Materials Letters 314 (2022) 131825
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Materials Letters
journal homepage: www.elsevier.com/locate/matlet
Zinc coating on steel by atmosphere plasma spray and their
anti-corrosion behavior
Xiqiang Zhong , Ruijie Liang , Guanghua Liu , Wei Pan *
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Atmosphere plasma spray
Zn coating
Surface adhesion
Anti-corrosion
In this paper, we report a simple process for Zn coatings based on atmosphere plasma spray (APS) techniques.
The Zn coating, prepared at the optimal spraying current of 120A, shows excellent structure characteristics with
low porosity and high adhesion strength of 10.42 MPa. After 3000 h of NaCl solution immersion test, no obvious
corrosion damage occurred on the surface. The corrosion evolution of Zn-coated steel was mainly transformed
from spontaneous passivation of the oxide film to pitting damage by corrosion medium. However, due to the
isolation effect and lower self-corrosion potential, the Zn coating could protect steel from corrosion damages,
which demonstrates the excellent anti-corrosion property.
1. Introduction
Steel has been an irreplaceable material for industry and daily life
due to its high strength, durability, and manufacturing flexibility [1,2].
However, their service life is always plagued by corrosion that signifi­
cantly creates additional stress or cracks [3]. Hence, some counter­
measures are necessary to suppress corrosion, such as cathodic
protection, alloying, coating protection and corrosion inhibitor, etc.
[4,5]. Among them, the Zn coating prepared by electroplating or hotdipping process, with excellent mechanical and corrosion resistance
properties, is regarded as the most effective method [6,7]. In general,
the zinc could easily react with the oxidative mediums to form a dense
and adherent oxide film on the substrate surface. Moreover, the corro­
sion potential of the zinc is considerably lower than the steel materials,
thus providing cathodic protection by sacrificing zinc anode. Recently,
Vucko et.al had prepared a Zn-Ni composite coating by the electro­
deposited methods, and they found the forming of simonkolleite was
affected the dissolution of the Zn-Ni coating [8]. Yuan et.al also fabri­
cated a Zn layer on Mg alloy substrate by friction stir processing tech­
nique, the corrosion current density was only 2.41*10-6A/cm2 in NaCl
solution, which significantly improves the anti-corrosion property for
Mg alloy [9].
However, the pollution of those traditional processes is detrimental
to human health and the environment, such as pneumonia, mutage­
nicity, or carcinogenesis [10,11]. The thermal spray is one of the most
economical and viable processes to obtain metal or ceramic coatings at
an industrial scale [12]. The atmospheric plasma spray (APS) technique,
using high-temperature plasma as a heating source, has attracted
extensive attention due to its low preparation cycle, better control of
coating thickness, and processing flexibility [13]. Compared with other
processes, the APS technique directly sends coating powder into plasma
or plasma jet to accelerate, melt or partially melt, and then rapidly de­
posit on substrates, which could effectively avoid environment pollution
by solvent or high-temperature vapor [14]. Some metal coatings on steel
had been prepared by plasma spraying process, like the titanium coating
that appeared obvious pits damages only after immersing for 170 h in
NaCl solution [15]. In this present work, the Zn coating on steel with low
porosity, low oxidation, and high adhesion strength was prepared by
APS techniques. Moreover, the corrosion evolution of Zn-coated steel in
NaCl solution was systematically studied, and the mechanism behind the
remarkable durability of Zn-coated steel was further explained.
2. Results and discussion
2.1. Morphology and phase composition
The micromorphology of Zn coatings prepared at different spraying
currents is shown in Fig. 1. The surface morphology of all Zn coating at
different spraying currents displayed a typical liquid sputtering char­
acteristic. Under lower spraying currents of 100A or 110A, many mi­
cropores and cracks were irregularly distributed the coating inside
according to cross-section analyses. However, a representative Zn
* Corresponding author.
E-mail address: panw@mail.tsinghua.edu.cn (W. Pan).
https://doi.org/10.1016/j.matlet.2022.131825
Received 15 November 2021; Received in revised form 10 January 2022; Accepted 26 January 2022
Available online 1 February 2022
0167-577X/© 2022 Elsevier B.V. All rights reserved.
X. Zhong et al.
Materials Letters 314 (2022) 131825
coating with fewer micropores or cracks was obtained when spraying
current raised to 120A in Fig. 1(c, c1). And the BET surface area rapidly
decreases from 2.2691 to 06824 m2/g, and the BJH desorption (between
1.7000 nm and 300.0000 nm diameter) is also decreased from 1.3997 to
0.5833 m2/g, as shown in the S2 section. Under higher spray current, the
Zn powders could fully molten and deposit on the steel at a faster rate,
significantly reducing the surface undulation by air cooling and droplet
aggregation. We also found the coating thicknesses were all about
105um, confirming it has a negligible relationship with the spraying
currents.
Fig. 1 (d) shows the X-ray diffraction (XRD) patterns of Zn coatings.
All the coatings exhibited a typical diffraction peak of metal Zn ac­
cording to PDF#04–0831. Under a low spraying current of 100A, the Zn
coating current was pure phase composition and had no impurities or
oxidation state. Nevertheless, as the spraying current increased to 110A,
the coating gradually showed distinct peaks at 2Θ = 31.8◦ and 34.4◦
corresponding to the ZnO phase. The higher spraying current is usually
accompanied by a higher heating temperature, which directly causes the
Fig. 1. The surface and back reflection SEM images of Zn coatings were prepared at different currents: (a) 100A, (b) 110A, (c) 120A; and the XRD patterns of Zn
coatings (d).
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Materials Letters 314 (2022) 131825
Zn powder more susceptible to oxide, whereas it is more important to
reduce coating defects for the anti-corrosion field.
2.3. Anti-corrosion behavior and mechanism
The anti-corrosion test of Zn-coated steel prepared at 120A currents
was performed in 3.5 wt% NaCl solution, and the corrosion evolution
was recorded with respect to immersion time in Fig. 3 (a1-a5). For the
Zn-coated steel, even though the immersion time increased to 720 h, the
coating surface was just partially covered by white and delicate corro­
sion products in Fig. a3. When the immersed time was prolonged to
1400 h, yellow slight spots began to appear from the bottom of the
coating. And the agglomerated phenomenon of corrosion products was
further aggravated after soaking for 3200 h in Fig. a5.
The phase composition and morphology analysis of corrosion prod­
ucts was observed and presented in Fig. 3(b, c). The phase composition
of corrosion products was mainly zinc oxide structure according to the
standard card (PDF#36–1451). Another, the sputtering-liking surface of
Zn coating had disappeared, and the corrosion products were aggre­
gated on the coating surface with a polygonal shape, which might
weaken the shielding effect of the protective layer. In NaCl environment,
the Zn atom usually undergoes an oxidation reaction with oxide medium
to generate a base compound of Zn, further decomposed and convert to
chlorinated compounds [16].
Fig. 3(d, e) shows the electrochemical behavior of Zn-coated steel
after immersing for different times. For the steel substrate immersed for
2.2. Adhesion strength
The adhesion strength between Zn coatings and substrate substan­
tially affects its service life and anti-corrosion property. The adhesion
strength of Zn-coated steel was measured by scratch test (ASTM D33592009) and tensile measurement and shown in Fig. 2. Under a low spray
current of 100A, the surface appeared some jagged notch with less than
1.6 mm (3A grade according to the standard). As the spray current
increased to 120A, there was no trace shedding from the coating surface.
As shown in Fig. 2 (b1-b3), the Zn coatings were completely removed
from the substrate surface after tensile measurement, and the tensile
stress was 3108.88 N, 3515.32 N, 4166.6 N. The adhesion strength was
further calculated to reach 7.76 MPa, 8.79Mpa, and 10.42 MPa with
spraying currents of 100A, 110A, 120A, respectively, which proved the
Zn coating possessed better adhesion strength in higher spraying cur­
rents. The adhesion strength of Zn coatings is closely related to spraying
current, and higher spray current usually contributes to higher spraying
temperature and higher heat-treatment effect on coating and substrate,
thereby significantly reducing the interface stress and defects.
Fig. 2. The images of the surface scratch test (a), tensile results (b), and the curve of the stress–strain (c), adhesion strength (d) of Zn coatings at 2*2cm
steel substrate.
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Materials Letters 314 (2022) 131825
Fig. 3. The immersion test for 0 h, 240 h, 720 h, 1400 h, 3200 h (a1-a5) of Zn-coated steel; the XRD patterns (b) and surface SEM images immersed for 3200 h; their
kinetic polarization curve (c) and corrosion potential and current density (d).
different times over 240 h, shown in the S3 section, the corrosion po­
tential rapidly increases from − 0.444 V to − 0.369 V, and the corrosion
current density increased from 0.7009 uA/cm2 to 0.8899uA/cm2,
proving the corrosion rate gradually expedites with immersion times.
However, for the Zn-coated steel, the corrosion potential was reached −
1.064 V at an immersion time of 24 h. When the immersion time was
prolonged to 48 h, the corrosion potential turned negative to − 1.286 V
and maintained a dynamic balance. Concerning the corrosion current
density in Fig. 3e, a significant decrease occurred from 0.3065uA/cm2 to
0.01783 uA/cm2 in the early stage. Subsequently, the stable area began
to appear and was maintained between 0.14 uA/cm2 and 0.16 uA/cm2.
However, with immersion time surpassing 240 h, the corrosion current
density significantly increased, proving that the corrosion process began
to accelerate. Saadi etc. had proposed the evolution mechanism of
pitting corrosion damages for metallic coating: (i) pitting nucleation, (ii)
metastable growth of pitting, and (iii) stable pitting growth, which is
well proved in this immersion test [17]. This corrosion evolutions of Zncoated steel could be explained by the Nernst equation: φ = φ (standard)
+ (0.0592/n) lg ([Zn] / [Zn2+]). With the forming of oxide film, the
Zn2+ concentration decreases, and φ becomes larger. When the
decomposition rate exceeds the formation rate, the oxidation concen­
tration of Zn gradually decreases and φ becomes lower [18]. Essentially,
the anti-corrosion mechanism of Zn-coated steel is based on the
passivation effects of the ZnO film and sacrificial anode protection
(lower corrosion potential − 1.064 V than steel of − 0.44ev).
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Materials Letters 314 (2022) 131825
3. Conclusion
China (Grant No. 52073156, 91960104).
The Zn coating, with low surface roughness, low defect concentra­
tion, and high adhesion strength was obtained by APS techniques at
120A spray current. After long-time immersion in NaCl solution, the Zncoated steel surface gradually appeared uneven polygonal ZnO particles.
The corrosion potential of Zn-coated steel firstly became positive and
then shifted negative, and then kept dynamic balance. Simultaneously,
the corrosion current density was maintained at 0.14 uA/cm2. The Zncoated steel has displayed excellent anti-corrosion properties even
after serving for over 3000 h, proving the APS process is a feasible
method for Zn coatings.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.matlet.2022.131825.
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CRediT authorship contribution statement
Xiqiang Zhong: Methodology, Formal analysis, Investigation,
Writing – original draft. Ruijie Liang: Formal analysis, Investigation.
Guanghua Liu: Formal analysis, Investigation. Wei Pan: Supervision,
Writing – review & editing, Conceptualization, Methodology, Writing –
review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgement
This study was supported by the Natural Science Foundation of
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