Corrosion performance of Airware® aluminum solutions compared to legacy alloys for aerospace applications

Future generations of airliners or space launchers will require further weight reduction, improved durability and reductions of the costs and manufacturing cycles of aerostructures. These demands have generated for several years a major R&D effort on advanced aluminum-based materials. Constellium, a leading supplier of aluminum solutions, has responded to this challenge with the development of low density and high mechanical properties Airware^® Al-Cu-Li alloys.

The durability of the many structural configurations facing various environmental solicitations is one of the criteria conditioning the inspection intervals of aircraft structures. Constellium's sustained R&D efforts aim to offer a portfolio of alloys that meet customers’ needs including the compatibility with new chromate free surface treatments, the corrosion resistance in confined area and the stress corrosion cracking (SCC) performance. In this work, the higher corrosion capability of AA2198-T8 and AA2050-T8 Airware^® is demonstrated in comparison with legacy alloys like AA2024-T3 or AA2219-T8 for each of these key corrosion properties.

Several combined testing methods were used to properly characterize the corrosion performance and overcome potential limitations of existing standardized tests: (1) The compatibility with Trivalent Chromium Process (TCP) conversion coating was highlighted with more robust and quantitative alternatives to Neutral Salt Spray (ASTM B117) like the use of a NaCl containing reactive gel as well as the electrochemical impedance spectroscopy technique to investigate pitting corrosion susceptibility. (2) The risk of corrosion in occluded zones was investigated using an artificial clear glass slide/metal crevice coupled with a 3D profilometer analysis post-mortem. (3) SCC performance was assessed on the full “stress level”-“test duration” window. In addition, residual mechanical properties after alternate immersion exposure (ASTM G44) were measured by tensile test to decouple the respective role of SCC mechanisms and pitting corrosion.