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MATERIALS SELECTION & DESIGN FIGURE 1 TABLE 1
Degradation drivers and sensor availability for aircraft applications
Paint Degradation Drivers2
External stresses Internal stresses
Contaminants (e.g., fuel) Absorption/swelling Osmotic blistering
Increased permeability
Property degradation/aging Thermal breakdown UV radiation
Mechanical damage
Navy and Coast Guard aircraft operate in the most aggressive environments. Photo by Brandon Weeks.
FIGURE 2
Application issues (e.g., adhesion)
—conversion coating —primer —top coat
Metal Corrosion Drivers Time of wetness
Electrolyte concentration Temperature
No No No
Yes Yes Yes
This brings up the fact that airframe Schematic diagram of the galvanic corrosion process in a riveted structure.
dition of hydraulic systems, etc. More- over, the external surfaces of an aircraft are easily inspected visually, making condition-based inspections of internal compartments one of the prime focuses for corrosion sensors. The corrosion damage modes occur-
ring aboard aircraft are diverse and in- volve a variety of alloys. Currently, the major sensor focus is on aluminum air- frames typically constructed of AA7075 (UNS A97075) and AA2024 (UNS A92024) alloys, which are normally clad
NACE International, Vol. 51, No. 3 _Q\P I [IKZQÅKQIT bQVK ZQKP )) =6;
A97072) layer to drive corrosion laterally instead of deep into the structure. Types of corrosion to which these alloys are susceptible include pitting, stress corro- sion cracking (SCC), exfoliation, corro- [QWV NI\QO]M IVL KZM^QKM ÅTQNWZU IVL galvanic corrosion. Galvanic corrosion is I [QOVQÅKIV\ XZWJTMU QV ZQ^M\ML [\Z]K- tures; the process starts at stress-induced cracks in the protective coating around rivet heads (Figure 2). Moisture then en- ters the interface and the cell is complete.
structures are coated with various com- binations of conversion coatings, primers, and topcoats, which raises the question: Should we focus on detecting coating degradation or corrosion of the underly- ing metal? Sensing and repairing coating damage before the underlying metal cor- rodes can avoid more extensive repairs. A more conservative approach is to as- sume the coating system has preexisting defects and focus on conditions that corrode the structural alloy. Both of these approaches are currently being pursued and debated. Sensors for detecting metal corrosion are simpler and more mature, as are the models used for prognostics. In addition, coatings can be degraded by a large number of processes as listed in
March 2012 MATERIALS PERFORMANCE 57
Sensors No No No
Yes No
Yes No No No No