Corrosion of
Monel-400 in Aerated Stagnant Arabian Gulf Seawater after Different Exposure
Intervals
The
corrosion of Monel-400 after varied exposure periods in naturally aerated
Arabian Gulf seawater (AGS) has been carried out using gravimetric, cyclic
potentiodynamic polarization, chronoamperometry, open-circuit potential, and
impedance spectroscopy measurements along with SEM/EDX investigations.
Gravimetric data within 160 days showed that the weight loss increased, while
the corrosion rate decreased with time. SEM/EDX investigations after 160 days
immersion indicated that dissolution of Monel takes place due to the selective
dissolution of Ni. The electrochemical measurements confirmed the gravimetric
data and proved that severity of uniform corrosion of Monel decreases, while
pitting one increases with increasing the exposure period.
Keywords: corrosion; Monel-400 alloy; EIS; polarization; SEM;
weight loss
1.
INTRODUCTION
Monel-400
is one of the most important nickel based alloys that contains about 60-70
percent nickel, 20-29 percent copper and small amounts of iron, manganese,
silicon and carbon. It is a solid solution alloy that can only be hardened by
cold working. This alloy was discovered due to the efforts of Robert Crooks
Stanley, who worked for the International Nickel Company (INCO) in 1901. It was
installed as a sheet roofing membrane in 1908. In the late 1920s, Monel-400 was
begun to be used for grocery coolers, countertops, sinks, laundry and food
preparation appliances, roofing and flashing.
Monel-400
is characterized by its good corrosion resistance, good weldability and high
strength. Therefore, it has been used extensively in many applications such as
chemical processing equipment, gasoline and fresh water tanks, crude petroleum
stills, valves and pumps, propeller shafts, marine fixtures and fasteners,
electrical and electronic components, de-aerating heaters, process vessels and
piping, boiler feed water heaters and other heat exchangers, and etc [1-4]. For
that when a piece of equipment needs to stand up to interior or exterior
corrosive, Monel-400 is the fail-safe solution. It also has higher maximum
working temperatures than nickel (up to 540 °C, and its melting point is
1300‒1350 °C), which makes it the preferred metal for boiler feed water heaters
and other heat exchangers.
Although
Monel-400 is known for its ability to stand up to tough corrosive elements,
pitting corrosion of Monel-400 occurs when it is exposed to stagnant salt water
such as seawater [5]. The corrosion rate of this alloy decreases sharply with
increasing nickel content in the alloy. A series of Cu-Ni alloys have been
studied in natural sea water and in chloride solutions under different
conditions [6‒12]. Some of these studies have [7] reported that selective
electrodissolution of nickel is predominant; while others [11] have found that
the dissolution of copper depends on the composition of the alloy under
investigation.
The
objective of the present work was to study the anodic dissolution of Monel-400
in the aerated stagnant solutions of Arabian Gulf seawater after varied
exposure periods. The experimental work has been carried out using weight-loss
measurements after varied immersion periods of 5-160 days. The study was also
complemented by a variety of electrochemical techniques along with surface
morphology and elemental analysis investigations. Since, pitting corrosion is
one of the most destructive forms of localized corrosion and the ability of
corrosive ions that present in sea water on the breakdown of a passive film
might form on the surface of Monel-400, a particular attention was paid to the
effect of stagnant AGS solutions on the pitting corrosion of the alloy.
2.
EXPERIMENTAL PROCEDURE
The
natural sea water (AGS) was obtained directly from the Arabian Gulf at the
eastern region (Jubail, Dammam, Saudi Arabia), and was used as received. An
electrochemical cell with a three-electrode configuration was used for
electrochemical measurements. Monel-400 rod and sheet (were purchased from
Magellan Metals, USA, with the following chemical composition, Ni–63.0% min,
Cu– “28-34%” max, Fe–2.5% max, Mn–2.0%, Si–0.5% max, C–0.3% max, and S–0.024%)
were used in this study. The Monel rod was used as a working electrode. A
platinum foil and a Metrohm Ag/AgCl electrode (in 3 M KCl) were used as counter
and reference electrodes, respectively. The weight loss experiments were
carried out using rectangular Monel-400 coupons cut from the Monel sheet. The
coupons had dimensions of 4.0 cm length, 2.0 cm width, and 0.4 cm thickness and
the exposed total area of 54.02 cm2. were grinded successively with
metallographic emery paper of increasing fineness of up to 800 grits, and then
polished with 1, 0.5 and 0.3µm alumina slurries (Buehler). The electrodes were
then washed with doubly distilled water, degreased with acetone, washed using
doubly distilled water again and finally dried with pure air. The coupons were
weighed and then suspended in 300 cm3 solutions of Arabian Gulf seawater for
different exposure periods between 5 and 160 days. The losses in weight per
area (ΔW, g.cm-2) and the corrosion rates (KCorr, millimeters/year (mmpy)) over
the exposure time were calculated as has been reported before [13, 14]. The SEM
investigation and EDX analysis were obtained on the surface of a Monel-400
coupon after its immersion in AGS solution for 160 days. The SEM images were
obtained by using a JEOL model JSM-6610LV (Japanese made) scanning electron
microscope with an energy dispersive X-ray analyzer attached.
The
Monel-400 rods for electrochemical measurements were grinded and polished as
for the Monel coupons. The diameter of the working electrode was 1.2 cm with a
total exposed area of 1.13 cm2. Electrochemical experiments were performed by
using an Autolab potentiostat (PGSTAT20 computer controlled) operated by the
general purpose electrochemical software (GPES) version 4.9. The cyclic
potentiodynamic polarization (CPP) curves were recorded by scanning the
potential in the forward direction from -800 to +800 mV then backward from +800
to -800 mV against Ag/AgCl again at the same scan rate, 3.0 mV/s.
Chronoamperometric (CA) experiments were carried out by stepping the potential
of the Monel-400 electrode at +100 mV versus Ag/AgCl for 120 min. For the PPC
and CA experiments, the curves were recorded after the electrode immersion in
AGS for 0, 24, and 72 h before measurements. Electrochemical impedance
spectroscopy (EIS) tests were performed after 1, 24, and 72 h of the electrode
immersion at corrosion potentials (ECorr) over a frequency range of 100 kHz –10
mHz, with an ac wave of 5 mV peak-to-peak overlaid on a dc bias potential, and
the impedance data were collected using Powersine software at a rate of 10
points per decade change in frequency.
3. RESULTS AND
DISCUSSION
3.1. Weight-loss data and SEM / EDX
investigations
The
variations of (a) the weight loss (ΔW) and (b) the corrosion rate (KCorr) vs.
time for Monel-400 coupons in 300 cm3 of AGS are shown in Fig. 1. The values of
ΔW and KCorr were calculated as reported in the previous work [13, 14]. It is
clearly recognized that the values of ΔW increased from 510‒5 g/cm2 after 5
days to 5510‒5 g/cm2 after 160 days immersion in the AGS solution at the same
condition. This is due to the contentious dissolution of Monel-400 surface
under the influence of the high salinity of the Arabian Gulf (47000 ppm). On
the other side, the values of KCorr decreased with increasing time, which
indicates the development then the accumulation of corrosion products and/or
oxides on the Monel surface. These formed components partially protect the
surface by reducing its dissolution and so decreasing aggressiveness of AGS by
limiting the contact of the active Monel surface to AGS test solution. This
agrees with the previous studies [15, 16] and proves the poor performance of
Monel-400 in freely aerated stagnant AGS.
Figure
2 shows SEM/EDX investigations for Monel-400 surface after its immersion in AGS
solutions for 160 days where, (a) the SEM micrograph for a large area of the
surface and (b) the corresponding EDX profile analysis for the selected area on
the SEM image, respectively. It is obvious from Fig. 2a that the surface has a
flat area covered with corrosion products in addition to numerous pits, which
have almost similar round shapes and different diameters.
Figure 1. Variations of the weight loss (a) and corrosion rate
(b) versus time for Monel-400 coupons in open to air stagnant Arabian Gulf
seawater.
The
atomic percentage of the elements found in the selected area of image (a) by
the EDX profile, were 41.80% C, 35.28% O, 12.75% Ni, 6.33% Cu, 2.86% Cl, 0.49%
Fe, 0.29% S, and 0.19% Mn. The low contents of Ni and Cu and the high
percentages of C and O suggest that the alloy surface is covered with corrosion
products that have different compounds, complexes and oxides. The presence of chloride,
sulphur and iron besides carbon also suggest that the surface is having scales
deposited from the seawater.
In
order to understand the mechanism of pitting corrosion of Monel-400 in the AGS
at this condition, the SEM/EDX investigations were obtained for the corrosion
products around the pits as well as inside the pits. Fig. 3 depicts, (a) SEM
micrograph represents a pit on the Monel-400 surface after its immersion in
freely aerated stagnant AGS solution for 160 days and (b) the corresponding EDX
profile analysis taken for the corrosion products around the pit as selected on
the SEM image, respectively. The atomic percentage of the elements found around
the pit shown in the SEM image (a), were 46.31% O, 20.76% C, 5.01% Ni, 19.12%
Cu, 8.26% Cl, and 0.55% S with no Fe and Mn. The high level of O and C provide
that the surface of Monel-400 around the pit is covered with a thick oxide
layer with other corrosion products. The very low content of Ni compared to Cu
is due to the selective dissolution of Ni, while Cu tends to form oxide and
chloride. The presence of C and S indicate that the area around pits has scale
and corrosion products resulted from the components of the
seawater.
Figure 2. (a) SEM micrograph for a large area of Monel-400
surface after its immersion in freely
aerated
stagnant AGS solution for 160 days and (b) the corresponding EDX profile
analysis
taken
in the selected area of the SEM image, respectively.
Figure
4 shows (a) SEM micrograph represents an extended area inside a pit was formed
on the surface of Monel-400 that has been immersed in freely aerated stagnant
AGS solution for 160 days and (b) the corresponding EDX profile analysis taken
inside the pit as selected on the SEM image, respectively. The SEM image shows
that the formed pit is deep and wide with corrosion products deposited in some
areas inside it. The atomic percentages of the components found inside the pit
were found to be 58.92% Cu, 17.01% Ni, 15.73% O, 6.96% Cl, 0.95% Fe, and 0.43%
S. The very high content of copper (almost double of its natural presence in
the alloy) as well as the very low Ni content inside the pit confirms the
selective dissolution of Ni with copper enrichment. The poor presence of oxygen
inside the pit compared to its percentages on the surface (35.28%) and around
the pit (46.31%) also specifies that the aggressive ions such as Cl─ displace
the oxygen at its weakest bond with metal on the alloy surface and initiate
pitting corrosion.
Figure 3. (a) SEM micrograph represents a pit on the Monel-400 surface
after its immersion in freely aerated stagnant AGS solution for 160 days and
(b) the corresponding EDX profile analysis taken for the corrosion products
around the pit as selected on the SEM image, respectively.
The
presence of Cl─ increases the potential difference across the passive film,
thereby enhancing the rate of nickel ions diffusion from the nickel-film
interface to the film-solution interface, forming cation vacancies at the
Monel-film interface [5]. When the concentration of Cl─ is high enough, voids
develop at the nickel-film interface. Continued growth of such voids results in
the localized collapse of the passive film, which will dissolve faster than
other regions of the passive film, leading to pit growth and ultimately
substrate alloy dissolution [17]. It is worth to mention that the maximum pit
depth measured in stagnant natural seawater for Monel-400 was 1.067 mm deep as
reported in a three-year study at the Inco Test Facility [15]. The attacked
regions were copper-rich while the regions around the active sites had higher
Ni concentrations. This agrees with our work and also the work reported by
Gouda et al. [18] and Little et al. [19] that the corrosion of Monel-400
undergoes through the selective leaching of nickel from the alloy leaving a
spongy copper –rich material in the base of the pit.
Figure 4. (a) SEM micrograph represents an extended area for a
pit on the Monel-400 surface after its immersion in freely aerated stagnant AGS
solution for 160 days and (b) the corresponding EDX profile analysis taken
inside the pit as selected on the SEM image, respectively.
According
to Little et al. [19] chlorine and sulphur from seawater accumulate within the
pit and react with the iron and nickel in the alloy, which is why the
percentage of Fe and Ni are lower inside the pit compared to their natural
presence in the alloy.
3.2. Cyclic potentiodynamic polarization
(CPP) measurements
CPP
experiments were carried out after Monel immersion in AGS solutions for 0 h, 24
h, and 72 h in order to understand the mechanism of Monel dissolution after
varied exposure periods. This technique was also used to report the effect of
time on the change of corrosion potential (ECorr), Corrosion current (jCorr),
pitting potential (EPit), pitting current (jPit), polarization resistance (RP),
and corrosion rate (KCorr) of Monel-400 in the test medium.
Figure 5. Cyclic potentiodynamic polarization curves for
Monel-400 after 0 h (a), 24 h (b), and 72 h (c) immersion in Arabian Gulf
seawater, respectively.
The
CPP curves for the Monel electrode after (a) 0 h, (b) 24 h, and (c) 72 h
immersion in AGS solutions, respectively are shown in Fig. 5. Blundy and Pryor
[9] have reported that the anodic reaction of Monel-400 is the selective
dissolution of nickel, particularly at high potential values. Gouda et al. [18]
with Arabian seawater and Little et al. [19] with Gulf of Mexico water have
also demonstrated that the anodic dissolution of Monel-400 occurs due to the
intergranular corrosion and selective dealloying of iron and nickel, especially
in the presence of sulfate-reducing bacteria. It is clearly seen from Fig. 5a
that an active dissolution of the alloy occurred with increasing potential in
the anodic side. It is also seen that there is a peak on the anodic branch at
which the current decreased with increasing the applied potential. This peak
was appeared due to either the formation of a passive oxide film [20, 21] or
the accumulation of corrosion products on the electrode surface. The sudden
increase of the current after the formation of the peak is due to the breakdown
of the passive film formed on the Monel-400 surface by the attack of aggressive
ions presented in the seawater such as chlorides and lead to the occurrence of
pitting corrosion [22]. The further increase of the current with potential is
caused by the agglomeration of chloride ions inside the pits leading to pit
growth and ultimately substrate alloy dissolution [5].
Table 1. Corrosion parameters obtained from CPP curves shown
in Fig. 6 for the Monel-400 in aerated stagnant Arabian Gulf seawater after
different exposure intervals.
Increasing
the immersion time of the electrode in the AGS solution to 24 h before
measurements (Fig. 5b) decreased both the anodic and peak currents and even
eliminated the peak when the immersion time was increased to 72h as shown in
Fig. 5b and 5c, respectively. The corrosion parameters obtained from Fig. 5 are
shown in Table 1. It is seen from Table 1 that the values of jCorr, jProt and
jPit decreased with increasing the immersion time. Also, the values of ECorr,
EProt, and Epit increased to the more negative values. This effect also
increased the values of polarization resistance (RP) and decreased the values
of corrosion rate (KCorr), which were calculated as previously reported [13,
23-25].
3.3. Chronoamperometric measurements
The
variation currents versus time for Monel-400 electrode that has been immersed
in the AGS solutions for (1) 0 h, (2) 24 h, and (3) 72 h, respectively before
stepping the potential to 100 mV for 120 min are shown in Fig. 6. The highest
current values for Monel-400 in AGS solutions were recorded when the
measurements were carried out after the first moment of the electrode
immersion, curve 1. It is clearly observed that the current increased with time
till the end of the run. Increasing the immersion time to 24 h, (curve 2) led
to decreasing the absolute current and further current decreases were recorded
when the time was increased to 72 h (curve 3). This decrease in the absolute
current with increasing immersion time of Monel before applying the constant
potential can be explained by the formation of a passive oxide film and/or
corrosion products; this film gets even thicker as the time increases. The
formation of such species on the surface decreases the uniform attack of the
Monel-400 and so decreases the absolute current under the applied potential.
Figure 6. Chronoamperometric curves for the Monel-400 electrode
that has been immersed in Arabian Gulf seawater for 0 h (1), 24 h (2) and 72 h
(c) before stepping the potential to 100 mV vs. Ag/AgCl before measurements.
On
the other hand, the increase of current values with time when the potential was
stepped to 100 mV is due to the dissolution of the film formed on the Monel-400
surface, due to the preimmersion of the electrode in the solution, leading to
the occurrence of the pitting corrosion. The higher the absolute currents the
higher the number of small and narrow pits. This means that increasing the
exposure time before measurement leads to decrease the number of pits at the
same time it increases the width and depth of the pits formed. Pits develop [5,
26] at sites where oxygen adsorbed on the alloy surface is displaced by an
aggressive species such as Cl─ ions that are presented in the AGS solution.
This is because Cl─ ions have small diameters allows it to penetrate through
the protective oxide film and displace oxygen at the sites where metal-oxygen
bond is the weakest [5, 17].
3.4. Open-circuit potential (OCP) and
electrochemical impedance spectroscopy (EIS) measurements
The
variation of the OCP versus time (72 h) for Monel-400 electrode in AGS is shown
in Fig. 7. It is clearly seen that the potential values slightly increased
toward the more negative direction in the first 10 h due to the dissolution of
Monel-400 by the aggressive ions attack that present in the AGS on the
electrode surface. This slight negative potential shift decreased with
increasing the immersion time up to the first 45 h, which might be due to the
formation of corrosion products including oxides on the surface.
Figure 7. The change of the open-circuit potential versus time
for Monel-400 in Arabian Gulf seawater.
The
formation of such corrosion products partially protected the alloy surface,
which is why the potential decreased in the positive direction again and till
the end of the test. This very slight positive shift in the OCP values
decreased the corrosion rate by decreasing the uniform attack of the alloy with
time. This decrease in corrosion rate might result not only from the formation
of corrosion products but also because of the ability of AGS solution to form
scales on the Monel-400 surface. The EIS measurements were carried out to
determine kinetic parameters for electron transfer reactions at the
Monel-400/electrolyte interface and to confirm the data obtained by CPP and CA
measurements. Typical Nyquist plots (a), Bode (b), and phase angle (c) for
Monel-400 after its immersion for (1) 1 h; (2) 24 h; and (3) 72 h, respectively
in AGS are shown in Fig 8. It is clear from Fig. 8a that only single
semicircles are observed for the Monel electrode in AGS for the different
exposure intervals. The chord length pertaining to the high frequency (HF) loop
observed in Nyquist diagram increased as the immersion time before measurements
increased. This increase of the HF chord is due to the decrease in the
electrochemical active areas by the accumulation of corrosion products and
oxides at the Monel surface. The diameter of the semicircle is significantly
increased with increasing the time of exposure to 24 h and further to 72 h. It
has been reported that the semicircles at high frequencies are generally
associated with the relaxation of the capacitors of electrical double layers
with their diameters representing the charge transfer resistances [24, 27]. The
Nyquist spectra shown in Fig. 8a were analysed by fitting to the equivalent
circuit model shown in Fig. 9 and was also used previously to fit the impedance
data obtained for Monel-400 in simulated seawater [28]. The parameters obtained
by fitting the equivalent circuit shown in Fig. 9 are listed in Table 2.
Table 2. EIS parameters obtained by fitting the Nyquist plots
shown in Fig. 8a with the equivalent circuit shown in Fig. 9 for the Monel-400
in aerated stagnant Arabian Gulf seawater after different exposure intervals.
Figure 8. Nyquist (a), Bode (b) and phase angle (c) plots for
Monel-400 at OCP (ECorr ± 5 mV) after its immersion in Arabian Gulf seawater
for 1 h (1), 24 h (2), and 72 h (3).
Figure 9. The equivalent circuit used to fit the experimental
data presented in Fig. 8a.
According
to usual convention, RS represents the solution resistance between the Monel
surface and the platinum counter electrode, Q the constant phase elements (CPE)
and contain two parameters; a pseudo capacitance and an exponent (an exponent
close to 0.5), the RP1 accounts for the resistance of a film layer formed on
the Monel-400 surface, Cdl is the double layer capacitance, and RP2 accounts
for the charge transfer resistance at the alloy surface, i.e. the polarization
resistance. It is seen from Fig. 8 and Table 2 that the values of RS, RP1 and
RP2 increased as the immersion time of Monel in AGS before measurements was
increased. The polarization resistance measured by EIS in this case is a
measure of the uniform corrosion rate as opposed to tendency towards localized
corrosion. The increase of the resistances (RP = RP1 + RP2) in this case is
attributed to the formation of a passive film and/or corrosion products, which
gets thicker with time and could lead to the decrease in jCorr and KCorr and
also the increase in RP values we have seen in CPP experiments, Fig. 5 and
Table 1, under the same conditions. The CPE, Q, is almost like Warburg
impedance with its n-values smaller than 0.50, suggesting that the formed
corrosion products and oxide layer on the Monel-400 surface block the mass
transport acting like a resistor. The values of YQ decreased with time
indicating that the dissolution of Monel is also limited by mass transport.
That was again confirmed by the decrease of the double layer capacitance, Cdl, values
with increasing the immersion time. The results presented by Nyquist plots and
Table 2 were also supported by the increase in the impedance of the interface
with increasing the exposure interval of Monel before measurement as shown in
Fig. 8b. It has been reported [29‒37] that the increase of the impedance and
low frequency values means the increase of the passivation of the surface.
Further confirmation is also provided by the increase of the maximum phase
angle (Fig. 8c) with time at the same conditions. In general, the EIS data is
in good agreement with the electrochemical (CPP and CA) experiments and weight
loss measurements.
4. CONCLUSIONS
The
corrosion of Monel-400 alloy in stagnant Arabian Gulf seawater (AGS) has been
studied by using gravimetric and electrochemical measurements in addition to
SEM/EDX investigations. The loss in weight data indicated that Monel-400
suffers both general and localized corrosion. Pitting corrosion occurred for
Monel due to the attack of corrosive species such as Cl─ to the weakest
oxygen-Alloy bond and that a selective dissolution of Ni leads to the
propagation of the formed pits as shown by SEM/EDX investigations. Cyclic
polarization, OCP and EIS measurements revealed that the increase of exposure
time decreases the corrosion current and increases the polarization resistance
as well as shits pitting and protection potentials to the more negative
direction. Chronoamperometric curves proved that the severity of pitting
corrosion increases and the uniform attack decreases for Monel with increasing
immersion time. In general, the electrochemical measurements confirmed the data
obtained by gravimetric ones that the uniform attack decreases, while pitting
corrosion increases with increasing the exposure time of Monel in the AGS
solution.
