Corrosion at fasteners in aluminum presents a high risk and challenge to corrosion mitigation strategies. The objective is to model atmospheric galvanic corrosion and determine the stages of damage evolution for 2024-T3 around fasteners. The synthesis of three test methods, exposure testing, laboratory measurements, and software modeling, are used to increase understanding of corrosion modes and damage evolution in order to help mitigate risk for atmospheric galvanic corrosion from fasteners in aluminum.
An exposure study in Daytona Beach, Florida consisted of coupling aluminum coupons to metal fasteners for 24 months. Fastener metals used are 316L stainless steel, cadmium-plated low carbon 1018 steel, and cadmium-plated fasteners with Cd partially removed to simulate damaged plating. The corrosion damage modes at each stage are identified, and the impact of galvanic action is determined. A sequence of corrosion damage stages is developed and the transition through the stages is related to corrosion processes and their controlling factors. The results are related to corrosion mitigation for fasteners in Al.
Galvanic corrosion is studied in the lab through Mixed Electrode Theory and through analytical modeling. Data quantifies the impact of galvanic action from fastener metal on the corrosion of aluminum. The models provide visual and quantitative data to identify galvanic couples of higher risk. Models predictions are at time zero and without the influence of damage evolution or changing environments. Laboratory testing allows for samples to be damaged so corrosion modes can be identified. The combination of modeling for quantifying the galvanic impact and damage evolution from lab tests allow for more effective corrosion mitigation methods to be employed.
Models are becoming a valuable tool as user knowledge increases about the sensitivity of parameter inputs. Fasteners in aluminum, comparable to Daytona exposure tests, are used as examples for model applications. The effects of model inputs on the predictions of model outputs are analyzed for all inputs through quantified data. Models produce 3-D diagrams that highlight corrosion damage, making results easy to comprehend for non-subject experts and providing the ability to increase the effectiveness of material selection.
Knowledge pertaining to atmospheric galvanic corrosion from fasteners in aluminum is increased. Laboratory tests and software models correlated very well due to the controlling input parameter (polarization curves) for software modeling being generated from laboratory data. Exposure tested aluminum coupons go through a series of corrosion stages that include pitting, intergranular corrosion and stress corrosion cracking. A synergistic effect is generated when the three test methods, exposure testing, lab testing, and software modeling, are used in combination.
Keywords: Atmospheric galvanic corrosion, intergranular corrosion, stress corrosion cracking, mixed electrode theory, modeling, exposure testing, wedging, aluminum, 2024-T3, finite elemental analysis, coulometric reduction, rust, aerospace alloys