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, their characteristics, the extent of the electrolyte, and the type of mitigation methods employed all affect the extent and rate of corrosion. SICOM, a recent project involving aircraft manufacturers Airbus and EADS, BEASY, and a number of research centers, investigated how the “find it and fix it” approach can be replaced by a more fundamental approach based on computer modeling of the corrosion process that has the ability to predict its behavior.
Until the end of the 1960s, no significant attention was given to corrosion in the aircraft industry; thereafter, manufacturers and engineers became increasingly aware of the impact of different types of corrosion on the lifetime of structures—the costs of which were estimated in the U.S. alone to be in the order of $2.2 billion per year in 2001. More recent data from the U.S. Department of Defense suggest the cost is $5.4 billion per year for the Air Force and $2.1 billion for naval aircraft. Nowadays, corrosion is generally recognized as a key factor in limiting the operational lifetime of aircraft due to uneconomical high maintenance.
The currently applied “find it – fix it” maintenance concept is a common approach against corrosion where corrosion findings have to be maintained independent of their impact. However, it is widely recognized that scheduled measures are more efficient in the mid and long terms. In this respect, the benefit of modeling for prediction and optimal design becomes clear.
Galvanic corrosion (GC) occurs when different materials are in contact with a common electrolyte. GC on its own can cause severe damage in an aircraft due to highly accelerated corrosion rates compared to other mechanisms. Usually GC can be avoided by proper material selection and appropriate corrosion protection measures. However, combinations of dissimilar materials are often applied due to structural requirements that need to be fulfilled in the design. In this case, costly corrosion prevention systems have to be applied to avoid the ingress of electrolyte. Often, GC alone may not be directly the main cause of structural failures, but its occurrence may favor proper conditions for initiation of other types of corrosion such as pitting.
The progressive advance of computational modeling in the last few decades has today made it possible to model a variety of complex corroding systems, thus representing a leading-edge technology not only for research in the subject but also for direct application in engineering design. At present, a computational modeling approach is one of the most effective tools for design and optimization purposes, as well as for failure detection, monitoring, and quality performance assessment.
For this case study, the computational model was developed based on the BEASY software, which has been used for a number of years to model corrosion control solutions in the marine, defense, and offshore oil and gas industries. Having validated the model through an extensive program of experimental testing, a series of tests were carried out to evaluate its performance on typical aircraft structures for the case of a structure submerged in a bulk electrolyte and for the case of a structure covered in a thin electrolyte film.
The case studies focused on the extent and rate of corrosion and how it was impacted by the geometry of the connections, the characteristic and extent of the electrolyte, and the type of mitigation methods employed. A typical structure representative of a modern aircraft design was studied, which consisted of a CFRP (carbon-fiber-reinforced plastic) stringer and an aluminum rib.
The modeling approach is very similar to that used for structural and stress analysis in that the geometry of the structure is divided into finite elements, each element representing the surface of the material in contact with the corrosive electrolyte. In fact, the same CAD-based tools can be used to create the corrosion model as those used for the structural applications.
Having developed the model of the aircraft structure, the user can select the type of environmental conditions the structure is expected to experience and predict the location of potential corrosion sites and assess the severity of the corrosion. The model can also be used to identify the location and extent of corrosion protection measures required against GC (e.g., coatings, paints, etc.) and can also contribute to the reduction of materials and process development costs, as well as predicting the long-term impact of coating degradation and damage.
The development of the technology is ongoing, with the aim of extending the applications to a wide range of structures and components such as those found in aircraft, automobiles, ground vehicles, ships, and similar structures.
Dr. Robert Adey, Director of Strategic Development, CM BEASY Ltd., wrote this article for SAE Magazines.