Abstract
Organic coatings used in aerospace applications are the primary means of protecting structures from atmospheric corrosion in harsh environments. Corrosion often occurs in occluded areas buried within a structure, such as crevices at faying surfaces and fasteners, and may be difficult to measure directly. In addition to combating corrosion, which represents a risk to the safe operation of an asset, there are strong economic and environmental drivers to extend the service life of aerospace coatings. Repair and replacement of protective coatings that no longer meet performance requirements generate a significant volume of environmentally hazardous waste, that includes the coating material, media used for coating removal, as well as the waste materials generated in surface preparation and reapplication of the coating system. Development and selection of durable coating systems have often been limited by the ability to produce service-relevant failures in accelerated laboratory tests. Conventional coating testing relies on post-test analysis of corrosion damage and does not provide continuous measures of coating performance or alloy degradation. This makes quantifying interaction effects between environmental stressors (chemical, temperature, and relative humidity) and mechanical stressors (static and dynamic loads) to better understand the time-dependent degradation of the coatings and corrosion of the substrate difficult. To improve coating evaluations, a complete measurement system using electrochemical sensors and automated data acquisition has been developed to monitor coating performance and corrosion continuously during atmospheric tests. In this work, test methodologies that employ combined environmental and mechanical loading are used to excite cracking and corrosion failure modes of multilayer coating systems at sealant-filled lap joints and fasteners. Direct continuous measurements via electrode arrays are used to monitor the evolution of damage during tests. The kinetics of moisture ingress, coating cracking, and damage progression can be quantified during these mechanical tests performed under cyclic atmospheric conditions using in situ measurements of coating properties. A description of the sensors, electrochemical measurements, and methods for coating testing are reported along with the results of atmospheric tests using a range of conditions to produce coating degradation.
