This study investigated the effect of post-brazing cooling rate and Si addition on the intergranular corrosion (IGC) susceptibility of brazed Al-Mn-Cu alloys by electrochemical analysis and microstructure observation. Water-quenched samples after brazing exhibited no IGC susceptibility, whereas slowly-cooled samples were prone to IGC. The results suggest that IGC is caused by precipitation during cooling. In addition, it was observed that IGC susceptibility depended on the Si content. An alloy sample with a low Si-additive content exhibited high IGC susceptibility because Mn/Cu-depleted zone was formed near the grain boundaries as a result of the preferential precipitation of Al 6 (Mn,Fe) and CuAl 2 on the grain boundaries. In contrast, moderate Si addition inhibited IGC because the decrease of the Mn content in the grain interiors due to enhanced precipitation of Al 15 (Mn,Fe) 3 Si 2 in the grain. Additionally, Cu-depleted zone also disappeared because preferential precipitation of CuAl 2 on the grain boundaries was prevented. The excess-Si alloy exhibited high IGC susceptibility because Si-depleted zone formed around the grain boundaries as a result of the preferential precipitation of coarse Si particles on the grain boundaries although the Mn/Cu-depleted zones were not formed.

To investigate effect of additional Mn in thin brazing sheet core alloy on corrosion resistance, SWAAT and electrochemical measurements have been carried out with brazed Al-Mn-1.6 mass%Si alloys. Intergranular corrosion clearly occured in the alloys which Mn content is 1 mass% or less. In the case of these alloys precipitates of Si particle were observed at grain boundary, but they were not observed in grain after brazing. As a result, the potential difference between neighborhood of the grain boundaries and grain matrix became bigger. Therefore, selective dissolution occured at neighborhood of the grain boundaries which was lower solid solution of Si. On the other hand, intergranular corrosion were not observed in the alloys which Mn content is 1.6 mass% or more. In the case of these alloys fine precipitates of AlMnSi were observed in grain after brazing. As a result, the formation of Si-SDZ solute depleted zone by Si precipitation at grain boundary seen in the former alloys were inhibited. In conclusion, addition of Mn in thin brazing sheet had improving effect of corrosion resistance after brazing.

Aluminum alloy three-layered brazing sheets, clad with sacriˆcial anode alloy on one side of them, have widely been used for the tube of radiators for automotive in which cooling water circulates. Inner pitting corrosion resistance of the tube has been excellent both in acidiˆed and in neutral corrosive aqueous solutions by the eŠect of sacriˆcial anode. On the other hand, it has been reported that the sacriˆcial anode does not work eŠectively in alkaline solution, and perforation by pitting corrosion occurs in earlier testing period of time. In this report, localized corrosion characteristics in alkaline solution were investigated by immersion corrosion tests and electrochemical measurements. Based on the experimental results, new corrosion mechanism was proposed in terms of solution environment formed on cathode surface. Pitting corrosion of the core alloy grew not with the attack by the alkaline bulk solution, but with accelerated dissolution by strong alkaline solution derived from the electrochemical cell formation between sacriˆcial anode and cathodic core alloy. Corrosion products formed in the pit were eŠective for maintaining strong alkaline environment on the core alloy surface.

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To investigate the effects of Si and Cu on strength and corrosion rate of sacrificial anode fin stock of Al-1.5%Mn-0.2%Fe-1.5%Zn alloy for automotive heat exchangers, tensile test and SWAAT have been carried out. Strength increased with Si and Cu addition. Corrosion rate did not increase with Si addition in spite of increase of a-AlMnSi compound density, because the compounds did not work as effective cathode in the alloy. On the other hand, corrosion rate increased with Cu addition. This was explained in the existence of deposited Cu on the specimen surface during SWAAT worked as strong cathode in the alloy. Cathodic polarization characteristics in 3.5% NaCl solution rather than in SWAAT solution were effective to evaluate corrosion rate of the alloys in SWAAT.