Multi Scale COrrosion Testing: application to the prediction of the intergranular corrosion rate of a reference aluminium alloy for aeronautic. – M-SCOT

In WP1 a mass transport and reaction model simulating geometries of the grain boundary will be implemented. FEM simulation will be validated by specially designed experiments and simulation strategy will be applied for realistic sizes of the dissolving cavity. Attention will be paid to the definition of initial chemical conditions to test the environmental effects and to the discussion of the dissolution law of the PFZ in the expected chemical conditions found in the grain boundary cavity.
In WP2, the objectives will be to set-up a parametric study to improve the reliability of two laboratory-scale tests: (i) the tensile test on pre-corroded samples and (ii) the thin foil penetration technique test. Output data concerning the effect of local chemical changes on local mechanical properties and corrosion propagation will be used as input data in simulations in WP1.
In WP3 we will evaluate the robustness of the transfer of laboratory-scale tests to engineering scale tests which means for us testing in standardized atmospheric conditions. Coupon test specimens will be tested via classical standardized environmental tests (natural marine, salt spray, immersion-emersion exposures) in order to establish a reference for the corrosion behavior of the alloy. Tensile tests will be performed on samples corroded after marine natural exposure and submitted to a salt spray environment while thin foil penetration technique tests will be performed on samples submitted to a salt spray environment.
Finally in WP4, the outputs of WP2 and WP3, i.e. the average and maximum penetration depths, will be compared to the instantaneous propagation rate defined in WP1 (Task 4.1) to structure an “application guide”.

1. The machining of a 1D artificial electrode or pore electrode) (dissolving wire of pure Aluminium of a diameter of 100 µm) allows measurement of dissolution rates close to the instantaneous propagation rate of intergranular corrosion (IGC).
2. This growth rate was compared to the intergranular damage rate measured on thin foils of an aluminium alloy 2024 (thickness 30 microns) in a controlled potentiostatic regime.
3. The detection method originally proposed in the literature for the thin foil penetration method was revisited and allowed to explain the difficulties encountered in previous works described by partner P2.
4. In the frame of T2.1., typical corrosion defects were generated for different exposure conditions (continuous immersion and cycling). The average depth of corrosion defects and their densities were measured on the basis of observations by light microscopy. Different analysis techniques such as transmission electron microscopy combined with electrons energy loss spectroscopy were implemented to analyze locally these corrosion defects: the presence of a layer thickness of 50-100 nm copper nanoparticles on the walls of corrosion defects along these intergranular corrosion paths was especially highlighted.

M- SCOT project is proceeding according to a pre-defined schedule. Three significant results were obtained:
- The development of controlled electrochemical experiments on the electrode « 1-D Artificial pit «.
- The copper detection confirming the importance of electrochemical reduction reactions at the crack tip.
- The development of a new detection technique in «thin foil penetration « which was revisited.
One can reasonably think that the consortium will be able to address the following schedule, the key task of the project T4.1. on the comparison of the different approaches for measuring the propagation rate of intergranular corrosion.

R.OLTRA, R.BONZOM
Revisiting Thin Foil Penetration Technique for Intergranular Corrosion Tests on Aluminum Alloys, Journal of The Electrochemical Society, 163 (6) C1-C3 (2016)

Aging aircrafts are experiencing high level of maintenance costs to reduce the risk due to structural effect of existing corrosion damage for which the propagation rate cannot be predicted with a sufficient level of reliability. Indeed, in service, aircraft can suffer from cumulative corrosion and fatigue damage. Therefore, the evolution of corrosion in aluminium alloys, in particular the propagation of intergranular corrosion, remains not only a problem of fundamental interest but is also a key point in aircraft risk and reliability analysis as corrosion damage and mainly intergranular corrosion has a detrimental effect on integrity of aircraft structures by promoting fatigue crack initiation.
The concept of M-SCOT is driven by the important need in developing strategies for validating engineering scale tests that would allow the detrimental effect of an intergranular corrosion defect to be evaluated and the remaining lifetime of the corroded structural parts to be quantified. In M-SCOT, it is proposed to revisit laboratory scale tests completed by micro-environmental scale simulations.
To illustrate how the propagation rate of intergranular corrosion defects and therefore the validation of tests (at various scales) is strongly dependent of the role of the microstructure, M-SCOT project will be focused on the typical case of the propagation of the intergranular corrosion of AA2024 as it is largely described from the phenomenological point of view in literature and represents a significant cause of structural damage in ageing civilian and military aircrafts.
The final scientific objective is to reduce the lack of predictive data for intergranular corrosion which is a bottleneck in implementation of operational management of existing defects in structures. The scientific and technical work plan has been defined to cover all the relevant scales for this type of corrosion to develop an engineering testing methodology based on a mechanistic (laboratory scale tests) and kinetic understanding of intergranular corrosion propagation rate (modelling at the microstructural scale).
The key deliverables will be to predict quantitative estimations of the intergranular corrosion rate propagation on the basis of coherent results of tests (tests on specially designed specimens or using simulation based testing), conducted at microstructural scale in simulated environments, at laboratory scale in controlled environments and finally at engineering scale in standardized atmospheric environments (natural marine, salt spray, immersion-emersion exposures). It is planned to demonstrate that the prediction of the intergranular corrosion rate results from the benchmarking of the average and maximum penetration depths obtained from testing on metallic specimen compared to the instantaneous propagation rate defined from modelling.
Beyond the immediate project, the final technical deliverable will be structured as an application guide which will helps engineers to improve and rationalize their knowledge on corrosion testing, to propose durable and damage tolerant designs. The engineering models developed will predict the performance of structural parts of aircrafts during service life. This demand is and will be more and more important as metallic materials (aluminium alloys) are and will be combined with composite materials by hybrid joining generating specific features of corrosion damages.