Pulse and reverse plating effect on the structure and corrosion properties of Zn and Zn alloy coatings

22
CHEMIJA.
Laima Gudavièiûtë, Aleksandras Kalinièenko, R
2004. T. 15. Nr. 4. P. 22–28
emigijus Juðkënas, Rimantas Ramanauskas
© Lietuvos mokslø akademija, 2004
© Lietuvos mokslø akademijos leidykla, 2004
Pulse and reverse plating effect on the structure and
corrosion properties of Zn and Zn alloy coatings.
1. Zn–Ni
Laima Gudavièiûtë,
The influence of pulse plating parameters on the surface morphology,
Aleksandras Kalinièenko,
grain size, lattice imperfection and corrosion properties of Zn–Ni alloy
was studied. The coatings were electrodeposited in alkaline cyanide-free
Remigijus Juðkënas,
solution containing glycerine complex Ni ions. AFM was applied for
Rimantas Ramanauskas
surface morphology examination, XRD measurements were carried out
for phase composition and texture analysis, while electrochemical tech-
Institute of Chemistry, A. Goštauto 9,
niques were used for corrosion behaviour studies. The pulse plated Zn–
LT-01108 Vilnius, Lithuania
Ni coatings appeared to consist of the ?-Zn Ni phase; the composition
21
5
of the latter depended on the plating parameters. The alloy lattice im-
perfection and grain size were established to be the main factors deter-
mining corrosion behaviour of the coating.
Key words: Zn and Zn alloys, electrodeposits, structure, corrosion
INTRODUCTION
ter, which under certain conditions can be the main
factor determining corrosion behaviour of Zn allo-
Under increasing requirements of industry (especially
ys. Modification of the Zn–Ni alloys’ structure, the-
automotive) to reduce coating thickness and to pro-
refore, may be the way to create new more resis-
long the service life of a coated material, the cor-
tant coatings on steel.
rosion resistance of sacrificial coatings on steel ne-
Pulse electrodeposition can be used as a means
eds to be enhanced. Extensive attempts have been
to produce a unique structure, i.e. coatings with pro-
taken recently to develop highly corrosion resistant
perties not obtained by direct current (d.c.) plating.
coatings on steel. As a result, the conventional zinc
Pulse electrodeposition yields a finer-grained and
electroplates are being replaced by zinc alloys. It
more homogeneous surface appearance of the de-
has been stated in several studies [1–8] that corro-
posit, because a higher instantaneous current densi-
sion resistance of electrodeposited Zn–Ni alloy co-
ty is possible during deposition by using pulse con-
atings within a certain composition range (9–15
trol in comparison with that by d.c. plating.
wt.%) can be significantly higher (5–6 times) than
Acidic or alkaline plating baths can be used for
that for pure zinc [8].
Zn–Ni alloy deposition. Data on pulse plated Zn–
Electrodeposited Zn–Ni alloys exist in the form
Ni alloy published up to date deal with the coatings
of three dominant phases: ?, ? and ?. The ?-phase
deposited from acidic solutions [8, 12, 13]. Electro-
is a solid solution of Zn in Ni with an equilibrium
deposition of this alloy in acidic bath, especially un-
solubility of about 30% Zn. The ?-phase is a solid
der d.c. conditions, results in the dual phase struc-
solution of Ni in Zn, with a Ni solubility of less
ture formation [14], which implies an unsatisfactory
than 1%. The composition range of the pure ?-sin-
corrosion resistance of the coating. Meanwhile, de-
gle phase was determined to be between 10 and
position in alkaline solutions, in contrast to acidic
30% Ni. The amount of Ni in the alloy, which finds
ones, is less efficient but gives a more uniform pla-
industrial application in the corrosion protection
ting and results in the ?-single phase formation [9,
field, is around 15% and its dominant structure is
11].
the ?- phase Zn Ni [8].
21
5
The objective of the present investigation was to
It was revealed in our previous works, e.g., [5,
determine the influence of pulse electrodeposition
9–11], that metal structure is an important parame-
parameters on the surface morphology, grain size,
lattice imperfection and corrosion properties of Zn–
* Corresponding author: ramanr@ktl.mii.lt
Ni coatings deposited in alkaline bath.

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Pulse and reverse plating effect on the structure and corrosion properties of Zn and Zn alloy coatings...
23
EXPERIMENTAL
alloy are presented in Fig. 1. Electrodeposition con-
ditions affected significantly the composition of Zn–
Zn–Ni (12%) coatings 10 µm thick were electrode-
Ni alloy. The increase in i and t caused an inc-
posited on low carbon steel (C 0.05–0.12%, Mn
p
off
rease, while the increase in t and i , in contrary,
0.25–0.5%) samples, which had been polished me-
on
a
led to a decrease in Ni content in the alloy. The
chanically to a bright mirror. An alkaline cyanide-
values of i lower than 1 A cm–2 were not acceptab-
free plating solution contained ZnO 10 g l–1, NaOH
p
le, because they caused reduction of Ni content be-
100 g l–1, organic additives and a glycerine complex
low 10%, while application of t lower than 0.1 ms
of Ni ions. The coatings were deposited at 25 °C
on
and i lower than 0.1 A cm–2 resulted in alloy for-
and current density 3.5 A dm–2. A detailed descrip-
a
mation with a Ni content overcoming 15%. The lat-
tion of the plating bath and operating conditions
ter case was not acceptable in this study, either.
have been presented elsewhere [5, 9–11].
The presented dependences (Fig. 1) validate the op-
Surface morphology studies were carried out with
timal range of pulse plating parameters, which assu-
an AFM by an Explorer (VEECO-Thermomicrosco-
red the desirable composition of Zn–Ni alloy.
pes) scanning probe microscope at atmospheric pres-
sure and room temperature in a contact mode. A
Si N cantilever with the force constant of 0.032 N
11
18
3
4
%
m–1 was used and the resolution of the images ob-
16
tained was 300 × 300 pixels.
C, Ni 10
X-ray diffraction measurements were performed
14
with a D8 diffractometer equipped with a Göbel
9
12
mirror (primary beam monochromator) for Cu ra-
0
2
4
6
0
1
2
diation. A step-scan mode was used in the 2-theta
-2
t , ms
range from 30° to 75° with a step length of 0.02°
i , A cm
on
p
and a counting time of 5 s per step.
The corrosion behaviour of coatings was investi-
gated in 0.1 M NaCl + 0.1 M NaHCO solution
14
3
15
(pH 6.8) using a standard three-electrode system
i
,
%
with a Pt counter electrode, a saturated calomel re-
12
14
ference electrode and a PI-50 potentiostat.
C N
10
13
RESULTS AND DISCUSSION
0
4
8
0.0
0.2
0.4
t , ms
-2
Chemical composition
off
i , A cm
a
Data on the composition, structure and corrosion
Fig. 1. Effect of pulse plating parameters on Zn–Ni alloy
behaviour of Zn–Ni coatings deposited in an alka-
composition
line bath by the direct current were presented in
our previous studies [9, 11]. It was established the-
Phase composition
re that the mentioned coatings consisted of the ?-
The influence of pulse plating parameters on the
Zn Ni phase with the content of Ni close to ?12%,
21
5
phase composition of the deposited alloy can be
while their corrosion resistance under the conditions
determined from XRD data. Figure 2 shows XRD
when the passivating corrosion product, film, forms
patterns of Zn–Ni coatings deposited under various
(atmospheric corrosion or aqueous corrosion in a
Cl– ions containing buffered solution) appeared to
?-Ni Zn (330)
?-Ni Zn (222)
5
21
?-Ni Zn (444)
be significantly higher than of pure Zn coatings [9].
5
21
5
21
500
The principal requirements for the pulse plated
Zn–Ni alloy were absence of pitting damages and/
400
or dendrites on the surface of the coating and the
?-Fe(110)
?-Ni Zn (600)
5
21
s
stability of its composition. The latter condition im-
cp 300
/
plies a variation of Ni content in the alloy between
i
t
y
ns
10 and 15%. A lower amount of Ni in the alloy is
te 200
D
In
accompanied by a dual phase formation, while a
C
B
higher one causes the loss of the sacrificial protec-
100
A
tion effect of the coating.
40
50
60
70
The influence of pulse and reverse plating para-
2?
meters (cathodic peak current density (i ), current
Fig. 2. XRD patterns of Zn–Ni coatings obtained by pul-
p
on-time (t ), current off time (t ) and anodic peak
se plating under various cathodic current peak (i ) densi-
on
oof
p
current density (i )) on the amount of Ni in the
ties (A cm–2): A – 0.2, B – 1, C – 3, D – 6
a

---

24
Laima Gudavièiûtë, Aleksandras Kalinièenko, Remigijus Juðkënas, Rimantas Ramanauskas
i conditions. The variation of other pulse and re-
nodular crystallites (Fig. 3a). The dimension of such
p
verse plating parameters (t , t and i ) did not cau-
agglomerations varied between 1.1 and 3.2 µm, (ave-
on
off
a
se the appearance of any new characteristic featu-
rage 2.0 µm), meanwhile the dimensions of the indi-
res in the obtained diffractograms, therefore, they
vidual crystallites (?cr) ranged between 0.5 and 1.8
are not presented here.
µm (the average value 0.92 µm).
All the Zn–Ni samples showed sharp peaks cor-
The ultrafine-grained structure was determined
responding to the base metal (Fe) and to ?-Zn Ni
from the 500 × 500 nm scanned surface area (Fig. 4a)
21
5
phase. No other Zn–Ni alloy phases were detected
and the grain size (? ) of d.c. deposited coatings va-
gr
to be present in the electrodeposited coatings. This
ried between 70 and 180 nm, with the average 90 nm
fact implies that under applied pulse and reverse pla-
value.
ting conditions the deposited alloy consisted of a sin-
The influence of pulse plating parameters on the
gle ?-Zn Ni phase.
21
5
morphology of electrodeposited Zn–Ni coatings can
An additional diffraction line characteristic: a full
be observed from the images presented in Figs. 3 and
width at half-maximum (FWHM) W
was measu-
FWHM
4 b–d. The average crystallite dimensions, grain size,
red and the obtained data will be presented and dis-
as well as the root-mean-square roughness (R ) va-
cussed below.
rms
lues were determined from the AFM measurements
Surface morphology
and the obtained data are presented in Fig. 5 and
Surface morphology studies of Zn–Ni coatings were
Table.
based on AFM measurements. Investigations on the
The morphology of Zn–Ni alloys deposited by pul-
µm level (20 × 20 µm scanned surface area) have
se and reverse plating appeared to depend rigorously
shown that d.c. plated coatings (deposited at 3.5 A
on the plating parameters. The characteristic feature
dm–2 current density) consisted of agglomerations of
of these coatings was the lack of crystallite agglome-
20 µm
20 µm
1.22 µm
667.36 nm
0.00 µm
0.00 nm
10 µm
10 µm
a
b
0 µm
0 µm
0 µm
10 µm
20 µm
0 µm
10 µm
20 µm
20 µm
20 µm
398.58 nm
680.87 nm
0.00 nm
0.00 nm
10 µm
10 µm
c
d
0 µm
0 µm
0 µm
10 µm
20 µm
0 µm
10 µm
20 µm
Fig. 3. Surface morphology of Zn–Ni coatings deposited under various i values (A cm–2): a – d. c. plated coating,
p
b – 0.5, c – 3, d – 6. The scanned area 400 µm2

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Pulse and reverse plating effect on the structure and corrosion properties of Zn and Zn alloy coatings...
25
500 nm
500 nm
128.63 nm
45.21 nm
250 nm
250 nm
0.00 nm
0.00 nm
a
b
0 nm
0 nm
0 nm
250 nm
500 nm
0 nm
250 nm
500 nm
500 nm
500 nm
37.04 nm
33.37 nm
250 nm
250 nm
0.00 nm
0.00 nm
c
d
0 nm
0 nm
0 nm
250 nm
500 nm
0 nm
250 nm
500 nm
Fig. 4. Surface morphology of Zn–Ni coatings deposited under various i values (A cm–2): a – d. c. plated coating,
p
b – 0.5, c – 3, d – 6. The scanned area 0.25 µm2
ting conditions (the increase in i , t , t and i ) cau-
p
on
off
a
0.8
0.8
60
m
60
sed a significant reduction in their dimensions, and as
µ
m
,
m
a result the surface morphology underwent modifica-
µ


1

, 0.6
1
50
m
? cr 0.6
45
, n
tion from a globular to a fine-grained film.
? cr
, n
2
2
? gr
? gr
The increase in the overpotential enhances the free
0.4
40
0.4
30
energy to form new nuclei, which result in a higher
0
2
4
6
0
1
2
-2
t , ms
nucleation rate and a smaller grain size. Therefore,
i , A cm
on
p
an increase in i in the pulse plating of metals usually
p
causes a decrease in the grain size. A similar effect
2
60
0.8
m
0.8
1
was also observed for Zn–Ni pulse electrodeposits.
µ
45
,
m
m
An increase in i values from 1 to 3 A cm–2 caused a
p
µ
50
m

, 0.6
? cr

0.6

, n
, n
40
slight reduction in the average ? values from 0.58
? cr
cr
2
? gr
1
? gr
µm up to 0.42 µm and reduction on the average ?
0.4
40
0.4
gr
35
from 42 nm up to 40 nm (Fig. 5). The further increase
0
4
8
0.0
0.2
0.4
in i (>3 A cm–2) caused dendrite formation on the
-2
p
t , ms
off
i , A cm
a
coatings surface, which was accompanied by an incre-
Fig. 5. Effect of pulse plating parameters on the crystal-
ase in the corresponding ? and
values. The lowest
cr
?gr
lite radius (? , µm) (1) and grain size (? , nm) (2) of
cr
gr
R values of the mentioned samples varied between
rms
pulse plated Zn–Ni coatings
60 and 40 nm and were obtained when i exceeded 3
p
rations, which were observed for d.c. plated samples.
A cm–2 (Table).
The surface of pulse plated Zn–Ni coatings appeared
The variation in the t parameter enabled to
on
to consist of globular crystallites. The variation of pla-
obtain Zn–Ni alloys with a lower surface roughness.

---

26
Laima Gudavièiûtë, Aleksandras Kalinièenko, Remigijus Juðkënas, Rimantas Ramanauskas
Table. Surface roughness of pulse and reverse
It may be stated, therefore, that application of
plated Zn–Ni coatings
pulse and reverse current in electrodeposition of Zn–
Ni alloy in alkaline solution results in a significantly
Electrodeposition parameter
R , nm
rms
smoother surface with relatively indefinite grain
0.2
144
boundaries and a reduction in the grain size from
0.5
82
?130 nm for d.c. plated up to 30–40 nm for pulse
i , A cm–2
1
52
p
plated samples.
3
40
Corrosion behaviour and structure relationship
6
61
Corrosion behaviour of pulse plated Zn–Ni coatings
1
48
was investigated in a naturally aerated NaCl +
t ms
2
58
off,
NaHCO solution with pH 6.8 by means of anodic
3
5
45
polarization measurements. It is known, however,
10
30
that the corrosion rates measured electrochemically
0.1
84
are in error with atmospheric data, mainly because
0.2
21
of the presence of corrosion products on the surfa-
t ,ms
0.5
23
on
ce. Zn alloy corrosion in unbuffered Cl– solution
1
20
occurs with the formation of oxide film having a
2
37
porous structure [15]. However, in HCO– contai-
3
0.05
13
ning media the oxide film is supposed to be more
i , A cm–2
0.1
11.5
compact, adherent and less soluble, thus exhibiting
a
0.2
13.5
a passivating character [16]. Aqueous corrosion da-
0.5
15
ta for Zn alloys in HCO – containing media correla-
3
te well with atmospheric corrosion data [9]. The
The average R values around ?20 nm were obtai-
corrosion currents were determined from Tafel plot
rms
ned when t varied between 0.2 and 1 ms. The
extrapolation and the data obtained are presented
on
application of the t parameter higher than 0.2 ms
in Fig. 6. The same figure contains data on W
on
FWHM
produced Zn–Ni surfaces with spherical crystallites
of the diffraction line (110) of Zn–Ni alloys elec-
and relatively undefined grain boundaries. Meanw-
trodeposited under various conditions.
hile, the influence of t on
and ? appeared to
on
?cr
gr
0.70
40
0.5
be similar to that of the previous (i ) case, as a
12
W
p
M
W
M
FWHM
decrease in ? to 0.40 µm and ? to 35 nm was
H
H
FWHM
0.4
-2
FW
cr
gr
FW
-2
0.68
W
obtained at 1 and 0.5 ms t values, respectively.
W

cm

20

8

cm

on
0.3
, A r
The onset of dendrite formation on the alloy surfa-
i
, A r
i cor
corr
i cor
i
ce was observed when t higher than 1 ms was
0.66
corr
0.2
4
on
0
applied. A corresponding increase in ? and
va-
0
2
4
6
0
1
2
cr
?gr
i , A cm-2
lues was also observed under the same conditions.
p
t , ms
on
In pulse plating there is no applied current du-
0.8
20
ring the t period. Local corrosion cells can be for-
0.60
off
i
M
corr
i
med during this period if it is long enough, as has
M
H
H
corr
8
-2
-2
FW
FW 0.6
W 0.56
been stated in several studies [14]. The t varia-
W
10

cm



cm


off
WFWHM
, A
tions influenced formation of Zn–Ni alloy coatings
, A
4
r
r
i cor
0.52
i cor
with the lowest values of around ? and ? 0.4 µm
0.4
WFWHM
cr
gr
0
0
and 40 nm, respectively. Surface flaws typical of lo-
0
4
8
0.0
0.2
0.4
cal corrosion attack were observed on the surface
t , ms
-2
off
i , A cm
a
when t exceeded 5 ms. This fact limited the use
off
of higher t values.
Fig. 6. Effect of pulse plating parameters on the
off
Application of the reverse current peak produ-
corrosion current of Zn–Ni alloy coatings in
ced significantly smoother coatings with R close
NaCl+NaHCO solution and the values of a full width
rms
3
to 11–15 nm. The use of i in the deposition pro-
at half-maximum (FWHM) W
of the diffraction
FWHM
a
gram caused formation of spherical crystallites with
line (110)
relatively undefined grain boundaries. The smallest
crystallites (? ~ 0.4 µm) were obtained at i ? 0.2
The relationship of surface morphology and cor-
cr
a
A cm–2, while the lowest values of ? (?35 nm) we-
rosion behaviour (i ) of Zn–Ni coatings are not
gr
corr
re observed for coatings deposited under i 0.05 A
unambiguous. Generally, the lower values of ? and
a
cr
cm–2. However, the further increase in i (>0.2 A
? yields lower i values, as it can be observed
a
gr
corr
cm–2) caused formation of structural defects origi-
from i and t variations. However, several excep-
p
on
nated most probably by the alloy ionization process.
tions can be observed as well. In the case of toff

---

Pulse and reverse plating effect on the structure and corrosion properties of Zn and Zn alloy coatings...
27
and to some extent of i variations the increase in
CONCLUSIONS
a
these parameters was accompanied by the reduction
in ? and increase in ? . Under the same condi-
The composition of pulse plated Zn–Ni alloy coa-
cr
gr
tions the increase in coatings i
was detected.
tings depended on the plating parameters: an incre-
corr
This
fact implies the dominant influence of the grain si-
ase in cathodic current peak density (i ) and time
p
ze on the corrosion behaviour of alloy. The lowest
off period (t ) caused an increase, while an incre-
off
values of ? for electrodeposited Zn–Ni alloy under
ase in anodic (reverse) current peak density (i ) and
gr
a
all applied plating conditions varied around 35–40
time on period (t ) a decrease in Ni content in the
on
nm, while the corresponding i values ranged bet-
alloy. All pulse plated Zn–Ni coatings, in spite of
corr
ween from 1.5 to 4.0 10–6 A cm–2, being the mini-
the differences in their composition, consisted of a
mal ones for samples deposited with a pulse and
single ? Zn–Ni phase.
reverse current. It seems that not only the grain
Pulse and reverse plating of Zn–Ni alloy in alka-
size of the alloy is responsible for the corrosion
line solutions results in formation of a significantly
properties of the coating.
smoother surface with relatively indefinite grain
The corrosion process is essentially a surface phe-
boundaries and reduction in the grain size from ?90
nomenon, thus, it might be strongly related to crys-
nm for d.c. plated up to 35–40 nm for pulse plated
talline perfection, e.g., highly stepped metal surfa-
samples.
ces, since the presence of dislocations makes the
Zn–Ni alloys with lower grain size values and
steps indestructible. It is reasonable hence, to argue
without macrostructural defects (outset of dendryte,
that the lattice distortions must be important in the
areas of corrosion attack) exhibited a higher corro-
corrosion process.
sion resistance. Lattice imperfection, grain size and
As is evident from the data presented in Fig. 6
grain uniformity are the main factors that determi-
the higher values of W
usually are related to
ne the corrosion behaviour of Zn–Ni coatings.
FWHM
lower i values of Zn–Ni alloy and vice versa. This
Received 16 November 2004
corr
statement is clearly evident for i , t and t varia-
Accepted 26 November 2004
p
on
off
tions, however, only up to a certain number of the-
se parameters. The manifesting discrepancies are
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28
Laima Gudavièiûtë, Aleksandras Kalinièenko, Remigijus Juðkënas, Rimantas Ramanauskas
Rimantas Ramanauskas, Laima Gudavièiûtë, Aleksan-
Zn–Ni lydiniø korozinëms savybëms. Lydiniø dangos nuso-
dras Kalinièenko, Remigijus Juðkënas
dintos ið ðarminio necianidinio elektrolito, turinèio gliceri-
IMPULSINËS IR REVERSINËS ELEKTROS SROVËS
niná Ni jonø kompleksà. Pavirðiaus morfologiniams tyrimams
ÁTAKA ZN IR ZN LYDINIØ STRUKTÛRAI IR
taikyta atomo jëgos mikroskopija, faziø sudëties bei teks-
KOROZINËMS SAVYBËMS.
tûros analizei panaudoti rentgeno difrakciniai matavimai.
Nustatyta, kad Zn–Ni dangos, gautos impulsine elektros sro-
1. ZN–NI
ve, sudarytos ið ?-Zn Ni fazës, kurios sudëtis priklauso nuo
21
5
S a n t r a u k a
dengimo parametrø. Lydinio gardelës defektingumas ir kris-
Tirta impulsinës elektros srovës parametrø átaka pavirðiaus
talitø dydis yra pagrindiniai veiksniai, sàlygojantys dangos
morfologijai, kristalitø dydþiui, gardelës defektingumui ir
korozines savybes.

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