as the surface coverage of
inc-containing phases and is explained by the gradual formation
nd dominance of less soluble aluminum-rich phases such AlOOH
nd Al(OH)3 compared to zinc-rich phases.
Characteristic corrosion and runoff features of Galvalume com-
ared to bare zinc in the marine and the urban site investigated, the
ignificantly lower corrosion rate and the ever decreasing runoff
ate, are explained by the progressive uniform formation of first
l2O3, then by AlOOH and finally by Al(OH)3.
cknowledgements
The authors are grateful for the financial support of the field
xposure provided by Nordic Galvanizers Association, Sweden;
heinzink, Germany; Saferoad, Norway and SSAB, Sweden and
he funding from the European Union’s Research Program of the
esearch Fund for Coal and Steel (RFCS) research program Autocorr
nder grant agreement n◦ RFSR-CT-2009-00015.
Instrumental grants from Knut and Alice Wallenberg foundation
XPS) and from Nils and Dorthi Troëdsson Foundation (combined
onfocal Raman and AFM) are gratefully acknowledged.
Micrographic examinations by M. Sc. Oskar Karlsson of
on-exposed surfaces at the unit for metallic microstructure
haracterization (MEMIKA), a joint facility between KTH and
werea-KIMAB, Stockholm, Sweden, is highly acknowledged.
We acknowledge the French Corrosion Institute, Brest, France,
or their invaluable help in collecting runoff water at the marine
ite.
All AAS analytical efforts by Dr. Gunilla Herting and MSc David
indström are highly appreciated.
[
[
d Physics 133 (2012) 419– 428 427
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http://dx.doi.org/10.1002/maco.201106361
	Evolution of corrosion products and metal release from Galvalume coatings on steel during short and long-term atmospheric ...
	1 Introduction
	2 Material and methods
	2.1 Materials and surface preparation
	2.2 Laboratory and field exposure conditions
	2.3 Metal runoff measurements
	2.4 Surface analysis
	2.4.1 In situ infrared reflection absorption spectroscopy (IRAS)
	2.4.2 Confocal Raman microspectroscopy (CRM)
	2.4.3 Scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDS) mapping
	2.4.4 Scanning Kelvin probe force microscopy (SKPFM)
	2.4.5 Grazing incident X-ray diffraction (GIXRD)
	2.4.6 X-ray photoelectron spectroscopy (XPS)
	3 Results and discussion
	3.1 Bulk and surface characterization before exposure
	3.2 Initial corrosion of Galvalume in controlled laboratory exposures – CO2-reduced humidified air
	3.3 Initial corrosion of Galvalume in controlled laboratory exposures – humidified CO2-containing air and presence of sodi...
	3.4 Formation of corrosion products on Galvalume coatings at non-sheltered field conditions
	3.5 Release of zinc and aluminum from Galvalume coatings at non-sheltered field conditions
	4 Conclusions
	Acknowledgements
	References