Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 184 (2017 ) 418 422 Advances in Material & Processing Technologies Conference Development of Al-Zn-Cu Alloy for Low Voltage Aluminum Sacrificial Anode Deni Ferdian a, Yudha Pratesa a, Inez Togina a, Ira Adelia a a Department of Metallurgy and Materials Engineering, Universitas Indonesia, Kampus Baru UI Depok, 16424, Indonesia Abstract In general, a conventional aluminum anode for seawater service is Aluminum-Zinc-Indium (Al-Zn-In) alloys grade. This type of alloy offers high efficiency and high potential protection in a seawater environment. Nevertheless, there is a limitation for this alloy anode. High protection potential increases the possibility for hydrogen embrittlement in high-strength steel. Therefore, a new Al anode to produce a low voltage properties is being developed. Copper addition as an alloying element reduces the potential protection of aluminum anode. Based on polarization result, the copper addition could reduce the corrosion resistance of aluminum. Furthermore, result from the metallographic examination showed that higher copper content has a higher precipitation in the grain boundary of aluminum. This precipitate in aluminum reduced the protection of passive film and increase the corrosion rate of Al-Zn-Cu alloy as a new sacrificial alloy candidate. 2017 The The Authors. Published Published by Elsevier by Elsevier Ltd. This Ltd. is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of the Advances in Material & Processing Technologies Peer-review Conference. under responsibility of the organizing committee of the Urban Transitions Conference Keywords: Aluminum Anode; Low voltage, polarization 1. Introduction The aluminum sacrificial anode is usually used in marine environment due to their high efficiency and high current capacity. However, there is a limitation in aluminum anode development; the main concern is their passive layer which makes them nobler compared to carbon steel in sea water. Numerous researchers have been proposed to minimize this problem by adding active alloying elements that reduce the layer. The most common alloying element added to the aluminum sacrificial anode is Zn. Zinc addition ranged between 2.5% -5.75% wt. It is intended to damage the passive layer of aluminum by the formation of the second phase (ß particles) which prevent homogeneous passive layer on aluminum [1-3]. Common known alloy for the aluminum sacrificial anode is Al-Zn- In. Unfortunately, the electronegativity of these alloys can cause overprotection that triggered stress cracking (SCC) or hydrogen embrittlement (HE) on high-strength steel. This condition made the development of low voltage 1877-7058 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the Urban Transitions Conference doi:10.1016/j.proeng.2017.04.112
Deni Ferdian et al. / Procedia Engineering 184 ( 2017 ) 418 422 419 aluminum sacrificial anode is necessary.the addition of alloying elements such as Cu, Si, and Ge are proven to increase the anode potential and reducing the risk of overprotection [4]. Copper and silicon were added due to their higher potential than aluminum. Precipitation of copper at grain boundary of aluminum-copper alloy caused corrosion rate increase [5]. Copper will reduce corrosion resistance in aluminum with mechanism of metallic copper cathode. Copper will form a deposition thin layer in the surface and make double layer in the surface.[6] Another research used Ge as alloying element to achieved low voltage behavior. However this alloy needs heat treatment to activate the breakdown effect [7]. This research will introduce new composition for Al Sacrificial Anode that has lower voltage behavior. Zinc was added to increase the corrosion rate of aluminum by passive layer breakdown phenomenon while Copper was added in the low composition to produce metallic copper cathode phenomena and increase the potential protection hence make lower voltage behaviour 2. Experimental Procedures 2.1. Materials The alloy was cast from high purity Al, Zn and Cu ingot with 99.99% grade in a graphite crucible under atmospheric condition. Each of materials was weighed and prepared for Al-5Zn Al-5Zn-0.5Cu, Al-5Zn-1Cu. The melting furnace was set at 750 C and start with Al inside the crucible until complete melt. After the melting condition had achieved, zinc and copper were added subsequently to achieve desired alloy composition. The alloying process conducted for 20 minutes at 750 C. During the melting process, the molten metal was stirred using graphite bar in increase the alloying process. Slag was removed before the molten aluminum alloy poured into the mold. 2.2. Materials Characterization The alloying then characterized for the chemical composition, metallographic test, polarization and electrochemical impedance spectroscopy. Chemical composition was performed using optical emission spectrometer. The samples were then prepared by the standard method for metallographic examination follow with kellers etchant for 30 seconds. Corrosion behavior of the alloys was tested using electrochemical test technique in 3.5%NaCl water solution at room temperature. Cyclic polarization was conducted to analyzed pitting behavior of the alloy. Potentiodynamic Polarization test showed the corrosion alloy of the alloy while the Electrochemical Impedance Spectroscopy (EIS) analyzed the effect of alloying element on the aluminum passive film. Reduction of E pit could be observed using this methods. EIS were performed at a range of frequency 100 khz-10mhz. All of the electrochemical measure using Standard Calomel Electrode. Both of the experiment were done after immersed for 30 second in the 3.5% NaCl for the adaptation. 3. Result and Discussion 3.1 Cyclic Polarization Cyclic Polarization test in Figure 1 showed the pitting potential (E pit ) of Aluminum-Zinc reduced with the addition of copper. This condition indicated the passive layer of aluminum is easier to break after addition of copper in materials. At first, E pit shifted from the more noble potential into the active area after Zn added into the alloy. However, E pit increased again after addition of copper into Al-5Zn alloy. Based on these result, Zn makes passive layer of aluminum break down easier in chloride solution while copper provides more resistance to local corrosion. 3.2 Potentiodynamic Polarization
420 Deni Ferdian et al. / Procedia Engineering 184 ( 2017 ) 418 422 Linear polarization is a method to measure corrosion rate of materials. Figure 2 shows the potentiodynamic polarization result of pure aluminum, Al 5Zn Alloy, Al-5Zn-0.5Cu, and Al-5Zn-1Cu in 3.5 % NaCl solution. The electrochemical parameters result [I corr, E corr ] are listed in Table 1. Table 1 showed corrosion current density (Icorr) increased after alloying Zn and Cu added into the materials. Samples I corr (μa) E Corr (V) Corrosion Rate (mm/year) Al 37.8-1.21 0.4 Al-5Zn 118.9-1.27 1.37 Al-5Zn-0.5Cu 126-1.31 1.46 Al-5Zn-1Cu 214-1.45 2.4 Fig. 1 Cyclic polarization of Al pure (blue), Al-5Zn (brown), Al-5Zn-0.5Cu (green) and Al-5Zn-1Cu (Red). This condition could be a result of disruption of passive film (Al oxide) formation. Based on cyclic polarization examination, Aluminum will form a passive oxide layer when they are immersed in the NaCl solution. However, this passive layer is very dependent on the composition at materials surface condition. The presence of different phase in aluminum alloy could make a micro galvanic corrosion between of phase. The nobler phase will immune while the active site will corrode and leave a pitting in materials. Figure 2 also showed corrosion potential (E.Corr) shifted to the more active region after addition of more alloy. 3.3 Electrochemical Impedance Spectroscopy Fig. 2 Potentiodynamic result of Al, Al-5Zn, and Al5Zn-0.5Cu in sea water condition EIS test was conducted to analyzed surface electrochemistry behavior. Figure 3 showed Nyquist and Bode plot result from this study. The result found a reduction of resistivity as a copper percentage in alloying increased.
Deni Ferdian et al. / Procedia Engineering 184 ( 2017 ) 418 422 421 Charge transfer resistance (Rct) of aluminum reduced by Zn and Cu addition. The result indicated the charge transfer phenomena in Al-5Zn-Cu alloy were easier and the ability of passive layer to prevent corrosion reduced. This condition showed Al-5Zn-Cu alloy had higher corrosion rate than pure alloy. Low-frequency inductance showed at Al-5Zn-1Cu and Al-5Zn-0.5Cu. Low-frequency is an indication of absorption process of electrolyte in material surface, in particular with chloride adsorption in the case of aluminum pitting. Keddam et al. showed that the inductive loop formation related to a weakening of aluminum oxide effectivity as protection layer due to the anodic dissolution of the aluminum alloy [8]. Higher copper concentration in Al-5Zn alloy made inductive number increase which is indication of weakening effect of aluminum oxide in material surface as listed at table Fig. 3 EIS result from Pure Aluminum (Blue) and Al-5Zn-Cu with 0.5% Cu (Red) and 1%Cu (yellow) Table 2 EIS fitting circuit result Alloy Composition Rct N CPE L Al 13kΩ 0.798 19,4 - Al-5Zn-0.5Cu 1,32kΩ 0.826 30,3 851H Al-5Zn-1Cu 1,12kΩ 0.796 59,7 877H 3.4 Metallographic examination Metallographic examination of alloy showed different result between pure aluminum, Al-5Zn, Al-5Zn-0.5Cu and Al-5Zn-1Cu. The microstructure of Al-5Zn changed after copper was added into the alloy. Several white phases formed at the dendrite grain boundary. Grain boundary precipitates were thicker after addition more copper in Al- 5Zn alloy. This condition is an indication that copper will make precipitate at the Al-5Zn as showed in figure 4. This precipitate caused micro galvanic corrosion with the matrix. It makes aluminum passive layer is not uniform, and the local corrosion happens at the alloy. This evidence supports the electrochemical corrosion test using EIS and potentiodynamic polarization. 4. Conclusion This study showed 5% zinc alloying in aluminum would reduce the potential of alloy decreased significantly. However, addition 0.5% and 1% of copper will increase the potential of Al-5Zn alloy. Based on the EIS and potentiodynamic polarization, copper will increase corrosion rate of Al-5Zn alloy and reduce of Charge transfer resistances in Al-5Zn alloy. Reduction of charge transfer was affected by the precipitate formation in grain boundary
422 Deni Ferdian et al. / Procedia Engineering 184 ( 2017 ) 418 422 and center of the dendrite. The precipitate formation will make micro galvanic corrosion between the matrix and the precipitate phase. Al pure Al 5Zn Al 5Zn-0.5Cu Al 5Zn-1Cu Fig. 4 Metallographic result of various alloy with keller etchant showed copper precipitation (red arrow) (200x magnification) Acknowledgements The author would like to appreciate PT. Inalum Indonesia for the material support. Partial funding from Directorate for Research and Public Services - Universitas Indonesia (DRPM-UI) is highly acknowledged. References [1]. Muazu, A. and Yaro, S. A. Effects of Zinc Addition on the Performance of Aluminium as Sacrificial Anode in Seawater, Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.2, (2011) 185-198 [2]. S. Khireche, D. Boughrara, A. Kadri, L. Hamadou and N. Benbrahim, Corrosion mechanism of Al, Al Zn and Al Zn Sn alloys in 3wt.% NaCl solution. Corrosion Science 87, (2014) 504-516. [3] S. Lameche-Djeghaba, A. Benchettara, F. Kellou, and V. Ji, Electrochemical Behaviour of Pure Aluminium and Al-5%Zn Alloy in 3% NaCl Solution, Arab. J. Sci. Eng., vol. 39, no. 1,(2014) pp. 113 122. [4]. William (Jacob) Monzel, Alan P. Druschitz, Myrissa Maxfield, Development of New Low-Voltage Aluminum Sacrificial Anode Chemistries, NACE-4284 (2014). [5]. J.R. Galvele, S.M. de De Micheli, Mechanism of intergranular corrosion of Al-Cu alloys, Corrosion Science, Volume 10, Issue 11 (1970) 795-807 [6]. J.R. Davis, Alloying: Understanding the Basics, ASM International (2001), p351-416. [7].A. Druschitz, K. Tontodonato, Low Voltage Cast Al-Zn-BiSacrificial Anodes: Effect of Heat Treatment, MS&T Conference Proceedings, Pittsburgh (2014). [8] M. Keddam, C. Kuntz, H. Takenouti, D. Schustert, and D. Zuili, "Exfoliation corrosion of aluminium alloys examined by electrode impedance," Electrochimica Acta, vol. 42, (1997), pp. 87-97.