Reflections on 50 years with Aluminium: Corrosion Control, Novel Applications and Challenges of Recycling Post-Consumer Scrap

Prof. Geoffrey Scamans:  Abstract for the 2026 Henry Clifton Sorby Lecture

Reflections on 50 years with Aluminium:  Corrosion Control, Novel Applications and Challenges of Recycling Post-Consumer Scrap

Electron microscopy has been critical in developing an improved understanding of the corrosion and oxidation of aluminium alloys.  This technology has supported their increasingly widespread use in multiple applications, especially in transport, construction, and packaging industries.

In the 1970s, high voltage electron microscopes, with their greater penetrating power and use of heating stages and environmental cells, enabled the role of hydrogen embrittlement to be understood in the stress corrosion cracking of weldable AlZnMg and AlMg alloys and aerospace grades of AlZnMgCu alloys.  This was achieved through direct observation of hydrogen transport and bubble formation on grain boundaries, and from the recovery of embrittlement under vacuum. The role of minor alloy additions of elements such as beryllium and silver in controlling oxidation of AlMg alloys was also directly observed using combinations of in situ and ex-situ experimentation, combined with stereomicroscopy.

Advances in scanning electron microscopy in the 1980s led to observation of surface film blistering as a key part of the reaction of aluminium with water vapour.  This resulted in hydrogen penetration at grain boundaries.  Precise fracture surface matching demonstrated that stress corrosion cracking was a result of repeated cycles of a corrosion reaction and hydrogen embrittlement.  These processes could be controlled through development of more resistant grain boundaries, which in turn could be attained through regulated overaging practices. 

New business development initiatives in the 1990s changed the perspective of aluminium oxidation corrosion from a negative attribute by introducing a positive intent to exploit these processes as surface engineering of aluminium. This led to wide-ranging ideas for new application opportunities, many based on anodising to produce films of controlled porosity that could be stripped to make membranes or substrates for growth of cells for a range of biotechnical, filtration and sensing applications. In addition, high speed anodising processes were developed as improved pre-treatments, which could match the performance of chromium-based systems for adhesive bonding and coil coating applications. This required extensive use of ultramicrotomy to examine transverse sections through oxidised surfaces and the underlying aluminium alloy substrate.

Small reaction cells used for controlled electrochemical reactions, which could be stopped at critical points, permitted rapid sample transfer for scanning electron microscopy.  This led to advances in printed circuit lithography, and an understanding of how aluminium corrosion could be controlled by addition of activating elements such as gallium, indium, and tin.  This new technology found use in aluminium/air batteries for reserve-power applications.

The next developments centred on the need to control unsightly cosmetic, or ‘filiform corrosion’⁽¹⁾ of coated aluminium sheet in architectural and automotive applications.   A detailed understanding was obtained of the role high shear processing (rolling, grinding or machining) played in creating disturbed electrochemically-active layers on aluminium sheet surfaces by preferential precipitation of either dispersoids or grain boundary precipitates, and how this could be controlled by effective cleaning processes.    This opened the way for simple adhesion -promoting pre-treatments. The progressive combination of ion beam etching and electrochemical measurement with scanning electron microscopy was critical in understanding the reactivity of these deformed layers.

In recent years, developments have been mainly in the field of aluminium alloy formulation from end-of-life scrap sources:  learning to tolerate impurities, and to control cast microstructures using combinations of processing and alloy composition modification, and the exploitation of thermomechanical processing treatments to develop improved pairing of strength and ductility, especially for AlMgSiCu alloys for automotive applications. The more recent developments in high resolution transmission electron microscopy and atom probe tomography have led to a greatly-improved understanding of the nucleation and growth of cast microstructures, and of the precipitation hardening process.

The above paragraphs provide a brief summary of aluminium and aluminium alloy development over the past half-century to reflect on.  A number of these technical issues and advances will be reviewed in my lecture. 

(1). Filiform corrosion:  Localized thread-like corrosion that occurs under surface coatings and anodic               films.