Corrosion Behavior of Anodized Pure Zinc in Sea Water

In the present work an electrochemical anodizations of pure Zn sheet result in the ZnO porous film structure in alkaline solution (15% ammonium florid) by using constant voltage technique at a range of (2-10) volt, for period time of 10 minutes at room temperature was studied . Atomic force microscope (AFM) used to investigate the topography and roughness of anodic film formed on the surface, it was found that the roughness of the anodic film is mainly depended on the anodizing voltage and its increase from 0.2 (as polished) to (2.9) nm after anodizing. Coating thickness also increases with an increase in voltage from 0 to 285 μm after anodizing. The corrosion behaviors of anodized and non-anodized of zinc specimens in sea water are obtained. The corrosion current density and corrosion rate are decreased with an increase in supplying voltage which increases anodic film thickness.


1.Introduction
Zinc oxide (ZnO) is a widely used in semiconductor with high bound energy (60 meV) [1].It is also used in different instruments like sensors of gases, transparent electrode conducting (TEC ) in thin films of solar cells sheets, surface audio wave and microphone applications.It's only electro-optical properties, especially, permit it's use as ultraviolet (UV) lighttransmit diodes, also blue bright devices [2] , for this intention, wide articles was published towards to promote the band hiatus emission characteristics of the zinc oxide coating films [1][2][3].Recently , many procedure were published to produce zinc oxide (ZnO) nanostructures, for instance vapor phase transmit [4,5], metal organic, chemical vapor deposition [6][7], laser supply [8][9], thermal decomposition , a template-guide procedure , and chemical composition have been applied to product zinc oxide (ZnO) with hexagram shape, quantum points, nano tubes, nano wires, etc.These ZnO nanostructures in general produced by high temperature technique [4][5][6][7][8][9][10] by treatment procedure [11,12].Anodizing process is a famous way to produced selforganized structures with low dimension, like porous Al2O3, TiO2, and ZrO2.Suitable composition of produce zinc oxide (ZnO) nanostructures have an ability to get useful applications in producing materials with nanostructured shaps.Till now, little articles have reported a production procedure process to produce ZnO by anodization of zinc metal by applied high current density.Szpaek and Gabriel have explained the production of various ZnO structures by using different conditions in anodizing process [10][11].
Baugh [4] study the corrosion rate and polarization behavior of Zn in different aqueous solutions like NH4C1, NaClO4 and Na2 SO4 media.The study confirms that the create of oxide surface film in weak acidic solution by differential conditions capacity measure over a broad range of frequency.
The anodic coated layer work as protection film and their structure and composition are the important agent that define the photoelectric and electrical characteristic of the photo electrochemical solar cell.X-ray diffraction (XRD) results [5][6][7] have specified that the formation of anodic film layer on the metal surface with white color and its consist of ZnO as main composition of the surface which have a good properties of semiconducting.
Anodising is an electrochemical process that converts the surface of metals to an insoluble oxide coating.This coating film improves the corrosion rate and has wide applications such as decorative surfaces, a good basis for painting and othercoating methods, also special mechanical and electrical properties.Anodizing process has long been used in decoration and frilly metals surface or to increase the resistance of corrosion [1].This operation is a high cost-effective process to create regular and a good adhesive thin films oxide on metal's surface [2] also the surface treatment is an important method used to increase a different types of nanostructures on numerous surface of metals, with the films produced widely used in various applications [3].Metal oxides show a wide properties range of properties depending on interconnection between the reaction of metal ion and oxygen and the crystal structure of the metals.Therefore, the oxides film formed on the metal surface have been the excellent materials in different purposes like light emitting diodes, varied types of transistors and solar cells [7].The aim of the this work is to investigate the formation of zinc oxide film on zinc specimen by means of ammonium florid NH4F (as electrolyte) in anodizing process using constant voltage technique and study the properties of the anodic coating layer and the effect of this layer on corrosion resistance.

2.Experimental work I.Metal Preparation
Pure zinc sheet (99.98 % purity of 5mm thick) (chemical composition listed in Table 1), is used as working electrode in anodizing cell, the specimens were cut in to the desired size (3x2 cm).Zn sheet was immersed in acetone for 5 min.toremove oil, grease and common dirt.The specimens were rinsed in running water followed by distilled water to removal the traces of acetone on the specimen.

Table 1: Standard chemical compositions of pure zinc.
The surface of the pure zinc was ground using emery paper up to 1000 grit to produce a flat surface.The specimen therefore has to be polished with 0.5 μm diamond paste on red felt cloth.The specimens were neatly cleaned by water, rinsed with acetone and dried under air then store in a disecater over silica jel.

3.Anodizing procedure
The zinc specimen was attached to the positive terminal of the D.C. power supply where it becomes (anode) while the stainless steel sheet, was connected to the negative terminal to be (cathode).Figures ( 1) and ( 2) demonstrate a photo and a schematic outline for the entire structure of the anodizing process respectively.The zinc object (anode) and cathode electrode are immersed in a solution of 15% NH4F in the anodizing vessel.Before the power supply switched on, the voltage must be fixed at the desired value, when power supply switched on the current increased gradually and the voltage maintains at the fixed value for the remaining time of the experiment.
Figure1: Photograph of the complete assembly of the anodizing apparatus.When all necessaries conditions of experiment are set up, the power supply was switched on so that constant current was obtained .The parts were immersed in the electrolyte immediately when the power supply was switched on.Anodizing process provides a protection for the specimens from dissolution, also care have been taken during the loading specimen in order to prevent short circuit i.e. burning the surface.the power supply was switched off at the end of the required anodizing time, and the specimen (anode) was removed from anodizing cell immediately to prevent the dissolution of anodic  () = 0.13 *   * . ρ where: icorr : corrosion current density, μA/cm 2 E.W.: specimen equivalent weight (56 g. / equivalent).ρ: Specimen density (7.14 g./ cm 3 ) .

4.Results and Discussions I.Coating Thickness Measurement
Eddy current testing process (coating thickness gauge TT260 ITALY made) was used to investigate the coating thickness, as shown in Table 2, the coating thickness increases with an increase in the supplying voltage due to the thick layer that precipitates on the low carbon steel surface.
Table 2 Coating thickness at different voltage supply.
Figure 5 demonstrates the effect of supplying voltage on film thickness of pure zinc.Thickness of (ZnO) is Increasing directly with the increasing in supplying voltage.Normally, the oxide film thickness increases to a maximum value at which higher voltage promote the oxygen reaction with pure zinc, i.e. to create the zinc oxide.N. Diomidis (9) indicated that an increase in anodizing voltage leads to increase the rate of growth of anodic film thickness when study the composition of coating film create on Zn by anodizing process by using Silicated based as electrolyte.This study also shows that the influence of hydrodynamics on the structural properties and corrosion resistance of the new film formed on the surface of the metal.The dissolution of the Zn base is observed to be desired for the precipitation of silicate-rich coating layers.

I.AFM Investigation
The roughness of surface film before and after anodization process was determined by AFM as shown in Figs. 6 and 7.The AFM image revealed that a rough film formed on the zinc surface after anodizing.The root mean square (RMS) roughness increases as the time increase due to the anodic layer formed on the surface.The type of this layer appears as spherical or semi-spherical.Precipitation process depends on surface cleaning.The roughness increases by increasing the supplying voltage of anodizing.Table 3 shows the effect of anodizing voltage on surface roughness.This may be due to rapid film growth with increasing applied voltage which enhances growth of nanoporous film.This can be shown in Figure 7. A. chour, et.al. (10) Indicated in his study that the coating layer formed on the zinc surface is by anodizing process, consist of zinc oxide as mainly component and the surface becomes more rougher than the surface before anodizing due to the new layer create on the surface and the columnar structure becomes more obvious as the supply voltage value raises from 1.8 to 3.0 V.The increase in roughness of the surface film is due to the fact that the coating surfaces become nanoporous after electrochemical process.Basheer [12] indicate that the roughness of the anodic film is mainly depend on the anodizing time and coating thickness, the ZnO layer create on the surface by anodizing process for magnesium alloy by alkaline solution at constant voltage.Sami,and Basheer . [13], study the anodic layer formed on aluminum alloy (3003) by using (15% H2SO4) as electrolyte, the study shown that the micro hardness and surface roughness for the anodized samples are higher than that of unanodized samples due to very much thicker oxide film created on the surface for anodized specimens, while atmospheric oxide film formed on the surface of un-anodized specimens.The anodic film was improved different properties like physical and chemical properties.And, the rates of corrosion for anodized samples are decreasing compare with unanodized ones.The polarization curves for anodizing alloy demonstrate that the breaks down potentials are transfer to more noble way compare with uncoated specimen because of anodizing process.

Conclusions
Zinc material is protected from corrosion by anodic film at different voltage supply (2, 4, 6, 8 and 10 volt) at constant time which produced thick film of ZnO.Corrosion resistance at 10 volt is improved due to its anodic film.
The anodic film create on the surface at 10 V has a rough surface and dark gray color, it mainly consisted of zinc oxide, also has the best corrosion resistance due to the higher coating thickness

Figure 2 :
Figure 2: Schematic outline for the complete entire structure of the anodizing process.

Figure 5 :
Figure 5: The effect of supplying voltage on film thickness.
Figure6:The effect of supplying voltage on film roughness.

Figure 8 :
Figure 8: corrosion behavior of pure zinc before anodizing.

Figure 14 :
Figure 14: The effect of zing anodizing on corrosion rate