Identification, Testing and Evaluation of REACH Compliant Hexavalent Chromium free Aluminium Pre-treatments
TWI Industrial Member Report 1210-2025
By Kranthi Maniam and Ben Robinson
Industrial Need
The widespread use of hexavalent chromium (Cr(VI)) in industrial processes is of growing concern due to the strengthening evidence regarding its toxicity and role as a carcinogen. This growing concern has seen an evolution in regulatory frameworks such as Registration, Evaluation, Authorisation and Restriction of Chemical Substances (REACH) where Cr(VI) is now classified as a substance of very high concern (SVHC). This change in regulatory status means that industries face increasing pressure to seek authorization for its use or find viable alternatives. This imperative is particularly pronounced in sectors like aerospace and defence, where Cr(VI) has traditionally been prevalent.
One notable application of Cr(VI) compounds are their use in chromating treatments, where they play a critical role in producing corrosion-resistant layers on metal surfaces. The urgency to replace Cr(VI) with more sustainable, cost-effective, and environmentally friendly alternatives has prompted extensive research and development activity. Despite the availability of several commercial alternatives to chromating, the lack of comprehensive comparative studies and inconsistent performance claims presents significant challenges. To bridge these gaps, TWI has undertaken a comprehensive review to identify state-of-the-art Cr(VI)-free conversion coating technologies. These efforts aim to assess their suitability for mitigating corrosion of aluminium alloys, particularly within the aerospace sector.
TWI's literature review serves as a cornerstone for understanding the current landscape of Cr(VI)-free surface pre-treatments. It encompasses a wide range of published works, patent grants, and commercial product portfolios to discern trends, identify gaps, and highlight critical areas for further investigation. Notably, the review highlights the emergence of silane-based pre-treatments (such as AC-130) among available commercial products while emphasizing the scarcity of impartial data comparing their corrosion performance.
The culmination of these efforts is a comprehensive examination of Cr(VI)-free candidates, including a combination of conversion coatings and silane pre-treatments.
The review highlights the urgent need for the development of Cr(VI)-free coatings devoid of fluorine and cobalt, aligning with regulatory initiatives aimed at reducing toxic substances and critical raw materials. It also sheds light on the inadequately understood environmental impact of these alternative technologies, emphasizing the necessity for thorough evaluations of their environmental impact.
Building upon insights gathered from the literature review, the subsequent report addresses key objectives such as evaluating candidate performance through laboratory validation, assessing corrosion resistance, and providing controlled comparisons with Cr(VI)-based pre-treatments. By focusing on rigorous testing, validation, and alignment with regulatory requirements, these efforts contribute to the development of safer, more sustainable corrosion protection solutions, thus meeting industry demands while safeguarding environmental and human health.
While certain Cr(VI)-free alternatives demonstrate corrosion resistance, there is no clear comparison of the technologies that can be applied across various sectors which is essential for validation. This Core Research Programme project aims to provide insights into the performance of Cr(VI)-free surface pre-treatments including a combination of chemical conversion coatings, silane pre-treatments and multi-layer coating systems, creating the foundation for informed decision-making in industries reliant on these technologies. By addressing critical gaps and objectives, this report contributes to the ongoing efforts towards safer, more sustainable corrosion protection solutions.
Key Findings
- Surface Appearance and Structure: Coatings exhibited uniformity and good bonding, confirmed by visual and microscopic analyses. SEM images revealed homogeneous distribution, emphasizing coating quality.
- Corrosion Performance: Selected Cr(VI)-free alternatives showed corrosion resistance comparable to Alocrom 1200. Testing results met industry standards for durability and performance
- Top Performers: Bonderite MNT 65000 and Lanthane 613.3 emerged as top performers, with excellent corrosion resistance. They surpassed testing standards even after prolonged exposure.
- Silane-Based Coating System: Silane-based pre-treatments combined with REACH compliant paints met industry standards, demonstrating a Cr(VI)-free REACH compliant complete coating system.
- Contribution to Industry: The report provides insights into REACH compliant Cr(VI)-free surface pre-treatments aiding informed decision-making. It supports efforts for safer, sustainable and environmentally responsible solutions in aerospace and related sectors.
Impact
The findings of this work contribute significantly to addressing the industrial need for Cr(VI)-free surface treatments, particularly in the aerospace industry. By identifying promising Cr(VI)-free alternatives through a literature review and providing rigorous testing of these Cr(VI) alternatives, this project lays the foundation for informed decision-making. This work is aligned with existing and emerging European, UK, US and other global regulatory requirements, and supports ongoing efforts towards safer, more sustainable corrosion protection solutions. This work serves to de-risk the transition between coating technologies for TWI Industrial Members by narrowing the development options to the most promising candidate technologies.