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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 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.

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