Characterization of the Surface Chemistry of Corroded and Non-Corroded Sites on Aluminum Alloys 2024-T3 and 7075-T6 Exposed at a Coastal Location

Abstract

The last few decades have seen an improvement in the characterization of bare metal surfaces undergoing atmospheric corrosion, leading to a better understanding and consideration of the environmental parameters involved. However, in the quest for more reliable accelerated laboratory corrosion test methods, knowledge of the corrosion processes and the role that the environmental parameters play in those processes is critical in replicating the field exposure environment. The corrosion process occurs within a multiphase system that is rather complex involving chemical reactions and equilibria, ionic transport phenomena, and gaseous, aqueous and solid phases.1 Various corrosion products, specific to the metallic substrates in the system, and the corrosive species present (anions, cations, acidic and basic salts, particulates, etc.) which interact with each other all vary in amounts and residence time. It is well documented that corrosion behaviors of metal substrates in accelerated laboratory testsdo not correlate with the observed behaviors in an outdoor exposure environment. 2-6 Atmospheric constituents found on metal surfaces are a function of the atmosphere itself, with sulfates, nitrates, nitrites, chlorides, carbonates, hydrogen ions, ammonium, metal ions, particulates and organic compounds commonly found in electrolyte chemistries or corrosion layers. Each of these have an effect on the corrosion processes on exposed surfaces.7 These result either directly from the deposition processes or from aqueous phase reactions of the deposited atmospheric constituents. Sulfates, chlorides and cations such as sodium, magnesium, calcium, potassium and bromine can originate either directly from the wet deposition process, from particle deposition or from aerosol reactions of the gaseous air pollutants in the aqueous phase of the adsorbed electrolyte.1 In the present work, a matrix of epoxy-mounted AA2024-T3 and AA7075-T6 alloy samples were exposed to outdoor ambient environmental conditions at the Naval Research Laboratory-Key West (NRL-KW) facility for 18 months. Samples were retrieved for analysis at 3 month intervals. The field exposed samples were subjected to SEM/EDS for image and elemental analysis using a Zeiss Environmental Model EVO-50XVP scanning electron microscope integrated with an Energy Dispersive Analysis X-Ray (EDAX) Genesis 2000 EDS system. Images of the metal surfaces and compositional elemental data for the control (non-exposed) alloy samples were compared to the field exposed metal alloy samples in both the “as received” condition and after cleaning by sonication in deionized (DI) water. Pitted and non-pitted sites on each sample were analyzed for compositional elements of the alloy as well as non-compositional (i.e. environmentally-derived) elements. Any elements remaining on the surfaces after cleaning were considered to be well adhered, non-water-soluble corrosion products. The resulting concentrations of various environmental elements and their presence within or outside of pit sites were calculated as a percentage of their composition in natural seawater at NRL-KW and entered into a database. It was found that the elements chlorine, sulfur, potassium and calcium were present at higher concentrations on the metal surfaces than in natural seawater at NRL-KW; it was also found that sulfur concentrations were higher in the pits of the AA7075-T6 metal substrates than the surrounding non-pitted surfaces (Figure 1). Data analytics was performed to determine what environmental factors, if any, could be correlated with the observed presence of elements derived from atmospheric-seawater interaction and deposited onto the metal substrates. This work is currently supporting the development of an accelerated corrosion test protocol on identical bare metal alloy specimens using a modified laboratory atmospheric exposure chamber.References Cited Oesch, M. Faller, “Environmental Effects on Materials: The Effect of the Air Pollutants SO2, NO2, NO and O3 on the Corrosion of Copper, Zinc and Aluminum. A Short Literature Survey and Results of Laboratory Exposures,” Corrosion Science 39:9 (1997): p. 1505-1530. 2. Y. Wan, E. Macha, R. Kelly, “Modification of ASTM B117 Salt Spray Corrosion Test and Its Correlation to Field Measuremnets of Silver Corrosion,” Corrosion 68:3 (2012): p. 036001-1 to 036001-10. 3. Z. Feng, G. Frankel, W. Abbott, C. Matzdorf, “Galvanic Attack of Coated Al Alloy Panels in Laboratory and Field Exposure,” Corrosion 72:3 (2016): p. 342-355. 4. A. King, B. Kannan, J. Scully, “Environmental Degradation of a Mg-Rich Primer in Selected Field and Laboratory Environments: Part 1-Without a Topcoat,” Corrosion 70:5 (2014): p. 512-535. 5. D. Liang, H.C. Allen, G.S. Frankel, Z.Y. Chen, R.G. Kelly, Y. Wu, B.E. Wyslouzil, “Effects of Sodium Chloride Particles, Ozone, UV and Relative Humidity on Atmospheric Corrosion of Silver,” J. Electrochem. Soc. 157:4 (2010): p. C146-C156. Figure 1