Corrosion Behavior Of Aluminium Alloy Aa 2219 − T 87 Welded Plates in Sea Water

The corrosion behavior of aluminium alloy AA2219−T87 welded plates in seawater was investigated using potentiodynamic polarization and EIS techniques. The potentiodynamic polarization curves reveal that heat affected zone (HAZ) is more prone to corrosion than weld zone (WZ) and base metal (BM). This is further confirmed by EIS which showed a decrease of Rct. The microstructures of AA2219− T87 were made by optical microscopy and SEM. The higher environmental susceptibility of HAZ as compared to BM is caused by the dissolution and segregation of CuAl2 intermetallic particles along the grain boundaries. The corrosion rate of WZ is due to the presence of micro pores and copper rich areas in α-matrix.


Introduction
The most readily weldable high strength aluminium alloy 2219 (Metals Hand Book, 1990, Aluminium standards and data, 1982) in T87 temper condition finds extensive use for the structural construction of cryogenic fuel tanks to store liquid oxygen (−183°C/90K) and liquid hydrogen (−253°C/20K). Its only disadvantage is its poor corrosion resistance especially in chloride containing environment owing to higher copper content.
Over the years, a number of studies have been carried out in order to assess the effect of copper content and the distribution of second phase intermetallic particles on the corrosion behavior of aluminium alloys (Robinson et. al., 1982;Meletis et. al., 1991;Keddam et. al., 1997;Williams Stewart et. al., 2003;Paglia et. al., 2006;Birbilis et. al., 2005). The copper distribution in the microstructure affects the susceptibility to localized corrosion. Pitting corrosion is usually occurs in the Al matrix near copper containing intermetallic particles owing to galvanic interaction with Al matrix (Lunarska et. al., 1987). Intergranular corrosion (IGC) is generally believed to be associated with Cu containing grain boundary precipitates and the PFZ along grain boundaries (Hatch ASM 1984; Ramgopal et. al., 2002).
Attempts have been made to study the corrosion properties of various zones of friction stir welded (FSW), AA2024, AA7075 (Lumsden et. al., 1999;Paglia et. al., 2002;paglia et. al 2003;Lumsden et. al., 2003), AA2017−T4 (Kuznicka et. al., 2008) and the results have shown that the heat affected zone (HAZ) of these aluminium alloys were more susceptible to corrosion than the base metal. Corrosion behavior of Al-Si/Sic composite in sea water was analysed and reported (Gnecco et. al,. 1999). Frankel and Xia (Frankel et. al., 1999) investigated the pitting corrosion and stress corrosion cracking behavior of FSW AA5454 alloy and compared them with those of base alloy and in gas tungsten arc welded (GTAW) samples. Rao et al (Koteswara Rao et. al., 2005) have shown that the copper segregation to grain boundaries has significant effect on pitting corrosion behavior of AA2219 electron beam welds. Liquation in FSW and gas metal arc welded (GMAW) AA2219 has been investigated by Cao et al (Cao et. al., 2005;Huang C et. al., 2004). GMAW AA2219 showed that θ (CuAl 2 ) particles acts as insitu microsensors for detecting the onset of liquation by forming distinct composite like eutectic particles upon reaching the eutectic temperature. The general corrosion resistance of the weld nugget was better than that of the parent AA2219 friction stir weldment in 3.5% NaCl solution (Balasrinivasan et. al., 2010). Corrosion behavior of aluminium alloy 2219 treated with a chromate conversion coating exposed to 3. Most of the corrosion studies investigated so far is pertaining to FSW aluminium alloys. Electrochemical corrosion and SCC behavior of TIG welded AA2219 Aluminium alloy in 3.5 Wt. pct. NaCl solution was reported by Venugopal (Venugopal et. al., 2010). Among the available welding processes, GTAW process on AC mode with 2319 filler wire is largely followed for welding because of its inherent advantages of arc cleaning (Ghosh et. al., 2007). The present work elucidates the corrosion behavior of different zones namely, weld zone (WZ) and heat affected zone (HAZ) of gas tungsten arc welded AA2219−T87 plates in sea water and compared the results with base metal (BM) in T87 temper condition. Research article www.indjst.org

Experimental
Gas tungsten arc welded (GTAW) AA2219 rolled plate of size 570mm × 500mm × 7.4mm in T−87 temper condition ( Fig.1) was used in the present study. The temper designation T8 represents solution heat treatment and cold working followed by artificial aging and 7 represents the percentage of cold work employed (Aluminium standards and data'1982). The gas tungsten arc welding process on alternating current mode with ER 2319 alloy (filler wire) is used for welding the rolled plates and the procedure is described elsewhere (Venkata Narayana et al., 2004). The chemical composition of parent metal and weld filler wire are given in Table 1.
The working electrodes for corrosion studies were cut from the welded plates in transverse direction parallel to the rolling direction and perpendicular to welding direction in such a way the test specimens consisted of base metal (BM), weld zone (WZ) and heat affected zone (HAZ). The specimens were used in flat type after flush grinding both the crown and root beads with mirror smooth finish after polished with different grades of silicon carbide sheets followed by 1 µm finish using rotating disc with non-aqueous diamond paste, degreased by acetone, washed with double distilled water and dried. Except the zone under study, the rest of the zones were suitably masked with a red lacquer and further wrapped with Teflon tape. The Keller etchant [2.5ml HNO 3 (60%) + 1.5ml HCl (37%) + 1ml HF (48%) + 95 ml of double distilled water] was used to identify the HAZ and are located 3 mm away from weld zones on both sides. In all experiments 8mm×1mm was exposed in order to compare the results.
Potentiodynamic polarization tests were carried out according to ASTM standard G3-89 (ASTM G3-89,2004)) using software based Bio-analytical system [BAS−Zahner, make IM6electrochemical analyzer model using THALAS−Flink software]. The flat type working electrodes were categorized into BM, WZ and HAZ. A saturated calomel electrode coupled to a fine Luggin capillary as reference electrode and graphite electrode as counter electrode were used. The Luggin capillary was kept close to the working electrode to reduce the ohmic contribution. Sea water (pH = 8.23) collected from East Coast area, Chennai was used as electrolyte. The polarization curves were determined by stepping the potential at a scan rate of 0.5 mV/sec. from -250 mV (SCE) to +250 mV vs SCE. All the experiments analyzed in this paper were performed at room temperature (25°C) and repeated for at least two times to get reproducibility.
The electrochemical impedance spectroscopy (EIS) measurements were carried out in sea water using a potentiostat coupled to a frequency response analyzer system in the frequency range 100 k Hz. to 100 m Hz with a sinusoidal perturbation of 10 mV at OCP. The working electrode consists of either BM or WZ or HAZ and graphite electrode and saturated calomel electrode were used as counter electrode and reference electrode respectively. Potentiodynamic polarization and EIS measurements were performed after initial delay of 10 minutes for the sample to reach a steady state condition.  The microstructure of the AA2219−T87 welded plates were characterized by using optical microscopy (Olympus GX 71 Inverted Metallurgical Microscope, Japan) and Scanning Electron Microscopy (SEM). Optical metallography was performed on samples with transverse cross section. The samples were polished with different grade of silicon carbide sheets followed by 1 µm finish using rotating disc with non-aqueous diamond paste followed by etching with Keller's etchant for about 30 to 60 seconds and used. SEM characterization was performed on samples using scanning electron microscope (HITACHI model) operating at 15kV. The values of corrosion potential (E corr ) for BM when compared to WP and HAZ exhibits a little shift towards anodic side. This shift was confirmed in the reduction of corrosion current density (i corr ) 0.156 µA cm -2 . The E corr towards anodic side and reduction in i corr may be due to higher anodic to cathodic area distribution curves that, all regions showed cathodic control reaction since they have higher cathodic Tafel slopes than anodic Tafel slopes. The predominance of cathodic Tafel slope is due to the presence of main alloying element copper and second phase intermetallic particles CuAl 2 . A little passive region can be observed from the anodic side of all regions is due to the slightly higher pH of sea water. Table 2

. Corrosion parameters obtained from Tafel plots for different zones of AA2219-T87 welded plates in sea water
The E corr for WZ and HAZ exhibits a shift towards cathodic side from OCP (−659 mV to -688 mV for WZ and -645 mV to -653 mV for HAZ). This shift indicates that corrosion of these regions follow via cathodic dissolution mechanism. This factor is further supported by i corr which is 0.555 µA cm -2 for WZ and 1.22 µA cm -2 for HAZ. During welding the temperature in the HAZ varies from 490°C to 570°C. Due to this high temperature the dissociation and segregation of hardening precipitates, CuAl 2 takes place along the grain boundaries (Koteswara Rao et. al., 2005). Further this causes redistribution and enrichment of copper in HAZ when exposed to sea water. These enriched copper acts as pure cathode and supports oxygen reduction at higher rates (Kuznicka et al., 2008), which greatly lowers alloys corrosion resistance particularly in the HAZ. Fig.3a illustrates the Nyquist plot obtained for different zones of AA2219−T87 welded plate in sea water. The various corrosion parameters obtained from Nyquist plots are given in Table 3.

EIS measurements
The diameter of low frequency capacity loop corresponding to charge transfer resistance indicates the corrosion rate. The BM showed maximum R ct (63 kΩ cm 2 ) than WZ (23 kΩ cm 2 ) and HAZ (9 kΩ cm 2 ). This higher corrosion resistance of parent metal is due to the more homogeneous distribution of CuAl 2 intermetallic particles in α-solid solution. This is in agreement with the results provided by Rao et al (Koteswara Rao et. al., 2005). The capacitance connected in parallel to the charge transfer resistance corresponds to interfacial capacitance ( (C dl ) and therefore may approximately indicate the expanded surface area of the corroding electrode. This was calculated using the following formula where, is the maximum frequency, obtained from the Nyquist plots. The decrease of the capacitance for BM (3.41x10 -3 µF cm 2 ) further confirms the increased corrosion resistance in sea water. The relatively lower R ct and increased capacitance showed the lower corrosion resistance of HAZ. This is essentially due to the acceleration of cathodic reaction. Presence of shrinkage pores and copper rich phase in WZ is responsible for low R ct . Fig.3 (a). Nyquist plot for AA2219-T87 welded plate in seawater Fig.3b shows experimental EIS results in Bode magnitude diagram for different zones of AA2219−T87 welded plate in sea water. A higher impedance of 48.3 k Ohms cm 2 was obtained for BM compare to WZ (15.7 k Ohms cm 2 ) and HAZ (8.5 k Ohms cm 2 ). This is also well agreement with the previous results from Tafel and Nyquist plot. It is known that higher impedance values are conducive to nobler electrochemical behavior. In BM, it is due to more homogenous distribution of fine CuAl 2 precipitates in α-solid solution. BM WZ HAZ Fig.3 (b). Bode Magnitude plot for AA2219-T87 welded plate in seawater   Fig.5a shows the microstructure of HAZ. The SEM micrograph (Fig.5b) of HAZ clearly shows segregation of CuAl 2 intermetallic particles along grain boundaries. Before welding the AA2219 consists of an aluminium rich matrix, α-Al and numerous θ, CuAl 2 particles in it. During welding, the temperature in HAZ varies from 490°C to 548°C (eutectic temperature). Due to this welding temperature the fine θ precipitates dissolve and segregate along grain boundaries as higher dense precipitates and remain after cooling. The increasing fraction of CuAl 2 particles at the grain boundaries increases the susceptibility to corrosion action with respect to grain matrix (Wislei et. al., 2002;Wislei et. al., 2007). The increased rate of corrosion of HAZ in seawater environment hence is due to the selective dissolution of copper rich intermetallic particles followed by redeposition of copper on the surface matrix. Fig.6a shows the weld microstructure consisting of a population of copper rich areas in α-solid solution. SEM micrograph (Fig.6b) of WZ exhibits a randomly distributed copper rich area and shrinkage pores. The presence of copper in α-matrix and micro pores is responsible for the higher corrosion rate in WZ when compared to BM. Fig.4 (a). Microstructure of base metal Fig.4 (b). SEM image of base metal

Conclusion
In the present work the general corrosion behavior of different zones of AA2219-T87 welded plates in sea water was investigated. Electrochemical results showed that HAZ is more prone to corrosion than WZ and BM.
The results of the microstructural survey performed by SEM confirmed that the increased rate of corrosion of HAZ in seawater is due to the selective dissolution of copper rich intermetallic particles followed by redeposition of copper on the surface matrix. On the other hand, the presence of copper in α-matrix and micro pores is responsible for the higher rate corrosion in WZ when compared to BM.