A. A. ADEDIRAN et al.: ELECTRON-OPTICAL AND AUGER ELECTRON SPECTROSCOPY STUDIES ...
183–188
ELECTRON-OPTICAL AND AUGER ELECTRON SPECTROSCOPY
STUDIES OF A ZIRCONIUM CONVERSION COATING ON
ALUMINIUM
ELEKTRONSKO-OPTI^NA [TUDIJA IN AUGERJEVA
ELEKTRONSKA SPEKTROSKOPIJA KONVERZIJSKE PREVLEKE
NA OSNOVI CIRKONIJA, NANE[ENE NA ^ISTI ALUMINIJ
Adeolu Adesoji Adediran
1,3
, Makanjuola Oki
1*
, Sarah Abidemi Akintola
2
,
Bamidele Ogunsemi
1
, Esther Titilayo Akinlabi
3,4
1
Landmark University, College of Engineering, Department of Mechanical Engineering, Km 4, Ipetu, Omu-Aran, PMB 1001, Ipetu Road,
Kwara State, Nigeria
2
University of Ibadan, Faculty of Technology, Department of Petroleum Engineering, Oduduwa Road, PMB 5116, Ibadan, Oyo State, Nigeria
3
University of Johannesburg, Faculty of Engineering and the Built Environment, Department of Mechanical Engineering, P. O. Box 524,
Auckland Park Campus, 2006, Johannesburg, South Africa
4
Covenant University, Department of Mechanical Engineering, Km 10 Idi-Iroko Road, Canaanland, Ota, Ogun 112001, Nigeria
Prejem rokopisa – received: 2018-09-11; sprejem za objavo – accepted for publication: 2018-10-18
doi:10.17222/mit.2018.197
The techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultramicrotomy and energy
dispersive X-ray analysis (EDX) were employed to examine conversion coatings on aluminium developed from a zirconium
nitrate/fluoride solution. The coating developed slowly on a microscopic metal substrate with islands of zirconium-rich centres
within the coating matrix. According to the TEM assessment, the coating thickness was 30 nm for specimens treated for 60 s
and 50 nm for those treated for 900 s. The population of the zirconium-rich centers ranged from 2.8 × 10
12
m
–2
to6×10
12
m
–2
over the treatment period. According to Auger-electron-spectroscopy in-depth analyses, the coating is composed of three
diffused layers with an outer region of zirconium oxide followed by a layer of aluminium oxide/hydroxide and an inner layer at
the metal/coating interface comprising compounds of aluminium and fluorides. The corrosion-resistance and paint-adhesion
characteristics of the zirconium conversion coating are superior to those of bare aluminium.
Keywords: SEM/TEM/EDX, ultramicrotomy, zirconium conversion coating, corrosion
Avtorji so uporabili SEM in TEM/EDX tehnike za preiskavo konverzijske prevleke, nastale na podlogi iz tehni~no ~istega
aluminija z uporabo Zr nitrat/fluoridne raztopine. Prevleka je nastajala po~asi na mikroskopski kovinski podlagi s posameznimi
sredi{~i z Zr bogatih oto~kov znotraj matrice prevleke. S TEM ocenjena debelina prevleke je zna{ala 30 nm na vzorcih po
60-sekundni in 50 nm pri 900-sekundni obdelavi. Populacija z Zr bogatih oto~kov je bila med 2,8 x 10
12
m
–2
do6x10
12
m
–2
glede na ~as obdelave. Globinska analiza z Augerjevo spektroskopijo je pokazala, da je prevleka sestavljena iz treh difuzijskih
plasti: zunanje plasti Zr oksida, plasti Al oksida/hidroksida ter notranje plasti pri stiku med kovino in prevleko z vsebnostjo
spojin Al in fluoridov. Korozijska odpornost in adhezijske karakteristike za barve konverzijske prevleke na osnovi Zr so bolj{e v
primerjavi s tistimi na osnovi ~istega (neprevle~enega) aluminija.
Klju~ne besede: SEM/TEM/EDX, ultramikrotomija, konverzijska prevleka na osnovi cirkonija, korozija
1 INTRODUCTION
Conversion coatings on aluminium have been in
usage for decades to provide additional corrosion resist-
ance and good keying facilities for subsequently applied
organic finishes/coatings. However, in the last few
decades restrictive legislations have been passed in diffe-
rent countries relating to chromates, the major compo-
nents of various conversion coating baths. The environ-
mental toxicity and health-related issues of chromates
prompted the search for green substitutes that are
non-toxic and with an acceptable hazard status. Thus, in
the search for a chromate replacement, various re-
searchers examined conversion coatings developed in
baths containing permanganate,
1–3
molybdenum,
4–6
cerium
7–8
and zirconium compounds.
9,10
Though findings
suggest that these replacements have a lower corrosion
resistance compared to the chromate coatings, they
display fairly good paint-adhesion characteristics. Other
researchers
11–15
developed sol-gel coatings as alternatives
to chromates with a limited success while the improve-
ments in their corrosion performances were engineered
with nano-containerization of the green corrosion inhi-
bitors into the coating fabrics.
16–20
In order to avoid the contact of chromium com-
pounds with foodstuffs, the canning industries have
stringent policies with respect to chromates. The ex-
pected corrosion resistance in respect of containers,
made mostly of aluminium alloys, is limited to staining
protection and enhanced surface for an improved ad-
hesion of lacquer and protective internal coatings. In
general, any conversion coating employed in this indus-
Materiali in tehnologije / Materials and technology 53 (2019) 2, 183–188 183
UDK 543.428.2:669.296'71 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(2)183(2019)
*Corresponding authors e-mails:
makanjuola.oki@lmu.edu.ng, adediran.adeolu@lmu.edu.ng

try must display a very good paint-adhesion capability in
addition to being colorless. In view of these facts, the
current study examines the development of a zirconium-
based conversion coating on super-pure aluminium in
order to rank its performance with those of bare-alumi-
nium specimens.
2 MATERIALS AND METHODS
2.1 Materials
99.9 % pure aluminium was made into spade-like
electrodes. These were electropolished in a perchloric
acid/ethanol mixture, rinsed in water and dried. The
electropolished electrodes were treated for various times
ranging from 30 s to 900 s in a zirconium conversion
coating solution made up of 4 g/L of zirconium nitrate,
2 g/L of sodium fluoride and about 1 ml of nitric acid for
the pH adjustment. All the chemicals employed are of an
analytical grade from British Drugs House Chemicals,
Poole, UK, and doubly distilled water was used through-
out.
2.2 Methods
From the start of the immersion of the specimens in
the coating solution, the potential of each specimen was
measured relative to the saturated Calomel electrode and
monitored by means of a high-impedance voltmeter and
a potentiometric chart recorder. The specimens selected
for this study were examined with a Reichart optical mi-
croscope and an ISI DS130 scanning electron micro-
scope (SEM) with an elemental analysis of the speci-
mens obtained from the energy dispersive X-ray (EDX)
facility attached to the microscope. Ultramicrotomed
sections of the selected specimens were obtained in the
usual manner
21
using an LKB Ultratome III 8800
ultramicrotome and thin sections of the specimens with
the coatings attached were examined with a Philips 301
transmission electron microscope (TEM). AES depth
profiling of the coating obtained with a 300-s immersion
in the zirconium solution was obtained with an alternate
analysis, in-situ ion-beam etching at approximately
0.3 nm/s in a vacuum generator, and Auger electron
spectroscopy witha3keVprimary electron beam and a
beam current of 200 μA.
A set of specimens, pretreated in a coating bath for
300 s and bare, electropolished aluminium were coated
with a vinyl aerosol clear lacquer in a 20-mL beaker by
dipping them into it as near vertical as possible for 90 s.
A calibrated permascope (ECS, Fischer’s Instrument)
was used to determine the thickness of dried lacquer to
be 6000 ± 2000 nm. Individual specimens were exposed
at right angle to the base of a humidity cabinet set at
40 °C and maintained at 100-% relative humidity. At
24-hour intervals and at the end of the exposure, after
100 h, the specimens were dried and examined with
optical microscopy, SEM and TEM.
3 RESULTS AND DISCUSSION
3.1 General observations
The initial visual examination of the specimens
treated for various times in the zirconium coating bath
showed that the specimens remained reflective after 900
s of treatment indicating that the as-formed coatings
were colorless and relatively thin.
The variation of potential with the treatment time of
electropolished aluminium kept in the zirconium
conversion coating bath, Figure 1, indicates that upon
immersion the potential shifted to negative values and
stabilized at about –1.4 V after 360 s. This may be
related to the feature of the conversion coating forma-
tion, where fluoride species in the solution generally
activate the substrate prior to the deposition of conver-
sion coating materials, which, in this instance, are likely
to be zirconium oxide/hydroxide mixed with aluminium
species/compounds.
3.2 Surface and sectional morphologies of as-formed
conversion coatings
The scanning-electron micrographs of the specimens
treated for 30–900 s appeared relatively featureless, yet
containing particles which increased in population from
about 2.8 × 10
12
m
–2
for a specimen treated for 30 s,
Figure 2a,t o1 5×1 0
12
m
–2
for a specimen treated for
900 s, Figure 2b.
These particles are zirconium-rich compounds. The
EDX attachment to the SEM indicated the presence of
A. A. ADEDIRAN et al.: ELECTRON-OPTICAL AND AUGER ELECTRON SPECTROSCOPY STUDIES ...
184 Materiali in tehnologije / Materials and technology 53 (2019) 2, 183–188
Figure 1: Potential/time measurement for electropolished aluminium
in zirconium conversion coating bath
Figures 2: SEM micrographs of aluminium specimens treated for: a)
30 s and b) 900 s, respectively, in a zirconium conversion coating bath

zirconium and aluminium during the spot analyses
performed on the clusters of the particles. These are
probably hydrated zirconium oxides and/or hydroxides
deposited on the microscopic surface of the aluminium
substrate during the conversion coating formation. A
micrograph of the ultramicrotomed section of the speci-
men treated for 60 s revealed that the initial electro-
polishing film, about 10 nm in thickness, had been
replaced with a thin coating, about 20 nm in thickness.
The zirconium-rich particles observed during the SEM
examinations were not revealed, which may have been
due to the exact location of the section. The features
were not readily resolvable within the coating section,
indicating that it is made of micro-crystallites of less
than 2.5 nm in size. The increase in the treatment time to
300 s caused a coating growth to about 30 nm, as seen in
Figure 3a where the coating is displayed at the top of the
aluminium substrate spreading towards the bottom of the
micrograph. The coating stripped from the substrate with
a mercuric chloride solution is displayed in Figure 3b
where dark and light materials representing relatively
thick and thin coatings can be observed. Unfortunately,
the EDX analysis revealed only aluminium; as suggested
earlier, the precise locations of the sections excluded the
zirconium-rich particles, which may have dropped off
into the stripping solution.
A further increase in the coating thickness to about
50 nm with no change in the morphology of the coating
materials were observed for the specimens treated for
900 s in the conversion coating bath. The coating growth
can be described based on the schematic diagrams from
Figures 4a to 4c. The schematic electropolishing film
from Figure 4a was described variously elsewhere
21
; the
electropolished specimen treated for 60 s revealed the
formation of a coating of about 20 nm in thickness. Both
the metal/coating and the coating/solution interfaces
appeared to be undulating and any features of the coating
materials were not resolvable, hence they were smaller
than 2.5 nm. Further treatment for 300 s and 900 s re-
vealed a continuous coating growth to about 30 nm and
50 nm, respectively. Individual features were still not
easily resolved within the coating materials; however,
both the metal/coating and the coating/solution interfaces
flattened out. This indicates further increases in the deve-
lopment of the colorless zirconium conversion coating
on the aluminium substrate.
3.3 Auger electron spectroscopy (AES) analysis
The AES surface analysis of the specimen treated for
300 s in the zirconium coating bath indicated the pre-
sence of zirconium, oxygen and fluoride with a very low
yield for aluminium. The Auger peak for Zr appeared at
147 eV, which corresponds to zirconium in ZrO
2
im-
plying that the coating surface is mostly composed of
zirconium oxide with fluoride and some aluminium
species.
22
The deposition of the sparingly soluble zir-
conium oxide as a result of an increase in the interfacial
pH of the specimen followed the usual activation of the
aluminium substrate by fluoride in the coating bath with
the attendant anodic dissolution of aluminium and the
corresponding cathodic hydrogen evolution reaction in
the acidic bath. The AES depth profile of the specimen is
displayed in Figure 5 where it can be observed that the
intensity of the Zr yield is at its maximum in the coating
materials at the film/solution interface, decreasing
rapidly with the etching time, suggesting that it is present
mainly in the near-surface regions of the coating.
Fluorine has a peak in the outer region of the coating
where it may be adsorbed as fluoride species.
However, another peak appeared at 60 s of etching,
indicating that it was involved in the activation of the
substrate at the metal/coating interface. The aluminium
yield increased from a very low intensity at the coating/
solution interface to its maximum at the metal/coating
interface at 60 s of etching where it remained constant.
On the contrary, oxygen in the coating decreased in
intensity from the coating/solution interface to the
metal/coating interface. In the outer regions of the
A. A. ADEDIRAN et al.: ELECTRON-OPTICAL AND AUGER ELECTRON SPECTROSCOPY STUDIES ...
Materiali in tehnologije / Materials and technology 53 (2019) 2, 183–188 185
Figure 3: Transmission-electron micrographs of a zirconium conver-
sion coating developed after 300 s showing: a) the ultramicrotomed
section and b) stripped coating in plan view
Figure 5: AES depth profile of aluminium specimen treated for 300 s
in a zirconium conversion coating bath
Figure 4: Schematic representation of the growth of the zirconium
conversion coating on aluminium substrate

coating, it is associated with zirconium as ZrO
2
and
probably as water of hydration. Within the coating,
oxygen is likely to be associated with aluminium as
Al
2
O
3
which is corroborated by the shifts of Al and O
peaks around 1390 eV and 510 eV in the AES spectra.
Beyond the 90-s etching time, most of the Auger peaks
observed are likely to come from the background noise
in the equipment.
From the AES depth-profile analyses, it appears that
the outer region of the coating is essentially composed of
ZrO
2
with some fluoride species occluded within the
coating and an underlying coating material that is
essentially alumina. However, fluoride is generally pre-
sent within the coating with peaks at both the coating/
solution and metal/coating interfaces. Thus, the diffuse
three-layer coating may be diagrammatically described
as displayed in Figure 6, showing the outer ZrO
2
at the
top of the diagram, laced with F
–
and Al
3+
species.
Sandwiched between this outer region and the inner
fluoride Al
3+
-rich coating materials next to the metal/
coating interface, there is the Al
2
O
3
coating material as
indicated by the Auger analyses.
ZrO
2
F
–
ZrO
2
ZrO
2
Al
3+
ZrO
2
F
–
ZrO
2
F
–
Al
2
O
3
Al
2
O
3
Al
2
O
3
Al
3+
F
–
Al
2
O
3
Al
2
O
3
F
–
F
–
OAl
3+
OF
–
Al
3+
OOF
–
F
–
Al
3+
Aluminium substrate
Figure 6: Schematic illustration of aluminium substrate carrying zir-
conium conversion coating
3.4 Performance of zirconium conversion coating in a
100-% humidity cabinet at 40 °C
The untreated and unpainted aluminium specimen
was severely corroded after 48 h of exposure in the
humidity cabinet whereas the lacquer-coated specimens
and the bare conversion-coated specimens did not seem
corroded to the naked eye although the lacquer, which
must have absorbed some moisture, turned slightly
opaque. The lacquer coating formed a barrier between
the substrates and the warm, moist environment in the
cabinet. However, the scratched regions of the specimens
revealed some evidence of corrosion when observed with
SEM.
Figure 7 shows a macrograph of the bare aluminium
specimen after 48 h of exposure where there are cell-like
corrosion products of hydrated Al
2
O
3
and probably some
hydroxides of aluminium on the macroscopic metal sur-
face. The dark spots observed within the corrosion pro-
ducts are likely to be incipient pits that occurred within
the flawed regions of the corrosion products. A further
examination of an ultramicrotomed section of the
corroded region with the transmission electron micro-
scope revealed a featureless corrosion product, about
25 nm in thickness, at the top, with the aluminium
substrate lying at the bottom of the micrograph, Fig-
ure 8.
A further exposure of the specimens for 100 h in the
humidity cabinet revealed that the lacquer-coated bare
aluminium specimen corroded and the corrosion pro-
ducts formed a cell-like pattern under the lacquer, similar
to those observed on the bare aluminium specimen
A. A. ADEDIRAN et al.: ELECTRON-OPTICAL AND AUGER ELECTRON SPECTROSCOPY STUDIES ...
186 Materiali in tehnologije / Materials and technology 53 (2019) 2, 183–188
Figure 9: Scanning-electron micrograph of the lacquer-coated bare-
aluminium specimen exposed for 100 h in the 100-% relative humidity
cabinet
Figure 7: Optical macrograph of an aluminium specimen after expos-
ure in a humidity cabinet for 48 h, a 1000× magnification
Figure 8: Transmission-electron micrograph of the ultramicrotomed
bare aluminium specimen after exposure for 48 h in a 100-% relative
humidity cabinet

exposed for 48 h. The lacquer coating was lifted up at
several places, Figure 9, displaying corrosion products
marked with P.
The application of an adhesive tape fixed firmly on
the specimen (the tape was rapidly pulled off) revealed
that the lacquer was completely removed in contrast to
the specimen that had a composite zirconium conversion/
lacquer coating, which retained some of the lacquer,
Figure 10. Thus, complete adhesion failure of the
lacquer was not observed on the zirconium conversion-
coated specimen.
This observation is reinforced by the findings of
Golrua et al. who concluded that the adhesion strengths
of the coatings applied on Zr-treated aluminium samples
were greater than those of untreated samples.
23
However,
after an exposure of 100 h, the conversion-coated spe-
cimen without a top lacquer coating failed and the
examination revealed cracked corrosion products on the
aluminium substrate. The conversion coating without a
lacquer barrier was compromised after a relatively
lengthy period in a high-humidity cabinet.
4 CONCLUSIONS
The conversion coating, a colorless material, deve-
loped slowly over the treatment period, is composed of
featureless micro-crystallites with zirconium oxide in the
outermost region followed by an aluminium-oxide-rich
coating material, while the region next to the metal/
coating interface is rich in aluminium and fluoride
components.
The specimens coated in a zirconium nitrate/fluoride
coating bath performed better than bare aluminium when
exposed to 40 °C and 100-% relative humidity in terms
of corrosion-resistance and paint-adhesion characte-
ristics.
Acknowledgement
One of the authors, M. Oki, acknowledges the use of
the facilities at the Corrosion and Protection Centre,
University of Manchester, UK, and Landmark Univer-
sity, thanking them for their assistance.
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188 Materiali in tehnologije / Materials and technology 53 (2019) 2, 183–188