ADVANCED MATERIALS & PROCESSES | APRIL 2025 16 sites. As illustrated in Fig. 5, a corrosion pit was located at the center of one such site. In most cases, the initiation sites contained pitting. These pits were located along a thin layer containing higher oxygen content, as demonstrated by the darker surface layer in the backscattered electron micrographs. When probed using energy dispersive x-ray spectroscopy (EDS), the pits were found to be comprised of primarily aluminum oxide, with elevated levels of chlorine, sodium, potassium, and calcium. In several pits, cadmium was identified, consistent with spalled coating material from the adjacent snap ring. The fatigue crack and retaining ring groove were cross-sectioned, mounted, and polished for metallographic examination. Figure 6 shows a typical cross-section displaying cracking at the outboard corner(s). These cross-sections also revealed corrosion pits on other parts of the ring groove surfaces, including some with cracks. However, the largest cracks were consistently observed at the outboard groove corners. The chemical composition of the actuator barrel housing was inspected using EDS and x-ray fluorescence. In all instances, the composition was consistent with 7075 aluminum alloy. The hardness of the housing was examined per ASTM E18, and the electrical conductivity measurements were examined per ASTM E1004[1,2]. The hardness and conductivity results were always within those expected for a T6 peak hardened temper (30.5 to 36.0 %IACS and a hardness exceeding 84 HRB, per AMS 2658)[3]. ANALYSIS AND DISCUSSION For each accident, hydraulic fluid pressure was lost after the pressurized hydraulic actuator barrel ruptured following fatigue crack propagation. These cracks initiated at corrosion pits along the retaining ring groove that was machined into the housing inner surface. While additional pits were observed along all the surfaces of the retaining clip groove, those with the largest cracks had initiated at corrosion pits along the outboard corner, which had been rounded. Along this corner, the pits would act as additional stress concentration sites, increasing the probability of fatigue crack initiation during service of the actuator during each pressurization cycle[4]. The cracks would have propagated under the cyclic loading from the pressurization and depressurization of the hydraulic fluid inside the actuator. As there was a layer of aluminum oxide along the crack initiation sites, there may have been corrosion processes aiding the rate of propagation[5]. There were also cadmium remnants present at some of the initiation sites, consistent with material that had spalled from the adjacent cadmium- coated retainer ring. The remnants would most likely separate from the steel retainer ring due to wear and fretting from vibrations incurred[6]. While the cadmium-coated steel retainer ring was in contact with the aluminum housing along the groove, the pitting was likely due to chlorine ions (Cl-) as significant amounts of Cl were detected from EDS examination of the pitting[7,8]. The difference in electrochemical potentials of cadmium and 7075 aluminum alloys are small and produce little driving force for galvanic corrosion[9]. However, contact with a bare steel part inserted into an aluminum alloy could create such driving forces[10]. The spalling of the cadmium coating from the steel snap ring, creating direct contact with the corresponding groove in the aluminum housing may exacerbate local corrosion rates, which could be another factor decreasing mean time between failures. As of this writing, the NTSB Materials Laboratory has investigated at least five hydraulic actuator failures, with additional failures having been documented outside the agency’s purview. The features from all these accidents were identical, namely crack locations, sizes, and physical features. In the cases examined by the NTSB, the fatigue cracks had initiated from the ring groove on the interior of the actuator housing and propagated outward into the housing. When the cracks had grown large enough circumferentially, the housings fractured longitudinally, relieving hydraulic fluid pressure and causing the actuators to fail in service. The actuators from these investigations, manufactured by Electrol, were installed on the Cessna 210 Centurion from 1960 to 1964. As the Electrol actuators are no longer produced, when one must be replaced, an aircraft owner either must locate an airworthy Electrol actuator or install Fig. 5 — Backscattered electron (BE) micrograph of a corrosion pit and aluminum oxide layer at a fatigue crack initiation site on the retainer ring groove. Fig. 6 — Bright field optical micrograph of a cross-section through an intact portion of the retainer ring groove, showing a fatigue crack propagating from the lower left corner.
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