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Corrosion inhibition of aluminium–brass in 3.5% NaCl solution and sea water
Materials Chemistry and Physics 71 (2001) 12–16
Corrosion inhibition of aluminium–brass in 3.5% NaCl solution and sea water M.M. Osman∗ Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt Received 18 May 2000; received in revised form 12 July 2000; accepted 16 October 2000
Abstract Electrochemical characteristics of Al–brass are investigated in both 3.5% NaCl and sea water. At active potential, the alloy is dissolved directly or through an adsorbed metastable CuCl layer. At higher applied anodic potential, the active–passive transition leads to the formation of a thicker and more stable CuCl salt layer. With further increase in the anodic potential, the formation of CuO and/or Cu(OH)2 will take place. The inhibition of the corrosion process is studied by adding SDBS, DPh(EO)9 , and LAPACl as anionic, nonionic, and cationic surfactants. It is found that the inhibition efficiency increases in the following order SDBS > LAPACl > DPh(EO)9 , and increases with increasing their concentrations. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Al–brass; Potentiodynamic polarisation; Cyclic voltammetric; Corrosion inhibition
1. Introduction Al and its alloys are used in a wide range of industrial applications for different aqueous solutions, so it represents an important category of technological materials [1–5]. The relatively low cost and high resistance toward corrosion of Al–brass preferred its use for many years in tubes production for sea water, cooled heat exchangers and piping. This metal and its alloy have the tendency to form a stable oxide film naturally or by anodisation [6,7]. Generally, Al-oxide consists of a thin barrier film adjacent to the metal surface (∼25 nm thick) which is covered by a thicker porous oxide layer. In aggressive media as chloride solution, a localised corrosion can occur which leads to a break down of the passive layer and a pit formation [8–11]. One of the most important methods in corrosion protection is the utilisation of organic inhibitors. Many mechanisms have been proposed for the inhibition of metal corrosion by organic inhibitors which takes place via adsorption on the metal surface [12,13]. The adsorption process leads to an effective blocking of the active sites of metal dissolution and/or hydrogen evolution, which results in a considerable decrease in the corrosion rate. The inhibitive effect of some ethoxylated fatty acids on Al-alloys was investigated [14] and showed that the adsorption of these compounds increased with increasing the ethylene oxide units and by the presence of the double bond in their molecular structures. ∗
Fax: +202-2747433.
Also, organic acids or their salts are used to inhibit the corrosion rate of aluminium and its alloys [15,16]. Some authors studied the effect of hydrazine compounds, aldehydes, ketones and amines as corrosion inhibitors for Al-alloys in hydrochloric acid solution [17,18]. In the present work, the corrosion characteristics of Al–brass alloy was investigated in 3.5% NaCl and natural sea water. The polarisation techniques were used to explain the corrosion inhibition processes which occur at the electrode– electrolyte interface using some organic inhibitors.
2. Experimental 2.1. Samples and measurements The tested electrodes were cut from a sheet of aluminium brass of the following composition in percent by weight: Cu
Si
Al
Mn
Mg
Cr
Zn
78.25
0.5
4.5
0.6
1.5
0.1
14.65
Electrodes were abraded successively with 0, 00, and 000 grades of emery paper before degreasing with acetone and washing with distilled water. Electrochemical measurements were carried out using an EG&G Model 273A potentiostate/galvanostate with model
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 5 1 0 - 1
M.M. Osman / Materials Chemistry and Physics 71 (2001) 12–16
13
3. Results and discussion
Table 1 Composition of sea water Cations
Na+
K+
Mg2+
Fe2+
Cu2+
Li+
Concentration (ppm)
13357
1224
1752
0.0054
0.0029
15.19
352 Soft Corr III corrosion measurement software was used to control the potentiodynamic process. The three electrodes of the polarisation cell are: a Pt counter electrode, a saturated calomel reference electrode, and Al–brass electrode. The specimens were immersed in two different media 3.5% NaCl solution and a solution of natural sea water. The sea water is analysed by ICP where the obtained data has been given in Table 1. All tests were carried out at 30◦ C. 2.2. The corrosion inhibitors The types of the used inhibitor samples were: 1. Cationic surfactant: 1,1 -(lauryl amido)propyl ammonium chloride with a chemical formula [CH3 (CH2 )10 CONH3 ]+ Cl− . 2. Anionic surfactant: sodium dodecyl benzene sulfonate with a chemical formula C12 H25 C6 H4 SO3 Na. 3. Nonionic surfactant: dodecyl phenol ethoxylated with 9 units of ethylene oxide and its formula is C12 H25 C6 H4 O(CH2 CH2 O)8 CH2 CH2 OH These inhibitors were prepared according to the reported procedures [19,20].
Fig. 1 shows the potentiodynamic polarisation curves for Al–brass in: (a) 3.5% NaCl and (b) sea water, respectively, scanned from −2 V up to oxygen evolution potential with scan rate of 25 mV s−1 . Each anodic curve exhibits firstly, a small current peak A0 due to the formation of Cu2 O according to the equation [21]. Cu + OH− → Cu(OH)(ads) + e−
(1)
2Cu(OH)(ads) Cu2 O + H2 O
(2)
Secondly, a large anodic current peak AI which corresponds to CuCl salt layer [22,23]. This layer is mainly a result of a combined dissolution reaction. This involves the electrochemical adsorption of Cl− 2Cu + Cl− CuCl(ads) + e−
(3)
followed by chemical dissolution CuCl(ads) + Cl− CuCl− 2
(4)
or chemical adsorption of Cl− Cu(OH)(ad) + Cl− CuCl(ads) + OH−
(5)
Thirdly, a small peak AII can be correlated to the formation of either CuO or Cu(OH)2 [24]. From Fig. 1 the anodic current in sea water is higher than that in 3.5% NaCl. This behaviour reflects the effect of additional ions present in the sea water (Table 1) which helps the increase of metal corrosion. Also, this trend can be
Fig. 1. Potentiodynamic polarisation curves for Al–brass in: (a) 3.5% NaCl, (b) sea water.
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M.M. Osman / Materials Chemistry and Physics 71 (2001) 12–16
to Cu, respectively. The data reveal also that the cathodic peaks occur at potential (more negative) differ from that of the conjugated anodic peaks indicating that the reactions are irreversible [26]. The data show that the height of both the anodic and cathodic current peaks increase with increasing the scan number “n”. This change can be attributed to the activation process of electrode as a result of increasing the surface roughness by cyclic polarisation. The effect of adding sodium dodecyl benzene sulfonate (SDBS), dodecyl phenol ethoxylated with 9 units of EO [DPh(EO)9 ] and 1,1 -(lauryl amido)propyl ammonium chloride (LAPACl) on the corrosion behaviour of Al–brass in 3.5% NaCl and sea water are studied. From the potentiodynamic polarisation curves, the addition of these surfactants inhibits the anodic and cathodic reaction. It is clear that the presence of even trace amounts of these inhibitors suppresses completely the two anodic peaks A0 , AII and reduces the current peak of AI , their values are listed in Tables 2–4. Also, as the concentration of these compounds increases, Ecorr shifts to more positive range as shown in Tables 2–4. The inhibition efficiency, η, was calculated according to,
Fig. 2. Cyclic voltammograms in: (a) 3.5% NaCl, (b) sea water (first cycle) and (b ) sea water (second cycle).
due to the migration, adsorption, formation and dissolution of chlorides; all of which are facilitated by the other ions present in the sea water [25]. Fig. 2 represents cyclic voltammograms recorded for Al–brass in: (a) 3.5% NaCl and (b) sea water starting from −1 to +1 V with scan rate of 25 mV s−1 where the sweep was reversed. The voltammograms display an electrochemical spectrum which shows in addition to the anodic peaks mentioned, three cathodic peaks C0 , CI and CII . These peaks can be attributed to the electroreduction of Cu(II) to Cu(I)
η=
i0 − i × 100 i0
(6)
where η is the corrosion inhibition efficiency in percent, i0 and i are the corrosion current density without and with inhibitor. The relation between η and concentration of each inhibitor in both 3.5% NaCl and sea water inhibitor is plotted (Figs. 3–5). From these figures we notice that the efficiency increases with increasing the concentrations of these additives.
Table 2 Ecorr and iCorr of Al–brass as a function of SDBS concentration Concentration (ppm)
0 200 400 600 800 1000
3.5% NaCl
Sea water (A cm−2 )
ECorr (mV) (SCE)
iCorr
−750 −738 −735 −731 −729 −726
200.00 60.30 44.45 26.15 19.95 13.60
ECorr (mV) (SCE)
iCorr (A cm−2 )
−798 −788 −783 −779 −774 −770
350.14 115.33 85.60 62.57 40.27 31.51
Table 3 ECorr and iCorr of Al–brass as a function of DPh(EO)9 concentration Concentration (ppm)
0 200 400 600 800 1000
3.5% NaCl
Sea water
ECorr (mV) (SCE)
iCorr (A cm−2 )
ECorr (mV) (SCE)
iCorr (A cm−2 )
−750 −744 −742 −739 −738 −736
200.00 93.75 55.14 40.50 31.88 28.60
−798 −795 −790 −788 −787 −784
350.14 200.00 120.55 85.40 80.73 73.00
M.M. Osman / Materials Chemistry and Physics 71 (2001) 12–16
15
Table 4 ECorr and iCorr of Al–brass as a function of LAPACl concentration Concentration (ppm)
0 200 400 600 800 1000
3.5% NaCl
Sea water (A cm−2 )
ECorr (mV) (SCE)
iCorr
−750 −740 −737 −735 −732 −729
200.00 75.12 53.45 30.25 25.00 22.63
Fig. 3 shows that the inhibition action of SDBS may be a result of the blocking effect generated from the physical adsorption (not chemisorption) of the negatively charged C12 H25 C6 H4 SO− 3 on the positively charged Al–brass surface [27]. As the concentration of SDBS increases, the adsorption density may be enhanced through the interaction between its organic tails via van der Waals forces. This phenomenon leads to the formation of hemimicelle followed by multi-layers with more increasing in the surfactant concentration [27]. So, the inhibition efficiency increases with increasing the concentration SDBS. The effect of adding DPh(EO)9 on the inhibition efficiency is shown in Fig. 4. At low concentration, the surfactant ions are adsorbed on the surface by ion-pair and ion-exchange as well as the dispersion force acts between the hydrophobic chains [28]. By increasing the concentration, a significant increase of the adsorption is noticed, which can be explained by an aggregation of hydrophobic group at which the interaction takes place via van der Waals forces [29]. Consequently, a slight increase in η is shown in this region (Fig. 4). In case of LAPACl (Fig. 5), the chloride ions present in this compound are chemisorbed on the positively charged sites of the metal surface. This behaviour retards the metal
Fig. 3. η as a function of SDBS concentration in: (a) 3.5% NaCl, (b) sea water.
ECorr (mV) (SCE)
iCorr (A cm−2 )
−798 −791 −787 −783 −779 −773
350.14 140.50 101.74 70.90 63.25 59.11
Fig. 4. η as a function of DPh(EO)9 concentration in: (a) 3.5% NaCl, (b) sea water.
dissolution and hydrogen evolution. The metal surface may now be envisaged to have a negative charge; in the presence of positive cations a physical (electrostatic) adsorption is also favoured [30,31]. At high concentration levels, the positive ions LAPA+ could be expected to be adsorbed as
Fig. 5. η as a function of LAPACl concentration in: (a) 3.5% NaCl, (b) sea water.
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M.M. Osman / Materials Chemistry and Physics 71 (2001) 12–16
closely as possible with their cationic heads which oriented toward the surface and the alkyl chains oriented away from the surface. These chains would interact to form a layer above the head groups [32]. The inhibition efficiency for this inhibitor increases with the concentration up to a limiting value, then η% shows a nearly steady state due to the approximate coverage of the surface. According to the previous results we can deduce that the efficiency of the tested surfactants, as corrosion inhibitors for Al–brass in 3.5% NaCl and sea water, increases in the following order: SDBS > LAPACl > DPh(EO)9 This behaviour shows that the anionic surfactants SDBS, is more efficient than cationic, LAPACl, because the former adsorbed directly onto the Al surface which leads to quick adsorption and more inhibition efficiency. But in case of the latter, the Cl− counter ions adsorbed firstly onto the positive sites of the solid surface followed by LAPA+ as discussed before. This mechanism of adsorption needs a certain period of time to take place. For this reason, LAPACl exhibits a lowest value for corrosion inhibition than SDBS. The low inhibition efficiency appears in case of the nonionic surfactant, DPh(EO)9 , may be due to the molecular structure of this compound not bearing any charge and its adsorption onto the Al surface suffers some difficulty. Also, these compounds are more efficient as corrosion inhibitors in 3.5% NaCl than in sea water. This is due to the presence of different ions in sea water which depresses the inhibitor effect of these additives. 4. Conclusions 1. Three anodic peaks for Al–brass appeared in potentiodynamic polarisation due to the formation of Cu2 O, CuCl and CuO and/or Cu(OH)2 . 2. On reversing the sweep rate, another three peaks C0 , CI , CII will be formed due to the electrochemical reduction of the above peaks. 3. The corrosion behaviour of Al–brass in sea water is higher than that in case of 3.5% NaCl. 4. The inhibition properties of the used surfactants increase in the order SDBS > LAPACl > DPh(EO)9 and also increases with increasing the concentration of the investigated inhibitors.
References [1] F. Ovari, L. Tomcsanyi, T. Turmezey, Electrochim. Acta 33 (1988) 323. [2] L. Tomcsanyi, K. Varga, I. Bartik, G. Horanyi, E. Maleczki, Electrochim. Acta 34 (1989) 855. [3] R.M. Stevanovic, A.R. Despic, D.M. Drazic, Electrochim. Acta 33 (1988) 397. [4] A.R. Despic, D.M. Drazic, L.G. Krstajic, J. Electroanal. Chem. 242 (1988) 303. [5] S.E. Frers, M.M. Stefenel, C. Mayer, T. Chierchie, J. Appl. Electrochem. 20 (1990) 996. [6] L. Young, Anodic Oxide Films, Academic Press, New York, 1961, pp. 4–9. [7] A. Despic, V. Parkhutik, in: J.O’.M. Bockris, R.E. White, B.E. Canway (Eds.), Modern Aspects of Electrochemistry, Vol. 20, Plenum Press, New York, 1989. [8] J.W. Diggle, T.C. Downie, C.W. Goulding, Electrochim. Acta 15 (1) (1970) 79. [9] C.M.A. Brett, I.A.R. Gomes, J.P.S. Martins, Corros. Sci. 36 (1994) 915. [10] W.A. Badawy, F.M. Al-Kharafi, Bull. Electrochem. 11 (1995) 505. [11] F.M. Al-Kharafi, W.A. Baday, Electrochim. Acta 40 (1) (1995) 811. [12] I.L. Rozenfeld, Corrosion Inhibitors, McGraw-Hill, New York, 1981, p. 327. [13] D.M. Drazic, in: J.O’.M. Bockeris, R.E. White, B.E. Canway (Eds.), Modern Aspect of Electrochemistry, Vol. 19, Plenum Press, New York, 1989. [14] M.M. Osman, S.S. Abd-El Rehim, Mater. Chem. Phys. 53 (1998) 34. [15] H. Mihara, Y.H. Kawa, Denkikagaku 36 (1968) 657 [Chem. Abst. 70 (1969) 14022]. [16] D. Brasher, A.D. Mercer, Br. Corros. J. 3 (1968) 120. [17] Y.A. El-Awady, A.I. Ahmed, J. Ind. Chem. 24A (1985) 601. [18] M.N. Desai, R.R. Patel, D.K. Shah, J. Ind. Chem. Soc. L (1973) 341. [19] J.Y. Johnson, Brit. Pat. 378 (1931) 383. [20] R.K.S. Chan, J. Colloid Interf. Sci. 32 (1970) 492. [21] J.C. Hamilton, J.C. Farmer, R.J. Andreson, J. Electrochem. Soc. 133 (1986) 739. [22] H.P. Lee, K. Nobe, J. Electrochem. Soc. 133 (1986) 2035. [23] S.I. Pyun, Y.G. Chum, Br. Corros. J. 31 (1996) 147. [24] M.R.G. De Chialvo, R.C. Salvarezza, D. Vasquez Moll, A.J. Ariva, Electrochim. Acta 30 (1985) 1501. [25] J. Chavarin, Corrosion 47 (1991) 472. [26] J.C. Hamilton, J.C. Farmer, R.J. Andreson, J. Electrochem. Soc. 133 (1986) 739. [27] H. Luo, Y.C. Guan, K.N. Han, Corrosion 54 (1998) 619. [28] M.M. Osman, M.N. Shalaby, Anti-Corros. Meth. Mater. 44 (1997) 318. [29] M. Elachouri, M.S. Hajji, S. Kertit, E.M. Essassi, M. Salem, R. Coudert, Corros. Sci. 37 (1995) 381. [30] D.P. Schweinsberg, V. Ashworth, Corros. Sci. 28 (1988) 539. [31] M.N. Shalaby, M.M. Osman, A.A. El-Feky, Anti-Corros. Meth. Mater. 46 (1999) 254. [32] R.J. Meakins, M.G. Stevens, R.J. Hunter, J. Phys. Chem. 73 (1969) 112.
Corrosion inhibition of aluminium–brass in 3.5% NaCl solution and sea water M.M. Osman∗ Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt Received 18 May 2000; received in revised form 12 July 2000; accepted 16 October 2000
Abstract Electrochemical characteristics of Al–brass are investigated in both 3.5% NaCl and sea water. At active potential, the alloy is dissolved directly or through an adsorbed metastable CuCl layer. At higher applied anodic potential, the active–passive transition leads to the formation of a thicker and more stable CuCl salt layer. With further increase in the anodic potential, the formation of CuO and/or Cu(OH)2 will take place. The inhibition of the corrosion process is studied by adding SDBS, DPh(EO)9 , and LAPACl as anionic, nonionic, and cationic surfactants. It is found that the inhibition efficiency increases in the following order SDBS > LAPACl > DPh(EO)9 , and increases with increasing their concentrations. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Al–brass; Potentiodynamic polarisation; Cyclic voltammetric; Corrosion inhibition
1. Introduction Al and its alloys are used in a wide range of industrial applications for different aqueous solutions, so it represents an important category of technological materials [1–5]. The relatively low cost and high resistance toward corrosion of Al–brass preferred its use for many years in tubes production for sea water, cooled heat exchangers and piping. This metal and its alloy have the tendency to form a stable oxide film naturally or by anodisation [6,7]. Generally, Al-oxide consists of a thin barrier film adjacent to the metal surface (∼25 nm thick) which is covered by a thicker porous oxide layer. In aggressive media as chloride solution, a localised corrosion can occur which leads to a break down of the passive layer and a pit formation [8–11]. One of the most important methods in corrosion protection is the utilisation of organic inhibitors. Many mechanisms have been proposed for the inhibition of metal corrosion by organic inhibitors which takes place via adsorption on the metal surface [12,13]. The adsorption process leads to an effective blocking of the active sites of metal dissolution and/or hydrogen evolution, which results in a considerable decrease in the corrosion rate. The inhibitive effect of some ethoxylated fatty acids on Al-alloys was investigated [14] and showed that the adsorption of these compounds increased with increasing the ethylene oxide units and by the presence of the double bond in their molecular structures. ∗
Fax: +202-2747433.
Also, organic acids or their salts are used to inhibit the corrosion rate of aluminium and its alloys [15,16]. Some authors studied the effect of hydrazine compounds, aldehydes, ketones and amines as corrosion inhibitors for Al-alloys in hydrochloric acid solution [17,18]. In the present work, the corrosion characteristics of Al–brass alloy was investigated in 3.5% NaCl and natural sea water. The polarisation techniques were used to explain the corrosion inhibition processes which occur at the electrode– electrolyte interface using some organic inhibitors.
2. Experimental 2.1. Samples and measurements The tested electrodes were cut from a sheet of aluminium brass of the following composition in percent by weight: Cu
Si
Al
Mn
Mg
Cr
Zn
78.25
0.5
4.5
0.6
1.5
0.1
14.65
Electrodes were abraded successively with 0, 00, and 000 grades of emery paper before degreasing with acetone and washing with distilled water. Electrochemical measurements were carried out using an EG&G Model 273A potentiostate/galvanostate with model
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 5 1 0 - 1
M.M. Osman / Materials Chemistry and Physics 71 (2001) 12–16
13
3. Results and discussion
Table 1 Composition of sea water Cations
Na+
K+
Mg2+
Fe2+
Cu2+
Li+
Concentration (ppm)
13357
1224
1752
0.0054
0.0029
15.19
352 Soft Corr III corrosion measurement software was used to control the potentiodynamic process. The three electrodes of the polarisation cell are: a Pt counter electrode, a saturated calomel reference electrode, and Al–brass electrode. The specimens were immersed in two different media 3.5% NaCl solution and a solution of natural sea water. The sea water is analysed by ICP where the obtained data has been given in Table 1. All tests were carried out at 30◦ C. 2.2. The corrosion inhibitors The types of the used inhibitor samples were: 1. Cationic surfactant: 1,1 -(lauryl amido)propyl ammonium chloride with a chemical formula [CH3 (CH2 )10 CONH3 ]+ Cl− . 2. Anionic surfactant: sodium dodecyl benzene sulfonate with a chemical formula C12 H25 C6 H4 SO3 Na. 3. Nonionic surfactant: dodecyl phenol ethoxylated with 9 units of ethylene oxide and its formula is C12 H25 C6 H4 O(CH2 CH2 O)8 CH2 CH2 OH These inhibitors were prepared according to the reported procedures [19,20].
Fig. 1 shows the potentiodynamic polarisation curves for Al–brass in: (a) 3.5% NaCl and (b) sea water, respectively, scanned from −2 V up to oxygen evolution potential with scan rate of 25 mV s−1 . Each anodic curve exhibits firstly, a small current peak A0 due to the formation of Cu2 O according to the equation [21]. Cu + OH− → Cu(OH)(ads) + e−
(1)
2Cu(OH)(ads) Cu2 O + H2 O
(2)
Secondly, a large anodic current peak AI which corresponds to CuCl salt layer [22,23]. This layer is mainly a result of a combined dissolution reaction. This involves the electrochemical adsorption of Cl− 2Cu + Cl− CuCl(ads) + e−
(3)
followed by chemical dissolution CuCl(ads) + Cl− CuCl− 2
(4)
or chemical adsorption of Cl− Cu(OH)(ad) + Cl− CuCl(ads) + OH−
(5)
Thirdly, a small peak AII can be correlated to the formation of either CuO or Cu(OH)2 [24]. From Fig. 1 the anodic current in sea water is higher than that in 3.5% NaCl. This behaviour reflects the effect of additional ions present in the sea water (Table 1) which helps the increase of metal corrosion. Also, this trend can be
Fig. 1. Potentiodynamic polarisation curves for Al–brass in: (a) 3.5% NaCl, (b) sea water.
14
M.M. Osman / Materials Chemistry and Physics 71 (2001) 12–16
to Cu, respectively. The data reveal also that the cathodic peaks occur at potential (more negative) differ from that of the conjugated anodic peaks indicating that the reactions are irreversible [26]. The data show that the height of both the anodic and cathodic current peaks increase with increasing the scan number “n”. This change can be attributed to the activation process of electrode as a result of increasing the surface roughness by cyclic polarisation. The effect of adding sodium dodecyl benzene sulfonate (SDBS), dodecyl phenol ethoxylated with 9 units of EO [DPh(EO)9 ] and 1,1 -(lauryl amido)propyl ammonium chloride (LAPACl) on the corrosion behaviour of Al–brass in 3.5% NaCl and sea water are studied. From the potentiodynamic polarisation curves, the addition of these surfactants inhibits the anodic and cathodic reaction. It is clear that the presence of even trace amounts of these inhibitors suppresses completely the two anodic peaks A0 , AII and reduces the current peak of AI , their values are listed in Tables 2–4. Also, as the concentration of these compounds increases, Ecorr shifts to more positive range as shown in Tables 2–4. The inhibition efficiency, η, was calculated according to,
Fig. 2. Cyclic voltammograms in: (a) 3.5% NaCl, (b) sea water (first cycle) and (b ) sea water (second cycle).
due to the migration, adsorption, formation and dissolution of chlorides; all of which are facilitated by the other ions present in the sea water [25]. Fig. 2 represents cyclic voltammograms recorded for Al–brass in: (a) 3.5% NaCl and (b) sea water starting from −1 to +1 V with scan rate of 25 mV s−1 where the sweep was reversed. The voltammograms display an electrochemical spectrum which shows in addition to the anodic peaks mentioned, three cathodic peaks C0 , CI and CII . These peaks can be attributed to the electroreduction of Cu(II) to Cu(I)
η=
i0 − i × 100 i0
(6)
where η is the corrosion inhibition efficiency in percent, i0 and i are the corrosion current density without and with inhibitor. The relation between η and concentration of each inhibitor in both 3.5% NaCl and sea water inhibitor is plotted (Figs. 3–5). From these figures we notice that the efficiency increases with increasing the concentrations of these additives.
Table 2 Ecorr and iCorr of Al–brass as a function of SDBS concentration Concentration (ppm)
0 200 400 600 800 1000
3.5% NaCl
Sea water (A cm−2 )
ECorr (mV) (SCE)
iCorr
−750 −738 −735 −731 −729 −726
200.00 60.30 44.45 26.15 19.95 13.60
ECorr (mV) (SCE)
iCorr (A cm−2 )
−798 −788 −783 −779 −774 −770
350.14 115.33 85.60 62.57 40.27 31.51
Table 3 ECorr and iCorr of Al–brass as a function of DPh(EO)9 concentration Concentration (ppm)
0 200 400 600 800 1000
3.5% NaCl
Sea water
ECorr (mV) (SCE)
iCorr (A cm−2 )
ECorr (mV) (SCE)
iCorr (A cm−2 )
−750 −744 −742 −739 −738 −736
200.00 93.75 55.14 40.50 31.88 28.60
−798 −795 −790 −788 −787 −784
350.14 200.00 120.55 85.40 80.73 73.00
M.M. Osman / Materials Chemistry and Physics 71 (2001) 12–16
15
Table 4 ECorr and iCorr of Al–brass as a function of LAPACl concentration Concentration (ppm)
0 200 400 600 800 1000
3.5% NaCl
Sea water (A cm−2 )
ECorr (mV) (SCE)
iCorr
−750 −740 −737 −735 −732 −729
200.00 75.12 53.45 30.25 25.00 22.63
Fig. 3 shows that the inhibition action of SDBS may be a result of the blocking effect generated from the physical adsorption (not chemisorption) of the negatively charged C12 H25 C6 H4 SO− 3 on the positively charged Al–brass surface [27]. As the concentration of SDBS increases, the adsorption density may be enhanced through the interaction between its organic tails via van der Waals forces. This phenomenon leads to the formation of hemimicelle followed by multi-layers with more increasing in the surfactant concentration [27]. So, the inhibition efficiency increases with increasing the concentration SDBS. The effect of adding DPh(EO)9 on the inhibition efficiency is shown in Fig. 4. At low concentration, the surfactant ions are adsorbed on the surface by ion-pair and ion-exchange as well as the dispersion force acts between the hydrophobic chains [28]. By increasing the concentration, a significant increase of the adsorption is noticed, which can be explained by an aggregation of hydrophobic group at which the interaction takes place via van der Waals forces [29]. Consequently, a slight increase in η is shown in this region (Fig. 4). In case of LAPACl (Fig. 5), the chloride ions present in this compound are chemisorbed on the positively charged sites of the metal surface. This behaviour retards the metal
Fig. 3. η as a function of SDBS concentration in: (a) 3.5% NaCl, (b) sea water.
ECorr (mV) (SCE)
iCorr (A cm−2 )
−798 −791 −787 −783 −779 −773
350.14 140.50 101.74 70.90 63.25 59.11
Fig. 4. η as a function of DPh(EO)9 concentration in: (a) 3.5% NaCl, (b) sea water.
dissolution and hydrogen evolution. The metal surface may now be envisaged to have a negative charge; in the presence of positive cations a physical (electrostatic) adsorption is also favoured [30,31]. At high concentration levels, the positive ions LAPA+ could be expected to be adsorbed as
Fig. 5. η as a function of LAPACl concentration in: (a) 3.5% NaCl, (b) sea water.
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M.M. Osman / Materials Chemistry and Physics 71 (2001) 12–16
closely as possible with their cationic heads which oriented toward the surface and the alkyl chains oriented away from the surface. These chains would interact to form a layer above the head groups [32]. The inhibition efficiency for this inhibitor increases with the concentration up to a limiting value, then η% shows a nearly steady state due to the approximate coverage of the surface. According to the previous results we can deduce that the efficiency of the tested surfactants, as corrosion inhibitors for Al–brass in 3.5% NaCl and sea water, increases in the following order: SDBS > LAPACl > DPh(EO)9 This behaviour shows that the anionic surfactants SDBS, is more efficient than cationic, LAPACl, because the former adsorbed directly onto the Al surface which leads to quick adsorption and more inhibition efficiency. But in case of the latter, the Cl− counter ions adsorbed firstly onto the positive sites of the solid surface followed by LAPA+ as discussed before. This mechanism of adsorption needs a certain period of time to take place. For this reason, LAPACl exhibits a lowest value for corrosion inhibition than SDBS. The low inhibition efficiency appears in case of the nonionic surfactant, DPh(EO)9 , may be due to the molecular structure of this compound not bearing any charge and its adsorption onto the Al surface suffers some difficulty. Also, these compounds are more efficient as corrosion inhibitors in 3.5% NaCl than in sea water. This is due to the presence of different ions in sea water which depresses the inhibitor effect of these additives. 4. Conclusions 1. Three anodic peaks for Al–brass appeared in potentiodynamic polarisation due to the formation of Cu2 O, CuCl and CuO and/or Cu(OH)2 . 2. On reversing the sweep rate, another three peaks C0 , CI , CII will be formed due to the electrochemical reduction of the above peaks. 3. The corrosion behaviour of Al–brass in sea water is higher than that in case of 3.5% NaCl. 4. The inhibition properties of the used surfactants increase in the order SDBS > LAPACl > DPh(EO)9 and also increases with increasing the concentration of the investigated inhibitors.
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