Corrosion Testing of Anodised Aerospace Alloys: Correlation between Electrochemical and Technological Approaches

Abstract

Anodising is extensively used to improve the corrosion resistance of aerospace alloys. The process used to be performed in chromic acid, but hexavalent chromium (Cr (VI)) has been restricted because of environmental and toxicological hazards. For safety and environmental protection, serious research efforts are ongoing both in academia and industry to develop novel, chromate-free anodic oxide films. In addition to the nontoxicity requirement for an anodising procedure, the process should also be energy-efficient, cost-effective and produce corrosion-resistant anodic oxide films. When a new anodic oxide film is proposed, specific corrosion testing must be undertaken to ensure durability. Industrially, technological evaluation methods such as immersion and salt-spray test (SST) tests are the most popular. However, these tests provide little information on the fundamental corrosion processes, and the performance evaluation is somewhat subjective. In academia, electrochemical measurements such as electrochemical impedance spectroscopy (EIS) are often preferred since they provide quantitative and mechanistic information, which are helpful during the development of technology readiness processes. Nevertheless, the results obtained by technological corrosion tests are rarely compared with those obtained from electrochemical measurements. Thus, with the ever-increasing evolution of new anodic oxide films, it is essential to understand the fundamental differences between the corrosion evaluation methods, which help in understanding the implications of selecting a particular evaluation method, improving the existing approaches or proposing new corrosion evaluation methodologies, and subsequently improving reliability and confidence in the outcome of corrosion testing. This thesis is undertaken to determine the correlation between technological and electrochemical methods on the corrosion testing of anodised aerospace alloys. Because of its extensive application in aerospace industries, AA 2024-T3 is employed for this work. Initially, the corrosion behaviour of bare alloy, unsealed anodic oxides, and anodic films sealed by different post-treatments was investigated by immersion and salt spray tests and complemented with EIS and surface characterisation. The findings reveal a strong correlation between the EIS and surface analyses for corrosion processes of anodic oxides under immersion and salt spray tests. The work also discloses fundamental differences between the immersion and salt spray tests and that those differences strictly depend on the nature of the film surface, such as pore morphology due to the difference in anodising potential and sealing post-treatments. The salt spray test is generally more aggressive than the immersion test regardless of the morphology and sealing posttreatment, albeit this difference is not significant for the bare alloy and the hydrothermal sealing posttreatment where the porous skeleton is hugely modified and blocked. The second aspect of this thesis focused on the effect of temperature on the corrosion testing of various anodised alloys during both immersion and salt spray tests. The findings from this aspect reveal that the performance and relative ranking of the films depend not only on exposure conditions but also on the temperature selection of a specific test. The work also suggests that the relative ranking of anodic oxide films could be inverted due to temperature variation. After understanding the differences between immersion and salt spray tests, and the effect of temperature, the work further explores the potential of an AC-DC-AC approach for the accelerated corrosion testing of anodic oxides. This test is based on the cyclic application of a cathodic potential step followed by a step of free corrosion and, finally, the measurement of the electrochemical impedance response. A qualitative correlation is found between the AC-DC-AC approach and long-term immersion tests at the free corrosion

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Fabio Scenini (Supervisor) & Michele Curioni (Supervisor)