ABSTRACT
The most effective means to control atmospheric corrosion of aircraft is through the use of protective coatings. 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 exterior coating systems that no longer meet protective requirements generate a significant volume of environmentally hazardous waste, which 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 use of the most durable coatings systems has often been limited by the ability to predict service performance in accelerated tests. Existing accelerated test techniques do not adequately employ the chemical, thermal, or mechanical stressors that produce relevant damage mechanisms, such as cracking at structural discontinuities in coated airframes. Additionally, single coating layers may be qualified individually rather than as part of a representative multilayer stack-up. As a result, current test methods cannot be used for accurate quantification of coating performance and service life. In this work, test methodologies previously described that employ combined environmental and mechanical loading modes are utilized to excite relevant failure modes of a multilayer system, such as coating cracking at sealant-filled lap joints. The mechanisms and kinetics of damage progression are quantified throughout static and dynamic atmospheric tests using in situ measurements of coating system properties. It is observed that the coating barrier properties and resistance to cracking at a lap joint are dependent upon both the individual effects of stress, temperature, and humidity as well as the combined interaction effects of these stressors.
INTRODUCTION
Atmospheric corrosion represents a significant threat to the reliability and maintainability of aircraft for the commercial and defense aerospace industries. It costs billions of dollars annually, increases demand on labor, degrades operational readiness, and is a safety issue. 1 Intact coating systems effectively protect structures from atmospheric corrosion, but with time, mechanical and environmental stresses will degrade and eventually breech the coating. When the coating is breeched, it must be repaired to protect the substrate from corrosion. In addition to costs, a significant volume of environmentally hazardous waste is generated during repair and replacement of coating systems. Hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) are needed, often in large quantities, for most remediation activities. The waste streams are associated with chemical strippers and dusts generated during removal and also with pretreatment chemicals. The amount of physical blast media and chemicals needed for stripping a single aircraft can be on the order of tens-of- thousands of pounds and several hundred gallons, respectively.
