building’s interior climate, the most crucial is the
monitoring of temperature fluctuations in the interior
and the performance of a DSC analysis according to
the measured indoor-temperature-rate change. In this
way, the choice of a proper PCM will consider the
actual conditions of a building, and the thermal enve-
lope with the incorporated PCM will be "tailor-
made".
Acknowledgment
Authors gratefully acknowledge the financial support
of the Czech Science Foundation, under project No
P105/12/G059.
5 REFERENCES
1 World Energy Outlook 2010, OECD/IEA, SOREGRAPH, Paris
Cedex 2010, 731
2 Y. Zhang, S. Zhuang, Q. Wang, J. He, Experimental research on the
thermal performance of composite PCM hollow block walls and
validation of phase transition heat transfer models, Adv. Mater. Sci.
Eng., (2016), doi:10.1155/2016/6359414
3 A. G. Entrop, H. J. H. Brouwers, A. H. M. E. Reinders, Experimental
research on the use of micro-encapsulated phase change materials to
store solar energy in concrete floors and to save energy in Dutch
houses, Sol. Energy, 85 (2011), doi:10.1016/j.solener.2011.02.017
4 F. Agyenim, N. Hewitt, P. Eames, M. Smyth, A review of materials,
heat transfer and phase change problem formulation for latent heat
thermal storage systems (LHTESS), Renew. Sust. Energ. Rev., 14
(2010), doi:10.1016/j.rser.2009.10.015
5 J. M. Marín, B. Zalba, L. F. Cabeza, H. Mehling, Determination of
enthalpy–temperature curves of phase change materials with the
temperature-history method: improvement to temperature dependent
properties, Measurement Sci. Technol., 14 (2003), doi:10.1088/
0957-0233/14/2/305
6 L. Gao, J. Zhao, Q. An, D. Zhao, F. Meng, X. Liu, Experiments on
thermal performance of erythritol/expanded graphite in a direct
contact thermal energy storage container, Appl. Therm. Eng., 113
(2017), doi:10.1016/j.applthermaleng.2016.11.073
7 B. Zalba, J. M. Marín, L. F. Cabeza, H. Mehling, Review on thermal
energy storage with phase change: materials, heat transfer analysis
and applications, Appl. Therm. Eng., 23 (2003), doi:10.1016/S1359-
4311(02)00192-8
8 P. Losada-Pérez, C. S. P. Tripathi, J. Leyes, G. Cordoyiannis, C.
Glorieux, J. Thoen, Measurement of heat capacity and enthalpy of
phase change materials by adiabatic scanning calorimetry, Int. J.
Thermophys., 32 (2011), doi:0.1007/s10765-011-0984-0
9 C. Arkar, S. Medved, Influence of accuracy of thermal property data
of a phase change material on the result of a numerical model of a
packed bed latent heat storage with spheres, Thermochim. Acta, 438
(2005), doi:10.1016/j.tca.2005.08.032
10 B. He, V. Martin, F. Setterwall, Phase transition temperature ranges
and storage density of paraffin wax phase change materials, Energy,
29 (2004), doi:10.1016/j.energy.2004.03.002
11 H. Mehling, H. P. Ebert, P. Schossig, Development of standards for
materials testing and quality control of PCM, 7th IIR Conference on
Phase Change Materials and Slurries for Refrigeration and Air
Conditioning, Dinan 2006, 1–9
12 Product Overview, BASF company, http://product-finder.basf.com,
6.4.2017
13 PCM RT-Line, Rubitherm, https://www.rubitherm.eu, 6.4.2017
14 E. Gmelin, S. M. Sarge, Temperature, heat and heat flow rate
calibration of differential scanning calorimeters, Thermochim. Acta,
347 (2000), doi:10.1016/S0040-6031(99)00424-4
15 ISO 11357-1:2016 Plastics – Differential scanning calorimetry
(DSC) – Part 1: General principles, ISO Committee, Geneve
16 A. Shimkin, Optimization of DSC calibration procedure, Thermo-
chim. Acta, 566 (2013), doi:10.1016/j.tca.2013.04.039
17 ISO 11357-3:2011 Plastics – Differential scanning calorimetry
(DSC) – Part 3: Determination of temperature and enthalpy of
melting and crystallization, ISO Committee, Geneve
18 Z. Wang, H. Su, S. Zhao, N. Zhao, Influence of phase change ma-
terial on mechanical and thermal properties of clay geopolymer
mortar, Constr. Build. Mater., 120 (2016), doi:10.1016/j.conbuild-
mat.2016.05.091
19 G. Feng, K. Huang, H. Xie, H. Li, X. Liu, S. Liu, DSC test error of
phase change material (PCM) and its influence on the simulation of
the PCM floor, Renew. Energy, 87 (2016), doi:10.1016/j.renene.
2015.07.085
20 J. Jeon, S. G. Jeong, J. H. Lee, J. Seo, S. Kim, High thermal
performance composite PCMs loading xGnP for application to
building using radiant floor heating system, Sol. Energ. Mater. Sol.
C., 101 (2012), doi:10.1016/j.solmat.2012.02.028
21 N. Ukrainczyk, S. Kurajica, J. Sipusic, Thermophysical comparison
of five commercial paraffin waxes as latent heat storage materials,
Chem. Biochem. Eng. Q., 24 (2010)
22 Z. Pavlík, J. Foøt, M. Pavlíková, J. Pokorný, A. Trník, R. ^erný,
Modified lime-cement plasters with enhanced thermal and hygric
storage capacity for moderation of interior climate, Energ. Buildings,
126 (2016), doi:10.1016/j.enbuild.2016.05.004
23 X. Liu, H. Liu, S. Wang, L. Zhang, H. Cheng, Preparation and
thermal properties of form stable paraffin phase change material
encapsulation, Energ. Convers. Manage., 47 (2006), doi:10.1016/
j.enconman.2005.10.031
24 E. M. Anghel, A. Georgiev, S. Petrescu, R. Popov, M. Constanti-
nescu, Thermo-physical characterization of some paraffins used as
phase change materials for thermal energy storage, J. Therm. Anal.
Calorim., 117 (2014), doi:10.1007/s10973-014-3775-6
25 D. Zhou, C. Y. Zhao, Y. Tian, Review on thermal energy storage with
phase change materials (PCMs) in building applications, Appl.
Energ. 92 (2012), doi:10.1016/j.apenergy.2011.08.025
J. FOØT et al.: EFFECT OF THE MODE AND DYNAMICS OF THERMAL PROCESSES ...
924 Materiali in tehnologije / Materials and technology 51 (2017) 6, 919–924
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
925–931
CHARACTERIZATION OF Ni-P COATING PREPARED ON A
WROUGHT AZ61 MAGNESIUM ALLOY VIA ELECTROLESS
DEPOSITION
KARAKTERIZACIJA Ni-P PREVLEKE, PRIPRAVLJENE Z
NEELEKTRI^NO DEPOZICIJO NA KOVANI MAGNEZIJEVI
ZLITINI AZ61
Martin Buchtík1, Petr Kosár1, Jaromír Wasserbauer1, Pavel Dole`al1,2
1Brno University of Technology, Faculty of Chemistry, Purkyòova 464/118, 602 00 Brno, Czech Republic
2Brno University of Technology, Faculty of Mechanical Engineering, Technická 2896/2, 602 00 Brno, Czech Republic
xcbuchtik@fch.vutbr.cz
Prejem rokopisa – received: 2017-03-08; sprejem za objavo – accepted for publication: 2017-04-20
doi:10.17222/mit.2017.029
A low-phosphorous Ni-P coating was prepared on a wrought AZ61 magnesium alloy via electroless deposition for 1 h after an
adequate substrate-surface pre-treatment. The prepared coating with a thickness of 10 μm was characterized by the uniform
distribution of Ni (95.4 % mass fraction) and P (4.6 % mass fraction) in the cross-section. Microcavities present in the coating
resulted in quite a low corrosion resistance of the coated magnesium alloy in a 0.1 M NaCl solution. On the other hand, the
coating exhibits a high degree of adhesion, as evidenced by a scratch test, and significantly improves the AZ61
magnesium-alloy microhardness.
Keywords: electroless nickel, magnesium alloy, AZ61, characterization of Ni-P coatings
Malo porozna Ni-P prevleka je bila pripravljena na povr{ini kovane AZ61 magnezijeve zlitine. Prevleka je bila pripravljena z
enourno neelektri~no depozicijo na predhodno ustrezno obdelani povr{ini zlitine. Pripravljena prevleka debeline 10 μm je imela
v pre~nem prerezu enakomerno porazdelitev Ni (95,4 % masnih odstotkov) in P (4,6 % masnih odstotkov). Zaradi prisotnosti
mikropraznin v izdelani prevleki je korozijska odpornost prevle~ene magnezijeve zlitine v 0,1 M NaCl raztopini slaba. Preizkusi
razenja prevleke pa so po drugi strani pokazali zelo dober oprijem izdelane prevleke s povr{ino zlitine in znatno izbolj{anje
mikrotrdote AZ61 magnezijeve zlitine.
Klju~ne besede: brezelektri~no nikljanje, magnezijeva zlitina, AZ61, karakterizacija Ni-P prevlek
1 INTRODUCTION
Due to their low density, magnesium alloys are
ranked among the lightest constructional metallic mate-
rials. Magnesium alloys concurrently have a high
specific strength, toughness and good casting properties.
They find their application in the automotive and aero-
space industry.1–4 A high chemical reactivity, low corro-
sion resistance and low hardness are their negative
properties.4 Therefore, it is necessary to protect magne-
sium alloys against the effects of external environment.
There are several ways of protecting magnesium alloys
such as galvanic or electroless deposition of coatings,
conversion coatings, organic coatings and varnishing.
Electroless-deposited Ni-P coatings improve the
coated-material resistance to corrosive environments and
material mechanical and wear resistance. Deposited Ni-P
coatings have a higher corrosion resistance, physico-
mechanical and tribological properties compared to
non-treated magnesium alloys.5–7 Generally, the industry
identifies three groups of electroless Ni-P coatings
according to their phosphorus contents. Low-phosphorus
coatings contain 2–5 % mass fractions of phosphorus,
medium-phosphorus Ni-P coatings contain 6–9 % mass
fractions of phosphorus and high-phosphorus Ni-P
coatings contain 10–13 % mass fractions of phosphorus.8
Low-phosphorus Ni-P coatings are predominantly used
to increase the hardness of the coated substrate.1 In gene-
ral, the hardness, crystallinity and density of electroless
Ni-P coatings decrease with the increasing content of
phosphorus in the coatings. However, the corrosion
resistance of Ni-P coatings increases with the increasing
content of phosphorus.1,9
The hardness of deposited low-phosphorus Ni-P coat-
ings can be additionally increased by heat treatment.5
During heat treatment, the deposited coating consisting
of an amorphous Ni-P phase decomposes due to a
gradual increase in the temperature to form crystalline
particles of phosphide (Ni3P). Fine crystalline particles
of phosphide (Ni3P) are formed simultaneously, predo-
minantly in the form of precipitates.9 The highest value
of the hardness of Ni-P coatings is reached after the heat
treatment at 400 °C for 1 h, when the hardness reaches a
value of up to 1300 HV.10 The hardness of Ni-P coatings
can also be increased by reducing the phosphorus con-
tent by changing the ratio of the components contained
in the nickel bath and by changing the coating-process
Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931 925
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:621.793.3:669.721.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)925(2017)
conditions.1 However, the Ni-P coating corrosion resis-
tance increases with the increasing content of phos-
phorus.1 The phosphorus content in a deposited Ni-P
coating can be controlled by adding suitable surfactants
into the nickel bath. As mentioned in reference,11 an
addition of SDS (sodium dodecyl sulfate) causes an
increase in the phosphorus content from 6 % to 9 %. It
was observed that the hardness of the Ni-P coating
increased with an increase in the SDS concentration up
to its critical micelle concentration (CMC) and then
decreased. The hardness of the Ni-P coating without a
surfactant was 450 HV 0.2 and, in the presence of SDS,
the hardness increased up to 685 HV 0.2. The change is
due to the change in the coating crystalline structure. In
the case of the absence of SDS, the structure of the Ni-P
coating is purely crystalline. With an addition of SDS,
the structure changes into a mixture of nanocrystalline
and amorphous structure. At SDS concentrations higher
than CMC, the phosphorus content is 9–10 %. The same
effect was observed for CTAB (cetyltrimethylammonium
bromide). An addition of CTAB caused an increase of
phosphorus in the deposited coating of 7.5–12 % and the
hardness of the Ni-P coating increased up to 675 HV 0.2.
In general, the addition of some filler to the Ni-P
coating matrix is another possible way to increase the
hardness of Ni-P coatings. SiC, Al2O3,12–13 SiO2, TiO214–15
particles and carbon nanotubes can be used as a suitable
filler for the Ni-P matrix, improving the coating proper-
ties.
The adhesion of the coatings to the substrate is signi-
ficantly affected by appropriately selected pre-treatments
of the substrate surface.13 The presented paper deals with
the characterization of electroless-deposited Ni-P coat-
ings prepared on an wrought AZ61 magnesium alloy.
The influence of the coated-substrate pre-treatment and
the coating chemical composition on the coating tribo-
logical properties and corrosion resistance was the main
objective of the study. A 0.1 M NaCl solution was used
for immersion tests to analyze the coated magnesium
alloy corrosion resistance and corrosion mechanism.
2 EXPERIMENTAL MATERIAL AND
PROCEDURES
2.1 Experimental material
Specimens of the wrought AZ61 magnesium alloy
with dimensions of 20 mm × 20 mm × 0.8 mm were
used as substrates for the deposition of the Ni-P coating.
The EDS-measured chemical composition of the AZ61
magnesium alloy is shown in Table 1. The measured
chemical composition corresponds to standard ASTM
B107M.16 The microstructure of the alloy is shown in
Figure 1. The microstructure was documented using a
light optical microscope (LM) and the microstructural
features were identified using a scanning electron
microscope (SEM) with EDS. To reveal the magnesium
alloy microstructure, ground and polished metallogra-
phic samples were poured into a picral etchant (con-
sisting of 4.2 g picric acid, 10 mL acetic acid, 10 mL
water and 70 mL ethanol) for 5 s. The microstructure is
formed by  substitutional solid-solution grains (a solid
solution of Al in Mg) and  particles corresponding to
the chemical composition of Mg17(Al,Zn)12. As shown in
Figure 1b, the presence of AlMn-based intermetallic
phases (apparently Al4Mn) was also evident in the
microstructure.
Table 1: Measured elemental composition of the uncoated wrought
AZ61 magnesium alloy
Alloy
Elements (w/%)
Al Zn Mn Si Fe Ni Cu Mg others
AZ61 5.8-7.2
0.4-
1.5
0.15-
0.5
max.
0.15
max.
0.005
max.
0.005
max.
0.05 bal.
max.
0.3
2.2 Deposition of the Ni-P coating
Before the Ni-P coating deposition, a specific pre-
treatment was required to reach the adequate surface
roughness and activity. The samples of the AZ61 mag-
nesium alloy were ground using SiC paper no. 1200.
Next, the samples were degreased in an alkaline degreas-
ing bath containing soil-releasing agents. The following
pickling in an acid pickling bath was performed to
activate the surface with partial etching (removing of the
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
926 Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Microstructure of wrought AZ61 magnesium alloy, etched
with picral etchant: a) structure of AZ61 magnesium alloy, LM;
b) alloy phases, SEM
oxide layer). Rinsing the samples with distilled water
and isopropanol and drying them with a stream of hot air
were performed between the steps of the pre-treatment.
The electroless Ni-P coating deposition, following
the sample-surface pre-treatment, proceeded for 60 min.
The electroless nickel bath was composed by a nickel
source (NiSO4·6H2O), a reducing agent (NaH2PO2·H2O),
a complexing agent and an H2PO2– activating substance.
The samples were kept in the middle of the bath to
ensure a uniform coating formation.
2.3 Characterization of the Ni-P coating
The microhardness of the deposited Ni-P coating was
carried out using a Vickers microhardness tester LECO
AMH43. Loading of 25 g for 10 s was used for the
indentation according to the standard.17 To obtain the
average value, ten indentations were performed on the
coating cross-section. The average thickness of the
deposited Ni-P coating used for microhardness testing
was about 30 μm.
The physicochemical properties of the deposited
Ni-P coating were evaluated using a CSM Instruments
REVETEST scratch tester with the progressive-load-type
method and a Rockwell diamond indenter with the top
angle of 120° and the top radius of 200 μm. For the
evaluation of the physicochemical properties, the surface
of the substrate was polished to the roughness of
Ra  0.25 μm using diamond pastes during the pre-treat-
ment after the grinding. The friction force, the friction
coefficient, the penetration depth and the acoustic
emission were recorded along with the adhesion during
the scratch test. The applied normal force was set in a
range of 1–20 N. The speed of the indenter was 1.58 mm
min–1 and the total length of the trace was 3 mm.18
A Zeiss Evo LS-10 scanning electron microscope
(SEM) equipped with an EDS Oxford Instruments Xmax
80 mm2 detector and the AZtec software was used to
determine the average contents of nickel and phosphorus
in the deposited Ni-P coatings. SEM observations were
also used to evaluate the mechanism of the corrosion
degradation of the magnesium substrate and the depo-
sited Ni-P coating after an exposure to 0.1 M NaCl.
For the evaluation of the mechanism of the corrosion
degradation of the magnesium substrate and the depo-
sited Ni-P coating, the samples were immersed into a 0.1
M solution of NaCl for 20 min. After this time, the
surface of a sample was analyzed using the scanning
electron microscope and the mechanism of corrosion
degradation was determined on the cross-section of the
sample.
3 RESULTS AND DISCUSSION
3.1 Characterization of the deposited Ni-P coatings
Uniform Ni-P coatings were deposited on the AZ61
magnesium alloy substrate. Figure 2a shows the nodular
structure of the Ni-P coating with a typical cauliflower-
like pattern. Between these nodular cusps, a certain
amount of microcavities is present. These microcavities
are nucleation sites for micropitting in the case of
material exposure to a corrosive environment. However,
no macrodefects were observed in the deposited Ni-P
coatings, neither at the Ni-P/substrate interface. Figure
2b shows the cross-section morphology of the deposited
Ni-P coating. It shows that the Ni-P coating is uniform
and compact. The average thickness of the deposited
Ni-P coating used for the microhardness testing was
about 30 μm and the average thickness of the deposited
Ni-P coating used for the EDS analysis and the scratch
test was about 10 μm.
Using the EDS analysis, it was determined that the
content and distribution of individual components in the
deposited Ni-P coating were homogeneous throughout
the entire cross-section (Figure 3). The analysis showed
that the nickel content in the deposited Ni-P coating was
95.4±0.1 % mass fraction and the phosphorus content
was 4.6±0.1 % mass fraction. Based on references,8,10
this coating can be classified as a low-phosphorus Ni-P
coating. The homogenous distribution of individual
elements in the coating is a sign of a continual coating
growth.
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931 927
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Microstructure of Ni-P coating: a) surface morphology of
Ni-P coating, b) cross-section morphology of Ni-P coating on AZ61
magnesium alloy
conditions.1 However, the Ni-P coating corrosion resis-
tance increases with the increasing content of phos-
phorus.1 The phosphorus content in a deposited Ni-P
coating can be controlled by adding suitable surfactants
into the nickel bath. As mentioned in reference,11 an
addition of SDS (sodium dodecyl sulfate) causes an
increase in the phosphorus content from 6 % to 9 %. It
was observed that the hardness of the Ni-P coating
increased with an increase in the SDS concentration up
to its critical micelle concentration (CMC) and then
decreased. The hardness of the Ni-P coating without a
surfactant was 450 HV 0.2 and, in the presence of SDS,
the hardness increased up to 685 HV 0.2. The change is
due to the change in the coating crystalline structure. In
the case of the absence of SDS, the structure of the Ni-P
coating is purely crystalline. With an addition of SDS,
the structure changes into a mixture of nanocrystalline
and amorphous structure. At SDS concentrations higher
than CMC, the phosphorus content is 9–10 %. The same
effect was observed for CTAB (cetyltrimethylammonium
bromide). An addition of CTAB caused an increase of
phosphorus in the deposited coating of 7.5–12 % and the
hardness of the Ni-P coating increased up to 675 HV 0.2.
In general, the addition of some filler to the Ni-P
coating matrix is another possible way to increase the
hardness of Ni-P coatings. SiC, Al2O3,12–13 SiO2, TiO214–15
particles and carbon nanotubes can be used as a suitable
filler for the Ni-P matrix, improving the coating proper-
ties.
The adhesion of the coatings to the substrate is signi-
ficantly affected by appropriately selected pre-treatments
of the substrate surface.13 The presented paper deals with
the characterization of electroless-deposited Ni-P coat-
ings prepared on an wrought AZ61 magnesium alloy.
The influence of the coated-substrate pre-treatment and
the coating chemical composition on the coating tribo-
logical properties and corrosion resistance was the main
objective of the study. A 0.1 M NaCl solution was used
for immersion tests to analyze the coated magnesium
alloy corrosion resistance and corrosion mechanism.
2 EXPERIMENTAL MATERIAL AND
PROCEDURES
2.1 Experimental material
Specimens of the wrought AZ61 magnesium alloy
with dimensions of 20 mm × 20 mm × 0.8 mm were
used as substrates for the deposition of the Ni-P coating.
The EDS-measured chemical composition of the AZ61
magnesium alloy is shown in Table 1. The measured
chemical composition corresponds to standard ASTM
B107M.16 The microstructure of the alloy is shown in
Figure 1. The microstructure was documented using a
light optical microscope (LM) and the microstructural
features were identified using a scanning electron
microscope (SEM) with EDS. To reveal the magnesium
alloy microstructure, ground and polished metallogra-
phic samples were poured into a picral etchant (con-
sisting of 4.2 g picric acid, 10 mL acetic acid, 10 mL
water and 70 mL ethanol) for 5 s. The microstructure is
formed by  substitutional solid-solution grains (a solid
solution of Al in Mg) and  particles corresponding to
the chemical composition of Mg17(Al,Zn)12. As shown in
Figure 1b, the presence of AlMn-based intermetallic
phases (apparently Al4Mn) was also evident in the
microstructure.
Table 1: Measured elemental composition of the uncoated wrought
AZ61 magnesium alloy
Alloy
Elements (w/%)
Al Zn Mn Si Fe Ni Cu Mg others
AZ61 5.8-7.2
0.4-
1.5
0.15-
0.5
max.
0.15
max.
0.005
max.
0.005
max.
0.05 bal.
max.
0.3
2.2 Deposition of the Ni-P coating
Before the Ni-P coating deposition, a specific pre-
treatment was required to reach the adequate surface
roughness and activity. The samples of the AZ61 mag-
nesium alloy were ground using SiC paper no. 1200.
Next, the samples were degreased in an alkaline degreas-
ing bath containing soil-releasing agents. The following
pickling in an acid pickling bath was performed to
activate the surface with partial etching (removing of the
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
926 Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Microstructure of wrought AZ61 magnesium alloy, etched
with picral etchant: a) structure of AZ61 magnesium alloy, LM;
b) alloy phases, SEM
oxide layer). Rinsing the samples with distilled water
and isopropanol and drying them with a stream of hot air
were performed between the steps of the pre-treatment.
The electroless Ni-P coating deposition, following
the sample-surface pre-treatment, proceeded for 60 min.
The electroless nickel bath was composed by a nickel
source (NiSO4·6H2O), a reducing agent (NaH2PO2·H2O),
a complexing agent and an H2PO2– activating substance.
The samples were kept in the middle of the bath to
ensure a uniform coating formation.
2.3 Characterization of the Ni-P coating
The microhardness of the deposited Ni-P coating was
carried out using a Vickers microhardness tester LECO
AMH43. Loading of 25 g for 10 s was used for the
indentation according to the standard.17 To obtain the
average value, ten indentations were performed on the
coating cross-section. The average thickness of the
deposited Ni-P coating used for microhardness testing
was about 30 μm.
The physicochemical properties of the deposited
Ni-P coating were evaluated using a CSM Instruments
REVETEST scratch tester with the progressive-load-type
method and a Rockwell diamond indenter with the top
angle of 120° and the top radius of 200 μm. For the
evaluation of the physicochemical properties, the surface
of the substrate was polished to the roughness of
Ra  0.25 μm using diamond pastes during the pre-treat-
ment after the grinding. The friction force, the friction
coefficient, the penetration depth and the acoustic
emission were recorded along with the adhesion during
the scratch test. The applied normal force was set in a
range of 1–20 N. The speed of the indenter was 1.58 mm
min–1 and the total length of the trace was 3 mm.18
A Zeiss Evo LS-10 scanning electron microscope
(SEM) equipped with an EDS Oxford Instruments Xmax
80 mm2 detector and the AZtec software was used to
determine the average contents of nickel and phosphorus
in the deposited Ni-P coatings. SEM observations were
also used to evaluate the mechanism of the corrosion
degradation of the magnesium substrate and the depo-
sited Ni-P coating after an exposure to 0.1 M NaCl.
For the evaluation of the mechanism of the corrosion
degradation of the magnesium substrate and the depo-
sited Ni-P coating, the samples were immersed into a 0.1
M solution of NaCl for 20 min. After this time, the
surface of a sample was analyzed using the scanning
electron microscope and the mechanism of corrosion
degradation was determined on the cross-section of the
sample.
3 RESULTS AND DISCUSSION
3.1 Characterization of the deposited Ni-P coatings
Uniform Ni-P coatings were deposited on the AZ61
magnesium alloy substrate. Figure 2a shows the nodular
structure of the Ni-P coating with a typical cauliflower-
like pattern. Between these nodular cusps, a certain
amount of microcavities is present. These microcavities
are nucleation sites for micropitting in the case of
material exposure to a corrosive environment. However,
no macrodefects were observed in the deposited Ni-P
coatings, neither at the Ni-P/substrate interface. Figure
2b shows the cross-section morphology of the deposited
Ni-P coating. It shows that the Ni-P coating is uniform
and compact. The average thickness of the deposited
Ni-P coating used for the microhardness testing was
about 30 μm and the average thickness of the deposited
Ni-P coating used for the EDS analysis and the scratch
test was about 10 μm.
Using the EDS analysis, it was determined that the
content and distribution of individual components in the
deposited Ni-P coating were homogeneous throughout
the entire cross-section (Figure 3). The analysis showed
that the nickel content in the deposited Ni-P coating was
95.4±0.1 % mass fraction and the phosphorus content
was 4.6±0.1 % mass fraction. Based on references,8,10
this coating can be classified as a low-phosphorus Ni-P
coating. The homogenous distribution of individual
elements in the coating is a sign of a continual coating
growth.
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931 927
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Microstructure of Ni-P coating: a) surface morphology of
Ni-P coating, b) cross-section morphology of Ni-P coating on AZ61
magnesium alloy
3.2 Mechanical and physicochemical properties of the
deposited Ni-P coating
The resulting average microhardness value of the
prepared electroless-deposited low-phosphorous Ni-P
coating is 700±40 HV 0.025, as measured on 10 random
places on the coating cross-section. The microhardness
of the plain magnesium substrate is 79±6 HV 0.025, so
the microhardness of the Ni-P coating is around nine
times higher than that of the AZ61 magnesium substrate.
The obtained value is higher when compared to the
microhardness of the coating reported on in reference19.
On the other hand, a positive influence of the filler
addition and heat treatment on the coating microhardness
was observed in reference.19 An increase in the micro-
hardness from 380±10 HV 0.1 to 540±10 HV 0.1 was
observed when adding SiO2 nanoparticles to the Ni-P
coating19; however, this value is still lower when com-
pared to the presented coating. The microhardness of the
electroless-deposited composite coating reported on in
reference19 increased to 970±10 HV 0.1 when applying a
heat treatment to the coated component at 400 °C.
A similar effect was observed in the work reported in
reference20 where the hardness of the plain Ni-P coating
reached a value of 608±12 HV. With the addition of a
TiO2 colloidal solution to the nickel bath, followed by a
co-deposition of TiO2 particles into the Ni-P coating, the
hardness value of the Ni-P/TiO2 composite coating
increased to 685±18 HV, which is comparable with the
presented coating. The increase in the hardness of the
deposited Ni-P/TiO2 composite coating to 1325±40 HV
was observed by following the application of a heat-
treatment.20
The obtained values of critical normal forces Lc1 and
Lc2 and adequate values of friction forces Ft1 and Ft2 at
normal forces Lc1 and Lc2, respectively, are shown in
Table 2. The record of the scratch test of the Ni-P coat-
ing on the AZ61 magnesium alloy is shown in Figure 4.
Details of the Ni-P coating on the AZ61 magnesium
alloy after the scratch test for normal forces Lc1 and Lc2
are shown in Figures 5a and 5b, respectively.
The determined value of critical normal force Lc1 was
6.9 N. The formation of oblique and parallel cracks were
observed at Lc1, Figure 5a. The determined value of
critical normal force Lc2 was 11.9 N. The formation of
transverse arch cracks was observed at Lc2, Figure 5b.
Ductile failure of the coating due to the internal tensile
stresses occurs during the scratch test.
Table 2: Values of critical normal forces and friction forces of Ni-P
coatings and a comparison of these values with the published data
Lit. Sub-strate Coating Lc1 (N) Lc2 (N)
Ft1 at Lc1
(N)
Ft2 at Lc2
(N)
This
work AZ61 Ni-P 6.9 11.9 0.8 2.2
(21) AZ31 Ni-P 7.3 12.3 1.1 2.6
(2) AZ91 Ni-P - 17.6 (Lc) - -
(23) AZ61 PEO 2.69 ± 0.10 - - -
(24) AZ61
Ti/Ti
(C,N)/
(TiAl)N
3 10 - -
Comparing critical loads Lc1 and Lc2 of the experi-
mental Ni-P coating deposited on the AZ61 magnesium
alloy with the Ni-P coating deposited on the AZ31
magnesium alloy from21, it became clear that the Ni-P
coating on the AZ61 magnesium alloy achieved lower
critical values Lc1 and Lc2. Oblique, parallel and trans-
verse arch cracks were also observed on the Ni-P coating
on the AZ31 magnesium alloy.21 This effect can be
attributed to a slight difference between the methods of
the pre-treatment of the magnesium substrate before the
deposition process.
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
928 Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Mapping of Ni-P coating on AZ6 alloy: a) structure of Ni-P
coating, b) phosphorus, c) nickel, d) magnesium
Figure 4: Evaluation of scratch tests for Ni-P coating on AZ61 mag-
nesium alloy
The adhesion of the deposited heat-treated Ni-P coat-
ing prepared on the AZ91 magnesium alloy was evalu-
ated in reference.2 The first cracks on the heat-treated
Ni-P coating deposited on the AZ91 magnesium alloy
were observed at a load of 17.6 N and their characte-
ristics were the same for all the heat-treated samples.
However, the character of the cracks is slightly different
compared to the experimental samples (Ni-P on AZ61)
due to the heat treatment of the Ni-P coating. Heat-
treated coatings are more brittle compared to the non-
treated coatings.1 Moreover, it was observed that the
abundance of cracks increased with the increasing
applied load.
In literature2, critical load value Lc, where the first
cracks were observed, is higher than that of the Ni-P coat-
ings deposited on the AZ61 magnesium alloy (Table 2)
and the samples of AZ31 described in reference21. This
fact can be attributed to several factors. The pro-
gressive-load-type method of the scratch test was chosen
for the evaluation of the adhesion of the experimental
Ni-P coating on an AZ61 alloy and the Ni-P coating on
an AZ31 alloy in 21. However, the constant-load-type
method of the scratch test was chosen for the evaluation
of the adhesion of a heat-treated Ni-P coating in litera-
ture 2. Moreover, the initial load for the evaluation of the
heat-treated Ni-P coatings on the AZ91 magnesium alloy
was determined to be 8.80 N and the load was increased
five times to 44.0 N. The heat treatment can affect the
adhesion of the coating to the substrate. As indicated in
literature22, the creation of Al-Ni intermetallic phases can
significantly reduce the adhesion of Ni-P coatings to the
AZ91 magnesium alloy after the heat treatment.
However, this effect was not observed in the research
from reference2. This can be attributed to the fact that the
presence of these phases is limited to small areas of the 
phase (Mg17Al12) present on a substrate surface. The
content of the  phase is dependent on the Al content in
the substrate. Clearly, a low amount of Al in AZ91 used
in the research from reference2 did not have a detrimental
effect on the coating adhesion.
As shown in Table 2, it was observed that the
adhesion of the PEO (plasma electrolytic oxidation)
coating on the AZ61 magnesium alloy mentioned in
reference23 is lower with respect to the Ni-P coatings
deposited on the AZ61 magnesium alloy in this work. In
particular, a significantly lower value of critical load Lc1
resulting in coating damage was measured in the re-
search from literature23, with respect to the present work.
It can be noted that the deposited Ni-P coating
showed a higher adhesion to the magnesium-alloy
substrate with respect to the PEO or plasmatic composite
coatings.24 These composite coatings and plasma-depo-
sited layers are more brittle than the deposited coatings.
In addition, the adhesion of coatings to the substrate is
negatively affected by the stresses at the matrix/filler
interface created during the coating formation.25
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931 929
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Details of Ni-P coating after scratch tests: a)Lc1, b) Lc2
Figure 6: Ni-P coating on AZ61 magnesium alloy after an exposure to
0.1 M NaCl: a) surface, b) cross-section
3.2 Mechanical and physicochemical properties of the
deposited Ni-P coating
The resulting average microhardness value of the
prepared electroless-deposited low-phosphorous Ni-P
coating is 700±40 HV 0.025, as measured on 10 random
places on the coating cross-section. The microhardness
of the plain magnesium substrate is 79±6 HV 0.025, so
the microhardness of the Ni-P coating is around nine
times higher than that of the AZ61 magnesium substrate.
The obtained value is higher when compared to the
microhardness of the coating reported on in reference19.
On the other hand, a positive influence of the filler
addition and heat treatment on the coating microhardness
was observed in reference.19 An increase in the micro-
hardness from 380±10 HV 0.1 to 540±10 HV 0.1 was
observed when adding SiO2 nanoparticles to the Ni-P
coating19; however, this value is still lower when com-
pared to the presented coating. The microhardness of the
electroless-deposited composite coating reported on in
reference19 increased to 970±10 HV 0.1 when applying a
heat treatment to the coated component at 400 °C.
A similar effect was observed in the work reported in
reference20 where the hardness of the plain Ni-P coating
reached a value of 608±12 HV. With the addition of a
TiO2 colloidal solution to the nickel bath, followed by a
co-deposition of TiO2 particles into the Ni-P coating, the
hardness value of the Ni-P/TiO2 composite coating
increased to 685±18 HV, which is comparable with the
presented coating. The increase in the hardness of the
deposited Ni-P/TiO2 composite coating to 1325±40 HV
was observed by following the application of a heat-
treatment.20
The obtained values of critical normal forces Lc1 and
Lc2 and adequate values of friction forces Ft1 and Ft2 at
normal forces Lc1 and Lc2, respectively, are shown in
Table 2. The record of the scratch test of the Ni-P coat-
ing on the AZ61 magnesium alloy is shown in Figure 4.
Details of the Ni-P coating on the AZ61 magnesium
alloy after the scratch test for normal forces Lc1 and Lc2
are shown in Figures 5a and 5b, respectively.
The determined value of critical normal force Lc1 was
6.9 N. The formation of oblique and parallel cracks were
observed at Lc1, Figure 5a. The determined value of
critical normal force Lc2 was 11.9 N. The formation of
transverse arch cracks was observed at Lc2, Figure 5b.
Ductile failure of the coating due to the internal tensile
stresses occurs during the scratch test.
Table 2: Values of critical normal forces and friction forces of Ni-P
coatings and a comparison of these values with the published data
Lit. Sub-strate Coating Lc1 (N) Lc2 (N)
Ft1 at Lc1
(N)
Ft2 at Lc2
(N)
This
work AZ61 Ni-P 6.9 11.9 0.8 2.2
(21) AZ31 Ni-P 7.3 12.3 1.1 2.6
(2) AZ91 Ni-P - 17.6 (Lc) - -
(23) AZ61 PEO 2.69 ± 0.10 - - -
(24) AZ61
Ti/Ti
(C,N)/
(TiAl)N
3 10 - -
Comparing critical loads Lc1 and Lc2 of the experi-
mental Ni-P coating deposited on the AZ61 magnesium
alloy with the Ni-P coating deposited on the AZ31
magnesium alloy from21, it became clear that the Ni-P
coating on the AZ61 magnesium alloy achieved lower
critical values Lc1 and Lc2. Oblique, parallel and trans-
verse arch cracks were also observed on the Ni-P coating
on the AZ31 magnesium alloy.21 This effect can be
attributed to a slight difference between the methods of
the pre-treatment of the magnesium substrate before the
deposition process.
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
928 Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Mapping of Ni-P coating on AZ6 alloy: a) structure of Ni-P
coating, b) phosphorus, c) nickel, d) magnesium
Figure 4: Evaluation of scratch tests for Ni-P coating on AZ61 mag-
nesium alloy
The adhesion of the deposited heat-treated Ni-P coat-
ing prepared on the AZ91 magnesium alloy was evalu-
ated in reference.2 The first cracks on the heat-treated
Ni-P coating deposited on the AZ91 magnesium alloy
were observed at a load of 17.6 N and their characte-
ristics were the same for all the heat-treated samples.
However, the character of the cracks is slightly different
compared to the experimental samples (Ni-P on AZ61)
due to the heat treatment of the Ni-P coating. Heat-
treated coatings are more brittle compared to the non-
treated coatings.1 Moreover, it was observed that the
abundance of cracks increased with the increasing
applied load.
In literature2, critical load value Lc, where the first
cracks were observed, is higher than that of the Ni-P coat-
ings deposited on the AZ61 magnesium alloy (Table 2)
and the samples of AZ31 described in reference21. This
fact can be attributed to several factors. The pro-
gressive-load-type method of the scratch test was chosen
for the evaluation of the adhesion of the experimental
Ni-P coating on an AZ61 alloy and the Ni-P coating on
an AZ31 alloy in 21. However, the constant-load-type
method of the scratch test was chosen for the evaluation
of the adhesion of a heat-treated Ni-P coating in litera-
ture 2. Moreover, the initial load for the evaluation of the
heat-treated Ni-P coatings on the AZ91 magnesium alloy
was determined to be 8.80 N and the load was increased
five times to 44.0 N. The heat treatment can affect the
adhesion of the coating to the substrate. As indicated in
literature22, the creation of Al-Ni intermetallic phases can
significantly reduce the adhesion of Ni-P coatings to the
AZ91 magnesium alloy after the heat treatment.
However, this effect was not observed in the research
from reference2. This can be attributed to the fact that the
presence of these phases is limited to small areas of the 
phase (Mg17Al12) present on a substrate surface. The
content of the  phase is dependent on the Al content in
the substrate. Clearly, a low amount of Al in AZ91 used
in the research from reference2 did not have a detrimental
effect on the coating adhesion.
As shown in Table 2, it was observed that the
adhesion of the PEO (plasma electrolytic oxidation)
coating on the AZ61 magnesium alloy mentioned in
reference23 is lower with respect to the Ni-P coatings
deposited on the AZ61 magnesium alloy in this work. In
particular, a significantly lower value of critical load Lc1
resulting in coating damage was measured in the re-
search from literature23, with respect to the present work.
It can be noted that the deposited Ni-P coating
showed a higher adhesion to the magnesium-alloy
substrate with respect to the PEO or plasmatic composite
coatings.24 These composite coatings and plasma-depo-
sited layers are more brittle than the deposited coatings.
In addition, the adhesion of coatings to the substrate is
negatively affected by the stresses at the matrix/filler
interface created during the coating formation.25
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931 929
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Details of Ni-P coating after scratch tests: a)Lc1, b) Lc2
Figure 6: Ni-P coating on AZ61 magnesium alloy after an exposure to
0.1 M NaCl: a) surface, b) cross-section
3.3 Exposure of the samples to a corrosive environ-
ment
Figure 6 shows a corrosion degradation of the AZ61
magnesium alloy with the Ni-P coating after an exposure
to a 0.1 M NaCl solution for 20 min. The low-phos-
phorus coatings are characterized by a higher hardness
and crystallinity but also by a lower corrosion resistance
when compared to the high-phosphorous Ni-P coatings.10
The deposited Ni-P coating was characterized by a
structure with nodular cusps (Figure 2a). An imperfect
nickel deposition may occur among these cusps, causing
the Ni-P coating to have a certain amount of micro-
cavities in its volume. These microcavities are the
nucleation sites for a corrosion attack and micropitting in
the case of an exposure to a corrosive environment. The
transport of corrosive agents to the surface of the mag-
nesium substrate occurs through these microcavities. The
corrosion process starts at the magnesium substrate/Ni-P
coating interface. Chemical reactions occur due to the
interaction between the corrosive agents and the mag-
nesium-alloy substrate. The formed corrosion products
accumulating on the magnesium alloy under the Ni-P
coating lead to a local destruction of the coating as
shown in Figure 6. An EDS analysis revealed that these
corrosion products are predominantly formed by the
oxides and chlorides of magnesium.
The presence of microcavities in Ni-P coatings can
be eliminated or removed by adding suitable surfactants
to the nickel bath. Such suitable surfactants can be
sodium dodecyl sulfate, sodium benzenesulfonate or
CTAB.11,26–27
4 CONCLUSION
An electroless-deposited low-phosphorous Ni-P coat-
ing was successfully prepared on a wrought AZ61 mag-
nesium alloy by applying a pre-treatment process. The
deposited Ni-P coating was characterized in terms of
elemental composition, mechanical and physicochemical
properties.
In terms of the chemical composition, the deposited
10-μm-thick Ni-P coating treated for 1 h shows a high
degree of homogeneity over the entire cross-section with
a phosphorus content of 4.6±0.1 % and a nickel content
of 95.4±0.1 %. The microhardness of the deposited Ni-P
coating reached a value of 700±40 HV 0.025. In terms of
physicochemical properties, the deposited Ni-P coating
was characterized by a high degree of adhesion. The
adhesion was determined from the corresponding critical
normal forces Lc1 (6.9 N) and Lc2 (11.9 N). The mecha-
nism of corrosion degradation of Ni-P applied on the
AZ61 magnesium alloy substrate was determined based
on a metallographic observation of a corrosion attack
with the subsequent degradation of the magnesium sub-
strate due to the microcavities present in the coating,
allowing the contact of the corrosive environment with
the substrate.
Acknowledgement
This work was supported by project Nr. LO1211,
Materials Research Centre at FCH BUT – Sustainability
and Development (National Programme for Sustain-
ability I, Ministry of Education, Youth and Sports).
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10 G. O. Mallory, J. B. Hajdu, Electroless Plating: Fundamentals and
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11 R. Elansezhian, B. Ramamoorthy, P. K. Nair, Effect of surfactants on
the mechanical properties of electroless (Ni–P) coating, Surf. Coat.
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12 C. Li, Y. Wang, Z. Pan, Wear resistance enhancement of electroless
nanocomposite coatings via incorporation of alumina nanoparticles
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doi:10.1016/j.matdes.2012.12.021
13 Y. Xin, K. Huo, T. Hu, G. Tang, P. K. Chu, Mechanical properties of
Al2O3/Al bi-layer coated AZ91 magnesium alloy, Thin Solid Films,
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14 E. Georgiza, J. Novakovic, P. Vassiliou, Characterization and corro-
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15 S. Afroukhteh, C. Dehghanian, M. Emamy, Preparation of the Ni–P
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16 ASTM B107 / B107M-13 – Standard Specification for Magne-
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19 A. Sadeghzadeh-Attar, G. Ayubikia, M. Ehteshamzadeh, Improve-
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20 X. Wu, J. Mao, Z. Zhang, Y. Che, Improving the properties of 211Z
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21 M. Buchtík, P. Kosár, J. Wasserbauer, P. Dole`al, Characterization of
Ni-P coating prepared via electroless deposition on wrought AZ31
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22 M. Novák, D. Vojtìch, T. Vítù, Influence of heat treatment on
tribological properties of electroless Ni–P and Ni–P–Al2O3 coatings
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23 A. Nìmcová, P. Skeldon, G. E. Thompson, S. Morse, J. ^í`ek, B.
Pacal, Influence of plasma electrolytic oxidation on fatigue
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58–66, doi:10.1016/j.corsci.2013.12.019
24 T. Tañsky, Characteristics of Hard Coatings on AZ61 Magnesium
Alloys, J. Mech. Eng., 59 (2013) 3, 165–174, doi:10.5545/sv-jme.
2012.522
25 K. Tohgo, G. J. Weng, A Progressive Damage Mechanics in Parti-
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Tension, J. Eng. Mater. Technol., 116 (1994) 3, 414–420,
doi:10.1115/1.2904307
26 R. Elansezhian, B. Ramamoorthy, P. K. Nair, The influence of SDS
and CTAB surfactants on the surface morphology and surface
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27 B.-H. Chen, L. Hong, Y. Ma, T.-M. Ko, Effects of Surfactants in an
Electroless Nickel-Plating Bath on the Properties of Ni-P Alloy
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doi:10.1021/ie0105831
M. BUCHTÍK et al.: CHARACTERIZATION OF Ni-P COATING PREPARED ON A WROUGHT AZ61 ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 925–931 931
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
3.3 Exposure of the samples to a corrosive environ-
ment
Figure 6 shows a corrosion degradation of the AZ61
magnesium alloy with the Ni-P coating after an exposure
to a 0.1 M NaCl solution for 20 min. The low-phos-
phorus coatings are characterized by a higher hardness
and crystallinity but also by a lower corrosion resistance
when compared to the high-phosphorous Ni-P coatings.10
The deposited Ni-P coating was characterized by a
structure with nodular cusps (Figure 2a). An imperfect
nickel deposition may occur among these cusps, causing
the Ni-P coating to have a certain amount of micro-
cavities in its volume. These microcavities are the
nucleation sites for a corrosion attack and micropitting in
the case of an exposure to a corrosive environment. The
transport of corrosive agents to the surface of the mag-
nesium substrate occurs through these microcavities. The
corrosion process starts at the magnesium substrate/Ni-P
coating interface. Chemical reactions occur due to the
interaction between the corrosive agents and the mag-
nesium-alloy substrate. The formed corrosion products
accumulating on the magnesium alloy under the Ni-P
coating lead to a local destruction of the coating as
shown in Figure 6. An EDS analysis revealed that these
corrosion products are predominantly formed by the
oxides and chlorides of magnesium.
The presence of microcavities in Ni-P coatings can
be eliminated or removed by adding suitable surfactants
to the nickel bath. Such suitable surfactants can be
sodium dodecyl sulfate, sodium benzenesulfonate or
CTAB.11,26–27
4 CONCLUSION
An electroless-deposited low-phosphorous Ni-P coat-
ing was successfully prepared on a wrought AZ61 mag-
nesium alloy by applying a pre-treatment process. The
deposited Ni-P coating was characterized in terms of
elemental composition, mechanical and physicochemical
properties.
In terms of the chemical composition, the deposited
10-μm-thick Ni-P coating treated for 1 h shows a high
degree of homogeneity over the entire cross-section with
a phosphorus content of 4.6±0.1 % and a nickel content
of 95.4±0.1 %. The microhardness of the deposited Ni-P
coating reached a value of 700±40 HV 0.025. In terms of
physicochemical properties, the deposited Ni-P coating
was characterized by a high degree of adhesion. The
adhesion was determined from the corresponding critical
normal forces Lc1 (6.9 N) and Lc2 (11.9 N). The mecha-
nism of corrosion degradation of Ni-P applied on the
AZ61 magnesium alloy substrate was determined based
on a metallographic observation of a corrosion attack
with the subsequent degradation of the magnesium sub-
strate due to the microcavities present in the coating,
allowing the contact of the corrosive environment with
the substrate.
Acknowledgement
This work was supported by project Nr. LO1211,
Materials Research Centre at FCH BUT – Sustainability
and Development (National Programme for Sustain-
ability I, Ministry of Education, Youth and Sports).
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