Experimental Investigation of the Corrosion Behavior of Friction Stir Welded AZ61A Magnesium Alloy Welds under Salt Spray Corrosion Test and Galvanic Corrosion Test Using Response Surface Methodology
Extruded Mg alloy plates of 6？mm thick of AZ61A grade were butt welded using advanced welding process and friction stir welding (FSW) processes. The specimens were exposed to salt spray conditions and immersion conditions to characterize their corrosion rates on the effect of pH value, chloride ion concentration, and corrosion time. In addition, an attempt was made to develop an empirical relationship to predict the corrosion rate of FSW welds in salt spray corrosion test and galvanic corrosion test using design of experiments. The corrosion morphology and the pit morphology were analyzed by optical microscopy, and the corrosion products were examined using scanning electron microscope and X-ray diffraction analysis. From this research work, it is found that, in both corrosion tests, the corrosion rate decreases with the increase in pH value, the decrease in chloride ion concentration, and a higher corrosion time. The results show the usage of the magnesium alloy for best environments and suitable applications from the aforementioned conditions. Also, it is found that AZ61A magnesium alloy welds possess low-corrosion rate and higher-corrosion resistance in the galvanic corrosion test than in the salt spray corrosion test. 1. Introduction Magnesium alloys have received extensive recognition due to their excellent physical properties, including light weight, high strength/weight ratio, high thermal conductivity, and good electromagnetic shielding characteristics; thus, become promising candidates to replace steel and aluminum alloys in many structural and mechanical applications due to their attractive mechanical and metallurgical properties [1, 2]. The joining of magnesium components made from this alloy is, however, still limited. Unfortunately, conventional fusion welding of magnesium alloys often produces porosity and hot cracks in the welded joint. This deteriorates both the mechanical properties and corrosion resistance [3, 4]. Hence, it will be of extreme benefit if a solid state joining process, that is, one which avoids bulk melting of the base materials, hot cracking, and porosity, can be developed and carried out for the joining of magnesium alloys. FSW is a solid state welding process without emission of ration or dangerous fumes, and it avoids the formation of solidification defects like hot cracking and porosity. Moreover, it significantly improved the weld properties and had been extensively applied in the joining of magnesium alloys [5]. The application of Mg alloy in the structural members is still limited due to its conventional fusion
References
[1]
B. L. Mordike and T. Ebert, “Magnesium Properties, applications, potential,” Materials Science and Engineering A, vol. 302, no. 1, pp. 37–45, 2001.
[2]
R. C. Zeng, W. Dietzel, R. Zettler, J. Chen, and K. U. Kainer, “Microstructure evolution and tensile properties of friction-stir-welded AM50 magnesium alloy,” Transactions of Nonferrous Metals Society of China, vol. 18, no. 1, pp. s76–s80, 2008.
[3]
R. C. Zeng, J. Zhang, W. J. Huang et al., “Review of studies on corrosion of magnesium alloys,” Transactions of Nonferrous Metals Society of China, vol. 16, supplement 2, pp. s763–s771, 2006.
[4]
T. Nagasawa, M. Otsuka, T. Yokota, and T. Ueki, “Structure and mechanical properties of friction stir weld Joints of magnesium alloy AZ31,” in Magnesium Technology 2000, H. I. Kaplan, J. Hryn, and B. Clow, Eds., pp. 383–387, TMS, Warrendale, Pa, USA, 2000.
[5]
W. Xu, J. Liu, and H. Zhu, “Pitting corrosion of friction stir welded aluminum alloy thick plate in alkaline chloride solution,” Electrochimica Acta, vol. 55, no. 8, pp. 2918–2923, 2010.
[6]
M. Zhao, S. Wu, J. R. Luo, Y. Fukuda, and H. Nakae, “A chromium-free conversion coating of magnesium alloy by a phosphate-permanganate solution,” Surface and Coatings Technology, vol. 200, no. 18-19, pp. 5407–5412, 2006.
[7]
B. A. Shaw, “Corrosion resistance of magnesium alloys,” in ASM Handbook, vol. 13A: Corrosion, L. J. Korb, Ed., p. 692, ASM International Handbook Committee, Metals Park, Ohio, USA, 9th edition, 2003.
[8]
D. L. Hawke, J. E. Hillis, M. pekguleryuz, and I. Nkatusugawa, “Corrosion behavior,” in Magnesium and Magnesium Alloys, M. M. Avedesian and H. Baker, Eds., pp. 194–1210, ASM International, Materials Park, Ohio, USA, 1999.
[9]
G. Song and A. Atrens, “Recent insights into the mechanism of magnesium corrosion and research suggestions,” Advanced Engineering Materials, vol. 9, no. 3, pp. 177–183, 2007.
[10]
G. Song, B. Johanesson, S. Hagupoda, and D. StJohn, “Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc,” Corrosion Science, vol. 46, no. 4, pp. 955–977, 2004.
[11]
M. J？nsson, D. Persson, and D. Thierry, “Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D,” Corrosion Science, vol. 49, no. 3, pp. 1540–1558, 2007.
[12]
R. G. Song, C. Blawert, W. Dietzel, and A. Atrens, “A study on stress corrosion cracking and hydrogen embrittlement of AZ31 magnesium alloy,” Materials Science and Engineering A, vol. 399, no. 1-2, pp. 308–317, 2005.
[13]
M. B. Kannan, W. Dietzel, C. Blawert, S. Riekehr, and M. Ko？ak, “Stress corrosion cracking behavior of Nd:YAG laser butt welded AZ31 Mg sheet,” Materials Science and Engineering A, vol. 444, no. 1-2, pp. 220–226, 2007.
[14]
R. Baboian, “Electrochemical techniques for corrosion engineering,” in Corrosion '76, p. 114, NACE, 1976.
[15]
H. Altun and S. Sen, “Studies on the influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of AZ63 magnesium alloy,” Materials and Design, vol. 25, no. 7, pp. 637–643, 2004.
[16]
Y. Song, D. Shan, R. Chen, and E. H. Han, “Effect of second phases on the corrosion behaviour of wrought Mg-Zn-Y-Zr alloy,” Corrosion Science, vol. 52, no. 5, pp. 1830–1837, 2010.
[17]
K. H. Goh, T. T. Lim, and P. C. Chui, “Evaluation of the effect of dosage, pH and contact time on high-dose phosphate inhibition for copper corrosion control using response surface methodology (RSM),” Corrosion Science, vol. 50, no. 4, pp. 918–927, 2008.
[18]
N. Aslan, “Application of response surface methodology and central composite rotatable design for modeling and optimization of a multi-gravity separator for chromite concentration,” Powder Technology, vol. 185, no. 1, pp. 80–86, 2008.
[19]
J. S. Cowpe, J. S. Astin, R. D. Pilkington, and A. E. Hill, “Application of response surface methodology to laser-induced breakdown spectroscopy: Influences of hardware configuration,” Spectrochimica Acta B, vol. 62, no. 12, pp. 1335–1342, 2007.
[20]
A. Dhanapal, S. R. Boopathy, and V. Balasubramanian, “Developing an empirical relationship to predict the corrosion rate of friction stir welded AZ61A magnesium alloy under salt fog environment,” Materials and Design, vol. 32, no. 10, pp. 5066–5072, 2011.
[21]
B. D. Craig and D. B. Anderson, Handbook of Corrosion Data, ASM International, 1995.
[22]
H. H. Uhlig, The Corrosion Handbook, John Wiley, 1948.
[23]
N. Hara, Y. Kobayashi, D. Kagaya, and N. Akao, “Formation and breakdown of surface films on magnesium and its alloys in aqueous solutions,” Corrosion Science, vol. 49, no. 1, pp. 166–175, 2007.
[24]
G. Song, A. Atrens, and M. Dargusch, “Influence of microstructure on the corrosion of diecast AZ91D,” Corrosion Science, vol. 41, no. 2, pp. 249–273, 1998.
[25]
Z. M. Zhang, H. Y. Xu, and B. C. Li, “Corrosion properties of plastically deformed AZ80 magnesium alloy,” Transactions of Nonferrous Metals Society of China, vol. 20, no. 2, pp. s697–s702, 2010.
[26]
Y. Song, D. Shan, R. Chen, and E. H. Han, “Effect of second phases on the corrosion behaviour of wrought Mg-Zn-Y-Zr alloy,” Corrosion Science, vol. 52, no. 5, pp. 1830–1837, 2010.
[27]
Corrosion Tests and Standards: Application and Interpretation, ASTM international, 2005.
