Electrochimica Acta 51 (2006) 4011–4018

Hydrodynamic effect on the behaviour of a corrosion inhibitor film: Characterization by electrochemical impedance spectroscopy Pascale Bommersbach, Catherine Alemany-Dumont ∗ , Jean-Pierre Millet, Bernard Normand Laboratoire de Physico-Chimie Industrielle, INSA de Lyon, Bˆat. L´eonard de Vinci, 20, avenue Albert Einstein, F 69621 Villeurbanne Cedex, France Received 28 July 2005; received in revised form 28 October 2005; accepted 4 November 2005 Available online 27 December 2005

Abstract Influence of hydrodynamic conditions on the behaviour of an inhibitor film used in the high speed mechanical field is studied. Experimentations are realized with a Rotating Disk Electrode (RDE). Electrochemical Impedance Spectroscopy (EIS) is carried out at the corrosion potential and also at anodic and cathodic over voltages under various rotation rates and after different immersion times. EIS technique is used to determine the beneficial effect of hydrodynamic conditions on the film formation. This technique characterizes also inhibitive layer modifications for high electrode rotation speeds and after long immersion times. Different analyses, XPS and AFM, were carried out and have confirmed film structure evolution with flow conditions. © 2005 Elsevier Ltd. All rights reserved. Keywords: Corrosion inhibitor; EIS; Hydrodynamic conditions; XPS; AFM

1. Introduction In the industry of mechanical machining, cutting tools and cooling systems are often submitted to aggressive conditions (turbulent flow, high temperature, etc.), involving the early apparition of corrosion. The addition of inhibitor in lubricating or rinsing solutions presents nowadays good results to prevent corrosion, and then to prolong lifetime of systems. The evolution of regulation concerning industrial waste implies the formulation of new inhibitors compatible with environmental norms. The aim of this work is to evaluate flow effect on the efficiency of an environment-friendly inhibitor (composed of amine and carboxylic acid) used in this field. Few studies deal with the influence of hydrodynamic conditions on inhibitors performance in laminar and turbulent flows [1,2]. Some authors have recently studied the inhibitor behaviour under multiphase turbulent flow [3,4]. The intensity of the flow seems to decrease the inhibitor efficiency. Ochoa et al. have studied the behaviour of the interface carbon steel/NaCl 200 mg L−1 in addition with inhibitor (fatty amines associated with phos-



Corresponding author. Tel.: +33 4 72 43 81 87; fax: +33 4 72 43 87 15. E-mail address: [email protected] (C. Alemany-Dumont). 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.11.020

phonocarboxylic acid salts) using a rotating disk electrode [5]. Particularly, they have shown that properties of the protective layer were dependent on the electrode rotation rate: current densities decrease when the electrode rotation rate increases. A previous work was undertaken by focusing the study on inhibition mechanisms using Electrochemical Impedance Spectroscopy associated to surface analyses [6]. Two phenomena intervene in the inhibition mechanisms: an adsorption induced by amine and a complexation between carboxylic acid and ferrous ions. Amine acts synergistically with carboxylic acid to form a three dimensional organic layer on the steel surface which incorporates corrosion products. Works have revealed that a good efficiency was maintained with time for 0.5 wt.% of inhibitor. For 0.3 wt.% of inhibitor, the adsorbed layer is sensitive to a water-uptake risk and the film would swell. The influence of temperature was also evaluated: above 60 ◦ C, the absence of film was connected to an important increase of iron dissolution which disturbs protective layer formation. Below 50 ◦ C, a film morphology evolution would induce the decrease of the film efficiency. The goal of this present work was focused on the importance of hydrodynamic effects on formation, structure and stability of the inhibitor film. The influence of flow was investigated by electrochemical impedance spectroscopy with a rotating disk electrode. Electrochemical results were compared with topog-

4012

P. Bommersbach et al. / Electrochimica Acta 51 (2006) 4011–4018

Table 1 Chemical composition (wt.%) of SAE 1038 C Mn Si S P Ni Cr Mo Cu Al

2.2. AFM studies 0.36 0.66 0.27 0.02 0.015 0.02 0.21 0.02 0.22 0.06

raphy analysis (AFM) and film chemical composition analysis (XPS). 2. Experimental conditions 2.1. Electrochemical experiments Corrosion tests were performed with classic three-electrodes configuration: a graphite rod as counter-electrode, a saturated calomel electrode (SCE) as reference electrode. A SAE 1038 sample constitutes the studied material, which composition and properties are very close to those of XC38 (non-allied steel of fine nature). The chemical composition of the carbon steel specimen is shown in Table 1. The rod was embedded in epoxy resin delimiting the working surface of 0.28 cm2 (∅ = 6 mm). The cylinder rod was previously covered by cataphoretic epoxyamine base paint to avoid electrolyte infiltrations between metal and resin interface. Deposit conditions were described elsewhere [7]. This system was machined to form a rotating disk electrode and constituted the working electrode. Just before each test, the surface was polished by emery-paper up to 2400 grade, rinsed with distilled water and dried under an air flow. The aqueous electrolyte (called solution A) was prepared with de-ionized water and 100 ppm of each salt: Na2 SO4 , Na2 CO3 and NaCl. These contents correspond to a “synthetic” corrosive solution that increases the severity of the test over that produced by distilled water [8]. Studied inhibitor consists on a carboxylic acid and a tertiary amine. Electrolyte volume was fixed to 100 mL. According to previous studies, 0.3 wt.% of inhibitor would be sufficient to obtain a good protective efficiency after 2 h of immersion in inhibitive solution but a content equal to 0.5 wt.% is necessary to maintain a good efficiency at least until 60 h. In this present study inhibitor content was fixed to 0.5 wt.% for all tests [6]. Electrochemical Impedance Spectra were obtained with Gamry equipment (model FAS 1), controlled by software EIS 300 and DC 105 (Bioritech). Impedance data were collected at the open-circuit potential and also at different over voltages over a frequency range between 65 kHz and 10 mHz at 10 points per decade; a sine wave with 10 mV amplitude was applied. The disk rotation speed was varied from 0 to 3200 rounds per minutes (rpm); for these rotation rates, the laminar flow was conserved.

Topographic studies on the electrode surfaces were monitored by Atomic Force Microscopy (AFM, Digital Instrument 3100). AFM pictures were realized in tapping mode. The two analyzed samples were immersed during 24 h in solution A with 0.5 wt.% of inhibitor, the first one without agitation (sample 1), and the second one under a rotating rate of 3200 rpm (sample 2). On the second sample, three different localizations were analyzed respectively at the centre of the disk electrode, at its periphery and at an intermediate place, in order to determinate topographic profile evolution of the surface. 2.3. XPS studies Analyses were carried out on a disk mild steel SAE 1038 sample, immersed during 2 h in solution A with 0.5 wt.% of inhibitor under a rotating rate equal to ω = 3200 rpm. The sample was then removed from solution and dried under an air flow. X-ray photoelectrons spectra were recorded on a ESCALAB 200R-VG Scientific Spectrometer, using a Al KL2,3 X-ray source (1486.6 eV). Binding energies were referred to the binding energy of C 1s peak at 284.6 eV. Two studies were performed (with an analyzing angle α = 90◦ ): the first one was realized at the centre of the disk and the other one on the edge of the disk to compare the composition of the film at these two different localizations. 3. Results and discussion 3.1. Qualitative study of flow effect A corrosion potential evolution versus time constitutes a good qualitative method for monitoring interface modification between a metal and its environment [9]. The anodic nature of the inhibitor [10] induces a continuous shift towards nobler potentials, indicating the spontaneous adsorption of inhibitor on metallic surface (Fig. 1). Open circuit potential evolution versus long time (250 h) with and without rotation is presented in Fig. 2a. At ω = 0,

Fig. 1. Open circuit potential evolution vs. time during 2 h for mild steel SAE1038 in solution A + 0.5 wt.% of inhibitor at different rotation rates.

P. Bommersbach et al. / Electrochimica Acta 51 (2006) 4011–4018

4013

Fig. 3. Impedance Nyquist plots for mild steel SAE1038 in solution A in addition with 0.5 wt.% of inhibitor at different rotation rates after 2 h of immersion.

Fig. 2. (a) Open circuit potential evolution vs. time, and (b) focused plot during the first 40 h for mild steel SAE1038 in solution A + 0.5 wt.% of inhibitor.

corrosion potential stabilization occurs after 10 h of immersion. Even if potential values fall down of about 60 mV several times (behaviour connected to a slight deterioration of the film which forms again spontaneously [11]), film efficiency is keeping up for about 200 h. For a great electrode rotation rate (ω = 3200 rpm), film behaviour is noticeably different. On the focused plot (Fig. 2b), a marked potential increase is observed during the first 5 h, which can be related to the inhibitor transport increasing or to the mass transport increase of dissolved oxygen from electrolyte to interface (this last phenomenon increases the diffusion limit current and then moves potential towards nobler values). Potential drop (connected to film deterioration) occurs at once earlier and more frequently for this high rotation rate, revealing a film structure evolution or a partial rupture of the film under stresses applied by the flow.

spectra obtained in solution A in addition with 0.5 wt.% exhibit one capacitive loop: its size increases with rotation rate. The relatively high charge transfer resistance value (till 100 k cm2 ) reveals a satisfactory protective efficiency. At corrosion potential, both anodic and cathodic reactions occur simultaneously. The evolution observed can be due to a modification of film properties (related to anodic behaviour) or to the evolution of the mechanism of oxygen diffusion-convection (related to cathodic behaviour). In order to distinguish these two contributions impedance spectra were recorded at anodic and cathodic over voltages for different rotation rates. The values of these over voltages were established from preliminary potentiodynamic study (Fig. 4). The anodic contribution (located on the anodic plateau) was obtained by polarizing at +300 mV/Ecorr and the cathodic one by polarizing at 700 mV/Ecorr . In the cathodic domain, for the three considered rotation rates, impedance spectra describe two capacitive loops (Fig. 5). The high frequencies loop can be attributed to the charge transfer which takes place at interface, and the

3.2. Quantitative study of flow effect on the film formation To quantify flow influence on film formation, EIS was carried out at corrosion potential for different rotation rates, after 2 h of immersion (Fig. 3). Whatever the rotation rate, all impedance

Fig. 4. Cathodic and anodic current density–potential curves for mild steel SAE1038 in solution A in addition with 0.5 wt.% of inhibitor at different rotation rates.

4014

P. Bommersbach et al. / Electrochimica Acta 51 (2006) 4011–4018

Fig. 5. Impedance Nyquist plots at −700 mV/Ecorr for mild steel SAE1038 after 2 h of immersion in solution A in addition with 0.5 wt.% of inhibitor at different rotation rates.

low frequencies loop is relative to the diffusion-convection of dissolved oxygen from electrolyte to interface. As the studied inhibitor does not act on the cathodic process [6] (Fig. 6) low values of polarization resistance are observed in Nyquist plot. A rotation rate enhancement induces a size decreasing of the two loops. This observation was already mentioned and interpreted in literature [12]. In Fig. 7a, low frequencies in anodic domain exhibit a straight line inclined at an angle of 45◦ to the resistive axis: this is related to a diffusion phenomenon, which is better characterized on Bode plots presenting the phases angle (Fig. 7b). Same diffusion behaviour is obtained for all tested rotation rates. This characterizes the iron ions diffusion through the inhibitor film (diffusion does not depend on the rotation speed). Results obtained in cathodic and anodic fields inform that capacitive loop size increasing at corrosion potential with the

Fig. 7. Impedance diagrams (a) Nyquist plots and (b) Bode plots, at +300 mV/ Ecorr for mild steel SAE1038 after 2 h of immersion in solution A in addition with 0.5 wt.% of inhibitor at different rotation rates.

rotation rate can be connected to a film properties improvement or to an increase of the film formation speed. Evolutions of physical parameters concerning film and corrosion processes are discussed from an electric equivalent circuit (EEC), which models the interface. The general EEC presented in Fig. 8a is constituted of elements relative to the inhibitor film (Cf : film capacitance and Rf : film resistance) and the interfacial processes (RtA : anodic charge transfer resistance, RtC : cathodic charge transfer resistance, Cdl : double layer capacitance, W: diffusion impedance, and W␦ : diffusion convection impedance). A constant phase element composed of a component Qdl and a coefficient α is required. This latter quantifies different physical phenomena like surface inhomogeneousness resulting from surface roughness, impurities, inhibitor adsorption, porous layer formation, etc. [13,14]. Thus, capacitance is deduced from the following relation (1): Cdl = Qdl (ωmax )α−1

Fig. 6. Cathodic current density–potential curves for mild steel SAE1038 in solution A without and in addition with 0.5 wt.% of inhibitor, ω = 1600 rpm.

(1)

In this equation, ωmax = 2πfmax , where fmax represents the frequency at which imaginary value reaches a maximum on the Nyquist plot.

P. Bommersbach et al. / Electrochimica Acta 51 (2006) 4011–4018

4015

Fig. 9. Impedance spectra obtained for SAE1038 immersed in solution A + 0.5 wt.% of inhibitor at different rotation rates for two different immersion times. Fig. 8. (a) General electric equivalent circuit. (b) Simplified electric equivalent circuit.

So, after 2 h of immersion and at corrosion potential, following assumptions may be put forward. Accordingly, some elements will be removed: • impedance spectra evolution with rotation is connected to the only film evolution; • iron ions diffusion through the film is quantifiable only at anodic over voltage; • the film contribution is only quantifiable after 6 h of immersion [6]. So the cathodic contribution (RtC and W␦ ), the iron ions diffusion contribution (W) and the film one (Cf and Rf ) can be removed from the general EEC. Then, the simplified circuit is presented in Fig. 8b. Parametrical adjustment of this circuit with experimental impedance spectra gives access to the double layer capacitance and charge transfer resistance evolutions with rotation rate. Respective values evolution of these two parameters with rotation is presented in Table 2; the error of the fitted parameters is appreciated by the chi-squared χ2 factor, which value is specified for each fit. The decrease of Cdl and the increase of RtA with electrode rotation rate is related to an active surface decrease connected to the inhibitor adsorption. This last observation shows the beneficial effect of hydrodynamic conditions on film formation after 2 h of immersion. Table 2 Values of RtA and Cdl resulting from the fitting of EEC with experimental spectra for different rotation rates ω (rpm)

RTa (k cm2 )

α

Cdl (␮F cm−2 )

χ2 (error factor)

0 400 1600 3200

50 65 100 120

0.81 0.88 0.83 0.88

104 80 71 69

9.80E−4 6.96E10−4 9.56E10−4 7.99E10−4

The error factor χ2 is also specified.

3.3. Influence of hydrodynamic conditions after long time Impedance spectra obtained for 2 and 10 h of immersion are presented in Fig. 9. After 10 h, they still describe only one capacitive loop, but its size decreases with rotation rates that presage inhibitor efficiency decreasing. This observation shows a flow effect evolution on the film with immersion time (the inverse effect is observed at 2 h). Figs. 10a and 11a show impedance diagrams after 140 and 250 h, for respectively, ω = 0 and ω = 3200 rpm. Nyquist Impedance spectra obtained after 140 h are different from those at 2 and 10 h, showing an evolution of film properties with time. On Bode plots shown in Figs. 10b and 11b, a phase widening more or less marked according to the rotating rate after 10 h of immersion can be attributed to a second time constant relative to a supplementary contribution: the film one. When immersion time is still prolonged (140 and 250 h), the apparition of a constant angle equal to 45◦ is observed at low frequencies; as described previously this angle value is characteristic of the diffusion process of iron ions through the inhibitor film. In the high frequencies range, modulus value, describing the electrolyte resistance, decreases with immersion time. This decreasing could be associated to the electrolyte conductivity evolution due to the greater account of iron ions in solution. The metallic dissolution is more marked at ω = 3200 rpm. All these previous observations show a deterioration of the film, which would become weaker under great rotation rate of electrode. Thus hydrodynamic conditions would act not only on film formation kinetic (at short times) but also on film structure and stability at longer times. 3.4. AFM analyses Fig. 12 obtained by optical microscopy shows working electrode surface immersed for 24 h in solution A with 0.5 wt.% of inhibitor under a constant rotation rate of 3200 rpm. This visualization describes the typical profile of a rotating disk electrode in laminar flow (logarithmic spirals issued from the rotation axis

4016

P. Bommersbach et al. / Electrochimica Acta 51 (2006) 4011–4018

Fig. 10. Impedance diagrams (a) Nyquist plots, (b) Bode plots, for SAE1038 in solution A + 0.5 wt.% of inhibitor for different immersion times at ω = 0.

Fig. 11. Impedance diagrams (a) Nyquist plots, (b) Bode plots, for SAE1038 in solution A + 0.5 wt.% of inhibitor for different immersion times at ω = 3200 rpm.

3.5. XPS analyses

[15]): hydrodynamic conditions seem to influence surface morphology. For a better resolution of hydrodynamic effect on film structure for important times, AFM analyses were carried out on two samples. As shown in Fig. 13, the surface of the sample 1 (without rotation) looks relatively smooth: the statistic criteria rms representing the averaged height of unevenness of the surface is weak and equals to 7.5. Many analyses were made at different loci on this sample and the surface is homogeneous. On the contrary, in Fig. 14 obtained with sample 2 (under a rotation rate equal to 3200 rpm, during 24 h), topography at different surface places is not the same and depends strongly on the measurements localization (several analyses were made for each localization and the statistic criteria rms represents an averaged value). Film roughness increases from the edge to the centre of the electrode (and rms varies from 5.4 to 53). This evolution can be correlated to the evolution of hydrodynamic stresses along the electrode radius. In fact, film roughness decreases when hydrodynamic stresses increase, which underlines importance of parietal fluid velocity distribution on film morphology. This parameter would have an erosive action on the inhibitive layer.

Inhibitor adsorption on metallic surface was appreciated using the characteristic C 1s, N 1s, and O 1s peaks. These elements confirm the presence of amine and carboxylic acid. In a previous work, the presence of the inhibitor was displayed

Fig. 12. Visualization by optical microscopy of the disk electrode obtained after 24 h of immersion in solution A + 0.5 wt.% of inhibitor at ω = 3200 rpm.

P. Bommersbach et al. / Electrochimica Acta 51 (2006) 4011–4018

4017

Table 3 Binding energies for each component of spectrum (in eV)

ω = 3200 rpm (2 h) Sample centre (A) ω = 3200 rpm (2 h) Sample edge (C)

Fe 2p3/2

O 1s (O2− )

O 1s (OH− )

O 1s (H2 O or COO− )

706.8 710.2 706.5 710.0

529.7 (45%) 529.5 (44.5%)

531.7 (46%) 531.7 (48%)

533.6 (9%) 533.6 (7.5%)

In parenthesis, percentage data values correspond to peak area ratio. Table 4 Values issued from XPS analyses after 2 h of immersion, ω = 3200 rpm

Sample centre (A) Sample edge (C)

Fig. 13. AFM picture of the morphology of the disk surface obtained after 24 h of immersion in solution A + 0.5 wt.% of inhibitor at ω = 0.

C/Fe

O/Fe

N/Fe

5.9 9.1

3.2 3.5

0.3 0.3

mainly with the N peak (399.6 eV) for analyses realized with inhibitor [6]. Moreover, oxygen characterizes the presence of iron oxide-hydroxide (peaks at 530 and 531.5 eV) [16] and the presence of adsorbed H2 O (peak at 533 eV) [5]. According to several authors, this last peak can also be attributed to the carboxylic group (O C O− ) present in the molecule of carboxylic acid [17]. The quantitative analyses of oxygen (given in atomic ratio versus iron) are presented in Table 3. Table 4 presents ratios of

Fig. 14. AFM pictures of the morphology of the disk surface at the centre (A), at the middle (B) and at the edge (C) of the electrode obtained after 24 h of immersion in solution A + 0.5 wt.% of inhibitor at ω = 3200 rpm.

4018

P. Bommersbach et al. / Electrochimica Acta 51 (2006) 4011–4018

carbon, nitrogen and oxygen elements detected on steel surface. Nitrogen and oxygen are respectively identified in the same proportions at the centre and at the edge of the sample whereas more carbon is detected at the periphery of the sample than in its centre. In literature, different mechanisms are presented to explain the action of carboxylates [18,19]. These compounds form probably a complex with the metallic cation [20]. This complex would be incorporated in the layer of iron oxide-hydroxide to plug the pores. Alexander et al. [21] put forward that this kind of complex seems to form itself preferentially where the film is defective. So, this could explain the more significant presence of carbon at the edge of the sample, where hydrodynamic stresses are more important. Finally this later observation indicates that the peak at 533 eV would be attributed rather to adsorbed H2 O than to the carboxylic group.

Acknowledgements This work was supported by the French Ministry of Research and inhibitors have been proposed by ASCOTEC Comp which is supported by Rhˆone-Alpes Region. The authors express their gratitude to Robert Di Folco for his great help and know-how concerning experimental devices. References [1] [2] [3] [4] [5] [6] [7]

4. Conclusion [8]

The influence of the flow on formation and stability of an inhibitive layer was studied. Film formation depends on the electrode rotation rate: film forms faster under high rotation rates. This evolution was quantified thanks to the double layer capacitance and the charge transfer resistance evolutions determined by impedance spectroscopy practiced at corrosion potential after 2 h of immersion. For important immersion times (more than 140 h) hydrodynamic stresses on the sample surface are responsible of a protective layer modification. More important stresses at the periphery of the sample have an erosive effect on the film. This action (verified by AFM) involves the apparition of defects quantifiable on the diffusion part (characteristic of iron ions diffusion through the film) of impedance spectra. The inhomogeneous repartition of the complex formed between carboxylic acid and iron oxide-hydroxide (measured by XPS) confirms the predominance of defects at the periphery of the sample.

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

D. Klenerman, J. Hodge, M. Joseph, Corros. Sci. 36 (1999) 301. Y.J. Tan, S. Bailey, B. Kinsella, Corros. Sci. 38 (1996) 1681. Y. Chen, W.P. Jepson, Electrochim. Acta 44 (1999) 4453. X. Jiang, Y.G. Zheng, W. Ke, Corros. Sci. 47 (2005) 2636. N. Ochoa, F. Moran, N. Pebere, B. Tribollet, Corros. Sci. 47 (2005) 593. P. Bommersbach, C. Dumont-Alemany, J.P. Millet, B. Normand, Electrochim. Acta 51 (2005) 1076. K. Es-Salah, M. Keddam, K. Rahmouni, A. Shriri, H. Takenouti, Electrochim. Acta 49 (2004) 2771. ASTM. Standard Test Method for Corrosion Test for Engine Coolants in Glassware. ASTM D 1384–1393. Philadelphia: ASTM, 1993. I. Felhosi, J. Telegdi, G. Palinkas, E. Kalman, Electrochim. Acta 47 (2002) 2335. P. Bommersbach, C. Dumont-Alemany, J.-P. Millet, Proceedings of the Eurocorr 2004, Nice (France), 12–16 September, 2004. Y. Yamamoto, H. Nishihara, K. Aramaki, Corrosion 48 (1992) 641. M. Duprat, Thesis, Institut National Polytechnique de Toulouse, Toulouse, France, 1981. S. Veleda, A. Popova, S. Raicheva, Proceedings of the Seventh European Symposium on Corrosion Inhibitors, Ferrara, 1990, p. 149. W.J. Lorenz, Dechema Monographs 101, Verlag Chemie, Weinheim, 1986, p. 185. A. Bonnel, F. Dabosi, C. Deslouis, M. Duprat, M. Keddam, B. Tribollet, J. Electrochem. Soc. 130 (1983) 753. N. Ochoa, G. Baril, F. Moran, N. Pebere, J. Appl. Electrochem. 32 (2002) 497. N. Ochoa, Thesis, Institut National Polytechnique de Toulouse, Toulouse, France, 2004. A. Wiesstuch, D.A. Carter, C.C. Nathan, Mater. Perform. 10 (1971) 11. D.C. Zecher, Mater. Perform. 15 (1976) 33. T. Suzuki, H. Nishihara, K. Aramaki, Corros. Sci. 38 (1996) 1223. M.R. Alexander, G. Beamson, C.J. Blomfield, J. Electron Spectrosc. Relat. Phenom. 121 (2001) 19.

Hydrodynamic effect on the behaviour of a corrosion ...

Available online 27 December 2005. Abstract. Influence of hydrodynamic ... +33 4 72 43 81 87; fax: +33 4 72 43 87 15. E-mail address: catherine.alemany-[email protected] ..... trolyte conductivity evolution due to the greater account of iron.

781KB Sizes 1 Downloads 255 Views

Recommend Documents

Effect of resistivity on the corrosion mechanism of mild ... - Springer Link
corrosion mechanism and obtain representative values of corrosion rates in ... Low-resistivity soils, for instance, generally contain high ..... ions is of interest.

Effect of resistivity on the corrosion mechanism of mild ... - Springer Link
Effect of resistivity on the corrosion mechanism of mild steel in sodium sulfate solutions. S. ARZOLA1, M.E. PALOMAR-PARDAVEґ2 and J. GENESCA1,*.

A New Theory on the Behaviour of Light
et al. (1976) sent electromagnetic signals between standard clock stations on Earth and showed that these signals ..... the water by precisely the required amount. This requirement ...... and controller of theory and opinion”. It is interesting to 

The Influence of Admixed Micelles on Corrosion Performance of ...
The Influence of Admixed Micelles on Corrosion Performance of reinforced mortar.pdf. The Influence of Admixed Micelles on Corrosion Performance of ...

The Effect of a Prerecorded Conversation on ...
speakers on a laptop computer at the maximum volume level. Participants used ... effect of auditory distraction on reading comprehension, F(1,9). 0.630, p 0.448.

THE HYDRODYNAMIC LIMIT OF BETA ...
Consider the beta(a, b) coalescent (see definition in Section 2) ... as n → ∞ in [0, 1]d in the Skorokhod topology, where by Corollary 1 ci(t) = c(t)2−a i! Bi. ((. 1.

The Effect of Crossflow on Vortex Rings
The trailing column enhances the entrainment significantly because of the high pressure gradient created by deformation of the column upon interacting with crossflow. It is shown that the crossflow reduces the stroke ratio beyond which the trailing c

Effect Of Ecological Factors On The Growth And Chlorophyll A ...
Effect Of Ecological Factors On The Growth And Chlor ... ed Kappaphycus alvarezii In Coral Reef Ecosystem.pdf. Effect Of Ecological Factors On The Growth And ...

The effect of Medicaid on Children's Health: A ...
design. I exploit the discontinuity generated by Medicaid's eligibility rule, based on family income, on program participation rates. In contrast with a standard regression discontinuity approach, here there are ... reau's March 2009 and 2010 Current

The Effect of Medicaid Premiums on Enrollment: A ...
However, this result is not robust to other choices of bandwidth. These patterns in the data are consistent with the conclusion that treatment status is unrelated to sample composition and supports identification. As a final check, I perform a set of

The effect of culture on perception and cognition - A conceptual ...
self-perceptions, with English-speaking bi-cultural people reporting a. perception of the self as independent of others and Chinese-speaking. bi-cultural people ...

The Effect of Crossflow on Vortex Rings
University of Minnesota, Minneapolis, MN, 55414, USA. DNS is performed to study passive scalar mixing in vortex rings in the presence, and ... crossflow x y z wall. Square wave excitation. Figure 1. A Schematic of the problem along with the time hist

Role of hydrodynamic conditions on quantity and ... - Springer Link
All statistical analyses were performed using SPSS version 8.0 (SPSS, 1997). Results. Environmental characteristics. The profile and beach face slopes for each ...

On the Effect of Bias Estimation on Coverage Accuracy in ...
Jan 18, 2017 - The pivotal work was done by Hall (1992b), and has been relied upon since. ... error optimal bandwidths and a fully data-driven direct plug-in.

On the Effect of Bias Estimation on Coverage Accuracy in ...
Jan 18, 2017 - degree local polynomial regression, we show that, as with point estimation, coverage error adapts .... collected in a lengthy online supplement.

Role of hydrodynamic conditions on quantity and ... - Springer Link
diately after recovery, samples were vertically sliced into five ... Data were normalised to sediment dry weight. ..... Lowry, O. H. & N. J. Rosebrough, 1951. Protein ...

The effect of mathematics anxiety on the processing of numerical ...
The effect of mathematics anxiety on the processing of numerical magnitude.pdf. The effect of mathematics anxiety on the processing of numerical magnitude.pdf.

The Effects of Ethical Climates on Bullying Behaviour in ... - CiteSeerX
organizational commitment, supervisory support .... autonomy, trust, cohesiveness, support, recognition, ...... Manchester Institute of Science and Technology,.

The effect of mathematics anxiety on the processing of numerical ...
The effect of mathematics anxiety on the processing of numerical magnitude.pdf. The effect of mathematics anxiety on the processing of numerical magnitude.pdf.

The effect of ligands on the change of diastereoselectivity ... - Arkivoc
ARKIVOC 2016 (v) 362-375. Page 362. ©ARKAT-USA .... this domain is quite extensive and has vague boundaries, we now focused only on a study of aromatic ...

The Effect of Recombination on the Reconstruction of ...
Jan 25, 2010 - Guan, P., I. A. Doytchinova, C. Zygouri and D. R. Flower,. 2003 MHCPred: a server for quantitative prediction of pep- tide-MHC binding. Nucleic ...

NitrAdine Corrosion test on Metal parts of dental appliances.pdf ...
good overall oral health. The use of denture cleaners helps to remove food debris and plaque that are adherent on the. removable orthodontic appliance (ROA). However one major problem of the routine use of these agents is that several of. them cause

(Zn & Mg) on Corrosion Behavior of Shape.pdf
28, No.18, 2010 Effect of (Zn & Mg) on Corrosion Behavior of Shape.pdf. Eng. & Tech. Journal, Vol. 28, No.18, 2010 Effect of (Zn & Mg) on Corrosion Behavior of ...

The Effects of Ethical Climates on Bullying Behaviour in ... - CiteSeerX
study is to explore the effects of bullying behaviour upon ... Journal of Business Ethics (2009) 86:273–295. © Springer 2008 .... affect the definition of their workplaces (Altmann,. 2000). ...... Manchester Institute of Science and Technology,.