Failure of Jet Engine Fuel Control Unit Originated by Corrosion of Cadmium Coating
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CASE HISTORY—PEER-REVIEWED
Failure of Jet Engine Fuel Control Unit Originated by Corrosion of Cadmium Coating Mirko Sgambetterra
. Guido Zucca . Vincenzo di Francesco . Fabrizio De Paolis . Manuele Bernabei
Submitted: 12 February 2020 Ó ASM International 2020
Abstract Since 2015 the barometric fuel units of the aircraft jet trainer fleet were featured by an anomalous increase in the rate of failures, all caused by the presence of debris in the jet fuel system. Cadmium and sulfur based composition of debris observed in the fuel control unit revealed that they were originated by a reaction of jet fuel’s sulfur compounds with cadmium protection coating of few components in the unit. Analysis carried out on jet fuel samples did not find any value out of the specification and therefore further investigations were conducted on the cadmium coating process. Two fuel pumps were examined: a degradation of cadmium coating was observed on every components of both pumps, proportionally to their flight hours.A brown-yellow gelatinous deposit was found into the pumps components, mainly composed by hydrocarbons, cadmium, sulfur and carboxylic salts. The investigation on the cadmium surface treatment process revealed the anomalous absence of the chromate conversion coatings: it was responsible of cadmium availability in the fuel flow, causing the cadmium plating detachment that finally resulted in the solid particles production. Keywords Aircraft failures Chemical analysis Corrosion Engine failures Microanalysis
M. Sgambetterra (&) G. Zucca V. di Francesco F. De Paolis M. Bernabei Technology Materials for Aeronautics and Space Department, Italian Air Force, Flight Test Center, Military Airport M. De Bernandi, via Pratica di Mare, 00040 Pomezia, RM, Italy e-mail: [email protected]; [email protected]
Introduction A failure rate anomalous increase of the barometric fuel control unit (BFCU)1 was noted since the 2015. BFCU is an accessory of the fuel system in all of the jet engine aircrafts of Italian Air Force (ITAF)2 Training Fleet. Some malfunctions occurred in flight operation, such as ‘‘shut-off cock’’ stiffness, engine over speed, failure in engine startups and exhaust gas over temperature: they were all amenable to an incorrect fuel flow regulation, so to a BFCU failures. The investigations were initially addressed to verify the fuel quality respect to jet fuel specification [1]. Due to all examined samples were within the applicable specification limits, the investigation was also focused on BFCU. It revealed the presence of a gelatinous brownyellow smelly deposit, containing mainly sulfur and cadmium compounds and a large amount of debris, able to shrink the correct movement of the BFCU valves. The observed gelatinous deposit can be produced by reaction of cadmium of the inner surfaces treatment in the jet fuel system with sulfur compounds, such as thiols and organic sulfide, naturally present in aviation jet fuel [2]. The interaction of cadmium-coated steel with fuel, when spiked
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