MW-IR Reflectance Inspection Through Organic Coatings

Jack Benfer NAVAIR Jacksonville

ABSTRACT

NAVAIR Jacksonville and the Air force Research Laboratory, Materials Directorate, has led a successful Environmental Security Technology Certification Program (ESTCP) project to demonstrate/validate (Dem/Val) the reliability of nondestructive inspection employing an Infrared Reflectance Imaging Technique (IRRIT) to detect corrosion under typical standard aircraft coating systems. This project was initially funded by the Strategic Environmental Research & Development Program (SERDP) for the purpose of eliminating or minimizing pollution (hazardous materials, VOCs, etc.) from unnecessary paint removal operations on DoD aircraft. Concurrent Technologies Corp and Northrop Grumman Integrated Systems (Bethpage, NY) were the two key contactors developing the technique and performing program management and cost/benefit analyses.

INTRODUCTION

Surface corrosion on aluminum aircraft skins, near joints and around fasteners, is often an indicator of buried structural corrosion and cracking. Aircraft paints are routinely removed to reveal the presence of corrosion on the surface of metal structures, and the aircraft is subsequently treated and repainted. This process can be very expensive and time consuming, and results in the generation of air pollutants and process waste. A method was developed to detect the early onset of corrosion on metal substrates covered by protective coatings so that aircraft paints do not have to be stripped without cause. By employing nondestructive techniques to inspect the aircraft exterior/interior skin and structure without removing coatings, the amount of stripping and reapplication of coatings that occurs at the military rework facilities can be substantially reduced.

An advanced infrared (IR) spectral imaging (3.75-5.0 microns) camera system was employed. This system was used to Demonstrate/Validate the collecting of reliable technical data, while establishing cost and environmental benefits. The demonstrations/validations occurred on P-3 aircraft in 2006 at the Jacksonville Fleet Readiness Center and on B-52 and KC-135 aircraft at the Air Force’s Oklahoma City Logistics Center. A demonstration was previously performed on a Coast Guard HU-25 aircraft through coatings of greater than 8 mils with excellent results.

The inspection/corrosion analysis technique takes advantage of an optically transparent spectral band, or window, in most standard coating systems, within the Mid-IR range of the electromagnetic spectrum. Since corroded surfaces do not reflect IR energy as well as the non-corroded surfaces, the corrosion signal is suppressed for the corroded surfaces or areas, resulting in discernable and detectable dark areas of contrast on a monitor or LCD screen. The camera system (hardware and software) was optimized and performance boundaries established with feedback from both laboratory and field measurements. Spectral IR analyses of actual surface coating systems were also made to identify compatibility with camera configurations using pre-corroded laboratory standard specimens.

Figure 1 is a schematic showing how visible light is blocked by a coating’s pigments while IR energy passes through coatings with pigmentation. This phenomenon is due to the longer wavelength of the IR radiation.

Figure 1. Mid-IR radiation (3-5um) passing through an organic coating while visible light is absorbed and reflected by the coating system.

Figure 2 shows a visible image (standard photography) on the left of a painted JSTARS aircraft part that was corroded in service with limited visual corrosion. The image on the right shows the same part but using IR reflectance illumination and an IR camera as the detector. The dark areas indicate corrosion, as shown. Looking at the right side of Figure 2 it is observable that the black line and the green primer are transparent to the IR radiation, so much more corrosion can now be seen than is visible in the left-hand photograph.

Figure 2. Standard visual illumination on left and IR illumination on right

ESTCP IRRIT DEMONSTRATION AND VALIDATION AT FRC JACKSONVILLE, FL

The P-3 aircraft maintained at FRC Jacksonville, FL was approved as a demonstration platform by the ESTCP Program Office due to its age, and the IR characteristics of the current paint system lend itself to the IR inspection technique. The P-3 aircraft was selected as one of the demonstration and validation candidates because the coatings are stripped every five years to inspect for corrosion and re-coated after treatment. This allowed an analysis to determine potential environmental savings, as well as to conduct a comprehensive cost benefit analysis (CBA) to establish if the inspection process is acceptable from a cost and environmental perspective. Metrics studied included Hazmat savings in terms of Resource Conservation and Recovery Act (RCRA) waste, Volatile Organic Compounds (VOC), and chromate reduction, plus associated costs.

IR Painted Image

Visible Stripped Image

Corrosion Site Designation, 22W

Visible Painted Image

IR Stripped Image

Corrosion Site Designation, 22W

Figure 3. Showing the Navy team visually inspecting under the wing of a P-3 and the Northrop Grumman team using the IRRIT method with visual vs. IR comparisons

During the dem/val of the IR inspection process, the aircraft was inspected for corrosion by two independent teams. The Navy team inspected the aircraft visually for corrosion utilizing standard inspection practices per NA 01-1A-509. The Northrop Grumman team inspected the aircraft using the IRRIT process. Figure 3 illustrates the general process, which was to visually inspect the aircraft and record the corrosion sites, then to inspect the aircraft using the IRRIT process, and to compare both methods with the visual inspection of a stripped aircraft to validate the findings. As one example, the photographs at the bottom of Figure 3 were taken on an actual P-3 aircraft in the paint stripping hanger at Jacksonville. The upper left quadrant shows the corrosion as it is seen under the paint, prior to stripping using the IRRIT process. The upper right quadrant shows a standard photograph of how the corrosion looks after stripping off the organic coating system. The corrosion by the fasteners that could be clearly seen under the coating utilizing the IRRIT process could also be seen after stripping, hence validating the process. The two photographs in the lower quadrants were taken to show what the coated aircraft surface looked like prior to stripping with no apparent corrosion visible and what the stripped surface and corrosion looked like using the IRRIT process.

ENVIRONMENTAL AND COST CONSIDERATIONS

Using the example of the P-3, it was shown that 221,900 lbs of VOC’s and 2,954 lbs of chromates for a fleet of 100 P-3’s can be saved every 4 years if the life of the coating can be extended to an 8-year interval cycle. This would also result in a cost reduction of $13M for stripping and refinishing the fleet of 100 aircraft every 4 years. Other aircraft programs will have to review different condition-based maintenance scenarios for their specific aircraft, and establish their own environmental and cost savings projections.

CONCLUSIONS

The transitioning of this technology into the applicable DoD Logistics Commands should significantly and economically reduce pollution such as toxic solid/liquid waste and Hazardous Air Pollutants (HAPS). Once implemented by the DoD, this novel inspection technology will pave the way for condition-based maintenance of aircraft structures without the need to remove organic coatings to inspect for corrosion and other structural defects such as fatigue cracks around fasteners. Advantages include the reduction of hazardous waste and toxic air emissions as well as associated costs. Each weapon system program will be able to review the final engineering and cost data generated from the ESTCP Dem/Val to determine how the IRRIT system can be implemented on its specific weapon systems. The IRRIT demonstrated technique will be able to target and map specific areas that require maintenance due to corrosion, allowing for a migration from schedule-based to condition-based maintenance. This technology will significantly reduce pollution and costs by eliminating unnecessary coating stripping/recoating. Depot and field maintenance personnel will have a new and improved method to inspect for and assess surface corrosion while minimizing the removal of coating systems.

REFERENCES

Final Report SERDP Pollution Prevention Project 1137 Non-Destructive Testing of Corrosion under Coatings dated September, 2004- Authors: Program Manager John D. Weir, P.E. Principal Investigator, Dr. Don DiMarzio both of Northrop Grumman and Dr. Hugh Isaacs of Brookhaven National Laboratory Upton, N.Y.

ACKNOWLEDGEMENTS

The authors would like to sincerely acknowledge John Munyak, Kevin Cook, Allen Sinowitz of Northrop Grumman Technical Services and Mr. Chuck Pellerin of the SERDP/ESTCP Program Office for their support on this project. Additionally, the authors would like to thank Mr.John Speers, Major (Ret.), Mr. Brian Pollack, Major, (Ret), and Mr. David Allen, Master sergeant (Ret), for contractor program management and technical support through Wright-Paterson AFB, OH. Concurrent Technology Corporation employees Mr. Matthew Campbell and Mr. Scott McPherson are also acknowledged for their project management and cost/benefit analysis work.

ABOUT THE AUTHOR

Jack Benfer is the Senior Corrosion Engineer and Corrosion Control Program Manager for NAVAIR Jacksonville with 18 years of NAVSEA and NAVAIR engineering experience. He holds a Bachelor of Science Degree in Materials Engineering from Virginia Tech and a Masters of Science Degree in Materials Science & Engineering from the University of Florida. Mr. Benfer is an ITC certified thermographer and a certified member of the International Association of Corrosion Engineers. He has authored numerous technical engineering reports and presentations and is currently published within Vol. 13 of the ASM Handbook. Mr. Benfer provides In-service Engineering support to the FRC and fleet through his duties within NAVAIR and the Aging Aircraft IPT, he supports RDT&E programs through ESTCP, Y0817 and collaboration with the Naval Air Warfare Center. Corrosion Engineering acquisition support is also provided to the P-8A program office.

John Weir, P.E.: managed the technical effort for this ESTCP Project at the AEW/EW Technology Development Center in Bethpage, NY. He also managed the prior SERDP 2003 Pollution Prevention Award Winning Project. He is a registered professional engineer in the states of NY and PA with 34-years experience, supporting numerous aircraft and ship programs, as a materials engineering Subject Matter Expert (SME). He holds nine (9) U.S. Patents in the materials and process field. He has published over 70 presentations and articles, many of which were given at national and international conferences. Previously, John was the Chief Materials Engineer responsible for the A-10 Aircraft Program. He has served as past Chairman and Executive Director for the Metro-NY Chapter of SAMPE. He holds a Bachelor Degree in Engineering Science (B.E.S.), as well as a Masters Degree (M.S.) in Materials Science from Stony Brook University and an M.B.A. in Finance from Long Island University. Email: john.weir@ngc.com