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Isothermal Calorimetric Observations of the Effect of Welding on Compatibility of
Stainless Steels with High-Test Hydrogen Peroxide Propellant
Rudy Gostowski, PhD.
Marshall Space Flight Center, National Aeronautical and Space Administration, TD40,
Huntsville, AL
Introduction
High-Test Hydrogen Peroxide (HTP) is receiving renewed interest as a monopropellant
and as the oxidizer for bipropellant systems. HTP is hydrogen peroxide having concentrations
ranging from 70 to 98%. In these applications the energy and oxygen released during
decomposition of HTP is used for propulsion.
2 H202 (0 + 2 H20 (g) + 0 2 ( g ) AH^ = 2887.0 J g-' anhydrous HTP [ 11 (1)
However, incompatibility with structural materials will lead to decomposition of HTP in
areas of the propulsion system that does not result in useful energy production. In addition,
incompatibility may cause undesirable pressure and temperature rises and loss of capacity.
Compatibility is generally thought to be controlled by complex interactions of the surface area,
the chemical constituency and the surface finish of the material. Therefore, implementation of
HTP as a propellant requires testing to determine the compatibility of structural materials for
fabrication of the propulsion system. Compatibility has been expressed functionally with a series
of classes ranging from one to four with one being preferred [2]. Percent Active Oxygen Loss
per Week (%AOUwk) has been used to quantitatively express compatibility.
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%AOUwk= 1 0 0 (Wl C1- W2C2) / W1 C1 (2)
WI and W2 represent the initial and final masses of HTP solution respectively. C1 and C2 are the
initial and final HTP mass fractions. To relate these quantitative measurements to the functional
compatibility designations, metal samples having a %AOUwk of 55 are assigned [3] to Class 1,
while those having a value between 5 and 80 are considered Class 2. Class 3 materials have a
%AOUwk >80.
Isothermal heat-conduction microcalorimetry (IMC) permits measurement of the energy
released when "IF decomposes which indirectly provides %AOUwk data for the sample and
facilitates class ranking.
In propulsion systems components must be fabricated and connected using available
joining processes. Welding is a common joining method for metallic components. The goal of
this study was to compare the HTP compatibility of welded vs. unwelded stainless steel.
Experimental
Propulsion grade high-test hydrogen peroxide (go%, FMC, Inc), reagent hydrogen
peroxide (35%, Spectrum), nitric acid (70%, trace metal grade, Fisher Scientific) and sodium
hydroxide solution (50%, Fisher Scientific) were used as received. Stainless steel sheet (0.15 cm
thickness, CRES 316L and CRES 304L, Penn Stainless Products, Inc.) was cut into panels (10.2
x 10.2 cm). One panel of each alloy was Tungsten Inert Gas (TIG) welded with purge gas
(Helium), one welded without purge gas and one was not welded. The panels were milled into
coupons (3.8 x 1.0 cm). The dimensions of the coupons including the weld bead were
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determined using calipers and the surface area estimated (Table 1). The samples were then
evaluated with a Surface Roughness Tester (Surftest 402 Series 178, Mitutoyo) yielding the root
mean square of the deviations of the surface profile from the mean line (Rq) and the maximum
peak to valley height (Rmax). Results are also listed in Table 1. The coupons were cleaned in
acetone and detergent and rinsed with deionized water. Coupons were passivated by soaking in
70% nitric acid for five hours, rinsed, soaked in 35% hydrogen peroxide for four hours, rinsed in
deionized water and air-dried. Molded borosilicate serum bottles (30m1, Wheaton Science
Products) were passivated by soaking in sodium hydroxide (10%) for one hour, rinsed, soaked in
nitric acid (35%) for one hour, soaked in 35% hydrogen peroxide for twenty four hours, rinsed in
deionized water and air dried.
An Isothermal Microcalorimeter (Model 4400, Water Bath Model 7238, Calorimetry
Sciences Corp, Provo, UT) was used to obtain heat flow measurements at 60 “c. The unit was
permitted to thermally equilibrate at the set temperature for 24 hours and then calibrated against
an internal resistance heater standard. Three measurement cells were evaluated against a
reference cell holding a sealed vial containing deionized water (20mL). Aluminum closures
were used with trifluoroethylene (TFE)-faced silicone liners for the reference and sample vials.
Background heat flows (PB, pW g-’ anhydrous HTP) were measured for three vials containing
only hydrogen peroxide (90%, 20mL). The anhydrous HTP mass was taken as 90% of the total
fluid mass. Heat flows were constant over the 50-hour observation interval.
Vials were opened, metal sample coupons added and recapped. In all cases the heat flow
quickly attained a thermal equilibrium state ( 4 5 hours) followed by a slow rise to a peak value
(Ps+B, p W g-’ anhydrous HTP) within 70 to 320 hours and subsequently diminished. The
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background value (PB) representing a sum of the homogeneous HTP decomposition and the
heterogeneous HTP decomposition on the surface of the vial was subtracted from the peak heat
flow (Ps+B). The resulting value (Ps, pW g-' anhydrous HTP) corresponded to the maximum
heterogeneous HTP decomposition on the surface of the coupon.
As shown in Equation 2, PS was converted to a first order rate constant (k, s - I ) by
division with the heat of reaction for the decomposition of hydrogen peroxide (hHr = 2887.0 x
lo6 p W s g -') [l]. Using Equation 3 the resulting first order rate constant was converted from
reciprocal seconds to %AOUwk with the results listed in Table 1.
%AOL/wk = (6.048~10~) k
Results
Table 1
Surface area, finish of samples and percent active oxygen loss per week*
Material Joining Method Area (cm') Surface Roughness Maximum
%AOUwk
Rmax ( P d C Rq ( P I d
CRES 3 16L No weld 10.31M.02 0.97d.25 0 .3M.09 5 .9d .5
CRES 304L No weld
CRES 3 16L TIG, He purge 9.364.14 3.66A.02 1.07M.19 34.h10.0
CRES 304L TIG, He purge 9.434.04 2.29k1.02 1.83k1.11 6 8 . 2 4 1.5
CRES 3 16L TIG, No purge 9.4oLo. 1 1 32.26k14.02 15.42k2.39 36.1k4.1
10.3oJt0.04 1.14M. 1 8 0.33d.15 27.921.6
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CRES 304L TIG, No purge 9.46j.X). 1 1 106.93~61.21 17.8 k2.32 94.523.1
'Average of three coupons.
%O% confidence level.
'Maximum peak to valley height.
dRmt mean square of the deviations of the surface profile from the mean line.
Unwelded CRES 316L shows good compatibility and by the rating system is nearly Class 1.
CRES 304L is a Class 2 material. Welded CRES 316L and welded CRES 304L have
significantly less compatibility than their unwelded counterparts with the CRES 304L falling
beyond the upward limit of Class 2 materials. When welded without purge gas the CRES 316L
is not significantly different from the metal welded with the gas. However, CRES 304L welded
without purge gas has a much higher %AOUwk and would be considered a Class 3 material.
Discussion
As mentioned previously, compatibility is determined by the surface area, the chemical
constituency and the surface finish of a material. In this investigation exposed area is obviously
not at factor as the welded samples had a slightly smaller surface than the unwelded, but were
more reactive. The chemical makeup of welded CRES 3 16L and welded CRES 304L have been
observed in the literature to change from the parent material as chromium and iron are
segregated in zones [4-61. In particular, the ratio of chromium to iron [6] in CRES 316L
increased from 0.26 to 0.79 in the heat affected zone (HAZ) of the weld and to 1.52 in the weld
bead itself. In CRES 304L the ratio of chromium to iron increased from 0.28 to 0.44 in the HAZ
and to 0.33 in the weld bead. It is possible that the increased reactivity of the welded samples
and of those welded without purge gas is due to this segregation phenomenon. Likewise the
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reactivity increased in keeping with the greater roughness of the welded and welded without
purge gas samples. Therefore enhanced roughness may also be responsible for the increased
reactivity.
Conclusions
In summary, "IF reactivity increased due to welding concurrently with formation of segregation
zones and increased roughness. These effects were even greater when purge gas was not used.
Causality between these factors while reasonable was not established and their fractional
contributions to reactivity were not determined.
Acknowledgments
The author is grateful to Bill Stanton (Marshall Space Flight Center, Metallic Materials Group)
for welding the various specimens and to Yvonne Villegas (NASA-Undergraduate Student
Research Program), Genne Nwosisi (NASA-Minority Programs) and Jennifer Baldridge (NASA-
Undergraduate Student Research Program) for their help in preparation and execution of the
IMC analysis.
References
[ 11 W.C. Schumb, C.N. Satterfield and R.L. Wentworth, Hydrogen Peroxide, Reinhold,
Baltimore, 1955, p. 249.
[2] W.C. Schumb, C.N. Satterfield and R.L. Wentworth, Hydrogen Peroxide, Reinhold,
Baltimore, 1955, p. 163.
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I
,
[3] Bulletin #104: Materials of Construction for Equipment in Use with Hydrogen Peroxide,
FMC, 1966, p. 48.
[4] T. Takalo, N. Suutala and T. Moisio, Metall. Trans A. 1OA (1979) 1173.
[5] J. Foulds and J. Moteff, Metall. Trans. A. 13A (1982) 173.
[6] M. Ahmad, K.A. Shoaib, M.A. Shaikh and J.I. Akhtar, J. Mat. Sci. 29 (1994) 1169.
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