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Similar and dissimilar joints of AA6061-T6 and AA5086 have been exposed in mild and severe marine, volcanic (with acid rain), rainforest, and arid atmospheric environments in the Hawaiian islands. The severe marine environment on the Marine Corps Base Hawai'i (MCBH) shows high humidity and rainfall and high chloride ion concentrations; whereas, the mild marine site on Coconut Island is characterized by a significant lower chloride ion concentration. Samples near the Kilauea Volcano on the Big Island are under exposure of acid rain, variable humidity, and sulfur dioxide plume. The rainforest at the Lyon Arboretum in Manoa represents the wet climate with high rainfall, high humidity and time of wetness, and low chloride ion concentrations. In contrast, the dry climate in Waipahu shows low humidity and rainfall, low time of wetness, and low chloride ion concentrations. The impact of the atmospheric environment on the severity of the corrosion has been studied in different welding zones of each type of sample. The focus was to show the dissilimarities in morphology of the parent material, the weld nugget, the heat affected zone (HAZ), and the thermo-mechanically affected zone (TMAZ). The microstructure and chemical composition of the different welding zones before and after the exposure to the environment have been characterized using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Analysis (EDXA). In addition to the field tests, laboratory tests have been performed to study galvanic corrosion between the different welding zones using the zero-resistance ammeter (ZRA) technique.

Galvanic corrosion of aluminum alloys coupled to non-passivating and passivating alloys exposed in corrosive environments can cause severe corrosion damage in marine structures. The aim of this study is the development of a finite element model using COMSOL Multiphysics to predict the galvanic interaction of aluminum alloy AA6061-T6 coupled to noble materials. The numerical model is based on anodic dissolution kinetics of AA6061 and cathodic kinetics of Ti6Al4V, 316 stainless steel, and copper determined with potentiodynamic polarization. The model predicts that the cathodic material and the geometry of the galvanic couple both influence the corrosion rate of AA6061, and that the solution composition locally changes over time. In order to account for the time-dependent change in solution chemistry a thorough experimental investigation of the effect of pH and chloride ion (Cl-) concentration on the corrosion kinetics of AA6061 is presented using potentiodynamic polarization. Corrosion kinetics were obtained in sodium sulfate and sodium chloride solutions as a function of pH and chloride ion concentration and implemented in the numerical model. The validation of the model is in progress and will be compared to data of actual galvanic couples exposed in the laboratory and in different atmospheric environments. Acknowledgement: This material is based on research sponsored by the USAFA and University of Hawaii under agreement number FA7000-18-2-0004. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon.