The experimental phase was oriented to fulfil the targets given during the initial definition of the requirements for the anodised and sealed alloys. A maximum pit size of 0,8 mm together with a pit density < 5 pits/dm2 for a 500-hour salt spray test exposure.
Two sealing technologies were considered to be optimized, one coming from the background of Clean Sky programme and based on 2-step baths with two chemical compounds each and a second non-commercial solution proposed by the consortium. This last proposal was based in a state of the art of sealing technologies at low TRL and the results of corrosion resistance in Salt spray test (SST) of promising solutions selected from the state of the art. As a milestone during the first period, it was agreed to focus and concentrate the project only on the sealing technology proposed by the Topic Manager.
The following step of the work consisted in the optimization of the 2-step sealing technology employing small samples of the aluminium alloy 2024, a pilot plant for surface treatments with baths of 115 litres and Design of Experiments (DoE) strategies.
Firstly, the chemical products were classified according to their degree of influence on the corrosion resistance of samples in SST. A qualitative classification regarding the impact of the four chemical compounds has been stablished. This helped to understand that the first sealing bath had a greater influence on the final performance.
Secondly the influence of changes in the concentration of chemicals was also analysed and helped to reach the optimized values.
Finally the time and temperature of the sealing were too studied and optimized.
The global results of Salt Spray Test performance on 120x80 mm AA2024 panels unveiled that the sealing technology is quite robust, fulfilling the requirements in a wide process window. The robustness was tested on 52 different conditions using Design of Experiment (DoE) techniques.
The mechanism of formation and protection of the sealing layer were investigated by means of several characterization techniques: SST, SEM-EDX, EIS and wettability test. The SEM-EDX analysis on samples top view and cross section allowed to identify a chemical interaction between two of the chemical compounds which is responsible of a final better corrosion performance.
In parallel, it was analysed the environmental performance of the complete thin layer Sulfuric Acid Anodizing process using the tool Life Cycle Assessment (LCA). In addition, an economic analysis of the process has been carried out, applying the tool Life Cycle Costing (LCC) and the model of the Eco-costs.
The corrosion results and the ecolonomical results obtained, were employed to define the optimum sealing conditions for AA2024. The corrosion resistance of the samples sealed achieved the requirements defined in the beginning of the project an even resist considerable extension of time of SST.
However, when applying the optimum sealing conditions to cast alloys, the SST results needed to be improved. A large number of pits appeared on the sealed cast panels. Consequently, the concentration, temperature and sealing were further tuned making use of DoE. Since the protection behaviour of the coating remaining lower than required, a characterisation plan was executed to understand the differences between 2024 and cast alloys. The results showed that intermetallic particles in the casting alloy seems to be the reason behind the experimental differences, because they provoke inhomogeneous alumina layer during anodizing.
At the end of the project, the industrial scale up of anodizing and sealing treatments optimized in previous work packages was performed. 2024T3 wheels and AU5NKZr body actuators were selected as demonstrators for the optimized process parameters of the sealing technology. The demonstrators and aluminium panels were processed according to the requirements arranged. Baths with volumes from 1300 to 1600 litres were employed.
As an overview, the main conclusions of the project have been:
1. The SAA+sealing technology at pre-industrial scale is an optimum and robust treatment for the processing of AA2024 unpainted parts to enhance their corrosion resistance as alternative to CAA
2. The results from the industrial scale-up shows a weaker performance compared to pre-industrial processing
3. The implementation of SAA+sealing for cast alloys showed on the contrary a weaker corrosion performance.
4. An inhomogeneous anodic layer in the cast alloys, due to intermetallic presence, is the reason behind this different corrosion behavior.
The general objectives and framework of the project have been communicated to the general public via press notes in newspapers and partners´ websites. A video was also edited at the end of the project for communication purposes.
Selected technical results were disseminated in the 32nd International Conference on Surface Modification Technologies (San Sebastian, Spain, 27-29 June 2018).
