as by arc resistance or gas welding 28 to form an integral clamshell heat exchanger 20 at this point it is customary in the prior art construction to install the aluminized heat exchanger clamshells in a furnace housing the instant invention contemplates an additional step prior to this installation of the clamshells this added step includes passing the clamshells through an oven having an ordinary air atmosphere at elevated temperatures to thoroughly heat the clamshells to a temperature sufficiently high to melt the aluminum outer layer but lower than a temperature which would anneal the steel substrate this temperature would be between about 1065 u00b0 f and 1800 u00b0 f this may be accomplished by passing the clamshell 20 through oven 30 wherein the time within the heating atmosphere is in excess of 1 minute and less than 24 hours a practical time of 5 minutes may be selected during this step the outermost surface of the clamshell is converted from substantially iron free aluminum alloy to an iron aluminum alloy the exact nature of the metallurgical changes which take place have not been fully investigated however corrosion tests indicate that the heat exchanger 39 s resistance to corrosion is not markedly decreased the clamshell heat exchanger is then removed from the oven 30 and allowed to cool as at 32 subsequently the cooled heat exchanger is installed into a housing 34 of a.