Failure of Jet Engine Fuel Control Unit Originated by Corrosion of Cadmium Coating

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.