Volume 6 Preprint 9
Electrochemical Corrosion Behavior of Tubing Alloys in Simulated Space Shuttle Launch Pad Conditions
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
L. M. Calle1, R.D. Vinje2, and L.G. MacDowell1
NASA, Mail Code YA-F2-T, Kennedy Space Center, FL 32899, USA,
2 ASRC Aerospace, Mail Code ASRC-15, Kennedy Space Center, FL
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.
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).
Materials and Methods
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.
Low carbon austenitic stainless steel
Molybdenum-containing austenitic stainless steel
Molybdenum-containing austenitic stainless steel
Superaustenitic stainless steel
Superferritic stainless steel
Austenitic stainless steel
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.
Chemical Composition of Stainless Steels Alloys
AL-6XN AL29-4C 254 SMO
UNS Number S30403 S31603 S31703 N08367 S44735
71.567 69.053 63.525 48.118
10.140 13.200 23.88
16.240 18.100 20.470
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.
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.
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
Results and Discussion
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
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
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
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
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
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 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
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
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
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.
Polarization resistance of SS alloys in 3.55% NaCl in various
concentrations of HCl
Critical breakdown potential and repassivation potential for SS alloys
3.55% NaCl in different concentrations of HCl
Ebd (mV) Erp (mV)
Ebd (mV) Erp (mV)
Ebd (mV) Erp (mV)
FIGURE 3. Cyclic polarization for AL29-4C in 3.55% NaCl-1.0N HCl
FIGURE 4. Cyclic polarization for 316L and 254-SMO in 1.0N-HCl
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
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.
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
Area of hysteresis loop for SS alloys in 3.55% NaCl with various
concentrations of HCl
Area of Hysteresis Loop (coulombs)
Negative Hysteresis No Hysteresis
Difference between Ecorr and Ebd for SS alloys in 3.55% NaCl with
different concentrations of HCl
Neutral 0.1N HCl 1.0N HCl
254 SMO 1106
PREN numbers for stainless steel alloys
Alloy 304L 316L 317L AL-6XN AL29-4C 254 SMO
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.
Visual observations of tube specimens after two years of atmospheric
Visual Observations after Two Years of Atmospheric Exposure
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
With Acid-Alumina Slurry
with pits and brown
Pits went through the
and pits all over
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
with slight discoloration
Slight discoloration of
Tubes in good condition
the tubes. Over all in
Better condition than 316L
Tube is in good
condition. Some spots
along the seam weld
Tubes look very good except
for pits on the seam weld
FIGURE 5. Tubing after two years of natural seacoast atmospheric
exposure (no acid rinse.
FIGURE 5. Photographs of tubing sections after two years of
seacoast atmospheric exposure with acid rinse every two
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
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
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,
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.
6 Z. Szklarska-Smialowska, M. Janik-Czacho, Corros. Sci. 11 12(1971):
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.