Aging Aircraft

KC-135 Tanker
The KC-135 is the workhorse refueling tanker for the USAF. Based on the Boeing 707, the average age of the -135 fleet exceeds 40 years.	Corrosion in occluded regions (such as this lap joint) is difficult to detect, costly to repair, and results in decreased readiness.

Our work in aging aircraft corrosion dates to 1995 when we were approached by Dr. Bob Piascik of NASA/LaRC about the determining the chemical composition inside fuselage lap joints on the KC-135 which was suffering increasing amounts of corrosion without a means to estimate the rate of degradation. This work led to development of a corrosion model and recent work on validating its predictions. In addition, we have worked on test method development for corrosion protection compounds (CPC), as well as the effects of environmental spectra on subsequent fatigue crack growth rate. Along with Profs. Rick Gangloff and John Scully at UVa, we are assisting the USAF in the incorporation of corrosion effects into the Aircraft Structural Integrity Program (ASIP), the process by which structural damage in all of the USAF fleets are managed.

Measurement of the Chemical Composition of Solutions Inside Aircraft Lap-Splice Joints Developed During Service (NASA/LaRC)

K.S. Ferrer, R.G. Kelly, “Development of an Aircraft Lap Joint Simulant Environment,” Corrosion, 58(5): 452-459, 2002.

Abstract: The aging of aircraft fleet has led to significant concern about the development of corrosion. Occluded regions such as lap splice joints, which are difficult to inspect, are of particular importance. By analyzing the corrosion products found in these joints, a simulant solution has been developed. The lap joint simulant solution (LJSS) contains 20 mM sodium chloride (NaCl), 4 mM sodium bicarbonate (NaHCO3), 4 mM sodium nitrite (NaNO2), and 2 mM sodium fluoride (NaF) at pH 9. Exposure of AA2024-T3 (UNS A92024) to the LJSS results in a similar corrosion topography to that found on aircraft in the field.

Pit aspect ratio versus time for LJSS with and without bicarbonate system, showing the maintenance of pits in the absence of carbonate. In the standard LJSS, initial pitting is overcome by more generalized corrosion.

In-Situ, High-Resolution Probes of Organic Coating Failure

(O.M. Schneider, J. M. Williams)

	Time ----à
Confocal Microscope stage for electrochemistry	Delamination of organic coating and corrosion of intermetallic followed in real time with the CLSM. Total elapsed time was ca. 50 h.
Imaging of coating blister surface with CLSM.	Microelectrode EIS (MEIS) on and in coating blister on AA2024-T3
SRET image of blister on epoxy-coated AA2024-T3 undergoing anodic undermining of coating. Blue indicates net anodic activity, whereas red indicates net cathodic activity.	SRET image of blisters on epoxy-coated AA2024-T3 undergoing cathodic delamination of coating. Blue indicates net anodic activity, whereas red indicates net cathodic activity.

Corrosion Prevention Compounds: Test Method Development (S&K Technologies)

F. Gui, R. G. Kelly, “Performance Assessment and Prediction of Corrosion Prevention Compounds with Electrochemical Impedance Spectroscopy,” accepted for publication, Corrosion, June, 2004.

(Abstract): The performance of corrosion prevention compounds (CPC) on AA7075-T6 was assessed with electrochemical impedance spectroscopy (EIS). The good correlation between the protection performance of CPC and both the interfacial impedance and double layer capacitance allowed two assessment criteria to be defined; Excellent protection was exhibited by CPC-coated surfaces with interfacial impedances above 0.1 Mohms-cm² or double layer capacitances below 7.6 x 10^-8 F/cm². A correlation between double layer capacitance (C_dl) and corroded area was also obtained. Thus, one can also evaluate CPC performance by calculating the corroded area from the measured capacitance. In this way, CPC performance could be ranked not only by the time period of full protection, but also by the rate at which the degradation spreads. In addition, a prediction method was demonstrated based on impedance parameters that showed the feasibility of using data from less than 50 days to predict the performance of CPC after 180 days exposure. It was found that the CPC failure can be greatly accelerated without changing the relative ranking among the CPC used by introducing intentional scratches on CPC-coated specimens. In particular, the minimum time needed to rank various CPC was reduced to five days for scratched specimens from several months for unscratched ones. Other impedance parameters, such as coating capacitance and resistance did not correlate with performance for any of the CPC studied.

Environmental Spectra and CPC Effects on Corrosion/Fatigue (NIA/NASA/AFRL)

M. P. Ciccone, R. P. Gangloff, R. G. Kelly, “Test Environment Selection for Corrosion-Fatigue Testing of Aluminum Alloy 7075-T6,” submitted to Corrosion, July 2004

Abstract: The appropriateness of a variety of test environments was determined for studies of interactions between corrosion and fatigue crack propagation (FCP) in the context of damage tolerance and durability. Because many fatigue cracks in aerospace structures involve materials in occluded geometries, measurements included occluded region potential as a function of time, boldly exposed region polarization scans, corrosion morphology comparisons, and pH. The alloy of interest was aluminum alloy (AA) 7075-T6 (UNS A97075). The test environments included four common ASTM test solutions for localized corrosion susceptibility of aluminum alloys and the lap joint simulant solution (LJSS), which was designed to reproduce in-service corrosion morphologies. The use of the ASTM standard solutions severely limited the range of potentials that can be probed during FCP testing. The LJSS was more passivating, resulting in the ability to probe a wide range of potentials without inducing localized corrosion on the boldly exposed surface. Occluded region corrosion morphology and pH measurements near the mouth of the crevice were dependent on the severity of the bulk solution. However, at interior sections, all the solutions’ pH tended toward LJSS behavior, that is, mildly alkaline.

pH maps for isolated, occluded lap joint exposed to EXCO for 235 h.	Corrosion attack at mouth (left) and in the interior for exposure to LJSS for 235 h
pH maps for isolated, occluded lap joint exposed to EXCO for 235 h.	Polarization behavior for AA7075-T6 in various solutions. Note large potential window available in LJSS as opposed to no potential window in others.