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Evaluation of red mud as surface treatment for carbon steel prior painting
Progress in Organic Coatings 52 (2005) 351–358
Evaluation of red mud as surface treatment for carbon steel prior painting A. Collazo, D. Fern´andez, M. Izquierdo, X.R. N´ovoa, C. P´erez∗ Universidade de Vigo, ETSEIM, Lagoas-Marcosende 9, 36310 Vigo, Spain Received 12 December 2003; received in revised form 10 May 2004; accepted 16 June 2004
Abstract Red mud (RM) is the waste product of the Bayer process for obtaining alumina from bauxite. The alkaline nature of RM suspensions together with the presence of Fe3+ species point towards the possibility of using RM as a good corrosion inhibitor for carbon steel. Based on this idea, the possible use of RM suspensions as pre-treatment for carbon steel was studied by recording the electrode potential and the electrochemical impedance spectroscopy (EIS) evolution with immersion time. Different parameters regarding the steel surface finishing (grinding, pickling or degreasing) and RM suspension condition (stirred or steady, decanted or filtered) have been considered in the study; the passivation was obtained when ground samples were immersed in decanted RM suspensions and subjected to continuous stirring. The influence of chlorides and pH were analysed by potentiometric titration. The obtained results indicate that treated samples depassivate at lower Cl− /OH− ratio than untreated ones. Regarding the pH parameter, treated samples remain passive at lower pH values than the untreated ones. Finally, some treated and untreated samples were painted and subjected to cathodic polarisation experiments, including an artificial defect in the coating. The comparative study was done using EIS technique; the impedance diagrams would indicate an effective passivation of the steel surface for treated samples. © 2004 Elsevier B.V. All rights reserved. Keywords: Red mud; Passivation; Surface finishing; Potentiometric titration; Electrochemical impedance spectroscopy (EIS); Cathodic polarisation
1. Introduction Red mud (RM) is the insoluble residue remaining after the caustic digestion of bauxite, the ore used in the production of alumina through the Bayer process. This highly alkaline residue (pH = 10–12.5) is composed primarily of fine particles containing silica, aluminium, iron, calcium and titanium oxides and hidroxides (along with other minor components). The iron impurities are responsible for the brick red colour of the mud [1]. For every tonne of alumina produced, between 1 and 2 t (dry weight) of red mud residues are generated. This waste is a major environmental problem for areas where alumina industries are installed because of the alkaline nature and the chemical and mineralogical species present in RM [2].
∗
Corresponding author. Tel.: +34 986 81 26 03; fax: +34 986 81 22 01. E-mail address: [email protected] (C. P´erez).
0300-9440/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2004.06.008
Many attempts have been made over the years to find a use for this residue; some have been based on its possible usage as a partial substitute of clay in ceramic products (bricks, tiles, etc.) [3] or as an additive for mortar and concrete [4]. Attempts have also been made to use bauxite residue in agricultural applications, such as, in acidic soils or as a treatment for iron deficient soils [5]. Toxic heavy metals have been removed from aqueous solutions using red mud as an absorbent [6,7]. Unfortunately, those applications have proven to be economically unsatisfactory and research should be continued. The surface properties of red mud particles will strongly determine its behaviour, not only the composition but also the ˚ and 1 m. The caustic particle size that ranges between 50 A insoluble bauxite minerals are hematite (Fe2 O3 ), aluminium goethite ((Fe, Al)OOH), and titanium oxides; occasionally, boehmite (AlOOH) may be present depending on the extraction conditions. The aluminium loss in the Bayer process is due to the formation of a insoluble by-product named “Bayer sodalite”, 3(Na2 O·Al2 O3 ·2SiO2 ·nH2 O)·Na2 X, where X may
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be CO3 − , SO4 − , 2OH− , 2Cl− or a mixture of them, depending on the digesting liquor composition. The n value ranges from 0 to 2 [8]. Most of these minerals and oxides exhibit acid/base type behaviour [9], which is expected to occur on the surface of red mud particles. Surface charge properties of red mud have been studied by means of potentiometric titration, the RM particles in alkaline aqueous solutions carry ionised surface hydroxyl groups, S O− (S denotes red mud surface), these active sites can favour the adhesion between red mud particles and a metallic surface. On the other hand, when protons are added to RM aqueous solutions, the surface hydroxyl groups adsorb H+ ions, resulting in a nearly constant pH in the bulk solution, which indicates a certain buffering character [8,10–11]. Besides, Cl− ions can replace the surface OH groups, i.e., RM particles can capture aggressive ions, like Cl− , from the environment. In this field, previous studies have been carried out to evaluate the steel corrosion inhibition using RM aqueous solutions or red mud as an additive to cement paste (steel embedded in mortar) when Cl− ions are present [12–14]. The obtained results indicate that red mud is good corrosion inhibitor against chloride attack in this alkaline media. Finally, the presence of Fe3+ species indicates a certain redox activity; in this way, attempts to use RM as an anodic pigment in anticorrosive paints have been reported [15,16]. Following those experiences, and based on the previous works by our group, the present paper deals with the possible use of RM as an alternative pre-treatment of carbon steel. The traditional treatments, based on chromate conversion layers, are hazardous and new environment-friendly alternatives are being investigated [17,18]. Bearing this in mind, the first part of this study is focused on the assessment of the conditions which enable optimal surface passivation. Once those parameters were defined, the second part was devoted to the evaluation of such pre-treatment with a comparative study between painted samples previously treated, and grinded carbon steel without treatment.
2. Experimental Carbon steel samples (S = 4 cm2 ) were prepared with different surface finishing: ground, pickled with HCl acid or degreased with trichloroethylene. The passivation procedure was direct immersion in a red mud suspension. The RM was supplied by ALCOA factory located in San Cibrao (Northwest of Spain) with a chemical composition (w/w, %): Fe2 O3 (37%), TiO2 (20%), Al2 O3 (12%), SiO2 (9%), CaO (6%), Na2 O (5%), H2 O (1000 ◦ C) (11%). The RM suspensions were prepared by adding 20 g of solid to 1 l of distilled water. Once vigorously shaken and left overnight, the decanted suspension was taken (pH = 12) and the steel samples were immersed in it. Some tests were done with “filtered suspension” obtained by filtering the decanted one. All experiments were performed on stirred RM suspensions, unless “steady”
is specified. The electrode potential evolution with immersion time was recorded for the different situations. Surface characterization of treated steel samples was carried out by scanning electron microscopy (SEM) analysis using an Electroscan JSM-5410, equipped with an energy dispersive X-ray (EDX) detector Link ISIS 300. The compound identification was performed by X-ray diffraction (XRD) technique using a Siemens D5000 powder diffractometer. In order to study the chlorides and pH influence in passivity breakdown, potentiometric titrations were performed using treated and untreated samples. A Crison pH meter was used for pH measurements. Previously, the pH meter was calibrated using a set of 4.0, 7.0, 9.0 and 11.0 standard pH solutions. Periodically, impedance measurements were taken. The electrochemical experiments, potential evolution with immersion time and impedance measurements, were performed using an AUTOLAB PGSTST30/FRA from Ecochemie. The total immersion time for the potential evolution curves was 24 h, unless the samples exhibited generalised corrosion earlier. EIS measurements were taken in potentiostatic mode at the open circuit potential, except for the cathodic polarisation experiments. The frequency range covered was from 104 Hz down to 10−2 Hz and the signal amplitude was 10 mV r.m.s. A conventional three electrode arrangement was used: working electrodes of carbon steel or painted carbon steel, a graphite sheet as large counter electrode, and a saturated calomel electrode (SCE) or Hg/HgO 0.1 M KOH as reference electrodes. The potential of the Hg/HgO electrode used was +80 mV with respect to SCE. The steel samples were painted by air-spraying after being passivated. The primer was based on alkyd-phenol resin with 41 m average thickness and the topcoat was based on acrylic resin with an average thickness of 53 m. The same paint system was applied on ground non-passivated steel samples. In order to detect differential behaviours, an artificial defect (S ≈ 0.2 mm2 ) was made in both types of the samples and, in order to promote the cathodic disbonding, they were subjected to cathodic polarisation, using constant current I = 1 A with modulation I = 0.1 A r.m.s. for EIS measurements (galvanostatic mode). The frequency range was from 105 down to 10−2 Hz. The electrochemical cell has been described above and the electrolyte used was 5% NaCl solution.
3. Results and discussion 3.1. Surface passivation conditions Fig. 1 depicts the corrosion potential evolution with immersion time for carbon steel samples immersed in stirred and decanted RM suspensions, with different steel surface finishing. This parameter seems to be critical: the higher initial potentials correspond to the ground sample, where the pre-existing oxides are mechanically removed. In the case of degreased and pickled samples, cleaning is less effective
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Fig. 1. Corrosion potential evolution for carbon steel samples immersed in stirred RM suspensions with different surface finishing conditions.
and some oxides may remain on the steel surface. In alkaline media, those oxides, together with the RM particles, are charged negatively and the repulsive interactions hinder the adsorption of RM on the metallic surface [19]. Concerning the RM solution parameters, Fig. 2 shows the corrosion potential evolution of ground carbon steel samples immersed in different RM suspension conditions, all of them are decanted, unless “filtered” is specified. The samples have high initial potential values, which confirm the results obtained in Fig. 1 regarding the steel surface preparation. On the other hand, the best results are obtained with decanted and stirred solutions. It seems that a minimum concentration of RM particles is necessary to meet passivation requirements [10] and these particles should be moving to prevent the formation of large deposits on the metallic surface, which can induce phenomena such as local crevice corrosion. Figs. 3 and 4 show the impedance diagrams obtained on carbon steel samples immersed in RM suspensions, at the corrosion potential.
Fig. 2. Corrosion potential evolution of ground carbon steel samples using different RM suspensions conditions.
Fig. 3. Nyquist (a) and Bode (b) plots obtained for ground carbon steel samples immersed in stirred RM suspension at different immersion times.
The markedly high impedance values corresponding to the ground samples immersed in stirred and decanted suspensions with respect to those obtained in other situations (Fig. 4) are significant. The spectra exhibit only one time constant and the low frequency limit enables the polarisation resistance, Rp , to be obtained [20]. The Rp values are presented in Table 1.
Fig. 4. Nyquist and Bode plots obtained at the end of the potential vs. immersion time curve (Fig. 2) for ground carbon steel sample immersed in filtered RM suspension.
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Fig. 5. Picture of degreased carbon steel sample after 4 h immersion in stirred and decanted RM suspension.
Using the diameter of the capacitive arcs (Figs. 3a and 4) as Rp , the corrosion rate can be calculated from Eq. (1) and assuming that B = 26 mV. The results are depicted in Table 1. As can be seen, only the ground samples immersed in stirred and decanted RM suspensions have low corrosion rates, well below the threshold usually considered for a passive steel (0.1 A cm−2 ) [21]; moreover, these values decrease as im-
Fig. 6. X-ray diffractograms corresponded to red mud powder (upper) and passivated steel surface (lower).
mersion time increases icorr =
B Rp
(1)
Fig. 5 is a picture of a degreased sample immersed in stirred RM suspensions at the end of the experiment, corrosion signs are evident.
Fig. 7. (a) and (b) SEM images at different magnifications and (c) EDX spectrum of the metallic surface after the passivation process in RM suspension.
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Table 1 Polarisation resistance, Rp , and corrosion rate, icorr , obtained for the different testing conditions Sample
Rp (k cm2 )
icorr (A cm−2 )
Ground (1 h) Ground (7 h) Ground (24 h) Ground + filtered (24 h) Ground + steady (24 h) Degreased (4 h) Pickled (24 h)
156 607 2.6 × 103 2 0.93 0.73 5.3
0.17 0.04 0.01 12.8 27.8 35.5 4.8
Based on these experiences, it is possible to conclude that the optimum passivating conditions correspond to ground carbon steel samples immersed for about 24 h in decanted RM suspensions subjected to continuous stirring. So, from here on, the term “passivated samples” will refer to those samples treated in such conditions. Fig. 6 shows the X-ray diffractograms corresponding to red mud podwer and the passivated steel surface. It is possible to identify the main RM components: iron oxide
Fig. 8. Potentiometric titration with 5N NaCl of samples previously passivated in RM suspension and carbon steel without pre-treatment (control samples). Both are immersed in 250 mL 0.1N NaOH solution.
Fig. 9. Bode plots obtained at different Cl− /OH− ratio for (a) passivated samples and (b) control samples. The potential values are measured with respect to the Hg/HgO 0.1 M KOH.
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(hematite) and Fe oxo-hydroxide (goethite), titanium oxide (rutile) as well as aluminium oxide (gibbsite) and aluminium oxo-hydroxide (boehmite) together with the Bayer sodalite. Nevertheless, only the iron and aluminium oxo-hidroxides (goethite and boehmite) are detected on the steel surface. This fact can be explained in terms of the ability of these compounds to be negatively charged in alkaline media and, thus, having more active sites to adhere to the metallic surface. Fig. 7 corresponds to the SEM images and the associated EDX spectrum of the passivated metallic surface. It is noticeable the lack of a continous layer; instead of that, particles with heterogeneous sizes are found, distributed all over the surface (see Fig. 7a and b). The EDX spectrum confirm that only iron and aluminium compounds are present on the steel surface. One interesting aspect to analyse here is the effect that chloride additions have on these passivated surfaces. For that purpose, potentiometric titrations with NaCl 5N were performed. Fig. 8 depicts the results obtained on passivated samples immersed in 250 mL 0.1 M NaOH (pH = 13) solution and on ground carbon steel samples without any pre-treatment, denoted as control. Both were subjected to the same immersion conditions. Initially, anodic potentials were recorded in both cases, as corresponding to steel samples immersed in alkaline solutions. When the Cl− /OH− ratio reaches a critical value [22], the passivity breakdown occurs and the electrode potential falls to about − 500 mV versus Hg/HgO. In this sense, a big difference can be observed in the “critical” ratio depending on the type of samples. Thus, the control ones withstand a very high Cl− /OH− ratio with no corrosion signs, while in the case of passivated samples, a much lower Cl− /OH− ratio leads to passivity breakdown. This evolution seems surprising, because in previous studies [10,12] better behaviour has been observed for carbon steel immersed in RM suspensions than samples immersed in NaOH solution (pH = 12), both containing high chloride concentration. That fact has led us to think that RM is a good inhibitor against chloride attack. The explanation for the present results may come from the difference in experimental conditions. In the current experiment, where the samples are immersed in NaOH solution, there are discrete particles of iron and aluminium oxo-hydroxides on the steel surface, and the surrounding Cl− ions can replace the surface OH groups. The result is the formation of new chloride containing deposits on the metallic surface [10], which tend to depassivate it; in addition to the possible local crevice
Fig. 10. Acid/base potentiometric titration with 0.1N HCl of samples previously passivated in RM suspension and carbon steel without pre-treatment (control samples). Both are immersed in 250 mL 0.1N NaOH solutions.
corrosion phenomena. In the previous experiences [11,12] the samples were immersed in RM suspensions, so the huge amount of particles present favours chloride adsorption in the bulk suspension instead of in the particles deposited on the steel surface, avoiding the formation of the chloride containing deposits and crevice corrosion phenomena.
Table 2 Corrosion rates, icorr , obtained at different Cl− /OH− ratios for passivated and control samples Passivated sample Cl− /OH− 1 3.5
ratio
Control sample icorr 0.04 1.2
(A cm−2 )
Cl− /OH− ratio
icorr (A cm−2 )
1 3.5 14.5
0.06 0.06 7.1
Fig. 11. Bode plots for (a) passivated samples and (b) control samples at pHs higher and lower than passivity breakdown. The potential values are measured with respect to the Hg/HgO 0.1 M KOH.
A. Collazo et al. / Progress in Organic Coatings 52 (2005) 351–358
The impedance measurements performed at different Cl− /OH− ratios corroborate these results. Fig. 9 depicts the corresponding Bode plots obtained at the initial stage and at passivity breakdown. A dramatic decreasing in the impedance values at the low frequency limit is observed after passivity breakdown. Using Eq. (1) the corrosion rates can again be estimated, Table 2 shows some of representative values. The corrosion rate at Cl− /OH− = 3.5 ratio raises for the passivated sample, whereas the control sample remains passive, as its low icorr value reflects. The influence of pH was analysed using acid/base potentiometric titration of the passivated and control samples immersed in 250 mL 0.1 M NaOH solution and titrated with 0.1 M HCl. The results are shown in Fig. 10. As can be seen, the passivated samples withstand better the H+ addition than the control ones, which may be the result of the buffering effect of the red mud deposits [8]. Fig. 11 shows the impedance diagrams before and after passivity breakdown, which reflect the drastic decrease during this transition. The corrosion rates estimation gives high values, about 40 A cm−2 for both types of samples.
Fig. 12. Nyquist (a) and Bode (b) plots of painted passivated samples with artificial defect subjected to cathodic polarisation conditions. The potential values are measured with respect to saturated calomel electrode (SCE).
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3.2. Behaviour of painted samples under cathodic polarisation conditions In order to evaluate the effect of the pre-treatment, a paint system was applied on passivated samples and ground carbon steel specimens. An artificial defect was done in both types of the samples and they were subjected to cathodic polarisation experiment. Figs. 12 and 13 show the impedance spectra evolution with immersion time. Big differences are observed depending on the specimen types. Passivated samples (Fig. 12) show an important impedance increase with immersion time and the measured potential remains in well cathodic values, indicating that an effective cathodic protection of the metallic surface was achieved. Nevertheless, the evolution of samples not pre-treated (Fig. 13) indicates that a surface activation occurs with immersion time. Initially, the potential values fall in the cathodic domain (E = −1.13 VSCE ) but, as the immersion time increases, these values shift anodically up to reach the free corrosion potential of carbon steel. These results point towards the RM pre-treatment may be an interesting alternative to the traditional passive layers in confined and cathodic disbonding conditions.
Fig. 13. Nyquist (a) and Bode (b) plots of painted non-passivated samples with artificial defect subjected to cathodic polarisation conditions. The potential values are measured with respect to saturated calomel electrode (SCE).
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In spite of that, the mechanisms developed in both situations are not well understood and more detailed studies must be undertaken.
4. Conclusions The study of different parameters which can affect the steel surface passivation indicates that the better conditions correspond to ground surfaces immersed for about 24 h in stirred and decanted red mud suspensions. Based on the X-ray diffraction results, iron and aluminium oxo-hidroxides are the only RM components deposited on the metallic surface, not as a continuous layer but as discrete distribution of particles. The presence of RM particles promote Cl− ions adsorption on the steel surface and depassivation takes place at lower Cl− /OH− ratios. On the other hand, passivated steel samples withstand lower pHs than the non-passivated ones. The evaluation of RM pre-treatment into a paint scheme was analysed using painted samples with an artificial defect, subjected to cathodic polarisation. The results obtained indicate that the passivated samples improve their behaviour with the immersion time. Nevertheless, the non-passivated specimens tend to activate the surface as shown the impedance diagrams.
References [1] A.R. Hind, S.K. Bhargava, S.C. Grocott, Colloids Surf. A 146 (1999) 359. [2] D.J. Cooling, D.J. Glenister, Light Met. (1992) 25. [3] M. Patel, B.K. Padhi, P. Vidyasagar, A.K. Pattnaiik, Res. Ind. 37 (1992) 154. [4] J. Pera, R. Boumaza, J. Ambroise, Cem. Conc. Res. 27 (1997) 1513. [5] R.N. Summers, N.R. Guise, D.D. Smirk, Fert. Res. 34 (1993) 85.
[6] A.I. Zouboulis, K.A. Kydros, K.A. Matis, Water Sci. Technol. 27 (1993) 83. [7] E. L´opez, B. Soto, M. Arias, A. N´un˜ ez, D. Rubinos, M.T. Barral, Water Res. 32 (1998) 1314. [8] D. Chvedov, S. Ostap, T. Le, Colloids Surf. A 182 (2001) 131. [9] W. Stumm, Chemistry of the Solid–Water Interface, Wiley, New York, 1995 (Chapters 2–6). [10] G. Atun, G. Hisarli, J. Colloid Interf. Sci. 228 (2000) 131. [11] B. D´ıaz, S. Joiret, M. Keddam, X.R. N´ovoa, M.C. P´erez, H. Takenouti, Electrochim. Acta 49 (2004) 3039. [12] X.R. N´ovoa, C. P´erez, J.J. P´erez, M.C. P´erez, Proc. V Congr. Nac. Corros. y Prot., Madrid, Spain, June 19–22, 2000. [13] A. Collazo, M.J. Crist´obal, X.R. N´ovoa, G. Pena, M.C. P´erez, Electrochemical impedance spectroscopy as a tool for studying steel corrosion inhibition in concrete environments. Red mud used as rebar inhibitor, in: N.S. Berke (Ed.), Electrochemical Techniques for Evaluating Corrosion Performance and Estimating Service-Life of Reinforced Concrete, ASTM STP 1457, ASTM International, West Conshohocken, PA, 2004. [14] M. Cabeza, A. Collazo, X.R. N´ovoa, M.C. P´erez, Red Mud as Corrosion Inhibitor for Reinforced Concrete, vol. 6, JCSE, p. C077. Available at http://www2.umist.ac.uk/corrosion/Jcse/Volume6/ Preprints/V6Preprint32.pdf. [15] S. Ramanujan, Paintindia 12 (1962) 22. [16] T. Skoulikidis, P. Vassiliou, N. Diamantis, P.J. Tunturi (Eds.), Proc. of 12th Scand. Corros. Congr. Eurocorr’92, Corros. Soc. of Finland, 1992, p. 475. [17] T.L. Metroke, R.L. Parkill, E.T. Knobbe, Prog. Org. Coat. 41 (2001) 233. [18] T.R. Farhat, J.B. Schlenoff, Electrochem. Solid-State Lett. 5 (2002) 13. [19] A. Carnot, I. Frateur, S. Zanna, B. Tribollet, I. Dubois-Brugger, P. Marcus, Corros. Sci. 45 (2003) 2513. [20] G.K. Glass, C.L. Page, N.R. Short, J.Z. Zhang, Corros. Sci. 39 (1997) 1657. [21] C. Andrade, V. Castelo, C. Alonso, J.A. Gonz´alez, The determination of the corrosion rate of steel embedded in concrete by polarization resistance and AC impedance methods, in: V. Chaker (Ed.), Corrosion Effects of Stray Currents and the Techniques for Evaluating Corrosion of Rebars in Concrete, ASTM STP906, ASTM, Phladelphia, 1986, p. 46. [22] C. Alonso, M. Castellote, C. Andrade, Electrochim. Acta 47 (2002) 3469.
Evaluation of red mud as surface treatment for carbon steel prior painting A. Collazo, D. Fern´andez, M. Izquierdo, X.R. N´ovoa, C. P´erez∗ Universidade de Vigo, ETSEIM, Lagoas-Marcosende 9, 36310 Vigo, Spain Received 12 December 2003; received in revised form 10 May 2004; accepted 16 June 2004
Abstract Red mud (RM) is the waste product of the Bayer process for obtaining alumina from bauxite. The alkaline nature of RM suspensions together with the presence of Fe3+ species point towards the possibility of using RM as a good corrosion inhibitor for carbon steel. Based on this idea, the possible use of RM suspensions as pre-treatment for carbon steel was studied by recording the electrode potential and the electrochemical impedance spectroscopy (EIS) evolution with immersion time. Different parameters regarding the steel surface finishing (grinding, pickling or degreasing) and RM suspension condition (stirred or steady, decanted or filtered) have been considered in the study; the passivation was obtained when ground samples were immersed in decanted RM suspensions and subjected to continuous stirring. The influence of chlorides and pH were analysed by potentiometric titration. The obtained results indicate that treated samples depassivate at lower Cl− /OH− ratio than untreated ones. Regarding the pH parameter, treated samples remain passive at lower pH values than the untreated ones. Finally, some treated and untreated samples were painted and subjected to cathodic polarisation experiments, including an artificial defect in the coating. The comparative study was done using EIS technique; the impedance diagrams would indicate an effective passivation of the steel surface for treated samples. © 2004 Elsevier B.V. All rights reserved. Keywords: Red mud; Passivation; Surface finishing; Potentiometric titration; Electrochemical impedance spectroscopy (EIS); Cathodic polarisation
1. Introduction Red mud (RM) is the insoluble residue remaining after the caustic digestion of bauxite, the ore used in the production of alumina through the Bayer process. This highly alkaline residue (pH = 10–12.5) is composed primarily of fine particles containing silica, aluminium, iron, calcium and titanium oxides and hidroxides (along with other minor components). The iron impurities are responsible for the brick red colour of the mud [1]. For every tonne of alumina produced, between 1 and 2 t (dry weight) of red mud residues are generated. This waste is a major environmental problem for areas where alumina industries are installed because of the alkaline nature and the chemical and mineralogical species present in RM [2].
∗
Corresponding author. Tel.: +34 986 81 26 03; fax: +34 986 81 22 01. E-mail address: [email protected] (C. P´erez).
0300-9440/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2004.06.008
Many attempts have been made over the years to find a use for this residue; some have been based on its possible usage as a partial substitute of clay in ceramic products (bricks, tiles, etc.) [3] or as an additive for mortar and concrete [4]. Attempts have also been made to use bauxite residue in agricultural applications, such as, in acidic soils or as a treatment for iron deficient soils [5]. Toxic heavy metals have been removed from aqueous solutions using red mud as an absorbent [6,7]. Unfortunately, those applications have proven to be economically unsatisfactory and research should be continued. The surface properties of red mud particles will strongly determine its behaviour, not only the composition but also the ˚ and 1 m. The caustic particle size that ranges between 50 A insoluble bauxite minerals are hematite (Fe2 O3 ), aluminium goethite ((Fe, Al)OOH), and titanium oxides; occasionally, boehmite (AlOOH) may be present depending on the extraction conditions. The aluminium loss in the Bayer process is due to the formation of a insoluble by-product named “Bayer sodalite”, 3(Na2 O·Al2 O3 ·2SiO2 ·nH2 O)·Na2 X, where X may
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be CO3 − , SO4 − , 2OH− , 2Cl− or a mixture of them, depending on the digesting liquor composition. The n value ranges from 0 to 2 [8]. Most of these minerals and oxides exhibit acid/base type behaviour [9], which is expected to occur on the surface of red mud particles. Surface charge properties of red mud have been studied by means of potentiometric titration, the RM particles in alkaline aqueous solutions carry ionised surface hydroxyl groups, S O− (S denotes red mud surface), these active sites can favour the adhesion between red mud particles and a metallic surface. On the other hand, when protons are added to RM aqueous solutions, the surface hydroxyl groups adsorb H+ ions, resulting in a nearly constant pH in the bulk solution, which indicates a certain buffering character [8,10–11]. Besides, Cl− ions can replace the surface OH groups, i.e., RM particles can capture aggressive ions, like Cl− , from the environment. In this field, previous studies have been carried out to evaluate the steel corrosion inhibition using RM aqueous solutions or red mud as an additive to cement paste (steel embedded in mortar) when Cl− ions are present [12–14]. The obtained results indicate that red mud is good corrosion inhibitor against chloride attack in this alkaline media. Finally, the presence of Fe3+ species indicates a certain redox activity; in this way, attempts to use RM as an anodic pigment in anticorrosive paints have been reported [15,16]. Following those experiences, and based on the previous works by our group, the present paper deals with the possible use of RM as an alternative pre-treatment of carbon steel. The traditional treatments, based on chromate conversion layers, are hazardous and new environment-friendly alternatives are being investigated [17,18]. Bearing this in mind, the first part of this study is focused on the assessment of the conditions which enable optimal surface passivation. Once those parameters were defined, the second part was devoted to the evaluation of such pre-treatment with a comparative study between painted samples previously treated, and grinded carbon steel without treatment.
2. Experimental Carbon steel samples (S = 4 cm2 ) were prepared with different surface finishing: ground, pickled with HCl acid or degreased with trichloroethylene. The passivation procedure was direct immersion in a red mud suspension. The RM was supplied by ALCOA factory located in San Cibrao (Northwest of Spain) with a chemical composition (w/w, %): Fe2 O3 (37%), TiO2 (20%), Al2 O3 (12%), SiO2 (9%), CaO (6%), Na2 O (5%), H2 O (1000 ◦ C) (11%). The RM suspensions were prepared by adding 20 g of solid to 1 l of distilled water. Once vigorously shaken and left overnight, the decanted suspension was taken (pH = 12) and the steel samples were immersed in it. Some tests were done with “filtered suspension” obtained by filtering the decanted one. All experiments were performed on stirred RM suspensions, unless “steady”
is specified. The electrode potential evolution with immersion time was recorded for the different situations. Surface characterization of treated steel samples was carried out by scanning electron microscopy (SEM) analysis using an Electroscan JSM-5410, equipped with an energy dispersive X-ray (EDX) detector Link ISIS 300. The compound identification was performed by X-ray diffraction (XRD) technique using a Siemens D5000 powder diffractometer. In order to study the chlorides and pH influence in passivity breakdown, potentiometric titrations were performed using treated and untreated samples. A Crison pH meter was used for pH measurements. Previously, the pH meter was calibrated using a set of 4.0, 7.0, 9.0 and 11.0 standard pH solutions. Periodically, impedance measurements were taken. The electrochemical experiments, potential evolution with immersion time and impedance measurements, were performed using an AUTOLAB PGSTST30/FRA from Ecochemie. The total immersion time for the potential evolution curves was 24 h, unless the samples exhibited generalised corrosion earlier. EIS measurements were taken in potentiostatic mode at the open circuit potential, except for the cathodic polarisation experiments. The frequency range covered was from 104 Hz down to 10−2 Hz and the signal amplitude was 10 mV r.m.s. A conventional three electrode arrangement was used: working electrodes of carbon steel or painted carbon steel, a graphite sheet as large counter electrode, and a saturated calomel electrode (SCE) or Hg/HgO 0.1 M KOH as reference electrodes. The potential of the Hg/HgO electrode used was +80 mV with respect to SCE. The steel samples were painted by air-spraying after being passivated. The primer was based on alkyd-phenol resin with 41 m average thickness and the topcoat was based on acrylic resin with an average thickness of 53 m. The same paint system was applied on ground non-passivated steel samples. In order to detect differential behaviours, an artificial defect (S ≈ 0.2 mm2 ) was made in both types of the samples and, in order to promote the cathodic disbonding, they were subjected to cathodic polarisation, using constant current I = 1 A with modulation I = 0.1 A r.m.s. for EIS measurements (galvanostatic mode). The frequency range was from 105 down to 10−2 Hz. The electrochemical cell has been described above and the electrolyte used was 5% NaCl solution.
3. Results and discussion 3.1. Surface passivation conditions Fig. 1 depicts the corrosion potential evolution with immersion time for carbon steel samples immersed in stirred and decanted RM suspensions, with different steel surface finishing. This parameter seems to be critical: the higher initial potentials correspond to the ground sample, where the pre-existing oxides are mechanically removed. In the case of degreased and pickled samples, cleaning is less effective
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Fig. 1. Corrosion potential evolution for carbon steel samples immersed in stirred RM suspensions with different surface finishing conditions.
and some oxides may remain on the steel surface. In alkaline media, those oxides, together with the RM particles, are charged negatively and the repulsive interactions hinder the adsorption of RM on the metallic surface [19]. Concerning the RM solution parameters, Fig. 2 shows the corrosion potential evolution of ground carbon steel samples immersed in different RM suspension conditions, all of them are decanted, unless “filtered” is specified. The samples have high initial potential values, which confirm the results obtained in Fig. 1 regarding the steel surface preparation. On the other hand, the best results are obtained with decanted and stirred solutions. It seems that a minimum concentration of RM particles is necessary to meet passivation requirements [10] and these particles should be moving to prevent the formation of large deposits on the metallic surface, which can induce phenomena such as local crevice corrosion. Figs. 3 and 4 show the impedance diagrams obtained on carbon steel samples immersed in RM suspensions, at the corrosion potential.
Fig. 2. Corrosion potential evolution of ground carbon steel samples using different RM suspensions conditions.
Fig. 3. Nyquist (a) and Bode (b) plots obtained for ground carbon steel samples immersed in stirred RM suspension at different immersion times.
The markedly high impedance values corresponding to the ground samples immersed in stirred and decanted suspensions with respect to those obtained in other situations (Fig. 4) are significant. The spectra exhibit only one time constant and the low frequency limit enables the polarisation resistance, Rp , to be obtained [20]. The Rp values are presented in Table 1.
Fig. 4. Nyquist and Bode plots obtained at the end of the potential vs. immersion time curve (Fig. 2) for ground carbon steel sample immersed in filtered RM suspension.
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Fig. 5. Picture of degreased carbon steel sample after 4 h immersion in stirred and decanted RM suspension.
Using the diameter of the capacitive arcs (Figs. 3a and 4) as Rp , the corrosion rate can be calculated from Eq. (1) and assuming that B = 26 mV. The results are depicted in Table 1. As can be seen, only the ground samples immersed in stirred and decanted RM suspensions have low corrosion rates, well below the threshold usually considered for a passive steel (0.1 A cm−2 ) [21]; moreover, these values decrease as im-
Fig. 6. X-ray diffractograms corresponded to red mud powder (upper) and passivated steel surface (lower).
mersion time increases icorr =
B Rp
(1)
Fig. 5 is a picture of a degreased sample immersed in stirred RM suspensions at the end of the experiment, corrosion signs are evident.
Fig. 7. (a) and (b) SEM images at different magnifications and (c) EDX spectrum of the metallic surface after the passivation process in RM suspension.
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Table 1 Polarisation resistance, Rp , and corrosion rate, icorr , obtained for the different testing conditions Sample
Rp (k cm2 )
icorr (A cm−2 )
Ground (1 h) Ground (7 h) Ground (24 h) Ground + filtered (24 h) Ground + steady (24 h) Degreased (4 h) Pickled (24 h)
156 607 2.6 × 103 2 0.93 0.73 5.3
0.17 0.04 0.01 12.8 27.8 35.5 4.8
Based on these experiences, it is possible to conclude that the optimum passivating conditions correspond to ground carbon steel samples immersed for about 24 h in decanted RM suspensions subjected to continuous stirring. So, from here on, the term “passivated samples” will refer to those samples treated in such conditions. Fig. 6 shows the X-ray diffractograms corresponding to red mud podwer and the passivated steel surface. It is possible to identify the main RM components: iron oxide
Fig. 8. Potentiometric titration with 5N NaCl of samples previously passivated in RM suspension and carbon steel without pre-treatment (control samples). Both are immersed in 250 mL 0.1N NaOH solution.
Fig. 9. Bode plots obtained at different Cl− /OH− ratio for (a) passivated samples and (b) control samples. The potential values are measured with respect to the Hg/HgO 0.1 M KOH.
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(hematite) and Fe oxo-hydroxide (goethite), titanium oxide (rutile) as well as aluminium oxide (gibbsite) and aluminium oxo-hydroxide (boehmite) together with the Bayer sodalite. Nevertheless, only the iron and aluminium oxo-hidroxides (goethite and boehmite) are detected on the steel surface. This fact can be explained in terms of the ability of these compounds to be negatively charged in alkaline media and, thus, having more active sites to adhere to the metallic surface. Fig. 7 corresponds to the SEM images and the associated EDX spectrum of the passivated metallic surface. It is noticeable the lack of a continous layer; instead of that, particles with heterogeneous sizes are found, distributed all over the surface (see Fig. 7a and b). The EDX spectrum confirm that only iron and aluminium compounds are present on the steel surface. One interesting aspect to analyse here is the effect that chloride additions have on these passivated surfaces. For that purpose, potentiometric titrations with NaCl 5N were performed. Fig. 8 depicts the results obtained on passivated samples immersed in 250 mL 0.1 M NaOH (pH = 13) solution and on ground carbon steel samples without any pre-treatment, denoted as control. Both were subjected to the same immersion conditions. Initially, anodic potentials were recorded in both cases, as corresponding to steel samples immersed in alkaline solutions. When the Cl− /OH− ratio reaches a critical value [22], the passivity breakdown occurs and the electrode potential falls to about − 500 mV versus Hg/HgO. In this sense, a big difference can be observed in the “critical” ratio depending on the type of samples. Thus, the control ones withstand a very high Cl− /OH− ratio with no corrosion signs, while in the case of passivated samples, a much lower Cl− /OH− ratio leads to passivity breakdown. This evolution seems surprising, because in previous studies [10,12] better behaviour has been observed for carbon steel immersed in RM suspensions than samples immersed in NaOH solution (pH = 12), both containing high chloride concentration. That fact has led us to think that RM is a good inhibitor against chloride attack. The explanation for the present results may come from the difference in experimental conditions. In the current experiment, where the samples are immersed in NaOH solution, there are discrete particles of iron and aluminium oxo-hydroxides on the steel surface, and the surrounding Cl− ions can replace the surface OH groups. The result is the formation of new chloride containing deposits on the metallic surface [10], which tend to depassivate it; in addition to the possible local crevice
Fig. 10. Acid/base potentiometric titration with 0.1N HCl of samples previously passivated in RM suspension and carbon steel without pre-treatment (control samples). Both are immersed in 250 mL 0.1N NaOH solutions.
corrosion phenomena. In the previous experiences [11,12] the samples were immersed in RM suspensions, so the huge amount of particles present favours chloride adsorption in the bulk suspension instead of in the particles deposited on the steel surface, avoiding the formation of the chloride containing deposits and crevice corrosion phenomena.
Table 2 Corrosion rates, icorr , obtained at different Cl− /OH− ratios for passivated and control samples Passivated sample Cl− /OH− 1 3.5
ratio
Control sample icorr 0.04 1.2
(A cm−2 )
Cl− /OH− ratio
icorr (A cm−2 )
1 3.5 14.5
0.06 0.06 7.1
Fig. 11. Bode plots for (a) passivated samples and (b) control samples at pHs higher and lower than passivity breakdown. The potential values are measured with respect to the Hg/HgO 0.1 M KOH.
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The impedance measurements performed at different Cl− /OH− ratios corroborate these results. Fig. 9 depicts the corresponding Bode plots obtained at the initial stage and at passivity breakdown. A dramatic decreasing in the impedance values at the low frequency limit is observed after passivity breakdown. Using Eq. (1) the corrosion rates can again be estimated, Table 2 shows some of representative values. The corrosion rate at Cl− /OH− = 3.5 ratio raises for the passivated sample, whereas the control sample remains passive, as its low icorr value reflects. The influence of pH was analysed using acid/base potentiometric titration of the passivated and control samples immersed in 250 mL 0.1 M NaOH solution and titrated with 0.1 M HCl. The results are shown in Fig. 10. As can be seen, the passivated samples withstand better the H+ addition than the control ones, which may be the result of the buffering effect of the red mud deposits [8]. Fig. 11 shows the impedance diagrams before and after passivity breakdown, which reflect the drastic decrease during this transition. The corrosion rates estimation gives high values, about 40 A cm−2 for both types of samples.
Fig. 12. Nyquist (a) and Bode (b) plots of painted passivated samples with artificial defect subjected to cathodic polarisation conditions. The potential values are measured with respect to saturated calomel electrode (SCE).
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3.2. Behaviour of painted samples under cathodic polarisation conditions In order to evaluate the effect of the pre-treatment, a paint system was applied on passivated samples and ground carbon steel specimens. An artificial defect was done in both types of the samples and they were subjected to cathodic polarisation experiment. Figs. 12 and 13 show the impedance spectra evolution with immersion time. Big differences are observed depending on the specimen types. Passivated samples (Fig. 12) show an important impedance increase with immersion time and the measured potential remains in well cathodic values, indicating that an effective cathodic protection of the metallic surface was achieved. Nevertheless, the evolution of samples not pre-treated (Fig. 13) indicates that a surface activation occurs with immersion time. Initially, the potential values fall in the cathodic domain (E = −1.13 VSCE ) but, as the immersion time increases, these values shift anodically up to reach the free corrosion potential of carbon steel. These results point towards the RM pre-treatment may be an interesting alternative to the traditional passive layers in confined and cathodic disbonding conditions.
Fig. 13. Nyquist (a) and Bode (b) plots of painted non-passivated samples with artificial defect subjected to cathodic polarisation conditions. The potential values are measured with respect to saturated calomel electrode (SCE).
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In spite of that, the mechanisms developed in both situations are not well understood and more detailed studies must be undertaken.
4. Conclusions The study of different parameters which can affect the steel surface passivation indicates that the better conditions correspond to ground surfaces immersed for about 24 h in stirred and decanted red mud suspensions. Based on the X-ray diffraction results, iron and aluminium oxo-hidroxides are the only RM components deposited on the metallic surface, not as a continuous layer but as discrete distribution of particles. The presence of RM particles promote Cl− ions adsorption on the steel surface and depassivation takes place at lower Cl− /OH− ratios. On the other hand, passivated steel samples withstand lower pHs than the non-passivated ones. The evaluation of RM pre-treatment into a paint scheme was analysed using painted samples with an artificial defect, subjected to cathodic polarisation. The results obtained indicate that the passivated samples improve their behaviour with the immersion time. Nevertheless, the non-passivated specimens tend to activate the surface as shown the impedance diagrams.
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