Case inner diameter Spider pitting investigation, Process Simulation Article (PSA)

摘要

Case inner diameter (ID) pitting in the acreage has been noted during the refurbishment process since early in the SRM/HPM phase of the shuttle program. Corrosion under cured insulation has long been thought to be caused by the use of Methylchlorform (TCA). TCA can cause different acids (HCL and Nitric) to be formed during NBR insulation cure at elevated temperature and pressure (TWR-61501). More recently noticed is a corrosion/pitting phenomena (spider pitting) which causes extensive pitting of case hardware and appears black with legs and a large pit in the center - hence the term spider pitting. Pits have been recorded with the deepest being .O65 in. Process Simulation Article - 9 (PSA-9) was used to simulate the actual insulation process utilizing insulation processing parameters and associated materials. The different insulation lay-up processing anomalies for PSA-9, were developed by PTP0032/TWR-64407, and included scenarios (thin CHEMLOK, excessive TCA, TCA and water, no CHEMLOK 205, scratches in the chemlok surfaces) thought to cause pitting and corrosion in D6AC steel. The 32 witness panels accompanied the PSA-9 article, incorporating testing scenarios which could not be tested on flight hardware, including different quantities of acid, acid and water, localized charring of the insulation, and different inclusions under the insulation. All the data except for long-term aging test panels 24, 25, 31, and 32 are documented in this report.