Electrochemical behavior of 304L stainless steel in high saline and sulphate solutions containing alga Dunaliella Salina and β-carotene

Journal of Alloys and Compounds 491 (2010) 636–642

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Electrochemical behavior of 304L stainless steel in high saline and sulphate solutions containing alga Dunaliella Salina and ␤-carotene F. El-Taib Heakal a,∗ , M.M. Hefny a , A.M. Abd El-Tawab b a b

Chemistry Department, Faculty of Science Cairo University, Giza 12613, Egypt Egyptian Salts and Minerals Company (EMISAL), Egypt

a r t i c l e

i n f o

Article history: Received 7 July 2009 Accepted 3 November 2009 Available online 18 November 2009 Keywords: EIS Corrosion 304L Stainless steel Alga Dunaliella ␤-Carotene

a b s t r a c t Type 304L stainless steel alloy is a main constructional material in many factories for the production of several salts from the hyper saline water. In this water, such that of Lake Quaroun in Fayioum, Egypt, the sole surviving micro-organism is almost the unicellular alga Dunaliella Salina which secretes relatively massive amount of ␤-carotene. The present investigation deals with the role of this alga as well as its secreted ␤-carotene on the corrosion and passivation behavior of the 304L steel alloy in hyper saline water and sulphate solutions. The tested ␤-carotene was extracted from a culture alga solution. Standard biochemical, chemical and electrochemical methods were applied. The results reveal that this compound does not affect the mechanism of the corrosion process but merely decrease its rate by making a physical barrier, indicating that ␤-carotene acts as an adsorption inhibitor. Various electrochemical measurements were used including open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Analysis of the EIS results based on the dispersion formula approach, allowed estimation of the electrical parameters of the proposed equivalent circuit used to simulate the alloy/passive film/solution system. From the increase of the surface film resistance, the degree of surface coverage has been calculated as a function of the added ␤-carotene concentration, hence specifying the appropriate adsorption isotherm. The data were found to obey the Temkin adsorption equation. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Metallic materials suffer from corrosion in many processes such as acid pickling, industrial acid cleaning and oil well-acidizing, as well as other heavy-duty services. Salt-laden medium is highly aggressive agent as it enhances localized corrosion for many metallic alloys, especially stainless steels. These steels are interesting materials having a widespread use in corrosive environment, due to their excellent oxidation resistance for structural applications. One of the most practical methods for protecting metals and alloys against deterioration from corrosion is the use of organic inhibitors [1,2]. Nowadays, there is increased attention on the development of environmentally compatible, non-polluting corrosion inhibitors. Hence, naturally occurring biological materials are of considerable

∗ Corresponding author. Tel.: +20 102449048; fax: +20 237528099. E-mail address: [email protected] (F.E.-T. Heakal). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.028

interest as possible corrosion inhibitors. The present work deals essentially with the corrosion behavior of a technical grade type 304L stainless steel (SS) in artificial high salt solutions containing the caroteniod alga Dunaliella Salina and in sulphate solutions containing natural ␤-carotene secreted by the alga. This alga is found in Quaron Lake water, Fayioum, Egypt, and has a high economic value as it accumulates a relatively massive amount of the precious compound ␤-carotene [3]. The artificial culturing of this alga is undertaken in metallic ponds. ␤-Carotene (formula 1) is an important useful biological material. The present investigation showed that it (and expectedly compounds with similar structures) can act as a corrosion inhibitor for stainless steel. This result is promising because non-toxic corrosion inhibitors for stainless steel in highly saline solutions are scarce [4].

An application of these algae has been achieved by Montany’a et al. [5], who studied the growth of the alga Dunaliella Primolecta

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Fig. 1. The alga Dunaliella Salina photographed from Butcher [6].

in axenic conditions to assay herbicides. The results showed that microcultures of this alga was susceptible to assay a large number of samples such as metamitron, sethoxydin and alloxydim under saline environments. The alga Dunaliella Salina is unicellular and belongs to the class Chlorophyceae and the order Vlovocales [6]. It is found naturally in many aquatic marine habits containing more than 10% salt, such as concentrate water of Lake Quaron which is hyper saline. Microbiological investigation of this water revealed that the monoculture strain which predominates in it after appreciable natural evaporation is the Dunaliella Salina. This alga is ovoid, motile and halotolerant via an osmoregulation mechanism, being lacking a cell wall but has a mucus surface coat, and easily responds to external tension by secreting glycerol and ␤-carotene. Fig. 1 shows a scanning electron micrograph for this alga. It proliferates in sunlight in concentrated saline solutions. Dunaliella is probably the most halotolerant eukaryotic organism known and found in a wide range of marine habitats, including oceans, brine lakes, salt marches and salt diches near the sea [7]. Owing to the corrosion resistance of type 304L stainless steel, it has been used in chemical industries where severe conditions are encountered, as the extraction of salts from natural water [8]. The lower corrosion rate of this steel is due to the formation of a passive film on its surface [9]. However, this film is susceptible to localized attack and pitting by chloride ions. In natural saline waters some micro-organisms survive and could influence the corrosion process. Studies dealing with corrosion of 304L stainless steel are numerous with regard to its alloying, surface modification, coating, inhibition, pitting and stress corrosion cracking [10–15], but biocorrosion studies are scanty. Electrochemical behavior of steel samples taken from the heat-exchanger tubes used for the production of sodium sulphate salt from Lake Qaroun was studied [16–18] using potentiodynamic and ac impedance techniques. The results showed that pitting corrosion commences at [Cl− ]/[SO4 2− ] ratio of about 0.6. The total impedance was found to increase with increasing formation voltage passing through a maximum due to change in properties and/or passive film breakdown. In the presence of an aerobic bacteria the open circuit potential (OCP) of type 304 stainless steel was found [19] to be ennobled due to an increase in the rate of cathodic reaction. Furthermore, the presence of an aerobic biofilm led to a decrease in the polarization resistance of the steel sample due to micropit formation as well as thinning of the passive film. Scanning electron microscopy (SEM) for localized biological corrosion of X52 steel exposed to sea water revealed [20] large number of pits with different depth having a mean statistical value of 0.05 mm. These pits were associated with various aerobic, facultative and anaerobic bacteria species together with aggressive chemical species containing chlorine and sulphur as detected in the biofilm deposited on a bioelement surface. Gayosso et al. [21] studied also the influence of microbial consortium on steel corro-

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sion rate using polarization resistance and electrochemical noise techniques. They found that the consortium consists of five different bacteria species but only one of them is the most influential. The damage observed on the metal surface depends upon the sessile microorganism population. The transition between uniform and localized corrosion was observed after approximately 500 h exposition time. Pickling of oxidized 304 stainless steel in single or multiple electrolytes was investigated based on weight loss and corrosion potential measurements by Lian et al. [22]. Multi-step pickling in successive HCl and HF electrolytes allowed achieving a smooth surface finish free of any oxide scales. The effect of carbonate anion on the pitting corrosion and inhibition behavior of stainless steel samples has been studied using potentiodynamic and scanning electron microscopy techniques [23]. Additions of Cl− or Br− ions into the carbonate solutions increased the anodic dissolution rate and decreased the pitting corrosion resistance. The inhibition effect was argued to formation of [Fe,Cr] CO3 film. 2. Experimental 2.1. Specimen preparation Stainless steel 304L samples were supplied from Brembana Co., Italy. The heat exchangers in the Egypt Company EMISAL have been manufactured from this material, which has the following chemical composition in mass percent: 17.47 Cr, 9.8 Ni, 0.57 Ti, 0.53 Si, 0.21 Mn, 0.06 C, 0.025 P, 0.016 S and balance Fe. The working electrode used was machined in the form of a rod with 2.0 cm length and 0.24 cm2 cross sectional areas. Electrical connection was achieved by mechanical jamming to the upper end, a 20 cm stout copper wire of 1 mm diameter. The rod was then fixed by epoxy resin in a Pyrex glass tube with an appropriate diameter leaving only the base cross sectional area to face the test solution. Prior to each experiment the electrode surface was mechanically polished with successively finer grades of metallurgical emery paper, till the surface appears free from scratches and other apparent defects. It was then rinsed with a distilled water and dried in the air at room temperature. 2.2. Solutions Analytical grade chemicals and triple distilled water were used throughout for the preparation of different solutions. Stock 1.0 M H2 SO4 solution was used to adjust the pH of 1.0 M Na2 SO4 (pH 7.0) test solution, to the desired pH value. The solution pH was checked by a sensitive pH-meter, model 15 Denver, USA. Artificial high salt solutions (free media) in which the alga can survive, were prepared by sequential additive of 175.5 g NaCl, 42.6 g Na2 SO4 and 36.1 g MgSO4 ·7H2 O, to less than 1 l of distilled water. To ensure complete dissolution of the different salts, the solution was boiled, and then cooled to room temperature. Calculated amounts from the minor components, namely, urea, potassium dihydrogen phosphate, iron–EDTA and sodium bicarbonate were added to get the recommended final concentrations of 0.1, 0.2, 0.002 and 2.0 mM, respectively. Finally the solution was completed to 1 l with distilled water and become ready for being used as homogeneous salt media for the various experimental tests. 2.3. Culturing of the alga A crop of the alga Dunaliella Salina was supplied from SRI, China. A volume of 250 ml of the crop solution with a cell number concentration of 1.5 × 105 cell ml−1 , was added to each liter of the media in a tank. It was kept under net shade exposed to sun light and moved to a cool place when the atmospheric temperature exceeds 35 ◦ C and vice versa, being shaken once each hour during day-time. This culture was diluted with proper amounts of the media according to the growth rate so that the volumetric cell number does not exceed 20 × 105 cell ml−1 . Also, it was diluted with distilled water to compensate for evaporation, thereby keeping the salinity constant [24]. Changes in cell number and ␤-carotene content with the growth time were recorded daily over a period of 15 days, and the results are given in Fig. 2. 2.4. Extraction of ˇ-carotene The extraction of ␤-carotene from a culture of the alga Dunaliella Salina was made using 90% (by volume) acetone. The separation was carried out by manual shaking until two layers were formed, where the upper layer contained the ␤-carotene. The concentration was then determined using a manual operating spectrophotometer (USA) at a wave length of 450 ␮m [25]. The same instrument has been used for the determination of chlorophyll, which was extracted from the solution layer attached to the electrode surface exposed to a solution of alga Dunaliella Salina. This has been achieved by leaching and dissolving the layer in 80% (by volume) acetone solution [26].

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Fig. 2. Change in cell number and ␤-carotene content in the medium with the growth time.

2.5. Measurements Electrochemical measurements were performed using a proper threecompartment electrolytic cell [18]. The cell was kept inside an air thermostat. An electrochemical workstation IM6e Zahner-elektrik, GmbH (Kronach, Germany) controlled by personal computer was used as a measuring instrument for potentiodynamic polarization and electrochemical impedance investigations. The EIS were carried out at the open-circuit potential, with excitation amplitude of 10 mV peakto-peak. The method involves direct measurements of the cell impedance (Z) and the phase shift (∅) over the frequency domain 100 kHz down to 10 mHz. All potentials were measured against and referred to the saturated calomel reference electrode (SCE).

3. Results and discussion 3.1. Open circuit potential (OCP) The OCP (E) of the steel sample in salt medium free or containing different amounts from the alga (in cell ml−1 ) was recorded for a period of more than one hour until reached a quasi-steady value. In general, E shifts toward more noble values with time (t), as shown in Fig. 3a. In the alga-free medium, the positive shift of E can be interpreted as reflecting spontaneous passivation of the surface [18], due to development of a protective film on the 304L sample. In the algacontaining media, the positive shift of E increases with increasing cell number, which is simultaneously connected to retardation of the anodic process by blocking the active sites on the corroding surface, most probably due to a biofilm formation [27]. Fig. 3b shows that E increases with t according to a logarithmic rate law. This behavior is likely attributed to growth of the pre-immersion native film by a dissolution–precipitation mechanism [28]. Furthermore, the rate of film growth is higher in the alga-containing solution compared to the free medium as revealed from the overall change in the rate of potential variation (Fig. 3c), which can be defined as (Ef − Ei )/, where Ef and Ei are the measured OCP values after 3600 s and after 10 s from the moment of immersion and  = 3550 s, being the time elapse. The presence of a break in the E vs. log t plot (Fig. 3b) indicate that the growing passive film is of a duplex nature, which can be represented by a relatively thick porous outer layer on top of a thin compact and less hydrated inner layer [18,28]. The enhanced growth rate of spontaneously formed passive film in the presence of the alga can be ascribed to several factors, such as accumulation of the alga at the electrode surface, adsorption of secreted ␤-carotene, and production of oxygen due to the alga metabolic processes. All these changes were confirmed by chemical analysis of the solution after each test. Accumulation of the alga is suggested on account of a yellow to orange colouration observed with the naked eye around the electrode surface, which was further confirmed by counting the cell number in a layer close to the electrode surface, being usually referred to as the biofilm. The results reveal higher values by about 10% compared to the cell number in

Fig. 3. (a) Variation of the open circuit potential (E) of 304L stainless steel with the immersion time (t) in salt media free or containing different amounts from the alga. (b) Dependence of the open circuit potential (E) of 304L stainless steel on log t in salt medium free or containing different amounts from the alga. (c) Rate of potential variation as a function of the cell number.

the solution bulk. Spectrophotometric analysis involving the surface biofilm layer with respect to chlorophyll a, b and c extracted with 90% (by volume) acetone solvent was also performed and the results are given in Table 1. The other factor which enhances the rate of film growth in presence of the alga is the increase of oxygen concentration as a result of the photosynthesis process by the scattered light [29], as revealed from the increase in the amount of oxygen with the cell number (Table 2).

Table 1 Concentration of the chlorophyll a, b and c as well as the ␤-carotene in the surface biofilm formed on type 304L stainless steel and the respective bulk values in solution containing different cell numbers, after 1.5 h immersion time. (Cell/ml)

Chlorophyll (mg m−2 )

[␤-Carotene] (mg l−1 ) bulk/surface

A B C bulk/surface bulk/surface bulk/surface 5.0 × 104 1.0 × 105 2.5 × 105

0.27/0.30 0.56/0.62 0.85/1.0

0.18/0.19 0.37/0.42 0.44/0.50

0.19/0.21 0.5/0.51 0.6/0.66

1.6/1.75 3.2/3.5 8.0/9.0

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Table 2 Concentration of dissolved oxygen after immersing type 304L stainless steel for 1.5 h in media containing various cell numbers. (Cell/ml) [O2 ] (mg l−1 )

5.0 × 104 8.5

0.0 7.8

10 × 104 9.5

25 × 104 11.5

Furthermore, the presence of electrode surface in the alga containing media acts as an external tension that leads the alga to respond by secreting ␤-carotene and glycerol [24]. Tests were carried out to compare the influence of the whole alga cell and each of its secreted substances on the corrosion behavior of 304L steel sample in salt media using dc polarization and ac impedance measurements, as well as chemical analysis of the test solutions. The results showed that although glycerol has no effect, ␤-carotene exhibits anti-corrosion properties. 3.2. Potentiodynamic polarization To compare between the effect of the alga and its secreted substance ␤-carotene regarding the corrosion behavior of 304L stainless steel in the salt medium, potentiodynamic polarization traces were recorded after the electrode has reached its steady state potential value. Fig. 4a shows typical cathodic and anodic scans for the sample in salt media free or containing various alga cell numbers. All polarization curves are qualitatively similar and continuously displaced to more positive potentials as the cell number (or ␤-carotene concentration) increases associated with a gradual decrease in the anodic current density. The anodic curves display a large passive region extends to ∼0.4 V, beyond which the anodic current begins to increase, reaches a maximum, and a second passivation region appears before the oxygen evolution reaction commences at 1.2 V. All these features indicate that addition of the alga or extracted ␤-carotene to the salt media can reduce the rate of active dissolution of the sample without affecting its mechanism. The corrosion current density (icorr ) was determined by the intersection of the cathodic or the anodic Tafel line with the OCP (Ecorr ). Fig. 4b shows that the trend for the variation of icorr with the alga cell number is mirrored completely by its variation with ␤-carotene, where icorr decreases significantly fast at first and then slowly with increasing either the alga cell number or ␤-carotene concentration in the media. Hence the observed inhibitive effect of the alga is likely attributed to the effect of its secreted ␤-carotene. This tent was further confirmed from the observed decrease in concentrations of some dissolved corrosion products (Fe, Cr and Ni) shown in Table 3 as a function of the cell number and ␤-carotene concentration. Fig. 4c illustrates the dependence of the degree of surface coverage ( p ) based on polarization, as calculated from the relation: p = 1 −

i

corr



(1)

o icorr

o and icorr are the corrosion current densities (A cm−2 ) in where icorr the absence and presence of the additive, respectively. As can be

Table 3 Concentration (in mg l−1 ) of some dissolved metals from the alloy after potentiodynamic polarization tests, as a function of the alga and ␤-carotene concentrations. (Cell/ml)

[Fe]

[Cr]

[Ni]

[␤-Carotene] (mg l−1 )

[Fe]

[Cr]

[Ni]

0.0 2.5 5.0 10.0 25.0

15.8 14.1 13.1 10.2 10.0

0.90 0.85 0.70 0.60 0.60

1.20 1.15 0.90 0.78 0.75

0.0 5.0 10.0 30.0 50.0

15.8 13.5 12.0 9.0 8.5

0.90 0.85 0.80 0.62 0.60

1.20 1.15 1.10 0.61 0.60

Fig. 4. (a) Poteniodynamic polarization scans for 304L stainless steel in salt media free or containing different alga cell numbers. (b) Dependence of the corrosion current density on ␤-carotene concentration, the results for the alga are given for comparison. (c) Change of the degree of surface coverage () with the cell number and ␤-carotene concentration.

seen  p exhibits fast increase over the lower additions from either the alga or the ␤-carotene and then tends to a fixed value at higher additions giving to a typical S-shaped isotherms for both cases. 3.3. Electrochemical impedance spectroscopy (EIS) The efficiency of ␤-carotene, extracted from the unicellular alga Dunaliella Salina, as a corrosion inhibitor for type 304L SS in sulphate solutions of various pH values was also tested using EIS measurements. EIS as a technique has proved capable of accessing relaxation phenomena whose relaxation time varies over orders of magnitude and permits single averaging within an experiment to obtain high precision levels. Thereby, it is nowadays widely applied for investigating the corrosion and passivation phenomena due to its non-destructive nature suitable for corrosion monitoring [28,30]. It can further provide considerable information about the mechanism of corrosion and inhibition processes and the kinetics of these reactions [31,32]. 3.3.1. Behavior in 1.0 M H2 SO4 The electrochemical impedance behavior of stainless steel sample was recorded in 1.0 M H2 SO4 solution at different immersion

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the electrolyte and Luggin capillary resistance terms (cf. Fig. 5c). Two constant phase elements (CPE) were used instead of the two ideal capacitors to account for the deviations observed as capacitive slopes and phase angles lower than −1 and 90◦ , respectively. The impedance (ZCPE ) of a CPE is defined as [28,34,35]: ZCPE =

1 Y o (jω)

˛

(2)

with 0 ≤ ˛ ≤ 1 and j2 = −1, while Yo (s˛ −1 ) is a frequency independent constant of the CPE which will be identical to the idealize capacitance (C) value at ω = 1 [36], ω being the angular frequency (ω = 2f rad s−1 ). The use of the CPE is necessary since the corrosion product layer on the alloy surface has a heterogeneous structure and/or composition. The heterogeneity affects the roughness of the electrode surface. The smaller value of ˛ the higher is the surface roughness [37]. Excellent fits were obtained on analyzing the experimentally obtained impedance data using the above fiveelement EC and the following dispersion formula representing the electrode impedance: Z(ω) = Rs +

Fig. 5. (a) The Bode plots for 304L stainless steel in 1.0 M H2 SO4 solution after immersion for (1) 5 min, (2) 30 min and (3) 60 min. (b) The Bode plots for 304L stainless steel in 1.0 M H2 SO4 (1) free solution or containing, (2) 5 mg l−1 , (3) 10 mg l−1 , (4) 30 mg l−1 and (5) 50 mg l−1 ␤-carotene after 5 min exposure. (c) The equivalent circuit model used for impedance data fitting.

times up to 60 min and presented as Bode plots in Fig. 5a. The effect of ␤-carotene concentration within the range 5–50 mg l−1 on the stability of spontaneously formed passive films on the alloy in 1.0 M H2 SO4 solution was also examined as a function of the exposure period. Fig. 5b shows a typical set of Bode plots probed after 5 min immersion in the absence and presence of ␤-carotene. All diagrams exhibit capacitive region at medium frequency (MF) caused by the surface film due to corrosion [31]. As can be clearly seen the impedance at low frequencies (LF) is several orders of magnitude higher than that at higher frequencies (HF), and thus HF features are difficult to discern from Nyquist plots (complex plane plots). The absolute impedance (|Z|) and phase shift of the tested system are found to depend on both immersion time and ␤-carotene amount in the medium. At the LF range, log |Z| − log f shifted to higher values with increasing each of the two factors. On the other hand, ∅ − log f spectra exhibit a broad phase maximum (∅max ), which becomes much broader and shifts to lower frequencies as the exposure period or ␤-carotene concentration increases, indicating formation of a more protective film on the surface [28,33]. The results seem to reveal two merged phase lags with ∅max values smaller than 90◦ (∼70–76◦ ). Based on the above visual diagnostic criteria, the EIS results were modeled according to an equivalent circuit (EC) consisting from two parallel combination R1 C1 and R2 C2 pairs arranged in series and in series connection with the ohmic impedance (Rs ), comprising

1 1 ˛ + ˛ 1/R1 + C1 (jω) 1 1/R2 + C2 (jω) 2

(3)

The validity of this EC model to the data complies with the duplex nature of the formed passive films on 304L stainless steel substrate. The first inner barrier layer is a thin and compact, which is followed by a relatively thick porous outer layer facing the solution. The calculated circuit parameters are listed in Table 4. Simple inspection of the obtained results indicates that ␤-carotene, at any given exposure period, noticeably decreases the corrosion rate of the alloy, since the total film resistance (Rt = R1 + R2 ) increases while its total capacitance (Ct−1 = C1−1 + C2−1 ) decreases with increasing ␤-carotene concentration. The results also reveal that the fitting parameters ˛1 and ˛2 for the inner and outer layers of the passive film amount in the average to 0.86 and 0.80, respectively, indicating that the corrosion process is not diffusion controlled. Furthermore, Fig. 6a reveals that the corrosion process leads also to increase the oxide film thickness as inferred from the decrease of Ct value, which is inversely proportional to the oxide thickness (d) in accordance to the relation: 1/Ct = d/εo εA, where A is the film area, εo is the permittivity of the free space and ε is the dielectric constant of the passive film. The role of ␤-carotene in inhibiting the corrosion process seems to be via an adsorption process for which the degree of surface coverage ( R ) can be estimated using the following relation [38]:



R = 1 −

Rto Rt



(4)

where Rto and Rt are the total resistance values of the passive film formed on 304LSS sample in the absence and presence of ␤carotene inhibitor, respectively. Fig. 6b shows that  R increases with increasing ␤-carotene according to the Temkin isotherm (cf. Fig. 6c) as expected for a rough surface [39]. This confirms that the inhibitive influence of ␤-carotene is due to its adsorption on the active site at the flawed area in the surface hence suppressing the dissolution process of the sample and enhancing the growth rate of its oxide. 3.3.2. Effect of pH The impedance diagrams of 304L stainless steel sample in 1.0 M Na2 SO4 solution free or containing different ␤-carotene concentrations in the range 5–50 mg l−1 at pH 3 and 7 are shown in Fig. 7a and b. In general, the spectra probed after 60 min exposure period are all characterized by similar features and reveal an increase in both Rt and 1/Ct values with increasing solution pH, due to a decrease in the tendency of the passive film solubility. The

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Table 4 EIS simulated parameters for 304L stainless steel in 1.0 M H2 SO4 free or containing ␤-carotene, at different concentrations of at various immersion times. Time (min)

R1 (k cm2 )

C1 (␮F cm−2 )

˛1

C2 (␮F cm−2 )

˛2

Rs ( cm2 )

Free solution 5 30 60

35.544 71.016 115.584

20.51 20.30 20.80

0.86 0.87 0.87

1.29 1.19 0.86

59.75 55.08 50.08

0.80 0.73 0.70

3.37 3.45 3.56

5 mg l−1 ␤-carotene 5 30 60

31.608 112.56 170.088

12.60 12.58 11.74

0.81 0.86 0.86

2.50 3.75 5.44

44.21 37.541 33.94

0.72 0.77 0.79

3.32 3.06 3.06

10 mg l−1 ␤-carotene 5 30 60

41.832 119.824 184.44

7.60 6.24 4.81

0.69 0.84 0.83

8.72 8.82 22.41

6.59 7.61 5.98

0.77 0.77 0.80

2.57 1.12 1.10

30 mg l−1 ␤-carotene 5 30 60

60.096 125.952 187.656

3.99 3.98 3.97

0.97 0.80 0.81

17.96 0.034 0.046

2.39 6.25 4.44

0.74 0.75 0.72

3.74 0.61 0.95

50 mg l−1 ␤-carotene 5 30 60

60.160 191.656 235.44

4.113 3.075 2.233

0.82 0.87 0.84

18.31 13.70 0.093

15.73 15.92 14.88

0.86 0.82 0.72

3.72 2.92 2.17

R2 (k cm2 )

increase in the two parameters (Rt and 1/Ct ) is more pronounced in presence of ␤-carotene (cf. Fig. 8a) because of its inhibitive effect. Fig. 8b shows the effect of pH on the degree of surface coverage ( R ). It is evident that  R increases with increasing pH, which can be attributed to a dielectric replacement influence [40]. The results indicate that neutral species are more strongly adsorbed close to the isoelectric point, while positively charged species are

Fig. 6. (a) Variation with the immersion time of Rt and 1/Ct for 304L stainless steel in 1.0 M H2 SO4 solution free or containing 50 mg l−1 ␤-carotene. (b) The adsorption isotherm of ␤-carotene on 304L stainless steel surface in 1.0 M H2 SO4 at 298 K. (c) A test for Temkin isotherm.

Fig. 7. (a) The Bode plots for 304L stainless steel in 1.0 M Na2 SO4 solution adjusted to pH 3, (1) free solution, or containing, (2) 5 mg l−1 , (3) 10 mg l−1 , (4) 30 mg l−1 and (5) 50 mg l−1 ␤-carotene after 60 min exposure. (b) The Bode plots for 304L stainless steel in 1.0 M Na2 SO4 solution adjusted to pH 7, (1) free solution, or containing (2) 5 mg l−1 , (3) 10 mg l−1 , (4) 30 mg l−1 and (5) 50 mg l−1 ␤-carotene after 60 min exposure.

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References

Fig. 8. (a) Dependence of Rt and 1/Ct on pH for 304L stainless steel in 1.0 M Na2 SO4 solution free or containing 50 mg l−1 of ␤-carotene. (b) Dependence of the degree of surface coverage on pH for 304L stainless steel in 1.0 M Na2 SO4 solution containing 50 mg l−1 ␤-carotene.

more strongly adsorbed on a negatively charged surface and vice versa. 4. Conclusion 304L stainless steel is currently used as a constructional material in many factories for the production of various commercial salts (such as NaCl, Na2 SO4 , MgSO4 , etc.) from the highly saline water, where corrosion problem is of a major concern. Under these conditions, the present study showed that in this water the naturally living alga Dunaliella Salina secrets ␤-carotene, which acts as a retarding catalyst for steel corrosion, indicating that the medium is auto-inhibited. Moreover, the inhibition efficiency increases with increasing ␤-carotene concentration and pH of the medium, as inferred from OCP, polarization scans and EIS measurements. The results provide valuable information as non-toxic corrosion inhibitors for stainless steels in highly aggressive saline solutions are rarely found and badly needed for various industrial applications. Acknowledgements The authors would like to thank Dr. A.A. Dardir, previous president of Emisal Company, for his assistance during the biomaterials work at Fayioum, Egypt.

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