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PVD Aluminium Alloy Coatings: Environmentally Friendly Alternative to Protect Steel Parts Against Corrosion


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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 excellent 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 configurations (pure as well as composite targets) were investigated. In most cases the coatings were stratified and composed of pure aluminium and numerous hardened AlxCry intermetallic phases. Chromium enrichment of aluminium based coatings induces not only a beneficial hardening effect on the surface characteristics (> 700 HV) but also significantly improves the corrosion behaviour of the coated pieces (increasing lifetime by up to three times compared to pure Aluminium).
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Bodycote International Prize Competition: Shortlisted
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
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 ( 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.
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
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,
Zn – Sn
or Zn – Mn
but such
processes generate waste which is also harmful to the
environmental. In contrast, physical vapour deposi-
tion (PVD) processes are non-polluting elaboration
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.
A bene-
ficial effect on corrosion resistance is also reported
for PVD Zn – Al
and IBAD elaborated Zn – Cr
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.
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.
A structural
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
as well
as in multilayer
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.
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.
Characteristics of the tested
samples are given in Table 1.
X-ray diffraction, using Cu KL
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
) 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
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
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.
The Charlesworth Group, Huddersfield 01484 517077
Table 1 Characteristics of studied samples
Sample nature Identification
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
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
and Al
, while
co-deposited coatings are characterised by the absence
of Al rich compounds (Al
and Al
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.).
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*
Al – Cr
Al – Cr
Al – Cr
Pure Al zzzzz zzzzz zzzzz
zzz zz
zzz zzzz zzzz
zzzz zzz zzzz
*z~relative importance
5AFM phase image (4276427 nm
) 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
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.
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
~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
Steel/coating coupling
Ecor, mV(SCE) Icor, mAcm
Ecor, mV(SCE) Icor, mAcm
Ecor, mV(SCE) Icor, mAcm
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.
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
and Al
) 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.
1. m. gravila,j.-p. millet,h. mazille,d. marchandise and j.-m.
cuntz:Surf. Coat. Technol., 2000, 123, 164 – 172.
2. v. cossin:Galvano-Organo-Trait. Surf., 2001, 176 – 178.
3. c. j. helwig: Proc. 15th Int. Conf. on ‘Surface modification
technology’, 275 – 283; 2001, Indianapolis, USA. ;
4. s. sylla,j. creus,c. savall,o. roggy,m. gadouleau and
p. refait:Thin Solid Films, 2002to be published <
5. b. navinsek,p. panjan and i. milosev:Surf. Coat. Technol.,
1999, 116 – 119, 476 – 487.
6. c. bowden and a. matthews:Surf. Coat. Technol., 1995,
76 – 77, 508 – 515.
7. m. a. baker,w. gissler,s. klose,m. trampert and f. weber:
Surf. Coat. Technol., 2000, 125, 207 – 211.
8. e. calla and s. c. modi:Trans. Met. Finish. Assoc. India, 2001,
10, 21 – 26.
9. f. alonso,v. madina,j. l. viviente,t. mursell and j. i. onate:
Surf. Coat. Technol., 1998, 103 – 104, 87 – 92.
10. b. enders,s. krauss,k. baba and g. k. wolf:Surf. Coat.
Technol., 1995, 74 – 75, 959 – 965.
11. o. a. abu-zeid and r. i. bates:Surf. Coat. Technol., 1996,
86 – 87, 526 – 529.
12. f. sanchette and a. billard:Surf. Coat. Technol., 2001,
142 – 144, 218 – 224.
13. f. sanchette,b. comas,a. billard,b. normand,c.
frantz and j. pagetti:Vide, Sci. Tech. Appl., 1995, 275,
427 – 430.
14. f. sanchette,a. billard and c. frantz:Surf. Coat. Technol.,
1998, 98, 1162 – 1168.
15. j. creus,h. idrissi,h. mazille,f. sanchette and p. jacquot:
Thin Solid Films, 1999, 346, 150 – 154.
16. c. charrier,p. jacquot,e. denisse,j.-p. millet and h.
mazille:Surf. Coat. Technol., 1997, 90, 29 – 34.
17. f. sanchette: ‘Synthe`se et caracte´ risation de de´poˆ ts Al – Cr –
(N) et Al – Ti – (N) obtenus par pulve´ risation cathodique
magne´ tron en condition re´active’, PhD thesis, Institut National
Polytechnique de Lorraine, 1996.
18. p. sidky and m. g. hocking:Br. Corros. J., 1999, 34, 171–
19. s. l. gao and e. mA
¨der:Appl. Sci. Manuf., 2002, 33, 559 –
20. p. steyer,j.-p. millet,s. anderbouhr and p. jacquot:Surf.
Eng., 2001, 17, 327 – 331.
21. j. creus,h. mazille and h. idrissi:Surf. Coat. Technol., 2000,
130, 224 – 232.
6Araujo et al. PVD aluminium alloy coatings protecting steel parts against corrosion
Surface Engineering 2003 Vol. 19 No. 4
Journal: Surface Engineering
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Al–Wx alloys with the W content ranging from 0·6 to 11 at-% were sputter deposited at a low temperature on AA7075 alloy substrates. The sputtered Al–W alloys exhibit a remarkably broad range of microstructures and were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). For the alloys with a W content from 0·6 to 3·5 at-%, increasing tungsten additions lead to a decrease in the grain size from a few hundred nm to 10 nm. At 11 at-%W, a lamella-like microstructure forms, accompanied by an amorphous phase. Nanoindentation tests revealed that the increasing tungsten additions lead to an increase in the hardness. A polarisation corrosion test in a near neutral 0·1M NaCl solution revealed that increasing the tungsten additions leads to an increasing passive range and, therefore, the pitting potential.
The development of thin and environmental friendly coatings on steel surfaces constitutes a major objective of steelmaking industry. Within this objective it becomes necessary to develop analytical approach allowing characterization of so thin coatings on hot dip galvanized substrates in order to control the product quality. XPS, GDOES, SIMS, FTIR and Raman spectroscopies are compared in terms of capability to characterize SiO2-CVD and silane thin films on rough substrates.At the issue of this study, results show that the SiO2 coating of some tens of nm in thickness is continuous and appears to be homogeneous everywhere on the substrate surface. The layers deposited on the materials with sufficiently low roughness have almost similar thickness. In the case of significant surface roughness increase the SiO2 layer becomes thinner.The silane coatings fill up substrate cavities during application and the coating thickness on rougher substrates is significantly higher than on smoother ones. The chemical composition of the silane layers is more uniform for rougher surfaces, whereas on smoother ones the Si concentration gradient is detected in depth.
Corrosion resistance of nanomultilayered TiN/CrN coatings has been evaluated in saline aqueous neutral solution and compared with monolayered TiN and CrN references. Coatings have been deposited by magnetron sputtering on M2 tool steel, as well as on glass strip, to determine their intrinsic electrochemical behavior. Corrosion resistance afforded by CrN is better than TiN, which is explained by the formation of a thin oxide passive film with favorable semiconductive properties. Nanostratified structure improves the coating density, but the electrochemical behavior is however mainly controlled by the nature of the outer layer, which must be CrN to optimize protection ability.
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Aluminium-based coatings are potential candidates for a sacrificial protection of steel. Such coatings, elaborated by magnetron sputtering, offer a good corrosion protection but often present poor tribological properties [1,2]. Cathodic arc evaporation PVD process has outstanding potential to deposit corrosion-resistant coatings with relatively dense structure. Moreover, adherent coatings and large scale production can be obtained. Aluminium, titanium and multilayer titanium/aluminium coatings have been deposited on steel substrates. The coating uniformity and homogeneity analysis were performed by optical and scanning electron microscopy while energy dispersive X-ray microanalyses were carried out to check the coating composition. The corrosion behaviours of smooth and threaded coated samples were analysed from 5% salt spray exposures and electrochemically characterized with voltamperometric measurements. Titanium/aluminium multilayers on low carbon steel lead to better corrosion resistance than monolayer coatings.Vacuum coating tests, carried out on small parts by means of a barrel coater, led us to consider the process to be a possible alternative to the actual anti-corrosion treatments of steel substrates, especially as a substitute for cadmium coatings.
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The plastics industry commonly uses moulds made of steels containing MnS inclusions. These inclusions play the role of heterogeneities, which make manufacturing easier. Moreover, these types of alloy are thermal treatment free, cheap, and easily available. Unfortunately, their use in corrosive conditions is prevented owing to the poor corrosion behaviour of the MnS particles. In order to mask these highly sensitive inclusions and to enhance the surface mechanical properties, hard physical vapour deposition (PVD) CrN coatings, elaborated at two different temperatures, have been used. The coated specimens have been tested in saline corrosive solutions using electrochemical techniques. Corrosion results from dissolution of the anodic substrate through coating porosity, inherent to the PVD process, which leads to the deposit peeling off. The aim of the present study is to determine the corrosion resistance of an underlayer electroless nickel deposited coating on a 40CrMnMo8S substrate coated with PVD CrN.
The potentiostatic electrodeposition of zinc–manganese alloys on steel from MnCl and ZnCl solutions, was studied in an 2 2 aqueous KClqH BO matrix. Cyclic voltammetry was used to determine the potential ranges where the redox processes of 3 3 interest were taking place. The obtained Zn and Mn containing films were investigated by means of scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction. The effects of potential deposition and stirring were studied. At Es y1.65 VySCE without stirring, it proved possible to obtain Zn–Mn deposits constituted of a singlé phase, with approximately 13 at.% Mn. Decreasing the potential or agitating the electrolyte reduced the Mn proportion in the films. At 10.3 at.% Mn, the film obtained is composed of HCP Zn–MnándMnánd monoclinic z (MnZn) phases. At 3.4% Mn, the film consists of three phases, 13 Zn along with the two previous Zn–Mn phases. All films have a cauliflower morphology and appear to be compact and homogeneous except for the films with the largest Mn contents (at.% Mn)11) where columnar deposits are obtained. Polyethylene glycol was tested as a potential additive. Its main effects were a decrease in the Mn content and inhibition of the z phase formation.
Physical vapour deposition (PVD) Zn alloy coatings are being considered as possible alternatives to conventional hot dip and electrochemically deposited coatings on steel substrates. Variation of the deposition temperature leads to very different microstructures being observed by SEM, primarily due to the low melting point of zinc. In the present study this phenomenon has been investigated in more detail. Zn+5at.% Al coatings were deposited by magnetron sputtering using a homogeneously alloyed Zn–Al target. As the substrate temperature is increased, the coating morphology changed from a compact to an open sponge/woollen-like microstructure and the coating thickness increased by up to a factor of 10. The Zn–Al coatings displayed a hexagonal structure corresponding to that of Zn. The corrosion properties were investigated by potential–time and salt bath immersion tests. Interestingly, both the barrier and sacrificial protection afforded by the coating were independent of the microstructure.
For some years now manufacturing industries around the world have been prompted to seek alternatives to cadmium because of the hazardous nature of this material. With respect to this the corrosion of Zn-Ni deposits and an assessment of their potential as an alternative to cadmium electroplate has been studied.The Zn-Ni systems under study are Zn-(10%–12%)Ni magnetron-sputtered deposits, a graded Zn-Ni magnetron-sputtered deposit and a commercially electrodeposited Zn-(12%)Ni film. Corrosive performance and behaviour were monitored and compared with cadmium using both electrochemical techniques and more conventional testing. Galvanic suitability and protective properties were assessed, as was time to white rust and time to red rust. It was found that the free corrosion potential of the Zn-Ni deposits rises with tile, resulting in a loss in galvanic sacrificiality. PVD Zn-Ni deposits were found to have the lowest corrosion rate and gave the best barrier protection; sacrificially however they were inferior to both electrodeposited Zn-Ni and cadmium.
Aluminium alloy coatings are potentially interesting candidates for the cathodic protection of construction steels. This paper reviews the main features of Al−TM−(N) metallurgical coatings (TM: transition metal = Cr, Ti) deposited by reactive magnetron sputtering of composite Al−TM targets in Ar−N2 reactive gas mixtures. In particular, both binary and ternary amorphous coatings always present the best compromise between mechanical properties and corrosion resistance. Generally, they present favourable intrinsic compressive stress associated with good compactness, a microhardness around 6000 MPa, a Young modulus of at least 110 GPa, better ductility and wear resistance than crystallized alloys, while maintaining a corrosion potential around that of iron, and an excellent pitting resistance in a chlorine environment. The thermal stability of such coatings is guaranteed for applications up to 300 °C. Their crystallization at higher temperature yields stable (Al4Cr) or metastable (disordered AlTi) phases.
Ion-beam-assisted deposition (IBAD) was used for depositing pure aluminium coatings on low carbon steel in order to study the corrosion properties as a function of the energy of the incident argon ions, the ion-to-atom arrival ratio and the angle of ion incidence. With theoretical considerations about the localized corrosion behaviour of substrate-coating systems it is possible to evaluate the porosity, the relative number of coating defects and their mean diameter. Their dependence on the process parameters gives an insight into the initial stages of film growth and shows that the energy deposition in the near-surface region is the most important factor for modifying coating defects and therefore the corrosion properties of the steel-aluminium system. The modification potential of each parameter of IBAD is studied in comparison to pure vapour deposition for the corrosion in acetate buffer. In chloride-containing aqueous solutions the depassivation behaviour of aluminium can be improved by use of alloying elements. Aluminium and magnesium were deposited on inert glass substrates; the magnesium concentration and the bombarding energy were varied. The resulting alloys have equilibrium potentials less noble than pure aluminium and increased passive regions. The passive region decreases with increasing magnesium concentration. From glancing-angle X-ray diffraction measurements it can be seen that structural changes influenced by the ion beam go parallel with changes in the localized corrosion behaviour.