Abstract: A composite parting sheet is disclosed for separating hot and cold layers in a heat exchanger core. Known parting sheets being primarily aluminum are subject to leakage due to corrosion and failure resulting from thermal fatigue stress. The composite sheet of the present invention includes a first aluminum outer layer, a second aluminum outer layer and a central metallic layer between and in contact with the first and second aluminum outer layers so that the central metallic layer comprises a continuous, uninterrupted layer between the first and second outer layers, and the central metallic layer is a metal that is more noble than aluminum, such as nickel. Corrosion penetrating either of the outer layers to produce a pin hole preferentially corrodes aluminum over the central metallic layer. Therefore, the corrosion expands sideways upon contact with the central metallic layer, and the pin hole does not penetrate the layer or the parting sheet, thereby inhibiting corrosion induced leakage.