Long-term atmospheric corrosion behaviour of aluminium alloys 2024 and 7075 in urban, coastal and industrial environments
Introduction
Because of their high strength and low density, aluminium alloys 2024 and 7075 are extensively used as structural materials in the aircraft and aerospace industries [1]. In those fields 2024 and 7075 are usually exposed in outdoor atmosphere without protection and they can be affected by different forms of atmospheric corrosion like pitting corrosion, intergranular corrosion and even exfoliation corrosion.
In the laboratory, large numbers of atmospheric corrosion tests of 2024 and 7075 have been carried out by various accelerated tests or electrochemical methods due to the limited time [2], [3], [4], [5], [6], [7], [8]. The laboratory tests are particularly useful for the study of the corrosion mechanisms and obtaining the influence of specific pollutants and ions on those aluminium alloys, but have limited value for predicting actual service performance and sometimes tend to magnify differences among those alloys that are negligible under atmospheric conditions [9]. Thus extensive long-term evaluations of the effects of exposure in different environments need to be conducted.
Corrosivity of the atmosphere to 2024 and 7075 varies greatly from one geographic location to another depending on the weather factors [10]. Some countries have performed the weather exposure tests of those aluminium alloys in different atmospheres. Both bare 2024-T3 and 2024-T6 fall into the category of materials susceptible to severe loss of ductility on prolonged atmospheric exposure in many earlier studies [11], [12], [13]. However, a study of the effect of a sacrificial cladding layer indicated that loss of ductility can be completely eliminated when 1230 alloy was the cladding on 2024-T3 alloy exposed in industrial and marine atmospheres for 10 years [14]. Corrosion of bare 7075-T6 sheet exposed for 4 years was mild in marine and inland atmospheres, with shallow microscopic pitting being the dominant form [15]. But bare 7075-T3 exposed to Geermu salt lake atmosphere in China showed severe deterioration with the corrosion rate reaching up to 7.73 μm/a after one year exposure, which resulted from a higher concentration of salt content and chloride ions in this atmosphere [16]. Nevertheless, the corrosion rate of 7075 coated with 7072 was only 0.78 μm/a after 10 years exposure in the Durban area in South Africa [17].
This paper presents weight loss and loss in mechanical properties obtained for alclad 2024 and 7075, extruded 2024 and 7075 in urban, coastal and industrial atmospheres in China over 20 years. We also investigated cross-sectional corrosion morphology and pitting attack on alclad using metaloscope, SEM and EDS. Finally, we analysed chemical compositions and morphology of corrosion products on alclad by XRD, EDS and SEM.
Section snippets
Test materials and exposure methods
Extruded 2024 and 7075, alclad 2024 and 7075 were used for the field exposure studies. Chemical compositions and temper of materials exposed are given in Table 1. Extruded 2024 and 7075 were groove- and angle-shaped, respectively, and the size of specimens was 200 mm length × 3 mm thickness. Alclad 2024 and 7075 were panels, of dimensions 200 × 100 × 2 mm. The claddings of alclad 2024 and 7075 were 1350 and 7072, respectively. The thicknesses of those cladding layers were all about 80 μm.
Prior to the
Weight loss
Several studies [18], [19], [20], [21] have shown that long-term atmospheric corrosion data for outdoor exposure in a broad range of environments can be expressed as:where C is the weight loss of metal due to corrosion (g/m2), t is the exposure time in years, and A and n are constants. A represents the corrosion weight loss of the first year and n represents the propensity of corrosion.
Conclusion
(1) A small amount of Mg presented in inner cladding on alclad 2024 and 7075 enhances corrosion resistance of the inner cladding so that the cladding had not been penetrated by pitting after 20 years exposure in urban, coastal and industrial atmospheres. Meanwhile, alclad 2024 and 7075 exposed for 20 years still retained their mechanical properties very well in the three atmospheres.
(2) Exfoliation corrosion occurred on extruded 2024 and 7075 in coastal and industrial atmospheres, and mechanical
Acknowledgements
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (50499331_3) and help rendered by the staff of the test stations in this work.
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