L. M. Calle, R.D. Vinje and L.G. MacDowell
Keywords: 304L, 316L, 317L, 254-SMO, AL-6XN, AL29-4C, stainless steel, acidic NaCl, DC measurements, atmospheric exposure
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Volume 6 Paper C013 Electrochemical Corrosion Behavior of Tubing Alloys in Simulated Space Shuttle Launch Pad Conditions L. M. Calle1, R.D. Vinje2, and L.G. MacDowell1 NASA, Mail Code YA-F2-T, Kennedy Space Center, FL 32899, USA, Luz.M.Calle@nasa.gov 2 ASRC Aerospace, Mail Code ASRC-15, Kennedy Space Center, FL 32899, USA 1 Abstract At the Kennedy Space Center, NASA relies on stainless steel (SS) tubing to supply the gases and fluids required to launch the Space Shuttle. 300 series SS tubing has been used for decades but the highly corrosive environment at the launch pad has proven to be detrimental to these alloys. An upgrade with higher alloy content materials has become necessary in order to provide a safer and long lasting launch facility. In the effort to find the most suitable material to replace the existing AISI 304L SS (UNS S30403) and AISI 316L SS (UNS S31603) shuttle tubing, a study involving atmospheric exposure at the corrosion test site near the launch pads and electrochemical measurements is being conducted. This paper presents the results of an investigation in which stainless steels of the 300 series, 304L, 316L, and AISI 317L SS (UNS S31703) as well as highly alloyed stainless steels 254-SMO (UNS S32154), AL-6XN (N08367) and AL29-4C (UNS S44735) were evaluated using direct current (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 an environment that is less, similar, and more aggressive respectively than the conditions at the Space Shuttle launch pads. The electrochemical results were compared to the atmospheric exposure data and evaluated for their ability to predict the long-term corrosion performance of the alloys. Keywords: 304L, 316L, 317L, 254-SMO, AL-6XN, AL29-4C, stainless steel, acidic NaCl, DC measurements, atmospheric exposure. Introduction 304L stainless steel (304L SS) tubing is used in various supply lines that service the Orbiter at the Kennedy Space Center (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 produces hydrochloric acid. The acidic chloride environment is aggressive to most metals and causes severe pitting in some of the common stainless steel alloys. 304L SS tubing is susceptible to pitting corrosion that can cause cracking and rupture of both high-pressure gas and fluid systems.1 The failures can be life threatening to launch pad personnel in the immediate vicinity. Outages in the systems where the failure occurs can create schedule impact to normal operation and shuttle launches. 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 losses. The objective of this work was to study the electrochemical behavior of corrosion resistant tubing alloys to replace the 304L SS tubing at the Space Shuttle launch sites. The stainless steel alloys chosen for this investigation were: 304L, 316L, 317L, AL6XN, AL29-4C and 254 SMO. 304L SS was included in the study for comparison purposes. These alloys were tested in three different electrolytes that provided less severe, similar, and more aggressive conditions than those found at the launch pads at the Kennedy Space Center in Florida (USA). 2 Materials and Methods Alloys Table 1 lists the tubing alloys chosen for this investigation. Table 2 lists trade name, UNS number, and chemical composition of each material. The specimens were flat sample coupons, 3.2 cm in diameter, from Metal Samples Co. The test specimens were polished to 600-grit, ultrasonically degreased in a detergent solution, and wiped with acetone before testing. TABLE 1 Alloys Alloy Class 304L Low carbon austenitic stainless steel 316L Molybdenum-containing austenitic stainless steel 317L Molybdenum-containing austenitic stainless steel AL-6XN Superaustenitic stainless steel AL29-4C Superferritic stainless steel 254 SMO Austenitic stainless steel Experimental A model 352 SoftCorrTM III Corrosion Measurement System, manufactured by EG&G Princeton Applied Research, was used for all electrochemical measurements. The equipment includes the software that is designed to measure and analyze corrosion data. The electrochemical cell (flat cell) included a saturated calomel reference electrode (SCE), a platinum-on-niobium counter electrode, the working electrode, and a bubbler/vent tube. The specimen holder in the cell is designed such that the exposed metal surface area is 1 cm2. 3 TABLE 2 Chemical Composition of Stainless Steels Alloys Alloy 304L 316L 317L AL-6XN AL29-4C 254 SMO UNS Number S30403 S31603 S31703 N08367 S44735 S31254 Fe 71.567 69.053 63.525 48.118 66.594 55.162 Ni 8.200 10.140 13.200 23.88 0.260 17.900 Cr 18.33 16.240 18.100 20.470 28.750 20.000 Mo 0.500 2.070 3.160 6.260 3.780 6.050 Mn 1.470 1.780 1.510 0.300 0.260 0.490 C 0.023 0.019 0.017 0.020 0.020 0.012 N 0.030 0.050 0.030 0.330 0.031 0.196 Si 0.380 0.280 0.460 0.40 0.280 0.350 P 0.030 0.027 0.027 0.021 0.023 0.019 S 0.0002 0.001 0.001 0.0003 0.002 0.001 Cu 0.460 0.150 0.200 Co 0.340 0.680 0.240 Nb 0.290 Ti 0.360 Three different aerated electrolyte solutions were used: (1) 3.55% NaCl, (2) 3.55% NaCl–0.1N HCl and (3) 3.55% NaCl–1.0N HCl. These solutions emulate less than, similar to, and more aggressive conditions than those found at the launch pads at KSC. 4 Corrosion potential, linear, and cyclic polarization data were gathered for the alloys under the three different electrolyte conditions. Polarization resistance determinations were generally based on ASTM G59. Cyclic polarization data were gathered using ASTM G 61as a guideline. Duplicate and triplicate tests had essentially the same outcome. The reported results are the averages of two or more runs. The corrosion potential was monitored until the sample reached a potential that was stable within ±5 mV for a period of 10 minutes. Linear polarization measurements were performed immediately after. A potential range of ±20 mV versus open circuit potential was used for these measurements. The scan rate was 0.166 mV/sec. A linear graph of potential (E) versus current (I) was obtained and the polarization resistance (Rp) calculated. Cyclic polarization measurements were started at –250mV relative to the corrosion potential (Ecorr). The scan rate was 0.166mV/sec. The scans were reversed when the current density reached 5mA/cm2. The reverse potential scan continued until the potential returned to the starting potential of –250 mV relative to Ecorr. A graph of E versus Log (I) was obtained. From this graph, the breakdown potential (Ebd), repassivation potential (Erp), and the area of the hysteresis loop were obtained. Linear and cyclic polarization results were calculated using the SoftCorr III software. Atmospheric Exposure A beach atmospheric exposure site near the launch pads was used to evaluate the performance of the six alloys included in this study for their resistance to localized corrosion under atmospheric conditions similar to those at the launch pad. Three tubes of each alloy were exposed. A 10 percent (v/v) solution of HCl and 28.5 grams of alumina powder per 500 ml of solution was mixed into acid slurry to simulate solid rocket booster deposition. One set of tubes was sprayed every two weeks with the acid slurry to accelerate the corrosion effect. The other set was left exposed to the natural marine seacoast environment. Results and Discussion 5 Corrosion Potential Corrosion potential gives an indication of how noble a metal is in a given environment. In general, a more positive corrosion potential means that the metal can be expected to be more resistant to corrosion in that particular electrolyte than one with a more negative corrosion potential. Thus, most metals can be ranked according to resistance to corrosion based on corrosion potential. Stainless steels, as passive materials, can fluctuate from an active to passive state depending on the environment to which they are exposed, the velocity of the solution, and passivation treatments applied during manufacturing. The corrosion potential for each alloy was monitored from the initial time of immersion until a stable potential was observed. The alloys differed in the time it took for the potential to stabilize. For simplicity, Figure 1 shows only the open circuit potential for the SS alloys at times just prior to and during stabilization. Table 3 lists the average value of the stable open circuit potential. Contrary to what was expected based on the composition of the alloys, the highly alloyed SS 254-SMO, AL6XN and AL29-4C did not exhibit a more noble stable potential when compared to 316L and 317L in 3.55% NaCl (Figure 1a). This behavior did not correlate with the performance of the tubing samples exposed to the atmosphere at the corrosion test site. As it was expected, 304L was the most active alloy in this environment with a stable corrosion potential of –173 mV vs. SCE. In 3.55% NaCl-0.1N HCL, the three highly alloyed SS displayed a more noble potential than the 300 series SS as it was expected based on their composition. This behavior became more pronounced when the concentration of HCl in the 3.55% NaCl solution was increased to 1.0N. Figure 1c shows a clear distinction between the more noble behavior of the highly alloyed SS in the 3.55% NaCl-1.0N HCl and the more 6 (a) (b) 7 (c) FIGURE 1. Corrosion potential of SS Alloys in (a) neutral 3.55% NaCl, (b) 3.55% NaCl-0.1N HCl, and (c) 3.55% NaCl-1.0N HCl. active behavior of the 300 series SS. The ennoblement of the higher alloyed SS as the concentration of HCl in the 3.55% NaCl solution increased was most pronounced for AL29-4C (269 mV increase in the corrosion potential) followed by AL-6XN (153 mV increase) and 254 SMO (144 mV increase). This behavior correlated very well with the actual corrosion performance of the alloys under atmospheric exposure. The transition toward a more active corrosion potential of the 300 series SS as the concentration of HCl in the electrolyte increased can be attributed to the fact that these SS are easily attacked by HCl because the passive film is not easily attained.2 Chloride (Cl-) ions are well known for their ability to attack SS by penetrating the protective layer at any discontinuity of the oxide film. The addition of HCl, a reducing acid, exacerbates the attack by interfering with the formation of the oxide film.3 Polarization Resistance Figure 2 shows linear polarization plots for SS 316L in 3.55% NaCl with increasing HCl concentrations (neutral (a), 0.1N (b), and 1.0N (c)). It is evident from the figure that the slope of the line is decreases as the acidity of the 3.55% NaCl solution increases. This behavior is indicative of the decrease in the polarization resistance. Table 4 summarizes the polarization resistance, Rp, values in neutral 3.55% NaCl, 3.55% NaCl- 0.1N HCl, and in 3.55% NaCl-1.0N HCl for all the alloys. The Rp values show that increasing the HCl concentration in the 3.55% NaCl solution resulted in a significant decrease in the Rp values of the 300 series SS. The decrease in the Rp values, indicative of an increase in the corrosion rate, in the presence of increasing concentrations of HCl, can be attributed to the fact that the protective layer of the 300 series SS becomes unstable. This is illustrated by the drastic decrease in Rp from 8 1.36 Mohms.cm2 in neutral 3.55% NaCl to 159 ohms.cm2 in 3.55%NaCl-1.0N HCl for 316L (Figure 2 and Table 4). Rp values for AL-6XN, AL29-4C, and 254 SMO in neutral 3.55% NaCl were approximately of the same order of magnitude as those for the 300 series SS. However, the Rp values for these alloys remained high as the concentration of (a) (b) 9 (c) FIGURE 2. Linear polarization curves for 316L in (a) neutral 3.55% NaCl, (b) 3.55% NaCl-0.1N HCl, and (c) 3.55% NaCl-1.0N HCl. HCl in the 3.55% NaCl solution increased. AL-6XN and 254 SMO showed a slight decrease in Rp as the concentration of HCl increased while AL29-4C exhibited no change in Rp after the initial slight increase. The lower corrosion rates of AL6XN, AL29-4C, and 254-SMO SS can be attributed to the presence of greater amounts of chromium, nickel and molybdenum that result in a more stable protective layer on the surface of the alloy. The low corrosion rate of AL29-4C, which remained fairly unchanged with the increased concentration of HCl, can be attributed to its high (28.750%) chromium content. Cyclic Polarization Cyclic Polarization measurements were performed in order to determine the tendency of the alloys to undergo localized (pitting or crevice) corrosion when placed in the electrolyte solutions. The resulting plot of the potential-current function is strongly indicative of the tendency of the material to undergo localized attack. In effect, the function traces a hysteresis loop, with the area of the loop indicating the amount of localized corrosion of the material. From the area value, it is possible to analyze the performance of the alloys. Hysteresis loop area values should be very small for alloys that are highly resistant to 10 localized corrosion. In this case, the reverse scan traces almost exactly over the forward scan.4,5 Two important potentials, also used to characterize the hysteresis loop, are the critical breakdown potential, Ebd, defined as the potential forward scan “knee” potential. Pitting is characterized by a rapid increase in current with a very small change in potential. Above this potential, pits initiate and propagate. The repassivation potential, Erp, is defined as the point where the reverse scan intersects the forward scan. At this potential, localized attack stops and the current decreases significantly past the passive current density. The more positive the value of Ebd, the more resistant the alloy is to initiation of localized corrosion. Also, the more positive the value of Erp, the more resistant the alloy to corrosion. 6 Values of Ebd and Erp for the SS alloys in the three different electrolytes are shown in Table 5. Cyclic polarization scans for three of the alloys included in this investigation are shown in Figures 3-5. Hysteresis loop area values are given in Table 6. Figure 3 shows a cyclic polarization scan for SS AL29-4C in 3.55% NaCl-1.0N HCl. In this case, the reverse scan traced almost exactly over the forward scan resulting in TABLE 3 Corrosion potentials of SS alloys in neutral 3.55% NaCl, 3.55% NaCl0.1N HCl, and in 3.55% NaCl-1.0N 3.55% NaCl Alloy Ecorr (mV) Neutral 0.1N HCl 1.0N HCl 304L -155 -122 -349 316L -102 -130 -320 317L -111 -150 -318 AL-6XN -125 12 28 AL29-4C -132 110 137 11 254 SMO -132 -48 12 no hysteresis. This is characteristic of an alloy that is highly resistant to localized corrosion. Figure 4 shows the overlay of the cyclic polarization scans for SS 316L and SS 254-SMO in 3.55% NaCl-1.0N HCl. The hysteresis loop area values for these two alloys are very similar under these conditions (5.58 and 5.98 coulombs respectively) indicating a high resistant to localized corrosion. However, because of the significance of Ebd and Erp, it is important to take also into account the position of the scan in the E vs. Log (I) diagram when analyzing cyclic polarization data. Values for Ebd and Erp are shown in Table 6. While the area of the hysteresis loop is very similar (Figure 4), the position of the scans in the plot is very different. The values of Ebd and Erp for 316L are –42 mV and –37 mV respectively, while the values for 254-SMO are 877 mV and 890 mV. These results indicate that 254SMO is a superior alloy in its corrosion resistance to localized corrosion when compared to 316L under the same conditions. Similar results were obtained for AL-6XN and AL29-4C. TABLE 4 Polarization resistance of SS alloys in 3.55% NaCl in various concentrations of HCl Rp (ohms.cm2) Alloy Neutral 0.1N HCl 1.0N HCl 304L 6.37x105 7.05 x105 2.00 x102 316L 1.36x106 4.80 x105 1.59 x102 317L 1.49x106 2.99 x105 1.93 x102 AL-6XN 1.40x106 1.18 x106 0.615 x106 AL29-4C 0.882x106 1.09 x106 1.09 x106 254 SMO 1.08x106 1.01 x106 0.782 x106 12 TABLE 5 Critical breakdown potential and repassivation potential for SS alloys 3.55% NaCl in different concentrations of HCl Alloy Neutral Ebd (mV) Erp (mV) 0.1N HCl Ebd (mV) Erp (mV) 1.0N HCl Ebd (mV) Erp (mV) 304L 366 -136 167 -153 -60 -58 316L 380 -143 135 -164 -42 -37 317L 622 -131 432 -91 -90 -89 AL-6XN 922 906 816 835 902 904 AL29-4C 964 964 818 N/A 878 N/A 254 SMO 952 939 825 831 877 890 FIGURE 3. Cyclic polarization for AL29-4C in 3.55% NaCl-1.0N HCl 13 FIGURE 4. Cyclic polarization for 316L and 254-SMO in 1.0N-HCl 3.55% NaCl SS AL29-4C is an alloy very resistive to localized corrosion as indicated by the very small hysteresis loop area in the cyclic polarization scan obtained in neutral 3.55% NaCl. The increase in the acid concentration of the 3.55% NaCl solution to 0.1N resulted in a negative hysteresis. A further increase to 1.0N in the concentration of the acid resulted in no hysteresis (Figure 3). SS AL-6XN and 254 SMO exhibited small hysteresis loop areas in the three electrolytes indicative of their resistance to localized corrosion in neutral and acidic 3.55% NaCl. Figure 5 shows the effect of increasing HCl concentration on the cyclic polarization scans of SS 304L. The scan in neutral 3.55% NaCl solution displays a higher corrosion potential as well as lower current density. When the HCl concentration was increased to 0.1N, the corrosion potential became more negative and the current density increased. The metal still portrays passive behavior where the voltage increases with small changes in current density. However, increasing the acid concentration to1.0N HCl affects the alloy more drastically. Past the 14 corrosion potential, the material experiences anodic dissolution and then repassivates over a small voltage range and rapidly experiences breakdown of the passive film at Ebd. Similar behavior was observed for the other 300 series SS. For these alloys, a decrease in the hysteresis loop area cannot be interpreted as an indication of increased resistance to localized corrosion. A decrease in the difference between Ecorr and Ebd has been associated with increased susceptibility to localized corrosion.7 Table 6 shows the values for the difference between Ecorr and Ebd for the SS alloys in the three different electrolytes. The values for the 300 series SS are lower than those for the higher alloyed materials and their decrease as the concentration of acid in the electrolyte increases is greater than for the higher alloyed materials. These results are in agreement with results from visual observations of the samples as well as with the atmospheric exposure data on the susceptibility to localized corrosion of these alloys. 15 FIGURE 5. Cyclic polarization scans for 304L in neutral, 0.1N and 1.0N HCl-3.55% NaCl solutions Pitting Resistance Equivalent Number It is well established that the pitting corrosion resistance of stainless steels depends mainly upon their chromium, molybdenum, and nitrogen contents. This resistance is evaluated empirically through the pitting resistance equivalent number (PREN) defined as PREN = (%Cr) + (3.0) x (%Mo) + (15) x (%N) where the percentage corresponds to the weigh percentage of the element in the alloy.8,9,10 PREN numbers for the alloys investigated are shown in Table 8. These values are in good agreement with the experimental results. 16 TABLE 6 Area of hysteresis loop for SS alloys in 3.55% NaCl with various concentrations of HCl Alloy Area of Hysteresis Loop (coulombs) Neutral 0.1N HCl 1.0N HCl 304L 22.96 11.36 10.42 316L 15.99 12.35 5.58 317L 33.12 23.53 12.58 AL-6XN 5.07 3.23 1.69 AL29-4C 5.23 254 SMO 5.11 Negative Hysteresis No Hysteresis 4.85 5.98 TABLE 7 Difference between Ecorr and Ebd for SS alloys in 3.55% NaCl with different concentrations of HCl Ebd-Ecorr (mV) Alloy Neutral 0.1N HCl 1.0N HCl 304L 514 401 287 316L 510 424 271 317L 810 730 221 AL-6XN 1081 895 950 AL29-4C 1129 757 746 254 SMO 1106 978 944 17 TABLE 7 PREN numbers for stainless steel alloys Alloy 304L 316L 317L AL-6XN AL29-4C 254 SMO PREN 19 26 31 46 40 43 Atmospheric Exposure The most important criteria of any laboratory test for localized corrosion is that it must rate alloys consistently with service performance in environments that cause localized corrosion. In this study, the laboratory results were compared to the two-year atmospheric exposure data. Detailed results of the atmospheric exposure have been previously reported elsewhere.11 Photographs of the tubes after one year of atmospheric exposure with no acid rinse are shown in Figure 5. Photographs of the tubes after two years of atmospheric exposure with biweekly acid rinse are shown in Figure 6. A photograph of SS 304L is not shown because the tube failed prior to the two-year evaluation and was removed from the test rack. A summary of the visual evaluation of the tubing test articles after two years of atmospheric exposure is summarized in Table 8. 18 TABLE 8 Visual observations of tube specimens after two years of atmospheric exposure Visual Observations after Two Years of Atmospheric Exposure Alloy Natural 304L Tubes in poor condition Tubes failed due to pitting. spots all over thickness of the tube Tubes in poor condition 2 out 3 tubes failed. with pits and brown Remaining tube in bad stains. Better than 304L condition with brown spots 316L With Acid-Alumina Slurry Rinse with pits and brown Pits went through the and pits all over 317L Brown spots and pits on 1 out 3 tubes failed. Pits and the tube. Better than brown spots all over the tube. Light browning of the Tubes look in good condition tube. with slight discoloration Slight discoloration of Tubes in good condition 316L AL-6XN AL29-4C the tubes. Over all in Better condition than 316L good condition. 254-SMO Tube is in good condition. Some spots along the seam weld 19 Tubes look very good except for pits on the seam weld 304L 316L 20 317L AL29-4C 21 AL-6XN 254-SMO FIGURE 5. Tubing after two years of natural seacoast atmospheric exposure (no acid rinse. 22 FIGURE 5. Photographs of tubing sections after two years of seacoast atmospheric exposure with acid rinse every two weeks. Conclusions 23 Electrochemical measurements of the six alloys indicated that the higher alloyed SS 254 SMO, AL29-4C, and AL-6XN exhibited a significantly higher resistance to localized corrosion than the 300 series SS. The stable corrosion potential values obtained in neutral 3.55% NaCl did not correlate with the performance of the alloys under natural seacoast atmospheric exposure. A correlation was found between the stable corrosion potential values obtained in acidic 3.55% NaCl and the corrosion performance of the alloys under atmospheric exposure with and without acid rinse. There was a correlation between the corrosion performance of the alloys during the two year atmospheric exposure and the corrosion rates based on polarization resistance values. The area of the hysteresis loop cannot be used as the sole criterion to predict susceptibility to localized corrosion. There was a correlation between the atmospheric exposure data and the susceptibility to localized corrosion that was predicted based on the difference between Ebd and Ecorr. These predictions were in agreement with the expectations based on the PREN calculated for the alloys. References 1 S. McDanels, Failure Analysis of Launch Pad Tubing, Microstructural Science, 25, p. 125-129 (1998). 2 Metals Hanbook, 13, p. 557, (ASM International, Metals Park, OH, 1987). 3 N.G. Thompson and J.H. Pager, DC electrochemical Test Methods, p. 57, (Houston, TX: NACE International, 1986). 4 W.S. Tait, Corrosion, 34 (6) (1978): pp.214-217. 5 W.S. Tait, corrosion, 35 (7) (1979): pp. 296-300. 24 6 Z. Szklarska-Smialowska, M. Janik-Czacho, Corros. Sci. 11 12(1971): p. 901. 7 J. Beddoes and J. Gordon Parr, Introduction to Stainless Steels (ASM International, Materials Park, OH, 1999, p. 83. 8 M.J. Matthews, Metall. Mater. Technol. 5 (1982): p. 205. 9 C.A. Clark, P. Gentil, P. Guha, “Development of Improved Alloy Duplex Steel, ed. J. Van Liere (The Hague, The Netherlands: Netherlands instituut Voor Lastechniek, 1986). 10 A.J. Sedriks, Corrosion 42, 7 (1986): p. 376. 11 R.G. Barile, L.G. MacDowell, J. Curran, L.M. Calle, and T. Hodge, “Corrosion of Stainless Steel tubing in a Spacecraft Launch Environment,” Paper No. 02152, Corrosion/2002, 57th Annual Conference & Exposition, April 7-11, 2002, Denver, Colorado. 25