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     Int. J. Electrochem. Sci.,  10 (2015)  7596 - 7605 International Journal of ELECTROCHEMICAL SCIENCE   www.electrochemsci.org Early Stages of Zinc Corrosion and Runoff Process Induced by Caribbean Sea Water  E. Mena 1  , L. Veleva 1,*  , R. M. Souto 2   1 Department of Applied Physics, Research Center for Advanced Study (CINVESTAV- IPN), Mérida Carr. Ant. a Progreso Km. 6, 97310, Mérida, Yucatán, México   2 Department of Chemistry, University of La Laguna, 38071 La Laguna, Tenerife, Canary Islands, Spain * E-mail: veleva@mda.cinvestav.mx   Received: 16   June 2015   /  Accepted: 8 July 2015 /  Published: 28   July 2015   Flat samples of electrolytic zinc were immersed for periods of 8, 10, 30 and 90 days in Caribbean Sea water for further analysis of the corrosion behavior and runoff process using different techniques. The free corrosion potential (o.c.p.) were monitored and correlated with the runoff rate. The Zn 2+  ions released from anodic sites interact with the OH -  ions formed at the cathodic sites, giving srcin to the slightly soluble Zn(OH) 2  precipitated during the experiment. X-ray diffraction analysis revealed the formation of several zinc hydroxides. The main corrosion product was  simonkolleite  [Zn 5 (OH) 8 Cl 2 .H 2 O], and as minority phases two  zinc carbonate hydroxides,  [Zn 5 (CO 3 ) 2 (OH) 6 ] and [Zn 4 CO 3 (OH) 6 .H 2 O], and later stages Mg 9 Zn 4 (SO 4 ) 2 (OH) 22 .8H 2 O and Zn(OH) 2 . The changes of pH in the interface zinc/substitute ocean water were monitored in situ  with scanning electrochemical microscopy (SECM) technique and the local anodic and cathodic sites were mapped. Due to the corrosion reaction, the initial pH of sea water was diminished to 4.4 at the anodic sites as a consequence of metal ion hydrolysis. This fact leaded to the formation of a complex variety of zinc corrosion products, consuming at least a fraction of the produced OH -  ions. Keywords:   zinc, corrosion potential, sea water, runoff, SECM.  1. INTRODUCTION Zinc is a component of the earth’s crust and an inherent part of our environment. About 12 million tons of zinc are produced annually worldwide and half of this amount is used for galvanizing to  protect steel from corrosion, due to its standard redox potential (-0.76 V / SHE) that is more negative than that of steel (-0.44 V / SHE). Zinc, particularly as hot dip galvanized steel, is a common metal for   Int. J. Electrochem. Sci., Vol. 10, 2015  7597 corrosion protection of steel structures: laminated roofs, fences, containers and tubes for water transport, zinc sacrificial anodes for cathodic protection, zinc-rich coatings, etc. [1-4]. It is well known, from a thermodynamic point of view, that zinc is relatively stable in neutral and near neutral aqueous environments [5]. Initially a fresh zinc surface corrodes fairly rapidly until it is covered with corrosion  product layers (zinc oxide/hydroxides/carbonates), which then act as a physical barrier between the metal and the aggressive environment, and the corrosion then continues at a reduced rate [6-8]. However, this so formed rust layer can be transformed into a nonprotective layer due either to physical removal of the layer (under the action of wind, flows and sand erosion) or from partial dissolution of some soluble corrosion products. As result, metal ions can be released from the metal surface to the surrounding environment. This phenomenon is recognized today as a metal runoff. All life on earth has evolved in the presence of natural levels of zinc. Due to its general availability to organisms and unique characteristics, zinc has an essential role in various biological  processes. As such, zinc is an essential element for all forms of life, from the smallest micro-organisms to man. The zinc industry has supported numerous studies in aquatic, terrestrial and atmospheric systems to further understanding of the natural variations of zinc in the environment. Consideration of  background zinc concentrations has assisted environmental risk, providing a context for biological acclimation and adaptation [9]. Yet a high ionic zinc ion concentration in water can cause a toxic effect for aquatic life, for example [10]. The analysis of the literature shows that attention to the metal corrosion (in sea water) [11-13] and runoff is enormous in Europe and USA since 1990 [14-17]. Our  previous studies have been focused in monitoring zinc runoff and changes that occur on the metal surface during the corrosion process in urban, rural and marine coastal environments [18-21]. Since zinc behaviour in sea water is an increasing topic of study, the generation of new data is of interest to the international environmental scientists and civil engineering community. This work  presents the early stages of zinc runoff phenomenon in Caribbean sea water due to zinc corrosion during 8, 10, 30 and 90 days. The analysis was carried out with a systematic study of the free corrosion  potential (at o.c.p.) fluctuations and runoff rate. The tendency of their changes were correlated with the corrosion products formed on zinc samples by using scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques, to assess the composition and morphology of these products. The pH distribution at the interface zinc/sea water was carried out  in situ  with scanning electrochemical microscopy (SECM) technique, since the initial immersion of zinc sample (electrode). Local changes of pH were used for identification of cathodic and anodic sites during the first stages of zinc corrosion in sea water. The results obtained from each approach were correlated. To our knowledge, no other research has been undertaken to date on this aspect. 2. EXPERIMENTAL 2.1. Zinc exposure and characterization Electrolytic zinc flat samples (99.99 mass %; dimensions of 20 mm x 20 mm x 1.5 mm), three replicates were previously degreased with ethanol and immersed separately, each one in 65 mL of   Int. J. Electrochem. Sci., Vol. 10, 2015  7598 Caribbean seawater at 21°C , in plastic containers for 8, 10, 30 and 90 days. Samples of zinc, withdrawn at these periods of exposure in seawater, were dried at room temperature and analyzed using various techniques. The crystalline corrosion products were characterized by X-Ray Diffraction Technique (XRD, Siemens D- 5000, with grazing beam geometry, 3° angle, 34 kV / 25 mA and Cu kα  radiation). The spectra were processed with DIFRACT AT [22] software and the identification of the  phases with Powder Diffraction File [23]. Scanning electron microscopes (SEM-EDS, Philips and XL-30 ESEM JEOL JSM-7600F) were used to obtain images of the surface morphology of zinc samples after their exposure to seawater. The zinc ions (Zn 2+ ) released (runoff) from the sample surfaces after each period of exposure was determined with a Multiparameter Ion Specific Meter (Model HI 83200, Hanna Instruments). The lower and higher detection limits for zinc ions in aqueous solutions are 0.00 and 3.00 mg L -1 , respectively. Single zinc samples were used as working electrode for immersion in seawater, inside an electrochemical cell. The changes in the free corrosion potential (  E  corr  ) values were measured at the open circuit potential, o.c.p. (i.e., without imposed polarization of the sample) during 12,600 s (3.5 h), after various periods of exposure in seawater. A potentiostat/galvanostat computerized series G750 (Gamry instruments, Inc., Software PHE 200) was employed for the measurement of   E  corr  .  The electrochemical set-up was completed using a calomel electrode (SCE, E Hg2+ / Hg / sat. KCl  = 0.244 V/SHE) as the reference, and a platinum plate as auxiliary electrode. In order to study the local pH changes at the interface zinc/electrolyte in situ , substitute ocean water was used, that was prepared according to International Standard [24]. In this way, the specific sea contaminants and changes of environmental parameters (pH, temperature, flow rate, sea waves,  pressure, calcareous deposit, salinity and biofouling) are avoided and the obtained results can be compared with those reported by another researchers. High-resolution SECM equipment, supplied by Sensolytics (Bochum, Germany), was employed for the spatially-resolved characterization of the local  pH changes occurring at the zinc/seawater interface. The instrument was built around a PalmSens (Utrecht, The Netherlands) electrochemical interface, and precise positioning unit, all controlled with a  personal computer. The zinc sample working electrode (4.3 mm x 1.3 mm) was mounted horizontally facing upward at the bottom of a electrochemical cell, submerged in 3.5 mL of substitute ocean water (pH = 8.3), and tested at open circuit potential, using an Ag/AgCl/(3 M) KCl reference electrode. Disc-shaped tip (micro-electrode) made of antimony, coated by its metal oxide, and surrounded by glass micropipette, was a sensitive pH sensor. The antimony microelectrode had 40- μm -diameter active surface. In order to quantify the localized pH distribution, the antimony oxide tip was calibrated from the measurement of the potential response transients towards pH change of the solution, using a sequence of eight buffer solu tions covering the 4 ≤ pH ≤ 11 range. Positioning of the antimony tip close to the surface was assisted with a video camera. A homemade voltage follower based on a 10 12 Ω input impedance operational amplifier was connected between the electrochemical cell and the  potentiometric input of the system [25-27]. Scan maps and 2D images of pH changes were obtained by scanning the tip parallel to the sample surface, at 30 μm constant height operation. SECM images were recorded rastering an area away from the sample edges ( 1000 μm x 4000 μm ), using a scan rate of 50 μm s -1 . The data were plotted using Quickgrid software.   Int. J. Electrochem. Sci., Vol. 10, 2015  7599 2.2. Seawater chemistry Seawater is a very aggressive medium for the metals and can cause severe damage to metallic structures in a very short length of time. Usually this water contains the ions (in decreasing quantities) of Cl - , Na + , SO 42- , Mg 2+ , Ca 2+ , K  + , HCO 3- , Br  - , B 3+ , Sr  2+ , F - , and dissolved gases, such as O 2  y CO 2 [28]. The seawater was taken from the Caribbean Sea at the warm humid tropical climate marine test station of CINVESTAV- Merida (Telchac port, Yucatan Peninsula, Mexico 21°7’ N, 89°25’ W), at a depth of 10 m and a distance of 10 km from the coast. Rack stands holding zinc samples were submerged at that location for metal corrosion testing in marine environment. The seawater had total salinity of 37.48%, pH = 7.69, dissolved oxygen 1.1 ppm, and temperature of 21°C at that depth. Specific sea pollutants were (expressed in μM L -1 ): 1.75 ammonium; 2.61 silicates; 0.28 phosphates; 0.04 nitrites, and 1.84 nitrates. 3. RESULTS AND DISCUSSION 3.1. Corrosion potential evolution and zinc runoff Initially the free corrosion potential (o.c.p.) had value of -1.108 V (vs SCE) and at the end of the experiment (90 days), it shifted to less negative (-1.028 V). Figure 1 presents the free corrosion  potential (o.c.p) fluctuations of zinc sample (electrode) after 8 and 90 days immersed in sea water. The zinc ion release (runoff) was detected at 8 days and Table 1 presents the values of zinc runoff rates determined at different periods of exposure in Caribbean seawater and the corresponding  E  corr   values. 0 2000 4000 6000 8000 10000 12000-1.0305-1.0300-1.0295-1.0290-1.0285-1.0280-1.0275    E   l  e  c   t  r  o   d  e  p  o   t  e  n   t   i  a   l   (   V   /   S   C   E   ) Exposure time (s)  8 days 90 days   Figure 1.  Electrode potential at o.c.p. after 8 and 90 days of exposure in Caribbean sea water. The results (Table 1) showed that runoff rate of zinc ions increases from 3.20 g m -2  (at 8 days) up to 4.80 g m -2  at 30 days, when  E  corr   reached the most negative potential (increase of corrosion)
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