Challenges in the Mitigation of Corrosion Damage From Galvanic Coupling Through Anodic and Cathodic Coatings in Complex Geometries

In complex material systems and geometries used in aerospace applications, the structural material is often less noble, causing it to experience accelerated corrosion damage when coupled with a more noble fastener. This phenomenon has been observed in a Pyrowear 675 (P675)/anodized titanium system where the anodized titanium fasteners induce pitting which leads to environmental-assisted cracking (EAC) on the P675 structure inside the fastener holes. It has also been observed on AA7075 structures with mechanical fasteners made of SS316 or Ti-6Al-4V set in them. The galvanic corrosion damage on these material systems present risks for the readiness and longevity of these structures. The objective of the work outlined in this dissertation is to assess methods to mitigate the EAC of P675, a high-strength stainless steel, by evaluating possible coating applications that may slow the corrosion damage and cracking observed in such a construction. This assessment is approached through a combined experimental and modeling framework to assess and optimize the mitigation strategy based on the application of a sacrificial anodic coating on the P675. The scope of work encompasses the modification of a commercial ZnNi plating, the sacrificial anode of interest, and the evaluation of kinetics for the ZnNi of varying compositions. These kinetics for ZnNi and those determined for anodized titanium are used for computational modeling (through finite element method) to evaluate their suitability to mitigate the EAC observed on P675. Validation of the modeling predictions is conducted through fracture mechanics testing. The application of a coating to the noble fasteners to slow down their cathodic kinetics and thus the induced galvanic corrosion on AA7075 is also considered. Two coatings applied to the cathodic materials are considered and the impact of wetting, defects, and an additional coating applied to the anodic material are considered.