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Final Report Summary - PROAIR (Active PROtection of multi-material assemblies for AIRcrafts)

The project was devoted to development of the novel active corrosion protection systems for multi-material combinations used in the aircraft design. The development of such new approaches is only possible via deep mechanistic understanding of the degradation processes in confined environments formed at multi-material joints and supported by modelling. However, the availability of suitable localized techniques, which are capable to provide such information, is limited. The second essential stage was finding the synergistic inhibiting combinations, which can provide effective suppression of corrosion processes in the case of galvanically coupled dissimilar materials. The ultimate step was introduction of inhibiting compounds to the protective layers, adhesives and sealants used in multi-material design of aircrafts. The project also aimed at creation of stimulating and interdisciplinary R&D partnership, with actors from the academia and private sector, promoting the exchange of ideas, methods, techniques as well as enabling an accelerated technology transfer from academia to industrial scale and a continuous collaborations between the stakeholders.
The main objective of this project was development of basic strategies for active corrosion protection of multi-material assemblies in new “green aircraft” designs. In this way, the project was driven by a strong industrial demand, but has at the same time has a very important research component on a fundamental scientific level. An essential starting point is a detailed investigation of the reaction mechanisms of corrosion caused by galvanic coupling effects in different multi-material combinations including carbon fiber reinforced plastics (CFRP) and metallic materials. This knowledge is essential as a basis for simulation of the localized corrosion processes in critical zones such as micro-confined hybrid joint areas and defects in coatings for multi-material application.
The specific scientific objectives of the project were the following:
- Deeper understanding of galvanic corrosion mechanisms in multi-material structures including micro-confined defects;;
- Development of localized electrochemical methods capable to measure the corrosion currents in such areas;
- Development of finite element based models describing the localized corrosion attach in relevant multi-materials assemblies;
- Discovery of corrosion inhibitors and synergistic inhibiting mixtures for relevant galvanic couples;
- Understanding of inhibition mechanisms of the electrochemical reaction on CFRP;
- Development of encapsulation approaches for mixtures of corrosion inhibitors in different nanocontrainers or multi-inhibitor loading of nanocontainers;
- Development of active protective schemes containing mixtures of nanocontainers of different corrosion inhibitors for multi-material application;
- Revealing mechanisms of active corrosion protection in the micro-defects formed in vicinity of galvanically coupled materials.

The project resulted in numerous important outcomes, which can be systemized in three main groups. The first remarkable result is related to the fact that during the project a new localized electrochemical technique was developed on basis of Scanning Vibrating Electrode Technique, namely 3D SVET system (“LocalProber”, Fig. 1). The new system is considered to be commercialized and exploitation strategies are currently discussed. The new technique is capable providing a powerful tool to investigate localized electrochemical processes in confined areas of multi-material assemblies. The new concept for probe vibration and signal collection has been implemented. The main difference from previous SVET systems is the fully digital secondary signal processing. Which allows controlling better the actual tip vibrational motion during the measurement and collecting potential difference signal between different adjusted spatial points at the same time with the separation of selected phase shift. This allowed the improvement of the maximal lateral and spatial resolution of SVET and the estimation of corrosion process in confined zones, such as corrosion attack at the joint area of galvanic aircraft materials. Moreover the localized electrochemechical method is coupled with optical profilometry allowing acquisition of electrochemical signal on complex-shape specimens and confined defects. Two fully functioning prototypes were assembled and tested in different environments. These setups were also introduced and used during the lab sessions of two summer schools for young researchers organized in frame of PROAIR. Moreover the data obtained by these new 3D SVET was used for development and validation of FEM models for galvanic corrosion in different material combinations.
A new model describing the galvanic corrosion of multi-material assemblies was developed. The model includes both the acceleration of corrosion processes as a result of galvanic coupling and the potential effect of insoluble corrosion product layer formed on the actively corroding metals. However, in the case of aluminum alloys the corrosion process has a strongly localized nature. The principally new approach of “active spot” was introduced into the model, explaining the increased corrosion activity of aluminum near zones of intermetallics. Figure 2 demonstrates the distribution of corrosion currents at Ti/Al galvanic couple obtained on the basis of the developed model and validated by the localized electrochemical measurements. This approach opens many possibilities for the prediction of galvanic corrosion of aeronautically relevant structures, and for fundamental understanding of corrosion mechanisms, involved in galvanic corrosion. As a result, the traditional trial-and-error method can be replaced by scientific based prediction of corrosion failure, allowing the reduction of construction and service costs.
The development of active corrosion protection schemes for galvanically coupled multimaterial assemblies (incl. CFRP) was a third principal direction in frame of ProAir.
At the first stage of the project different corrosion inhibitors and their synergistic mixtures were tested on relevant multi-material assemblies using developed model multi-electrode cells. One of the promising approaches for screening of corrosion inhibitors for galvanically coupled materials is to use Scanning Vibrating Electrode Technique (SVET) to quantify the galvanic corrosion currents. The selection of candidates was performed keeping in mind the localization of cathodic and anodic processes in galvanically coupled materials. Therefore, the mixtures of inhibitors were selected in the way to have one efficient at the anodic side and the second one performing at the cathode. Several promising synergistic combinations were identified. One of the most efficient candidates is a mixture of Ce (III) and benzotriazole. The combination of RE cations with mecaptobenzothiazole can also be considered as efficient candidate.
As a strategy to introduce the selected mixtures of corrosion inhibitors into the coating, different encapsulation approaches were followed using various types of nanocontainers. Layered Double Hydroxides (LDH) and bentonite nano-clays were used as the main candidates thanks to efficient ion-exchange properties, which allow control of the inhibitor release on demand. In frame of ProAir project the proof-of-concept was performed for CFRP coupled with aeronautically important aluminum alloys AA6061 via the combinations of nanocontainers loaded with different corrosion inhibitors impregnated into the polymeric coating. This work was further extended for galvanically coupled AA2024 and CFRP. Figure 3 demonstrates effect of nanocontainers loaded with synergistic mixture of inhibitors on the corrosion processes in the defect of coated multi-material assembly between CFRP and AA2024. The best performance is achieved in the case of coatings containing synergistic mixtures of corrosion inhibitors. However, the first obtained results demonstrate that compatibility between the polymer matrices and the nanocontainers can be an important issue, which can lead to limited reproducibility of the results. This important issue should further followed in the future research efforts.
The first obtained results demonstrate that in spite of promising preliminary outcomes the compatibility between the polymer matrices and the nanocontainers can be an important issue, which can lead to limited reproducibility. It was shown, that “smart” nanocontainers developed and used during the project can not be simple mixed with commercially available polymeric coatings. This approach often leads to the blistering of coating and loss of the protective properties. However, the number of trials was performed in order to overcome this problem and the close collaboration between partners, settled during this work will definitely continue in future in frame of joint projects.
The main objectives of the project include the knowledge exchange and training of scientific personnel along with the main scientific objectives. The intensive exchange of researchers between the industry from one side and research and academic organizations from the other was performed. Moreover, three young researchers were hired for temporary contracts in frame of ProAir project. This fact had a strong impact on development of their scientific carriers. Two of the involved ESR got prestigious grants (Alexander von Humboldt research grant and Marie-Curie Individual Fellowship) immediately after finishing their ProAir contracts. Two ESRs involved in the exchange program were also promoted as result of the project: one has got a permanent research position at Airbus business unit and second one got a governmental grant to perform his PhD in topic closely following the ProAir activities.
ProAir project has served as an effective platform for knowledge exchange, which resulted in much better understanding between the academic and industrial partners. Moreover the exploitable results related with development of new scientific technique were produced which will be further commercialized.

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