JMEPEG (2016) 25:1773–1780 ÓASM International

DOI: 10.1007/s11665-016-2012-9 1059-9495/$19.00

Corrosion Behavior of Thermally Sprayed NiCrBSi Coating

on 16MnR Low-Alloy Steel in KOH Solution
Q. Zeng, J. Sun, W. Emori, and S.L. Jiang

(Submitted July 16, 2015; in revised form February 23, 2016; published online March 24, 2016)

NiCrBSi coatings were selected as protective material and air plasma-sprayed on 16MnR low-alloy steel
substrates. Corrosion behavior of 16MnR substrates and NiCrBSi coatings in KOH solution were evaluated
by polarization resistance (Rp), potentiodynamic polarization curves, electrochemical impedance spec-
troscopy, and immersion corrosion tests. Electrolytes were solutions with different KOH concentrations.
NiCrBSi coating showed superior corrosion resistance in KOH solution compared with the 16MnR. Cor-
rosion current density of 16MnR substrate was 1.7-13.0 times that of NiCrBSi coating in the given con-
centration of KOH solution. By contrast, Rp of NiCrBSi coating was 1.2-8.0 times that of the substrate,
indicating that the corrosion rate of NiCrBSi coating was much lower than that of 16MnR substrate.
Capacitance and total impedance value of NiCrBSi coating were much higher than those of 16MnR
substrate in the same condition. This result indicates that corrosion resistance of NiCrBSi coating was
better than that of 16MnR substrate, in accordance with polarization results. NiCrBSi coatings provided
good protection for 16MnR substrate in KOH solution. Corrosion products were mainly Ni/Fe/Cr oxides.

improve and enhance the quality and performance of boilers, heat

Keywords 16MnR, APS, corrosion, KOH, NiCrBSi coating
exchangers, turbines and plungers, etc. (Ref 10-13). Ni and Cr
could form Ni and Cr oxides, i.e., NiCr2O4, NiO, and Cr2O3 in
oxidative media during the remelting process (Ref 14), and
boride and carbide hard phases are formed as well, all of which
1. Introduction could improve the wear resistance and corrosion resistance, thus
reducing the influence of environment. Thus, NiCrBSi has been
chosen in coating experiments (Ref 4, 5, 15-18).
Nickel-based self-fluxing alloys (Ref 1, 2) are mainly used in As a self-developed typical special steel for pressure vessel
the chemical and petrol industries, as well as in hot working in China, 16MnR alloy steel was widely applied because of its
punches, among others, because of the excellent wear and good toughness, economic price, high strength, and good
corrosion resistances of these alloys in acid, alkali, salt, and other welding performance in digesters (240-265 °C), clarifiers
corrosive environments (Ref 3-5). However, the high cost of (90 °C), and precipitators (75 °C). However, it was fre-
nickel-base self-fluxing alloy limits its wide application. Surface quently exposed to hot Bayer solutions (containing about 232-
treatment, such as nickel-based alloy deposition on steel 387 g/L NaOH and 90-170 g/L Al2O3, together with several
substrate, is practical and economically feasible. Air plasma impurities and a room temperature pH above 15) during
spraying (APS) is one of the most widely used thermal spraying extraction of alumina from bauxite ores (Ref 19, 20). On the
techniques for surface treatment because of its low cost and high premise of pressure vessel material, 16MnR equipments would
quality of prepared coatings (Ref 6). APS is widely used in be corroded when working in KOH solution or other alkaline
manufacturing coatings, such as thermal barrier (Ref 7, 8), environment, which must be taken into consideration. Gener-
corrosion resistance (Ref 3), and abrasive resistance coatings ally, 16MnR low-alloy steel could be used in alkaline solution
(Ref 9). In particular, the Ni-Cr-B-Si colmonoy alloys are the with low concentration, but it would corrode when KOH
most commonly applied nickel-base self-fluxing alloys, from concentration increased (e.g., 50%). Thus, how to develop
which a series of alloys with different hardness can be obtained protective measures for 16MnR was imperative (Ref 21, 22).
via adjusting the content of each element. NiCrBSi coatings However, numerous studies on thermally sprayed NiCrBSi
exhibit superior oxidation resistance and abrasive wear resistance coatings have focused on the performances of abrasive
under high temperature. Therefore, they are widely employed to resistance (Ref 23, 24), oxidation resistance under high
temperature (Ref 25, 26), and fatigue resistance performance
Q. Zeng and J. Sun have contributed equally to this work. (Ref 27). Several studies have examined corrosion resistance in
acidic or NaCl media (Ref 10, 13, 28). Navas et al. (Ref 13)
Q. Zeng, Key Laboratory of Nuclear Materials and Safety Assessment,
Institute of Metal Research, CAS, Shenyang 110016, PeopleÕs evaluated the effect of localized laser melting of NiCrBSi
Republic of China; and School of Environmental and Chemical coatings, previously deposited by plasma spraying, on the
Engineering, Shenyang Ligong University, Shenyang 110159, People’s corrosion behavior in 0.03 M NaCl solution. According to the
Republic of China; J. Sun, School of Environmental and Chemical potentiodynamic polarization curves, as well as results from the
Engineering, Shenyang Ligong University, Shenyang 110159, PeopleÕs Rp and electrochemical impedance spectroscopy (EIS) tests,
Republic of China; and W. Emori and S.L. Jiang, Key Laboratory of localized laser melting of plasma-sprayed coatings had no
Nuclear Materials and Safety Assessment, Institute of Metal Research,
CAS, Shenyang 110016, PeopleÕs Republic of China. Contact e-mail:
effect on corrosion rate. Bergant et al. (Ref 28) found that the
sljiang@imr.ac.cn. icorr value of NiCrBSi coatings decreased after prolonged

Journal of Materials Engineering and Performance Volume 25(5) May 2016—1773

exposure to furnace heat, i.e., 20 min, and the Rp value was Specimens used in characterizing the microstructure were
almost 25 times higher than that of base-steel specimen. This polished using 2.5-lm diamond paste between grinding and
result indicates that NiCrBSi coating could provide protection rinsing. All tests were performed at room temperature (25 °C),
for base steel with much lower corrosion rate. Moreover, Zhao and the exposed area of the working electrode area was 1 cm2.
et al. (Ref 10, 29) investigated the corrosion behavior of A Gamry Interface 1000 electrochemical workstation with a
NiCrBSi coatings in 1.0 N H2SO4, 1.0 N HCl, 1.0 N NaOH, conventional three-electrode system was employed for all
3.5% NaCl, and NaCl solution with acetic acid. They found that electrochemical measurements. The sample was used as
corrosion first occurred on the surface of the coating around the working electrode, a platinum sheet with large area (2 cm 9
defects, such as pores, inclusions, and microcracks, and particles 2 cm to reduce current density on the surface of counter
that did not melt during spraying, thus accelerating corrosion
rate. In addition, coatings in sour solutions had higher icorr Table 2 Selected processing parameters for plasma
values than those in 3.5% NaCl. Corrosion caused by Cl was spraying
more severe than that caused by SO42 and Ac. Moreover, the
NiCrBSi coating showed superior resistance in alkali solution, Parameter Value
because the surface could attain a self-passivation condition and
thereby protected the steel substrate against corrosion. Primary gas flow rate, L/min Ar: 45
In this paper, containers made of 16MnR low-alloy steel H2: 6
Carrier gas flow rate, L/min Ar: 3
were used to produce potassium permanganate by liquid-phase
Spray distance, mm 120
method. This study attempts to select a material to protect or Spraying power, kW 32
prevent it from being corroded. NiCrBSi coating was chosen as Powder flow rate, g/min 50
protective material because of their excellent performance. The
coating was air plasma sprayed on 16MnR substrates. Corro-
sion behavior of NiCrBSi coating and 16MnR substrate dipped
in solutions with different KOH concentrations were studied -0.3
16MnR
and analyzed. -0.4 -0.25

a
-0.30
-0.5

-0.6 -0.35 b
2. Experimental Procedure
E/V vs SCE

-0.40 c
-0.7
2.1 Materials and Air Plasma Spraying Processes -0.8
a: 1% -0.45

b: 5%
-0.50 d
The substrate material used in the experiment was 16MnR -0.9 c: 10%
low-alloy steel that was cut to yield 10 mm 9 10 mm 9 10 mm d: 20% -0.55
0.0
-1.0 e: 50% -1.2x10-6 -8.0x10-7 -4.0x10-7 4.0x10-7
specimens. NiCrBSi alloy powder (C: 7.51 wt.%, Si:
e
4.56 wt.%, Fe: 6.50 wt.%, Cr: 5.42 wt.%, B: 3.15 wt.% Ni: -1.1
-5 -5 -6 -6 -5
Bal.) was used as spraying material. Prior to spraying, the -1.5x10 -1.0x10 -5.0x10 0.0 5.0x10 1.0x10
surface of 16MnR low-alloy steel was carefully degreased in (a) i /A cm
-2

acetone and alcohol solution. Then, the steel was sandblasted -0.3
using corundum powder to produce a rough surface. This a
NiCrBSi b
operation allowed the mechanical bonding between the coating c
-0.4 d
and the substrate. Technical parameters of sand blasting are
shown in Table 1. Then, the samples were placed on a rotary
holder, and the floating ash on the sandblasted surface was -0.5
blown off before the samples were swept by the plasma gun.
E/V vs SCE

The samples were preheated uniformly up to 80-120 °C in a -0.6 a: 1%

drying oven or using a spray gun before spraying. Coating b: 5%
c: 10%
feedstock material was injected vertically into the plasma jet by -0.7
d: 20%
Ar carrier gas. Relevant APS processing parameters are listed in e: 50%
Table 2. The coating was sprayed to a thickness of approxi- e
-0.8
mately 1 mm.
-7 -7 -7 -7 -7
-6.0x10 -4.0x10 -2.0x10 0.0 2.0x10 4.0x10
(b) i /A cm
-2
2.2 Corrosion Tests
Surfaces of all samples in the corrosion tests were ground Fig. 1 Linear polarization resistance curves of 16MnR low-alloy
with 240 grit through 800 grit SiC paper in water suspension. steel and NiCrBSi coating in solutions with different KOH concen-
The samples were rinsed in water and alcohol and then dried. trations: (a) 1%, (b) 5%, (c) 10%, (d) 20%, and (e) 50%

Table 1 Technical parameters of sand blasting

Sand blasting machine Abrasive Abrasive grain Wind pressure, MPa Spray distance, mm

PHYM/suction type White corundum powder 20-80 mesh 0.20-0.60 80-150

1774—Volume 25(5) May 2016 Journal of Materials Engineering and Performance

electrode and avoid polarization) as counter electrode, and a polarization curves were performed with a scan rate of 0.5 mV/
saturated calomel electrode (SCE) as reference electrode. All s, starting at 0.5 V (versus Eoc) ending at 1.2 V (versus Eoc).
potentials described in this work were relative to the SCE Immersion tests were performed under 25 °C in KOH
unless stated otherwise. The plasma-sprayed and substrate solutions (1-50%) for 240 h. Prior to immersion tests, the
surfaces were tested. Electrolytes were 1, 5, 10, 20, and 50% surface of the specimens were polished mechanically with SiC
KOH solution (wt.%). Linear polarization resistance (LPR) paper up to 2000 grit in water suspension, rinsed with alcohol,
measurements were first performed after an open circuit and dried in air. Specimens were rinsed carefully and dried after
potential (Eoc) stabilization period of 1 h. Scan range was immersion tests. An XL-30FEG scanning electron microscope
±0.02 V relative to the Eoc, and a scan rate of 0.166 mV/s was was applied to analyze the morphology of the samples after
used. EIS test was performed with frequency of 100 kHz- immersion for 240 h, and chemical constituent analyses were
0.01 Hz and amplitude of 10 mV at Eoc. Potentiodynamic conducted with the built-in energy-dispersive spectrometer.

0.8 16MnR 3. Results and Discussion

0.4
3.1 LPR and Potentiodynamic Polarization Curves
0.0 The LPR and potentiodynamic polarization curves of
E /V vs SCE

a 16MnR low-alloy steel and NiCrBSi coating in solutions with

-0.4 b a: 1% different KOH concentrations are shown in Fig. 1 and 2,
c
d b: 5% respectively. Rp can be obtained from the slope of LPR curves.
-0.8 c: 10% Parts of the polarization curves closer to the Ecorr showing
d: 20% linear regions on cathodic and anodic portions of the polariza-
-1.2 e: 50% tion curves were the Tafel area, from which Ecorr and icorr can
e be obtained. Electrochemical parameters, such as Ecorr, icorr, and
-1.6 Rp, are summarized in Table 3. Comparative results from icorr
and Rp obtained from solutions with different KOH concentra-
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
(a) lg(i /A cm )
-2 tions are shown in Fig. 3(a) and (b).
The potentiodynamic polarization curves of NiCrBSi coating
and 16MnR low-alloy steel had similar shapes in the given range
0.8 NiCrBSi
250 14
0.4
12
200
Rp: 16MnR
E /V vs SCE

0.0 10
Rp: NiCrBSi
150
icorr: 16MnR 8

icorr /
2

a
Rp /k cm

-0.4 a: 1% icorr: NiCrBSi

b b: 5% 100 6
c d c: 10%
-0.8 4

cm
d: 20% 50
e
e: 50% 2
-1.2
0 0
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1
-2
(b) lg(i /A cm ) 0 10 20 30 40 50
CKOH /%
Fig. 2 Potentiodynamic polarization curves of 16MnR low-alloy
steel and NiCrBSi coating in solutions with different KOH concen-
trations: (a) 1%, (b) 5%, (c) 10%, (d) 20%, and (e) 50% Fig. 3 Relationship of icorr, Rp and concentration of KOH solution

Table 3 Parameters from the curves of linear polarization resistance and potentiodynamic polarization
16MnR NiCrBSi

CKOH (%) Ecorr, V (vs. SCE) icorr, lA/cm2 Rp, kX cm2 Ecorr, V (vs. SCE) icorr, lA/cm2 Rp, kX cm2

1 0.379 0.688 199.8 0.472 0.404 235.0

5 0.424 1.270 112.0 0.504 0.482 142.0
10 0.476 3.130 85.07 0.608 0.536 103.9
20 0.611 9.930 43.06 0.689 0.584 110.2
50 1.280 12.90 8.230 0.894 1.10 63.72

Journal of Materials Engineering and Performance Volume 25(5) May 2016—1775

 60000 b Fig. 4 Electrochemical impedance spectra of 16MnR low-alloy
16MnR a steel and NiCrBSi coating in solutions with different KOH concen-
50000 trations: (a) 1%, (b) 5%, (c) 10%, (d) 20%, and (e) 50%. (a) and (c)
Nyquist plots, (b) and (d) Bode plots of 16MnR low-alloy steel and
40000 b NiCrBSi coating
2
Zim/ cm

30000
c in the range of 1-20%. The icorr values further increased to
20000
a 1% 1.1 lA/cm2 in 50% KOH, which was approximately twice that at
d b 5% lower concentrations. The Rp values showed decreasing trend
10000 c 10% with increasing KOH concentration, such that Rp decreased
d 20%
e sharply before 10% and slowly afterward. In addition, Ecorr
0 e 50%
values decreased from 0.472 V to 0.894 V with increasing
0 20000 40000 60000 80000 KOH concentration in the test range. These results indicate that
2
(a) Zre/ cm the corrosion rate increased with KOH concentration. All
100 polarization curves exhibited shapes similar to that under the
5 16MnR NiCrBSi system (Fig. 2b). Active dissolution area was from
80 point of Ecorr to the first inflection point on the anodic region. The
4
60
icorr value increased rapidly as applied polarization potential in
this region increased, but became almost stable after further
3 40 increase in the potential, i.e., the surface of the coating was in
cm )
2

some self-passivating state (Ref 10).

2 20
Largest corrosion rate occurred in 50% KOH, in which the
lg(|Z|/

a 1% 0 Ecorr value shifted by 200 mV than that of 20% KOH

1 b 5%
c 10% solution. The icorr value was approximately twice as much as
-20
0 d 20% that at 1-20% KOH, and Rp value decreased to 63.72 kX cm2.
e 50% -40 Comparative results of Fig. 1, 2, 3 and Table 3 show that the
-1 icorr values of both NiCrBSi and 16MnR increased rapidly, and
-2 -1 0 1 2 3 4 5
(b) lg(f /Hz) relevant Rp values reduced obviously with increasing KOH
concentration. However, icorr value of NiCrBSi coating was
100000 much lower than that of 16MnR in all KOH solutions, whereas
NiCrBSi Rp value was much higher than that of 16MnR. These results
a
80000 imply that the corrosion resistance of NiCrBSi coating was
superior to that of 16MnR, because the existing Ni, Cr, B, and
Si improved corrosion resistance. Ni shows excellent chemical
60000
stability and is the best anti-corrosion material in concentrated
2
Zim / cm

b
alkaline solutions (Ref 30, 31). Alloying elements Cr, B, and Si
40000
could greatly improve oxidation and corrosion resistances of
c a 1%
b 5% the coatings (Ref 32, 33). NiCrBSi coating exhibited great
20000 d
c 10% stability in 5-20% KOH solution as the concentration increased,
e d 20% whereas corrosion resistance of 16MnR in 50% KOH solution
e 50%
0 was remarkably different from that at lower concentrations.
0 10000 20000 30000 40000 50000 60000 These characteristics caused considerable threat to the security
(c) Zre / cm
2
of industrial manufacture. In this condition, the icorr value of
16MnR reached up to 12.9 lA/cm2, which was approximately
100
NiCrBSi 13 times higher than that of NiCrBSi coatings. The Rp value of
5
16MnR was only 8.23 kX cm2, whereas that of the Rp value of
80
4
NiCrBSi coatings was approximately eight times as much as
60 that of the former. These findings fully illustrate the superiority
3 of NiCrBSi coatings in high concentrations of KOH solution,
lg(|Z| / cm )
2

40 which would ultimately provide good protection for 16MnR in

2 KOH solution.
/

20
a 1%
1
b 5% 3.2 Electrochemical Impedance Spectroscopic
0
c 10% Measurements
0 d 20%
e 50% -20 Nyquist plots, with corresponding Bode plots, of 16MnR
-1 low-alloy steel and NiCrBSi coating in solutions with different
-2 -1 0 1 2 3 4 5
(d) lg(f / Hz) KOH concentrations are shown in Fig. 4. Nyquist plots of both
materials show semicircles. This result indicates that both
materials possessed capacitance characteristics, i.e., an oxida-
of KOH concentration (1-50%, mass fraction; Fig. 3). Moreover, tion film appeared on the corrosion surface. The semicircle at
Fig. 3 and Table 3 show that icorr values of NiCrBSi coating high frequencies is attributed to the faradaic charge-transfer
increased slowly at around 0.5 lA/cm2 with KOH concentration process, and the size of radius reflects the size of the charge-

1776—Volume 25(5) May 2016 Journal of Materials Engineering and Performance

transfer resistance (Rct) (Ref 34). Nyquist plots, with corre- This evolution suggests that a homogeneous and compact film
sponding Bode plots, of 16MnR low-alloy steel and NiCrBSi was present on the surface of NiCrBSi coating, strongly
coating in solutions with different KOH concentrations are hindering corrosion processes.
shown in Fig. 4. Nyquist plots of both materials show Impedance results can be interpreted by equivalent circuits
semicircles. This result indicates that both materials possessed shown in Fig. 5 with well-fitting results. In the circuits above,
capacitance characteristics, i.e., an oxidation film appeared on Rs represents the solution resistance, Rct represents charge-
the corrosion surface. The semicircle at high frequencies is transfer resistance of the electric double layer, and Rf is the film
attributed to the faradaic charge-transfer process, and the size of resistance. Thus, uniformity of electrode surface was destroyed
radius reflects the size of the charge-transfer resistance (Rct) as shown by roughness, insufficient polishing, grain bound-
(Ref 35). These results are consistent with the potentiodynamic aries, impurities of solid electrode surface, and corrosion
polarization data. products accumulating on the surface. This characteristic
Figure 4(d) indicates that the overall impedance of NiCrBSi caused dispersion effect, and maldistribution of electric field
coating in KOH solutions of different concentrations were all on the surface of electrode. Thus, constant phase elements
approximately 100 kX cm2 with no remarkable difference. The (CPEs) were included to replace the capacitors in the equivalent
system presented characteristics highly similar to those of ideal circuits to get better fitted results. CPEdl and CPEf were used to
capacitance, whereas the phase angle was much closer to 90°. replace the electric double-layer and film capacitances, respec-
The h-lgf curves of NiCrBSi coating in 1-20%KOH solutions tively (Ref 39, 40). Impedance of CPE is frequency-dependent,
show two peaks, which indicate two time constants. The one at which can be mathematically expressed using Eq 1, as follows
low frequencies is related to the charge-transfer reactions in the (Ref 34, 41):
pores and defects of the protective film. This characteristic
1
corresponds to the redox process on the electrode surface, ZCPE ¼ ; ðEq 1Þ
which can be characterized by Rct. The peak at high frequencies Y0 ðjwÞn
is associated with the behavior of the protective film formed on where Y0 is the CPE constant, CPEdl is composed of Cdl, and
the surface of NiCrBSi coating (Ref 13, 36, 37). Figure 4(d) dispersive exponent n1. CPEf is composed of Cf and n1.
demonstrates that the phase angle in the low-frequency region Additionally, w is the angular frequency (rad/s), and j2 = 1.
decreases with increasing KOH concentration, revealing that The dispersive exponent n reflects roughness and nonunifor-
the system changes from capacitive to resistive behavior. This mity, with 1 < n < 1. A porous or rough surface can cause
phenomenon demonstrates that the uniformity and protective an electric double-layer capacitance to appear as a CPE with
properties of the coating are wearing off (Ref 38). The two time n varying between 0.5 and 1 (Ref 39, 42, 43).
constants at 50% KOH solution overlapped, suggesting the Equivalent circuit with two time constants is frequently used
existence of two mutually interfering processes with identical to successfully fit EIS curves and analyze the electrochemical
relaxation times. The system shows a preferable capacitive behavior of carbon steels in alkaline environment (Ref 35, 36,
behavior as confirmed by the increase in phase angle in the 44-47). The EIS fitting results for 16MnR and NiCrBSi coating
intermediate-frequency area, which reached values around 90°. in different concentrations of KOH solution are shown in

(1) (2) CPEdl CPEf

Rs CPEdl
Rs
CPEf

Rct
Rf Rct Rf

Fig. 5 Equivalent circuits used for fitting the electrochemical impedance spectra: (1) 16MnR and (2) NiCrBSi

Table 4 EIS fitting results for 16MnR and NiCiBSi coating in solutions with different KOH concentrations
CPEdl CPEf
2 2
Materials/model CKOH (%) Rs, X cm Y0CPEdl ; lF=cm 2
n1 Rct, X cm Y0CPEf ; lF=cm2 n2 Rf, kX cm2

16MnR/R(Q(R(QR))) 1 3.236 24.41 0.9908 52.52 39.58 0.7204 150.5

5 0.733 26.54 1 41.51 77.11 0.7466 109.5
10 0.4136 32.07 1 35.04 94.93 0.7478 63.82
20 0.3228 41.25 1 37.16 103 0.6934 40.02
50 0.3034 69.66 1 13.2 649.8 0.6361 115
NiCrBSi/R(QR)(QR) 1 4.801 332.2 0.6944 37 99.76 0.9019 242.8
5 0.8439 376.4 0.7342 12.35 154.4 0.8984 140.2
10 0.536 481.9 0.7571 6.778 201 0.9004 70.67
20 0.4001 287 0.8424 3.88 256.3 0.87 36.28
50 0.3871 136.2 1 0.9355 137.6 0.8992 37.18

Journal of Materials Engineering and Performance Volume 25(5) May 2016—1777

Fig. 6 Scanning electron micrographs of NiCrBSi before and after immersion for 240 h in solutions with different KOH concentrations (a) be-
fore immersion, (b)-(f) immersed for 240 h in (b) 1%, (c) 5%, (d) 10%, (e) 20%, and (f) 50% KOH solution

Table 4. The Rct and Rf values decreased with increasing KOH results also indicate that the corrosive ions were faster and
concentration, suggesting the declining corrosion resistance. easily reached the surface of substrate through the electric
Moreover, the values of CPEdl and CPEf increased, implying a double layer, and that the active area of the coupled dissolution
reduction in the thickness of the electric double layer. These reaction increased. Consequently, corrosion of working elec-

1778—Volume 25(5) May 2016 Journal of Materials Engineering and Performance

Table 5 Elemental composition of the surface of NiCrBSi the film formed quickly thus had more defects, which can be
coating after immersion in 50% KOH less protective. Although more iron oxide was produced in the
reaction under high KOH concentration, the corrosion rate
Elements wt.% at.% increased (Table 3) due to defects in the corrosion product film.
For NiCrBSi coating, the main cations are Ni2+, Cr3+, and
BK 4.51 9.69 2+
Fe , which are converted to the corresponding oxides
CK 5.44 10.51
OK 39.68 57.60
thereafter. In the NiCrBSi coating, the contents of Fe and Cr
Si K 0.60 0.49 are 6.50 and 5.42 wt.%, respectively. But in the film, the
Cr K 39.15 17.49 contents of Cr and Fe are 39.15 and 1.12 wt.%, respectively
Fe K 1.12 0.46 (Table 5), which is out of proportion to that in the coating. This
Ni K 9.51 3.76 means protective Cr oxide is the main composition of the film
which can be considered as a passive film. In fact, the
chromium always plays important roles on composition and
corrosion resistance of material (Ref 51). That is why thermally
trode was accelerated (Ref 35). Rct is closely related to anodic sprayed NiCrBSi coating exhibited lower corrosion rate than
dissolution, reflecting the change in the dissolution rate of that of 16MnR low-alloy steel.
metal. The larger the Rct is, the higher the corrosion resistance
will be. The decrease in Rf indicates that compactness and
uniformity of the protective film on the electrode surface
became increasingly worse. The reduced Rct was caused by the 4. Conclusions
accelerating rate of the charge-transfer reactions in the protec-
tive film as the concentration of KOH solution increased. The The Ecorr values of 16MnR low-alloy steel and plasma
elevated CPEf values may be related to the displacement of spraying NiCrBSi coating became more and more negative as
corrosion products (oxides and/or hydroxides), whereas in- KOH concentration increased. Meanwhile, icorr values in-
creased CPEdl can be attributed to an enhancement in the creased, and Rp values decreased, resulting in accelerated
dielectric constant or a reduction in the electric double layer corrosion rate.
(Ref 47, 48). The icorr and Rp values of NiCrBSi coating were remarkably
lower and higher than those of 16MnR, respectively, at the
3.3 Microstructural Analysis of the Coating Before and After same condition. EIS results show that the capacitance and the
Immersion Tests total impedance values of NiCrBSi coating were much higher
than those of 16MnR at the same condition. Thus, the corrosion
The microstructure of NiCrBSi coating before immersion is resistance of NiCrBSi coating was approximately 10 times
shown in Fig. 6(a), whereas the corresponding microstructures superior to that of 16MnR. NiCrBSi coating could provide a
of NiCrBSi coating after immersion for 240 h in different good protection for 16MnR in KOH solution.
concentrations of KOH solution are shown in Fig. 6(b)-(f).
Corrosion of NiCrBSi coating surface worsened with increas-
ing KOH concentration. Moreover, the corrosion pores and area
grew increasingly larger, suggesting the reduction in the References
corrosion resistance of NiCrBSi coating. These results are 1. F. Otsubo, H. Era, and K. Kishitake, Structure and Phases in Nickel-
consistent with the electrochemical data obtained. EDS was Base Self-Fluxing Alloy Coating Containing High Chromium and
applied to analyze the corrosion products on the surface of Boron, J. Therm. Spray Technol., 2000, 9, p 107–113
NiCrBSi coating immersed in 50% KOH for 240 h (Table 5). 2. S. Shrestha, T. Hodgkiess, A. Neville et al., The Corrosion Behaviour
EDS results showed that Cr oxide and Ni oxide are the main of High Velocity Oxy-Fuel (HVOF) Sprayed Ni-Cr-Si-B Coatings,
Proceedings of the International Thermal Spray Conference, ITSC,
compositions in the film, which is beneficial to corrosion Dusseldorf, Germany, 2002, p 763–785
resistance. 3. N. Serres, F. Hlawka, S. Costil et al., Corrosion Properties of In Situ
Laser Remelted NiCrBSi Coatings Comparison with Hard Chromium
3.4 Corrosion Mechanism Coatings, J. Mater. Process. Technol., 2011, 211, p 133–140
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1780—Volume 25(5) May 2016 Journal of Materials Engineering and Performance