The 304L stainless-steel (304L SS) tubing is used in various supply lines that service the Orbiter at the KSC launch pads. The atmosphere at the launch site has a very high chloride content caused by the proximity of the Atlantic Ocean. During a launch, the exhaust products from the fuel combination reaction in the Solid Rocket Boosters include concentrated hydrochloric acid. The acidic chloride environment is aggressive to most metals and causes severe pitting in some of the common stainless-steel alloys. The 304L SS tubing is susceptible to pitting corrosion that can cause cracking and rupture of both high-pressure gas and fluid systems. The failures can be life-threatening to launch pad personnel in the immediate vicinity. Outages in the systems where the failure occurs can affect the safety of Shuttle launches. The use of a new tubing alloy for launch pad applications would greatly reduce the probability of failure, improve safety, lessen maintenance costs, and reduce downtime losses.

The objective of this investigation was to study the electrochemical behavior of 10 corrosion-resistant tubing alloys to replace the 304L SS tubing at the Space Shuttle launch sites. The alloys included 317L, 316L, 2205, C-276, 625, 254 SMO, C-2000, AL-6XN, AL29-4C, and 2507. For comparison purposes, 304L SS was included in the study. The specimens were flat panels of various sizes obtained by cutting the tubes and flattening them. Three electrochemical techniques were used to evaluate the performance of the materials: corrosion potential, polarization resistance, and cyclic polarization. The samples were placed in an electrochemical cell designed to expose a metal surface area of 1 square centimeter to the electrolyte solution. These solutions emulate conditions that are less aggressive than, similar to, and more aggressive than those found at the launch pads at KSC.

Analysis of the electrochemical data showed that the nickel-based alloys C-2000, C-276, and 625, along with the iron-based alloys 254 SMO and 2507, exhibited a corrosion resistance superior to the other alloys included in this investigation. The 304L, 316L, and 317L alloys exhibited the lowest resistance to corrosion. No conclusive cyclic polarization data were obtained for AL29-4C, but the corrosion potential and the polarization resistance measurements indicated an intermediate resistance to corrosion among the alloys tested. AL-6XN and 2205 also showed intermediate resistance to corrosion.

A comparison between the results from electrochemical measurements under conditions emulating those found at the launch pad and those from long-term atmospheric exposure revealed a strong correlation between the predictions based on cyclic polarization measurements and actual performance under atmospheric exposure. It was also determined that the area of the hysteresis loop was the best parameter to predict the actual performance of the tubing alloys.

Contacts: Dr. L.M. Calle (Luz.Calle-1@ksc.nasa.gov), YA-C2-T, (321) 867-3278; and L.G. MacDowell, YA-C2-T, (321) 867-4550
Participating Organization: Dynacs Inc. (R.D. Vinje)