Thick, hard-anodised AAO is vital for wear-resistant aluminium extrusions, says Dr Judy Runge

Dr Judy Runge when approached for her key insights into the importance of anodisation for aluminium extrusions as part of AL Circle’s exclusive campaign on “Aluminium Extrusion Growth Markets”, she extensively spoke about the advantages that anodisation offers to aluminium extruded products, how it can be developed further, and what sustainability measures it should take.

Dr Judy Runge is a metallurgical engineer and surface scientist specialising in the surface treatment and interfacial science of aluminium and its alloys. Her career spans more than 40 years, beginning with her work in electronic materials and processes at Northrop Corporation, and culminating with the last 11 1/2 years at Apple, from where she retired in 2023.

Dr. Runge is an independent consultant at her company, CompCote International, Inc., providing failure analysis and metallurgical support to the aluminium finishing industry. She is an internationally recognised expert in the science and technology of anodic oxidation an https://www.alcircle.com/tag/aluminium-extrusion-growth-marketsd has given presentations and classes on the subject all over the world. Judy has received numerous awards for her contributions to anodising science, and was honored to receive the 2022 Scientific Achievement Award from the National Association of Surface Finishers (NASF).

Dr. Runge is on the faculty at SECAT in Lexington KY, where she shares the teaching of basic classes for the Aluminium Metallurgy Certification Program. In February of 2025, she was honored to be placed by the U.S. Department of States’ Bureau of Educational and Cultural Affairs on the Fulbright Specialist Roster to assist universities worldwide in developing Engineering Surface Science Curricula. She is Co-Chair of the AAC Education Committee and also serves on the AAC Academic Committee. Her book, “The Metallurgy of Anodizing Aluminium”, was published in April of 2018 by Springer Nature, the second edition will be published in 2026.

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AL Circle: Anodising is an electrochemical process that enhances aluminium's natural oxide surface layer by forming an even more durable anodic oxide surface. What key advantages does anodising offer to aluminium extruded products, and how does it enhance the functionality, aesthetics and lifecycle value of end-user applications?

Judy Runge: The Anodic Aluminium Oxide (AAO) is very different from the passive layer. If one considers aluminium’s “natural oxide surface layer”, aka the characteristic passive layer – the name alone identifies this type of oxide formation as the result of an equilibrium or “passive” process, which yields an oxide layer that is nanometers thick.  Anodising is far more than an “enhancement” and yields an oxide very different from the passive layer.

Anodising is a manufacturing process that has three distinct parts:  pretreatment; anodic oxidation; and post-treatment that yields Anodic Aluminium Oxide (AAO).  For the most part, with the exception of electropolishing (pretreatment) and electro-colouring (post-treatment), pretreatment and post-treatment are equilibrium processes, driven by the thermodynamics of the surface to oxidation products with the aluminium (in pretreatment) and with the anodic aluminium oxide during post-treatment.  This distinction is important because the electrochemical oxidation of aluminium is a non-equilibrium process that produces an oxide with a unique structure of discrete nano-scale columns that sustain ion exchange in order to grow well past the thickness of a passive layer.  

It is important to view the Anodic Oxide as a stand-alone product of the Anodising process, a true manufacturing process.  When thought of in these terms, it is easier to think of the Anodic Oxide as a manufactured integral aluminium oxide layer whose structure can be tuned in manufacturing to meet various design applications.  Anodic Oxidation can be engineered to yield highly ordered, robust oxides up to several microns thick.  In contrast, there are applications which utilise uniformly thin and continuous anodic oxides that exceed the limited functionality of the passive layer.  Consequently, Anodic Aluminium Oxide, as a design choice on a variety of substrates, both cast and wrought, has a broad array of engineering and scientific applications in many industries.  

Specific to extrusions, the primary functions that the anodic oxide (AAO) performs are:  Decorative Corrosion Protection; Corrosion Protection; Wear Resistance; and Fatigue Resistance.  The distinction is made between the first two functions because not all oxides are used in aesthetic applications that require colour or texture.  Sealed AAO, on its own, can be an effective barrier to various application environments, found in Automotive, Aerospace, Aircraft, Architectural and many other industries that use aluminium extrusions.  Thin AAO, less than 1 – 3 microns, is often used as a base for paint or other coatings, and for the adhesion of labels or other coatings.  AAO is an effective barrier to filiform corrosion, which can take place at the interface of a non-integral coating, such as paint, causing blistering and peeling of the surface layer.  Thick, hard-anodised AAO is a staple finish for aluminium extrusions in various wear applications:  under-the-hood automotive applications such as extruded 6082 pistons; machine components and dies for the food industry rely on aluminium alloys for their light weight and strength and rely on the AAO to provide wear protection and corrosion resistance.  The most obvious applications for decorative AAO are in Architecture and Consumer Electronics for which the dyed or colored AAO highlights the beauty of the metal from which it has grown.  Whether bright and reflective or textured and matte, the anodic oxide finish is a beautiful, reliable finish that can be dyed or colored to meet the design intention.

AL Circle: Cast aluminium alloys, particularly those with higher silicon content, often present challenges during anodising compared with extrusion alloys such as 6061 and 6063. How significant is the cost and technical gap between anodising cast and extruded aluminium? What solutions can industries consider when seeking high-performance finishes for cast aluminium components?

Judy Runge: Cast products are usually near-net shape as produced, requiring little finish machining.  This makes castings very inexpensive when considering the amount of mechanical finishing some extruded components require to achieve the detail of a specific design shape.  This often makes castings much less expensive than extrusions.  The question specifically asks to compare anodising high Si cast components to anodising extrusions from alloys 6061 and 6063, without commenting about application.  The anode microstructure governs AAO appearance.  Cast alloys, because they have more complex compositions, typically yield AAO that is not as attractive as that on low-alloyed extrusions. But in applications in which the appearance of the oxide is not key to operation – such as in helicopter rotors or on aircraft landing gear- a hard-anodised finish on strong, tough aluminium die casting meets the impact, wear and fatigue resistance requirements.  In order to make recommendations for solutions, the bottom line is design application and expectations for appearance.

AL Circle: The aluminium extrusion industry has experienced supply-chain disruptions, fluctuating demand and production uncertainties in recent years. How resilient is the anodising sector to this market volatility, and what impact could prolong market fluctuations have on anodising capacity and investments?

Judy Runge: The anodising industry requires product to finish in order to be viable.  I imagine that prolonged market fluctuations may reduce capacity.  A smart anodising facility might utilise the reduction in capacity as an opportunity to modernise, upgrade and modify their anodising lines, to optimise best practices in the plant and reduce their environmental footprint.  It may also be an opportunity to expand the current anodising capability to handle different products and more complex alloys for components from different industries.  A factory prepared for the future will be ready to handle the anodising demands for current and future markets with higher quality and more efficiency.  Aluminium and its alloys are important today for applications in many industries and will be more important in the future; therefore, so will the need for quality AAO finishes.

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AL Circle: The architectural sector has traditionally been one of the largest consumers of anodised aluminium. With construction activity facing challenges in several markets, which emerging industries like automotive, renewable energy, electronics or industrial applications could become key growth drivers for anodised aluminium in the coming years?

Judy Runge: In order to have anodising in the future, there has to be aluminium to anodise, regardless of the market.  I would like to get into the heads of product designers in every industry that uses aluminium and its alloys that have the pulse on the “New Best Thing” to give you the best answer!  However, from where I sit today, looking back at traditional products that use aluminium, and the challenges facing us now and how they will look in the future, we can be sure that there will be variations of the same aluminium products, or an expansion/transition from composite and polymer materials to aluminium alloys.  Because truly, the first step in being able to implement AAO as the finish of choice is to have the components of the future, regardless of the market, manufactured from aluminium.

I definitely think there will be a reversal from using polymers in our future; they have a profound environmental impact in molecular stability alone, and the difficulty in recycling them may shift the future of polymer components that were made from aluminium back to being made from aluminium. Consider food packaging alone; we are going to see more and more aluminium food containers and wrappers.  How much of that will be anodised is an open question.  I also suspect there will be more and different electronic devices housed in aluminium alloys that will require an AAO finish.

The amount of energy required for recycling and reprocessing iron and steel also greatly exceeds the amount of energy required to reprocess aluminium. So, in these respects alone there will be a greater demand for aluminium.  For example, as electric cars continue to develop and find a bigger place in the automotive market, we are sure to see more anodised aluminium.

AL Circle: What technological advancements are shaping the future of aluminium anodising, particularly in terms of coating performance, colour consistency and process efficiency?

Judy Runge: This question, like many of the others, deals more with the question:  How much aluminium will be used/consumed for products in the future?  The fact remains:  Aluminium (alloy) microstructure governs the oxide structure and drives the appearance of the finish after anodising.  New manufacturing methods such as Additive Manufacturing and Rapidly Solidified Material bring a lot of anodising challenges with them, remember: only aluminium anodises; alloying elements and empty space don’t anodise.  Both AM and RSM materials have a high grain boundary density, which will impact the appearance of the oxide, so while the oxide might perform without a problem, the colour will not be bright, and the finish won’t be shiny.  Other methods to consolidate and anneal the microstructures of these materials would need to be implemented to grow the grains to obtain a more conventional appearance.  On the other hand, designers might need to adjust their appearance perspective and find that a satin finish is indeed a beautiful thing.

We are going to continue to see increasing amounts of post-consumer scrap, albeit sorted, in our secondary alloy process stream.  This means that we will see variations in trace elements in our alloys that we used to, by choice, minimise for a variety of processing reasons, including anodising.  Iron seems to be the most difficult to keep to a minimum, and as iron content climbs in recycled alloys, so will the formation of intermetallic particles (IMPs).  Keeping in mind the statement above, we are definitely going to see differences in oxide colour consistency, with variations in appearance across the same component.  Here, the implementation of best practices will be necessary for the metal finisher.  Making sure that all other potential sources for resistance to anodic oxide growth are minimised, which tend to exacerbate inherent colouring effects in the AAO, is key to the anodising process contributing to the variations in finish appearance after anodising.

AL Circle: According to you, how important is collaboration between extruders, alloy producers and anodisers in developing next-generation aluminium products?

Judy Runge: Collaboration and education are critical as we move toward the next-generation aluminium products. I firmly believe that if component industrial designers incorporate all parts of the manufacturing team when developing a product, challenges of all types, from how the billet is produced, to how the material is extruded and anodised, would be minimised.  The involvement and education of all partners in the manufacturing team is key for setting realistic expectations for the product appearance and performance following anodising.  

Alloy producers must recognise the impact of increasing amounts of post-consumer scrap, with different types and amounts of trace and unexpected, tramp elements on the casting process.  Extruders need to recognise the impact of variations in billet quality that they might have been able to tolerate in the past, on the extruded product.  Anodisers must exercise best practices to understand their own process well enough to know how to tune their process to meet the challenge of a different alloy, whether or not it's recycled, or manufactured by a different process.  All team members must recognise and communicate when something is different or wrong so intelligent adjustments can be made to any of the contributing processes.

One can find a forum for these discussions and education at organisations such as the Aluminum Extruders Council (AEC) and the Aluminum Anodizers Council (AAC).

AL Circle: As sustainability becomes a key priority, what energy sources are increasingly being adopted to power anodising operations globally? Beyond renewable electricity adoption, what other measures are anodisers implementing to reduce carbon emissions, improve resource efficiency and minimise environmental impact?

Judy Runge: Maintaining the anodising electrolyte to operate at a sustained Al³⁺ content is a sure way to maintain the resistance of the electrolyte.  This is a sure way to sustain the quality of the AAO; fluctuations in aluminium-ion content are a source for temperature fluctuations in the bath that impact oxide growth.  This can be done by implementing ion exchange on the anodising electrolyte.  Other metal ions, that also impact electrolyte resistance are also removed with this method.

Anodising is the metal finishing process that by far uses the most water.  Minimising water usage and recycling aqueous process baths is one of the most efficient ways to improve resource efficiency and minimise environmental impact while reducing carbon emissions associated with the manufacture of the chemistry used for cleaning, acids for etching and brightening, and the electrolyte formulations required to grow the AAO.  There are companies who manufacture “Cleaner Cleaners” – YES – a process that filters and maintains the cleaning solutions used to remove the residues of deformation processing and machining.  Acids and acid solutions can be recycled with ion exchange devices – this is brilliant – with these devices, we can look ahead to when sulfuric acid is no longer sold as a commodity, as a by-product of petroleum manufacture, and avoid rising costs.  There are so many other ways to reduce water and other necessary resources for the anodising process, and many companies who specialise in this.

But the basic commitment to all this starts in the metal finishing plant, on the anodising line.  For example, instead of running tanks until they are ineffective and dumping them, implement “Bleed and Feed” programmes, in which a portion of an ageing tank is bled off and replenished with fresh solution.  This can be done on all tanks with purchased chemistry; the waste/discharge will be reduced, as will the amount of new chemistry purchased.  Rinse tanks must also be included as a part of the process to monitor and control – continuously running water and overflow to drains needs to be reviewed, and the water recycled.

There are supplier members with AAC who are experts in these areas. I encourage anodisers who see these steps toward a “greener” anodising process as something to investigate to contact AAC at anodizing.org.

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