New coatings may dramatically cut aircraft fuel tank repairs

  • Published
  • By Mindy Cooper
  • AFRL Materials and Manufacturing Directorate
WRIGHT-PATTERSON AIR FORCE BASE, Ohio --  Researchers with the Air Force Research Laboratory here developed a program to signicantly reduce costly repairs of aircraft fuel tanks.
 
By using a diethylene glycol mono-methyl ether (DIEGME)-resistant coating for the interior of aircraft fuel tanks, AFRL Materials and Manufacturing Directorate researchers said repairs will be required less frequently, resulting in dramatic cost savings. 

The research responds to an undesired side effect of fuel additives designed to prevent jet fuel from freezing in the fuel lines of high-flying aicraft.

In recent years, the Fuel System Ice Inhibitor (FSII) additive in military aircraft fuel has caused fuel tank topcoat peeling in which the protective coating on the interior surfaces of aircraft fuel tanks has delaminated. The problem has been observed in Air Force B-52, KC-135, C-17 and Navy P-3 aircraft. The FSII compound DIEGME is added to JP-8 and JP-5 jet fuels at concentrations of 0.1 percent to 0.15 percent.
 
"Because the DIEGME is more volatile than the JP-8 fuel, its vapors dominate the head space of the fuel tank in concentrations detrimental to paint, causing the fuel tank coating to swell and peel, which clogs the aircraft filters," explained Mr. Mike Spicer, Chief of the Coatings Technology Integration Office here, "This leads to additional costs and reduces aircraft availability due to the additional scheduled and unscheduled maintenance stops to clean paint chip contamination from fuel filters and tanks." 

Researchers wanted to identify a new protective fuel tank coating that is resistant to DIEGME. They collaborated with Applied Coating Technologies Incorporated, Eden Prairie, Minn., Axon Products Incorporated, Greenville, SC, NIC Industries Incorporated, White City, Ore., Boeing, Wichita, Kan., Naval Air Systems Command and the B-52 Program Office on the project. 

According to Spicer, the research effort, funded by Office of the Secretary of Defense Corrosion Prevention and Control Group and guided by AFRL, consisted of two phases. The first phase objective was to work with commercial coating manufacturers to identify a DIEGME-resistant fuel tank coating that was compatible with the current fuel tank coatings BMS 10-39 and AMS-C-27725. Another objective of this phase was to develop a test method for determining DIEGME resistance, as well as test parameters and requirements for inclusion into the AMS-C-27725 coating specification. 

"During this phase, three coatings were developed: FT-9-Y4, by Axon Products Incorporated; Phoenix Exo-Kote, by Applied Coating Technologies Incorporated; and CERAKOTE C556, by Micro Dyne, a division of NIC Industries Incorporated," Spicer said. 

In order to test the coatings, the following high-exposure conditions were selected: three DIEGME to water ratios for immersion (70:30, 80:20, and 90:10) and four different environments (standard laboratory conditions, 170 degrees Fahrenheit, 200 degrees Fahrenheit and a cyclic environment ranging in temperature from 10 degrees Fahrenheit to 200 degrees Fahrenheit), Spicer said. 

The test coatings were applied to chrome conversion treated panels and allowed to cure for 21 days at standard laboratory conditions of 77 degrees Fahrenheit. Two ovens were allowed to stabilize at 170 degrees Fahrenheit and 200 degrees Fahrenheit. For the cyclic testing, a Thermotron was programmed to go from 10 degrees Fahrenheit to 200 degrees Fahrenheit in the span of four hours. The DIEGME to water ratios were prepared in quart-size mason jars and panels were placed inside them, with the lower half submerged in the liquid and the top half exposed to the vapors. The jars were sealed so that no vapors could escape. 

Spicer explained the effects of the solution and vapors were determined by monitoring the loss of coating thickness, hardness and adhesion to the panel. These were checked every two weeks for a period of eight weeks. The results showed that all three test coatings performed better than the BMS 10-39, with the FT-9-Y4 by Axon Products Incorporated performing the best. 

Researchers plan to capture the requirements for DIEGME-resistant fuel tank coatings under the AMS-C-27725A coating specifications. They selected the DIEGME to water ratio of 80:20 at 170 degrees Fahrenheit for a period of six weeks as the testing parameter for the specification. Future DEIGME resistant coating candidates will be tested within those parameters. 

Researchers have begun the second phase of the testing which involves all three test coatings being applied in the fuel tank of an active B-52 for further monitoring and testing. The coating will be evaluated at the one year mark to see if it is suitable for long term use. In the meantime, researchers will continue to search for funding to qualify the test coatings to AMS-C-27725, thus authorizing the use of these test coatings on DOD aircraft. 

According to Spicer, the use of a new, DIEGME-resistant coating on the interior of B-52 aircraft fuel tanks will dramatically reduce maintenance costs. Rather than refurbishing B-52 fuel tanks every four years, at a cost of $120,000 per aircraft, this new coating would require only a single application. With the current fleet-size, a projected aircraft service-life to the year 2040 and a repair cycle of every four years, the savings would be close to $90 million.