Determination of the role of bicarbonate in the corrosion of aircraft lap splice joints

The extension of the service life of aircraft such as the KC-135 has led to concern about the effects of corrosion on airframe structures. Lap splice joints are a region of particular interest because their occluded nature makes detection of corrosion damage difficult and expensive. Research into the development of corrosion in these regions will provide information necessary for the maintenance of these aircraft for their remaining service life.; The lap joint simulant solution (LJSS) is a model environment for the corrosion and fatigue testing of lap joint alloys such as AA2024-T3. Exposure of model lap joints to the LJSS in the laboratory environment produced general corrosion on the faying surfaces. General corrosion is not well defined in the literature and often a precise definition is omitted from corrosion standards. By coupling microscopic evaluation of corrosion topography with image analysis techniques, a mathematical method was developed to describe general corrosion and classify corrosion topography on lap joints. Confocal Laser Scanning Microscopy (CLSM) was used to generate three-dimensional images of the corrosion surfaces. These images were then analyzed with surface metrology techniques such as the calculation of the autocorrelation function and the corresponding correlation length.; Corrosion topography within lap joints has been shown to depend on the carbon dioxide system (CO₂, HCO₃−, CO₃2−). Minimizing the carbon dioxide system changed the observed corrosion topography from general corrosion to pitting. The development of each of these corrosion topographies was determined over time using confocal microscopy and cross-sectional metallography. Metastable pits with diameters on the order of ten microns in diameter were found to form in the LJSS throughout twelve weeks of exposure. However, after three weeks of exposure, general corrosion began to form and dominated the development of corrosion for the rest of the exposure time. In the minimal CO_x environment, pits grew throughout the exposure time, reaching 100 microns in diameter after 12 weeks.; The carbon dioxide system impacted both the chemistry and electrochemistry of the corrosion environment. It serves as a buffer at pH 6.35 which increases the pitting potential of AA2024-T3 and prevents metastable pits from forming the acidified environment necessary for stabilization. The carbon dioxide system also decreased the cathodic kinetics which resulted in a decrease in the open circuit potential of AA2024-T3 in solutions containing the system. By decreasing the cathodic kinetics, the ability of the cathodic reaction to support continued pit growth was also reduced. In combination with the buffering effect, this reduction in the strength of the cathodic reaction led to a larger passive region for AA2024 exposed to the carbon dioxide system and prevented pit stabilization and growth.