The health risks associated with the use of chromium (Cr6+) in the surface treatment of aluminium (i.e. anodizing) for the structure of aircrafts makes finding alternatives a critical issue. Up until now, it has proven difficult to find alternatives that are as effective, especially for application in heavily corrosive conditions. Researchers have now provided fresh insights into the interplay between the anodizing conditions and the formation and durability of bond strength, showing that morphology and chemistry of the surface oxide both play an essential role. The results have been published in Nature’s new partner journal Materials Degradation.
Protection against corrosion
For over 60 years, chromic acid anodizing (CAA) has been the dominant electrochemical process used to create a thin oxide layer upon aluminum alloys as a base layer for subsequent coating or adhesive application. These thin oxide layers are critical to the corrosion protection and durability of airplane structures. However, the associated environmental and health risks concerning the use of chromium in its hexavalent state (Cr6+) have led to new European regulations (REACH, EC n°1907/2006) limiting and prohibiting its use in the near future (September 21st has been set as ‘sunset day’).
Interplay between morphology and chemistry
Finding alternatives that have the same adhesive properties and offer the same kind of protection against corrosion have proven hard to find up until now. The team of researchers reveal the fundamental adhesion and degradation mechanism at the interface between the oxide on aluminum and the overlaying organic resin. Aided by both imaging and spectroscopic characterization, this study provides fresh insights into the interplay between the anodizing conditions and the formation and durability of bond strength, showing that morphology and chemistry of the surface oxide are the two factors that should be considered in the selection of Cr6+-free surface treatments.
Cr6+ is a toxic and carcinogenic substance. Prolonged exposure to this substance is a known cause for lung cancer; an issue that frequently came up in the Dutch news after several former military and NS employees that used to work with Cr6+-containing chemistry were reported and claimed to suffer health issues related to this. One of the most famous cases worldwide was made public after Erin Brockovich won a lawsuit against PG&E oil and gas company for contaminating ground water with the substance, a story that was made into a Hollywood movie in 2000 starring Julia Roberts. Although nowadays employees are protected with masks, gloves and further protective work gear, application and wastewater storage and treatments still present health and environmental risks.
Interface Strength and Degradation of Adhesively Bonded Porous Aluminum Oxides, npj Materials Degradation, DOI: 10.1038/s41529-017-0007-0. Available at: rdcu.be/u9S8 (open access)
Authors: Shoshan Abrahami, Arjan Mol (TU Delft), Herman Terryn (Vrije Universiteit Brussel/TU Delft), John de Kok (Fokker Aerostructures), Visweswara Gudla en Rajan Ambat (Technical University of Denmark)
Contact Arjan Mol, Associate professor Corrosion Technology and Electrochemistry, TU Delft J.M.C.Mol@tudelft.nl