Wall shear stress mapping in the rotating cage geometry and evaluation of drag reduction efficiency using an electrochemical method

Corrosion Science 51 (2009) 1809–1816

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Wall shear stress mapping in the rotating cage geometry and evaluation of drag reduction efficiency using an electrochemical method L. Chaal a,b, B. Albinet c, C. Deslouis a,*, Y.T. Al-Janabi d, A. Pailleret a, B. Saidani b, G. Schmitt e a

UPR 15 CNRS Interfaces et Systèmes Electrochimiques, Université P. et M. Curie, 4 Place Jussieu, 75252 Paris Cédex 05, France Laboratoire de Technologie des Matériaux et de Génie des Procédés, Equipe Electrochimie et Corrosion, Faculté de Technologie, Université A. MIRA, Béjaia 06000, Algeria c TOTAL France, Affectation: RM/STD/RECH/CReG/PR/MCER, Z.I. du port autonome du Havre, route industrielle, carrefour N°4, 76700 Rogerville, France d Saudi Aramco, Research and Development Center, Box 62, Dhahran 31311, Saudi Arabia e Laboratory for Corrosion Protection, Iserlohn University of Applied Sciences, Frauenstuhlweg 31, D-58644 Iserlohn, Germany b

a r t i c l e

i n f o

Article history: Received 22 September 2008 Accepted 8 May 2009 Available online 19 May 2009 Keywords: B. AFM B. Erosion B. Electrochemical calculation B. Potentiostatic

a b s t r a c t The local wall shear stress (WSS) mapping in the rotating cage (RC) has been obtained from measuring the diffusion current due to the electrochemical reduction of methyl viologen used as tracer system for both Newtonian and non-Newtonian solutions. The latter were achieved with oleyltrimethyl ammonium, a cationic surfactant, and sodium salicylate as counter-ion, forming thread-like micelles which induce drag reduction conditions. The maximum WSS values were detected in the middle of the coupon in agreement with observation of corroded coupons in erosion–corrosion tests. Maximum values of 65% for drag reduction were measured at the same locations. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Turbulent flow is used frequently in industry in preference to laminar flow, in order to increase the transportation efficiency of fluids e.g. oil and water and possibly to minimize its cost. Unfortunately, the energy consumption to achieve this may be too high and the flow can induce a type of material degradation, erosion–corrosion, also referred to as flow induced localized corrosion (FILC). It is well known that FILC can be mitigated by adding corrosion inhibitors acting directly on the flow in order to reduce this deleterious effect. The first step in selecting corrosion inhibitors is the evaluation at the laboratory scale, followed by testing and/or use in field. For this reason, laboratory methodologies are essential to simulate and to optimize inhibitors selection for field applications. Flow geometries commonly used for studying this phenomenon are the pipe flow loop, the impinging jet, the rotating cylinder [1,2] or the rotating disk [3]. More recently, the Rotating Cage (RC) was proposed and developed as a promising and reliable laboratory methodology for screening the susceptibility of materials or the efficiency of inhibitors under turbulent flow conditions [4–6]. The flow pattern features as well as the corrosion rates in the RC geometry have been investigated by Papavinasam et al. [7]. This flow geometry has * Corresponding author. Tel.: +33 1 44 27 41 48; fax: +33 1 44 27 40 74. E-mail address: [email protected] (C. Deslouis). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.05.013

been recently introduced as a base for an ASTM standard intended for inhibitors screening (ASTM G184-06 ‘‘Standard practice for evaluating and qualifying oilfield and refinery corrosion inhibitors using rotating cage”). The application of the RC for simultaneous determination of inhibitor efficiency and drag reduction properties of chemicals was also demonstrated in [8,9]. In fact, some cationic surfactants, when mixed with an appropriate counter-ion, are known to self-assemble into thread-like micelles in water. Such behaviour causes drastic changes in rheological properties even of very diluted solutions, allowing them to be used as drag reducing agents in turbulent flow circulating systems. It is generally believed that only this type of micelles is required for surfactant to be drag reducing [10–12]. An increasing attention has been devoted in the recent years to study this phenomenon because of its widespread occurrence and the associated high costs for its remediation. Moreover, several papers reported that such compounds have a beneficial action on the mitigation of erosion–corrosion as well as on the drag reduction effects [13,14]. Compared to other methods used for investigating erosion–corrosion, the RC has the following advantages: the simple experimental setup, the easy and low cost preparation of test coupons, the easy experimental implementation under high temperature and high pressure conditions, the possibility of simultaneous materials screening, and the reliable information on corrosion resistance and inhibitor performance. Its only disadvantage is the ill-defined local flow kinematics due to the complex flow geometry

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List of symbols diffusion coefficient (m2 s1) faraday number (F = 96485° C) diffusional current (A) surface of the microelectrode (m2) concentration (mol L1) microelectrode diameter (m) number of electrons angular velocity (rad s1)

D F Id S C1 d n

x

X

s q m

Dimensionless parameters Id d Sh ¼ nFDSC Sherwood number 1

of the cage and hence the wall shear stress could not be predicted from Navier–Stokes equations. In addition, flow velocimetry techniques like Laser Doppler Anemometry are not adequate because one needs to describe the velocity distribution with regards to the rotating blades of the cage as a reference frame. However, several attempts have been reported to formulate empirical equations for very rough estimation of geometry-related WSS at coupons in the rotating cage. It was shown only recently that this hydrodynamic parameter could be determined more precisely for this flow geometry with the polarographic method [15]. Microelectrode arrays were inserted flush with the surface of dummy test probes made from plastic and local WSS was electrochemically measured with a convenient redox system such as hexacyanoferrate (III/ IV). In a recent work, it was also shown that this electrochemical technique can be used to probe the drag reduction effect using the oxygen reduction reaction which has the advantage to avoid introducing an external redox couple in the medium [16]. In this paper, the results obtained on the WSS mapping for a Newtonian solution are first briefly reported so as to establish the reference conditions on apparatus dimension and the parameters influencing the flow. The same approach based on the use of the polarographic method is developed further using a hydrodynamic drag reducing solution containing surfactants. Mass transport data obtained by measuring the limiting current required to choose another tracer reaction, the methyl viologen reduction, which gives a better sensitivity than the oxygen reduction reaction does, due to the low oxygen solubility in water. The choice of this system was also justified by the absence of interactions with the studied micelles [17]. The investigation was focused on quantifying local WSS at different positions of the microelectrodes in the RC arrangement and on determining the efficiency of hydrodynamic drag reduction provided by cylindrical micelles. 2. Experimental 2.1. Solution The electrochemical redox system commonly used for electrochemical WSS measurements is the hexacyanoferrate system. It is nevertheless not appropriate in our experimental conditions [14,17] because oxidized and reduced species have a tendency to act preferentially as the counter-ions of the cationic surfactant used, as they are polyvalent anions and therefore spherical micelles, which have no drag reducing effect, would be formed instead of thread-like micelles. Other tracer systems have been tested. Finally, the methyl viologen dication reduction into the corresponding radical cation was chosen according to the following reaction: H3C

CH3 MV2+

rotation speed (rpm) wall shear stress (Pa) volumic weight (kg m3) kinematic viscosity (m2 s1)

CH3

H3C MV

ð1Þ

In this case, the formation of the thread-like micelles in the presence of surfactant is expected. Moreover, due to its positive charge, methyl viologen under either form cannot be inserted into the micelles. At room temperature, the MV2+ diffusion coefficient in water is around 8.1  1010 m2 s1 [18,19]. The diffusion plateau corresponding to reaction (1) is located around E = 900 mV/SCE and this potential was chosen for all measurements. The drag reducing system tested in this work was Arquad S50, a commercial product from Akzo–Nobel. It is a cationic surfactant with 50% of active matter which consists of quaternary ammonium compound, oleyltrimethyl ammonium chloride also noted as OTAC. In the product there are also 36% isopropyl alcohol and water. An appropriate counter-ion for drag reduction efficiency, sodium salicylate (NaSal) (from Aldrich) was used. The concentrations of surfactant and counter-ion were 5 and 7.5 mM, respectively. In these conditions, i.e. when the counter-ion concentration is higher than the surfactant concentration (here the ratio is 1.5) the formation of cylindrical micelles is favoured [20]. 2.2. Rotating cage (RC) characteristics The RC system used in this work was the experimental setup reported by Deslouis et al. [15]. Some modifications have been made and will be outlined hereafter. The set-up is displayed in Fig. 1. Gold microelectrodes (diameter £ = 200 lm) were elaborated from the cross section of cylindrical gold wires inserted flush with the coupons of the RC according to the procedure described below. Six coupons were installed equally spaced in the cage. The wiring connections from the microelectrodes to the electrochemical interface passed through the hollow cover and the hollow shaft and through a multi-channel mercury contactor. All measurements have been performed with 42 microelectrodes (six coupons with seven microelectrodes each). The vessel was made of Plexiglas for visual observation of the flow. In order to reduce the bulk rotation motion of the liquid exerted by the RC and hence for more reproducible hydrodynamic conditions (i.e. eliminate a vortex of growing importance with the rotation speed), a set of 18 baffles was arranged at the wall of the vessel. An additional checking that this set-up introduced no significant secondary flow was given by measuring in this vessel the diffusion currents on microelectrodes embedded in a rotating disk of the same diameter as the RC. The current values found were consistent with those predicted from the flow over an ideal rotating disk [3]. One advantage of this arrangement is that the diameter of the vessel is not critical with respect to the shear stresses applied to the cage. An example of the effect of the baffles on the gross flow is displayed in Fig. 2 with a vessel of smaller diameter (£ = 123 mm). An electrochemical interface (SOTELEM potentiostat) was used to set the microelectrodes at a potential corresponding to diffusion-controlled reduction of methyl viologen (900 mV/SCE). The measurements were performed using the three electrode set-up with a saturated calomel electrode (SCE) and a platinum

L. Chaal et al. / Corrosion Science 51 (2009) 1809–1816

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Fig. 1. The rotating cage set-up with the electrochemical interface.

Fig. 2. The rotating cage has an external diameter of 123 mm. On the left picture, the cage is baffle-free showing the formation of a deep vortex. On the right picture, the rotating cage equipped with baffles (internal diameter 100 mm) shows no vortex formation. X = 1000 rpm in both cases. The baffle system ensures the boundary conditions corresponding to an infinite vessel for a finite distance (i.e. at the inner baffles diameter).

grid, as reference and counter electrodes, respectively. They were attached to the vessel and therefore their relative position with respect to the microelectrodes changed periodically during the rotation of the cage. Prior to each experiment, the working microelectrodes were polished with emery-paper up to 1200 grade, finished with alu-

mina then degreased with acetone and rinsed with deionised water. 2.2.1. Microelectrodes configurations The microelectrodes have been positioned on the coupons at various locations either vertically or horizontally; four faces of

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Toward the

FLOW

cage axis (inner)

DIRECTION

10

2

Opposite to the cage axis (outer)

45

2.5 1

4

20 5

11

17

17

23

23

29

35

35

41

41

9.5 8 6 5 3.5

configuration1

configuration2

configuration3

Fig. 3. Positions of the inserted microelectrodes on coupons.

the coupons are exposed to the flow, the leading and leaving faces (i.e. the thinnest ones) and the inner and outer ones (i.e. oriented inwards or outwards with respect to the cage axis), see the Fig. 3 for accurate locations (measured at ±0.2 mm) of the microelectrodes. The total height of the coupons is 63 mm but their fastening to the upper and lower covers of the Rotating Cage hides both ends so that the usable height (i.e. that actually exposed to the flow) is 45 mm (see the Fig. 3). For sake of comparison with the results of a previous study [15] the following configurations for the microelectrodes locations on the coupon have been chosen: Configuration 1: Configuration 2: Configuration 3:

Vertically on the leading face of the coupon and at 0.5 mm from the outer edge. Vertically on the outer face of the coupon and at 1 mm from the outer edge of the leading face. Horizontally in the middle of the coupon on the outer face of the coupon.

Depending on the rotation direction, two results could be obtained with the same arrangement: in one direction the electrodes were in the leading face region, in the other direction, in the trailing face region. 2.2.2. Microelectrodes preparation The coupons with flush-mounted microelectrodes in the surface were prepared according to the following steps: – A printed circuit board (thickness 1.4 mm) was chosen as a dummy coupon (63  10  1.4 mm) in which cataphoretically coated gold wires were inserted flush with the coupon surface. The electrical connections were designed by a photolithography process (Fig. 4). – On the backside of the printing circuit board the microelectrodes were soldered to the electrical connections.

Fig. 4. Details of the insertion of microelectrodes.

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– An epoxy resin layer was then applied on the back of the printing circuit board up to 2 mm full thickness of every coupon, so as to achieve proper insulation of the contacts. In such a way, each coupon could be equipped with up to seven micro-electrodes.

2.3. AFM experiments

Table 1 Wall shear stress for different flow geometries. Flow geometry

Wall shear stress

Cylindrical pipe [22] Cylindrical elbow [22] Rotating disc [23,24] Rotating cylinder [21,22]

s ¼ 0:03325qu 7=4 m1=4 R1=4 c s ¼ 0:01225qm0:11 u 2:11 R0:29 r 0:18 c c 1=5 8=5 9=5 s ¼ 0:027qm Rd x 0:3 1:7 1:4 s ¼ 0:0791qm x Rc  2 1:818 r1 0 s ¼ 0:1578qm0:182 Ur0:182 r0 0 3 qDm s ¼ 1:5614  Sh  d2

Impinging jet [25]

Atomic Force Microscopy (AFM) measurements were performed in air using a Molecular Imaging instrument in the acoustic resonant mode. This set-up was composed of a Pico-LE base equipped with a micro-positioning device aimed at the accurate positioning of the AFM tip in the x–y plane of the sample, a large zone scanner (100  100 lm) bearing a photo detector and the AFM nose adequate for TM-AFM experiments. A PicoScan 2100 controller connected to a computer was used to drive the scanner and to collect the data generated by the laser impact on the photo detector. In this purpose, rectangular silicon cantilevers bearing conical silicon tips were used. Their resonance frequency was in the range 280–365 kHz and their spring constant between 25 and 50 N m1. The surfactant solution specimens were prepared for AFM imaging by applying a small drop of the studied solution on mica substrates coated with gold film. The substrates were carefully rinsed with alcohol and double distilled water before use. 2.4. Drag reduction efficiency The drag reduction efficiency was calculated by using the following equation:

DR% ¼

sw  s  100 sw

ð2Þ

where sw and s are the values of the WSS in the absence and in the presence of surfactant, respectively.

Rotating cage [15]

allows reaching high WSS comparable to those effective in industrial conditions. 3. Results and discussion 3.1. WSS mapping As the influence of parameters on flow system is quite complex in the conditions of RC, geometry, effects have been investigated in this work only by changing the position of microelectrodes on the coupons. For some selected microelectrodes positions, the WSS were calculated by using Eq. (3). The area of each microelectrode was calculated by measuring precisely the microelectrode diameter with a scanning electron microscope. The distribution of WSS in the absence and in the presence of surfactant was measured for the three configurations described in the experimental part and are discussed below. For a better comparison, the data reported in the mappings below were normalized with respect to the maximum WSS value found during the measurements among the three configurations. 3.2. WSS for configuration 1

2.5. Calculation of WSS To characterize the flow of a system, geometry-independent fluid flow parameters such as the Reynolds number are usually evaluated. This is the case for Newtonian fluids. For Non-Newtonian viscoelastic fluids such as surfactant solutions forming cylindrical micelles, it is more appropriate to use the WSS, s [1,21]. In the RC, Eq. (3) was developed to quantify the flow intensities encountered at coupons; the method of calculation was explained in detail elsewhere [14,15]:

s ¼ 1:5614:Sh3 

qDm 2

d

ð3Þ

where Sh is the dimensionless Sherwood number, q the density (kg m3), D the diffusion coefficient of the electrode active species (m2 s1), m the kinematic viscosity (m2 s1) and d the diameter of microelectrode (m). The Sherwood number is defined from the diffusion current measured on a circular microelectrode as previously defined:

Sh ¼

Id d nFDSC 1

ð4Þ

Id is the diffusion current, n the number of exchanged electrons (here n = 1 according to Eq. (1)), F is the Faraday number (F = 96485° C), S is the microelectrode area (m2), C1 the tracer bulk concentration (mol m3). From Sh measurements, one is therefore able to compare the RC efficiency to that of different flow geometries issued from literature database [15,17,21–25] and reported in Table 1. Quantitative simulations not given here show that for typical solution parameters values and flow characteristics, the RC system

Fig. 5(a and b) represent the WSS mapping over the microelectrodes located vertically on the leading face of the coupon and at 0.5 mm from the outer edge in the absence and in the presence of surfactant, respectively. As expected, the highest WSS were measured for this electrodes arrangement. The maximum value was reached at about 15 mm from the upper cover and it was chosen for normalizing all subsequent data. This result is in qualitative agreement with the results of the previous study [15] which locate this maximum WSS around the middle of the coupon and also with most of empirical observations on metallic coupons submitted to extreme conditions of erosion–corrosion [26]. The lowest values have been detected near the bottom cover. The position of the maximum is strongly dependent on the medium composition. One can observe a decrease in the values of the WSS in the presence of surfactant (Fig. 5b). In Fig. 6, the drag reduction efficiency obtained with this configuration of electrodes at various rotation speeds shows a maximum value around 65% corresponding to the location of the maximum value of WSS in the absence of surfactant and thus confirms the results of the rheological studies [27]. When the cage is rotated in the opposite direction, the previous electrodes were close to the trailing edge (Fig. 7). For this configuration, the values obtained are much lower than those encountered at the leading edge. 3.3. WSS for configuration 2 For the vertical arrangement when the electrodes were placed vertically on the outer face of the coupon and at 1 mm from the outer

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(a)

1.2

max

0.8

/

0.6 0.4 0.2

35 30

0.8 0.6 0.4

1600 1200 800 25

20 15 Dist 10 5 ance from uppe cove r r/m m

400

0.0 45 40

00

2000 1600 1200 800 400

35 30 25

20 15 Dista nce 10 cove from up per r/m m

5

0 0

Fig. 7. Distribution of the wall shear stresses obtained on the gold microelectrodes placed on the trailing edge close to the inner corner in the absence of surfactant. Electrolyte: MV2+ 5 mM/KCl 0.1 M.

(b)

1.2

0.2

/r pm

0.0 45 40

1.0

/r pm

1.0

max

1.2 1.1 1.0 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0

/

1.2

1.2 1.1 1.0

1.0

0.90 0.80

1.2

0.70

max

0.8

(a)

0.60 0.50 0.40

1.0

0.30

0.6

0.20

/

0.10

0.8

0

0.2

τ/τmax

0.4

400 5

0.4 0.2

00

2000 1600 1200 800 400

0.0 40 35 30 25 20 D

/r pm

om u 10 cove pper r/m m

0.6

15 is 10 uppe tance fr r cov om er / mm

Fig. 5. Distribution of the wall shear stresses obtained on the gold microelectrodes placed on the leading edge close to the inner corner (configuration 1) in the absence and in the presence of surfactant. (a) MV2+ 5 mM/KCl 0.1 M, (b) MV2+ 5 mM/KCl 0.1 M/NaSal 7.5 mM/OTAC 5 mM.

Ω

0.0 45 40 Dist35 30 25 ance 20 15 fr

/r pm

1600 1200 800

5

0

0

1.2

(b)

70 =1000 rpm =1200 rpm = 1600 rpm

1.0 0.8 0.6 0.4

40

0.2

30

0.0 40 35 30 25 20

0

5

10

15

20

25

30

35

40

45

Distance from upper cover / mm Fig. 6. Drag reduction efficiency obtained in strong flows on gold microelectrodes placed at different distance on the section of the coupons (configuration 1) for different rotation speeds of the cage.

edge of the leading face (leading face or trailing face according to the rotation direction of the cage), the general aspect of the WSS map-

Dist

1600 1200 800 400

15

10 ance 5 cove from up per r/m m

0

0

/r pm

50

Ω

τ/τmax

DR / %

60

Fig. 8. Distribution of the wall shear stresses obtained on the gold microelectrodes positioned in vertical arrangement at the outer surface of the coupon close to the leading edge (configuration 2) in the absence and in the presence of surfactant. (a) MV2+ 5 mM/KCl 0.1 M, (b) MV2+ 5 mM/KCl 0.1 M/NaSal 7.5 mM/OTAC 5 mM.

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70 60

DR(%)

50 40 30 20 10

( ( (

0 0

5

10

15

20

25

30

) ) )

1000 rpm 1200 rpm 1600 rpm

35

40

Distance from upper cover (mm) Fig. 9. Drag reduction efficiency obtained in strong flows on gold microelectrodes placed at different distances on the outer surface of the coupon close to edge (configuration 2) for different rotation speed of the cage.

0.6 0.4 0.2

/r pm

8

2000 1600 1200 800 400

6

4 Dist a the e nce from dge / mm

Ω

0.0 10

2 0

0

(b)

1.2 1.0 0.8 0.6 0.4 0.2

8

6 Dist the

4 ance 2 from edg e/m m

2000 1600 1200 800 400

/r pm

0.0 10

00

Ω

In horizontal arrangement on the outer surface of the coupon the situation looks different (Fig. 10). A concave shaped distribution surface is obtained with a local minimum in the middle of the coupon. Anyhow, the highest measured WSS are lower than in the absence of surfactant. These values are by a factor 1.7 lower compared to those obtained in configuration 1. Although the difference between these two electrode arrangements (configuration 1) and close to the outer side (configurations 2 and 3) is quite low, the WSS distributions and therefore the flow regimes showed significantly different behaviours. Two aspects can be taken into account to explain this. Firstly, a flow interference of the coupon creates a turbulent region which hits the leading edge not necessarily symmetrically. On the other hand, the flow regimes inside and outside of the rotating cage are completely different and this also influences the flow on the leading edge close to the corners. The qualitative variations, which can be significant in some conditions, between WSS distributions in the situations with and without micelles might be explained by the fact that the kinematics of the gross flow can be affected by the presence of the surfactant. By merging those results with those in [14], the local WSS was shown to depend on many factors, including the rotation speed of the cage, the viscosity of the fluid and the geometry of the system (e.g. geometric position of the microelectrodes on the coupon surface, the coupon thickness, the number and arrangement of coupons in the rotating cage, the presence of baffles). Concerning the first two parameters only, the results demonstrated that the local WSS increases with the rotation speed of the cage as expected, with an exponent close to 3/2 and with the liquid viscosity with an exponent close to 3/4. Drag reduction by surfactants is assigned to the presence of thread-like micelles formed at low concentrations in the pres-

(a)

0.8

τ/τmax

3.4. WSS for configuration 3

1.0

τ/τmax

ping was similar to that for configuration 1. Only one set of data corresponding to the results of the leading edge (Fig. 8) is shown. Highest values of WSS were obtained again around 15 mm from the upper cover. However, these values are by a factor 1.6 lower that those obtained for the configuration 1. Fig. 9 allows estimating the efficiency of drag reduction which amounts up to 50% with this configuration.

Fig. 10. Distribution of the wall shear stresses obtained on the gold microelectrodes positioned in horizontal arrangement at the outer surface of the coupon (configuration 3) in the absence and in the presence of surfactant. (a) MV2+ 5 mM/KCl 0.1 M, (b) MV2+ 5 mM/KCl 0.1 M/NaSal 7.5 mM/OTAC 5 mM.

ence of specific counter-ions containing a lipophilic part such as the ‘‘NaSal”, used in this work. However, except from the cryo-TEM technique used so far [20], these structures have not been directly observed. AFM has also recently been suggested to reveal these structures after complete evaporation of the solvent in so far as the substrate used for these observations exerts weak interactions with the micelles [28]. It was undoubtedly shown that micellar structures are obtained and hence preserved on the gold substrates after a complete evaporation of the solvent. Indeed, rod-like structures deposited randomly at the surface of gold could be observed in Fig. 11. Topographic AFM image allowed to determine accurately their geometrical characteristics like diameter and length in the nanometer range in agreement with the data reported in reference [28] by the authors. The existence of cylindrical micelles responsible for drag reduction is then confirmed. As for linear polymers efficient from the standpoint of drag reduction, it is expected that the length of the micelles is a decisive parameter for the efficiency of micelles with respect to drag reduction. The structures, visible in Fig. 11 corresponding to the cylindrical micelles, show lengths significantly high (i.e. in the lm range) in agreement with their drag reduction efficiency.

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2.

3.

4.

5.

detailed mapping can be implemented by using microelectrode arrays. The methyl viologen redox couple proved to be an appropriate tracer system for polarographic measurements and a good alternative to the conventional hexacyanoferrate system for measurements in Newtonian as well as in surfactant solutions. The highest measured wall shear stresses were detected at the leading face of the coupon near the outer edge and at 1/3 distance to the upper cover of the cage. The expected cylindrical structure for OTA+/Sal micelles (the drag reducing surfactant system studied) was confirmed by AFM observations in air. The drag reducing efficiency of surfactant was estimated in electrochemical wall shear stress measurements and values up to 65% were obtained for the higher investigated wall shear stresses.

Acknowledgement This work received partial financial support from the ‘‘TASSILI” program between Algeria and France (N° 03 MDU 572). References

Fig. 11. AFM images (5  5 lm) obtained from micellar solution containing OTAC (5 mM) and NaSal (7.5 mM) deposited on gold coated mica. (a) Phase, (b) topography.

4. Conclusions The main findings of the study are as follows: 1. Wall shear stress of specimens in the rotating cage can be easily calculated thanks to the polarographic technique and a

[1] S. Papavinasam, R.W. Revie, M. Attard, D. Dermoz, K. Michaelian, Corros. Eng. Sect. 5 (10) (2003) 897–912. [2] S. Papavinasam, A. Doiron, G. Shen, R.W. Revie, CORROSION’ 2004, NACE International, Houston/Texas, 2004 (Paper 04622). [3] S. Goldstein, Proc. Camb. Philos. Soc. 31 (Part 2) (1935) 232. [4] G. Schmitt, W. Bruckhoff, CORROSION’ 88, NACE International, Houston/Texas, 1988 (Paper 88357). [5] S. Papavinasam, R.W. Revie, M. Attard, A. Dermoz, J.C. Donini, K. Michaelian, CORROSION’ 2001, NACE International, Houston/Texas, 2001 (Paper 01061). [6] R. Hausler, G. Schmitt, CORROSION’ 2004, NACE International, Houston/Texas, 2004 (Paper 04402). [7] S. Papavinasam, A. Doiron, R.W. Revie, CORROSION’ 2003, NACE International, Houston/Texas, 2003 (Paper 03333). [8] S. Papavinasam, M. Attard, R.W. Revie, J. Bojes, CORROSION’ 2003, NACE International, Houston/Texas, 2003 (Paper 03333). [9] C. Deslouis, Electrochim. Acta 48 (2003) 3279. [10] H.W. Bewersdorff, H. Thiel, Appl. Sci. Res. 50 (1993) 347. [11] R. Oda, J. Narayanan, P.A. Hassan, C. Manohar, R.A. Salkar, F. Kern, S.J. Candau, Langmuir 14 (1998) 4364. [12] Z. Lin, Y. Zheng, H.T. Davis, L.E. Scriven, Y. Talmon, J.L. Zakin, J. Non-Newtonian Fluid Mech. 93 (2000) 363. [13] G. Schmitt, Mater. Corros. 52 (2001) 329. [14] L. Chaal, C. Deslouis, A. Pailleret, B. Saidani, Electrochim. Acta 52 (2007) 7786. [15] C. Deslouis, A. Belghazi, Y.T. Al-Janabi, P. Plagemann, G. Schmitt, CORROSION’ 2004, NACE International, Houston/Texas, 2004 (Paper 04727, published in the framework of a collaborative Research Agreement G02E01010 between AOC Overseas Company B.V., TOTALFINAELF France, CNRS France & Fachhochschule Südwestfalen, Iserlohn University, Germany). [16] M.S. Boutoudj, A. Ouibrahim, F. Barbeu, C. Deslouis, S. Martemianov, Chem. Eng. Process. Process Intensification 47 (5) (2008) 793–798. [17] L. Chaal, M.S. Boutoudj, A. Ouibrahim, B. Saidani, R.P. Nogueira, C. Deslouis, J. Non-Newtonian Fluid Mech. 121 (2004) 143. [18] N. Winograd, T. Kuwana, J. Am. Chem. Soc. 92 (1) (1970) 224. [19] R.D. Webster, R.A.W. Dryfe, J.C. Eklund, C.W. Lee, R.G. Compton, J. Electroanal. Chem. 402 (1–2) (1996) 167. [20] B. Lu, X. Li, L.E. Scriven, H.T. Davis, Y. Talmon, J.L. Zakin, Langmuir 14 (1) (1998) 8. [21] K.D. Efird, E.J. Wright, J.A. Boros, T.G. Hailey, Corrosion 49 (12) (1993) 992. [22] D.C. Silverman, Corrosion 40 (5) (1984) 220–226. [23] Th.V. Karman, ZAMM 1 (1921) 233–252. [24] G. Liu, D.A. Tree, M.S. High, Corrosion 50 (1994) 584. [25] (a) F. Giralt, O. Trass, Can. J. Chem. Eng. 53 (1975) 505–511; (b) F. Giralt, O. Trass, Can. J. Chem. Eng. 54 (1976) 148–155. [26] G. Schmitt, W. Bruckhoff, K. Faessler, G. Blümmel, CORROSION’ 90, NACE International, Houston/Texas, 1990 (Paper 90023). [27] B. Lu, X. Li, J.L. Zakin, Y. Talmon, J. Non-Newtonian Fluid Mech. 71 (1997) 59. [28] L. Chaal, F. Pillier, B. Saidani, S. Joiret, A. Pailleret, C. Deslouis, J. Phys. Chem. B 110 (43) (2006) 21710.