An aluminum alloy is lightweight and has high thermal conductivity, and also achieves high corrosion resistance when properly treated. Thus, aluminum alloys are used for heat exchangers for automotive and other applications, such as radiators, condensers, evaporators, heaters, intercoolers, and oil coolers. Conventional tube materials for automotive heat exchangers include, for example, a two-layer clad material and a three-layer clad material, where the two-layer clad material has a core material made of an Al—Mn based alloy, such as 3003 alloy, and a brazing filler metal made of an Al—Si based alloy or a sacrificial anode material made of an Al—Zn based alloy that is clad on one surface of the core material, while the three-layer clad material additionally has a brazing filler metal made of an Al—Si alloy that is clad on the other surface of the core material. For a heat exchanger, in general, such clad material and a corrugated fin material are combined and brazed at a temperature as high as about 600° C. to be joined together.
Oil coolers, for example, usually employ a water cooling system in which heat is exchanged between engine oil and cooling water to cool the engine oil. Nowadays some intercoolers also employ such water cooling system. As the cooling water, an LLC containing an anti-corrosive agent as an additive should be used, but tap water or well water may sometimes be used in developing countries, for example. The tap water or well water can contain chloride ions, and thus may destroy a film of aluminum oxide to cause pitting corrosion, resulting in corrosion perforation in the passage for cooling water.
A typical countermeasure against this problem is cladding a sacrificial anode material made of an Al—Zn based alloy to give the sacrificial protection effect, by which the corrosion develops laterally to prevent the corrosion perforation caused by pitting corrosion. However, since tap water or well water generally contains chloride ions at a concentration as low as about 1,200 ppm or less, pitting corrosion is more likely to occur when the cooling water is at a low temperature around room temperature. At a high temperature, pitting corrosion is less likely to occur because a thick film of aluminum oxide is formed; however, a corrosion product, such as aluminum hydroxide, is created between the inside and outside of the pitting corrosion that occurs at room temperature, the corrosion product occludes the inside of the pitting corrosion, and thus local alkalization is expedited inside the pitting corrosion. When the inside of the occluded pitting corrosion becomes strongly alkaline, the corrosion turns into uniform corrosion with a higher corrosion speed, which means the sacrificial anode effect is not fully exerted, resulting in creation of corrosion perforation extending not laterally but deeply.
One method for forming a passage for cooling water in a heat exchanger includes stacking plates 1, which are made by forming a clad material into a passage for cooling water, via corrugated fins 2 as illustrated in FIG. 1. This method is advantageous because a heat exchanger can be changed in size merely by changing the number of stacks, thus providing higher design flexibility. However, to join plates together, the plate material itself is needed to supply a brazing filler metal during brazing.
In light of the foregoing, in order to apply the stacked-plate type to a water-cooling heat exchanger, it is necessary to clad layers that have a plurality of functions including: supplying a brazing filler metal during brazing to the inner side of the passage made of a material used for the passage forming component; having a sacrificial protection feature against pitting corrosion; and preventing the corrosion that may be caused by local strong alkalization.
Techniques for supplying a brazing filler metal during brazing and giving a sacrificial protection feature against pitting corrosion are described in Patent Literatures 1 and 2. These patent literatures describe methods suggested for achieving an excellent sacrificial protection feature by containing Zn and low-concentration Si in a clad layer to form a liquid-phase brazing filler metal during brazing to allow for the joining, while leaving part of the clad layer in the solid phase. In the techniques described therein, the molten clad layer creates through brazing a solidified structure in two phases: primary and eutectic. These patent literatures address the problem of preferential corrosion of the eutectic part due to a less noble potential of the eutectic phase than the primary phase, causing early separation of the primary part supposed to act as a sacrificial anode material, resulting in impaired corrosion resistance. Solutions to this problem are also disclosed in the literatures. However, these patent literatures neither recognize the problem of corrosion caused by local strong alkalization nor describe any method for preventing such corrosion.