FX.A. Wahyudianto, M.N. Ilman, P.T. Iswanto and Kusmono
Keywords: Corrosion, EIS, Inhibitor, Molybdate, Chromate, Dissimilar friction stir welding
Abstract:
Aluminium alloys AA5083 and AA6061 have been used in the construction of marine ships, high speed trains, and car body structures. In this study, both aluminium alloys were joined using a friction stir welding (FSW) method. A rotating tool with a cylinder-shaped pin was used with a rotation speed of 2280 rpm. The corrosion behaviours of AA5083/AA6061 dissimilar joints in 3.5% NaCl solution containing 0.5% sodium chromate (Na2CrO4) and 0.5% sodium molybdate (Na2MoO4) were analysed using polarization resistance and electrochemical impedance spectroscopy (EIS) techniques. The results showed that the EIS spectra of the dissimilar metal weld joints were between those of the parent materials, i.e. AA5083 and AA6061. Consistent with the EIS results, it was also shown that the corrosion resistance of the weld joints was measured to be between the resistances of the parent materials. The addition of the chromate inhibitor to the 3.5% NaCl solution effectively reduced the corrosion rate of the FSW weld.
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Analysis of Corrosion Inhibition in Friction Stir Welded Dissimilar Aluminium Alloys in 3.5% NaCl Solution Using Polarization Resistance and Electrochemical Impedance Spectroscopy (EIS) FX.A. Wahyudianto*,1, M.N. Ilman2, P.T. Iswanto2 and Kusmono2 1Jurusan Kemaritiman, Politeknik Negeri Samarinda, Jl. Cipto Mangunkusumo Samarinda, Indonesia, 75131 2Departemen Teknik Mesin dan Industri, Universitas Gadjah Mada, Jl. Grafika No. 2, Yogyakarta, Indonesia, 55281 wahyudie_fx@yahoo.co.id Abstract Aluminium alloys AA5083 and AA6061 have been used in the construction of marine ships, high speed trains, and car body structures. In this study, both aluminium alloys were joined using a friction stir welding (FSW) method. A rotating tool with a cylinder-shaped pin was used with a rotation speed of 2280 rpm. The corrosion behaviours of AA5083/AA6061 dissimilar joints in 3.5% NaCl solution containing 0.5% sodium chromate (Na2CrO4) and 0.5% sodium molybdate (Na2MoO4) were analysed using polarization resistance and electrochemical impedance spectroscopy (EIS) techniques. The results showed that the EIS spectra of the dissimilar metal weld joints were between those of the parent materials, i.e. AA5083 and AA6061. Consistent with the EIS results, it was also shown that the corrosion resistance of the weld joints was measured to be between the resistances of the parent materials. The addition of the chromate inhibitor to the 3.5% NaCl solution effectively reduced the corrosion rate of the FSW weld. Keywords: Corrosion, EIS, Inhibitor, Molybdate, Chromate, Dissimilar friction stir welding. Introduction Friction stir welding (FSW) is a welding method used for joining alloys that are difficult to weld, i.e. 2xxx series aluminium alloys. FSW is a solid-state welding process that was patented by The Welding Institute (TWI), UK, in 1991. The FSW weld joints of dissimilar aluminium alloys have high efficiencies in terms of mechanical properties [1–3]. Aluminium alloys (AA5083 and AA6061) have been used in many applications. This is due to their low 1 density, moderate strength and resistance to corrosion. However, they can also be susceptible to corrosion. Aluminium alloys corrode when they are exposed to corrosive environments, such as the chloride in seawater. In most studies, [4–9]. The use of inhibitors has been shown to effectively reduce corrosion rates in seawater environments. However, there has been limited research on the effect of inhibitors on the corrosion behaviour of a dissimilar FSW weld between AA5083 and AA6061-T6. The aim of this study was to investigate the effect of chromate and molybdate on the corrosion of a dissimilar FSW weld (AA5083/AA6061) in 3.5% NaCl solution using polarization resistance and electrochemical impedance spectroscopy (EIS). Experimental Materials Two aluminium alloy sheets (AA5083 and AA6061T6) with the dimensions (length x width x thickness) 300 x 100 x 3 mm were joined using a Friction Stir Welding (FSW) technique. The alloys’ chemical compositions are given in Table 1. The electrolyte used for resistance polarization was 3.5% NaCl, with the addition of various inhibitors. In this case sodium chromate and sodium molybdate (Na2CrO4 and Na2MoO4) were used. Table 1. Chemical composition of alloys Elements (wt.%) Alloys AA5083 AA6061-T6 Mg Mn Cu Cr Si Fe Al 4.3 0.50 0.04 0.06 0.11 0.30 Balance 1.10 0.02 0.28 0.22 0.64 0.33 Balance 2 Instrumentation The FSW welds and parent material microstructures were observed using optical microscopy with a JEOL-JSM 6510 LA (Japan) to analyse the specimens after a 24-hour immersion in 3.5% NaCl with and without inhibitors. Resistance polarization tests were carried out in a three-electrode electrolytic cell using a PGS-201 ST potentiate. Electrochemical impedance spectra (EIS) measurements were carried out with the VersaSTAT4 (Ametek, USA). Procedure The FSW process was done using a milling machine with a tool rotating speed of 2280 rpm, a tool traveling speed of 30 mm/min, and a tilt angle of 3o. The cross section of the weld area was used for microstructure investigation and the corrosion test specimens were taken from the upper surface of the weld area (nugget zone). These specimens were prepared by a wet sanding method using a polishing machine with 2000 grit SiC papers of and an etchant of Keller reagent made of 5ml HNO3, 2ml HF, 3ml HCl, and 190 ml of H2O. Electrochemical polarization measurements were carried out in a three-electrode cell. The test materials used were a working electrode, a saturated calomel electrode (SCE) as the reference electrode, and platinum (Pt) as the auxiliary electrode. Resistance polarization and EIS analysis were carried out for the samples in various solutions (3.5% NaCl, 3.5% NaCl + 0.5% chromate, and 3.5% NaCl + 0.5% molybdate). The EIS measurements were taken with the VersaSTAT4 (Ametek, USA) controlled with Versa Studio software and using a frequency band ranging from between 10-2 Hz and 105 Hz and a disturbance in amplitude of 10 mV. The equivalent circuit parameters (Rs, Rc and C) were calculated using Zsimpwin software. Results and Discussion Microstructures Fig. 1. shows micrographs of the microstructure of parent materials (AA5083 and AA6061) and the FSW nugget zone. It can be seen that the grains in the FSW nugget zone were smaller and finer than the parent material grains. This due to recrystallization and deformation during high-temperature stirring processes [4]. The etchant showed that AA5083 was darker in colour than AA6061, and the nugget zone was a combination of both alloys’ colours. At the same time, the density of the FSW weld decreased and after a large deformation, the nugget zone became chemically homogenous. 3 Figure 1. Structure of alloys (a) AA5083, (b) AA6061-T6 and (c) FSW welded Resistance Polarization Figure 2 shows the defects of FSW weld specimens after the resistance polarization corrosion test with and without an inhibitor. It can be seen that more defects were found in the FSW weld specimen in 3.5% NaCl solution (Fig.2a), while the specimens in 3.5% NaCl solution with inhibitors (chromate and molybdate) showed fewer defects (Fig. 2b and 2c). The results of electrochemical measurements of the FSW welds and parent materials are shown in Table 2. The corrosion current density (Icorr) values of the FSW weld in 3.5% NaCl solution with inhibitors (0.5% Na2CrO4 and 0.5% Na2MoO4) at room temperature were between the corrosion current density values of its parent materials as well as corrosion rates. 4 Figure 2. The macrograph of specimen after corrosion test in (a) 3.5% NaCl , (b) 3.5% NaCl + 0.5% Chromate and (c) 3.5% NaCl + 0.5% Molybdate solutions. Fig. 3 shows the polarization resistance curves for the parent materials and FSW welds in various solutions. The addition of chromate and molybdate caused changes in the polarization activity of the specimens examined. The corrosion potential (Ecorr) shifted to more negative potential densities and the corrosion current (Icorr) decreased due to the 5 Figure 3. Polarization resistance curves in 3.5% NaCl solution without and with inhibitors. Figure 4 is the SEM micrograph of the FSW weld specimen surfaces after a 24-hour immersion. It can be seen that pitting corrosion occurred on the specimen’s surface after immersion for 24 h in 3.5% NaCl solution without an inhibitor (Fig. 4a). However, the specimens immersed in the solution containing inhibitors did not experience pitting corrosion (Fig. 4b and 4c). This shows that the addition of chromate and molybdate in a 3.5% NaCl solution can inhibit corrosion on FSW welds. It’s possible that a protective layer was formed on the surface of the specimen due to the addition of the inhibitors [10]. Figure 4. The SEM micrograph at 500x magnification of welded in (a) 3.5% NaCl , (b) 3.5% NaCl + 0.5% Chromate and (c) 3.5% NaCl + 0.5% Molybdate solutions. 6 Electrochemical Impedance Spectroscopy (EIS) Figure 5.a. shows Nyquist diagrams for the parent materials and FSW welds in 3.5% NaCl solution with and without inhibitors. The impedance diagram, obtained in a semicircle, was analyzed by ZsimpWin software. The capacitance loop radius of the FSW welds was between those of the two parent materials in the 3.5% NaCl solution and larger than those of the FSW welds in the 3.5% NaCl solution containing inhibitors. The yield indicated that the corrosion resistance of the FSW weld was increased using inhibitors (chromate and molybdate). The chromate inhibitor was shown the have the highest corrosion resistance. Table 2. Electrochemical parameters in 3.5% NaCl solutions Environment icorr R Rc C (µA/cm2) Corr-rate (mm/year) AA5083 0.86 1.630 7.38E+4 5.19E-6 0.009666 AA6061 2.25 1.669 3.18E+4 7.88E-6 0.025251 1.5 1.787 5.45E+4 4.71E-6 0.017292 FSW + chromate 0.03 1.730 5.71E+5 4.68E-6 0.000337 FSW + molybdate 0.16 1.759 2.26E+5 4.61E-6 0.001798 FSW The EIS equivalent circuit of a FSW weld, a Randles circuit, is shown in Fig. 5.b. The Randles circuit is one of the most common and simplest circuit models in EIS analysis, where (Rs) is solution resistance, (Rc) is a coating resistance, and (C) is interfacial capacitance. Zsimpwin software was used to fit the exact index from all the equivalent circuit parameters, as shown in Table 2. Rc has a negative correlation with the corrosion rate and polarization resistance. A high in Rc I would indicate a low corrosion rate. According to EIS results, the lowest corrosion rate was found in the solution made up of 3.5% NaCl+0.5% Chromate. This result was similar to those obtained in previous polarization resistance experiments. 7 Figure 5. EIS results (a) Nyquist diagrams for parent material and FSW weld in 3.5% NaCl without and with inhibitors and (b) Equivalent circuit. Conclusion Corrosion inhibition by chromate and molybdate in 3.5% NaCl solution were investigated in this experiment using dissimilar FSW welds (AA5084/AA6061). The microstructures of the dissimilar FSW weld showed finer grains compared to the parent materials. This difference is caused by recrystallisation and deformation during the stirring process at high temperatures. The FSW weld samples showed a resistance to corrosion between those of their parent materials. Adding an inhibitor to a 3.5% NaCl solution was found to significantly reduce the corrosion rate. The polarization resistance and EIS curve presented similar results. The addition of inhibitors significantly reduced the corrosion rate, or increased the corrosion resistance. 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