19 S. Maitra, G. English, Mechanism of localized corrosion of 7075
alloy plate, Metallurgical and Materials Transactions A, 12 (1981) 3,
535–541, doi:10.1007/BF02648553
20 H. Bakker, H. Bonzel, C. Bruff, M. Dayananda, W. Gust, J. Horvath,
I. Kaur, G. Kidson, A. LeClaire, H. Mehrer, Diffusion in solid metals
and alloys/Diffusion in festen metallen und legierungen: vol. 26,
Springer, Berlin 1990
21 E. Hollingsworth, H. Hunsicker, Corrosion of aluminum and alumi-
num alloys, ASM Handbook, 13 (1987), 583–609
22 J. T. Staley, S. Byrne, E. Colvin, K. Kinnear, Corrosion and stress-
corrosion of 7xxx-w products, Materials Science Forum, 217 (1996),
1587–1592, doi:10.4028/MSF.217-222.1587
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
836 Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
837–842
CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED
ELECTROLESS DUBLEX Ni-P/Ni-B-W COATINGS
KOROZIJSKA ODPORNOST PLATIRANIH IN NEELEKTRI^NO
TOPOLOTNO OBDELANIH DUPLEKS Ni-P/Ni-B-W PREVLEK
Behiye Yüksel1, Garip Erdogan2, Fatih Erdem Bastan2, Rasid Ahmed Yýldýz3
1Istanbul Aydin University, Mechanical Engineering Department, 34668 Istanbul, Turkey
2Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering, 54187 Sakarya, Turkey
3TU Istanbul, Mechanical Engineering Faculty, 34437 Istanbul, Turkey
behiyeyuksel@aydin.edu.tr
Prejem rokopisa – received: 2016-10-20; sprejem za objavo – accepted for publication: 2017-03-10
doi:10.17222/mit.2016.304
In this study, Ni-P/Ni-B-W dublex coatings were deposited on carbon steel substrates (AISI 1020) using the electroless plating
process and their microstructure and corrosion properties were systematically evaluated based on different heat-treatment
temperatures. Both, the surface morphology and cross-sectional morphology of the Ni-P/Ni-B-W coatings were studied using a
scanning electron microscope (SEM), while X-ray diffraction (XRD) was applied for examining the structural modifications.
The amorphous coating began to crystallize at a heat-treatment temperature of 350 °C. Potentiodynamic polarization
measurements were carried out in an aqueous medium containing 3.5 % NaCl for evaluating the corrosion resistance of
as-plated and heat-treated dublex coatings. The corrosion potentials of dublex coatings were observed to shift toward more
positive values with increased heat-treatment temperatures. Depending on the heat-treatment temperature, it was identified that
the crystallized dublex coatings generally had better corrosion resistance than the amorphous coating.
Keywords: Ni-P/Ni-B-W coating, corrosion, heat treatment
V {tudiji so bile dvojne Ni-P/Ni-B-W prevleke nane{ene na substrate iz ogljikovega jekla (AISI 1020) s postopkom neelek-
tri~nega platiranja. Sistemati~no so bile ocenjene mikrostruktura ter korozijske lastnosti izdelanih prevlek, glede na razli~ne
temperature toplotne obdelave. Morfologije povr{in in profili izdelanih Ni-P/Ni-B-W prevlek so bili pregledani z vrsti~nim
elektronskim mikroskopom (SEM), medtem ko so bile z rentgensko difrakcijsko analizo (XRD) ugotovljene strukturne
spremembe. Amorfna prevleka je za~ela kristalizirati pri temperaturi toplotne obdelave 350 °C. Korozijska odpornost platiranih
in toplotno obdelanih dupleks prevlek je bila ocenjena s pomo~jo meritev potenciodinami~ne polarizacije v 3,5 % vodni razto-
pini NaCl. Opazovali so korozijske potenciale dupleks prevlek, da bi dobili bolj pozitivne vrednosti z zvi{animi temperaturami
topolotne obdelave. Ugotovljeno je bilo, da imajo kristalizirane dupleks prevleke na splo{no bolj{o korozijsko odpornost kot
amorfne prevleke.
Klju~ne besede: Ni-P/Ni-B-W dvojna prevleka, korozija, toplotna obdelava
1 INTRODUCTION
Studied for the first time in 1946 by Brenner and
Riddell, the electroless plating process has been used in
many industrial applications because of its superior
characteristics over electroplating, such as the ability to
plate insulation materials and having a homogeneous
coating-thickness distribution.1 Among electroless coat-
ing types, electroless nickel coating is the most popular
for having good hardness, wear and corrosion resistance
properties.2 If the electroless Ni coating family is re-
viewed, besides pure Ni coatings, Ni-P and Ni-B coat-
ings deposited using reducing agents such as hypo-
phosphite, borohydride or dimethylamine borane stand
out. The major advantages of Ni-P coatings are their low
cost, high corrosion resistance and easy process control.3
Nonetheless, a borohydride reduced nickel coating is
harder and has higher wear resistance than tool steel and
hard chrome coatings.4,5 On the other hand, some studies
– although they are limited in number – have highlighted
the addition of tungsten to this coating system for in-
creasing its corrosion resistance, which is lower than that
of a Ni-P coating.6–9 As a result of the co-deposition of
this refractory material (which cannot be reduced in an
aqueous solution as metallic tungsten) together with iron
group metals, developing coatings with attractive corro-
sion and tribological properties is possible.10–12 Recently,
research for obtaining multilayer coatings to increase
existing coating corrosion resistance to higher levels has
also been conducted.13,14
The aim of the present study was to evaluate the
effect of different heat-treatment temperatures on the
microstructure and corrosion properties of electroless
Ni-P/Ni-B-W coatings. To the best of our knowledge, no
others studies are available on this topic.
2 EXPERIMENTAL PART
For this experiment, 10 mm × 10 mm × 60 mm plain
carbon steel (AISI 1020) plates were used as a substrate
material for developing Ni-P/Ni-B-W dublex coatings.
Prior to the plating process, the surfaces of all the
Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842 837
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 620.193:621.78:669.058 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)837(2017)
samples were mechanically cleaned (up to 1200 grade)
and then soaked in trichloroethylene and cleaned with
detergent in an ultrasonic bath at 70 °C. Finally, sample
surfaces were activated in 30 % of volume fractions. HCl
for two minutes and then rinsed in distilled water. The
cleaned substrates were then soaked in Ni-P and Ni-B-W
electroless plating baths for two hours, respectively.
Reagent-grade chemicals and distilled water were
used for the preparation of all the electrolytes and the pH
was adjusted to 4.8±0.2 and 13.5±0.2 with H2SO4 or
NaOH in Ni-P and Ni-B-W plating baths, respectively.
The temperature of the baths was maintained at 90±2 °C
for Ni-P and 88±2 °C for Ni-B-W.
Deposition of Ni-P and Ni-B-W was carried out in an
aqueous bath containing 15 g/L NiSO4.6H2O, 26 g/L
Na2H2PO2·H2O, 13 g/L NaC2H3O2, 12 mL/L HF and
8 g/L NH4HF2 for the Ni-P coating, and 24 g/L NiCl2,
60 mL/L EDTA, 26.5 g/L KOH, 120 g/L NaBH4,
263 g/L NaOH, 2.6g/L PbWO4, 13 g/L EDTA and 40 g/L
Na2WO4·2H2O for Ni-B-W coatings, respectively.
The coated specimens were heat treated at 250 °C,
300 °C, 350 °C and 650 °C for 1 h each at a heating rate
of 5 °C/min in an air-circulated furnace, followed by
slow furnace cooling to room temperature.
The composition of the dublex coatings was deter-
mined by energy-dispersive X-ray analysis (EDX) using
a Link Analytical QX-2000 attached to the SEM appa-
ratus. The crystal structures of the dublex coatings were
examined using a Philips PW 3710 grazing incidence
x-ray diffractometer (Cu-K radiation). A Jeol
JSM-7000F FE-SEM was used to characterize the
microstructures and morphology of the coatings. The
corrosion behavior was investigated using a PGZ 301
Dynamic Voltammetry and VoltaMaster4 software.
Electrochemical experiments were carried out in a
3.5 % NaCl aqueous solution in a three-electrode cell at
room temperature. In this cell, a platinum electrode was
used as a counter electrode and a saturated calomel
electrode was used as a reference electrode; the dublex
coating was used as a working electrode and by masking
with silicon, only a 1 cm2 surface area was left on the
samples for exposure to the electrolyte. The dynamic
potential scanning technique was used for obtaining the
polarization curves of dublex coating samples. Prior to
the potentiodynamic measurement, samples were held
for approximately 15 min in the electrolyte and then
electrode potential was raised from -600mV to +200 mV
at a rate of 10 mV/min.
3 RESULTS
The XRD patterns of the as-plated and heat-treated
Ni-P/Ni-B-W coatings, and their chemical compositions
are in Figure 1 and in Table 1. SEM micrographs of
Ni-P/Ni-B-W coatings before and after the heat treat-
ments are in Figure 2.
Table 1: Chemical compositions of the Ni-P/Ni-B-W dublex coatings
Type of
coating Ni (w/%) P (w/%) B (w/%) W (w/%)
Ni-P 87.27 12.73 - -
Ni-B-W 87.45 - 7.86 4.69
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
838 Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Surface morphology of dublex coating a), b) as-plated
Ni-P/Ni-B-W, c) heat treated at 300 °C, d) heat treated at 650 °C
Figure 1: XRD spectra for a): as-plated Ni-P/Ni-B-W, b) 250 °C, c)
300 °C, d) 350 °C and e) 650 °C
The cross-sections of the Ni-P/Ni-B-W dublex coat-
ings are shown in Figure 3.
Potentiodynamic curves of as-plated and heat-treated
Ni-P/Ni-B-W coatings at different temperatures obtained
in a 3.5 % NaCl aqueous medium are shown in Figure 4.
The changes in the corrosion current density (Icorr)
values with heat treatment are given in Table 2.
Table 2: Corrosion resistance of as-plated and heat-treated
Ni-P/Ni-B-W dublex coatings at different temperatures in 3.5 % NaCl
solution
NiP/NiBW Icorr (μA/cm2) Ecorr (mV)
As-plated 1.55±0.59 -506±32
300 °C 1.70±0.41 -470±23
350 °C 0.71±0.20 -382 ±18
650 °C 0.57±0.09 -280±11
The surface morphologies of the as-plated sample
and heat-treated sample at 650 °C following potentio-
dynamic polarization measurements are shown in Fig-
ure 5.
The corrosion current density and corrosion potential
values obtained by Tafel interpolation are shown in
Table 3.
Table 3: Corrosion resistance of substrate and Ni-B-W and
Ni-P/Ni-B-W dublex coatings heat treated at 650 °C in 3.5 % NaCl
solution
Sample Icorr (μA/cm2) Ecorr (mV)
Carbon steel
substrate 4.59±1.01 -598±46
NiBW 3.40±0.35 -298±17
NiP/NiBW 0.57±0.09 -280±11
Figure 6 shows the polarization curves of the
Ni-B-W and dublex Ni-P/Ni-B-W coatings heat treated
at 650 °C and the steel substrate obtained in a 3.5 %
NaCl aqueous medium.
4 DISCUSSION
The Ni-P/Ni-B-W coating diffraction patterns up to
300 °C reveal a single broad peak indicating an amor-
phous coating structure under these conditions (Fig-
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842 839
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Cross-section of Ni-P/Ni-B-W dublex coating: a) as-plated,
b) heat treated at 250 °C c) heat treated at 350 °C, d) heat treated at
Figure 4: Polarization curves in a 3.5 % NaCl aqueous medium of
as-plated and heat-treated Ni-P/Ni-B-W coatings: a) as-plated
NiP/NiBW, b) heat treated at 300 °C, c) heat treated at 350 °C d) heat
treated at 650 °C
Figure 5: Surface morphology of NiP/NiBW dublex coatings detected
by SEM after potentiodynamic measurement: a) as-plated, b) heat
treated at 650 °C
samples were mechanically cleaned (up to 1200 grade)
and then soaked in trichloroethylene and cleaned with
detergent in an ultrasonic bath at 70 °C. Finally, sample
surfaces were activated in 30 % of volume fractions. HCl
for two minutes and then rinsed in distilled water. The
cleaned substrates were then soaked in Ni-P and Ni-B-W
electroless plating baths for two hours, respectively.
Reagent-grade chemicals and distilled water were
used for the preparation of all the electrolytes and the pH
was adjusted to 4.8±0.2 and 13.5±0.2 with H2SO4 or
NaOH in Ni-P and Ni-B-W plating baths, respectively.
The temperature of the baths was maintained at 90±2 °C
for Ni-P and 88±2 °C for Ni-B-W.
Deposition of Ni-P and Ni-B-W was carried out in an
aqueous bath containing 15 g/L NiSO4.6H2O, 26 g/L
Na2H2PO2·H2O, 13 g/L NaC2H3O2, 12 mL/L HF and
8 g/L NH4HF2 for the Ni-P coating, and 24 g/L NiCl2,
60 mL/L EDTA, 26.5 g/L KOH, 120 g/L NaBH4,
263 g/L NaOH, 2.6g/L PbWO4, 13 g/L EDTA and 40 g/L
Na2WO4·2H2O for Ni-B-W coatings, respectively.
The coated specimens were heat treated at 250 °C,
300 °C, 350 °C and 650 °C for 1 h each at a heating rate
of 5 °C/min in an air-circulated furnace, followed by
slow furnace cooling to room temperature.
The composition of the dublex coatings was deter-
mined by energy-dispersive X-ray analysis (EDX) using
a Link Analytical QX-2000 attached to the SEM appa-
ratus. The crystal structures of the dublex coatings were
examined using a Philips PW 3710 grazing incidence
x-ray diffractometer (Cu-K radiation). A Jeol
JSM-7000F FE-SEM was used to characterize the
microstructures and morphology of the coatings. The
corrosion behavior was investigated using a PGZ 301
Dynamic Voltammetry and VoltaMaster4 software.
Electrochemical experiments were carried out in a
3.5 % NaCl aqueous solution in a three-electrode cell at
room temperature. In this cell, a platinum electrode was
used as a counter electrode and a saturated calomel
electrode was used as a reference electrode; the dublex
coating was used as a working electrode and by masking
with silicon, only a 1 cm2 surface area was left on the
samples for exposure to the electrolyte. The dynamic
potential scanning technique was used for obtaining the
polarization curves of dublex coating samples. Prior to
the potentiodynamic measurement, samples were held
for approximately 15 min in the electrolyte and then
electrode potential was raised from -600mV to +200 mV
at a rate of 10 mV/min.
3 RESULTS
The XRD patterns of the as-plated and heat-treated
Ni-P/Ni-B-W coatings, and their chemical compositions
are in Figure 1 and in Table 1. SEM micrographs of
Ni-P/Ni-B-W coatings before and after the heat treat-
ments are in Figure 2.
Table 1: Chemical compositions of the Ni-P/Ni-B-W dublex coatings
Type of
coating Ni (w/%) P (w/%) B (w/%) W (w/%)
Ni-P 87.27 12.73 - -
Ni-B-W 87.45 - 7.86 4.69
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
838 Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Surface morphology of dublex coating a), b) as-plated
Ni-P/Ni-B-W, c) heat treated at 300 °C, d) heat treated at 650 °C
Figure 1: XRD spectra for a): as-plated Ni-P/Ni-B-W, b) 250 °C, c)
300 °C, d) 350 °C and e) 650 °C
The cross-sections of the Ni-P/Ni-B-W dublex coat-
ings are shown in Figure 3.
Potentiodynamic curves of as-plated and heat-treated
Ni-P/Ni-B-W coatings at different temperatures obtained
in a 3.5 % NaCl aqueous medium are shown in Figure 4.
The changes in the corrosion current density (Icorr)
values with heat treatment are given in Table 2.
Table 2: Corrosion resistance of as-plated and heat-treated
Ni-P/Ni-B-W dublex coatings at different temperatures in 3.5 % NaCl
solution
NiP/NiBW Icorr (μA/cm2) Ecorr (mV)
As-plated 1.55±0.59 -506±32
300 °C 1.70±0.41 -470±23
350 °C 0.71±0.20 -382 ±18
650 °C 0.57±0.09 -280±11
The surface morphologies of the as-plated sample
and heat-treated sample at 650 °C following potentio-
dynamic polarization measurements are shown in Fig-
ure 5.
The corrosion current density and corrosion potential
values obtained by Tafel interpolation are shown in
Table 3.
Table 3: Corrosion resistance of substrate and Ni-B-W and
Ni-P/Ni-B-W dublex coatings heat treated at 650 °C in 3.5 % NaCl
solution
Sample Icorr (μA/cm2) Ecorr (mV)
Carbon steel
substrate 4.59±1.01 -598±46
NiBW 3.40±0.35 -298±17
NiP/NiBW 0.57±0.09 -280±11
Figure 6 shows the polarization curves of the
Ni-B-W and dublex Ni-P/Ni-B-W coatings heat treated
at 650 °C and the steel substrate obtained in a 3.5 %
NaCl aqueous medium.
4 DISCUSSION
The Ni-P/Ni-B-W coating diffraction patterns up to
300 °C reveal a single broad peak indicating an amor-
phous coating structure under these conditions (Fig-
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842 839
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Cross-section of Ni-P/Ni-B-W dublex coating: a) as-plated,
b) heat treated at 250 °C c) heat treated at 350 °C, d) heat treated at
Figure 4: Polarization curves in a 3.5 % NaCl aqueous medium of
as-plated and heat-treated Ni-P/Ni-B-W coatings: a) as-plated
NiP/NiBW, b) heat treated at 300 °C, c) heat treated at 350 °C d) heat
treated at 650 °C
Figure 5: Surface morphology of NiP/NiBW dublex coatings detected
by SEM after potentiodynamic measurement: a) as-plated, b) heat
treated at 650 °C
ure 1). For heat-treatment temperatures up to 300 °C, the
dublex coating preserved its amorphous structure.
However, after heat treatment at and above 300 °C, the
diffraction patterns showed the formation of Ni3B inter-
metallic crystals. For the higher heat-treatment tempe-
ratures up to 650 °C, formation of the nickel borides
progressively increased. Drovosekov et al., reported
chemical bond formations between boron and tungsten
in the Ni-B-W coating system. However, the major
nickel borides formed during the heat treatments might
have prevented the formation of the B-W bond.9,15 In
other words, the quantity of the boron in the coating was
high enough to prevent the formation of Ni-W com-
pounds in the structure.16
SEM micrographs of the Ni-P/Ni-B-W coatings be-
fore and after the heat treatments showed the formation
of typical spherical nodular structures (Figure 2). Each
nodule appears to be formed by the unification of many
grains existing as colonies (Figure 2b), similar to the
case suggested by S. Ruan et al.17 Some microcracks
were noticeable on the surfaces of the as-plated
Ni-P/Ni-B-W samples. The microcracks may be due to
the internal stress caused by the relatively larger tungsten
atoms in the coating structure.18,19
With the heat treatments, the structure progressively
crystallized and, as consequence, a substantial reduction
of the microcracks was noticed (Figure 2c and 2d). In a
Ni-B-W coating system, W atoms may selectively exist
at the boundaries of the nodules, from there, they may
segregates to the coating surface, during the heat treat-
ments.16,17 This process may decrease the internal stress
inside the coating, resulting in a substantial decrease of
the microcracks on the coating surface.
The thickness of the Ni-P coating in the dublex
coating system was measured as 2 μm, while the average
thickness of the Ni-B-W (the other coating in the
system) was measured as 11 μm (Figure 3). On the other
hand, it was obvious that the nodular structure seen in
the surface morphology of the Ni-B-W coating showed
columnar growth starting from the Ni-P layer to the
topmost surface of the coating (Figure 3a). Furthermore,
it was observed that adherence of the Ni-P coating to the
substrate was good and there was no pore formation
between the substrate and Ni-P coating. Pores seen in the
interface of Ni-P/Ni-B-W, in addition to pores partially
observed inside the Ni-B-W coating had a columnar
structure and they were considered to have been formed
due to the capturing of released hydrogen – even if in
small amounts – during the oxidation reaction of sodium
borohydride used as a the boron source.5 The pores
disappear with increasing heat-treatment temperatures.
This may be a result of the diffusion of hydrogen atoms
to the surface. They may promote better crystallization
of the coating and ease the formation of the Ni3B and
Ni2B phases (Figure 3b and 3c). After 650 °C heat
treatment the major phase of the coating was noticed to
be Ni3B. At that stage adherence between the coatings
reached its best level, while the Ni-B-W coating showed
columnar grains with a denser structure (Figure 3d).
As Figure 4, a difference of approximately 220 mV
was detected between corrosion potentials (Ecorr) of
as-plated and 650 °C heat-treated dublex coatings. This
is the shifting of Ecorr values of the coatings toward
nobler values with increased heat treatments applied to
the coatings. The anodic branch of the polarization curve
of the coating before heat treatment showed the break-
down potential at values around -310 mV (Figure 4a).
But the current density increased at a constant rate,
indicating that the unstable passive film formation on the
coating has disappeared rapidly. For heat treatment at
300 °C, the breakdown potential was observed to shift
toward -220 mV. At 300 °C, which is the onset tem-
perature of crystallization, corrosion potential shifted
towards a positive end approximately 40 mV, compared
to the as-plated sample. At 350 °C, crystallization was
nearly completed and the corrosion potential had shifted
towards more positive values. However, the breakdown
potential was observed (as with other samples) at around
-180 mV. Nonetheless, it was detected that the current
density of this sample became higher as the potential
value increased compared with that for the to as-plated
and heat-treated samples at 300 °C. For the heat treat-
ment at 650 °C, a passive film formation on the coating
surface was seen at around -190 mV, suggesting that
corrosion rate was restrained at such potential range.
However, as this protective film began to dissolve at
around -90 mV, a steep increase in the current density
was observed.
The major reason for the difference between the
corrosion resistance of as-plated and heat-treated
Ni-P/Ni-B-W coatings exposed to chloride ions was
undoubtedly associated with the transformation of the
microstructure from an amorphous to a crystalline mor-
phology. The main reason for having a higher Ecorr value
the heat-treated coating at 300 °C (where an amorphous
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
840 Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Polarization curves: a) steel substrate, b) Ni-B-W coating
heat treated at 650 °C, c) dublex Ni-P/Ni-B-W coating heat treated at
650 °C
structure still prevailed) compared to the as-plated
sample was the result of the borides formed during the
onset of crystallization. The heat-treated sample at
350 °C (where the microstructure had completely been
transformed to a crystalline structure) had a higher
corrosion resistance compared to the sample heat-treated
at 300 °C.20 On the other hand, it is a known fact that
corrosion resistance generally increases with addition of
tungsten to a electroless nickel-boron coating system,
because of the tendency to form an oxide film on the
surfaces.7,18 However, as seen in Figure 4, the corrosion
resistance of the as-plated Ni-P/Ni-B-W sample having a
nodular structure did not increase, with the addition of
tungsten. This is because the tungsten atoms were stuck
in the colony boundaries, not to be segregated to the
surfaces during the corrosion process and therefore,
could not be oxidized to promote the formation of a
mixed oxide film. However, depending on the increased
heat treatment temperatures segregation of W atoms
toward surfaces at 650 °C was more likely to occur. The
mixed oxide film a passive zone formed this way may be
the main reason for the substantial decrease of the
current density as observed here.
It can be seen that uniform corrosion occurred on the
surface of the as-plated sample in the amorphous
structure that has the lowest corrosion resistance (Fig-
ure 5a). Conversely, on the surface of the heat-treated
sample at 650 °C complete crystallization were noted
and the pits (indicated by arrows) in the grains within the
colonies were observed (Figure 5b).
To interpret the effect of the dublex coating on
corrosion resistance, the potentiodynamic polarization
measurements of the Ni-P/Ni-B-W and Ni-B-W
coatings, as well as which were deposited under the
same process conditions and heat-treated at 650 °C, and
carbon steel substrate were studied (Table 3).
As can be observed from Figure 6, while the Ni-B-W
coating and the dublex coating almost had the same
corrosion potential, more positive values at about 300
mV, compared with that of the substrate. Among the
coating studied the dublex coating has the lowest corro-
sion current density. The anodic branches of polarization
curves of both the dublex and the Ni-B-W coatings have
a passive zone, and both coatings had an Epit value of
approximately -90 mV. Additionally, the current density
of the Ni-B-W coating at a constant rate increases,
depending on the increased potential value. On the other
hand, the anodic branch of the dublex coating indicated
that an entrance to a second passive zone at approxi-
mately 20 mV. This passive zone may be related to the
existence of Ni-P in the dublex coating. A similar inter-
pretation was reported by Zhang et al. for the corrosion
of Ni-B-W surfaces.14 In general, corrosion starts at the
outermost surface of the substrate and proceed inward.
However, the stable passive film formed on the Ni-P
layer on the substrate may act as a barrier for corrosion
propagation, causing the dublex coating to have better
corrosion resistance than the single-layer Ni-B-W
coating.
5 CONCLUSIONS
The effect of heat treatments on the corrosion resis-
tance of a Ni-P/Ni-B-W dublex coating deposited on
carbon steel was studied. The Ni-P/Ni-B-W coatings
have an amorphous structure for a heat treatments up to
300 °C. The coatings start to crystallize at about 350 °C
and completed at about 650 °C with the major phases
being the nickel borides. The heat treatment, besides
causing nickel boride formation within Ni-B-W, also
caused obtaining a denser Ni-P/Ni-B-W coating when
increasing heat treatment temperature.
The corrosion resistance of the dublex coating in-
creases substantially with increasing the heat treatment
temperature. The dublex coating and the single-layer
Ni-B-W coating heat-treated at 650 °C indicated higher
corrosion resistance as compared to that of steel sub-
strate. When these two coatings were compared in terms
of corrosion resistance, the dublex coating showed better
performance.
6 REFERENCES
1 A. Brenner, G. Riddell, Nickel Plating on Steel by Chemical
Reduction, J. Res. Nat. Bur. Std., 37 (1946), 31–35
2 J. Sudagar, J. Lian, W. Sha, Electroless nickel, alloy, composite and
nano coatings-A critical review, J. Alloys Compd., 571(2013),
183–204, doi:10.1016/j.jallcom.2013.03.107
3 Y. F. Shen, W.Y. Xue, Z.Y. Liu, L. Zuo, Nanoscratching Deformation
and Fracture Toughness of Electroless Ni-P Coatings, Surf. Coat.
Technol., 205 (2010), 632–640, doi:10.1016/j.surfcoat.2010.07.066
4 K. M. Gorbunova, M.V. Ivanov, V. P. Moissev, Electroless Depo-
sition of Nickel-Boron Alloys – Mechanýsm of Process, Structure,
and Some Propertýes of Deposits, J. Electrochem. Soc., 120 (1973),
613–618, doi:10.1149/1.2403514
5 R. N. Duncan, T. L. Arney, Operatýon and Use of Sodium-Boro-
hydride-Reduced Electroless Nickel, Plat. Surf. Finish., 71 (1984)
12, 49–54
6 R. A. C. Santana, S. Prasad, A. R. N. Campos, F. O. Arau´ jo, G. P.
DA Silva, P. Lýma-Neto, Electrodeposition and Mechanical Pro-
perties of Ni–W–B Composites from Tartrate Bath, Prot Met Phys
Chem., 46 (2010) 1, 117–122, doi:10.1134/S207020511001017X
7 M. G. Hosseini, M. Abdolmaleki , H. Ebrahimzadeh, S. A. Seyed
Sadjadi, Effect of 2-Butyne-1, 4-Diol on the Nanostructure and
Corrosion Resistance Properties of Electrodeposited Ni-W-B
Coatings, Int. J. Electrochem. Sci., 6 (2011) 4, 1189–1205
8 T. Osaka, N. Takano, T. Kurokawa, T. Kaneko, K. Ueno, Characteri-
zation of chemically-deposited NiB and NiWB thin films as a
capping layer for ULSI application, Surf. Coat. Technol., 169–170
(2003), 124–127, doi:10.1016/S0257-8972(03)00186-5
9 G. Graef, K. Anderson, J. Groza, A. Palazoglu, Phase evolution in
electrodeposited Ni-W-B alloy, Mater. Sci. Eng., B41 (1996),
253–257, doi:10.1016/S0921-5107(96)01656-X
10 E. Lassner, W. D. Schubert, Tungsten: Properties, Chemistry, Tech-
nology of the Element, Alloys, and Chemical Compounds, Kluwer
Academic, New York, 1999
11 E. Beltowska-Lehman, Kinetics of Induced Electrodeposition of
Alloys Containing Mo from Citrate Solutions, Phys. Stat. Sol., 5
(2008) 11, 3514–3520, doi:10.1002/pssc.200779404
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842 841
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
ure 1). For heat-treatment temperatures up to 300 °C, the
dublex coating preserved its amorphous structure.
However, after heat treatment at and above 300 °C, the
diffraction patterns showed the formation of Ni3B inter-
metallic crystals. For the higher heat-treatment tempe-
ratures up to 650 °C, formation of the nickel borides
progressively increased. Drovosekov et al., reported
chemical bond formations between boron and tungsten
in the Ni-B-W coating system. However, the major
nickel borides formed during the heat treatments might
have prevented the formation of the B-W bond.9,15 In
other words, the quantity of the boron in the coating was
high enough to prevent the formation of Ni-W com-
pounds in the structure.16
SEM micrographs of the Ni-P/Ni-B-W coatings be-
fore and after the heat treatments showed the formation
of typical spherical nodular structures (Figure 2). Each
nodule appears to be formed by the unification of many
grains existing as colonies (Figure 2b), similar to the
case suggested by S. Ruan et al.17 Some microcracks
were noticeable on the surfaces of the as-plated
Ni-P/Ni-B-W samples. The microcracks may be due to
the internal stress caused by the relatively larger tungsten
atoms in the coating structure.18,19
With the heat treatments, the structure progressively
crystallized and, as consequence, a substantial reduction
of the microcracks was noticed (Figure 2c and 2d). In a
Ni-B-W coating system, W atoms may selectively exist
at the boundaries of the nodules, from there, they may
segregates to the coating surface, during the heat treat-
ments.16,17 This process may decrease the internal stress
inside the coating, resulting in a substantial decrease of
the microcracks on the coating surface.
The thickness of the Ni-P coating in the dublex
coating system was measured as 2 μm, while the average
thickness of the Ni-B-W (the other coating in the
system) was measured as 11 μm (Figure 3). On the other
hand, it was obvious that the nodular structure seen in
the surface morphology of the Ni-B-W coating showed
columnar growth starting from the Ni-P layer to the
topmost surface of the coating (Figure 3a). Furthermore,
it was observed that adherence of the Ni-P coating to the
substrate was good and there was no pore formation
between the substrate and Ni-P coating. Pores seen in the
interface of Ni-P/Ni-B-W, in addition to pores partially
observed inside the Ni-B-W coating had a columnar
structure and they were considered to have been formed
due to the capturing of released hydrogen – even if in
small amounts – during the oxidation reaction of sodium
borohydride used as a the boron source.5 The pores
disappear with increasing heat-treatment temperatures.
This may be a result of the diffusion of hydrogen atoms
to the surface. They may promote better crystallization
of the coating and ease the formation of the Ni3B and
Ni2B phases (Figure 3b and 3c). After 650 °C heat
treatment the major phase of the coating was noticed to
be Ni3B. At that stage adherence between the coatings
reached its best level, while the Ni-B-W coating showed
columnar grains with a denser structure (Figure 3d).
As Figure 4, a difference of approximately 220 mV
was detected between corrosion potentials (Ecorr) of
as-plated and 650 °C heat-treated dublex coatings. This
is the shifting of Ecorr values of the coatings toward
nobler values with increased heat treatments applied to
the coatings. The anodic branch of the polarization curve
of the coating before heat treatment showed the break-
down potential at values around -310 mV (Figure 4a).
But the current density increased at a constant rate,
indicating that the unstable passive film formation on the
coating has disappeared rapidly. For heat treatment at
300 °C, the breakdown potential was observed to shift
toward -220 mV. At 300 °C, which is the onset tem-
perature of crystallization, corrosion potential shifted
towards a positive end approximately 40 mV, compared
to the as-plated sample. At 350 °C, crystallization was
nearly completed and the corrosion potential had shifted
towards more positive values. However, the breakdown
potential was observed (as with other samples) at around
-180 mV. Nonetheless, it was detected that the current
density of this sample became higher as the potential
value increased compared with that for the to as-plated
and heat-treated samples at 300 °C. For the heat treat-
ment at 650 °C, a passive film formation on the coating
surface was seen at around -190 mV, suggesting that
corrosion rate was restrained at such potential range.
However, as this protective film began to dissolve at
around -90 mV, a steep increase in the current density
was observed.
The major reason for the difference between the
corrosion resistance of as-plated and heat-treated
Ni-P/Ni-B-W coatings exposed to chloride ions was
undoubtedly associated with the transformation of the
microstructure from an amorphous to a crystalline mor-
phology. The main reason for having a higher Ecorr value
the heat-treated coating at 300 °C (where an amorphous
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
840 Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Polarization curves: a) steel substrate, b) Ni-B-W coating
heat treated at 650 °C, c) dublex Ni-P/Ni-B-W coating heat treated at
650 °C
structure still prevailed) compared to the as-plated
sample was the result of the borides formed during the
onset of crystallization. The heat-treated sample at
350 °C (where the microstructure had completely been
transformed to a crystalline structure) had a higher
corrosion resistance compared to the sample heat-treated
at 300 °C.20 On the other hand, it is a known fact that
corrosion resistance generally increases with addition of
tungsten to a electroless nickel-boron coating system,
because of the tendency to form an oxide film on the
surfaces.7,18 However, as seen in Figure 4, the corrosion
resistance of the as-plated Ni-P/Ni-B-W sample having a
nodular structure did not increase, with the addition of
tungsten. This is because the tungsten atoms were stuck
in the colony boundaries, not to be segregated to the
surfaces during the corrosion process and therefore,
could not be oxidized to promote the formation of a
mixed oxide film. However, depending on the increased
heat treatment temperatures segregation of W atoms
toward surfaces at 650 °C was more likely to occur. The
mixed oxide film a passive zone formed this way may be
the main reason for the substantial decrease of the
current density as observed here.
It can be seen that uniform corrosion occurred on the
surface of the as-plated sample in the amorphous
structure that has the lowest corrosion resistance (Fig-
ure 5a). Conversely, on the surface of the heat-treated
sample at 650 °C complete crystallization were noted
and the pits (indicated by arrows) in the grains within the
colonies were observed (Figure 5b).
To interpret the effect of the dublex coating on
corrosion resistance, the potentiodynamic polarization
measurements of the Ni-P/Ni-B-W and Ni-B-W
coatings, as well as which were deposited under the
same process conditions and heat-treated at 650 °C, and
carbon steel substrate were studied (Table 3).
As can be observed from Figure 6, while the Ni-B-W
coating and the dublex coating almost had the same
corrosion potential, more positive values at about 300
mV, compared with that of the substrate. Among the
coating studied the dublex coating has the lowest corro-
sion current density. The anodic branches of polarization
curves of both the dublex and the Ni-B-W coatings have
a passive zone, and both coatings had an Epit value of
approximately -90 mV. Additionally, the current density
of the Ni-B-W coating at a constant rate increases,
depending on the increased potential value. On the other
hand, the anodic branch of the dublex coating indicated
that an entrance to a second passive zone at approxi-
mately 20 mV. This passive zone may be related to the
existence of Ni-P in the dublex coating. A similar inter-
pretation was reported by Zhang et al. for the corrosion
of Ni-B-W surfaces.14 In general, corrosion starts at the
outermost surface of the substrate and proceed inward.
However, the stable passive film formed on the Ni-P
layer on the substrate may act as a barrier for corrosion
propagation, causing the dublex coating to have better
corrosion resistance than the single-layer Ni-B-W
coating.
5 CONCLUSIONS
The effect of heat treatments on the corrosion resis-
tance of a Ni-P/Ni-B-W dublex coating deposited on
carbon steel was studied. The Ni-P/Ni-B-W coatings
have an amorphous structure for a heat treatments up to
300 °C. The coatings start to crystallize at about 350 °C
and completed at about 650 °C with the major phases
being the nickel borides. The heat treatment, besides
causing nickel boride formation within Ni-B-W, also
caused obtaining a denser Ni-P/Ni-B-W coating when
increasing heat treatment temperature.
The corrosion resistance of the dublex coating in-
creases substantially with increasing the heat treatment
temperature. The dublex coating and the single-layer
Ni-B-W coating heat-treated at 650 °C indicated higher
corrosion resistance as compared to that of steel sub-
strate. When these two coatings were compared in terms
of corrosion resistance, the dublex coating showed better
performance.
6 REFERENCES
1 A. Brenner, G. Riddell, Nickel Plating on Steel by Chemical
Reduction, J. Res. Nat. Bur. Std., 37 (1946), 31–35
2 J. Sudagar, J. Lian, W. Sha, Electroless nickel, alloy, composite and
nano coatings-A critical review, J. Alloys Compd., 571(2013),
183–204, doi:10.1016/j.jallcom.2013.03.107
3 Y. F. Shen, W.Y. Xue, Z.Y. Liu, L. Zuo, Nanoscratching Deformation
and Fracture Toughness of Electroless Ni-P Coatings, Surf. Coat.
Technol., 205 (2010), 632–640, doi:10.1016/j.surfcoat.2010.07.066
4 K. M. Gorbunova, M.V. Ivanov, V. P. Moissev, Electroless Depo-
sition of Nickel-Boron Alloys – Mechanýsm of Process, Structure,
and Some Propertýes of Deposits, J. Electrochem. Soc., 120 (1973),
613–618, doi:10.1149/1.2403514
5 R. N. Duncan, T. L. Arney, Operatýon and Use of Sodium-Boro-
hydride-Reduced Electroless Nickel, Plat. Surf. Finish., 71 (1984)
12, 49–54
6 R. A. C. Santana, S. Prasad, A. R. N. Campos, F. O. Arau´ jo, G. P.
DA Silva, P. Lýma-Neto, Electrodeposition and Mechanical Pro-
perties of Ni–W–B Composites from Tartrate Bath, Prot Met Phys
Chem., 46 (2010) 1, 117–122, doi:10.1134/S207020511001017X
7 M. G. Hosseini, M. Abdolmaleki , H. Ebrahimzadeh, S. A. Seyed
Sadjadi, Effect of 2-Butyne-1, 4-Diol on the Nanostructure and
Corrosion Resistance Properties of Electrodeposited Ni-W-B
Coatings, Int. J. Electrochem. Sci., 6 (2011) 4, 1189–1205
8 T. Osaka, N. Takano, T. Kurokawa, T. Kaneko, K. Ueno, Characteri-
zation of chemically-deposited NiB and NiWB thin films as a
capping layer for ULSI application, Surf. Coat. Technol., 169–170
(2003), 124–127, doi:10.1016/S0257-8972(03)00186-5
9 G. Graef, K. Anderson, J. Groza, A. Palazoglu, Phase evolution in
electrodeposited Ni-W-B alloy, Mater. Sci. Eng., B41 (1996),
253–257, doi:10.1016/S0921-5107(96)01656-X
10 E. Lassner, W. D. Schubert, Tungsten: Properties, Chemistry, Tech-
nology of the Element, Alloys, and Chemical Compounds, Kluwer
Academic, New York, 1999
11 E. Beltowska-Lehman, Kinetics of Induced Electrodeposition of
Alloys Containing Mo from Citrate Solutions, Phys. Stat. Sol., 5
(2008) 11, 3514–3520, doi:10.1002/pssc.200779404
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842 841
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
12 A. I. Aydeniz, A. Göksenli, G. Dil, F. Muhaffel, C. Calli, B. Yüksel,
Electroless Ni-B-W Coatings for Improving Hardness, Wear And
Corrosion Resistance, MTAEC9, 47 (2013) 6, 803–806
13 T. S. N. Sankara Narayanan, K. Krishnaveni, S. K. Seshadri, Elec-
troless Ni–P/Ni–B Dublex Coatings: Preparation and Evaluation of
Microhardness, Wear and Corrosion Resistance, Mater. Chem. Phys.,
82 (2003), 771–779, doi:10.1016/S0254-0584(03)00390-0
14 W. X. Zhang, Z. H. Jiang, G. Y. Li, Q. Jiang, J. S. Lian, Electroless
NiP/NiB Dublex Coatings For Improving The Hardness and The
Corrosion Resistance of AZ91 D Magnesium Alloy, Appl. Surf. Sci.,
254 (2008), 4949–4955, doi:10.1016/j.apsusc.2008.01.144
15 A. B. Drovosekov, M.V. Ivanov, V. M. Krutskikh, E. N. Lubnin, Y.
M. Polukarov, Chemically Deposited Ni-W-B Coatings: Composi-
tion, Structure and Properties, Prot. Met., 41(2005) 1, 55–62,
doi:10.1007/s11124-005-0008-1
16 F. Z. Yang, Z. H. Ma, L. Huang, S. K. Xu, S. M. Zhou, Electro-
deposition and Properties of an Amorphous Ni-W-B Alloy before
and after Heat Treatment, Chin. J. Chem., 24 (2006), 114–118,
doi:10.1002/cjoc.200690004
17 S. Ruan, C. A. Schu, Mesoscale Structure and Segregation in
Electrodeposited Nanocrystalline Alloys, Scripta Mater., 59 (2008)
11, 1218–1221, doi:10.1016/j.scriptamat.2008.08.010
18 R. A.C. Santana, S. Prasad, A. R. N. Campos, F. O. Arau´ jo, G.P.
Sýlva, P. Lima-Neto, Electrodeposition and Corrosion Behaviour of a
Ni–W–B Amorphous Alloy, J. Appl. Electrochem., 36 (2006),
105–113, doi:10.1007/s10800-005-9046-2
19 R. A. Yildiz, A. Göksenli, B. Yüksel, F. Muhaffel, A. Aydeniz, Effect
of Annealing Temperature on The Corrosion Resistance of Elec-
troless Produced Ni-B-W Coatings, Adv Mat Res., 652 (2013),
263–268, doi:10.4028/www.scientific.net/AMR.651.263
20 Z. Z. Hamid, H. B. Hassan, A. M. Attyia, Influence of Deposition
Temperature and Heat Treatment on The Performance of Electroless
Ni–B Films, Surf. Coat. Technol., 205 (2010) 7, 2348–2354,
doi:10.1016/j.surfcoat.2010.09.025
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
842 Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
J. ZHE et al.: SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW STRESSES ...
843–847
SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW
STRESSES AND AN INSTANTANEOUS-STRESS-CHANGE
TESTING
KRATKOTRAJNO LEZENJE TOPLOTNO ODPORNEGA JEKLA P91
PRI NIZKIH NAPETOSTIH IN NENADNI MENJAVI NAPETOSTI
OBREMENJEVANJA
Jiang Zhe, Shen Junjie, Zhao Pengshuo
Tianjin University of Technology, Tianjin Key Laboratory for Advanced Mechatronic System Design and Intelligent Control, National
Demonstration Center for Experimental Mechanical and Electrical Engineering Education, 391 Binshui Road, Xiqing District, Tianjin, China
sjj1982428@sina.com, j211209977@live.com
Prejem rokopisa – received: 2016-1'-20; sprejem za objavo – accepted for publication: 2017-03-21
doi:10.17222/mit.2016.305
For the short-term creep behavior to be evaluated and the creep mechanism of P91 heat-resistant steels at low stresses and high
temperatures to be clarified, stress-change testing was conducted with a “helicoidal-spring creep test” demonstrating a high
strain resolution. The creep deformation consists of the primary creep stage, whereas no secondary creep stage was observed.
Blackburn’s law was suggested to be the best choice for a short-term-creep-behavior description because it provides a good
representation of an experimental creep curve. An anelastic backflow at a low stress was confirmed, following a high reduction
in the stress. The absolute value of the instantaneous strain for a load increase was equal to the value for a load decrease and the
creep of the P91 steels at low stresses might have been controlled by the viscous glide of dislocations.
Keywords: P91 heat-resistant steel, creep, anelastic, stress change
Da bi bilo mo~ oceniti kratkotrajno lezenje in njegov mehanizem toplotno obstojnega jekla P91 pri nizkih napetostih in visokih
temperaturah, so bili izvedeni preizkusi spremembe napetosti z uporabo t.i. testa spiralne vzmeti, ki omogo~a veliko lo~ljivost
deformacije. Deformacija lezenja sestoji iz primarne in sekundarne faze lezenja, vendar sekundarne faze niso analizirali oz.
opazovali. Blackburnov zakon je najbolj uporaben zakon za opis obna{anja jekla med kratkotrajnim lezenjem, ker se z njim
najbolj pribli`amo rezultatom, dobljenim z eksperimenti. Neelasti~ni povratni tok pri nizki napetosti je bil potrjen z ve~jim
zmanj{anjem napetosti. Zaradi absolutne vrednosti trenutne deformacije za dano obremenitev je bil narastek deformacije enak
zmanj{anju obremenitve, medtem ko je lezenje pri nizkih napetostih jekla P91 kontrolirano z viskoznim drsenjem dislokacij.
Klju~ne besede: P91 jeklo za delo v vro~em, lezenje, neelasti~nost, sprememba napetosti
1 INTRODUCTION
The P91 ferritic heat-resistant steel is utilized for the
material production of thermal-power-plant components,
which are exposed to elevated temperatures for extensive
periods. This requires that the P91 structural materials
resist creep deformation at both high temperatures and
low stresses. The creep strength obtained with the extra-
polation method based on the data of the creep deforma-
tion at a high stress is higher than the true creep strength
under these conditions (at low stresses).1,2 The creep-
strength degradation is not only based on the microstruc-
tural degradation3–6, but it is also related to the change in
the creep mechanism.7–9 The creep-mechanism clarifica-
tion has to depend on the instantaneous creep behavior
because the long-term creep is associated with the micro-
structural degradation. The stress-change test constitutes
an effective method for a creep-deformation-mechanism
clarification based on the instantaneous-strain and
creep-rate-variation evaluation at the stress changes
during the creep testing.10–12
At a high strain rate (a high stress) and a high tem-
perature, a sudden-stress-change experiment has been
widely utilized for pure metals and solution-strengthened
alloys.10–12 In contrast, at a low strain rate 0, especially
the ultra-low 0, lower than 10–10 s–1, it is usually impossi-
ble for a conventional tension-creep technique to dist-
inguish the instantaneous plastic strain from the total
strain under a sudden stress change, due to the unsatis-
factory strain resolution, existing between 10–6 and 10–5.
As an additional creep technique, the helicoidal-spring
creep test based on torsion deformation provides a
significantly high strain resolution, even up to 10–9.13
Due to this high strain resolution, it is possible for very
low instantaneous strains to be measured during a
sudden stress change.14
In the present study, the instantaneous creep of the
P91 ferritic heat-resistant steels was studied during
sudden-stress-change experiments using the helicoidal-
spring-specimen technique for the creep mechanism
under both a low stress and a high temperature.
Materiali in tehnologije / Materials and technology 51 (2017) 5, 843–847 843
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.9.011:620.172.24:691.714 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)843(2017)