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Coating electroaccretion on galvanized iron and aluminum in seawater
Electrochimica Acta 50 (2004) 169–178
Coating electroaccretion on galvanized iron and aluminum in seawater G. Salvagoa,∗ , S. Maffib , A. Benedettib , L. Magagnina a
Dip. Chimica, Materiali e Ingegneria Chimica-Politecnico di Milano, Via Mancinelli, 7-20131 Milano, Italy b CNR-IENI Sezione di Milano, Via R. Cozzi, 53-20125 Milano, Italy Received 22 December 2003; received in revised form 8 April 2004; accepted 24 July 2004 Available online 28 August 2004
Abstract Coating electroaccretion on galvanized iron and aluminum 1100 under cathodic polarization in artificial and natural seawater was investigated through electrochemical tests and optical imaging techniques. Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. On galvanized iron in ASTM and natural seawater at potential <−1.2 V versus Ag/AgCl, coating is composed of aragonite and brucite as calcareous deposits on pure iron, aragonite forming before the growth of brucite. Even when coupled to a magnesium anode, the zinc layer can corrode and large aggregates of brucite and aragonite form on the bare steel. Coatings are composed of zinc hydroxychloride Zn5 (OH)8 Cl2 ·H2 O and aragonite without brucite if electroaccretion is performed in natural seawater at potential >−1.2 V versus Ag/AgCl. Coatings grown on aluminum 1100 are different from those on galvanized iron. In ASTM seawater, the coating on aluminum 1100 is composed of aluminum oxide and Mg4 Al2 (OH)14 ·2H2 O; in natural seawater, only of aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg6 Al2 (OH)18 ·4H2 O islands. © 2004 Elsevier Ltd. All rights reserved. Keywords: Calcareous deposit; Electroaccretion; Galvanized iron; Aluminum; Seawater
1. Introduction Cathodic protection is widely used to protect steel structures in seawater. High initial current densities for fast structure polarization as well as for calcareous deposit formation are normally imposed in order to obtain good protection conditions [1,2]. The growth of calcareous deposit should be related to alkalization phenomena at the surface, deriving from cathodic processes [3–7]. The protection offered by calcareous deposits has been related to their capability to hinder the oxygen diffusion toward the metallic surface [8,9]. The shielding action of deposits could encourage the crevice corrosion of stainless steels [10], or can be protective even against localized corrosion [11,12], probably due to calcareous deposit capability of neutralizing localized acid∗
Corresponding author. Tel.: + 390223993133; fax: +390223993180. E-mail address: [email protected] (G. Salvago).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.07.028
ification phenomena, which accompany the corrosion processes [13]. Nowadays, cathodic polarization is applied not only to prevent corrosion, but it is specifically directed towards coating growth as well. If calcareous deposits are generally composed of constituents from the sea, coatings could be composed of relevant amounts of materials coming from the metallic substrate besides from the sea. Electroaccretion of coatings was proposed as a technique to build artificial reefs [14–18], to induce tigmotropism on surfaces [19] and to improve the visual impact of bodies immersed in natural environments [20]. As conductive substrate for coating electroaccretion, rolled steel parts, structural steels, nets made of metals even different from bare mild steel can be employed. Recovered materials, e.g. cars, train wagons, airplanes, ships [21], cathodically polarized with particularly high current densities to induce both oxygen cathodic reduction and hydrogen evolution have been used as well [14,16].
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In the typical conditions of steel under cathodic protection in seawater, coating growth is attributed to alkalization at the surface, where the cathodic process is the oxygen reduction, and it is usually considered that the so-called calcareous deposit is basically composed of aragonite grown after a thin brucite layer adherent to the metal surface [22–26]. Some authors observed the formation of calcite [2,27,28], the formation of calcite modified with magnesium and the growth of calcite monohydrate and pyroaurite [29]. Chemical analysis of coating on mild steel has often shown the presence of iron compounds in amounts larger than 2 wt.% as metal, even after cathodic protection performed at high current densities [30,22]. Even though, the restriction by coating growth in pH increases at the surface under cathodic polarization, in the case of amphoteric metals like zinc and aluminum, the alkaline environment could lead to the dissolution of zinc or aluminum oxides and to the their entrapment inside the coating [31]. The oxygen reduction is a rather complex process, since it is not only limited to the increase of hydroxyl ions, but it can also promote H2 O2 formation [32–34], which can interact with the biofilm growth [35]. In turn, the bacterial activity can interact both with alkalization processes [36–38] and directly with calcareous deposits [39,40]. Some authors have shown the possibility to delay the biofouling growth by pulsed cathodic protection [41]; others have shown how the hydrogen formation during cathodic protection can be related to the growth of sulfate reducing bacteria, SRB [42,43]. Some other authors have emphasized how hydrogen evolution can interact with a coating in a deeply different way, even from a pure physical point of view, from the oxygen reduction process, with removal of preformed coating by gas bubbles [44], which were absent in the oxygen reduction process. Therefore, coating electroaccretion on metal surfaces, in the range of electrochemical potentials where both oxygen and hydrogen reduction process in presence or absence of biofilm are important, deserves more investigations by traditional electrochemical tests and optical imaging technique.
2. Experimental Tests were carried out on commercially available galvanized iron (Fe/Zn) and aluminum 1100 (Al) wires with diameter Ø = 3–6 and 4 mm, respectively. The coating on iron was about 600 g m−2 and consisted, from the outer to the inner layer, of eta phase (100% Zn) > 25 m, then zeta, delta, and gamma phase. Experiments in natural seawater directly extracted from the sea (pH = 8.3 ± 0.05, T = 13–27 ◦ C, O2 = 8–10 mg L−1 , total organic carbon T.O.C. <3 mg L−1 , heavy metals <0.1 mg L−1 ) were performed at MARECO Marine Research Center of the IENI Institute of the CNR located in Bonassola on the Mediterranean Ligurian coast (Italy). For measurements in abiotic environments, artificial seawater was prepared according to ASTM D1141 specification for substitute ocean water. To allow the growth of a mature
biofilm on metallic surfaces, Fe/Zn and Al specimens were pre-exposed to natural seawater (T ≥ 18 ◦ C) for 18 days. Potentiodynamic tests with optical image recording were carried out for both natural and artificial seawater in flowing conditions in a 0.2 L electrochemical cell with quartz window to allow in situ and real time visualization of specimen surface. Flow rate of the solution was 1.3 mL s−1 and specimen exposed area was 1.5–6 cm2 . Cyclic potentiodynamic experiments were performed from open circuit potential, OCP, to minimum potential −2.5 V versus Ag/AgCl with scan rate of 0.6 mV s−1 . The specimen was left at OCP for 5 min before each experiment. Potentiostatic tests in artificial seawater were performed in a 2-L polyethylene beaker, while for potentiostatic tests in flowing natural seawater 200-L tanks with 1 turnover/h were used. As counter-electrode, a platinum ring wire was used. All potentials are referred to as Ag/AgCl/seawater reference electrode. Electrochemical measurements were performed with a programmable AMEL potentiostat with IR drop automatic correction. Equipment for optical imaging consisted of an optical microscope with an external illuminator and Sony charged couple device (CCD) camera. An image acquisition card digitalized the analog signal of the camera and stored the images in the computer. During the tests, acquisition card captured images (768 × 576 pixels) at a rate of 10 frames s−1 . Diffraction (XRD) measurements with Bragg-Brentano configuration were performed in a Philips PW 1830 instrument, with a goniometer Philips PW 3020 and a control unit Philips PW 3710 (Cu K␣ radiation with wavelength ˚ Scanning Electron Microscopy (SEM) with en1.5406 A). ergy dispersive X-ray analysis (EDX) was performed with a Cambridge Stereoscan 360.
3. Results The hydrogen evolution process and the morphology of gas bubbles detaching from the surface during potentiodynamic tests, as well as coating growth and interaction between coating and gas bubbles, were strongly dependent on the metallic material and applied potential. Gas bubble evolution became macroscopically evident only at potentials much lower than the equilibrium potential of hydrogen. During cathodic potentiodynamic tests, as hydrogen evolution started, small bubbles moving along the metal surface first appeared and afterwards, coating formation was evident. As the potential decreased, the density of bubbles at the surface increased and they were few moving bubbles, which increased greatly in size before detaching. The presence of coating microparticles on detached gas bubbles was also observed [44]. The size reached by the gas bubbles seemed at first to increase and then to decrease as potential decreased. As the potential decreased, the interaction between gas bubbles and coating became evident. Hydrogen bubbles appeared nucleating with spherical shape on the top of the coating. While growing, the
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Fig. 1. Optical images during potentiodynamic tests in natural seawater: deformation of growing hydrogen bubble at −1.7 V on Fe [44].
interaction with the mineral coating could lead to the bubble shape deformation, as shown in the sequence of image in Fig. 1 [44]. At lower potentials (more cathodic and at higher currents), gas bubbles caused the preformed coating to detach from the metal surface as in Fig. 2 [44]. At much lower potentials, the maximum size reached by the gas bubbles decreased significantly and the mixed layer of liquid, gas and solid
particles close to the metal surface was affected by strong turbulence. In Fig. 3, a potentiodynamic curve on Fe/Zn and an acquired sequence of images during test after immersion in natural seawater is shown. The bubble size at the surface was scattered in a large range, but there was a clear size dependence on applied potential. At the end of the cyclic potentio-
Fig. 2. Two series of optical images during potentiodynamic tests in natural seawater: detachment of deposit scales from Al surface at −2.23 V. Deposit: (), breaking scale: (+), detaching scale: ( ).
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Fig. 3. A: potentiodynamic cathodic polarization of Fe/Zn in natural seawater; 1–3: relative optical images sequence of specimen surface.
dynamic test, the surface was covered by a rather heterogeneous coating. In ASTM seawater, hydrogen evolution and coating formation on galvanized iron were very similar to the natural seawater case, even if bubbles with Ø ≥ 1 mm were
observed at a potential close to −1.8 V instead of −1.7 V. On Fe/Zn surfaces with presence of biofilm, cyclic potentiodynamic test results differed in electrochemical aspects as well as bubble development from those obtained in the absence
Fig. 4. A: potentiodynamic cathodic polarization of Al in natural seawater; 1–3: relative optical images sequence of specimen surface.
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Fig. 5. A: potentiodynamic cathodic polarization of prefouled Al in natural seawater; 1–3: relative optical images sequence of specimen surface.
of biofilm. Furthermore, bubbles with Ø > 0.25 mm were not observed in the whole examined potential range and at the end of the cyclic potentiodynamic test, the coating on the Fe/Zn surface had a morphology similar to that obtained in the absence of preformed biofilm. Also in the case of Al, the size of the bubbles seemed to depend on potential, Fig. 4 . The spatial distribution of the bubbles on the surface appeared more uniform with respect to galvanized iron, the bubble mean size was higher and the maximum size was observed at lower potentials, i.e., −2 V. At the end of the cyclic potentiodynamic tests, the surface was covered by a thick and compact coating, contrary to the case of Fe/Zn. Hydrogen evolution on Al with a biofilm was substantially similar to that obtained on specimens without biofilm (Fig. 5); the size of the bubbles depended on potential and its maximum value (<0.5 mm) was much lower than the value reached by gas bubbles in absence of biofilm (≥1 mm). Composition and morphology of the coating were found to depend on substrate material, i.e., galvanized iron or aluminum, on environment, i.e., ASTM solution or natural seawater, and on polarization mode. Potentiostatic tests were performed on Fe/Zn at −1.05 and −1.1 V. In natural seawater, current density values were twice as much as those in ASTM seawater in the first 20 h with values up to 50 A cm−2 . After about 20 h, current densities were similar with values next to 20 A cm−2 . At the end of the potentiostatic tests, coatings obtained in ASTM seawater were mainly composed of zincite ZnO, aragonite CaCO3 and some brucite Mg(OH)2 . Electroaccreted coatings in natural
seawater were composed of zincite, zinc hydroxychlorides Zn5 (OH)8 Cl2 ·H2 O and aragonite. No brucite seemed to be formed, at least for our tests, as confirmed by the analysis of magnesium. Fig. 6 shows the results of potentiostatic tests on Al. Current densities were much lower than those observed on Fe/Zn at the same potential. Furthermore, it can be noted that in the case of aluminum contrary to Fe/Zn, current densities in natural seawater were lower than those obtained in ASTM seawater at the same potentials. This behavior was observed in tests at other cathodic potentials as well. X-ray diffraction analysis for the coating on Al in ASTM seawater in potentiostatic conditions at −1.05 V showed the presence of similar amount of Mg4 Al2 (OH)14 ·2H2 O and corindone Al2 O3 , while coatings obtained in natural seawater were very poor and only
Fig. 6. Potentiostatic tests at −1.05 and −1.10 V on Al in natural and ASTM seawater.
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the presence of corindone was detected. Calcium was not detected in coating obtained on aluminum at −1.05 V. Potentiostatic tests were performed on Fe/Zn at −1.5 V. Even at this potential, current densities in natural seawater were higher than those obtained in artificial seawater. Current density initially decreased, then, after 20 h, its value was about 200 A cm−2 . At the end of the tests in ASTM seawater, the coating on Fe/Zn was composed of aragonite with brucite agglomerates as shown in Fig. 7. Such three-dimensional agglomerates differed, at least morphologically, from the brucite underlayer usually observed in coating grown on iron [22–26]. Coatings obtained in natural seawater were substantially similar to that in ASTM seawater with presence of zincite as well. In coatings obtained on Al at the end of potentiodynamic tests in natural seawater, aragonite but no brucite was detected, perhaps due to the short time of experiment. In potentiostatic tests at −1.5 V on Al in ASTM seawater, current density, after an initial decrease, reached a value of about 100 A cm−2 after 20 h. At the end of these tests, the presence of aragonite and some brucite in the coating was observed. No mixed Mg–Al-hydroxide was observed even though it was found in the same environment at less cathodic potentials as previously reported. In coatings on aluminum in potentiostatic tests at −1.5 V in natural seawater, aragonite but no brucite was detected, perhaps because the current densities were lower than those in ASTM seawater. The alkalization at the surface is related to the current density and could cause the dissolution of the aluminum oxide passive layer as aluminate. This reaction could hinder the pH increase, which can not reach the value required for a large precipitation of brucite at low current densities. However, it is interesting to observe that both on Fe/Zn and Al, aragonite did not form only after brucite, as stated by many authors about calcareous deposits on steel [22–26] and gold [45]. Fig. 8 shows results in natural seawater for Fe/Zn specimens coupled with magnesium anodes. The trend of the cou-
Fig. 7. SEM images of the coating on Fe/Zn after potentiostatic test at −1.5 V in ASTM seawater; 1: aragonite, 2: brucite.
pling current versus time depended on coupling resistance. The increase of current density with time observed after an initial decreasing in the case of low coupling external resistance, as reported in Fig. 8a, was unexpected. In both cases, as shown in Fig. 8b, the electroaccreted coating showed a particular morphology with large spherical agglomerates on top of a nearly uniform layer which covered the metallic surface. The presence of brucite as well as aragonite, detected by XRD and EDX analysis, were observed variously distributed in the deposit thickness; even if close to the metal surface, the brucite amount prevailed if compared to the aragonite content as shown in Fig. 9 . Under the agglomerates, Fig. 10 marked area 1, the zinc layer of the Fe/Zn was corroded even if the coupon was coupled with magnesium anode, as shown by the SEM image and by the different ratio of Fe and Zn peaks intensity in the marked areas. This result was at least qualitatively in agreement with the corrosion rate of zinc coupled with magnesium in marine environment observed by other authors [46] and probably related to pH increase at the cathodic surface and to the amphoteric behavior of zinc. The increase of hydrogen evolution due to hydrogen overvoltage decrease on iron with respect to zinc induced the detachment of the calcareous deposit from area where hydrogen evolution was higher, and its reprecipitation nearby. The lower overvoltage for hydrogen evolution on iron with respect to
Fig. 8. Fe/Zn specimens coupled with Mg anode through different external ohmic resistance in natural seawater; a: potential and current density vs. time plots; b: optical image of specimens at the end of the test.
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Fig. 9. Fe/Zn coupled with Mg anode through 10 external resistance in natural seawater; a: X-ray diffraction pattern of the coating, brucite: (X), aragonite: (); b: cross-section SEM image of the coating; b1 and b2: concentration maps of Ca and Mg, respectively.
zinc can explain both the increase with time of the mean current densities and the localized increase of current density with formation of globular coating on galvanized iron where zinc was dissolved. In the case of Al specimens, current density values related to the galvanic coupling with Mg anode were much lower
than those of Fe/Zn and the external coupling ohmic resistance did not appear critical. The topology and morphology of coatings on Al were very different from those of the deposits grown on Fe/Zn, Fig. 11 . It was possible to observe a dense coverage of Chlorophiceae seaweed; the mineral deposit on Al was characterized by a more uniform thickness
Fig. 10. SEM images of Fe/Zn surfaces coupled with Mg anode through 10 external resistance and EDX composition of the marked areas after removal of agglomerates of marked area 1.
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Fig. 11. Al coupled with Mg anode through different external ohmic resistances in natural seawater; A: optical image at the end of the test; B: SEM image of Al specimen surface (10 ); B1–B4: concentration maps of Al, Ca, Mg and O of B, respectively.
than that obtained in the same experimental conditions on Fe/Zn. The deposit was mainly composed of aragonite with Mg6 Al2 (OH)18 ·4H2 O islands. It is reasonable that the alkalization at the surface could cause the aragonite precipitation, and also the depassivation and dissolution of aluminum oxide as aluminate. The aluminate subsequently precipitated as hydroxide together with magnesium at the surface where the alkalinity was reduced due to hindrance of hydroxide ions diffusion by biofilm or to calcium carbonate precipitation with hydroxide ions subtraction to bicarbonate/carbonate equilibrium in seawater. The particular chemical composition of the electroaccreted coating on Al could help to explain why this material promotes algal growth, Fig. 11A, more than Fe/Zn alloys, Fig. 8b, and that the type of flora and fauna growth on artificial reefs in seawater remarkably differs as function of the em-
ployed materials: car bodies (with high content of galvanized iron) or airplane bodies (with aluminum alloys prevalence) [21].
4. Discussions Our results in short period tests in ASTM seawater can be not representative of what happens in natural seawater. The biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and electroaccreted coating. The mineral composition of coating can be remarkably different if the coating is grown in natural or artificial seawater. This is probably related to the different behavior of hydroxyl ions diffusion phenomena if biofilm is
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present and to corrosion phenomena on amphoteric materials related to surface alkalization. On Fe/Zn polarized at −1.05 ÷ −1.10 V versus Ag/AgCl, coating is composed of aragonite and brucite if electroaccretion is performed in ASTM seawater and of zinc hydroxychloride Zn5 (OH)8 Cl2 ·H2 O and aragonite without brucite if electroaccretion is performed in natural seawater. In both environments, on Fe/Zn polarized at low potentials, i.e., <−1.2 V versus Ag/AgCl, the coating is composed of brucite and aragonite as on pure iron; the formation of aragonite can come before the growth of brucite. The zinc layer, even if the specimen is coupled with magnesium anode, can corrode leaving uncovered iron areas where hydrogen evolution and coating formation are favored with growth of big aggregates composed of a mixture of brucite and aragonite. Brucite is prevalent inside and aragonite outside. In this case, the galvanic coupling current of galvanized iron–magnesium couple can significantly increase and not decrease as usual with time. On Al, hydrogen evolution and coating formation take place at potentials much lower than the potentials observed on galvanized iron. Current densities during potentiostatic tests performed in natural seawater are lower than those in ASTM seawater at the same potentials. Coatings on Al are far different from deposit on Fe/Zn, both in morphology and chemical composition. In ASTM seawater, the coating on aluminum at potentials >−1.1 V versus Ag/AgCl does not contain calcium but only aluminum oxide and Mg4 Al2 (OH)14 ·2H2 O. At lower potentials, the coating is composed of brucite and aragonite and mixed hydroxides of aluminum and magnesium disappear. In natural seawater, the coating on Al at potentials >−1.1 V versus Ag/AgCl does not contain calcium and magnesium, but only aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg6 Al2 (OH)18 ·4H2 O islands. 5. Conclusions Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. The coatings on the examined materials, i.e., galvanized iron and aluminum 1100, can contain substrate corrosion products, i.e., ZnO, Zn5 (OH)8 Cl2 ·H2 O, Mg4 Al2 (OH)14 ·2H2 O, Al2 O3 and Mg6 Al2 (OH)18 ·4H2 O, even if the coupons are galvanically coupled with magnesium anode, and can not contain CaCO3 . Than, these coatings cannot be properly defined as calcareous deposits. References [1] K.P. Fischer, J.E. Finnegan, Corrosion89, NACE, Houston, TX, 1989, no. 582.
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[2] K.P. Fischer, W.H. Thomason, S. Eliassen, Corrosion96, NACE, Houston, TX, 1996, no. 548. [3] H.J. Engell, P. Forchhammer, Corros. Sci. 5 (1965) 479. [4] S.L. Wolfson, W.H. Hartt, Corrosion 37 (1981) 70. [5] W.H. Hartt, C.H. Culberson, S.W. Smith, Corrosion83, NACE, Houston, TX, 1983, no. 59. [6] J.R. Ambrose, A.E. Yaniv, U. Rupert Lee, Corrosion83, NACE, Houston, TX, 1983, no. 60. [7] B.B. Chernov, T.B. Pustovskikh, Zashchita Metallov. 25 (1989) 506. [8] S.C. Dexter, S.H. Lin, Corrosion91, NACE, Houston, TX, 1991, no. 499. [9] J.R. Scully, H.P. Hack, Corrosion85, NACE, Houston, TX, 1985, no. 214. [10] A. Neville, T. Hodgkiess, A.T. Morizot, Br. Corros. J. 32 (1997) 277. [11] G. Salvago, G. Bollini, E. Negri, I. Bona, Proceedings of the Symposium on Corrosion Control by Coating, Cathodic Protection and Inhibitors in Seawater, 223rd Event of the European Federation of Corrosion, Dubrovnik, Croatia, 4–7 October 1998. [12] G. Salvago, G. Bollini, Corrosion 55 (1999) 397. [13] P.O. Gartland, Corrosion96, NACE, Houston, TX, 1996, no. 311. [14] W.H. Hilbertz, IEEE J. Ocean Eng. 3 (1979) 94. [15] L. LeBlanc, Offshore (1983) 42. [16] M.B. Gravens, J. Waterway Coastal Ocean Eng. 114 (1988) 519. [17] W.H. Hilbertz, Building Res. Inf. 19 (1991) 242. [18] F. Pierce, New Scientist 2047 (1996) 22. [19] G. Salvago, A. Granelli, Incontro con l’ecologia marina, Lions Club Levanto, Italy, 1991. [20] G. Salvago, V. Mezzanotte, L. Magagnin, E. Poli, A. Benedetti, E. Olzi, Proceedings of the 10th International Congress on Marine Corrosion and Fouling, Melbourne, Australia, February 1999, p. 248. [21] R.L. Lukens (Ed.), in: Artificial Reef Subcommittee of the Technical Coordinating Committe Gulf States Marine Fisheries Commission, Guidelines for Marine Artificial Reef Materials, 1997. [22] W.H. Hartt, C.H. Culberson, S.W. Smith, Corrosion 40 (1984) 609. [23] M.M. Kunjapur, W.H. Hartt, S.W. Smith, Corrosion 43 (1987) 674. [24] J.S. Luo, R.U. Lee, T.Y. Chen, W.H. Hartt, S.W. Smith, Corrosion88, NACE, Houston, TX, 1988, no. 36. [25] J.S. Luo, R.U. Lee, T.Y. Chen, W.H. Hartt, S.W. Smith, Corrosion 47 (1991) 189. [26] K.E. Mantel, W.H. Hartt, T.Y. Chen, Corrosion 48 (1992) 489. [27] S.H. Lin, S.C. Dexter, Corrosion 44 (1988) 615. [28] C.G. Peng, J.K. Park, Corrosion 50 (1994) 669. [29] J.E. Finnegan, K.P. Fischer, Corrosion89, NACE, Houston, TX, 1989, no. 581. [30] L.F. Laque, Marine Corrosion: Causes and Prevention, WileyInternational Publications, 1975, p. 106. [31] G. Salvago, S. Maffi, A. Benedetti, S. Pasqualin, Proceedings of the 5th Latincorr 2003, Santiago, Chile, 20–24 October 2003. p. 863. [32] H.P. Dhar, J.O’M. Bockris, D.H. Lewis, J. Electrochem. Soc. 128 (1981) 229. [33] N.P. Dhar, D.W. Howell, J.O’M. Bockris, J. Electrochem. Soc. 129 (1982) 2178. [34] D. Gervasio, I. Song, J.H. Payer, J. Appl. Electrochem. 28 (1998) 979. [35] B.E. Christensen, K. Trønnes Vollan, O. Smidsrød, R. Bakke, Biofouling 2 (1990) 165. [36] J.L. Crolet, S. Daumas, M. Magot, Corrosion93, NACE, Houston, TX, 1993, no. 303. [37] B. Little, P. Wagner, D. Daquette, Corrosion87, NACE, Houston, TX, 1987, no. 370. [38] G. Salvago, L. Magagnin, Corrosion 57 (2001) 759. [39] S.C. Dexter, Corrosion90, NACE, Houston, TX, 1990, no. 157. [40] J.B. Little, P.A. Wagner, Corrosion93, NACE, Houston, TX, 1993, no. 525. [41] E. Littauer, D.M. Jennings, Proceedings of the 2nd International Congress of Marine Corrosion and Fouling, Athens, Greece, 1968.
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Coating electroaccretion on galvanized iron and aluminum in seawater G. Salvagoa,∗ , S. Maffib , A. Benedettib , L. Magagnina a
Dip. Chimica, Materiali e Ingegneria Chimica-Politecnico di Milano, Via Mancinelli, 7-20131 Milano, Italy b CNR-IENI Sezione di Milano, Via R. Cozzi, 53-20125 Milano, Italy Received 22 December 2003; received in revised form 8 April 2004; accepted 24 July 2004 Available online 28 August 2004
Abstract Coating electroaccretion on galvanized iron and aluminum 1100 under cathodic polarization in artificial and natural seawater was investigated through electrochemical tests and optical imaging techniques. Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. On galvanized iron in ASTM and natural seawater at potential <−1.2 V versus Ag/AgCl, coating is composed of aragonite and brucite as calcareous deposits on pure iron, aragonite forming before the growth of brucite. Even when coupled to a magnesium anode, the zinc layer can corrode and large aggregates of brucite and aragonite form on the bare steel. Coatings are composed of zinc hydroxychloride Zn5 (OH)8 Cl2 ·H2 O and aragonite without brucite if electroaccretion is performed in natural seawater at potential >−1.2 V versus Ag/AgCl. Coatings grown on aluminum 1100 are different from those on galvanized iron. In ASTM seawater, the coating on aluminum 1100 is composed of aluminum oxide and Mg4 Al2 (OH)14 ·2H2 O; in natural seawater, only of aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg6 Al2 (OH)18 ·4H2 O islands. © 2004 Elsevier Ltd. All rights reserved. Keywords: Calcareous deposit; Electroaccretion; Galvanized iron; Aluminum; Seawater
1. Introduction Cathodic protection is widely used to protect steel structures in seawater. High initial current densities for fast structure polarization as well as for calcareous deposit formation are normally imposed in order to obtain good protection conditions [1,2]. The growth of calcareous deposit should be related to alkalization phenomena at the surface, deriving from cathodic processes [3–7]. The protection offered by calcareous deposits has been related to their capability to hinder the oxygen diffusion toward the metallic surface [8,9]. The shielding action of deposits could encourage the crevice corrosion of stainless steels [10], or can be protective even against localized corrosion [11,12], probably due to calcareous deposit capability of neutralizing localized acid∗
Corresponding author. Tel.: + 390223993133; fax: +390223993180. E-mail address: [email protected] (G. Salvago).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.07.028
ification phenomena, which accompany the corrosion processes [13]. Nowadays, cathodic polarization is applied not only to prevent corrosion, but it is specifically directed towards coating growth as well. If calcareous deposits are generally composed of constituents from the sea, coatings could be composed of relevant amounts of materials coming from the metallic substrate besides from the sea. Electroaccretion of coatings was proposed as a technique to build artificial reefs [14–18], to induce tigmotropism on surfaces [19] and to improve the visual impact of bodies immersed in natural environments [20]. As conductive substrate for coating electroaccretion, rolled steel parts, structural steels, nets made of metals even different from bare mild steel can be employed. Recovered materials, e.g. cars, train wagons, airplanes, ships [21], cathodically polarized with particularly high current densities to induce both oxygen cathodic reduction and hydrogen evolution have been used as well [14,16].
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In the typical conditions of steel under cathodic protection in seawater, coating growth is attributed to alkalization at the surface, where the cathodic process is the oxygen reduction, and it is usually considered that the so-called calcareous deposit is basically composed of aragonite grown after a thin brucite layer adherent to the metal surface [22–26]. Some authors observed the formation of calcite [2,27,28], the formation of calcite modified with magnesium and the growth of calcite monohydrate and pyroaurite [29]. Chemical analysis of coating on mild steel has often shown the presence of iron compounds in amounts larger than 2 wt.% as metal, even after cathodic protection performed at high current densities [30,22]. Even though, the restriction by coating growth in pH increases at the surface under cathodic polarization, in the case of amphoteric metals like zinc and aluminum, the alkaline environment could lead to the dissolution of zinc or aluminum oxides and to the their entrapment inside the coating [31]. The oxygen reduction is a rather complex process, since it is not only limited to the increase of hydroxyl ions, but it can also promote H2 O2 formation [32–34], which can interact with the biofilm growth [35]. In turn, the bacterial activity can interact both with alkalization processes [36–38] and directly with calcareous deposits [39,40]. Some authors have shown the possibility to delay the biofouling growth by pulsed cathodic protection [41]; others have shown how the hydrogen formation during cathodic protection can be related to the growth of sulfate reducing bacteria, SRB [42,43]. Some other authors have emphasized how hydrogen evolution can interact with a coating in a deeply different way, even from a pure physical point of view, from the oxygen reduction process, with removal of preformed coating by gas bubbles [44], which were absent in the oxygen reduction process. Therefore, coating electroaccretion on metal surfaces, in the range of electrochemical potentials where both oxygen and hydrogen reduction process in presence or absence of biofilm are important, deserves more investigations by traditional electrochemical tests and optical imaging technique.
2. Experimental Tests were carried out on commercially available galvanized iron (Fe/Zn) and aluminum 1100 (Al) wires with diameter Ø = 3–6 and 4 mm, respectively. The coating on iron was about 600 g m−2 and consisted, from the outer to the inner layer, of eta phase (100% Zn) > 25 m, then zeta, delta, and gamma phase. Experiments in natural seawater directly extracted from the sea (pH = 8.3 ± 0.05, T = 13–27 ◦ C, O2 = 8–10 mg L−1 , total organic carbon T.O.C. <3 mg L−1 , heavy metals <0.1 mg L−1 ) were performed at MARECO Marine Research Center of the IENI Institute of the CNR located in Bonassola on the Mediterranean Ligurian coast (Italy). For measurements in abiotic environments, artificial seawater was prepared according to ASTM D1141 specification for substitute ocean water. To allow the growth of a mature
biofilm on metallic surfaces, Fe/Zn and Al specimens were pre-exposed to natural seawater (T ≥ 18 ◦ C) for 18 days. Potentiodynamic tests with optical image recording were carried out for both natural and artificial seawater in flowing conditions in a 0.2 L electrochemical cell with quartz window to allow in situ and real time visualization of specimen surface. Flow rate of the solution was 1.3 mL s−1 and specimen exposed area was 1.5–6 cm2 . Cyclic potentiodynamic experiments were performed from open circuit potential, OCP, to minimum potential −2.5 V versus Ag/AgCl with scan rate of 0.6 mV s−1 . The specimen was left at OCP for 5 min before each experiment. Potentiostatic tests in artificial seawater were performed in a 2-L polyethylene beaker, while for potentiostatic tests in flowing natural seawater 200-L tanks with 1 turnover/h were used. As counter-electrode, a platinum ring wire was used. All potentials are referred to as Ag/AgCl/seawater reference electrode. Electrochemical measurements were performed with a programmable AMEL potentiostat with IR drop automatic correction. Equipment for optical imaging consisted of an optical microscope with an external illuminator and Sony charged couple device (CCD) camera. An image acquisition card digitalized the analog signal of the camera and stored the images in the computer. During the tests, acquisition card captured images (768 × 576 pixels) at a rate of 10 frames s−1 . Diffraction (XRD) measurements with Bragg-Brentano configuration were performed in a Philips PW 1830 instrument, with a goniometer Philips PW 3020 and a control unit Philips PW 3710 (Cu K␣ radiation with wavelength ˚ Scanning Electron Microscopy (SEM) with en1.5406 A). ergy dispersive X-ray analysis (EDX) was performed with a Cambridge Stereoscan 360.
3. Results The hydrogen evolution process and the morphology of gas bubbles detaching from the surface during potentiodynamic tests, as well as coating growth and interaction between coating and gas bubbles, were strongly dependent on the metallic material and applied potential. Gas bubble evolution became macroscopically evident only at potentials much lower than the equilibrium potential of hydrogen. During cathodic potentiodynamic tests, as hydrogen evolution started, small bubbles moving along the metal surface first appeared and afterwards, coating formation was evident. As the potential decreased, the density of bubbles at the surface increased and they were few moving bubbles, which increased greatly in size before detaching. The presence of coating microparticles on detached gas bubbles was also observed [44]. The size reached by the gas bubbles seemed at first to increase and then to decrease as potential decreased. As the potential decreased, the interaction between gas bubbles and coating became evident. Hydrogen bubbles appeared nucleating with spherical shape on the top of the coating. While growing, the
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Fig. 1. Optical images during potentiodynamic tests in natural seawater: deformation of growing hydrogen bubble at −1.7 V on Fe [44].
interaction with the mineral coating could lead to the bubble shape deformation, as shown in the sequence of image in Fig. 1 [44]. At lower potentials (more cathodic and at higher currents), gas bubbles caused the preformed coating to detach from the metal surface as in Fig. 2 [44]. At much lower potentials, the maximum size reached by the gas bubbles decreased significantly and the mixed layer of liquid, gas and solid
particles close to the metal surface was affected by strong turbulence. In Fig. 3, a potentiodynamic curve on Fe/Zn and an acquired sequence of images during test after immersion in natural seawater is shown. The bubble size at the surface was scattered in a large range, but there was a clear size dependence on applied potential. At the end of the cyclic potentio-
Fig. 2. Two series of optical images during potentiodynamic tests in natural seawater: detachment of deposit scales from Al surface at −2.23 V. Deposit: (), breaking scale: (+), detaching scale: ( ).
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Fig. 3. A: potentiodynamic cathodic polarization of Fe/Zn in natural seawater; 1–3: relative optical images sequence of specimen surface.
dynamic test, the surface was covered by a rather heterogeneous coating. In ASTM seawater, hydrogen evolution and coating formation on galvanized iron were very similar to the natural seawater case, even if bubbles with Ø ≥ 1 mm were
observed at a potential close to −1.8 V instead of −1.7 V. On Fe/Zn surfaces with presence of biofilm, cyclic potentiodynamic test results differed in electrochemical aspects as well as bubble development from those obtained in the absence
Fig. 4. A: potentiodynamic cathodic polarization of Al in natural seawater; 1–3: relative optical images sequence of specimen surface.
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Fig. 5. A: potentiodynamic cathodic polarization of prefouled Al in natural seawater; 1–3: relative optical images sequence of specimen surface.
of biofilm. Furthermore, bubbles with Ø > 0.25 mm were not observed in the whole examined potential range and at the end of the cyclic potentiodynamic test, the coating on the Fe/Zn surface had a morphology similar to that obtained in the absence of preformed biofilm. Also in the case of Al, the size of the bubbles seemed to depend on potential, Fig. 4 . The spatial distribution of the bubbles on the surface appeared more uniform with respect to galvanized iron, the bubble mean size was higher and the maximum size was observed at lower potentials, i.e., −2 V. At the end of the cyclic potentiodynamic tests, the surface was covered by a thick and compact coating, contrary to the case of Fe/Zn. Hydrogen evolution on Al with a biofilm was substantially similar to that obtained on specimens without biofilm (Fig. 5); the size of the bubbles depended on potential and its maximum value (<0.5 mm) was much lower than the value reached by gas bubbles in absence of biofilm (≥1 mm). Composition and morphology of the coating were found to depend on substrate material, i.e., galvanized iron or aluminum, on environment, i.e., ASTM solution or natural seawater, and on polarization mode. Potentiostatic tests were performed on Fe/Zn at −1.05 and −1.1 V. In natural seawater, current density values were twice as much as those in ASTM seawater in the first 20 h with values up to 50 A cm−2 . After about 20 h, current densities were similar with values next to 20 A cm−2 . At the end of the potentiostatic tests, coatings obtained in ASTM seawater were mainly composed of zincite ZnO, aragonite CaCO3 and some brucite Mg(OH)2 . Electroaccreted coatings in natural
seawater were composed of zincite, zinc hydroxychlorides Zn5 (OH)8 Cl2 ·H2 O and aragonite. No brucite seemed to be formed, at least for our tests, as confirmed by the analysis of magnesium. Fig. 6 shows the results of potentiostatic tests on Al. Current densities were much lower than those observed on Fe/Zn at the same potential. Furthermore, it can be noted that in the case of aluminum contrary to Fe/Zn, current densities in natural seawater were lower than those obtained in ASTM seawater at the same potentials. This behavior was observed in tests at other cathodic potentials as well. X-ray diffraction analysis for the coating on Al in ASTM seawater in potentiostatic conditions at −1.05 V showed the presence of similar amount of Mg4 Al2 (OH)14 ·2H2 O and corindone Al2 O3 , while coatings obtained in natural seawater were very poor and only
Fig. 6. Potentiostatic tests at −1.05 and −1.10 V on Al in natural and ASTM seawater.
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the presence of corindone was detected. Calcium was not detected in coating obtained on aluminum at −1.05 V. Potentiostatic tests were performed on Fe/Zn at −1.5 V. Even at this potential, current densities in natural seawater were higher than those obtained in artificial seawater. Current density initially decreased, then, after 20 h, its value was about 200 A cm−2 . At the end of the tests in ASTM seawater, the coating on Fe/Zn was composed of aragonite with brucite agglomerates as shown in Fig. 7. Such three-dimensional agglomerates differed, at least morphologically, from the brucite underlayer usually observed in coating grown on iron [22–26]. Coatings obtained in natural seawater were substantially similar to that in ASTM seawater with presence of zincite as well. In coatings obtained on Al at the end of potentiodynamic tests in natural seawater, aragonite but no brucite was detected, perhaps due to the short time of experiment. In potentiostatic tests at −1.5 V on Al in ASTM seawater, current density, after an initial decrease, reached a value of about 100 A cm−2 after 20 h. At the end of these tests, the presence of aragonite and some brucite in the coating was observed. No mixed Mg–Al-hydroxide was observed even though it was found in the same environment at less cathodic potentials as previously reported. In coatings on aluminum in potentiostatic tests at −1.5 V in natural seawater, aragonite but no brucite was detected, perhaps because the current densities were lower than those in ASTM seawater. The alkalization at the surface is related to the current density and could cause the dissolution of the aluminum oxide passive layer as aluminate. This reaction could hinder the pH increase, which can not reach the value required for a large precipitation of brucite at low current densities. However, it is interesting to observe that both on Fe/Zn and Al, aragonite did not form only after brucite, as stated by many authors about calcareous deposits on steel [22–26] and gold [45]. Fig. 8 shows results in natural seawater for Fe/Zn specimens coupled with magnesium anodes. The trend of the cou-
Fig. 7. SEM images of the coating on Fe/Zn after potentiostatic test at −1.5 V in ASTM seawater; 1: aragonite, 2: brucite.
pling current versus time depended on coupling resistance. The increase of current density with time observed after an initial decreasing in the case of low coupling external resistance, as reported in Fig. 8a, was unexpected. In both cases, as shown in Fig. 8b, the electroaccreted coating showed a particular morphology with large spherical agglomerates on top of a nearly uniform layer which covered the metallic surface. The presence of brucite as well as aragonite, detected by XRD and EDX analysis, were observed variously distributed in the deposit thickness; even if close to the metal surface, the brucite amount prevailed if compared to the aragonite content as shown in Fig. 9 . Under the agglomerates, Fig. 10 marked area 1, the zinc layer of the Fe/Zn was corroded even if the coupon was coupled with magnesium anode, as shown by the SEM image and by the different ratio of Fe and Zn peaks intensity in the marked areas. This result was at least qualitatively in agreement with the corrosion rate of zinc coupled with magnesium in marine environment observed by other authors [46] and probably related to pH increase at the cathodic surface and to the amphoteric behavior of zinc. The increase of hydrogen evolution due to hydrogen overvoltage decrease on iron with respect to zinc induced the detachment of the calcareous deposit from area where hydrogen evolution was higher, and its reprecipitation nearby. The lower overvoltage for hydrogen evolution on iron with respect to
Fig. 8. Fe/Zn specimens coupled with Mg anode through different external ohmic resistance in natural seawater; a: potential and current density vs. time plots; b: optical image of specimens at the end of the test.
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Fig. 9. Fe/Zn coupled with Mg anode through 10 external resistance in natural seawater; a: X-ray diffraction pattern of the coating, brucite: (X), aragonite: (); b: cross-section SEM image of the coating; b1 and b2: concentration maps of Ca and Mg, respectively.
zinc can explain both the increase with time of the mean current densities and the localized increase of current density with formation of globular coating on galvanized iron where zinc was dissolved. In the case of Al specimens, current density values related to the galvanic coupling with Mg anode were much lower
than those of Fe/Zn and the external coupling ohmic resistance did not appear critical. The topology and morphology of coatings on Al were very different from those of the deposits grown on Fe/Zn, Fig. 11 . It was possible to observe a dense coverage of Chlorophiceae seaweed; the mineral deposit on Al was characterized by a more uniform thickness
Fig. 10. SEM images of Fe/Zn surfaces coupled with Mg anode through 10 external resistance and EDX composition of the marked areas after removal of agglomerates of marked area 1.
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Fig. 11. Al coupled with Mg anode through different external ohmic resistances in natural seawater; A: optical image at the end of the test; B: SEM image of Al specimen surface (10 ); B1–B4: concentration maps of Al, Ca, Mg and O of B, respectively.
than that obtained in the same experimental conditions on Fe/Zn. The deposit was mainly composed of aragonite with Mg6 Al2 (OH)18 ·4H2 O islands. It is reasonable that the alkalization at the surface could cause the aragonite precipitation, and also the depassivation and dissolution of aluminum oxide as aluminate. The aluminate subsequently precipitated as hydroxide together with magnesium at the surface where the alkalinity was reduced due to hindrance of hydroxide ions diffusion by biofilm or to calcium carbonate precipitation with hydroxide ions subtraction to bicarbonate/carbonate equilibrium in seawater. The particular chemical composition of the electroaccreted coating on Al could help to explain why this material promotes algal growth, Fig. 11A, more than Fe/Zn alloys, Fig. 8b, and that the type of flora and fauna growth on artificial reefs in seawater remarkably differs as function of the em-
ployed materials: car bodies (with high content of galvanized iron) or airplane bodies (with aluminum alloys prevalence) [21].
4. Discussions Our results in short period tests in ASTM seawater can be not representative of what happens in natural seawater. The biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and electroaccreted coating. The mineral composition of coating can be remarkably different if the coating is grown in natural or artificial seawater. This is probably related to the different behavior of hydroxyl ions diffusion phenomena if biofilm is
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present and to corrosion phenomena on amphoteric materials related to surface alkalization. On Fe/Zn polarized at −1.05 ÷ −1.10 V versus Ag/AgCl, coating is composed of aragonite and brucite if electroaccretion is performed in ASTM seawater and of zinc hydroxychloride Zn5 (OH)8 Cl2 ·H2 O and aragonite without brucite if electroaccretion is performed in natural seawater. In both environments, on Fe/Zn polarized at low potentials, i.e., <−1.2 V versus Ag/AgCl, the coating is composed of brucite and aragonite as on pure iron; the formation of aragonite can come before the growth of brucite. The zinc layer, even if the specimen is coupled with magnesium anode, can corrode leaving uncovered iron areas where hydrogen evolution and coating formation are favored with growth of big aggregates composed of a mixture of brucite and aragonite. Brucite is prevalent inside and aragonite outside. In this case, the galvanic coupling current of galvanized iron–magnesium couple can significantly increase and not decrease as usual with time. On Al, hydrogen evolution and coating formation take place at potentials much lower than the potentials observed on galvanized iron. Current densities during potentiostatic tests performed in natural seawater are lower than those in ASTM seawater at the same potentials. Coatings on Al are far different from deposit on Fe/Zn, both in morphology and chemical composition. In ASTM seawater, the coating on aluminum at potentials >−1.1 V versus Ag/AgCl does not contain calcium but only aluminum oxide and Mg4 Al2 (OH)14 ·2H2 O. At lower potentials, the coating is composed of brucite and aragonite and mixed hydroxides of aluminum and magnesium disappear. In natural seawater, the coating on Al at potentials >−1.1 V versus Ag/AgCl does not contain calcium and magnesium, but only aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg6 Al2 (OH)18 ·4H2 O islands. 5. Conclusions Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. The coatings on the examined materials, i.e., galvanized iron and aluminum 1100, can contain substrate corrosion products, i.e., ZnO, Zn5 (OH)8 Cl2 ·H2 O, Mg4 Al2 (OH)14 ·2H2 O, Al2 O3 and Mg6 Al2 (OH)18 ·4H2 O, even if the coupons are galvanically coupled with magnesium anode, and can not contain CaCO3 . Than, these coatings cannot be properly defined as calcareous deposits. References [1] K.P. Fischer, J.E. Finnegan, Corrosion89, NACE, Houston, TX, 1989, no. 582.
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