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Transcript of 1-s2.0-S0c635-main
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Wear 256 (2004) 537544
An examination of the electrochemical characteristics oftwo stainless steels (UNS S32654 and UNS S31603)
under liquidsolid impingement
Xinming Hu, Anne Neville
Corrosion and Surface Engineering Research Group, School of Engineering and Physical Sciences,
Heriot-Watt University, Edinburgh, Scotland, UK
Abstract
The erosioncorrosion resistance of high alloy stainless steel UNS S32654 and standard stainless steel UNS S31603 has been assessedunder liquidsolid impingement conditions. The electrochemical characteristics of the two stainless steels have been examined via free
corrosion potential measurements, anodic polarisation, linear polarisation and potentiostatic control in erosioncorrosion.
It has been shown in this paper that high alloy stainless steel UNS S32654 exhibits better corrosion and erosioncorrosion performance
than lower grade alloy UNS S31603. A general linear relationship between two electrochemical parameters (Ecorr and Rp) has been shown
in this study. A critical solid loading between 60 and 100 mg/l, at which there is a transition from corrosion to erosioncorrosion for the
two stainless steels under different conditions, has been determined.
2003 Elsevier B.V. All rights reserved.
Keywords: Stainless steels; Corrosion; Erosion; Electrochemical
1. Introduction
It is common for components subjected to liquid flows
containing solid particles to experience high degradation
rates. In particular, for components where the flow experi-
ences a sudden diversion (e.g. in pumps, valves, tees and
elbows in pipework) high rates of material loss can occur
as a result of combined mechanical erosion and abrasion
and electrochemical corrosion.
Flow-induced corrosion describes a process whereby the
corrosion rate of a material is increased in a moving fluid.
It was reviewed by Weber [1] in 1992 who defined the ef-
fects of flow velocity to be in three categories. At low flow
velocities and in the absence of induced convection, natural
convection is responsible for mass transfer and can affect
corrosion rates. When induced convection leads to increased
mass transfer at moderate flow velocities the corrosion rate
can increase but in this regime there are no mechanical ef-
fects of flow. At high velocities mechanical flow effects can
result and in this case the damage mechanisms become in-
creasingly complex. The phenomenon of erosioncorrosion
has received widespread study over the last two decades
Corresponding author. Tel.: +44-131-451-4365;
fax: +44-131-451-3129.
E-mail address: [email protected] (A. Neville).
where the focus has been on assessment of material perfor-
mance under varying conditions and assessment of how ero-sion affects corrosion rates and vice versa e.g. [26]. During
this time it has become generally appreciated that significant
interactions exist between electrochemical and mechanical
effects and these result in sometimes very large synergistic
[79] or additive [10] effects where the combined pro-
cesses result in much greater material loss than the sum
of their individual components. In ASTM G119-93 (1998)
guidelines are given for the calculation of synergism between
wear and corrosion [11]. Erosioncorrosion maps are of-
ten used to give a visual representation of these interactions
and to isolate regimes, in terms of flow parameters, which
are erosion-dominated or corrosion-dominated [12,13].
Electrochemical techniques are often used to assess the
effect of tribological processes in tribo-corrosion [1417].
The free corrosion potential was monitored by Huang and
Chuang [18] in a rotating arrangement where a load was
applied to the surface to form an abrasive contact. Under
rotation with no load the free corrosion potential showed a
trend of ennoblement and in contrast once a load of 3.92 N
was applied a shift in the active direction (signifying loss of
passivity) was observed. Oltra et al. [19] coupled electro-
chemical measurements with acoustic emission measure-
ments to monitor erosioncorrosion in aggressive slurries
to capture the mechanical response of the surface and the
0043-1648/$ see front matter 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0043-1648(03)00563-5
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538 X. Hu, A. Neville / Wear 256 (2004) 537544
electrochemical response. Acoustic emission in isolation
enabled a critical flow velocity on steel in sulphuric acid
to be determined [20]. In the case of stainless steels and
other similar materials which rely on their passive film for
corrosion protection in static environments, the effect of
the flow of a solid-containing stream of liquid can be to
cause mechanical removal of the protective layer and chargetransfer is temporarily enhanced. As stated by Li et al. [21]
in circumstances where the surface material is removed by
impingement of a liquidsolid stream the generation rate of
fresh oxide and the repassivation ability of the material are
two parameters that are of importance. Such depassivation
and repassivation effects are known to be of importance
also in wear-accelerated corrosion caused by sliding wear
in a corrosive media [21].
In this paper the overall erosioncorrosion damage rates
of the superaustenitic stainless steel (UNS S32654) and
austenitic stainless steel (UNS S31603) are presented but
the main focus of the work is to look at the corrosion
rates and the detailed electrochemical response of the alloysunder impingement conditions. Electrochemical measure-
ments were used to assess the transition for different regimes
(flow-induced corrosion to erosioncorrosion). The paper
also enables more detailed understanding of the generic dif-
ferences between a high grade and a standard austenitic
stainless steels to be obtained.
2. Materials and experimental methods
Two stainless steels are included in this study and their
compositions are given in Table 1. The two stainless steelsare chosen to represent a super grade (UNS S32654) and a
standard austenitic (UNS S31603). The additional alloying
of Mo, N and Cr are known to be important for localised
corrosion resistance in static saline environments [22] and
in this work a comparison is made of their resistance to
flow-induced corrosion and erosioncorrosion. Also shown
in Table 1 are the average Vickers microhardness values
taken from 10 measurements on each surface.
The impingement apparatus comprised a submerged
liquidsolid jet generated using a recirculating rig and the
electrochemical apparatus used for in situ monitoring as
described in [23]. The rig comprised a dual nozzle system
each nozzle diameter being 4 mm. The exit velocity of the
jet for this study was kept constant at 17 m/s which is a
relatively high velocity for applications of stainless steels in
pump impeller and casing etc. The nozzle-to-specimen dis-
tance was kept constant at 5 mm. The area of the specimen
Table 1
Nominal compositions and microhardness of UNS S32654 and UNS
S31603
Cr Ni Mo Mn C N Hv
UNS S32654 24 22 7.3 3.5 0.01 0.5 337
UNS S31603 1618 7 3.5 0.8 0.03 265
is 4cm2. The solid loading of silica sand in the 3.5% NaCl
fluid was varied between 5 and 6000 mg/l. The solid load-
ing was tested during every test by extracting water samples
(three replicates) from the nozzles, filtering and weighing
the solids collected. The size distribution of the silica sand
is given in Fig. 1. The temperature of the liquid was kept
at 18
C by using a cooling system. For all tests the angleof impingement was 90. Tests were typically conducted
for 8 h for three times for each of the solid loading and the
specimens were weighed before and after the experiment
to determine the total material loss. Errors of results are
determined from three replicated experiments in this study.
The in situ corrosion rate was measured using a
three-electrode electrochemical cell comprising a Ag/AgCl
reference electrode connected by means of a salt bridge and
a platinum counter electrode. DC anodic polarisation tests
involved scanning the potential of the working electrode
(the specimen under examination) from the free corrosion
potential (Ecorr) in the more noble (positive) direction at a
fixed rate of 25 mV/min. The potential was scanned in thepositive direction until the current flowing in the external
circuit between the working and counter electrodes reached
a value of 500A/cm2. The anodic polarisation tests were
started after 30 min exposure to the impinging jet. To mea-
sure changes in the corrosion rate as a function of solid load-
ing, linear polarisation tests were conducted. In these tests
the potential of the working electrode (the sample under
erosioncorrosion) was shifted at a rate of 15 mV/min from
0.02 V negative to the free corrosion potential to 0.02 V pos-
itive to the free corrosion potential. The applied potential is
then a linear function of the current density in the external
cell. Changes in the polarisation resistance (Rp) can be cal-culated from the slope E/i. In this work the assumption
is made that the change in the slope and hence the change in
Rp is inversely proportional to the change in corrosion rate.
The free corrosion potential (Ecorr) of the two alloys were
measured in situ on adding solids into the recirculating sys-
tem with a range of solid loading between 0 and 3000 mg/l,
the sampling rate was 1 Hz.
In order to determine the critical solid loading at
which there is a transition from flow-induced corrosion
to erosioncorrosion and to investigate the anodic current
transients in liquidsolid impingement conditions, potentio-
static tests were carried out at an applied (anodic) potential
of 0 V (Ag/AgCl) and the solid loading was incrementally
increases in the range 06000 mg/l. The current density was
monitored for 5 min at each solid loading and the data was
recorded at a rate of 1 Hz.
3. Results
3.1. Total weight loss (TWL)
In Fig. 2, the TWL measured after exposure to the im-
pinging jet for 8 h at 17 m/s is shown as a function of solid
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Fig. 1. Distribution of sand size for erosioncorrosion tests.
loading for the two stainless steels. For the high alloy stain-
less steel UNS S32654 the TWL and solid loading exhibit
an exponential relationship which has also been confirmed
by other high alloy stainless steels [24]. It is clear from the
figure that high alloy stainless steel UNS S32654 has shown
superior overall erosioncorrosion resistance compared with
UNS S31603. This is in accordance to results presented
under cavitationerosion and wear-corrosion conditions by
other workers [25,26].
Fig. 2. Weight loss tests on materials in erosioncorrosion after 8 h, 17 m/s, 18 C in 3.5% NaCl.
3.2. Anodic polarisation
In situ electrochemical monitoring using DC anodic po-
larisation enables the corrosion characteristics under the in-
fluence of liquidsolid impact to be determined. The com-
plex anodic polarisation behaviour of high alloy stainless
steels has been reported previously [24]. The in situ corro-
sion current (icorr) can be obtained via the Tafel extrapolation
technique [27] and this enables the material loss due to pure
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Fig. 3. icorr determined from AP for UNS S32654 and UNS S31603 at various solid loadings.
electrochemical effects (C) to be determined. The icorr values
are shown in Fig. 3 and it is clear that UNS S31603 has much
greater anodic current densities at the three solid loadings.
3.3. Linear polarisation
Anodic polarisation tests enabled the corrosion current
density to be determined. However, this method is generally
Fig. 4. K (Equation 2) determined on UNS S32654 and UNS S31603 in erosioncorrosion at 18 C, 17 m/s in 3.5% NaCl.
destructive given the extent of potential change imposed on
the surface during the measurement, and as such only one
measurement per experiment can be made. Linear polari-
sation is an alternative method for measuring the corrosion
rate, which permits rapid corrosion rate measurements and
can be used to monitor the corrosion rate in various con-ditions. Using this method, the applied potential is approx-
imately a linear function of current density in the region
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X. Hu, A. Neville / Wear 256 (2004) 537544 541
adjacent to Ecorr within which the tested specimens are
not suffering serious corrosion attack due to high applied
potential as used in Tafel extrapolation method. From mea-
surement of the E/i relationship over the small potential
range from 20 mV more negative to 20 mV more positive
than Ecorr the changes in polarisation resistance (Rp) as a
Fig. 5. Free corrosion potential at various solid loadings for (a) UNS S31603 and (b) UNS S32654 at 18 C, 17 m/s in 3.5% NaCl.
function of solid loading can be determined from Eq. (1):
E
i=
ac
2.3icorr(a + c)= Rp (1)
where a and c are the Tafel constants [16] for the anodic
and cathodic reactions, respectively (Fig. 4).
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In this work it has been shown that there is little change in
the absolute value of the grouping Kas defined in Eq. (2) as
solid loading increases but there is a substantial difference
between the two different stainless steels as shown in Fig. 7
K =ac
2.3(a + c)(2)
As a result the variation in Rp can be used to determine the
change in icorr for the two materials. And these results of Rpwill be correlated with the free corrosion potential results in
the discussion section. In agreement with the measurement
of icorr by Tafel extrapolation, the corrosion resistance at
all solid loadings is lower on UNS S31603 than on UNS
S32654. However, there is a difference between the ratios
of corrosion rate determined from linear polarisation and
anodic polarisation and this is probably due to the experi-
mental processes. Anodic polarisation tests were conducted
on a fresh specimen surface at one constant solid load-
ing, while the linear polarisation measurements were made
during a continuous experimental process where the solidsare progressively added to the recirculating system. The
specimen corrosion rate, determined by linear polarisation
is therefore the corrosion rate as a result of the progressive
increase in solids. This may therefore vary the specimen
surface and implied that some history effect is evident
which leads to anomalies in the corrosion rates.
3.4. Free corrosion potential (Ecorr) measurements
Fig. 5a and b shows free corrosion potential measure-
ments made on UNS S31603 and UNS S32654 while pro-
Fig. 6. Mean values of current density as a function of solid loading for UNS S32654 and UNS S31603, at 0 V, 17 m/s, 18 C in 3.5% NaCl.
gressively adding solids at 10 min intervals to reach a solid
loading of 3129 mg/l. The following trends emerge on both
materials:
TheEcorr shifts in the negative (active) direction on adding
solids.
Ennoblement is observed at the lowest solid loadings
(most evident on UNS S32654 as shown in Fig. 5b) dur-ing the 10 min periods but as the solid loading increases
the ennoblement effect is no longer evident.
The shift in the active direction on adding solids is reduced
as the solid loading is increased.
The oscillations (noise) in the Ecorr values are enhanced
as the solid loading increases.
3.5. Potentiostatic measurements
The current density, while the sample is maintained at
a potential of 0 V (Ag/AgCl) for a period of 150 min with
solids progressively added into the system was monitored.The mean value of the current density is plotted as a function
of solid loading for the two stainless steels in Fig. 6. It is
clear that there is an increase in current density as solids
is increased and UNS S31603 exhibits higher values in all
conditions. At lower solid loadings (
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Fig. 7. Potentiostatic measurements as a function of solid loading for (a) UNS S31603 and (b) UNS S32654 at 0 V, 17 m/s, 18 C in 3.5% NaCl.
4. Discussion
The overall erosioncorrosion resistance of higher alloy
stainless steel UNS S31654 is greatly improved over UNS
S31603 in both mechanical and electrochemical terms. From
Table 1 it is clear that UNS S32654 is a harder material which
improves the mechanical erosion resistance of this alloy. In
this study UNS S31603 also exhibits a much greater cor-
rosion rate than UNS S32654. Development of high-grade
stainless steels has been driven by the need to improve re-
sistance to localised corrosion attack and loss of passivity
in the form of pitting and crevice corrosion [22]. This has
been achieved through additions of N, Mo and some other
elements which have been found to promote passivation. In
erosioncorrosion conditions it has been shown here and
elsewhere [25] that even high grade alloys can be in the ac-
tive corrosion regime, casting doubt on the benefits of the
additional alloying for resisting degradation. However, it is
clear in this study that there is a substantial benefit in terms of
lowering the electrochemical charge transfer during erosion.
Better resistance to corrosion under erosioncorrosion con-
ditions (which shows UNS S32654 better resistant than UNS
S31603) correlates to better overall performance measured
by TWL for the two alloys. This implies that corrosion and
its subsequent effect on erosion is of great importance and it
is more significant for austenitic stainless steel UNS S31603.
In terms of electrochemical corrosion the most readily
available parameter which can be measured is Ecorr and this
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was done in situ during tests under liquidsolid erosion con-
ditions with various solids. However, generally Ecorr cannot
be used as a direct way to quantify the corrosion rate. Ecorrand Rp have been measured at different solid loadings and
these data enabled the relationship between the two electro-
chemical parameters for UNS S32654 and UNS S31603 to
be determined under erosioncorrosion conditions. Detailedstudied on the Tafel constants have shown that the values of
K (Eq. (2)) remain almost constant at different solid load-
ings as shown in Fig. 4. It is noticed that in this study only
solid loading was varied as the erosioncorrosion parame-
ter and other important parameters have not been taken into
consideration such as temperature, impinging velocity and
impingement angle.
The current study is also focused on determining the crit-
ical solid loading where the material degradation regime
changes from corrosion or flow-induced corrosion to a more
severe stage of erosioncorrosion which can result in greater
material loss. Flow-induced corrosion has been reported
[28,29] to result in an enhancement of corrosion rate due toimpingement effect on active alloys. According to the cur-
rent work carried out on the stainless steels at a velocity of
17 m/s, the two stainless steels exhibit low current densities
under potentiostatic control under solid-free and low solid
loading (