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Chromium free surface pre-treatments and sealing of Tartaric Sulphuric Anodizing

Final Report Summary - CHROMFREE (Chromium free surface pre-treatments and sealing of Tartaric Sulphuric Anodizing)

Executive Summary:
Corrosion of Al has to be counteracted by anodizing the Al parts and subsequently applying further protective coatings. Anodized aluminium is normally further processed with a sealing as a final step after anodizing. Processes, which use hazardous Cr(VI)-compounds, such as chromic acid anodizing (CAA), but also chromium-free anodic processes with subsequent Cr(VI)-based sealing such as a hot-water sealing processes with chromates as additives, are still commonly used. Cr(VI)-based sealing solutions have been employed for several decades, but remain one of the most effective and commonly-used methods to improve corrosion resistance of anodized aluminium.
State of the Art new anodizing technologies have been developed as alternatives of CAA and applied on conventional structural Al alloys, joined by riveting. These technologies are Chromium free regarding the compounds of Anodization bath but include Chromium on the surface preparation and sealing steps.

The gaps that ChromeFree CfP covered can be summarized on the following points:
1. Development of a TSAA technology fully free of Chromium. This development has been based on previously existing TSAA technology involving hexavalent Chromium on surface preparation steps as well as on sealing steps. The new ChromeFree technology eliminates the use of hexavalent and/or trivalent Chromium from all the steps of anodizing.
2. The new technology applied on new lighter structural Al alloys (Al-Li 2198/2196) for airframe parts with improved mechanical properties in comparison to the conventional Al alloys (2024/7075).
3. Application of the new TSAA technology on Laser Beam Welded structures instead of conventional heavier riveted structures.
4. The new technology applied and tested on demonstrators resulting in the upgrading of TRL. After ChromeFree project, the achieved TRL is 5.

In the scope of the Project ChromFree a new REACH-compliant TSAA process (Tartaric Sulphuric Acid Anodization) was developed, which uses no chromium compounds in pre-treatment, anodization and sealing steps, and thus gives health and environmental benefits and reduces costs for disposal of chemical waste and recycling costs. The quality characteristics are equal to chromic acid anodized parts and in accordance with quality requirements as established by AIRBUS (AIPS 02-01-003). Thus the outcome of the project provides a process which grants equal properties such as corrosion protection as state-of-the-art Cr(VI)-based processes. As it is almost mature, it will soon be implemented in aircraft production lines and reduce their ecological footprint.
Project Context and Objectives:
Chromic acid anodizing (CAA) has traditionally been widely used for aluminium alloy protection in aircraft structures due to its anticorrosive properties and excellent paint adhesion. However, Chromium(VI)-compounds such as chromic acid and chromates bear toxicological and environmental hazards. Therefore processes which are using Cr(VI) compounds should be replaced by more environmentally friendly processes.
Recent research has brought up alternatives for chromic acid anodization which are free of hexavalent chromium. However, chromium(VI)-containing chemicals like Alodine 1200, chromates, dichromates and chromic acid are still widely in use for sealing anodized aluminium surfaces in order to achieve corrosion protective properties.

The goal of the project ChromFree was to establish a chromium(VI)-free process for pre-treatment, anodization and sealing of novel aluminium-lithium alloys, which should perform equally or better than the currently in used Cr(VI)-containing processes. The new technology should be applicable on new Al-Li structural alloys which are lighter, weldable and perform better in comparison of the conventional 2024/7075 Aluminium Alloys.
To achieve these goals, the following tasks have been performed, including extensive testing and characterization in each step:
• Development and optimization of a green pre-treatment process, comprising steps for degreasing, pickling and desmutting. These process steps aim for complete removal of any grease, oxides and smut from the surface, which otherwise can interfere with the oxide layer formed in the subsequent anodization step. On the other hand it is mandatory to make sure that no additional irregularities are introduced, which can act as starting points for cracking.
• Optimization of the TSA anodization process and adaption for the selected aluminium-lithium alloys. It is well known, that the potential profile (such as continuous, pulsed, pulsed with offset, etc.) can have a significant effect on the resulting oxide layer. Potentiodynamic investigation in combination with extensive characterization of the resulting oxide layers were to be performed to gain detailed understanding in the dependence between electrical parameters and layer growth, especially growth of the barrier layer and influence of the parameters on the porosity / pore size of the porous layer.
• Establishing a Cr(VI)-free “green” sealing process in combination with testing the corrosion protective properties of the sealed layers. Additives to be tested were selected from various promising inorganic and organic compounds. The corrosion tests to be performed included tests with specimens which have been undergone the full process including a subsequent painting.
• Testing for strippability and repairability
• As a final step, a test panel (demonstrator) was to be treated with the full process including painting and to be undergone certified testing to make sure that the process established in the scope of the ChromFree project meets all requirements.

Summarized, the following quality requirements were to be met by the process established in the scope of the ChromFree project:

Qualitative requirements
- Total coverage, smooth, and aesthetically pleasing
- Excellent substrate-coating adhesion
- Applicable after Cr free surface preparation (e.g. cleaning, desmutting, etc.)
- Repairable (local touch up)
- Compatible with REACH
- Process, control and testing safe for environment and workers

Quantitative requirements:
Film properties:
- Anodic film weight more than 22 mg/dm² (tested according to EN 12373-2)
- Film thickness between 2 mm and 7 mm (tested according to ISO 2360 and by SEM on a cross section)
Corrosion resistance of painted specimen:
- Filiform corrosion shall not exceed 2 mm at 960 h exposure (tested in accordance with EN 3665)
- No blister or corrosion extending 1.25 mm at 3000 h exposure (tested in accordance with ISO 9227)
Corrosion resistance of unpainted specimen:
- Less than 2 pits/dm2 and no pit exceeding 0.8 mm diameter (tested in accordance with ISO 9227)
- No patchy dark grey areas after minimum 96 h exposure for 2024 T351 unclad and 7175 T7351 unclad
Assessment of Paint adhesion:
- Maximised paint adhesion, GT0 (dry) and GT1 (wet) (tested according to ISO 2409)
Assessment of Fatigue properties:
- Plotted as Wöhler curves (mean and minimum curves) (tested according to EN6072)

All the initially stated goals have been achieved. As a consequence the new technology performs at least equally in comparison to CAA, is totally Chromium Free, can be applied on new substrate materials of higher mechanical efficiency and is applicable for welded parts. The achieved TRL is 5 and after further maturation of the technology, the new TSAA can be applied on industrial level.

Project Results:
In the scope of the ChromFree project a new process has been developed for corrosion protection of structural aircraft parts, where all process steps use solutions which are free of environmentally hazardous hexavalent chromium compounds. This process comprises steps for pre-treatment, TSAA anodization, sealing and subsequent painting. Validation of this process has been performed through extensive testing to ensure that the coated parts meet all requirements as established by AIRBUS (AIPS 02-01-003).
Hence all steps of the process developed in the scope of the ChromFree project are free of Cr(VI)-compounds, it covers a technological gap of previously developed TSAA processes, which included Cr(VI)-compounds in pre-treatment, sealing and painting steps.

The main achievements and benefits of the ChromFree process – compared to conventional processes – can be summarized as follows:
• Pre-treatment
o Conventional: Use of chemical solutions containing Cr(VI)
o ChromFree: Use of chemical solutions free of Cr(VI)
• Anodization
o Conventional: Chromic Acid Anodization (CAA)
o ChromFree: Optimized Tartaric Sulphuric Acid Anodization (TSAA) process
• Sealing
o Conventional: Use of solutions based on Cr(VI) or containing Cr(VI) compounds as additives
o ChromFree: Use of chemical solutions which are free of hexavalent chromium
• Coating / Paint
o Conventional: Use of primer containing hexavalent chromium
o ChromFree: Use of chemical solutions free of Chromium
• Strippability
o Conventional: Chemical stripping with dangerous solutions
o ChromFree: Use of environmental and health friendly strippers
• Demonstrator Panel

o Conventional: Riveted structure out of Conventional Al Alloys, protected from corrosion with treatments and coatings involving hazardous materials and processes.
o ChromFree: Laser beam Welded structure out of lighter and high efficient new Al alloys, protected from corrosion with treatment and coating involving only environmentally friendly materials and processes

Benefits of the ChromFree process
• Quality characteristics of the ChromFree process are equal to chromic acid anodized parts
• Quality characteristics meet all present requirements
• Health- and environmental benefit
• REACH compliance
• Avoidance of using hazardous chemical compounds including Cr(VI), which will be shortly prohibited and not available in the market

Through the application in a fuselage demonstrator panel, the technology is almost mature to be implemented in a production line. The remaining steps to further mature the technology are:
• Application on parts and systems with more complex geometries
• Full scale testing

Potential Impact:
Within the Clean Sky JTI, the employment of ECO friendly cleaning processes is going to be thoroughly studied, analysed and applied in the aircraft production processes. One major focus of the technological competition in aeronautics is the increased use of novel superior lightweight materials processed in ECO friendly manufacturing. Clean Sky contributes to technological and scientific leadership in the area of aeronautics light-weight design, and therewith strengthens the competitiveness of the European industry.

In the scope of the ChromFree project, a process for corrosion protection of novel aluminium-lithium alloys for structural parts of aircraft has been developed, where all steps, namely pre-treatment, TSAA anodization, sealing and subsequent painting, are free of hexavalent chromium. Due to the avoidance of hazardous chemicals, in particular Cr(VI), which will soon be prohibited, the process will reduce the ecological footprint of aircraft manufacturing and it will reduce the costs for disposal of hazardous chemical wastes. Compared to state of the art, where aircraft parts are treated/painted with chromic acid anodization, sealing with Cr(VI)-containing chemicals and Cr(VI)-containing primers, parts treated with the ChromFree process will contain no hexavalent chromium. Thus the implementation of the process in industrial scale will reduce costs concerning disposal and/or recycling of materials during repair processes as well as recycling costs at the end of the lifetime of such parts. Additionally the ChromFree process gives benefits in terms of health and safety for employees who are employed in the field of the respective galvanic and repair processes.
The CleanSky process is almost mature and therefore it is expected that it can soon be implemented in real life production lines. Final work remaining to further mature the process are application and testing on parts and systems with more complex structures, and full scale testing, including application on the integrated technology demonstrator (ITD).

The processes developed in the scope of ChromFree have high performance potentials for anodizing steps using environmentally friendly chemicals fulfilling the REACH requirements. Application of high know-ledge based processes from pulse plating techniques to anodic oxidation processes mainly for high-end micro components opens a new field of business for Happy Plating. Process solutions for the selective coating of aluminium based microelectronic components can be provided together with the industrial partners involved in this project, giving access to a new key market for the micro system industry. Additionally the new technology developed gives rise to a broad variety of possible applications simultaneously introducing an environmentally friendly chrome free process route. The high-end European surface finishing industry as well as electronic and automotive industry can be addressed as possible customers. The company’s portfolio of core competences, especially current pulse related process solutions, can be strengthened and the position of Happy Plating as one of the leading companies in the highly innovative sector of pulse plating can be further enhanced. The deep scientific and technical understanding for anodic pulse processes gained within this project will help Happy Plating successfully working with customers in different fields of surface technology.

The project results of ChromFree will enlarge the understanding, knowledge and capabilities of CEST concerning the surface treatment of Light metal alloys. CEST is interested to exploit its increased competence in other research projects with other aeronautical partners for the benefit of the European aeronautical community.
Experience gained in other projects concerning TSAA and SAA processes, such as ValidateTSAA (JTI-CS-2012-3-ECO-01-058), TARTASEAL (JTI-CS-2011-1-ECO-01-020) and SAA-Seal (SP1-JTI-CS-2010-05) shows that novel chromium-free processes have the potential to yield equal results in comparison to Cr(VI)-based processes, but on the other hand such a process optimized for a particular type of alloy is not such an all-round solution as Cr(VI)-based processes. This means in particular, that the type of the alloy, such as the aluminium-copper alloy AA2024, the aluminium-zinc-magnesium alloy AA7075 or the aluminium-lithium alloy AA2198, as well as the production process, such as sheets, machined specimens or cast alloys, have significant influence on the results of the finishing process, which means that in certain circumstances the process parameters need to be adapted and optimized for certain alloys of types of alloys in order to achieve optimum results. Aiming to continue the work within the CleanSky / Horizon2020 programme and to gain further and deeper understanding in chromium-free anodization and sealing processes on various types and production states of aluminium alloys, a proposal for the OptiSeal project (JTI-CS2-2015-CFP02-SYS-02-12) has recently been applied.