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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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X. Hu, A. Neville / Wear 256 (2004) 537544 539

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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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 (

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