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 protective coatings that no longer meet performance 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 selection of durable coating systems have often been limited by the ability to produce service-relevant failures in accelerated laboratory tests. Existing accelerated test techniques do not adequately employ the chemical, thermal, or mechanical stressors that produce damage mechanisms such as coating cracking at structural discontinuities in airframes. Additionally, individual coatings may be qualified separately rather than as part of a multi-layer system.
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 are used to excite relevant failure modes of multi-layer systems such as coating cracking at sealant-filled lap joints. The kinetics of moisture ingress, coating cracking, and damage progression are quantified throughout static and dynamic mechanical tests performed under cyclic atmospheric conditions using in situ measurements of coating properties. It is observed that the coating barrier properties and cracking are dependent on stress, temperature, and humidity as well as the interaction effects of these parameters.
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.
