ABSTRACT

This paper presents the results of an investigation in which several 300-series stainless steels (SS): AISI S30403 SS (UNS S30403), AISI 316L SS (UNS S31603), and AISI 317L SS (UNS S31703), as well as highly-alloyed: SS 254-SMO (UNS S32154), AL-6XN (N08367) and AL29-4C (UNS S44735), were evaluated using DC electrochemical techniques in three different electrolyte solutions. The solutions consisted of neutral 3.55% NaCl, 3.55% NaCl in 0.1N HCl, and 3.55% NaCl in 1.0N HCl. These solutions were chosen to simulate environments that are less, similar, and more aggressive, respectively, than the conditions at the Space Shuttle launch pads. The electrochemical test results were compared to atmospheric exposure data and evaluated for their ability to predict the long-term corrosion performance of the subject alloys. The electrochemical measurements for the six alloys indicated that the higher-alloyed SS 254-SMO, AL29-4C, and AL-6XN exhibited significantly higher resistance to localized corrosion than the 300-series SS. There was a correlation between the corrosion performance of the alloys during a two-year atmospheric exposure and the corrosion rates calculated from electrochemical (polarization resistance) measurements.

INTRODUCTION

Type S30403 stainless steel, (S30403 SS), tubing is used in various supply lines that service the Orbiter at the Kennedy Space Center, (KSC), launch pads in Florida (USA). 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 combustion reaction in the solid rocket boosters produces hydrochloric acid, (HCl). The acidic chloride environment is aggressive to most metals and causes severe pitting in some of the common stainless steel alloys. Type S30403 SS tubing is susceptible to pitting corrosion (Figure 1) that lead to cracking and rupture (Figure 2) of high-pressure gas and fluid systems.1 Such failures can be life-threatening to launch pad personnel in the immediate vicinity. Outages in the systems where failures occur can impact normal operation and shuttle launch schedules. The use of a better tubing alloy for launch pad applications would greatly reduce the probability of failure, improve safety, lessen maintenance costs, and reduce downtime. The objective of this work was to study the electrochemical behavior of several candidate corrosion-resistant tubing alloys to replace the existing S30403 SS tubing at the Space Shuttle launch sites. The stainless steel alloys chosen for this investigation were: S30403, S31603, S31703, N08367, S44735 and S32154. Type S30403 SS was included in the study as the control. These alloys were tested in three different electrolytes that provided less severe, similar, and more aggressive conditions, respectively, than those found at the launch pads at the Kennedy Space Center in Florida (USA).

MATERIALS AND METHODS

Samples

Table 1 lists the tubing alloys chosen for this investigation. Table 2 lists common name, UNS number, and chemical composition of each material. The specimens were flat sample coupons, 3.2 cm in diameter, supplied by Metal Samples Co. (Munford, AL). The test specimens were polished to 600-grit, ultrasonically degreased in a detergent solution, and wiped with acetone before testing.

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