Influence of flow on the corrosion inhibition of St52-3 type steel by potassium hydrogen-phosphate

Corrosion Science 51 (2009) 1828–1835

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Influence of flow on the corrosion inhibition of St52-3 type steel by potassium hydrogen-phosphate H. Ashassi-Sorkhabi *, E. Asghari Electrochemistry Research Laboratory, Physical Chemistry Department, Faculty of Chemistry, University of Tabriz, 29 Bahman Blvd., Tabriz 51664, Iran

a r t i c l e

i n f o

Article history: Received 24 December 2008 Accepted 10 May 2009 Available online 18 May 2009 Keywords: A. Steel B. EIS B. Polarization C. Neutral inhibition C. Oxygen reduction

a b s t r a c t Influence of hydrodynamic conditions on the corrosion of St52-3 type steel rotating disc electrode, RDE, in 3.5% NaCl and its corrosion inhibition using K2HPO4 have been studied. Results showed that by rotating the electrode in blank and inhibited solutions, corrosion current density, icorr, increased, corrosion potential, Ecorr, shifted toward more positive values and charge transfer resistance, Rct, decreased. The inhibition efficiencies increased with electrode rotation rate. This increase was attributed to the enhanced mass transport of inhibitor molecules toward the metal surface and formation of more protective films. Little decrease of efficiencies at higher rotation speeds was probably because of the separation of protective films due to high shear stresses. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The seawater with approximately 3.5% salinity is used by many industries such as shipping, offshore oil and gas production, power plants and coastal industrial plants. The main use of seawater is for cooling purposes but it is also used for fire-fighting, oil-field water injection and for desalination plants [1–5]. Corrosion and corrosion inhibition of iron and steels in NaCl containing solutions have fundamental academic and industrial importance. Phosphorous compounds are commonly used to inhibit metals corrosion in aqueous electrolytes. Their use is relatively risk free due to their low toxicity [6–8]. For example, sodium monofluorophosphate (Na2PO3F) [9], calcium monoflourophosphate (CaPO3F) [10], combination of zinc phosphate/molybdate (Actirox) and calcium ion exchange silica (Shieldex) [11] and phosphate anions [12,13] have been used to decrease the corrosion rate of different metals in aqueous solutions. In the industries, the equipments are often submitted to aggressive conditions (flow conditions, high temperature, etc.), involving the early apparition of corrosion. Thus corrosion and corrosion inhibition studies under hydrodynamic conditions are very important for industrial applications. However, there are few studies in literatures about the effect of hydrodynamic conditions on performance of organic and inorganic inhibitors under laminar or turbulent flow. The effect of hydrodynamic conditions on corrosion and corrosion inhibition of some metals and alloys such as copper [14], * Corresponding author. Tel.: +98 411 3393136; fax: +98 411 3340191. E-mail addresses: [email protected], [email protected] (H. AshassiSorkhabi). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.05.010

steel [15–18], Cu–Ni alloys [19–23] and nickel–aluminum–bronze (NAB) [24] has been investigated in different media. Ochoa et al. [25] have reported the influence of flow on corrosion inhibition of carbon steel RDE by fatty amines in association with phosphonocarboxylic acid salts. They showed that the inhibitor film formed at high rotation rate (2000 rpm) was thinner than that formed at low rotation rate (100 rpm). They also deduced that increasing the electrode rotation rate leads to an increase in the cathodic current densities, a decrease in the anodic current densities and corrosion potential shifted toward anodic direction. Cáceres and co-workers [26] have reported the electrochemical parameters and corrosion rate of carbon steel in different concentrations of un-buffered NaCl solutions under hydrodynamic condition using a superposition model. Hamdy et al. [27] have investigated corrosion and erosion–corrosion resistance of mild steel in sulfide-containing NaCl aerated solutions. Corrosion inhibition of carbon steel using Na3PO4 in simulated interstitial solution of concrete (pH 12.5) contaminated by chloride ions has been studied. The influence of steel pretreatment time and electrode rotation rate on its corrosion behavior was investigated. It was found that for steel electrode immersed in inhibited solution, the inhibition efficiency decreased as electrode rotation rate increased, but when the steel specimen was pretreated in inhibitor containing solution for different immersion times, the inhibition efficiency was not significantly affected by rotation speed [28]. In our previous work, we studied the effect of hydrodynamic conditions on corrosion inhibition of steel in acidic solution using methionine as a green organic inhibitor. It was found that the inhibition performance of methionine improves with increasing the electrode rotation rate [29].

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The aim of present research is to investigate the influence of fluid flow on corrosion inhibition of St52-3 type steel in NaCl solution using hydrogen phosphate ion. The St52-3 steel is widely used in most of industrial and constructional works, such as mineral processing equipments, petrochemical and oil industries, power plants, storage tanks etc. Electrochemical methods including potentiodynamic polarization and electrochemical impedance spectroscopy were performed to identify the effect of flow on inhibition efficiencies; scanning electron microscopy, SEM, technique was also used for morphological studies. 2. Experimental 2.1. Working electrode construction The working electrode was made of a St52-3 steel rod which was mounted in a polyester resin in such a way that only the end side of electrode was left uncovered and the exposed area was 0.283cm2 (Ø = 6 mm). This assembly was machined to form a rotating disc electrode RDE. An AFMSRX rotator (PINE Instruments Co.) was used to control the electrode rotation speed between 0 and 2400 rpm. 2.2. Electrochemical measurements All electrochemical tests were performed with conventional three-electrode configuration: a Pt rod as counter electrode, an Ag–AgCl (3 M KCl) as reference electrode and the constructed St52-3 RDE, with chemical composition (wt.%) Mn: 1.180%; Si: 0.490%; C: 0.175%; Al: 0.067%; Cu: 0.054%; Ta: 0.052% and Fe: Balance, as working electrode. Just before each test the working electrode was polished by SiC paper up to 2500 grade to have a mirror-finished surface, then degreased with ethanol and rinsed with distilled water. The temperature was maintained at 25 ± 1 °C, using a Memmert Thermostat. The aggressive solution was 3.5% NaCl and the inhibitor was potassium hydrogen phosphate. Inhibitor in three concentrations, 104, 5  104 and 103 M, was examined in static conditions and the highest inhibition efficiency was obtained for 103 M of K2HPO4. Therefore, a concentration of 103 M inhibitor was selected for studies under hydrodynamic conditions. Autolab PGSTAT30 potentiostat–galvanostat and frequency response analyzer (FRA) were used to perform the polarization and electrochemical impedance spectroscopy (EIS) measurements. The

potentiodynamic polarization measurements were performed by sweeping the potential from 1000 to +250 mV versus open circuit potential (OCP) at a scan rate of 2 mVs1. Potentials sweeping from cathodic region ensured that any oxide present prior to the experiments was reduced and the corrosion rate measurements are performed under rust-free conditions [26]. The obtained polarization curves were corrected for uncompensated IR drop using the GPES 4.9005 software. Then, the corrosion current density, icorr and corrosion potential, Ecorr, were obtained for steel corrosion in both blank (3.5% NaCl) and inhibited (3.5% NaCl + 103 M inhibitor) solutions at different electrode rotation rates. The corrosion behavior and the variation of inhibition efficiencies with rotation rate were also studied using EIS measurements. The EIS tests were performed after immersion of working electrode in test solution for 60 min at open circuit potential with frequency ranged between 10 kHz and 3 mHz, using a 10 mV (rms) sinusoidal potential perturbation. EIS data were analyzed using Zview2 software and fitted to an appropriate equivalent circuit model. All electrochemical experiments were repeated at least three times and the average values were reported. 2.3. Scanning electron microscopy (SEM) studies In order to evaluate the effect of hydrodynamic conditions on surface morphology of samples, scanning electron microscopy (SEM) technique was used. 3. Results and discussion 3.1. Effect of inhibitor concentration Fig. 1 shows the Nyquist plots for corrosion of St52-3 in the presence of three concentrations of K2HPO4 (103, 5  104 and 104 M) under static conditions. It is clear that the corrosion resistance of the steel sample increases with the increase of inhibitor concentration. According to these results the concentration of 103 M K2HPO4 was selected for studies under hydrodynamic conditions. 3.2. Potentiodynamic polarization measurements Fig. 2a and b shows typical potentiodynamic polarization curves of St52-3 samples under static conditions and at some electrode

5000.00 1 mM K2HPO4 0.5mM K2HPO4 4000.00 0.1 mM K2HPO4

-Z″ (Ω.cm2)

3000.00

2000.00

1000.00

0.00 0.00

2000.00

4000.00

6000.00

8000.00

-1000.00

Z′ (Ω.cm2) Fig. 1. Nyquist plots for corrosion of St52-3 specimen in solutions containing different concentrations of K2HPO4 under static conditions.

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a

1.00E+00

0 rpm 200 rpm 1800 rpm

1.00E-01 1.00E-02

i (Acm-2)

1.00E-03 1.00E-04 1.00E-05

1.50E-03 1.00E-03

1.00E-06

5.00E-04

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

1.00E-07

0.00E+00 0.00 -5.00E-04 -1.00E-03 -1.50E-03

1.00E-08 -1.7

-1.5

-1.3

-1.1

-0.9

-0.7

-0.5

-0.3

-0.1

E (V vs. Ag/AgCl)

b

1.00E+00

0 rpm 200 rpm 1800 rpm

1.00E-01 1.00E-02

i (Acm-2)

1.00E-03 1.00E-04 1.00E-05

4.00E-04 3.00E-04 2.00E-04

1.00E-06

1.00E-04

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00E+00 0.00 -1.00E-04

1.00E-07

-2.00E-04 -3.00E-04

1.00E-08 -1.6

-4.00E-04

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

E (V vs. Ag/AgCl) Fig. 2. Typical potentiodynamic polarization curves for St52-3 type steel in (a) 3.5% NaCl and (b) 3.5% NaCl + 103 M K2HPO4 under static conditions and some rotation speeds (insets: The linear polarization plots, I–E, for St52-3 type steel in (a) 3.5% NaCl and (b) 3.5% NaCl + 103 M K2HPO4 under static conditions and some rotation speeds).

rotation speeds in blank and inhibited solutions, respectively. The insets in this figure indicate the linear representation of polarization curves. It is well known that the main anodic sub-process during the corrosion of steel is:

Fe oxidation : Fe ! Fe2þ þ 2e

ð1Þ

But the cathodic sub-process should be considered in details. It is clear from Fig. 2a that, in the case of static conditions, corrosion potential of the steel sample is very negative so that the cathodic reaction is water reduction:

Water reduction : 2H2 O þ 2e ! H2 þ 2OH

ð2Þ

In this case, a pseudo passive region is seen in polarization plots (the plateau observed in anodic branch of polarization plot) [30]. However, it was observed that the rotation of electrode led to a significant shift in Ecorr toward more positive potentials (Fig. 2a). Therefore, under hydrodynamic conditions the dominant cathodic reaction is oxygen reduction and the observed cathodic current

plateaus can be attributed to diffusion-controlled reduction of dissolved oxygen [31,32]. The cathodic stationary current, iL, measured in the potential of 0.78 V vs. Ag/AgCl increases with square root of electrode angular velocity, x, in accordance with the Levich’s criteria (Fig. 3). The Levich equation for a fully smooth RDE in laminar flow is [33]:

iL ¼ 0:62nFD0:66 t0:166 C b x0:5

ð3Þ

where n, F, D and Cb are the stoichiometric number of electrons exchanged in the mass transport limited reaction, the Faraday constant, diffusion coefficient and the bulk or interfacial concentration of the electroactive species, respectively. Calculation of n from the experimental Levich slope (4.36  105 A cm2 s0.5 rad0.5) and published data for 298 K with C O2 = 2.48  107 mol cm3 (oxygen concentration in water), tH2 O = 102 cm2 s1 (water kinematic viscosity) and DO2 = 1.9  105 cm2 s1 (oxygen diffusion coefficient) [34] showed a two-electron reduction mechanism.

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H. Ashassi-Sorkhabi, E. Asghari / Corrosion Science 51 (2009) 1828–1835 8.00E-04 7.00E-04 y = 4.36E-05x R2 = 0.9771

iL (Acm-2)

6.00E-04 5.00E-04 4.00E-04 3.00E-04 2.00E-04 1.00E-04 0.00E+00 0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

ω 0.5(rad0.5.s-0.5) Fig. 3. The limiting current densities (iL) for oxygen reduction reaction versus square root of angular velocity of the St52-3 RDE in 3.5% NaCl obtained in E = 0.78 V vs. Ag/ AgCl.

A simplified relationship for the complete reduction of oxygen, (Eq. (4)), involves an overall exchange of four electrons resulting in the production of hydroxyl ions or, at low pH, water molecules.

O2 þ 2H2 O þ 4e ! 4OH

ð4Þ

The generalized scheme describing the possible reduction mechanisms involving intermediate peroxide is shown in the following reaction [35]: k6 (+4e)

O2

(b)

diff.

k7(+2e)

O2 *

k10 (+2e) H2O2 (a)

OHk-10 (-2e)

k-7 (-2e) k-9

k9

ð5Þ

k8 H2O2 * diff. H2O2 (b)

The indices indicate: (a) species adsorbed at the electrode surface, (b) species considered to be within the bulk of the electrolyte, and * species within the immediate vicinity of the electrode surface.

It is clear that the complete, four-electron reduction of oxygen may occur through a direct or indirect route. Hydroxyl ions or water molecules can be products of a single four-electron step or the result of cumulative two-electron reduction steps where oxygen is reduced to peroxide which in turn is reduced to hydroxyl ions [35]. Hence, if k6, k10, k9 are small and k8, k9 large, oxygen reduction may only involve an overall two-electron change. Clearly, this explanation is simplified. The kinetics of the reversible electrochemical and chemical reactions along with the rates of the adsorption/desorption processes may be equivalent to the active reduction processes. This will result in a complication of the overall reduction mechanism. Because of these considerations, the kinetics of oxygen reduction is expected to be very specific to the system under study. The character of the substrate, surface condition, temperature and electrolyte conditions all have an influence over each step in the reduction mechanism [35]. The corrosion parameters including corrosion current density, icorr, corrosion potential, Ecorr, and the inhibition efficiency, gp%, were also obtained from the polarization curves (Fig. 2) and are given in Table 1. The icorr for a non-ideal diffusion controlled polarization plot is obtained from the intersection of linear extrapolations for both anodic and cathodic branches at OCP [30,36]. The inhibition efficiencies were calculated using the following equation [21]:

gp % ¼

  icorr  icorr  100  icorr

ð6Þ

Table 1 Electrochemical parameters for St52-3 dissolution in the absence and presence of inhibitor in 3.5% NaCl at different rotation rates. Rotation rate (rpm)

Reynolds numbers

icorr (lA/cm2)

Ecorr (V vs. Ag/AgCl)

gp%

3.5% NaCl

0 200 600 1200 1800 2400

0 843.4 2530.2 5060.4 7590.6 10120.8

45.6 ± 4.1 122 ± 20.5 280 ± 20.0 330 ± 24.9 315 ± 20.9 323 ± 24.7

0.986 0.571 0.569 0.520 0.510 0.501

– – – – – –

3.5% NaCl + 103 M K2HPO4

0 200 600 1200 1800 2400

0 843.4 2530.2 5060.4 7590.6 10120.8

7.40 ± 0.3 18.1 ± 1.1 20.0 ± 2.9 23.3 ± 1.8 25.0 ± 2.0 27.7 ± 2.0

0.951 0.831 0.781 0.780 0.800 0.844

84 85 93 93 92 91

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where icorr and icorr are the corrosion current densities in the absence and presence of inhibitor, respectively. The Reynolds numbers, Re, are also calculated using Eq. (7) [33,37] and presented in Table 1.

Re ¼

r 2 :x

jt

< 105 for laminar flow

ð7Þ

where r, x and jm are the radius of the RDE active area in mm, angular velocity in rad/s and kinematics viscosity in stokes (mm2/s), respectively. The Reynolds numbers show that the flow is laminar in all studied rotation speeds. From Table 1, it is clear that the corrosion current density for corrosion of St52-3 in blank solution increases under hydrodynamic conditions compared to stagnant solution; but at higher rotation speeds, the corrosion rate is not controlled by diffusion and its variation with rotation rate is less significant. The significant shift of Ecorr toward more positive potentials is due to the increasing of mass transfer of oxygen from electrolyte to the metal surface, causing an increase in limiting current density. Fig. 4 illustrates in a schematic way the effect of hydrodynamic conditions on the Ecorr and icorr. The anodic sub-process, dissolution of iron, was well described in terms of a pure charge transfer controlled reaction, while the cathodic sub-process, oxygen reduction on iron, was well described in terms of mixed mass transfer and charge transfer control. An increase in electrode rotation rate raises the limiting current that causes an enhancement of the specimen corrosion rate [26]. It can be also seen from Table 1 that the presence of phosphate in stagnant solution (0 rpm) decreases the icorr values and shifts the Ecorr toward more positive potentials. Phosphates are known as indirect anodic inhibitors, which act through prevention of anodic dissolution of metal only in the presence of oxygen. According to the literatures, the inhibition effect of phosphate ions may be due to specific passivation of steel by the deposition of metal phosphate [38]. The accumulation of a poorly soluble iron phosphate on steel surface creates conditions favorable for ordinary oxide passivation [38]. Hancock et al. [39] showed that the protective film formed on steel surface in the presence of phosphate ions is consisting of a mixture of c-Fe2O3 and FePO32H2O. However, hydrodynamic conditions may also affect the inhibition of metal corrosion. It is clear from data presented in Table 1 that the gp% increases when the electrode rotates. Under different

E

Eca E2corr Rotation

Stagnant

E1corr

rotation speeds, the gp% is approximately constant. However, a little decrease is observed at high rotation speeds. In fact, flow conditions may have several different effects on inhibition performance: (i) Flow can increase mass transport of inhibitor molecules toward metal surface, which causes the formation of more insoluble phosphate on metal surface. This effect can improve the inhibition performance [29,40]. (ii) The high shear stress resulted from high velocity can also separate the protective films from metal surface, thus the inhibition efficiency may decrease [40]. The balance of above mentioned effects lead to changes of inhibition efficiency (g%) with electrode rotation rate. 3.3. EIS studies Fig. 5a and b shows typical Nyquist plots at some rotation speeds in blank and inhibited solutions, respectively. Evolution of physical parameters concerning corrosion process is discussed using an electric equivalent circuit model for the alloy–solution interface. When the steel electrode is immersed in the blank solution, the Nyquist plots show a depressed capacitive loop, which have an asymmetrical form in high frequency region. The asymmetrical shape of the capacitive loop can be attributed to a diffusion impedance process with a finite thickness of diffusion layer [28]. An appropriate equivalent circuit, fitting data properly, is shown in Fig. 6a in which Rs is solution resistance, Rct is charge transfer resistance, CPE shows constant phase element related to the double layer capacitance and Zdiff represents the finite-diffusion impedance. Because of the depressed shape of capacitive loop, a constant phase element, CPE, including a component Qdl and a coefficient, n, is required. This latter quantifies different physical phenomena like surface inhomogenity resulting from surface roughness, impurities, porous layer formation etc. The double layer capacitance is calculated by the following relationship [41,42]:

C dl ¼ Q dl ðxmax Þn1

In this equation, xmax = 2pfmax, where fmax represents the frequency at which imaginary value reaches a maximum on the Nyquist plot. In contrast with that observed for the blank solution, in the presence of inhibitor, the Nyquist plots show a depressed non asymmetrical capacitive loop. Thus, the equivalent circuit shown in Fig. 6b was used to fit these spectra. By fitting the experimental data for all rotation rates, using the Zview2 software, impedance parameters were obtained that are given in Table 2. As can be seen, the Rct values are decreased as the electrode rotation rate increased. Such an observation is in agreement with the increase of icorr with rotation rate enhancement, which was found from potentiodynamic polarization measurements. A similar trend is observed in the presence of inhibitor. The inhibition efficiencies can also be calculated from Rct values using the following equation [21]:

gz % ¼ ð

Ean i1corr

i2corr

Log i

Fig. 4. Schematic diagram illustrating the effect of rotation on limiting current, corrosion current and corrosion potential.

ð8Þ

Rct  Rct Þ  100 Rct

ð9Þ

where Rct and Rct are the charge transfer resistances in absence and presence of inhibitor, respectively. It is clear from data presented in Table 2 that there is an acceptable agreement between inhibition efficiencies obtained from polarization, gp%, and EIS measurements, gz%. As described in Section 3.2, fluid flow has several effects on the behavior of inhibitor. It can improve the performance of inhibitor through increasing the mass transport, and worsen the inhibition

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a

700.00 0 rpm 200 rpm 1800 rpm

600.00

-Z″ (Ω.cm2)

500.00

400.00

300.00

200.00

100.00

0.00 0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

Z′ (Ω.cm2)

b

5000 0 rpm

4500

200 rpm 1800 rpm

4000

-Z″ (Ω.cm2)

3500 3000 2500 2000 1500 1000 500 0

0

1000

2000

3000

4000

5000

6000

7000

8000

Z′ (Ω.cm2) Fig. 5. Typical Nyquist plots for St52-3 type steel in (a) 3.5% NaCl and (b) 3.5% NaCl + 103 M K2HPO4 under static conditions and some rotation speeds.

are dominant and an increase in g% is observed. But at higher speeds the interference of high shear stresses causes a little decrease in inhibition efficiencies. 3.4. SEM studies

Fig. 6. The electrochemical equivalent circuits fitting EIS data for St52-3 in (a) 3.5% NaCl and (b) 3.5% NaCl + 103 M K2HPO4.

action by separation of protective films. The balance of these opposite effects leads to the overall changes of inhibition efficiency (g%) with rotation rate. Therefore, as the electrode rotates, the increase of inhibitor supply on metal surface cause an increase in inhibition efficiency. Under these circumstances, the beneficial effects of flow

The surface morphology of steel sample immersed in 3.5% NaCl in the absence and presence of K2HPO4 was studied by scanning electron microscopy (SEM). Fig. 7 shows the SEM images from typical areas of St52-3 specimens at static conditions and under electrode rotation at 600 rpm. Under static conditions, the surfaces obtained in the absence and presence of phosphate ions show that the latter one is smoother. It means that the presence of phosphate ions has decreased the corrosion of specimen. At 600 rpm, there is a large amount of corrosion products on metal surface in blank solution. It is due to the increase of steel corrosion rate under hydrodynamic conditions. Whereas, in the presence of inhibitor a quite smooth surface was obtained (Fig. 7d). The smooth surface of metal at 600 rpm in the inhibited solution deals with the inhibition of the metal corrosion under hydrodynamic conditions.

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Table 2 Electrochemical impedance parameters for St52-3 corrosion in the absence and presence of inhibitor in 3.5% NaCl at different rotation rates. Rotation rate (rpm)

Rct (X cm2)

Cdl (lFcm2)

n

Ws (X cm2/s0.5)

gz%

3.5% NaCl

0 200 600 1200 1800 2400

1200.0 ± 45.7 235.4 ± 14.3 120.2 ± 15.8 113.2 ± 6.0 110.0 ± 5.7 106.4 ± 6.0

79.5 57.6 56.1 53.0 47.3 47.0

0.71 0.68 0.65 0.63 0.59 0.58

288.7 242.6 220.5 211.0 152.6 75.73

– – – – – –

3.5% NaCl + 103 M K2HPO4

0 200 600 1200 1800 2400

7600.3 ± 91.0 3373.4 ± 42.0 1452.9 ± 29.4 1205.3 ± 31.5 1100.8 ± 20.0 976.4 ± 18.6

27.5 38.7 45.9 47.0 49.5 50.1

0.82 0.83 0.83 0.84 0.83 0.84

– – – – – –

84 93 92 91 90 89

Fig. 7. SEM images from typical areas of St52-3 samples in (a) 3.5% NaCl, X = 0 rpm; (b) 3.5% NaCl + 103 M K2HPO4, X = 0 rpm; (c) 3.5% NaCl, X = 600 rpm; (d) 3.5% NaCl + 103 M K2HPO4, X = 600 rpm.

4. Conclusion The influence of flow on corrosion inhibition of St52-3 steel using K2HPO4 in 3.5% NaCl solution was studied. It was shown that all corrosion parameters were dependent on the electrode rotation rate. Ecorr had a strong shift toward more positive values under electrode rotation. This shift was attributed to the increased mass transfer of oxygen from bulk of solution to the electrode surface. However, in the presence of phosphate ions, the displacement of Ecorr was not as high as the blank. The inhibition efficiencies calcu-

lated from both polarization and impedance measurements increased as the electrode rotated. Then, the efficiencies remained almost constant under low rotation speeds but showed a little decrease at high speeds. It was demonstrated that the flow has several different effects on inhibition performance. By increasing the inhibitor mass transport from bulk to electrode surface, flow raises the inhibition efficiency; but at higher rotation rates high shear stress eliminates the protective films from metal surface causing a decrease in efficiency. Therefore the balance of these factors will be predominant.

H. Ashassi-Sorkhabi, E. Asghari / Corrosion Science 51 (2009) 1828–1835

Acknowledgements The authors thank Eng. M. Rahimi for preparation of steel samples and University of Tabriz for financial support.

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