REACh Regulation and its Impact on Anodizing in Aerospace Sector Qualification Test of REACh-Compliant Thin Sulfuric Acid Anodizing as an Alternative to Chromic Acid Anodizing

Due to European REACh regulation hexavalent chromium-based substances must be removed from surface treatments to improve environmental sustainability and workers safety. Thin Film Sulfuric Acid Anodizing sealed with a 3-steps process was identified to be a suitable Cr VI-free alternative to Chromic Acid Anodizing sealed in hot water. The TFSAA was tested in accordance with MIL-PRF-8625 and EN4827 and it shall have no regression if compared to CAA. The tests included visual inspection, paint adhesion, coating weight, and thickness. Corrosion resistance was assessed using salt fog, intergranular attack, and end grain pitting tests. The mechanical properties were examined through static tensile tests, low and high cycle fatigue tests. Most tests were performed on panels, while specific specimens were required for IGA, EGP, fatigue, and static tensile tests. The AA2024 alloy was predominantly tested due to its extensive use in the aerospace industry. However, AA6061 and AA7075 alloys were also evaluated on panels. The following tests were conducted on panels of AA2024, AA6061 and AA7075 in accordance with MIL-DTL-5541 and EN4827: Visually inspection: all panels shown a compliant coating. Coating thickness by means of Fisher method: all the measures were within the required range of 3-6 μm and constant upon different measurements. They were also confirmed by cut up. Coating weight: The AA2024 panels were the only that met the requirements of 75-115 mg/dm2, while AA6061 and AA7075 had a slightly heavier coating than expected. Salt spray test: All panels shown almost no pits after 552 hours. Paint adhesion: eight different primers and wash primers were tested in dry and wet conditions, in accordance with ISO2409 and MIL-PRF-8625 respectively. In dry conditions, TFSAA shown a good paint adhesion, comparable with CAA. But after immersion, separation occur on the REACh-compliant alternative. The following tests were conducted on dedicated samples of AA2024: IGA and EGP: no intergranular attack was detected, while end grain pits size was comparable between CAA and TFSAA. A static tensile test was performed to ensure minimum mechanical properties and to confirm that anodic layer does not impact static characteristics. All the tested specimens had complaint results, no regression between treated and bare samples was highlighted. Regarding HCF: in longitudinal direction, the curves related to the 3 treatment conditions are almost superimposable at low cycles. In transverse direction TFSAA curve is even higher than CAA ones, but still lower than bare material. Regarding LCF: TFSAA is comparable to CAA at higher cycles only. However, during component design, a very conservative fatigue curve is considered. Indeed, several derating factors were used to contemplate the surface treatment, surface roughness and safety factor. This curve was then compared with all the experimental data, more than 200, all were above the design curve. Anodic layer thickness was measured after fatigue test since it could favour crack initiation and propagation. All the results fall inside the required range of 3-6 μm. In conclusion, TFSAA can be considered a suitable REACh-compliant alternative to CAA. Indeed, TFSAA shown fulfilment to MIL-PRF-8625 and EN4827 requirements and good paint adhesion on 5 out of 8 primers tested. While the very conservative approach on fatigue design curve can justify the change of anodic treatment