The scientific and technological objectives of SICOM are:

• Definition of modelling parameters that represent corrosion condition and in-service experience of aircraft.
• Development of a numerical micro-scale model to simulate localised corrosion of Al- Alloys with regard to micro-structure and the micro-electrochemical condition.
• Determine the corrosion rate of Al-Alloys in the meso-scale of occluded cells by means of numerical calculation as a function of physical and geometrical factor for given macro-environment.
• Development of a basic numerical model for prediction of galvanic corrosion behaviour and upscale it for application to structural elements of aircraft.
• Analysis and integration of the impact of surface treatment of modelling results with regard to inhibition release effect on corrosion rate, clad layer influence and oxide degrading effects.
• Development of a decision tool to link different model of different scales and model validation.

Objective I: Definition of parameters that represent corrosion condition and in-service experience of aircraft

The definition of requirements and specifications for optimal corrosion modelling including experience of in-service findings for corrosion problems related to Al alloys will the base line for model development. This should result in formulating relevant input parameters for the other WP.

Objective II: Development of a numerical micro-scale model to simulate localised corrosion of Al- Alloys with regard to micro-structure and the micro-electrochemical condition

The dominating factors that trigger localized corrosion on aluminium alloys will be analysed. The purpose is to evaluate the influence of the microstructure of an Al alloy on the onset of localized corrosion. Numerical models - a mechanistic, mass transport model and a probabilistic, cellular automaton model - will be developed that allow simulating electrochemical reactions at the micrometer scale. The mass transport model will be used to simulate local dissolution processes, such as those occurring during pitting and crevice corrosion. The cellular automaton model will simulate the corrosion propagation path when intergranular corrosion occurs. In order to determine the required input parameters for the numerical models, different kind of micro-electrochemical experiments will be performed in combination with various types of surface analytical investigations. The input parameters required for the models include:
a) the micro-geometry of local corrosion sites;
b) the microchemistry that has been generated near corrosion sites;
c) the dissolution kinetics of active surface micro-zones. In order to validate the simulated data specific configurations will be selected, that allow a quantitative classification of the computed results.

Objective III: Determine the corrosion rate of Al-Alloys in the mesoscale of occluded cells by means of numerical calculation as a function of physical and geometrical factor for given macro-environment

A basic model is developed to understand and quantify the possible chemical and electrochemical changes that occur in an occluded cell resulting from joint design (e.g. crevice). The purpose is to model the (electro) chemical evolution leading to the localized corrosion (intergranular) inside a narrow gap of electrolyte encountered in a sensitive assembly as a function of the relevant physical parameters. The role of a prior surface treatment on the exposed metallic part of the occluded cell will be considered.
Defects will be assumed to exist and it will be demonstrated that it is possible to predict a possible critical chemical and electrochemical change which can be due to the confinement of the electrolyte in specific situations. The numerical calculation of the corrosion rate in an occluded cell can provide a tool to estimate corrosion e.g. occurring in hidden non accessible. This can be used as valuable input for a repair or maintenance decision tool.

Objective IV: Development of a basic numerical model for prediction of galvanic corrosion behaviour and upscale it for application to structural elements of aircraft

Galvanic corrosion in aircraft is important as it occurs whenever dissimilar metals or certain types of composites are joined or are located close to each other. The geometry of the connections, the characteristic and extent of the electrolyte, and the type of mitigation methods employed all affect the extent and rate of corrosion. Previous maintenance approaches of “find it and fix it” must be replaced by a more fundamental approach that is based on an understanding of the corrosion process and the ability to predict and monitor its behaviour.
Starting with basic model elements the critical parameters that dominate the galvanic corrosion of aircraft structure will be analysed. A mechanistically based computational model for galvanic corrosion will be developed and validated by comparison with experimental results. Further improvement and optimisation of the model for the application on typical structural elements will aim to deliver a model capable of predicting galvanic corrosion properties and their dependence on materials, protection systems, design and environmental conditions.

Objective V: Analysis and integration the impact of surface treatment of modelling results with regard to inhibition release effect on corrosion rate, clad layer influence and oxide degrading effects

The corrosion band damage behaviour can be significantly changed with different types of surface protection systems. The influence of surface coatings on the localized corrosion of aerospace Al alloys will be studied. The micro-scale modelling approach used in this project will be applied on cases where the aluminium is protected by different surface coatings.
Investigation of the failure mechanism of different model coatings and their influence on the microscale corrosion phenomena can be implemented into a modified reaction model with new boundary conditions for example. Different cases can be identified depending on the nature of the coating, oxide isolator case, conductive clad layer or a inhibitor release. A scratch might hit only the Al bulk matrix, or the Al bulk matrix and second phase particles. Since pitting is more likely initiated in the second case, the influence of released inhibitor on scratched second phase particles will mainly be modelled. The results are "advanced" numerical models, which allow complex localised corrosion processes to be modelled on aluminium alloys.

Objective VI: Development of a decision tool to link different model of different scales and model validation

A tool summarising the detailed results of the project in a form suitable to support decision making and its validation against experimental tests will be developed. The corrosion models will be validated with typical technological specimens and assumption of corrosion severity according to the variation of the applied environment of experimental testing.
Further validation of the models will be performed with the use of available results obtained form in-flight exposed specimens and service findings.
This knowledge will be incorporated in the decision support tool to enable the engineer to view the data generated by the models but also examine the trends of the data with the main design variables as it would not be possible with the detailed models to consider every possible case.
The encapsulation of the project results in this form will provide a convenient mechanism for the exploitation and dissemination of the project results.