2017 International Conference on Mechanical and Mechatronics Engineering (ICMME 2017) ISBN: 978-1-60595-440-0 Formation of Corrosion Resistant Mg(OH) 2 Film Containing Mg-Al Layered Double Hydroxide on Mg Alloy Using Steam Coating Takahiro ISHIZAKI 1,*, Mika TSUNAKAWA 2, Yuta SHIMADA 2, Ryota SHIRATORI 2, Kae NAKAMURA 2 and Ai SERIZAWA 1 1 Department of Materials Science and Engineering, College of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, 135-8548 Tokyo, Japan 2 Department of Materials Science and Engineering, Graduate School of Engineering and Science, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, 135-8548 Tokyo, Japan *Corresponding author Keywords: Mg alloy, Steam coating, Corrosion resistance, Magnesium hydroxide, Mg-Al layered double hydroxide. Abstract. In recent years, a reduction in weight of materials is highly desirable in the fields of transport industry and information technology devices. Magnesium (Mg) alloys have excellent physical and mechanical properties such as low density, good electromagnetic shielding, and high strength/weight ratio. Unfortunately, they have a great issue that is low corrosion. Thus, it is very important to develop surface treatment technologies to improve the corrosion resistance of magnesium alloys. Many surface treatments such as chemical conversion, anodic oxidization, and electroplating have been developed. However, there processes require multi-step pretreatments and waste liquid treatments. Thus, the development of a simple, easy, and low-cost surface treatment is highly desirable. In this paper, we report a novel preparation method of anticorrosive film on Mg alloy by steam coating and the corrosion resistance of the film. The prepared films were characterized by XRD, SEM, EDX, and electrochemical measurements. Polarization curve measurements demonstrated that the E corr and i corr values were found to be most positive potential and lowest current density in all the samples, indicating that the sample had best corrosion resistant performance. Introduction Magnesium and its alloys are superior materials in lightweight property. Recently, the weight saving in the fields of the transportation application would be one of the most important issues to improve energy conservation. Thus, they are expected to be applied to transportation such as aerospace, aircraft, auto-mobile, and railway [1-4]. However, the low corrosion resistant property of the magnesium alloys limits the use at large scale, so it is necessary to improve the corrosion resistant properties of the magnesium alloys. To improve the fault, various surface treatments for the preparation of protection films such as anodized film, chemical conversion film, and polymer film on magnesium alloys have been developed [5-8]. Among them, the chemical conversion films have been practically applied to Mg components such as parts of automotive components because of easy operation and low cost. Chromate system chemical conversions are well known as the most popular and effective conversion process to produce a protective layer on the various metal surfaces. However, the use of chromate system is being progressively regulated due to the high toxicity of the hexavalent chromium compounds. Therefore, an environmentally friendly surface treatment method to improve the low corrosion resistance of the magnesium alloys is highly desirable. Recently, a chemical conversion method using Mg-Al layered double hydroxide (LDH) has been used as a protect film to improve significantly corrosion resistance [9-13], because the LDHs could be effective agents for delaying the corrosion reaction because of the entrapment functionality of corrosion-relevant anions such as chloride ions in LDHs [14,15]. Thus, the LDHs are promising materials for improving corrosion resistance. For industrial application, it is important to develop 201
environmentally-friendly surface treatment methods to prepare anticorrosive film containing LDHs. Recently, we developed a novel preparation method of anticorrosive film on Mg alloy by steam coating [16,17]. The steam coating is a simple, environmentally friendly, and inexpensive surface treatment method. In addition, the film prepared by steam coating was mainly composed of magnesium hydroxide and Mg-Al LDH. In this paper, we report the preparation method of the corrosion resistant Mg(OH) 2 film containing Mg-Al LDH by steam coating and the corrosion resistance of the film coated magnesium alloys.. Experimental Procedures Magnesium alloy AZ31 (composition: 2.98 mass% Al, 0.88 mass% Zn, 0.38 mass% Mn, 0.0135 mass% Si, 0.001 mass% Cu, 0.002 mass% Ni, 0.0027 mass% Fe, and the rest is Mg) specimens were used as substrate. Substrates were ultrasonically cleaned in ethanol for 10 min. The cleaned AZ31 substrates were introduced in a Teflon-lined autoclave with a 100 ml capacity. 20 ml of ultrapure water was introduced at the bottom of the autoclave to produce steam. The autoclave was heated to a temperature of 433 K, and then held at this temperature for 2 to 6 h, and was subsequently cooled naturally to room temperature. After the steam treatment, the samples were ultrasonically cleaned in ethanol for 10 min. The surface morphologies and of the corrosion resistant films were observed using a scanning electron microscopy. The crystal phase of the obtained film was investigated using a glancing angle X-ray diffraction (GAXRD) at a glancing angle of 1o with CuKα radiation (40 kv, 40 ma) within the range of 5 and 80 and scanning rate of 2θ = 4 (min 1 ). The corrosion resistance was estimated by electrochemical measurements and immersion tests in a 5 mass% NaCl aqueous solutions. All electrochemical measurements were performed in a 5.0 mass% NaCl aqueous solution at room temperature. The film coated AZ31 and a platinum mesh were used as the working and counter electrodes, respectively. An Ag/AgCl was used as the reference electrode. Potentiodynamic polarization curves were measured with respect to the open circuit potential (OCP) at a scanning rate of 0.5 mv/s from 200 to +800 mv. Results and Discussion Figure 1 shows XRD patterns of the film coated AZ31 by steam coating for (a) 2 h, (b) 4 h, and (c) 6 h. Some peaks attributable to Mg alloy substrate are clearly observed in all the patterns. In particular, the peak intensity attributable to Mg ally substrate for the film coated AZ31 for 2 h was high. This indicates that the film thickness is thin. All peak intensities assigned to Mg alloy substrate decrease with an increase in the treatment time, indicating that the film thickness increases gradually with an increase in the treatment time. A diffraction peak of [101] reflection at around 2θ = 38 becomes stronger with an increase in the treatment time. With an increase in treatment time, several peaks at approximately 2θ = 18, 33, 38, 51, 58, 62, 68, and 72 assigned to the 001, 100, 101, 102, 110, 111, 200, and 201 diffraction peaks of brucite type Mg(OH) 2 are also clearly observed in all the GAXRD patterns. In addition, the two peaks at around 2θ = 11 and 22 assigned to the [003] and [006] reflections of Mg 1-x Al x (OH) 2 (CO 3 ) x/2 nh 2 O (Mg-Al LDH), respectively, can be clearly observed in the XRD patterns of the film prepared for 2 and 4 h. These results indicate that Mg(OH) 2 is initially formed on the Mg alloy substrate and Mg-Al LDH is then grown in the Mg(OH) 2 film. 202
Figure 1. XRD patterns of the obtained sample by steam coating for (a) 2, (b) 4, and (c) 6 h. Figure 2 shows SEM images of the sample surfaces treated at 433 K for (a) 2 h, (b) 4 h, and (c) 6h. In the case of 2h of the treatment time, aggregated particles were clearly observed. The particles were composed of nanoparticles with sizes of 50 500 nm. When the preparation time was prolonged to 4 h, granular structures were clearly observed. The particle size of the sample prepared for 4 h became larger and the film was found to be dense and the surface was perfectly covered with the film. The film treated for 6 h was found to be denser than the sample surface treated for 4 h, and exhibited small amounts of nanosheets that are aligned at fairly inclined angles with respect to the surface. Figure 2. SEM images of the obtained sample by steam coating for (a) 2, (b) 4, and (c) 6 h. Figure 3 shows cross-sectional SEM images and elemental Mg and O mapping images of the samples prepared by steam coating for 2, 4, and 6 h. The film thicknesses for the samples prepared for 2, 4, and 6 h were estimated to be 3.1, 20.2, and 68.0, respectively. The film thickness increases with an increase in the treatment time. The elemental mapping images revealed that the film mainly consisted of Mg and O, which indicates that it was comprised of magnesium hydroxide. This is in agreement well with the XRD results. 203
Figure 3. Cross-sectional and elemetal Mg and O mapping images of the obtained sample by steam coating for (a) 2, (b) 4, and (c) 6 h. Figure 4 shows potentiodynamic polarization curves of the samples prepared by steam coating for 2, 4, and 6 h. The potentiodynamic polarization curve for untreated AZ31 is also shown. The corrosion potential, E corr, and corrosion current density, i corr, values for the untreated AZ31 were estimated to be -1.435 V vs. Ag/AgCl and 5.395 X 10-5 A/cm 2, respectively. The anodic region of the curve for the bare AZ31 can be divided into three fields: (i) first region beginning from E corr (low anodic overpotential) that exhibits a linear increase in current density with potential; (ii) a range of abrupt increase in current density from -1.4 V, where the dissolution of Mg to Mg + or Mg 2+ ions predominantly occurred [18]; (iii) a current plateau at more positive potentials, where the electrode surface is covered with a film identified as Mg(OH) 2 [19]. In the cathodic branch, hydrogen evolution is more dominant at negative potentials than Ecorr, resulting in an increase in the cathodic current density. The current densities of the anodic and cathodic branches of all samples decreased with film formation, which indicates that the films are effective in improving the corrosion resistance of AZ31 alloy. The E corr for the film coated AZ31 tends to shifted to positive direction with an increase in the treatment time. In addition, the i corr values were lowered with an increase in the treatment time. Clear passive regions can be observed on the curve for the samples treated at 433 K for 4 and 6 h. The E corr and i corr values of the sample treated at 433 K for 6h were estimated to be -0.161 V and 4.824 X 10-11 A/cm 2, respectively. The E corr and i corr values were found to be most positive potential and lowest current density in all the samples, indicating that the sample had best corrosion resistant performance. Figure 4. Potentiodynamic polarizaiton curves of the obtained sample by steam coating for (a) 2, (b) 4, and (c) 6 h. Summary The corrosion resistant Mg(OH) 2 film was successfully formed on AZ31 Mg alloy by steam coating. 204
XRD patterns revealed that the film thickness became thicker with an increase in the treatment time tthe film thicknesses of the samples prepared for 2, 4, and 6 h were estimated to be 3.1, 20.2, and 68.0, respectively. Polarization curve measurements demonstrated that the E corr and i corr values for the film coated AZ31 tends to shifted to positive and lower direction, respectively, with an increase in the treatment time. As Mg alloys are one of the more promising materials for reducing vehicle weight, thereby lowering fuel consumption and reducing CO 2 emission, any improvement in their inherently low corrosion resistance is of great value to increasing their wider-scale use. We believe future work should focus on developing the steam coating process described here to greatly improve the corrosion resistance of a range of Mg alloys. Acknowledgement This research was partially supported by a Grant for Advanced Industrial Technology Development from the New Energy and Industrial Technology Development Organization (NEDO) of Japan (No. 11B06024d), a Grant-in-Aid for Young Scientists (B) (No. 16K18249) from the Japan Society for the Promotion of Science, and the Japan Science and Technology Agency (JST), Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-step: No. AS251Z02997K & AS2815047S) References [1] S.R. Agnew, M.H. Yoo, C.N. Tome, Acta Mater. 49 (2001) 4277. [2] M. Furui, C. Xu, T. Aida, M. Inoue, H. Anada, T.G. Langdon, Mater. Sci. Eng. A, 410 (2005) 439. [3] M. Kawasaki, K. Kubota, K.I. Higashi, T.G. Langdon, Mater. Sci. Eng. A, 429 (2006) 334. [4] M.L. Zhang, Y.D. Yan, Z.Y. Hou, L.A. Fan, Z. Chen, D.X.Tang, J. Alloy Compd. 440 (2007) 362. [5] Z. Liu, W. Gao, Surf. Coat. Technol., 200 (2006) 5087. [6] J.S. Lian, G.Y. Li, L.Y. Niu, C.D. Gu, Z.H. Jiang, Q. Jiang, Surf. Coat. Technol. 200 (2006) 5956. [7] C.-E. Barchiche, E. Rocca, C. Juers, J. Hazan, J. Steinmetz, Electrochim. Acta, 53 (2007) 417. [8] M.F. Montemor, M.G.S. Ferreira, Electrochim. Acta, 52 (2007) 7486. [9] A. Collazo, M.Hernández, X.R. Nóvoa, C. Pérez, Electrochim. Acta, 56 (2011) 7805. [10] J.K. Lin, J.Y. Uan, Corros. Sci., 51 (2009) 1181. [11] J. Chen, Y. Song, D. Shan, E.-H. Han, Corros. Sci., 53 (2011) 3281. [12] J.K. Lin, K.L. Jeng, J.Y. Uan, Corros. Sci., 53 (2011) 3832. [13] J. Chen, Y. Song, D. Shan, E.-H. Han, Corros. Sci., 63 (2012) 148. [14] J.K. Lin, C.L. Hsia, J.Y. Uan, Scripta Mater., 56 (2007) 927. [15] J. Tedim, A. Kuznetsova, A.N. Salak, M.F. Montemor, D. Snihirova, M. Pilz, M.L. heludkevich, M.G.S. Ferreira, Corros. Sci., 55 (2012) 1. [16] T. Ishizaki, S. Chiba, H. Suzuki, ECS Electrochem. Lett., 2 (2013) C15-C17. [17] T. Ishizaki, S. Chiba, K. Watanabe, H. Suzuki, J. Mater. Chem. A, 1 (2013) 8968-8977. [18] D.K. Tanaka, G.G. Long, J. Kruger, Structure of the Passive Films on Cast and Rapidly Solidified Mg Alloys. In: Proceedings of 11th International Corrosion Congress; 2-9, April, 1990; Florence. London: The International Corrosion Council. 1990. 605-610. [19] G. Baril, G. Galicia, C. Deslouis, N. Pébère, B. Tribollet, V. Vivier, M.X. Quondam. J. Electrochem. Soc., 154 (2007) C108 C113. 205