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Journal-Name: Corrosion Reviews
Article-DOI: https://doi.org/10.1515/corrrev-2022-0071
Article-Title: Effect of environmental variables and main alloying elements on the repassivation potential of Ni–Cr–
Mo–(W) alloys 59 and 686
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Corros Rev 2023; ▪▪▪(▪▪▪): 1–12
Original Article
Q1 Q2
Edgar C. Hornus* and Martín A. Rodríguez
Effect of environmental variables and main
alloying elements on the repassivation potential
of Ni–Cr–Mo–(W) alloys 59 and 686
https://doi.org/10.1515/corrrev-2022-0071
Received August 2, 2022; accepted November 25, 2022;
published online ▪▪▪
Abstract: Chloride-induced crevice corrosion (ER,CREV) of alloys UNS N06059 and UNS N06686 was studied at different
temperatures in 0.1, 1 and 10 M chloride solutions. Crevice
corrosion occurred several degrees below the reported critical crevice temperatures obtained through standard immersion tests. The repassivation potential of the tested alloys
as a function of temperature and chloride concentration was
given by ER,CREV = (A + BT)log[Cl−] + CT + D for a range of
environmental conditions. When temperature and chloride
concentration increased ER,CREV showed a lesser dependence
on the environmental variables. The repassivation potential
of Ni–Cr–Mo–(W) alloys was described by a new proposed
equation in terms of [Cl−], T, Cr, Mo and W, alloys in wt%. The
dependence of ER,CREV with the weight % of main alloying
elements was 5–6 mV/%Cr, 17–18 mV/%Mo and ∼9 mV/%W, at
85 °C in chloride solutions. An optimal main alloying elements
relationship was noted that maximizes the ER,CREV value. The
optimal alloy ratio would be 1:3.3:1.65 for wt%Cr, wt%Mo and
wt%W, the same factors as in the PRE equation. The optimal
alloying ratio would be independent of the alloy composition
since it is not a function of the content of main elements.
Keywords: chloride; crevice corrosion; nickel alloys;
temperature.
1 Introduction
Nickel-based alloys are widely used in the chemical process,
oil and gas, nuclear industries because of their resistance to
*Corresponding author: Edgar C. Hornus, Gerencia Materiales, Comisión
Nacional de Energía Atómica, Instituto Sabato, UNSAM/CNEA, San Martín,
B1650KNA Buenos Aires, Argentina, E-mail: hornus@cnea.gov.ar
Martín A. Rodríguez, Gerencia Materiales, Comisión Nacional de Energía
Atómica, Instituto Sabato, UNSAM/CNEA, San Martín, B1650KNA Buenos
Aires, Argentina; and Consejo Nacional de Investigaciones Científicas y
Técnicas, Buenos Aires, Argentina
highly aggressive environmental conditions. Because of
their FCC structure, nickel-based alloys have excellent
ductility, malleability and formability. These alloys are
selected due to their outstanding resistance to localized
corrosion and stress corrosion cracking in hot chloride solutions (Rebak 2000, 2005). Nickel-based alloys are classified
into two groups: those designed for low-temperature or
aqueous solution applications and those designed to resist
high-temperatures or dry gaseous corrosion. The alloys
resistant to high temperature are known as heat-resistant
alloys (HRA), and those designed for the low-temperatures
are known as corrosion resistant alloys (CRA).
From a chemical point of view, the CRAs may be grouped
as (1) commercial Ni pure alloys, (2) Ni–Cu alloys, (3) Ni–Mo
alloys, (4) Ni–Cr–Mo alloys, and (5) Ni–Cr–Fe alloys (Rebak
2000). Ni–Cr–Mo alloys are the most versatile nickel alloys
since they contain molybdenum, which imparts resistance
against corrosion in reducing acidic media and chromium,
which provides protection under oxidizing conditions
(Agarwal and Kloewer 2001; Rebak and Crook 2000). Chromium addition helps to form a protective chromium oxide
film (Cr2O3) on the material surface and protects against
further environmental degradation. The major applications
of Ni–Cr–Mo alloys are in hot-chloride-containing solutions,
where most stainless steels suffer pitting corrosion, crevice
corrosion and stress corrosion cracking. Thus, the Ni–Cr–Mo
alloys are highly resistant, if not immune, to hot chlorideinduced attacks in most industrial applications (Rebak 2000,
2008). Pitting Resistant Equivalent (PRE) is a parameter used
to characterize the localized corrosion resistance of nickelbased alloys (Szklarska-Smialowska 1986). Equation (1) defines the value of PRE concerning the weight percentages of
key alloying agents Cr, Mo and W.
PRE = %Cr + 3.3(%Mo + 0.5 % W)
(1)
Several researchers state that Cr is the main element
that protects against the initiation of the localized corrosion by its resistance to the passivity breakdown, whereas
Mo and W act on the repassivation of the localized corrosion once it has been generated (Kehler et al. 2001;
CORRREV-2022-0071_proof ■ 17 January 2023 ■ 10:55 pm
UNCORRECTED PROOF
2
E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
Maristany et al. 2016). Gruss et al. (1998) applied the Cyclic
Potentiodynamic Polarization (CPP) technique to contrast
the repassivation potential of alloys 825 (UNS N08825), 625
(UNS N06625) and C-22 (UNS N06022). They reported an
increase in repassivation potential with the PRE value.
Since alloys 625 and C-22 have similar Cr content, the superior corrosion resistance of alloy C-22 was due to its
higher Mo content. Zadorozne et al. (2012) observed that Cr
is necessary on the alloy to form a passive protective film,
but when the passive film is destroyed, the presence of Mo
in the alloy is the crucial element that controls a quick
repassivation.
Alloys 59 (UNS N06059) and 686 (UNS N06686) were
designed to outperform alloy C-22 under reducing conditions without sacrificing its properties under oxidizing
conditions (Heubner et al. 1989). Mishra and Frankel (2008)
contrast the initiation of crevice corrosion and the repassivation of alloys C-276 (UNS N10276) and 686, which
contain 16 wt% Cr and 21 wt% Cr, respectively, and both
containing 16 wt% Mo and 4 wt% W. They concluded that
whereas Cr affected the critical crevice corrosion potential
for crevice corrosion initiation, Cr content had little effect
on the repassivation potential. The effect of W on localized
corrosion resistance has not been studied to the same
magnitude as other alloying elements such as Cr and Mo.
Some researchers agree that there is an optimal concentration range of W, and that outside this range, W would
become ineffective or harmful (Sedriks 1986; Kim and
Kwon 1999). Szklarska-Smialowska (1986) mentions a
particular W-to-Mo ratio where a synergistic effect leads to
maximum corrosion performance.
Above the Critical Crevice Temperature (CCT), the
crevice corrosion resistance of an alloy in a given environmental condition is usually measured by its Repassivation Potential (ER,CREV) (Carranza 2008; Rebak 2005).
Occurrence of localized corrosion, such as pitting or
crevice corrosion, depends on the medium and metallurgical parameters combined effects. At elevated temperature/acidic medium, presence halogens and salt formation
deposits lead to local depassivation in the form of cracks or
pinholes (Sosa Haudet et al. 2015). The higher the crevice
corrosion resistance of an alloy, the higher its ER,CREV.
Among the many methods used to obtain ER,CREV, the
Potentiodynamic-Galvanostatic-Potentiodynamic (PD-GSPD) test with polytetrafluoroethylene (PTFE)-wrapped
ceramic formers has shown the most conservative results
(Giordano et al. 2011).
This study aimed to investigate the effects of the temperature, chloride solutions and alloying elements on the
repassivation potential of the nickel-based alloys N06059
and N06686.
2 Materials and methods
The alloy specimens were obtained from wrought mill annealed (MA)
plate stock. Their chemical composition are listed in Table 1, including
their PRE value. The specimens were prepared following ASTM G192
(2008) standard, with creviced spots formed by two ceramic washers (24
artificially creviced sites) wrapped with a 70 µm-thick PTFE tape. The
tested surface was approximately 14 cm2. The applied torque to the
artificial crevice formers was 5 N m. Specimens were polished down to
600 grit SiC paper, rinsed with ultrapure water, degreased with acetone
and ethanol and forced dried.
The PD-GS-PD method was used to determine the crevice corrosion
repassivation potentials. The PD-GS-PD method is a modification of the
Tsujikawa-Hisamatsu Electrochemical (THE) method (Giordano et al.
2011; Mishra and Frankel 2008; Rincón Ortíz et al. 2010), and it consists of
three main stages (Figure 1):
– A potentiodynamic polarization (at a scan rate of 0.167 mV/s) in the
anodic direction up to an anodic current of 20 μA/cm2.
– A galvanostatic step with an application of 20 μA/cm2 for 2 h.
– A potentiodynamic polarization (at a scan rate of 0.167 mV/s) in the
cathodic direction, from the end potential of the previous setup to
reaching alloy repassivation (ER,CREV or cross-over potential).
Previous to the test, the Open-Circuit Potential (OCP) was measured for
600 s and afterwards, a galvanostatic pre-treatment of 5 μA during 300 s
was applied. The post-test analysis included Light Optical Microscopy
(LOM) and Scanning Electron Microscopy (SEM).
All the electrochemical experiments were led in a 3-electrode glass
vessel with a 1 L capacity. The testing solutions were 0.1 M NaCl, 1 M NaCl
and 5 M CaCl2, and the applied temperatures varied from 50 to 117 °C.
Evaporation of the solution was avoided by a water-cooled condenser.
The entire cell was immersed in a constant temperature water bath to
control the test temperature. Pure nitrogen (N2) gas was purged into the
solution 1 h before and throughout the electrochemical test. A water trap
prevented oxygen re-ingress from the air. The reference electrode
employed was a Saturated Calomel Electrode (SCE), which has a potential of +0.242 V with respect to the Standard Hydrogen Electrode
(SHE) at room temperature. A high area platinum foil and/or a graphite
bar were used as counter-electrodes.
Potentiodynamic polarization tests were performed using prismatic specimens in similar deaerated neutral chloride solutions as
the PD-GS-PD tests. The parallelepiped specimens were mounted with
a PTFE compression gasket (ASTM G5 2002). A minimum torque was
applied to the gasket to ensure a leak-proof assembly. The exposed
area of the specimen was 8 cm2. Polarization test were performed at
constant temperatures from 30 to 117 °C. The potential scan was
started 50 mV below the OCP in the anodic direction at a scan rate of
0.167 mV/s. The test finished when the anodic current density reached
a value of 1 mA/cm2.
3 Results and discussion
3.1 Polarization curves
Polarization curves for alloys N06059 and N06686 recorded
in NaCl and CaCl2 solutions at different temperatures are
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UNCORRECTED PROOF
E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
3
Table : Summary of electrochemical behaviors observed under different studied conditions, concerning Figure .
Alloy
Ni
Cr
Mo
W
Fe
Co
Si
Mn
C
PRE
N
N

a

a

a
–
.a
.
b
.
–
.
.b
.
.b
.
.b


a
a
a
b
b
b
b
b
a
Average value; bmaximum value.
Figure 1: Steps of the PD-GS-PD technique used for determining crevice
corrosion repassivation potentials.
shown in Figure 2. These tests were carried out to determine
the anodic behavior of the alloys, avoiding crevice corrosion
phenomena, although, in some instances, it could not be
avoided (Rodríguez et al. 2010). The curves showed an
extensive passive zone with low passive currents approximately independent of temperature. The potential at which
the current increased after the passive domain (breakdown
potential) depended on the temperature and the solution.
The breakdown potential was associated with the onset of
transpassive film breakdown in most cases, and with crevice
corrosion initiation in other cases. For both alloys, a pseudosecondary passive region (current density plateau before the
transpassive domain) was observed between ∼0.4 VSCE and
0.6 VSCE. This was attributed to surface enrichment with
Ni(OH)2 in the passive film (Mishra and Shoesmith 2013). The
most striking difference among polarization curves in the
tested solutions was the transpassive potential, where a
wider passive zone was observed in CaCl2 solutions. With
increased potential, a crevice corrosion breakdown and
further repassivation were observed on the alloy N06059 at
60 °C. Crevice corrosion without further repassivation was
observed for alloy N06059 at higher temperatures than 60 °C.
Crevice corrosion was also found on the alloy surface under
the top PTFE compression gasket. No crevice corrosion was
observed on alloy N06686 under any tested condition. The
absence of localized corrosion on alloy N06686 is consistent
with its higher PRE value (Table 1). Although the Ca2+ solution used here corresponds to the highest aggressive
ion concentration, a particular inhibitory effect of Ca2+
concerning Na+ is already known (Jiang et al. 2006). This
inhibitory effect becomes less evident with increasing temperature, to the point that Carranza and Rebak (2009) did not
observe any effect on ER,CREV at 90 °C. The polarization curve
of alloy N06686 in CaCl2 5 M at 117 °C deviates from the
behavior observed at lower temperatures and shows a
similar shape to those in NaCl solutions (Figure 2b). Deviation behavior is supposed that corresponds to a suppression
of the Ca2+ inhibition by the high temperature on alloy
N06686, and crevice corrosion on alloy N06059 may have
overshadowed the apparent suppression of the Ca2+
inhibition.
3.2 Polarization curves
Figure 3 shows PD-GS-PD tests performed on alloy N06686 in
chloride solutions at different temperatures. The outcome of
the PD-GS-PD tests depended on the alloy, the chloride concentration and the temperature. For both alloys and solution/temperature conditions, the measured PD-GS-PD curves
exhibited a shape similar to one of the five reference curves
shown in Figure 3 (Types 1, 2, 3, 4 and 5). The experimental
conditions and the electrochemical behavior exhibited by
each alloy are summarized in Table 2.
At temperatures higher than 80 °C, both alloys showed
a Type 1 behavior (Figure 3a), which is the most common
outcome of the PD-GS-PD method. In those conditions, the
crevice corrosion initiation potential (ECREV) is indicated by
a sudden increase of the current density. The potential
decreases during the galvanostatic step, with a final
repassivation and a positive hysteresis loop during the
potentiodynamic polarization in the cathodic direction.
Post-test LOM/SEM examination revealed crevice corrosion
damage occurred in all the specimens. For specimens tested
in CaCl2 solutions, the appearance of the alloys was clean
and without corrosion products, Figures 4a and 5a for alloys N06686 and N06059 at 90 °C. The localized attack on the
specimen tested in CaCl2 5 M spread out from the covered
area by the crevice former teeth. The localized attack was
CORRREV-2022-0071_proof ■ 17 January 2023 ■ 10:55 pm
Q3
UNCORRECTED PROOF
4
E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
Figure 2: Polarization curves in deaerated
0.1–1 M NaCl and 5 M CaCl2 at 30, 60, 90 and
117 °C for alloys. (a) N06059 and (b) N06686.
Potential scan rate: 0.6 V/h.
crystalline where grains and stacking faults were discernible in SEM images (Rebak 2005). For specimens tested in
NaCl 0.1 and 1 M solutions, the attack was discontinued and
full of corrosion products on the alloy surface and below
the crevice former teeth, as shown in Figure 5b, c.
Type 2 curves (Figure 3a) were observed in tests performed in CaCl2 5 M solution at temperatures from 60 to 90 °C
for the alloy N06059 and at 90 °C for alloy N06686. The PD-GSPD tests with Type 2 behavior showed similar ECREV to Type 1
(Figure 3a), but a sudden increase in the recorded potential
during the second step (galvanostatic step). The potential
reached high values up to the transpassivity range and
remained there throughout the entire stage. Mishra and
Shoesmith (2014) observed similar behavior on the galvanostatic step of the PD-GS-PD test on the 686 alloy on NaCl 1 M
at T < 80 °C. The potentiodynamic polarization in the
cathodic direction (third stage of the method) showed a
decreased current density (negative hysteresis loop) up to a
reactivation peak and subsequent repassivation. The reactivation peak exceeded the current density value of the
previous galvanostatic stage, up to 200 μA/cm2 at +50 m VSCE.
Repassivation potentials were similar to those obtained on
Type 1 (Figure 3a). LOM/SEM examination revealed a more
profound and rough attack than Type 1 (Figure 4b). The
unbright surface under the crevice formers showed an
aggressive localized corrosion as the elevated charge circulated. Rebak (2005) characterized that surface with a spotty
dull grey appearance, with isolated crystallographic etch
pits. Rebak argued that this type of corrosion occurs when
potentials above transpassivity are applied to the specimen
in non-aggressive solutions, this is low chloride concentration or low temperatures. In contrast, this type of attack was
observed with the highest chloride concentration in this
work.
Type 3 curves (Figure 3b) were also observed in tests
performed in CaCl2 solution at 60 °C for the alloy N06059 and
at temperatures of 80 and 90 °C for alloy N06686. The PD-GSPD tests that showed Type 3 behavior did not show an ECREV
as Type 1 and 2. Instead, a wide passive range is observed
until the current density threshold is obtained at transpassive potentials. The recorded potential during the second
step (galvanostatic) did not show changes over time. The
potentiodynamic polarization in the cathodic direction
(third stage of the method) showed a similar behavior than
in Type 2, following the current density path from the first
stage of the technique until the reactivation peak. The
reactivation peak did not exceed the galvanostatic current
density value of the previous step. LOM/SEM examination
revealed a rough attack similar to that of Type 2 (Figure 4c),
even with a low-peak current density, and it is supposed that
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E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
5
Figure 3: Examples of behaviors exhibited
during PD-GS-PD tests on chloride solutions:
Types 1–5. The results correspond to alloy
N06686, and the multiple curves are replicate
tests to illustrate reproducibility.
Table : Summary of electrochemical behaviors observed under
different studied conditions, concerning Figure .
Alloy
N
−
T (°C)/[Cl ]








N
. M
M
 M
. M
M
 M
–
–
–
①
④
④
④
⑤
–
–
–
①
④
④
④
⑤
①
①
①
①②
①②
②
②③
⑤
–
–
–
①
④
④
⑤
–
–
–
–
①
④
④
④
⑤
①
①
①
①②③
③
⑤
–
–
the charge circulated by the current density peak contributes to the observed crevice corrosion.
Type 4 curves (Figure 3c) were observed in tests performed in NaCl solution at temperatures from 60 to 80 °C for
alloy N06059 and alloy N06686 at temperatures from 70 to
80 °C in NaCl 0.1 M and 60–80 °C in NaCl 1 M. The recorded
potential during the galvanostatic step (second step) did not
show changes over time, the same as Type 3 behavior. The
potentiodynamic polarization in the cathodic direction
(third stage of the method) showed higher current densities
than the first stage, obtaining a positive hysteresis loop. No
reactivation peaks were observed on NaCl solutions. LOM/
SEM examination revealed a similar attack to Figure 5b, c
(not shown).
Type 5 curves (Figure 3d) indicated the absence of
localized corrosion. Type 5 curves were observed at 50 °C for
alloy N06059 and at 60 °C in NaCl 0.1 M, 50 °C in NaCl 1 M and
70 °C in CaCl2 5 M for alloy N06686. LOM/SEM examination
revealed no crevice corrosion. The recorded potential did
not show any change over time during the galvanostatic step.
No significant hysteresis between the forward (step 1) and
reverse (step 3) curves were observed. This was indicated by
the current density retracing the forward scan until finally
achieving values lesser than those recorded during the forward scan.
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6
E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
Figure 4: SEM and macroscopic images of alloy
N06686 specimens after testing in 5 M CaCl2 at
90 °C (a and b are Type 1 and 2 respectively)
and at 80 °C (c is Type 3).
Figure 5: SEM and macroscopic images of alloy
N06059 specimens after testing in 5 M CaCl2
(a), 1 M NaCl (b) and 0.1 M NaCl (c) at 90 °C. All
of them with Type 1 behavior.
Some PD-GS-PD tests performed in alloys N06059 and
N06686 reached transpassive potentials during stage 1 and
remained at those high potentials during stage 2. The
transpassive zone of potentials is characterized by a loss of
stability of the passive layer, where the electrochemical
conversion of the Cr (III) oxide into soluble Cr (VI) species
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UNCORRECTED PROOF
E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
occurs (Hayes et al. 2006; Mishra et al. 2016). Henderson et al.
(2019) observed that Ni (II) is the principal cation released
during transpassive dissolution on Ni–Cr–Mo alloys, while
some Cr but mainly Mo are retained, enriching the surface in
these elements. The same authors observed that during
repassivation, the Mo that enriched the metallic surface is
subsequently released during repassivation. This release of
Mo surface could be related to the sudden increase in current density, and the peak observed at [Cl−] = 10 M (Figure 3a,
b) by a hypothetical depletion of this alloying element on the
metallic surface after it was released during repassivation.
Figure 6 shows ER,CREV from the PD-GS-PD method on
alloys N06059 and N06686, with the temperature for NaCl
0.1 M, NaCl 1 M and CaCl2 5 M. The symbols are the average
values, and the error bars represent the standard deviation.
The CCT inferred for alloy N06059 from the present results
was between 50 and 60 °C. For alloy N06686, crevice corrosion was observed at T ≥ 60, 70, and 80 °C for [Cl−] = 1 M,
[Cl−] = 0.1 M and [Cl−] = 10 M solutions, respectively. From the
present results, CCT for alloy N06686 was estimated between
50 and 60 °C, as for alloy N06059. A considerable lower CCT
was obtained with the PD-GS-PD method compared to those
obtained by Agarwal et al. (2000), which following ASTM G48
(10% FeCl3 solutions), report CPT > 85 °C for both alloy
N06059 and alloy N06686.
Equation (2) states a relationship between ER,CREV chloride concentration and temperature. This empirical equation was used at first by Dunn et al. (2006) for alloy C-22 (UNS
N06022). In previous works, Equation (2) shows a good fitting
for ER,CREV of Ni–Cr–Mo–W alloys with temperature and
chloride solutions (Hornus et al. 2014, 2015; Maristany et al.
2015). It was fitted to a range of the collected data for alloys
N06059 and N06686. A, B, C and D are constants parameters
that depend on each alloy. Figure 6 also shows the fits of
7
Equation (2) with dash-line for the two alloys. The comparison of the ER,CREV values of the tested alloys as a function of
temperature and chloride concentration indicates that their
corrosion resistance increased according to their corresponding PRE, being N06059 < N06686. When the temperature and chloride concentration increased ER,CREV showed a
lesser dependence on the environmental variables.
E R, CREV = (A + B · T)log[Cl− ] + CT + D
(2)
The parameters obtained by least-square fits of Equation (2) are shown in Table 3 for alloy N06059 and Table 4 for
alloy N06686. The correlation coefficients (R 2) obtained were
0.77 and 0.90 for alloys N06059 and N06686, respectively,
which indicates that Equation (2) successfully represented
ER,CREV as a function of temperature and chloride concentration for the tested alloys. Although the correlation coefficient on alloy N06059 was lower than that of alloy N06686,
the standard deviations observed in the fitted parameters (A,
B, C, D) were at least one order of magnitude lower than the
mean values, in all cases. Regardless of that a similar normal
distribution of ER,CREV is expected around the average values
given by the fits, the high error bars on ER,CREV values for
alloy N06059 in NaCl 1 M at 70 °C and NaCl 0.1 M at 60 °C
(Figure 6) could contribute to its low correlation coefficients.
Table : Fitted parameters for the N alloy from Equation ()
(R = .).
Parameter
Value
Standard error
A
B
C
D
−. VSCE
. V/K
−. V/K
. VSCE
. V
. V/K
. V/K
. V
Figure 6: ER,CREV of alloys N06059 and N06686
as a function of temperature. Average ER,CREV
and standard deviation values are represented
along with the fit of Equation (2).
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E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
Table : Fitted parameters for the N alloy from Equation ()
(R = .).
Parameter
Value
Standard error
A
B
C
D
−. VSCE
. V/K
−. V/K
. VSCE
. V
. V/K
. V/K
. V
3.3 Alloying effects on ER,CREV
Dunn’s empirical equation for ER,CREV has shown an
excellent correlation for the repassivation potential with
environmental variables such as temperature and chloride
solution concentration. Their parameter A indicates the
variation of ER,CREV with log[Cl−] independently of the
temperature, the parameter C indicates the variation of
ER,CREV with T independently of the log[Cl−], B shows the
temperature-dependent variation of ER,CREV with log[Cl−]
and D an independent parameter. Nevertheless, all parameters depend on the alloy composition.
Obtaining an equation for ER,CREV that involves the
environmental parameters and the main alloying elements
would help to quantify the individual effects of main alloying
elements on ER,CREV. Linear behaviors of different corrosion
parameters such as Critical Pitting Temperature (CPT) and
Critical Crevice Temperature (CCT) with the PRE value were
observed by various researchers on Ni alloys and stainless
steels (Sarmiento Klapper et al. 2017; Sedriks 1996). Sosa
Haudet et al. (2012) obtained ER,CREV by the PD-GS-PD method
for several Ni–Cr–Mo–W alloys in chloride solutions. They
observed a linear behavior of the ER,CREV with the PRE values
on NaCl 1 M and CaCl2 5 M at 60 °C. Similar linear shapes of
ER,CREV with PRE were reported by Zadorozne et al. (2012) on
Ni–Cr–Mo–W alloys on NaCl solutions. The linear relationships observed by Sosa Haudet et al. and others could be
extended from a phenomenological point of view to different
temperatures and chloride concentrations as follows:
E R, CREV = ER, CREV A1, B1, C1, D1 + PRE{ER, CREV A2, B2, C2, D2}
(3)
where ER, CREV A1, B1, C1, D1 and E R, CREV A2, B2, C2, D2 represent Equation (2) and PRE corresponds to Equation (1). The extended or
explicit form of Equation (3) for fitting is shown as follows:
ER, CREV = (A1 + B1 T)log[Cl− ] + C 1 T + D1
+ PRE{(A2 + B2 T)log[Cl− ] + C 2 T + D2 }
(4)
Figure 7 shows the fitted Equation (4) for alloys N06059
and N06686 (this work) and alloys N06625, N06022, N07022,
and N10362 (Hornus et al. 2014) as a function of temperature
Figure 7: Individual ER,CREV for alloys N06059 and N06686 (this work) and
for alloys N06625, N06022, N07022, and N10362 (Hornus et al. 2014) alloys
as a function of temperature. Dash lines correspond to the fit of
Equation (4).
and chloride solution concentration. Equation (4) shows a
complex interdependence of environmental variables ([Cl−]
and T) and the content of alloying elements (PRE) to determine ER,CREV. The parameters obtained by least-square fits of
Equation (4) are shown in Table 5. A similar fit shape was
observed with Equation (2) for alloys N06059 and N06686
and the other Ni–Cr–Mo–W alloys. The correlation coefficient (R 2) obtained with Equation (4) was 0.83 for the six
alloys, which is better than the one obtained for alloy N06059
(R2 = 0.77) with Equation (2) but lower than that observed for
alloys N06625 (R2 = 0.90), N06022 (R2 = 0.92), N07022 (R2 = 0.92),
N10362 (R2 = 0.98) (Hornus et al. 2014) and N06686 (R2 = 0.90).
Figure 7 shows a high dispersion of the repassivation potential for all alloys with a chloride concentration less than
or equal to 1 M. The statistical dispersion of the repassivation
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E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
Table : Fitted parameters for all the alloys from Equation () (R = .).
Parameter
Value
Standard error
A
B
C
D
A
B
C
D
. VSCE
−. V/K
−. V/K
−. VSCE
−. VSCE
. V/K
−. V/K
. VSCE
. V
. V/K
. V/K
. V
. V
. V/K
. V/K
. V
potentials observed in 10 M chloride solutions is considerably lower than for the other chloride concentrations. Miller
and Lillard (2019) studied the nucleation and growth stages
of crevice corrosion by potentiostatic methods measuring
50–1000 µA in alloy 625 in 1 M NaCl, and they observed that
the completion of the growth stage exceeded 48 h. According
to them, the repassivation during the growth stage is the
reason for the high dispersion in repassivation potentials.
The galvanostatic step on the PD-GS-PD method lasts only 2 h,
and a rapid decrease in potential has been observed for high
chloride concentrations, getting slower for the more dilute
solutions, as in Figure 8. In general, for [Cl−] ≤ 1 M, the potential recorded during the galvanostatic stage does not
finally stabilize, contributing to the high dispersion of
ER,CREV. Figure 7 shows the fitted Equation (4) successfully
represented ER,CREV as a temperature and chloride concentration function for the tested alloys depending on its PRE
value.
The terms of Equation (4) were regrouped to compare its
fitted parameters with those of Equation (2) (individual fitted
for each alloy) according to:
ER, CREV = (A′ + B′ T)log[Cl− ] + C ′ T + D′
(5)
where A′ = A1 + A2 * PRE, B′ = B1 + B2 * PRE, C′ = C1 + C2 * PRE
and D′ = D1 + D2 * PRE. Figure 9 shows the fitted parameters
of Equations (2) and (5) as a function of PRE. It can be seen
that the parameters A (affected by chloride concentration)
and D (independent of chloride concentration and temperature) showed a specular behavior with a different sign. The
same is observed with parameters B and C. Figure 9 shows
that the parameters of the fitted Equation (2) are very similar
to those of Equation (5), except for those corresponding to
the N06059, N06686 and N10362 alloys. The main difference
between the three alloys that did not follow Equation (5)
parameters is their composition’s null or low Co content. Co
alloy is added to Ni alloys to increase the resistance to
carburization and sulfidation as Co increases the solubility
of C in Ni-based alloys (Davis 2000). The unmatched parameters for alloys N06059, N06686 and N10362 (indicated in
Figure 9 with dot-lines) could suppose a Co effect that is not
contemplated in the PRE equation.
From Equation (4) and the fitted parameters (Table 5),
we analyzed the effect of incremental values of Cr, Mo and W
on ER,CREV. The derivatives of ER,CREV concerning Cr, Mo and
W were calculated and evaluated for each alloy, chloride
concentration and temperature. Figure 10 shows dER,CREV/d
[%Cr], dER,CREV/d[%Mo] and dER,CREV/d[%W] in weight
percent for 0.1–10 M chloride concentration as a function of
temperature. The three derivate behaviors are the same
with the Y-axis modification. The largest effect on repassivation potentials with increasing Cr, Mo or W alloys would
be observed at low temperatures and chloride ion concentrations. In a particular case, there is a temperature for
which the increase in repassivation potential would be independent of the chloride ion concentration (∼84.1 °C) and a
Figure 8: Galvanostatic step of the PD-GS-PD
method for alloys N06059 and N06686 at 90 °C
in chloride solutions with different concentrations.
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E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
Figure 9: Fitted parameters of Equations (2)
and (5) as a function of PRE. Dot lines show the
PREs of alloys N06059, N06686 and N10362.
chloride ion concentration for which the repassivation potential would be independent of temperature (∼3.36 M). At
85 °C (where slopes are independent of chloride concentration), the Cr–Mo–W contribution to the ER,CREV on Ni-based
alloys was about 5–6 mV/%Cr, 17–18 mV/%Mo and ∼9 mV/%W.
Sosa Haudet et al. (2012) obtained dER,CREV/dPRE from
several Ni-based alloys in NaCl 1 M and CaCl2 5 M at 60 °C. As
we could not represent directly dER,CREV/dPRE on Figure 10,
we can obtained them as dER,CREV/d[alloy] = dER,CREV/
dPRE × dPRE/d[alloy] with alloy as %Cr, %Mo or %W. The
variation of ER,CREV with the alloying elements from Sosa
are in excellent agreement with this work and almost
Figure 10: dER,CREV/d[%Cr] (black), dER,CREV/d[%Mo] (red) and dER,CREV/d
[%W] (green) in weight percent for 0.1–10 M chloride concentration as a
function of temperature.
superimposed on our results (Figure 10, Sosa Haudet 2012).
Sosa Haudet et al. (2015) also studied the effect of the alloying
elements in ER,CREV for NaCl 1 M at 60 °C with artificial neural
networks. Their results are in Figure 10 (Sosa Haudet 2015)
with an overestimation effect for W and sub-estimation for
Cr and Mo concerning the present work. It is important to
note that a large amount of data and training are necessary
for artificial neuronal networks to obtain reliable values.
Igual Muñoz et al. (2004) studied the ER,CREV on the alloys 33
and 31 in LiBr solutions at a scan rate of 0.5 mV/s and
different temperatures. The variation of ER,CREV with Mo are
close to those obtained in the present work, even increasing
with the temperature (Figure 10, Muñoz). The Mo effect on
ER,CREV reported by Muñoz is higher than in the present
work. The less aggressive bromide solutions plus the overlapped effect of increased Mo with a Cr falling could explain
its behavior even when Mo is not as effective in inhibiting
Br− localized corrosion as it is for inhibiting Cl− localized
corrosion (Kappes 2019). Ha et al. (2017) observed a higher W
effect on ER,CREV than the actual work on duplex stainless
steels at 90 °C with a scan rate of 2 mV/s.
Each derivate from Equation (4) gives us information
about the behavior of ER,CREV with the different variables and
parameters. Classical mathematical analysis tells us that the
gradient of a function allows us to observe the direction of
maximum growth of the function (Marsden et al. 1993).
Considering the triple Cr–Mo–W as if it were a vector entity,
it is possible to obtain the vector components (in this case,
the relation among them) that maximize ER,CREV. The “alloy
vector” of maximum growth of ER,CREV are dependent on the
environmental variables (chloride concentration and temperature) that become independent of them when normalized. The ratio of alloys that maximizes the value of ER,CREV is
1:3.3:1.65 for Cr, Mo and W, respectively. It should be noted
that the ratio of the alloy that maximizes ER,CREV has the
same factors as the PRE (Equation 1). The optimal alloying
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E.C. Hornus and M.A. Rodríguez: Repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
ratio would be independent of the initial composition of the
alloy since it has constant values, i.e., an alloying increase of
1:3.3:1.65 in Cr, Mo and W on alloy N06022 would increase the
ER,CREV values in a maximum way, the same as it would on
alloy N10362.
4 Summary and conclusions
The crevice corrosion resistance of alloys N06059 and
N06686 was assessed in 0.1–10 M chloride solutions at temperatures ranging from 50 °C to 117 °C. Alloy N06059 suffered
crevice corrosion at T ≥ 60 °C, while N06686 alloy suffered
crevice corrosion at T ≥ 60 for [Cl−] = 1 M, at T ≥ 70 °C for
[Cl−] = 0.1 M and at T ≥ 80 °C for [Cl−] = 10 M. These results
showed that the studied Ni alloys may suffer crevice corrosion tens of degrees below the reported critical crevice
temperatures obtained through immersion tests.
The repassivation potential of the tested alloys as a
function of temperature and chloride concentration is given
by ER,CREV = (A + BT)log[Cl−] + CT + D for a range of environmental conditions. When the temperature and chloride
concentration increased ER,CREV showed a lesser dependence
on these environmental variables.
The repassivation potential of nickel-based alloys depends on temperature, chloride concentration, and PRE. The
following equation was proposed: ER,CREV = (A1 + B1T)log
[Cl−] + C1T + D1 + PRE{(A2 + B2T)log[Cl−] + C2T + D2} for a
range of Ni–Cr–Mo–W alloys. The dependence of ER,CREV
with the content of main alloying elements is included in the
PRE (PRE = %Cr + 3.3(%Mo + 0.5%W)). When the temperature and chloride concentration decreased ER,CREV showed a
higher dependence on the content of main alloying
elements.
Dependence of ER,CREV with the contents of alloying elements is described by the slopes dER,CREV/d(wt%Cr) ≈ 5–
6 mV/wt%Cr, dER,CREV/d(wt%Mo) ≈ 17–18 mV/wt%Mo and
dER,CREV/d(wt%W) ≈ 9 mV/wt%W, at 85 °C in chloride solutions. The ratio of alloys that maximizes the ER,CREV value is
1:3.3:1.65 for wt%Cr, wt%Mo and wt%W, the same factors as
in the PRE equation. The optimal alloying ratio would be
independent of the alloy composition since it is not a function of the content of main elements.
Acknowledgments: The authors are grateful to Dr. Mariano
Kappes for his valuable comments.
Author contributions: All the authors have accepted
responsibility for the entire content of this submitted
manuscript and approved submission.
11
Research funding: This work was supported by Agencia
Nacional de Promoción de la Investigación, el Desarrollo
Tecnológico y la Innovación of Argentina (grant PICT2020-SERIEA-00149), National Scientific and Technical
Research Council of Argentina (grant PIP CONICET 2021-23
11220200101057CO), and National Commission of Atomic
Energy of Argentina.
Conflict of interest statement: The authors declare no
conflicts of interest regarding this article.
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CORRREV-2022-0071_proof ■ 17 January 2023 ■ 10:55 pm