EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The use of Cr(VI) is restricted by regulations such as REACH, and its use will be forbidden in 2024. This is valid not only for surface treatments but also for repair and maintenance, when the removal of oxide layers needs to be done with Cr free solutions. The solutions currently in use are Cr(VI) based ones and NaOH as Cr free alternative. A suitable stripping process will assure a uniform and total removal of oxide layers, assuring the same performance in protecting the aluminium against corrosion after the re-treatment step.
Pre-treatment of aluminium substrates (2024 and 7075) was done with an alkaline cleaner followed by acid etching. To produce the oxide layers the Al substrates were anodized or conversion coatings were formed.
Stripping of the aluminium oxide layers was performed using state of the art and commercially available solutions. Surface morphology of the aluminium alloys was investigated by mean of scanning electron microscopy (SEM with EDX) before and after the stripping process. Cross-section investigations of the samples after stripping process were performed using focused ion beam (FIB) in order to quantify the oxide layer removal and its uniformity. Stripping solution temperature and stripping time were tailor to investigate their effect on oxide layer removal. For short stripping time (few minutes), oxide layer is not or just partially removed from the surface while for longer stripping time (e.g. 2 hours) a total removal of the anodic layer is achieved. Samples weight loss as a function of the stripping time was evaluated. Raman spectroscopy was used for detection of Cr on the samples surface after stripping step. Comparison of the commercially available solutions with the classical Cr(VI) based solution regarding their performance on uniform removal of oxide layers without damaging the Al substrates was done.