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Influence of Silicon on Intergranular Corrosion for Aluminum Alloys

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Influence of Silicon on Intergranular Corrosion for Aluminum Alloys Materials Transactions, Vol. 54, No. 7 (2013) pp. 1200 to 1208 ©2013 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE Influence of Silicon on Intergranular Corrosion for Aluminum Alloys Yoshiyuki Oya1, Yoichi Kojima1 and Nobuyoshi Hara2 1Technical Research Div., Furukawa-Sky Aluminum Corp., Fukaya 366-8511, Japan 2Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan In an effort to improve the tensile strength of aluminum­silicon (Al­Si) alloys used in heat exchangers, we investigated the influence of Si concentration and heat-treatment at 453 K on the susceptibility of Al­Si alloys to intergranular corrosion. It was found that the susceptibility to intergranular corrosion increased with an increase in Si concentration. It also initially increased with heat-treatment at 453 K, but then decreased with long-term heat-treatment at 453 K. The addition of Mg and Mn, which affect the precipitation of Si, promoted precipitation and reduced the susceptibility of the Al­Si alloys to intergranular corrosion. With longer heat-treatment at 453 K, large Si precipitates were observed in the grains and at the grain boundaries, which reduced the susceptibility to intergranular corrosion. Short-term heat-treatment at 453 K formed a continuous Si-depleted layer along the grain boundaries, which increased the susceptibility to intergranular corrosion. It is suggested that the susceptibility to intergranular corrosion was dependent on the addition of Mg and Mn. [doi:10.2320/matertrans.M2013048] (Received February 5, 2013; Accepted April 11, 2013; Published May 24, 2013) Keywords: aluminum alloy, intergranular corrosion, brazing process, heat treatment 1. Introduction potential (EPIT) of the aluminum alloy noble, the EPIT of the grain boundary is lower than that of the grains. The Aluminum­manganese (Al­Mn) series aluminum alloys difference in EPIT between the grains and grain boundaries such as 3003, 3103 and 3203 are widely used for heat causes intergranular corrosion, which means that the addition exchangers because of their high tensile strength and of Cu in aluminum alloys increases intergranular corrosion. corrosion resistance. Heat exchangers in automobile air However, the tensile strength of aluminum­manganese alloys conditioners are produced by a brazing process, and CFC- without Cu is unacceptably low for usage in heat exchangers 134a (CH2FCF3) is used as a refrigerant. The refrigerant may with CO2 refrigerant. Thus, the addition of other elements to change to carbon dioxide (CO2), which has lower global increase tensile strength is imperative. warming potential than the alternative fluorocarbon refriger- Si is typically added to aluminum alloys because it 1) ant. If CO2 is used as the refrigerant, both the pressure and contributes to an increase in tensile strength due to solid- the temperature in the heat exchanger would increase. Copper solution and precipitation strengthening. The precipitation of (Cu) and Si are often added to Al­Mn alloys in order to the various intermetallic compounds containing Si is affected increase the tensile strength. However, when the high- by heat-treatment, meaning that susceptibility of the alloy to strength Al­Mn series aluminum alloys containing Cu and intergranular corrosion also changes.6­11) Intergranular corro- 8) Si are applied to a heat exchanger with CO2 refrigerant, sion was not observed for water-quenched Al­Si or Al­Si­ solute elements precipitate preferentially at the grain Mg6,7,9) alloys, but it was observed for air-cooled Al­Si,8) Al­ boundaries when the operating temperature reaches 453 K.1) Si­Mg6,10) and Al­Si­Mn alloys.11) Heat-treatment increases This precipitation induces a concentration difference between susceptibility to intergranular corrosion for Al­Si­Mg6,7,9) the grains and the grain boundaries, possibly leading to and Al­Si­Mn.10,11) This intergranular corrosion is caused intergranular corrosion. by dissolution of either Mg2Si intermetallic compound at Al­Mn series aluminum alloys have comparatively low the grain boundaries in Al­Si­Mg7,9) or the Si-depleted susceptibility to intergranular corrosion, although the sus- layer along the grain boundaries in Al­Si and Al­Mn­Si ceptibility increases as a result of heat-treatment and the alloys.6,8,10,11) This means that the cause of the intergranular addition of alloy elements.2­4) Heat-treatment at more corrosion depends on the type of alloy. However, there are than 673 K causes Al6Mn and/or Al6(MnFe) to precipitate very few reports providing a systematic study of the influence preferentially on the grain boundaries, forming a Mn- of Si concentration in various alloys and the heat-treatment depleted layer along the boundaries. Subsequent preferential conditions on the susceptibility to intergranular corrosion. corrosion of the Mn-depleted layer causes intergranular In this study, we investigated how the Si concentration corrosion. In an Al­Mn alloys with Cu as an alloy element, and heat-treatment time at 453 K after brazing affects the the presence of Fe as an impurity leads to enhanced susceptibility of various alloys to intergranular corrosion. susceptibility to intergranular corrosion,2,3) while the pres- ence of Si inhibits susceptibility to intergranular corrosion.4) 2. Experimental Procedure The mechanism responsible for intergranular corrosion has been investigated carefully for Al­Cu alloys.5) Heat- 2.1 Process and materials treatment, by which an Al2Cu intermetallic compound The chemical composition of the specimens is shown in preferentially precipitates on grain boundaries, forms a Cu- Table 1. All specimens were cast in a rectangular parallele- depleted layer along the grain boundaries. This is the reason piped mold, homogenized at 873 K for 1.08 © 104 s, hot why the diffusion rate of Cu on the grain boundaries is higher rolled at 793 K to a 3.5 mm thickness, and then cold rolled to than that in the grains. Because solute Cu makes the pitting a 1 mm thickness. The sheets were annealed at 673 K for Influence of Silicon on Intergranular Corrosion for Aluminum Alloys 1201 Table 1 Chemical composition of specimens. and 2.59 © 104 s after anodic dissolution tests. At HTT = 0s, Composition (mass%) the corrosion morphology depends on the Si concentration. Specimen Pitting corrosion was observed for Al­0.4Si and ­0.8Si alloys Si Fe Cu Mn Mg Al and intergranular corrosion was observed for Al­1.2Si alloy. 0.4Si 0.4 0.4 0.0 0.0 0.0 Bal. At HTT = 8.64 © 104 s, pitting corrosion was observed for 0.8Si 0.8 0.4 0.0 0.0 0.0 Bal. Al­0.4Si alloy, whereas intergranular corrosion was observed 1.2Si 1.2 0.4 0.0 0.0 0.0 Bal. for Al­0.8Si and ­1.2Si alloys. The corrosion depth at 1.4Si 1.4 0.4 0.0 0.0 0.0 Bal. HTT = 8.64 © 104 s was deeper than that at HTT = 0s. ­ 0.2Mg 0.9Si 0.9 0.4 0.0 0.0 0.2 Bal. However, the corrosion morphology was independent of Si ­ 0.2Mg 1.3Si 1.3 0.4 0.0 0.0 0.2 Bal. concentration, showing pitting corrosion at HTT = 2.59 © ­ 1.1Mn 0.4Si 0.4 0.4 0.0 1.1 0.0 Bal. 106 s. ­ 1.1Mn 0.8Si 0.8 0.4 0.0 1.1 0.0 Bal. Figure 2 shows variations of corrosion depth with HTT ­ 1.1Mn 1.2Si 1.2 0.4 0.0 1.1 0.0 Bal. for Al­0.4Si, ­0.8Si, ­1.2Si and ­1.4Si alloys after anodic ­ 1.1Mn 1.4Si 1.4 0.4 0.0 1.1 0.0 Bal. dissolution tests. The open and solid symbols show pitting ­ ­ 1.1Mn 0.2Mg 0.6Si 0.6 0.4 0.0 1.1 0.2 Bal. corrosion and intergranular corrosion, respectively. If the ­ ­ 1.1Mn 0.2Mg 0.8Si 0.8 0.4 0.0 1.1 0.2 Bal. current efficiency is constant in anodic dissolution regardless ­ ­ 1.1Mn 0.2Mg 1.2Si 1.2 0.4 0.0 1.1 0.2 Bal. of corrosion morphology and the volume of dissolved ­ ­ 1.1Mn 0.2Mg 1.4Si 1.4 0.4 0.0 1.1 0.2 Bal. aluminum is constant with a constant current density, the corrosion depth would show degree of intergranular corro- sion susceptibility. 7.2 © 103 s. The annealed sheets were heat-treated at 873 K The corrosion depth of Al­0.4Si alloy is independent of for 180 s, which corresponds to a brazing process. Finally, HTT, approximately 50 µm, and the corrosion morphology is the sheets were reheated at 453 K, which is the maximum pitting corrosion. The corrosion depth and morphology of working temperature for CO2 air conditioners, for 0­ Al­0.8Si, ­1.2Si and ­1.4Si alloys depend on HTT. 7.20 © 106 s. The heat-treatment time at 453 K after the For Al­0.8Si alloy, the corrosion morphology is pitting heat-treatment simulating the brazing process is denoted as corrosion at HTT = 0 s. The corrosion depth increases at HTT in this paper. 0 ¯ HTT ¯ 8.64 © 104 s, although intergranular corrosion is observed at 7.2 © 103 ¯ HTT ¯ 6.05 © 105 s. Furthermore, 2.2 TEM observation the corrosion morphology is pitting corrosion again at The distribution of precipitated intermetallic compounds HTT = 2.59 © 106 s. near the grain boundaries of the specimens heat-treated at For Al­1.2Si and ­1.4Si alloys, intergranular corrosion is 453 K was observed by transmission electron microscopy observed at HTT = 0­6.05 © 105 s and pitting corrosion is (TEM, JEOL Ltd., JEM-3100FEF, accelerating voltage: observed at HTT = 2.59 © 106 s.
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