Corrosion is the natural process that takes place when elemental materials degrade to their lowest energy equilibrium states. Raw materials, such as bauxite or iron, are refined into aluminums and steels to improve their structural properties. If not adequately protected through coatings, anodization, plating, or other measures, these refined materials will begin to degrade from their higher energy refined states back to their naturally occurring oxidized states.
Corrosion adds a multibillion cost to the DoD each year, comprising approximately 25% of all maintenance costs and totaling nearly $20 billion annually. Corrosion initiates when a coating system fails, leaving a surface exposed to the environment. For the example of an aircraft, corrosion most commonly occurs at locations on the exterior surface where non-aluminum structural fasteners (rivets) are used to attach aluminum skins (typically 2024-T3) to the aircraft structural frame. After many operational hours, fatigue cycles can lead to cracking of the UV- embrittled coating, enabling water and atmospheric contaminants to permeate the damaged topcoat and porous primer, making the ingredients for corrosion available between the two dissimilar metals.
Once the inhibitors in the primer are fully expensed and the pretreatments are no longer providing further protection, the substrates are at the mercy of the environment and galvanic corrosion of the active substrate rapidly commences. This and other forms of corrosion result in costly repairs that must take place in order to maintain military readiness and ensure safety of crew members during operations.
Because of corrosion’s cost, prevention of corrosion results in millions of man-hours of labor each year, resulting in decreased military readiness due to platform downtime and extensive taxpayer expenditures. The use of a CBM+ approach to predicting corrosion based on extended levels of environmental exposure could eliminate many of the extraneous preventative maintenance actions performed by military maintainers, reducing environmental footprint and helping military leadership to better allocate resources to assets that are more likely to corrode than others.
Degradation of coatings is not necessarily just a function of time, but rather the sum total of all exposures that adversely impact the coating. The use of models can help quantify the degradation of a coating’s integrity as a function of the cumulative environmental and dynamic exposure that a coating has observed. This enables corrosion maintenance and inspection intervals to exit an interval-based approach that is a function of operational hours or elapsed time and enter a condition-based approach where maintenance actions are performed on the basis of need.
SERDP and ESTCP have a portfolio of projects that focus on environmental aspects of corrosion mitigation for weapons and platforms. For example, work undertaken in ESTCP project WP-201710 seeks to identify the point at which chromates in MIL-PRF-23377 are fully exhausted. Additionally, many DoD corrosion activities, service-specific activities, commercial, and academic activities focus on related technical aspects of this problem and should be utilized as much as possible to establish a DoD-relevant solution. This SON will aid in identifying the starting point at which chromate leaching begins during initial topcoat failure, and thus enables a data-driven approach to making smarter corrosion inspection actions by leveraging work in WP-201710.