Fundamentals of Intergranular Corrosion in Aluminum Alloys

The project "Fundamentals of Intergranular Corrosion in Aluminum Alloys" (FICAL) was about understanding intergranular corrosion (IGC) in aluminum alloys. IGC spreads along the grain boundaries of an alloy and is a limiting factor for the industry when it comes to finding new areas for the use of aluminum, such as new car parts. Being able to reduce IGC without a reduction of mechanical properties will have high economic significance. FICAL was a five-year KPN project that established a new understanding of the mechanisms associated with IGC. The research partners were NTNU and SINTEF, and the project was partly funded and carried out in collaboration with the industry, which represents the entire aluminum value chain, from raw materials to component manufacturers. The industry partners were Hydro, Gränges, Benteler and Steertec Raufoss. Two PhD students and a postdoc have been educated in the project, in addition to six MSc students. 15 scientific articles have been published in the project, and 6 more are expected to follow. 27 presentations have been given at conferences. To understand IGC, we had to 1) Explore the nanoscale electrochemical causes of IGC; 2) Measure grain boundary chemistry and microstructure; 3) Understand the initiation and spread of IGC; and 4) Model grain boundaries to be able to predict how dependent IGC is on alloy composition and thermal history. This knowledge will then be implemented in the development and design of new aluminum alloys. The alloys studied in the project were selected in close collaboration with and delivered by the industry partners. In this way, the results had direct relevance to the industry. The corrosion properties have been studied quantitatively down to the nanometer scale using advanced characterization instruments - we have used transmission electron microscopy (TEM), focused ion beam (FIB) and atom probe tomography (APT) - all of these are instruments in national infrastructures (NORTEM, NORFAB and MiMaC. One of the studies performed was of an industrial Al-Mg-Zn alloy from Benteler showing intergranular stress corrosion (IGSCC). Here, microstructure and chemistry around grain boundaries have been studied in detail using TEM, and linked this with corrosion properties. This was used to provide good analytical datasets which in turn were used to set up models. Nucleation and spread of IGC are key components for obtaining a good picture of IGC and establishing parameters for predictability and control. Here we have done a thorough study of extruded AA6005 alloys related to Hydro. IGC is related to surface treatment, secondary phases in the alloy and the size and orientation of grains on the surface. The extent to which secondary phases initiate and promote IGC by acting as external cathodes on the metal surface has been studied in detail. The main purpose of the modeling activities has been to predict the chemistry and structural development of grain boundaries during various thermomechanical heat treatments. Looking at the connection to mechanical properties was also one of the main goals. Significant progress was made in the development of the modeling framework, as well as physical models for the segregation of dissolved atoms and precipitates in grain boundaries. In particular, the development of vacancy density as a function of heat treatment is included in the model, and a model for grain boundary precipitation was implemented. We now hope to combine the vacancy, segregation and precipitation models to be able to predict chemistry and structure in grain boundaries and expand the framework to include dynamic IGC. The effect of thermomechanical treatment on the IGC properties of extruded profiles and rolled sheets of Al-Mg-Si with low Cu content for the automotive industry has been a major activity in the project. We have observed high IGC sensitivity in almost copper-free (<0.01% by weight) alloys, and that small changes in the composition of extruded products significantly affect IGC resistance under accelerated test conditions. We have also looked at alloys without Mn and Fe, as well as the effect of texture with respect to IGC. We observed a large variation in the results obtained by standardized corrosion tests. A Round-Robin test was therefore conducted in all laboratories associated with the project to check how different IGC test practices affect the amount and type of corrosion attack. This systematic study shows small variations and is summarized in a report, which also ranks the IGC resistance for a number of Al-Mg-Si alloys. Although there have not yet been developed any new industrial processes or patents from the project, the industry has expressed that they have gained a much better understanding of IGC, and that the project has led to a lot of new knowledge. Industrial partners will use this new knowledge to develop new products and processes. However, we have also shown that there is a need to continue this type of research.

We provide new info about the role of external and internal cathodes in initiation and propagation of IGC. We also highlight the importance of grain boundary (GB) properties and chemistry for IGC and that 6xxx alloys can be optimized to obtain desirable changes in the GB nanostructure and chemistry, grain size, geometry, the surface microstructure and also maintain the protective oxide formed during extrusion. The microstructure study involves new understandings concerning the precipitation behavior in 6xxx and 7xxx alloys. It is shown that corrosion can be reduced by altering the processing parameters, and that the fundamental reason for this is closely related to the precipitates. Methodologies for improved investigation of GBs have been presented. From the modelling side, a model of the GBs, including vacancies and solutes has been developed and tested. The combined findings will be important in producing Al alloys with excellent mechanical properties and corrosion resistance.

Susceptibility to Intergranular Corrosion (IGC) is a limiting factor in finding new applications for aluminium alloys, such as for components for the automotive industry, that require improved mechanical properties as well as good resistance to IGC. FICAL is a 5 year KPN project that has the objective of establishing a new understanding of the mechanisms of IGC and how to control it, by relating composition, processing and microstructure to corrosion properties. Industrial funding is provided by a substantial consortium of companies who represent the entire value chain from alloy production to component manufacture. Particular attention will be paid to the behaviour of segregating elements at and around grain boundaries (GB) and precipitation (at GB and bulk) and how these affect the electrochemical properties at the nano-scale. The relationship between IGC and Stress Corrosion Cracking (SCC) will also be addressed. Advanced experimental infrastructure such as Transmission Electron Microscopy (TEM) and Focussed Ion Beam (FIB) will be used to provide information about the structure and chemistry of the alloys that has not previously been available. Advanced thermodynamic, ab-initio, molecular dynamic and Monte-Carlo modelling tools will be developed to provide a framework for predicting the IGC behaviour of alloys. The relationship between corrosion and mechanical properties will be addressed, establishing principles for optimising alloy design and performance. The project will interact with international experts. The project will provide a focus for a national and international R&D network. Two PhD students and one Postdoctoral worker will be educated within the project.