In a method for joining a steel tube with an aluminum rib, a zinc-aluminum
alloy layer, having an aluminum content of 0.5% by wt. to 20% by wt., is
applied to the surface of the steel tube or the aluminum rib, and then a
fluxing agent in the form of cesium-aluminum tetrafluoride is applied
between the steel tube and the aluminum rib at room temperature before or
during the mechanical contact of the aluminum rib with the steel tube. The
steel tube provided with the aluminum rib is then heated to a soldering
temperature of between 370.degree. C. and 470.degree. C. and subjected to
room temperature to cool down.
