Influence of Silicon on Intergranular Corrosion for ... · Influence of Silicon on Intergranular Corrosion for Aluminum Alloys Yoshiyuki Oya 1, Yoichi Kojima and Nobuyoshi Hara2
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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, Japan2Graduate 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 Siconcentration and heat-treatment at 453K on the susceptibility of AlSi alloys to intergranular corrosion. It was found that the susceptibility tointergranular corrosion increased with an increase in Si concentration. It also initially increased with heat-treatment at 453K, but then decreasedwith long-term heat-treatment at 453K. The addition of Mg and Mn, which affect the precipitation of Si, promoted precipitation and reduced thesusceptibility of the AlSi alloys to intergranular corrosion. With longer heat-treatment at 453K, large Si precipitates were observed in the grainsand at the grain boundaries, which reduced the susceptibility to intergranular corrosion. Short-term heat-treatment at 453K formed a continuousSi-depleted layer along the grain boundaries, which increased the susceptibility to intergranular corrosion. It is suggested that the susceptibility tointergranular 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)
Aluminummanganese (AlMn) series aluminum alloyssuch as 3003, 3103 and 3203 are widely used for heatexchangers because of their high tensile strength andcorrosion resistance. Heat exchangers in automobile airconditioners are produced by a brazing process, and CFC-134a (CH2FCF3) is used as a refrigerant. The refrigerant maychange to carbon dioxide (CO2), which has lower globalwarming potential than the alternative fluorocarbon refriger-ant.1) If CO2 is used as the refrigerant, both the pressure andthe temperature in the heat exchanger would increase. Copper(Cu) and Si are often added to AlMn alloys in order toincrease the tensile strength. However, when the high-strength AlMn series aluminum alloys containing Cu andSi are applied to a heat exchanger with CO2 refrigerant,solute elements precipitate preferentially at the grainboundaries when the operating temperature reaches 453K.1)
This precipitation induces a concentration difference betweenthe grains and the grain boundaries, possibly leading tointergranular corrosion.
AlMn series aluminum alloys have comparatively lowsusceptibility to intergranular corrosion, although the sus-ceptibility increases as a result of heat-treatment and theaddition of alloy elements.24) Heat-treatment at morethan 673K causes Al6Mn and/or Al6(MnFe) to precipitatepreferentially on the grain boundaries, forming a Mn-depleted layer along the boundaries. Subsequent preferentialcorrosion of the Mn-depleted layer causes intergranularcorrosion. In an AlMn alloys with Cu as an alloy element,the presence of Fe as an impurity leads to enhancedsusceptibility to intergranular corrosion,2,3) while the pres-ence of Si inhibits susceptibility to intergranular corrosion.4)
The mechanism responsible for intergranular corrosionhas been investigated carefully for AlCu alloys.5) Heat-treatment, by which an Al2Cu intermetallic compoundpreferentially precipitates on grain boundaries, forms a Cu-depleted layer along the grain boundaries. This is the reasonwhy the diffusion rate of Cu on the grain boundaries is higherthan that in the grains. Because solute Cu makes the pitting
potential (EPIT) of the aluminum alloy noble, the EPIT ofthe grain boundary is lower than that of the grains. Thedifference in EPIT between the grains and grain boundariescauses intergranular corrosion, which means that the additionof Cu in aluminum alloys increases intergranular corrosion.However, the tensile strength of aluminummanganese alloyswithout Cu is unacceptably low for usage in heat exchangerswith CO2 refrigerant. Thus, the addition of other elements toincrease tensile strength is imperative.
Si is typically added to aluminum alloys because itcontributes to an increase in tensile strength due to solid-solution and precipitation strengthening. The precipitation ofthe various intermetallic compounds containing Si is affectedby heat-treatment, meaning that susceptibility of the alloy tointergranular corrosion also changes.611) Intergranular corro-sion was not observed for water-quenched AlSi8) or AlSiMg6,7,9) alloys, but it was observed for air-cooled AlSi,8) AlSiMg6,10) and AlSiMn alloys.11) Heat-treatment increasessusceptibility to intergranular corrosion for AlSiMg6,7,9)
and AlSiMn.10,11) This intergranular corrosion is causedby dissolution of either Mg2Si intermetallic compound atthe grain boundaries in AlSiMg7,9) or the Si-depletedlayer along the grain boundaries in AlSi and AlMnSialloys.6,8,10,11) This means that the cause of the intergranularcorrosion depends on the type of alloy. However, there arevery few reports providing a systematic study of the influenceof Si concentration in various alloys and the heat-treatmentconditions on the susceptibility to intergranular corrosion.In this study, we investigated how the Si concentrationand heat-treatment time at 453K after brazing affects thesusceptibility of various alloys to intergranular corrosion.
2. Experimental Procedure
2.1 Process and materialsThe chemical composition of the specimens is shown in
2.2 TEM observationThe distribution of precipitated intermetallic compounds
near the grain boundaries of the specimens heat-treated at453K was observed by transmission electron microscopy(TEM, JEOL Ltd., JEM-3100FEF, accelerating voltage:300 kV).
2.3 Evaluation of susceptibility to intergranular corro-sion
Figure 2 shows variations of corrosion depth with HTTfor Al0.4Si, 0.8Si, 1.2Si and 1.4Si alloys after anodicdissolution tests. The open and solid symbols show pittingcorrosion and intergranular corrosion, respectively. If thecurrent efficiency is constant in anodic dissolution regardlessof corrosion morphology and the volume of dissolvedaluminum is constant with a constant current density, thecorrosion depth would show degree of intergranular corro-sion susceptibility.
The corrosion depth of Al0.4Si alloy is independent ofHTT, approximately 50 µm, and the corrosion morphology ispitting corrosion. The corrosion depth and morphology ofAl0.8Si, 1.2Si and 1.4Si alloys depend on HTT.
Figure 5 shows variations of corrosion depth with HTTfor Al0.2Mg0.9Si and 0.2Mg1.3Si alloys after anodicdissolution tests. The open and solid symbols show pittingcorrosion and intergranular corrosion, respectively. Both thecorrosion depth and corrosion morphology depend on HTT.
Fig. 2 Variations of corrosion depth with HTT for Al0.4Si, 0.8Si, 1.2Siand 1.4Si alloys after anodic dissolution tests. Pitting corrosion andintergranular corrosion is denoted as PC and IGC, respectively.
diameters of approximately 0.1 µm on the grain boundariesand small ones with diameters of approximately 0.01 µm inthe grains are observed. This indicates that Si precipitates and
Figure 8 shows variations of corrosion depth with HTTfor Al1.1Mn0.4Si, 1.1Mn¹0.8Si, 1.1Mn1.2Si and1.1Mn1.4Si alloys after anodic dissolution tests. The openand solid symbols show pitting corrosion and intergranular
Fig. 5 Variations of corrosion depth with HTT for Al0.2Mg0.9Si and0.2Mg1.3Si alloys after anodic dissolution tests. Pitting corrosion andintergranular corrosion is denoted as PC and IGC, respectively.
Influence of Silicon on Intergranular Corrosion for Aluminum Alloys 1203
corrosion, respectively. The corrosion depth and morphologyof Al1.1Mn0.4Si alloy are independent of HTT, whereasthe corrosion depth and morphology of Al1.1Mn0.8Si,1.1Mn1.2Si and 1.1Mn1.4Si alloys depend on HTT.
Figure 11 shows variations of corrosion depth withHTT for Al1.1Mn0.2Mg0.6Si, 1.1Mn0.2Mg0.8Si,1.1Mn0.2Mg1.2Si and 1.1Mn0.2Mg1.4Si alloys afteranodic dissolution tests. The open and solid symbols showpitting corrosion and intergranular corrosion, respectively.For Al1.1Mn0.2Mg0.6Si alloy, the corrosion depth isindependent of HTT and the morphology is pitting corrosionin spite of HTT.
The corrosion depth and morphology of Al1.1Mn0.2Mg0.8Si, 1.1Mn0.2Mg1.2Si and 1.1Mn0.2Mg1.4Si alloys depend on HTT.
Fig. 8 Variations of corrosion depth with HTT for Al1.1Mn0.4Si,1.1Mn0.8Si, 1.1Mn1.2Si and 1.1Mn1.4Si alloys after anodicdissolution tests. Pitting corrosion and intergranular corrosion is denotedas PC and IGC, respectively.
Fig. 11 Variations of corrosion depth with HTT for Al1.1Mn0.2Mg0.6Si, 1.1Mn0.2Mg0.8Si, 1.1Mn0.2Mg1.2Si and 1.1Mn0.2Mg1.4Si alloys after anodic dissolution tests. Pitting corrosion andintergranular corrosion is denoted as PC and IGC, respectively.
Y. Oya, Y. Kojima and N. Hara1206
corrosion is preferential corrosion on the grain boundariescaused by the difference of EPIT between the grains and theCu-depleted layer along the grain boundaries.5) Thus, it issuggested that the intergranular corrosion in AlSi alloys isgenerated by the Si-depleted layer along the grain boundariesbecause diffusion coefficient of Si on the grain boundaries ishigher than in the grains. It is expected that the depth ofintergranular corrosion will increase with increasing Siconcentration because of the formation of a continuous Si-depleted layer caused by increase driving force of both Siprecipitation and Si diffusion with increasing Si concentrationand an increase in the difference of EPIT between the grainsand the Si-depleted layers along the grain boundaries leadingto preferential dissolution of the grain boundaries. Thistendency is found in Fig. 2.
As shown in Figs. 6 and 9, regardless of the alloyelements, precipitation and growth of Si precipitates occur bythe heat-treatment at 453K. It is thought that the generationand disappearance of the intergranular corrosion are causedby the Si-depleted layer along the grain boundaries for Al0.2mass% MgSi alloys, as mentioned in section 3.2, Al1.1mass% MnSi alloys in 3.3, and Al1.1mass% Mn0.2mass% MgSi alloys in 3.4 in the same manner as withthe AlSi alloys. Figure 13 shows influences of Si content onHTT leading to the maximum corrosion depth (A) and thetransition from intergranular corrosion to pitting corrosion(B). These characteristic HTT values depend little on Siconcentration when the corrosion depth is the deepest or thecorrosion morphology changes from intergranular corrosionto pitting corrosion. These HTT values are the longest for AlSi alloys and are shortened by the use of other alloy elements.This is why the addition of Mn and Mg induces precipitationof some kinds of intermetallic compounds as shown Figs. 6and 9. These intermetallic compounds promote precipitationof Si because these intermetallic compounds becomenucleation sites.
Fig. 13 Influences of Si content on HTT leading to the maximum corrosion depth (A) and the transition from intergranular corrosion topitting corrosion (B).
Influence of Silicon on Intergranular Corrosion for Aluminum Alloys 1207
5. Conclusion
We investigated that susceptibility to intergranular corro-sion of AlSi, Al0.2mass% MgSi, Al1.1mass% MnSiand Al1.1mass% Mn0.2mass% MgSi alloys heat-treatedat 453K after the heat-treatment simulating the brazingprocess. In terms of increasing and decreasing the suscepti-bility of these alloys to intergranular corrosion the followingconclusions can be drawn:(1) The addition of Si increased susceptibility to intergra-
nular corrosion for each alloy series. Susceptibility tointergranular corrosion was observed for the as-brazedspecimens containing more than 1.2mass% Si.
(2) Susceptibility to intergranular corrosion initially in-creased with an increase in the heat-treatment time at453K and then decreased.
(3) Short-term heat-treatment at 453K caused precipitationand growth of Si precipitates on grain boundaries, andthen, a continuous Si-depleted layer was formed alongthe grain boundaries. Si also precipitated in grainswith long-term heat-treatment at 453K and the soluteSi concentration in the grains decreased to that on thegrain boundaries. This is the reason why the suscepti-bility to intergranular corrosion was time-dependent.
(4) The addition of Mg and Mn increased susceptibilityto intergranular corrosion because Mg2Si intermetalliccompounds and AlMn series intermetallic compoundspromote precipitation of Si.
(5) The addition of Mg and Mn decreased the susceptibilityto intergranular corrosion, in the same manner as theincrease in susceptibility.
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