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Corrosion tests in nitric acid were made using a freshly vacuum-distilled (18°C, 55 mbar) nitric acid diluted to concentrations of 10 to 99.8%, with varying nitrogen tetraoxide (N2O4) contents. The duration of testing in the various nitric acid concentrations was from 6 days up to 1 year. The corrosion rates were determined by atomic absorption (AAS), using the percentage of aluminum (and alloying elements) dissolved in the nitric acid. Several wrought aluminum materials—such as A199.99 (3 ppm copper), A199.5 of three different strength levels, Al-Mg1, Al-Mg2.5, Al-Mg-Si0.5, Al-Mg-Si1, Al-Mn, Al-Mn1-Mg0.5, Al-Mg2-Mn0.8, Al-Mg2.7-Mn, G-A199.99, G-A199.8, G-A199.5, G-Al-Mg2.7-Mn, and G-Al-Mn—were used in the investigation. The alloys Al-Mg1, Al-Mg-Si1, Al-Mg2-Mn0.8, Al-Mg-Si0.5, and Al-Mn1-Mg0.2 showed corrosion rates comparable to those of the aluminum grades. Cast aluminum samples were taken from sections of continuously cast bar of the following materials: G-A199.5, G-A199.8, G-A199.99, G-Al-Mn, G-Al-Mg2.7-Mn, and G-Al-Mg2-Mn0.8. The corrosion rates show the advantage of the purer grades with respect to the weight loss. A marked influence of heat treatment, however, can be observed with samples of aluminum alloys produced by continuous casting. The investigation has clearly demonstrated that pure and high-purity grades of aluminum, as well as several aluminum alloys, are well suited for handling highly concentrated nitric acid.
aluminum, aluminum alloys, corrosion, concentrated nitric acid
Head, Bayer AG, Leverkusen,
Mechanical engineer, Mobay Corp., Pittsburgh, PA