Influence of the alloying component on the protective ability of some zinc galvanic coatings

Article abstract of DOI:10.1016/j.electacta.2005.03.049

The composition of the corrosion products of pure Zn galvanic coatings as well as of some zinc alloys (Zn-Mn and Zn-Co) after treatment in selected free aerated model media (5% NaCl and 1N Na₂SO₄) is studied and discussed. X-ray diffraction and X-ray photoelectron spectroscopy investigations are used for this purpose. It is concluded that the corrosion products (zinc hydroxide chloride hydrate in 5% NaCl and zinc hydroxide sulfates hydrates in 1N Na₂SO₄) play a very important role for the improved protective ability of the zinc alloys toward the iron substrate, compared to the pure Zn coatings. Another result is that, for a given medium, the corrosion products are one and the same for both alloys independently of the fact that the alloying component is electrically more positive or negative than the zinc. Some suggestions about the models of the appearance of these products and their protective influence are also discussed.

Full text of DOI:10.1016/j.electacta.2005.03.049

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Electrochimica Acta 51 (2005) 77–84
Influence of the alloying component on the protective ability
of some zinc galvanic coatings
N. Boshkova,∗, K. Petrovb, D. Kovachevab, S. Vitkovaa, S. Nemskab
a
Institute of Physical Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev”,
Bl. No. 11, Sofia 1113, Bulgaria
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev”,
b
Bl. No. 11, Sofia 1113, Bulgaria
Received 2 January 2005; received in revised form 2 March 2005; accepted 19 March 2005
Available online 17 May 2005
Abstract
The composition of the corrosion products of pure Zn galvanic coatings as well as of some zinc alloys (Zn–Mn and Zn–Co) after treatment in
selected free aerated model media (5% NaCl and 1N Na2SO4) is studied and discussed. X-ray diffraction and X-ray photoelectron spectroscopy
investigations are used for this purpose. It is concluded that the corrosion products (zinc hydroxide chloride hydrate in 5% NaCl and zinc
hydroxide sulfates hydrates in 1N Na2SO4) play a very important role for the improved protective ability of the zinc alloys toward the iron
substrate, compared to the pure Zn coatings. Another result is that, for a given medium, the corrosion products are one and the same for both
alloys independently of the fact that the alloying component is electrically more positive or negative than the zinc. Some suggestions about
the models of the appearance of these products and their protective influence are also discussed.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Corrosion; X-ray diffraction; Photoelectron spectroscopy; Zinc; Zinc alloys
1. Introduction
the zinc itself, for example, Co, Ni, Sn, Fe and Cr. The Co
is more preferred from an economical viewpoint. Consid-
erably high protective ability in corrosion media contain-
ing Cl− ions or SO2 has been reported for Co contents as
low as 1–5 wt% [5,11–13]. This alloy is a solid solution of
cobalt in the zinc (␩-phase) with a hexagonal close packed
structure.
Contrary to all above-mentioned metals, manganese has
electrically more negative potential compared to the zinc and
is the only metal that can be co-deposited with Zn from wa-
ter solutions. High protective ability of this alloy is usually
achieved at manganese amounts in the range from 40 up to
60 wt% [8,14], although lower concentrations have been also
successfully used [9,10,15–17].
Zinc galvanic coating on steel substrates provides good
mechanical properties, weldability, paintability as well as
good corrosion resistance [1–4]. Generally, in the case of
corrosion attack, zinc protects the iron or steel substrate
by sacrificial protection—its layers are covered with oxi-
dized products known as “white rust”. The corrosion re-
sistance of zinc could be improved by using additional
treatment—chromating or phosphating films, another type
surface finishing or by alloying with some 3D-metals like
Co, Ni, Mn, Cr and Fe [2–5]. All these alloys exhibit higher
corrosion resistance (protective ability toward the substrate)
compared to the individual metals [6–10].
Most of the zinc galvanic alloys used in practice con-
tain metals that show electrically more positive potential than
This article describes and specifies the protective ac-
tion of the alloying component in two representative types
of zinc galvanic alloys (namely, Zn–Mn and Zn–Co) dur-
ing the corrosion treatment compared to the pure zinc
coatings.
∗
Corresponding author. Tel.: +359 2 979 3920; fax: +359 2 971 26 88.
E-mail address: nbojkov@ipchp.ipc.bas.bg (N. Boshkov).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.03.049

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N. Boshkov et al. / Electrochimica Acta 51 (2005) 77–84
2. Experimental
- model medium of free aerated 5% NaCl solution with
pH ∼ 6.7 at 22 ◦C—causes mainly local corrosion;
2.1. Galvanic coatings (thickness ∼12 µm, hexagonal
- model medium of free aerated 1N Na2SO4 solution with
pH ∼ 6.0 at 22 ◦C—causes local and general corrosion.
close packed structure)
2.1.1. Zn–Mn alloy coatings
2.3. Sample characterization
Galvanic Zn–Mn alloys were electrodeposited from
a starting electrolyte (SE) (in g/l): ZnSO4·7H2O 10.0;
MnSO4·H2O 100.0 and (NH4)2SO4 60.0. The process was
carried out in a double-chamber cell (500 ml volume), current
density 2 A/dm2, pH value 5, 22 ◦C and continuous circula-
tion of 150 rpm. Metallurgical zinc was taken for the anodes
[18]. The phase composition of these alloys is discussed and
described elsewhere [18,19]. Following alloy coatings were
electrodeposited and investigated:
2.3.1. X-ray diffraction (XRD)
The phase composition of the corrosion products was
determined using X-ray diffractometer DRON-3 (Bragg–
Brentano arrangement, Cu K␣ radiation and scintillation
counter).
2.3.2. X-ray photoelectron spectroscopy (XPS)
TheXPSmeasurementswerecarriedoutonanESCALAB
MkII (VG Scientific) electron spectrometer at base pressure
in the analysis chamber of 1 × 10−8 Pa using Mg K␣ X-ray
source. Pass energy of the analyzer was 20 eV and the in-
strumental resolution measured as the full-width at a half-
maximum (FWHM) of the Ag3d5/2 photoelectron peak is
1.2 eV. Energy scale is corrected to the C1s peak maxima at
285 eV. Sample surfaces were studied after etching with ac-
celerated argon (Ar) ions (for depth profiling) with energy of
3 keV and ionic current of 20 mA/cm2.
(a) Zn–Mn (∼6 wt%), obtained by SE and two additives
[18] with trade names AZ-1 (wetting agent 40 ml/1)
and AZ-2 (brightener 10 ml/l). The additive AZ-1
contains poly-ethylene glycol and benzoic acid and
AZ-2—benzalaceton and ethyl alcohol. This alloy forms
a poly-phase coating—it consists generally in a pure
zinc matrix with dispersed small zones of manganese
and intermetallic compound MnZn7 (known also as
␦1-phase from the phase diagram of metallurgical
Zn–Mn alloys) [15,18].
2.3.3. Microprobe analysis
The elemental composition of the samples was determined
using micro-probe analyzer JEOL Superprobe 733, Japan.
(b) Zn–Mn (∼11 wt%), obtained by SE and AZ-1
(20 ml/1)—the alloy contains mainly the intermetallic
␦1-phase and small of pure zinc inclusion zones [15,18].
2.1.2. Zinc–cobalt alloy coatings
3. Results and discussion
Galvanic Zn–Co (1–5 wt%) alloys are obtained by us-
ing a starting electrolyte with a composition (in g/l):
ZnSO4·7H2O 100.0; CoSO4·7H2O 120.0; NH4Cl 30.0 and
H3BO3 25.0. The electrodepositing conditions were: cur-
rent densities 2–5 A/dm2, pH value 3.0–4.0, room temper-
ature 22 ◦C and metallurgical zinc anodes. Two laboratory
additives (similar to AZ-1 and AZ-2), named ZC-1 (wetting
agent 20 ml/l) and ZC-2 (brightener 2 ml/l) were also used
[13].
3.1. Model medium of 5% NaCl
3.1.1. Zn–Mn alloys
3.1.1.1. X-ray diffraction. The diffraction patterns of both
alloy coatings treated for 6 days in this model corrosion
medium – Fig. 1B and C (Fig. 1A shows the spectra of non-
treated␦1-phase)–containlinesofZn, NaClandzinchydrox-
idechloridehydrateZn5(OH)8Cl2·H2O(ZHC). Thelatterhas
very low product of solubility (10−14.2) [20–22] that could be
the most probable reason for the increased protective ability
of this alloy, compared to the pure Zn [9,10,14,15]. It is ob-
vious, that the coatings of the ␦1-phase Zn–Mn (∼11 wt%),
Fig. 1B – transform more easy to ZHC than the samples
Zn–Mn (∼6 wt%) – Fig. 1C. Probably, the homogeneous
distribution of Mn in the intermetallic coating causes the nu-
cleation and growth of uniform ZHC layer over the whole
surface.
2.1.3. Zinc coatings from a slightly acidic electrolyte
Zinc galvanic coatings were obtained from a sulfate bath
containing (in g/l): ZnSO4·7H2O 175.0; (NH4)2SO4 25.0
and H3BO3 30.0 and deposition conditions: current density
2 A/dm2; pH value 4.5–5.0; room temperature 22 ◦C and
metallurgical zinc anodes. The additives used were AZ-1
(50 ml/l) and AZ-2 (10 ml/l) [13,15].
2.2. Sample sizes and corrosion media
3.1.1.2. X-ray photoelectron spectroscopy. XPS spectra of
zinc and oxygen for Zn–Mn (11%) alloy before and after
corrosion treatment are presented in Fig. 2. It can be seen
from the Zn spectra that the peak of this metal for corro-
sionally non-treated sample (No. 1) occurs at binding en-
ergy Ebind = 1022.5 eV. The literature data used [23,24], cor-
Both sides of steel plates with sizes 20 mm × 10 mm ×
1 mmweregalvanicallycoatedwithpureZnorwiththealloys
Zn–Mn and Zn–Co, respectively.
The protective ability of the coatings has been studied in
two different corrosion media:

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N. Boshkov et al. / Electrochimica Acta 51 (2005) 77–84
79
Fig. 1. Diffraction patterns in 5% NaCl solution: (A) corrosionally non-
treated Zn–Mn (11 wt%); (B) Zn–Mn (11 wt%) after 6 days exposure at
E
corr; (C) Zn–Mn (6 wt%) after 6 days exposure at Ecorr. (*) Diffraction
lines of ZHC and (+) diffraction lines of NaCl (all plots are of the same
intensity scale).
respond mainly to Zn O bond and indicates the presence of
the compound ZnO. The other sample – No. 2, corrosion-
ally treated during 6 days at open circuit potential (OCP)
– demonstrates significant changes and “splitting” of the
Zn peak combined with a shift to higher Ebind values of
1024.4 eV. Thelattercouldbecomparedtothebindingenergy
found for Zn(OH)2 (at 1022.6 eV) and ZnCl2 (at 1022.5 eV)
[23,24] that compounds present in ZHC (see Section
3.1.1.1).
Similar arguments could be given also for the oxygen
peaks of both samples investigated. The corrosionally treated
– No. 2 – coating shows a peak with greater area that suggests
the appearance of newly formed compounds such as different
manganese oxides as well as H2O [23,24] (that also present
in ZHC).
Fig. 2. Zn2p3/2 and O1s spectra (sputter time 10 min) of Zn–Mn (11%) alloy:
No. 1, corrosionally non-treated sample and No. 2, after 6 days treatment at
Ecorr in 5% NaCl.
3.1.1.3. Double protective action of manganese. The forma-
tion of ZHC in 5% NaCl is possible only at a slight increase
of the pH value of the medium [22]. Since Mn is electrically
more negative element than the Zn, it dissolves first as Mn2+.
The free electrons from the dissolution processes will react
with the hydrogen ions from the medium causing a hydrogen
evolution [17,22], while some of the hydroxide OH− ions ac-
cumulate near the dissolved zones and breaches. As a result,
the pH of the corrosion medium increases, mainly near the
surface.

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The spatial distribution and relative content of the different
crystalline phases in the protective layer depend on the type
of the deposited galvanic alloy:
- The ␦1-phase has a very good protective ability toward
the iron substrate [9,10]. Since the manganese atoms are
randomly distributed in this structure they cause a forming
of a compact layer of ZHC that covers almost the whole
sample surface [17,19].
- In the case of galvanic alloy Zn–Mn (∼6 wt%), the coating
is poly-phase. The corrosion process is very intensive at
the Mn/Zn interface and the manganese regions begin to
dissolve. This process starts here on local surface centers
and then spreads out over the whole sample. The MnZn7
regions, which are covered with a ZHC layer, remain
relatively stable—from corrosionally viewpoint they, al-
though in small quantities, act as cathode zones, scattered
into the zinc phase, which is an anodic zone. Due to this
distribution, considerable damages appear on the sample
surface and some of the corrosion products precipitate
on the cell bottom. The overall effect of the different
corrosion behavior of each of the phases, consisting the
coating, is the lower protective ability [17,19].
- ZHC is registered also in the presence of pure zinc during
treatment in salt-spray chamber [13,17]. One of the most
probable reasons for its appearance at these conditions
seems to be the surface morphology. Since the galvanic
coating is not perfectly smooth micro-galvanic couples
appear between the protruded and concaved zones or as
a result of the cathodic inclusions from the additives.
The difference in this case is that the process is slower
compared to both investigated Zn–Mn alloys. In addition,
it can be concluded, that at these conditions of corrosion
treatment and for the experimental period, ZHC does not
appear in amounts that allow its XRD registration.
Fig. 3. Diffraction patterns in 5% NaCl solution: (A) corrosionally non-
treated Zn; (B) corrosionally non-treated Zn–Co (1%); (C) galvanic Zn after
6 days exposure at Ecorr. (+) Diffraction lines of NaCl (all plots are of the
same intensity scale).
samples contains additional lines of NaCl and Fe substrate,
in comparison with the non-treated ones (compare Fig. 3A
and C).
All these results demonstrate that the single-phase inter-
metallic alloy has higher protective ability against corrosion.
The manganese in this coating could be formally regarded as
anode protector that plays this role until it is totally dissolved.
The latter process increases the pH values and causes a for-
mation of a protective layer of ZHC. This particular mecha-
nism, which combines anodic protection, accompanied by a
formation of a layer with low product of solubility, could be
regarded as a double-protective mechanism [17,19].
XRD patterns of corrosionally treated Zn–Co (1–5%) al-
loy samples are shown in Fig. 4. They contain lines of the
Zn–Co ␩-phase of the substrate, ␣-Fe of NaCl and also of
ZHC. Its presence enhances the corrosion resistance and
protective ability of the Zn–Co alloy [13] (like for Zn–Mn)
compared to the pure zinc coating. The correlation “Co con-
tent/ZHC amount” can be also observed here—the intensity
of the diffraction lines of ZHC gradually increases with in-
creasing Co content (compare Fig. 4A with Fig. 4B and C).
The presence of the diffraction lines of NaCl is due to the
sample preparation. After the treatment, the samples were
only dried in order to keep all corrosion products intact.
3.1.2. Zn–Co alloys
3.1.2.1. X-ray diffraction. The results obtained by X-ray
diffraction analysis are represented on the Figs. 3 and 4 [13].
Fig. 3 shows the diffraction patterns for corrosionally non-
treated samples of pure Zn (Fig. 3A) and of Zn–Co (1%)
(Fig. 3B); the corrosionally treated (during 6 days expo-
sure in 5% NaCl) Zn is shown in Fig. 3C. XRD pattern on
Fig. 3A shows lines of the hexagonal zinc phase and that
on Fig. 3B—of the ␩-phase of Zn–Co alloys (solid solution
of cobalt in the matrix of hexagonal zinc), respectively. It
is seen that the diffraction pattern of corrosionally treated
3.1.2.2. X-ray photoelectron spectroscopy. Fig. 5 represents
the XPS spectra of zinc and oxygen for Zn–Co (3%) al-
loy after treatment in 5% NaCl. It can be seen that the zinc
peak occurs at the binding energy Ebind = 1022.9 eV. The lit-
erature data [23,24] corroborate the presence of Zn2+ ions,
hexa-co-ordinated by OH− groups and of Zn2+ ions, tetra-co-