Heat exchanger

Abstract

A heat exchanger includes exchange tubes formed from an aluminum extrudate and fins made of an aluminum bare material. The wall of each heat exchange tube is composed of a main body portion made of an Al alloy forming the aluminum extrudate, and a covering layer made of an Al—Si—Zn alloy and covering the main body portion. A diffusion layer containing Zn and Si diffused from the Al—Si—Zn alloy is formed in an outer surface layer portion of the main body portion. A low potential portion whose spontaneous potential is the lowest, and a high potential portion whose spontaneous potential is 60 mV or more higher than that of the low potential portion, are present within a range between an outermost surface of the wall and a deepest portion of the diffusion layer such that the low potential portion is located toward the outermost surface of the wall.

What is claimed is: 
     
         1 . A heat exchanger comprising:
 a plurality of heat exchange tubes formed from an aluminum extrudate; and   fins made of an aluminum bare material, each disposed between adjacent heat exchange tubes, and joined to the corresponding heat exchange tubes by a brazing material, wherein   each heat exchange tube has a wall composed of a main body portion made of an Al alloy forming the aluminum extrudate, and a covering layer made of an Al—Si—Zn alloy and covering an outer surface of the main body portion;   a diffusion layer in which Zn and Si contained in the Al—Si—Zn alloy forming the covering layer are diffused is formed in an outer surface layer portion of the main body portion of the wall of each heat exchange tube; and   a low potential portion whose spontaneous potential is the lowest, and a high potential portion whose spontaneous potential is 60 mV or more higher than that of the low potential portion, are present within a range between an outermost surface of the wall of each heat exchange tube and a deepest portion of the diffusion layer such that the low potential portion is located toward the outermost surface of the wall.   
     
     
         2 . The heat exchanger according to  claim 1 , wherein within the range between the outermost surface of the wall of each heat exchange tube and the deepest portion of the diffusion layer, the spontaneous potential of the wall lowers from the outermost surface of the wall toward the main body portion up to the low potential portion, and the spontaneous potential of the wall increases from the low potential portion toward the main body portion up to the high potential portion. 
     
     
         3 . The heat exchanger according to  claim 1 , wherein the brazing material for joining the heat exchange tubes and the corresponding fins is composed of Al contained in the Al alloy forming the aluminum extrudate, and Si of Si powder caused, before joining, to adhere to surfaces of the heat exchange tubes. 
     
     
         4 . The heat exchanger according to  claim 1 , wherein the Al—Si—Zn alloy serving as the covering layer of each heat exchange tube is composed of Al contained in the Al alloy forming the aluminum extrudate, Si of Si powder caused, before joining, to adhere to the surface of the heat exchange tube, and Zn of Zn powder caused, before joining, to adhere to the surface of the heat exchange tube.