(191aa) The Effect of Various Sodium Chloride Concentrations On the Wettability and Surface Energy of Aluminum 7075

            Aluminum 7075 is commonly used in the skin of aircraft, due to its light weight and high strength.  However, environmental effects are known to induce deleterious effects and damage in metals.  Corrosion of aluminum, especially in areas of high concentrations of chloride ions, begins with pit formation and growth, which increase the local stresses around the tip of the pit.  In addition, corrosion can lead to the embrittlement of the aluminum surface, due to the diffusion of hydrogen released during the corrosive reaction.  This local stresses around the tip of the pit and the hydrogen embrittlement ultimately leads to the development and propagation of cracks, which can be further accelerated by the sodium chloride environment.  In environmentally assisted cracking, the surface energy changes as the crack grows, due to development of two surfaces from the original single surface.  Determining how the surface energy of aluminum 7075-T6 is affected by aqueous salt concentrations over time is highly important, especially for determining and modeling the underlying damage mechanisms.  Currently, continuum fracture mechanics can describe the overall damage behavior, but they lack describing the underlying damage mechanisms (for example, whether fracture is controlled by anodic dissolution, hydrogen embrittlement, or some combination of both).

           In the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University, the relationship between surface energy, time, and aqueous salt concentration is being examined both experimentally and through modeling techniques.  Contact angles over a range of aqueous sodium chloride concentrations, ranging from 0% - 5%, are measured.  In addition, the water droplets will be left for at least 30 minutes, to determine how the surface and the water droplet interact, through diffusion, chloride reactions, and wetting of the aluminum surface.  The changes observed in the resulting contact angles will be used to determine the surface energy of aluminum 7075-T6. Young’s Equation allows for the conversion of the measured contact angle values to surface energy, while Griffith’s criterion describes the creation of new free surfaces during crack extension.