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Bodycote International Prize Competition: Shortlisted
PVD ALUMINIUM ALLOY COATINGS:
ENVIRONMENTALLY FRIENDLY ALTERNATIVE
TO PROTECT STEEL PARTS AGAINST
CORROSION
P. de Araujo, P. Steyer, J.-P. Millet, E. Damond, B. Stauder and P. Jacquot
In the automotive and aeronautic industries, security is
of prime importance and so corrosion prevention is
essential. Steel parts were once protected with deposits
such as cadmium or zinc – nickel produced using a
chromatation treatment but recent environmental
directives dictate that these methods are no longer
acceptable. Aluminium sacrificial coatings have excel-
lent corrosion behaviour but unfortunately they have
poor mechanical characteristics and a fast dissolution
rate. The present study investigated a possible solution:
alloying the aluminium with a more noble element,
chromium, in order to decrease the sacrificial galvanic
effect of the deposit and therefore improve its lifetime.
The corrosion protection afforded by such coatings in
relation to their structure and mechanical properties
was investigated. The coatings were produced on
carbon steel by a vacuum PVD arc evaporation process
at a pilot scale. Intrinsic electrochemical properties
were determined on pure materials and on layers
deposited on glass strip. Several elaboration configura-
tions (pure as well as composite targets) were
investigated. In most cases the coatings were stratified
and composed of pure aluminium and numerous
hardened Al
x
Cr
y
intermetallic phases. Chromium
enrichment of aluminium based coatings induces not
only a beneficial hardening effect on the surface
characteristics (w700 HV) but also significantly
improves the corrosion behaviour of the coated pieces
(increasing lifetime by up to three times compared to
pure Aluminium). SE/481
Dr Araujo, Dr Steyer (steyer@insa-lyon.fr) and
Dr Millet are at the Laboratoire de Physico-Chimie
Industrielle, INSA de Lyon, 21 av. J. Capelle, baˆt
L. de Vinci, F69621 Villeurbanne Cedex, France.
Dr Damond, Dr Stauder and Dr Jacquot are with
Bodycote hit, 25 rue des Fre`res Lumie`re, F69680
Chassieu, France. Contribution to the 2002 Bodycote
International Prize Paper Competition.
#2003 IoM Communications Ltd. Published by Maney for
the Institute of Materials, Minerals and Mining.
INTRODUCTION
Anodic coatings are important for preventing cor-
rosion in the automotive and aeronautic industries.
Until the 1990s protection was mainly achieved by
using cadmium coatings which have good corrosion
properties and wear behaviour. However, the use
of cadmium has been strictly regulated because it
is considered to be hazardous to the environment
(CEE recommendation no. 91/338/CEE), and there-
fore, environmentally friendly alternatives need to be
developed.
In the case of sacrificial deposits, the coated piece
immersed in an aggressive medium leads to the pre-
ferential dissolution of the deposit while the substrate
is electrochemically protected. There are two methods
that can then improve the lifetime of the part:
increasing the thickness of the coating (although
this has economical drawbacks), or associating a
more noble element into the sacrificial coating to
decrease the dissolution rate by reducing the galvanic
effect between coating and substrate. Efficient solu-
tions are provided with electrodeposited zinc based
alloys, Zn – Ni,
1,2
Zn – Sn
3
or Zn – Mn
4
but such
processes generate waste which is also harmful to the
environmental. In contrast, physical vapour deposi-
tion (PVD) processes are non-polluting elaboration
methods,
5
extensively used to depose metastable
alloys. Comparing Zn – Ni coatings obtained with
different techniques, Bowden et al. concludes that
the best electrochemical behaviour is obtained from
specimens which are magnetron sputtered.
6
A bene-
ficial effect on corrosion resistance is also reported
for PVD Zn – Al
7,8
and IBAD elaborated Zn – Cr
9
alloys.
Research has also been devoted to aluminium
based binary alloys. For instance, the addition of
10 – 20 wt-% magnesium to aluminium decreases the
potential of coated mild carbon steel in acetate buffer
and enlarges its passive domain.
10
Abu-Zeid and
Bates, studying both the tribological and electro-
chemical behaviour of Al – Mo coatings deposited by
magnetron sputtering, observed a significant reduc-
tion in the friction coefficient but without any effect
on the corrosion properties.
11
A structural
12
and
electrochemical
13,14
study focused on Al – Cr and Al –
Ti deposited by magnetron sputtering onto steel,
proposed by Sanchette et al., confirms the improve-
ment afforded by chromium alloying but warns
against the risk of obtaining too large an amount
of alloying element, a cathodic coating. Some results
concerning Al – Ti deposits on steel in bilayer
15
as well
as in multilayer
16
structures have also been reported.
With such configurations, authors emphasise the
importance of the nature of the external layer in
contact with the aggressive medium, as well as the
internal layer in contact with the substrate.
(gamma) SUR27232.3d 11/8/03 17:30:07 Rev 7.51n/W (Jan 20 2003)
DOI 10.1179/026708403225006122 Surface Engineering 2003 Vol. 19 No. 4 1
The purpose of the present study was to evaluate,
on steel coated parts, the protective effect of Al – Cr
alloys deposited by cathodic arc evaporation in
relation to their deposition parameters, composition,
morphology and hardness. Intrinsic electrochemical
behaviour of deposits is also characterised with
coatings deposited onto glass substrate.
EXPERIMENTAL
Materials
The substrate material was a mild carbon steel
with composition Fe – 0.704Mn – 0.171Si – 0.166C –
0.024S – 0.01P. Aluminium based coatings were depos-
ited in a pilot BMI-PVD 64 type reactor at the
Chassieu Bodycote site (France). Cathodic arc eva-
poration is a versatile process and the elaboration
parameters can be easily adapted for industrial
plants. Two different configurations were adopted to
favour either successive deposition of Cr or Al ele-
ments from the evaporation of pure metal targets,
or co-deposition of both metals simultaneously. Co-
deposition results were obtained from either the
‘immersion’ of the substrate inside a zone subject
to the vapour of both metals (co-deposition) or from
the evaporation of a duplex target made of Cr ‘nails’
inserted into a pure aluminium matrix (coevaporation).
Similar types of ‘prealloyed’ targets have already
been successfully tested by Sanchette for the magne-
tron sputtering process.
17
Characteristics of the tested
samples are given in Table 1.
X-ray diffraction, using Cu K–L
2,3
radiation, was
used to identify the growth direction and the nature of
the deposited phases. Specimens were characterised
using optical and electronic microscopy (Philips XL30
SEM). Submicrometer investigations of the dispersed
intermetallic phases were carried out with an AFM
(Veeco Nanoscope Multimode). Mechanical reinfor-
cement was evaluated through microhardness mea-
sured using a 50 g load.
Electrochemical procedure
Corrosion tests were carried out in a neutral aqueous
NaCl (15 g L
21
) solution, naturally aerated and main-
tained at room temperature. The test device was
composed of an EGG 273 potentiostat and a 1 L
electrolytic cell together with three electrodes: the
calomel reference electrode saturated in KCl(SCE); a
large platinum counter electrode; and the specimen
as the working electrode. Investigated specimens were
cylinders (12 mm in height and diameter), stirred at
500 rev min
21
with a rotating device.
Open circuit potential versus time is recorded
during 72 h of immersion of the steel substrate and
during 8 h of the coating deposited onto glass strips,
potential stabilisation being, in these latter conditions
more quickly reached. Corrosion rate was measured
from potentiodynamic curves, plotted, after poten-
tials recording, from 250 mV versus the corrosion
potential up to 150 mV in the anodic side, with a
sweep rate of 10 mV min
21
.
RESULTS AND DISCUSSION
Metallurgical analysis
Figures 1 and 2 illustrate metallographic observations
of evaporated Al – Cr coatings. The structure appears
stratified and composed of alternated Al and Cr rich
layers. X-ray images show aluminium throughout the
coating, even occurring in its pure state. External
surface heterogeneity results from the splash caused by
droplet impacts and appears rough and porous, similar
to deposits made using thermal spray processes.
18
The
The Charlesworth Group, Huddersfield 01484 517077
Table 1 Characteristics of studied samples
Sample nature Identification
Coating
thickness, mm
Bare steel Steel …
Pure Al Al 6
Evaporated Al – Cr Evap. Al – Cr 8
Co-deposition Al – Cr Codep. Al – Cr 6
Coevaporated Al – Cr,
from duplex targets
Coevap. Al – Cr 3.5
aco-deposited cross-section; bco-deposited surface; ccoeva-
porated cross-section; dcoevaporated surface
1Backscattered electron images of Al – Cr coatings
2Araujo et al. PVD aluminium alloy coatings protecting steel parts against corrosion
Surface Engineering 2003 Vol. 19 No. 4
presence of such droplets can be explained by the
significant local overheating produced in the arc
impact area, leading to a large number of melted
particles. This phenomenon is important because the
melting and evaporating temperatures are quite dif-
ferent. Aluminium is particularly subject to this
problem because it melts at a relatively low tempera-
ture (660uC at atmospheric pressure), and vaporises at
above 2500uC.
Simultaneous co-deposition leads to a better distri-
bution of both elements throughout the whole coating
thickness, whereas coevaporation from duplex targets
brings no evident enhancement (Fig. 3).
Whatever the configuration adopted for elabora-
tion, the composition and structure of coatings are
complex, composed of numerous well crystallised
intermetallic metastable phases (Fig. 4). Sanchette
17
observed relatively amorphous compounds when using
magnetron sputtering. This difference in the nature
and crystallinity of deposits has to be correlated with
the high energetic arc evaporation process, which
was able to promote the diffusion of both metals and
abackscattered electron image; bAl X-ray image; cCr
X-ray image
2SEM metallagraphic cross-sections of evaporated Al –
Cr coating
3Backscattered electron image of sample surface (evaporated Al – Cr coating)
4Diffractograms of aevaporated Al – Cr coating and b
enlargement of [36u,46u] angular domain
Araujo et al. PVD aluminium alloy coatings protecting steel parts against corrosion 3
Surface Engineering 2003 Vol. 19 No. 4
the crystallisation of intermetallic phases (during the
deposition process parts can be heated to more than
200uC). Taking into account the large number of
formed compounds, most of them diffracting into the
same angular domain, only major present phases are
considered in Table 2 and classified in terms of their
relative importance.
Pure aluminium is always the predominant phase.
Chromium is combined as a binary compound, which
corroborates with the qualitative analysis based on
X-ray images. Evaporated and co-evaporated depos-
its are of a similar chemical nature, with a pre-
dominance of AlCr
2
,Al
8
Cr
5
and Al
86
Cr
14
, while
co-deposited coatings are characterised by the absence
of Al rich compounds (Al
80
Cr
20
and Al
86
Cr
14
).
AFM phase images taken using intermittent
contact mode, allow a description of the tip surface
interactions. No correlation appears between both
topographic and phase images (Fig. 5), so that phase
contrasts are mainly interpreted in terms of the
difference in their mechanical properties (hardness,
elasticity, etc.).
19
Considering that the mechanical
characteristics modification is due to a different phase
nature, it can be assumed that coatings are composed
of well dispersed fine intermetallic phases of several
tens of nanometers in size.
The coated parts used in the mechanical industry
also require high mechanical properties, which cannot
be met by using pure aluminium. Hardness values,
given in Table 3, correspond to the mean of at least
five different measurements. Important dispersion for
alloyed coatings results from the surface heterogene-
ity. However, whatever the sample, chromium alloy-
ing induces great mechanical reinforcement,
particularly significant for coatings elaborated with
the co-deposition configuration.
Electrochemical behaviour
The free corrosion potential of bare steel quickly
decreases from 2450 mV(SCE) down to a plateau at
2650 mV(SCE), corresponding to the continuous
dissolution of iron in electrolyte (Fig. 6). Potential
values of coated pieces are somewhat ‘unstable’ in
Table 2 Coating composition*
Evaporated
Al – Cr
Co-deposited
Al – Cr
Coevaporated
Al – Cr
Pure Al zzzzz zzzzz zzzzz
Al
86
Cr
14
zzz …zz
Al
80
Cr
20
z…z
Al
8
Cr
5
zzz zzzz zzzz
AlCr
2
zzzz zzz zzzz
*z~relative importance
5AFM phase image (4276427 nm
2
) obtained with intermittent contact mode
Table 3 Hardness of different samples
Hardness, HV
Bare steel 269¡9
Pure aluminium 95¡12
Evap. Al – Cr 552¡70
Co-dep. Al – Cr 789¡173
Coevap. Al – Cr 457¡47
6Free corrosion potential evolution versus duration time
of samples immersed in saline solution
4Araujo et al. PVD aluminium alloy coatings protecting steel parts against corrosion
Surface Engineering 2003 Vol. 19 No. 4
relation to the important chemical heterogeneity of
the external surface in contact with saline environ-
ment. Pure aluminium coating remains sacrificial
throughout a protracted test but is affected by pitting
corrosion, whereas the co-deposited sample promotes
the same advantageous electrochemical behaviour
without developing any localised attack (Fig. 7aand
b). In contrast, the free corrosion potential of Al – Cr
evaporated coatings slowly increases so that after
about 30 h of immersion, and even earlier with the
coevaporated deposit, coated parts become more
noble than the substrate. This ennoblement may be
due to the selective dissolution of the more anodic
phases of the deposit. The parts then suffer from
classical corrosion attack encountered with cathodic
coatings,
20
consisting of corrosion of the substrate
through open porosity (Fig. 7c).
The corrosion of coated parts results from galvanic
effect between coating and subjacent substrate.
Determination of the overall corrosion rate requires
knowledge of the intrinsic electrochemical behaviour
of each structure. For this purpose, electrochemical
measurements have been carried out on bare steel and
pure aluminium as well as on co-deposited Al – Cr
coating produced on a glass plate. Potential time
curves indicate, for all samples, a stable evolution and
an ennoblement of about 40 mV compared to pure
aluminium is seen on the chromium enriched sample
(Table 4). Assuming that anodic and cathodic reac-
tions occur respectively, on coating and substrate, the
corrosion rate of the coated steel can be evaluated by
the intersection of the two corresponding potentio-
dynamic curves (Fig. 8, Table 4). However, as an
important part of the substrate surface is protected
by the coating, corrections are required to ensure
that only the actual substrate area in contact with
environment is taken into account.
21
An open
porosity rate of 10% has been chosen for further
calculations, the high value being justified by the
‘spray coating-like structure’. A correction is then
needed: the steel current density is reduced by one
decade (curve ‘steel S
cathodic
/S
anodic
~0.1’ in Fig. 8,
Table 4).
Once both anodic and cathodic structures were
associated, there was a significant increase in the
apure aluminium coating; bco-deposited Al – Cr coating;
ccoevaporated Al – Cr coating
7Macroscopic observations of corroded samples after
72 h of immersion in electrolyte
Table 4 Electrochemical parameters deduced from potentiodynamic curves after 8 h of immersion, with and without cou-
pling effects, and for two cathode/anode area ratios
Intrinsic corrosion
Steel/coating coupling
S
cathodic
/S
anodic
~1
Steel/coating coupling
S
cathodic
/S
anodic
~0.1
Ecor, mV(SCE) Icor, mAcm
22
Ecor, mV(SCE) Icor, mAcm
22
Ecor, mV(SCE) Icor, mAcm
22
Bare steel 2607 25
Bare Al 2730 0.32675 80 2695 16
Co-dep. Al/Cr on glass 2693 1 2625 10 2653 6
8Potentiodynamic curves of bare materials (steel and
aluminium) and of co-deposited Al – Cr coating on
glass strip
Araujo et al. PVD aluminium alloy coatings protecting steel parts against corrosion 5
Surface Engineering 2003 Vol. 19 No. 4
anodic materials dissolution rate, particularly for
pure aluminium, for which the anodic part of i(E)
curve is steep. Galvanic effects are then deleterious in
terms of corrosion rate but for the chromium alloyed
coating, a much less harmful galvanic effect is
noticeable. For example, an open porosity of 10%
enhancement, provided by chromium alloying, leads
to a deposit dissolution rate three times lower than for
pure aluminium.
Therefore, compared to pure aluminium coatings,
Al – Cr alloys produced on steel by co-deposition
brings not only a great mechanical reinforcement but
also provides an extended lifetime resulting from the
improvement of its corrosion behaviour.
CONCLUSION
Using the arc evaporation process with pure alumi-
nium and chromium targets, coatings are heteroge-
neous in both composition and structure. Nevertheless,
a more homogeneous distribution is obtained in co-
deposition conditions with either a composite binary
target or by vaporising simultaneously the two pure
metals. In all cases, coatings are composed of
numerous well crystallised nanometric intermetallic
particles, the influence of which appears crucial to
corrosion resistance. Aluminium rich phases (Al
86
Cr
14
and Al
20
Cr
20
) observed in the evaporated and co-
evaporated deposits are probably responsible for the
evolution of the free corrosion potential towards
cathodic values when they are immersed in saline
solutions. In contrast, co-deposited coatings, charac-
terised by chromium rich compounds, remain sacrifi-
cial even in long duration tests. Chromium alloying
also induces significant hardening, suitable for mechan-
ical applications. The protection of steel substrate
afforded by Al – Cr co-deposited coatings is three
times longer than that obtained with pure aluminium
with the same deposit thickness.
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