10.7 Orbiter Corrosion

¶Removing and replacing Thermal Protection System tiles sometimes results in damage to the anti-corrosion primer that covers the Orbitersʼ sheet metal skin. Tile replacement often occurs without first re-priming the primed aluminum substrate. The current repair practice allows Room Temperature Vulcanizing adhesive to be applied over a bare aluminum substrate (with no Koropon corrosion-inhibiting compound) when bonding tile to the Orbiter.

¶A video borescope of Columbia prior to STS-107 found corrosion on the lower forward fuselage skin panel and stringer areas. Corrosion on visible rivets and on the sides and feet of stringer sections was also uncovered during borescope inspections, but was not repaired.

¶Other corrosion concerns focus on the area between the crew module and outer hull, which is a difficult area to access for inspection and repair. At present, corrosion in this area is only monitored with borescope inspections. There is also concern that unchecked corrosion could progress from internal areas to external surfaces through fastener holes, joints, or directly through the skin. If this occurs beneath the tile, the tile system bond line could degrade.

¶Long-Term Corrosion Detection

¶Limited accessibility renders some corrosion damage difficult to detect. Approximately 90 percent of the Orbiter structure (excluding the tile-covered outer mold line) can be inspected for corrosion.²⁵²⁵ Julie Kramer, et al., "Minutes from CAIB / Engineering Meeting to Discuss CAIB Action / Request for Information B1-000193," April 24, 2003. CAIB document CTF042-00930095. Corrosion in the remaining 10 percent may remain undetected for the life of the vehicle.

¶NASA has recently outlined a $70 million, 19-year program to assess and mitigate corrosion. The agency fore- sees inspection intervals based on trends in the Problem Resolution and Corrective Action database, exposure to the environment, and refurbishment programs. Development of a correlation between corrosion initiation, growth, and environmental exposure requires the judicious use of long-term test data. Moreover, some corrosion problems are uncovered during non-corrosion inspections. The risk of undetected corrosion may increase as other inspections are removed or intervals between inspections are extended.

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¶Observations:

¶O10.7-1 Additional and recurring evaluation of corrosion damage should include non-destructive analysis of the potential impacts on structural integrity.

¶O10.7-2 Long-term corrosion detection should be a funding priority.

¶O10.7-3 Develop non-destructive evaluation inspections to find hidden corrosion.

¶O10.7-4 Inspection requirements for corrosion due to environmental exposure should first establish corrosion rates for Orbiter-specific environments, materials, and structural configurations. Consider applying Air Force corrosion prevention programs to the Orbiter.

10.8 Brittle Fracture of A-286 Bolts

¶Investigators sought to determine the cause of brittle fractures in the A-286 steel bolts that support the wingʼs lower carrier panels, which provide direct access to the interior of the Reinforced Carbon-Carbon (RCC) panels. Any misalign- ment of the carrier panels affects the continuity of airflow under the wing and can cause a "rough wing" (see Chapter 4). In the end, 57 of the 88 A-286 bolts on Columbiaʼs wings were recovered; 22 had brittle fractures. The fractures occurred equally in two groups of bolts in the same locations on each wing. Investigators determined that liquid metal embrittlement caused by aluminum vapor created by Columbiaʼs breakup could have contributed to these fractures, but the axial loads placed on the bolts when they separated from the carrier panel/box beam at temperatures approaching 2,000 degrees Fahrenheit likely caused the failures.

¶Findings:

¶F10.8-1 The present design and fabrication of the lower carrier panel attachments are inadequate. The bolts can readily pull through the relatively large holes in the box beams.

¶F10.8-2 The current design of the box beam in the lower carrier panel assembly exposes the attachment bolts to a rapid exchange of air along the wing, which enables the failure of numerous bolts.

¶F10.8-3 Primers and sealants such as Room Temperature Vulcanizing 560 and Koropon may accelerate corrosion, particularly in tight crevices.

¶F10.8-4 The negligible compressive stresses that normally occur in A-286 bolts help protect against failure.

¶Observations:

¶O10.8-1 Teflon (material) and Molybdenum Disulfide (lubricant) should not be used in the carrier panel bolt assembly.

¶O10.8-2 Galvanic coupling between aluminum and steel alloys must be mitigated.

¶O10.8-3 The use of Room Temperature Vulcanizing 560 and Koropon should be reviewed.

¶O10.8-4 Assuring the continued presence of compressive stresses in A-286 bolts should be part of their acceptance and qualification procedures.