Compatibility of Afnor 7020 AI–Zn–Mg alloy with demineralised water and nitrogen tetroxide

Laboratory immersion studies on Afnor 7020 aluminium alloy used in the fuel tanks of space launch vehicles were conducted in demineralised water and in nitrogen tetroxide. Parent material in the T6 heat treatment condition (solution treated and artificially aged) showed a weight gain of 10·2 g m⁻² after 17 days’ immersion in demineralised water, which then remained constant up to 87 days. A weight gain of 16·4 g m⁻² was observed after 62 days for the weldment in the T4 heat treatment condition (solution treated and naturally aged). A weldment in the T6 heat treatment condition showed little weight gain (2·24 g m⁻²) after 28 days, increasing to 8·3 g m⁻² after 47 days. The weight gain was found to be caused by adherent corrosion products formed by reation with dissolved oxygen presentin the water. Coupons exposed to demineralised water for 17 days and then to nitrogen tetroxide for 1 and 72 h showed weight losses of 1·15 and 12·0 g m⁻² respectively; the corrosion products formed in the demineralised water were found to be insoluble and to become loose and nonadherent in nitrogen tetroxide. No corrosion products were observed on parent material and weldments exposed to deoxygenated demineralised water for 62 days: weight gains of only 90 and 190 mg m⁻² were recorded respectively. The removal of dissolved oxygen from the demineralised water used to calibrate the tanks was found to eliminate the danger of fuel line blockages arising from obstruction by insoluble corrosion products. The nature of the corrosion products was examined using scanning electron microscopy, X -ray energy dispersive spectroscopy, and X -ray diffraction analysis techniques.