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WADC TECHNICAL REPORT 55-109
CORROSION STUDIES IN FUMING NITRIC ACID
EDSON H. PHELPS
FREDRICK S. LEE, 1ST f.T.
RAYMOND B. ROBINSON
MATERIALS LABORATORY
OCTOBER 1955
WRIGHT AIR DEVELOPMENT CENTER
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FOMEJRD
This report was prepared by the Protective Processes Branch and
wasinitiated under Project No. 7312, "Finishes and Materials
Preservation",Task No. 73122, "Corrosion and Corrosion Prevention",
formerly RDO 611-13,"Corrosion and Corrosion Prevention" and was
administered under the di-rection of the Materials Laboratory,
Directorate of Research, Wright AirDevelopment Center, with Mr. E.
H. Phelps as Senior Project Engineer.
This report covers work conducted from June 1953 to April
1954.
WAX TR 55-109
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ABSTRACT
Tests were conducted to determine the corrosion behavior of
Armco 17-7PHstainless steel alloy in red fuming nitric acid
containing various amounts ofhydrofluoric acid as an inhibitor. It
was found that hydrofluoric acid addi-tUms to red fuming nitric
acid markedly reduced the corrosion rate of thealloy. Inhibition
was obtained in both liquid and vapor phases at 1200F and1600F, for
periods up to 42 days duration. It was found that a
hydrofluoricacid concentration of 0.25% by weight was marginal for
good inhibition, andthere was some indication that a hydrofluoric
acid concentration of 0.75% wasoptimum. The presence of glass in
the system diminishes the inhibiting effectof the hydrofluoric
acid.
The corrosion potential of aluminum and stainless steel
electrodes wasobserved in fuming nitric acids with various nitrogen
dioxide and water con-tents, and with added fluoride. It was found
that water additions shift thepotential of both aluminum and
stainless steel in the anodic direction. Ni-trogen dioxide content
up to 30% did not produce a significant change in thepotential-of
either electrode. Fluoride additions caused a definite anodicshift
in the stainless steel potential, and had a tendency to shift the
alu-minum potential in the cathodic direction. The observed effects
were cor-related in a qualitative manner with the previously
reported corrosion rateeffects of the variables tested.
PUBLICATION REVIEW
This report has been reviewed and is approved.l
FOR THE COMMANDER:
M, R. WhitmoreTechnical DirectorMaterials LaboratoryDirectorate
of Research
WADC TR 55-109
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TABLE OF CONT.NTS
Pap
TINTRODUCTION ... . . . . . . . . . . . . . .
Section I - Hydrofluoric Acid as a Corrosion Inhibitor for Armco
17-7PHStainliss Steel in Red Fuming Nitric Acid . . a . . o e . o
2
A. INTRODUCTION. o . . . . . .o o .o o 2
1. 3pecimen Preparation. . .0. . . . . . . . . a . p .o s .o s
22. rest Media. . . . . . . .* * 0 * * * * * * 0 e 43. Test
Containers . o . .0. . . . ..0. . . . . . . . . * . . 44.
Calculation of MPY Values ..... . .o..o. .4
C. DISCUSSION Z'D IMSULTS . .. . .. . ..... o o.• 41.
Uninhibited Corrosion Tests .,. . . .o * * o o o e . . o 42. Short
Time Tests with Inhibited Acid. .. 9 9 90 53. Longer Time Tests
with Inhibited Acid . ...... o...6
D. SUL•RY . .a. .o s .. . . . . . . a o e. e o * o o * 7
Section II - Potential Measurements in Fuming Nitric Acid * o e
o e o .o o
A. INTRODUCTION**., e o 9 9 s * o * a @ o e & o o s, o e *
8
B. EXPEnWTAI ROEDTULR CESD.. o S o. .... o..o..o .... 91. Test
Arrangement. . . . o .. .. 0 . . . .4. .: . .: . 0 92. Test
Procedure. . o . ..o. . . . ..0. . . .* 6 a . . 0103. Electrode
Preparation o . . .0. . . . . # o o # e * o .a o .04. Acid
Distillation Apparatus 99999. 99999oo 99 s10
C. RESULTS AND DISCUSSIONoo. .99 9. 99 .9 9 00 * . 9flo.oo s1.
Effect of Temperature 9.... .o99....9 9 90 , 012. Effect of
Fluoride Additions. .* *999 ooe9 # o .123. Effect of Water
Additions .a d i . . . .ee 34. Effect of Nitrogen Dioxide. . .o. .
. . . . 9 v e .# o o o145. Effect of Agitation . . o . 0 0 0 . . o.
. . . . . . . .o 146. Reproducibility of Measurements e o e e. . .
. . . . . . .147. Results with the Platinum Electrode . . . . . . .
. . . . .15
De SU)RY . e o . . . . . . . *. . . . . . . . . o . o . o . .
&15
BIBLIOGRAPHY... .o . e . .. .o . .. .. .. .o e.. .* * o o . .
.a. . . . .16
WADC TR 55-109 -dv-
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LIST OF ILLUSTRATIONS
Figure Page
1. Test Apparatus for Corrosion Tests, , , , o . . 182.
Corrosion Behavior of Armco 17-7 PH SS in Uninhibited R17A(12% N02)
. . .. . ........... aa00aa000a90 193. Corrosion Behavior of Armco
17-7 PH SS in Uninhibited RJNA(12% N02) . . .. .. 204. Effect of HY
Concentration on the Corrosion of 17-7 PH 8S in
RNA (12% NO 2 ) for One Day Tests at 160?F . * s o e a e o * e o
a * * 215. Effect of HF Concentration on the Corrosion of 17-7 PH
88 In
RINA (12M N02) for Two Day Tests at 12007 . e e . e * @ a e * .
a * * * 226. Corrosion Behavior 17-7 PH 3S in RIMA (12% NO2 )
Containing
0.75%FHF. • 0 ,* ** * • . * -.... 237. Corrosion Behavior of
17-7 PH 38 in RINA@(12%NO2 Containing*&0.75%[ HF**e*9..
0*0*28., Effect of HF Concentration on the Corrosion of 17-7 PH SS
in
R]NA (12% NO2 ) for a 20 Day Test Period at 12007. . . . . . . .
.. 259. Specimen Exposed to RIM for 4 DVs at 1200?........... .. "
26
10. Surface of Specimen Exposed to RiMA at 16007 for4 Days , , ,
, . 2711. Root of Crack on Specimen Exposed to R•NA at. 1200?. . ..
, .. . 2712. Diagram of CellArrangement , * . , , . . . .a . . . .a
.e .s o .2813. Diagra of Sitching ArraXeient.. .. ... .* e. . .
*
4. Diagram of Distillation Apparatus , , * * & a , . ,
,3015o Effect of Time on Potential at 210 C (4% N0 2 Acid), . . . .
. 3116. Effect-of Tim on Potential at 190 C in 12% N0 2 Acid 0* 0 *
* , . 3217. Effect of Time on Potential at 190C in 12% N02 Acid *
.0.000.3318& Effect of Time on Potontlal at 550 C in 1%N0 Acid
........ 3419. Effect of Fluoride on Potential at 190C in 1% NO
Acid* , , , , 3520, Effect of Fluoride on Potential at 19 0 C in
15% N52 Acid . . . . , . , 3621, Effect of Fluoride on "Coe.rt.al
at 190C in 15% NO.2 Acid. * * , . & . 3722, Effect of Fluoride
on Potential at 170C in 30% N02 Acid ,. . . . . 3823. Effect of HF
on Potential at 210 C in5% NO02 Acid. . . . ,,.3924e Effect of
Water on Potential at l9C in5% N02 Acid. .. 00 .. 040
VADO 7R 55-109 i'
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DITRDUCTION
Fuming nitric acid is a highly corrosive liquid which attacks
moststructural metals readily. The use of fuming nitric acid as an
oxidizer inliquid rocket engines has necessitated a considerable
amount of research onthe corrosion behavior of various metals in
the acid, and on methods of pre-venting the corrosion obtained.
This report covers two portions of the Materials Laboratory
program onfuming nitric acid corrosion. Section I covers an
investigation of hydroflu-oric acid as a corrosion inhibitor on
Armco 17-7PH stainless steel. This in-vestigation was prompted by
previous studies which established hydrofluoricacid as a very
promising corrosion inhibitor for both aluminum and austenitictype
stainless steels in fuming nitric acid. The Armco 17-7PH alloy was
usedbecause of its desirable high strength properties. Section 2
covers an in-vestigation of the solution or corrosion potential of
stainless steel and alu-minum in fuming nitric acid. The purpose of
these studies was to determineany possible correlation between the
observed potential and the previously re-ported corrosion rate
behavior of these metals under conditions of varyingnitrogen
dioxide and water contents, and with added fluoride.
WADC TR 55-109 -1-
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SECTION I
Hydrofluoric Acid as a Corrosion Inhibitor
Aruco 17-7PH Stainless Steel in Red Fuming Nitric Acid
(RFKA)
A. INTRODUCTION:
The purpose of this work was to determine the inhibiting effect
of bydro-fluoric acid additions on the corrosion rate of 17-7PH
stainless steel in redfuming nitric acid containing 12% nitrogen
dioxide.
Corrosion tests with uninhibited and inhibited RFNA were
conducted inclosed, Teflon lined pressure bombs at 1209F. The
effect of added HF was e-valuated primarily on the basis of weight
loss versus time curves. The cor-rosion weight loss data were
supplemnted by microscopic examination of thecorroded specimens.
The results presented are based on the initial HF con-centration in
the acid, since difficulties were encountered in
accuratelydeterminug the HF content after the corrosion test
periods.
This report covers results obtained on hardened 17-7PH stainless
steel,an alloy of interest because of its high strength
characteristics.
This investigation was prompted by previous studies (l,2)* which
indicatedthat bydrofluoric acid (HF) additions markedly reduced the
corrosion rates ofboth austenitic type stainless steel alloys and
aluminum alloys.
1. Specimen Preparation: The 17-7PH stainless steel used in the
cor-rosion tests was obtained from the Armco Steel Corporation,
Middletown, Ohio.The metal was received as .042" X 12" X 12"
samples in the annealed condi-tion. The analysis of the material,
as reported by Armco, is given in Table I.Sections 6" X 6" were cut
from the samples and given the following two stageheat
treatment:
(1) Held at 1400OF for 90 minutes - Air cooled at 2000F, Water
quenchedto 60-F.*
(2) Held at 1070F for 90 minuites - Air cooled.
The ,properties obtained with this heat treatment are given in
Table 1.After heat treatment the sections were sand blasted to
remove scale, andsheared into specimens 1cm X 3cm. Holes were
punched in the specimens formounting and identification marks
stencilled into the lower edges. Thespecimens were then degreased
with menthanol, washed with soap and water,
* Numbers in parentheses refer to listings in the
Bibliography.
WADC TR 55-109 -2-
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TABLE I
Analysis of Armco 17-7PH Stainless Steel Used inCorrosion
Tests
C - 0.073%Mn - 0.68P - 0.025S - 0.015Si - 0.47Cr - 16.99Ni -
7.20NO - 0.09Cu - 0.15Al - 1.08
TAN.E 2
Physical Properties of Amoo 17-7PH Stainless Steel after Heat
Treatment
Yield Strength* Tensile Strength % Elongation Rockwell C
Hardness
161,900 psi 192,400 psi 8%42
* All values are averages of three specimens cut from sheet
subjected to sameheat treatment as was given the corrosion
specimens.
WADC TR 55-109 -3-
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and abraded, .first with 180 grit and then with 240 grit alundum
paper to pro-duce a reproducible surface. After a final water wash
and degreasing operation,the specimens were weighed to the nearest
one tenth (0.1) milligram. The actualsurface area of the test
specimens was 6.66 square Centimeters. After the cor-rosion tests,
the specimens were rinsed with water and the corrosion
productsremoved with a rubber eraser. They were then rinsed with
water and menthanoland reweighed.
2. Test Media: All of the corrosion tests of this section were
conductedin a red funjing nitric acid containing 12% N02, obtained
from the General Chemi-cal Company in a 55 gallon aluminum drum.
Chemical analysis of this acid accord-ing to the procedures
outlined in W,-ADC Technical Report 53-6 showed that the acidas
taken from the drum contained 1.1% H20, 11.98% N02, 0.063% Al.
Hydrofluoric acid additions were made by pipetting measured
volumes of a 48%by weight solution of the HF into krown volumes of
the RFNA. The density of theRFNA was taken as 1.54 gms/cm3 and the
HF solution as 1.15 gms/cm3 . The HFconcentration figures given in
this report were calculated on the basis of gramsHF/gram RFHA X
100, and are therefore weight percentage of anhydrous HF.
3. Test Containers: All of the tests covered in this report were
conductedin sealed stainless steel bombs with Teflon liners. Two
size bombs ware used.The Teflon liners were machined from bar
stock. Kel-F sample holders were cutfrom sheet stock. Figur,: I
shows the large and small bombs, the Teflon liners,and the Kel-F
sample holders. The large size bombs permitted evaluation of
bothliquid and vapor phase corrosion. The liquid phase only was
evaluated in thesmall bombs. One hundred and twenty-five cc of acid
was used in the large bombsand 75cc in the small bombs. Since two
specimens were exposed to the liquidphase jn each case, the
specimen surface to volume of test media ratio was 0.160cm2/cm' in
the large bombs and 0.178 cm2/cm3 in the small bombs. The ullage
(rateof gas volume to total volume of container) was approximately
55% in the largebombs and 43% ix the small bombs.
4. Calculation of MPY Values: The following formula was used to
convertweight loss to corrosion rate in terms of mils per year:
MPY MW 1. 1 10j3A D _d1
Where W - weight loss of a specimenA - area of specimen (6.66
cm2 )D - time of exposure - drysd1- density of 17-7PH alloy - 7.69
gns/o3
C. DISCUSSION AND RESULTS:
1. Uninhibited Corrosion Tests: The corrosion behavior of the
17-7WPalloy in RFNA at 120°F was determined to provide a valid base
point for the
WADC TR 55-109 -4-
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inhibition tests. Weight loss versus time curves were obtained
by starting aseries of bomb tests at one time, and then removing a
single bomb from test atintervals for weight loss determinations.
The results of this type of test at160OF and 120.F are shown in
Figure 2 and 3. Each point is the average of dup--licate specimens,
exposed simultaneously.
Weight loss versus time curves have the advantage of showing
how, the cor-rosion rate varies with time. The slope of the curve
at any given time is thecorrosion rate at that time.
High corrosion rates were obtained in both liquid and vapor
phases at 1600F.The leveling off of the 1600F liquid phase curve is
attributed to build-up ofcorrosion products in the system. Linear
weight loss-time curves were obtainedin both liquid and vapor
phases at 120°F during the 8 day test period (Figure 3).The vapor
phase weight loss apparently remains low until after an incubation
periodof about 1 1/2 days.
The corrosion visually observed on the specimens exposed to
uninhibited RFNAat 120OF and 1600F was uniform in nature. A
photomicrograph ot the surface ofa specimen exposed to the liquid
phase for 4 days at 160"F is shown in Figure10. The corrosion
apparently proceeds in an intergranular manner. The alloyappeared
to be somewhat more susceptible to attack in the cold worked
areas(around stencil marks and close to the sheared edge). A thin,
non adherent whitefilm was visible on the specimens after they had
been removed from the acid,washed with water, and allowed to
dry.
Stress corrosion cracks occurred in the stencilled areas at
1200F, whenspecimens were exposed to either the liquid or vapor
phase. Practically everyspecimen tested at 120OF in uninhibited
acid was cracked to sane extent. Nocracks were found at 1600F,
probably because the excessive corrosion obtainedrelieved the
residual stresses in the stencil marks. A representative
crackedspecimen is shown in Figure 9, and a photomicrograph of the
root of one of thecracks is shown in Figure ll.
2. -Short Time Tests with Inhibited Acid: A series of short time
(one andtwo day) tests were conducted at 1206F and 160°F with a
range of HF concentration.The purpose of these tests was first to
determine the order of. magnitude of in-hibition to be expected,
and secondly, to determine if there was a concentratimrange over
which inhibition would be at a maximum.
The results of these tests, along with the uninhibited corrosion
rates, aregiven in Figures 4 and 5. It can be seen that HF very
effectively inhibits thecorrosion of the 17-7PH alloy at 120F and
1600F, in both liquid and vapor phases.The use of glass sample
holders nullified the inhibiting effect of the HF in theliquid
phase at 160.F (Figure 4). The HF probably reacts with the Olass,
forminga product which is non-inhibiting.
At 160"F, Figure 4, and HF concentration of 0.25% appears to be
insufficientto produce good inhibition. Progressively more
inhibition was obtained in theliquid phase as the HF concentration
was increased from' 0.50% to 1.5%. Theresults at 120OF (Figure 5)
give some indication that the inhibitor is mosteffective in the
concentration range of from 0.5 to l.%.
WADC TH 55-109 -5-
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At the completion of above tests, the appearance of the
specimens was essen-tially unchanged. Examination with A binocular
microscort, sh-Mi"d ! "i'ry slightetching type attack on the
surfaces. Very small, colorless, crystalline depositsat scattered
points were visible on some of the specimens. The specimens
testedwith the glass holders were covered with a heavy transparent
coarse deposit.The acid media were clean in all cases, whereas even
one day test with un-inhibited acid at 1600F produced the typical
dark discoloration.
3. Longer Time Test with Inhibited Acid: A group of experiments
were con-ducted to determine the inhibiting effect of the HF over
longer time periods.Based on the results of the short time tests,
an XF concentration of 0.75% wasused in most of the longer time
tests.
Weight loss-time curves in inhibited acid at 160°F and 120OF are
shown inFigure 6. The 160"F data shown were obtained with large
bombs, the 120OF datawith the small bombs. The W/A values for the
duplicate samples compared within5%, but some scatter between
samples in different bombs is apparent; especiallyweight loss and
time in both the liquid and vapor phase. This means that
thecorrosion rate in the inhibited acid decreases with increasing
time of exposure.The curves shown in Figure 5 indicate that
temperature does not have too mucheffect on the corrosion rate of
the 17-7PH alloy in inhibited acid.
The 1209F data were extended to a total time of 42 days in a
test in whichbombs were removed after 11, 21, 32 and 42 &dys of
test. The results are shownin Figure 7, along with the 8 day 120°F
results of Figure 6 for comparison.The 10-42 day weight loss time
curve was steady and corresponds to a corrosionrate of 1.7 MPY. The
corrosion rate was also constant in the vapor phase andcorresponded
to only 0.16 MPY. It can be seen that the best curve based on the8
day test results (dash line) does not coincide with the curve
through the10-40 day test results. This may have been due to fact
that the larger bombswere used in the latter test, whereas small
bombs had been used in the 8 daytest. Due to difficulties
encountered in the analysis of the inhibited acids,the HF
concentrations at the end of the above test are not known.
Experiments in small bombs extending over 21 days were conducted
with acidscontaining 0.25 and 1.25% HF. The results are presented
in Figure 8, along withthose obtained with 0.75% HF. The Ul day
test result with 0.25% HF definitelyindicates that this
concentration is border line for producing good inhibition.The
corrosion rate obtained with 1.25% HF is slightly higher than that
obtainedwith 0.75%.
Specimens exposed for the longer periods with 0.75% HF were
almost identicalin appearance to those described previously for the
one and two day tests. Athin, glassy, bluish film was visible over
portions of the vapor phase specimens.The specimens exposed to acid
containing 0.25% HF for li days were visiblycorroded. A thin, dark
green deposit was visible on the specimens exposed for21 days to
acid containing 1.25% HF.
Stress Corrosion cracks were found on only one of the specimens
exposed tothe inhibited acid. This specimen had been exposed to
RFNA containing 0.75% HFfor two days at 1600F. When this is
compared to the large number of cracksobtained in the uninhibited
acid tests, it is concluded that HF additions reducethe
susceptibility of the alloy to stress corrosion cracking in
RFNA.
MiADC TR 55-109 -6-
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SECTION II
Potential Measurements in Fuming Nitric Acid
A. INTRODUCTION:
The purpose of this work was to observe the corrosion potentials
of alu-minum alloy, 1100 (2S), and type 347 stainless steel in
fuming nitric acids ofvarious nitrogen dioxide and water contents,
and with fluoride additions. Itwas also the purpose of this
investigation to determine if there is any corre-lation between the
observed potentials and the reported corrosion behavior ofthe above
metals in fuming nitric acids of varying nitrogen dimxide and
watercontents, and with added fluoride.
Numerous investigations have been conducted on the resistance of
variousmetals to fuming nitric (1 4, 5, 6). Usually only the
corrosion rate isdetermined, since the suitability of the metal for
service depends on thisvalue. A number of investigations have also
included determination of theeffect of specific additives or
components of nitric acid on the corrosionrates of aluminum and
stainless steel alloys.
With stainless steel alloys, the following effects have been
reported:
a. Effect of water: In tests at 160"F with type 347 stainless
steel, thecorrosion rate decreased in a nearly linear manner from
270 mils per year(0PY) to 50 MPY as the total water content of
white fuming nitric acid wasincreased from 2% to 6% (7).
b. Effect of nitrogen dioxide (NO2 ): In tests at room
temperature withtype 347 stainless steel, a corrosion rate of
approximately 16 MPT was obtainedin acids containing 1% and 7% N02.
In an acid containing 23% ND2 the cor-rosion rate was 1 MP!
(8).
c. Effect of fluoride: Numerous investigations have shown that
fluorideis an exceptionally good corrosion inhibitor for stainless
steels in fumingnitric acids at all temperatures tested (1, 2, 9)
(see also Section I of thisreport).
The following effects have been reported on aluminum alloys:
a. Effect of water: In tests at 1600F with aluminum alloy, 3003
(3S), thocorrosion rate remained low in white fuming nitric acid
with total water con-tents up to about 6%. At water contents above
6% progressively higher cor-rosion rates were obtained (7).
In tests with aluminum alloy 1100 at room temperature, t-he
corrosion rateincreased in nitric acid with water content above
about 10% (10).
WADC TR 55-109
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Because of the difficulty in achieving reproducible values, a
comparisoncell was constructed parallel to the test cell. Any
variation of parameterswas always performed in the test cell.
The electrodes were supported in the fuming nitric acid from
transparentKel-F cell lids by means of Tygon gaskets. These gaskets
were replaced beforeeach run. The cell lids kept out moisture.
2. Test Procedure: The test cell and the comparison cell were
filled withthe fuming nitric acid purified by distillation via the
junction tubes fromthe junction cell by siphoning action. Zhis
procedure eliminated any bubbleformation in the junction tubes. In
the runs made at 550C, however, difficultywas encountered by bubble
formation in the junction tubes. On one occasion, apotential was
observed across the junction tubes of the order of 40
millivolts,Since both junction tubes were of the same length, a
constant error may havebeen introduced which did not affect the
trend of the observed potentials
The HF was produced in all runs but one by carefully placing
Mallinckrodt'sAR grade KF.2H20 into the test cell. This addition
was accompanied by anexothermic reaction in the evolution of some
NO2 fumes.
Different NO2 concentrations in the fuming nitric acid were made
by addingsolid N20 4 to the distilled, white, fuming nitric acid
cooled to OC. Thesolid N204 was prepared by cooling N02 to the dry
ice temperature. The NO2was approximately 99% pure as delivered
from the Nitrogen Division of the AlliedChemical and Dye
Corporation. No attempt was made toward further purification.
3. Electrode Preparation: The electrodes used consisted of
aluminumalloy 1100, type 347 stainless steel, and bright platinum.
The aluminum andstainless steel electrodes were conditioned in the
following manner-
a. Abraded with coarse emery followed by number "0" emery
paper.b. Washed in water.c. Washed in methyl alcohol.d. Dried and
pre-soaked in white fuming nitric acid for 7 hours.
It was found that the above pretreatment gave more consistent
potentialreadings than any others attempted. The pre-soak in WFVA
was particularlyimportant from the standpoint of obtaining
consistent readings. This treat-ment produced a dark film on the
stainless steel electrodes. Interferencecolors were noted on the
aluminum electrodes. The platinum electrode waswashed in water and
solvent prior to each experiment.
4. Acid Distillation Apparatus: Figure 14 shows the distillation
ap-paratus which served minly to remove metallic contaminants found
in thefuming nitric acid coming from storage. Before distillation
the acid waspretreated with a quantity of concentrated sulfumic
acid. Approximately 5%by weight was added which caused a yellow
green precipitate to form.
WADC TI 55-109 -10-
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The fuming nitric acid was then decanted and placed in the
distillation flaskwhere a pressure of 25 mm. of mercury was
acheived with a water aspirator.At this pressure the fuming nitric
acid boiled in the region of room temp-erature. The dry ice
condenser served in a dual capacity, vix., as a condenserand a trap
which prevented the frequent replacing of pressure tubing.
C. RESULTS AND DISCUSSION:
In this investigation, potentials of the following couples were
observed;type 347 stainless steel, aluminum alloy 1100, bright
platinum, and saturatedcalomel. The convention adopted was to write
the cathodic electrode to theleft and the anodic electrode to the
right, e. g., aluminum vs saturatedcalomel. In this system, if
current were allowed to flow, it would flow fromthe aluminum
electrode to the calomel electrode, and the observed potentialis
taken as a positive value.
The potential measurements made have been reduced to graphical
form inwhich the observed potential is shown as a function of time.
Ten typicalruns are depicted in Figures 15 through 24. The analysis
given at the bottomof each Figurp represents the initial condition
of the fuming nitric acid.These analyses may be somewhat in error
because of inherent difficulties inpresent analysis procedures.
In the following considerations, it is assumed that the observed
potentialmeasurement arises from the half cell potential of the
corroding metal andthe calomel electrode with its junction
potential. It is assumed that thecalomel and the potassium nitrate
solution - fuming nitric acid electrolytejunction potential remain
constant even though it is suspected that there maybe some change
as the composition of the fuming nitric acid in the test oellis
changed. In runs where additions to the test cell are made there ih
aothe possibility of a junction potential across the connecting
tube betweenthe test cell and the junction cell. This is also
assumed to be negligiblesince in every case the total concentration
change due to the addition issmall.
1. EFFECT OF TEMFRTRAE:
Figures 15 and 16 show the effect of temperature on the observed
potentialof the aluminum and the stainless steel electrodes. The
same fuming nitricacid was used in both runs. Change in temperature
had little effect on theobserved potential of stainless steel. With
aluminum, the potential wassteady at 210C, while a definite anodic
shift was obtained at 556C.
Figure 17 and 18 show the effect of temperature with an acid of
higherN02 content. Again, temperature had no significant effect on
the stainlesssteel potential. The anodic shift of the aluminum
potential at 55"C is notas pronounced as in the previous case.
WADC TR 55-109 -i-
-
An idea of the reproducibility of the data can be obtained by
comparingthe results on each pair of electrodes.
The anodic shift of the aluminum potential at the higher
temperature maybe indicative of film breakdown conditions (11).
This would correlate withthe higher corrosion rates obtained at
elevated temperatures. The factorsresulting in increased corrosion
rates of stainless steels at elevated temrrp-eratures apparently do
not affect the half cell potential.
2. EFFECT OF FLUORIDE ADDITIONS:
Figure 19 is a clear example of the effect of fluoride on the
observedpotentials of aluminum and stainless steel in fuming nitric
acid of low N02content. Figure 20 also shows the effect of fluoride
addition, but in thiscase the N02 content was higher and the
fluoride additions were made in in-crements of 0.2%0 0.4%, and 0.7%
HF which was produced by the addition ofKF.2H20 to the test
cell.
Figure 21 shows an attempt to duplicate and extend the work
shown inFigure 20 (15% N02 acid).
The measurements shown in Figure 22 indicate the results
obtained whenfluoride was added to an acid of very high NO2 content
(30%).
As the fluoride concentrations in the above runs were produced
by theaddition of pure solid potassium fluoride dihydrate (KF.2H2
0), a check wasmade to see if any anomalous results would occur if
hydrofluoric acid per sowere added. Figure 23 shows the results of
this addition and should be com-pared to Figure 19. It should be
noted that the electrolyte composition wasnot exactly the same in
each case. With stainless steel, similar resultswere obtained,
indicating that the source of the fluoride is not too impor-tant.
This conclusion was reached in a previous investigation (9)
whichincluded both weight loss and potential measurements. The
potential of thealuminum electrode showed little change as HF was
added, while a noticeablechange was observed in Figure 19. The same
discrepancy in the effect offluoride on the aluminum potential was
noted in th0 duplicate runs shown inFiggres 20 and 21, where the
source of fluoride was the same.
The corrosion inhibiting effect of hydrofluoric acid additions
in fumingnitric acids has been attributed to the formation of a
tightly adhprent filmof insoluble metal fluorides on the surface of
the metal (1). If this is thecase, fluoride would be expected to
function primarily atthe anodic areas of the corroding metal
surface, and should be classed as ananodic inhibitor. The normal
attributes of an anodic inhibitor are increasedpolarization of the
anodic areas, resulting in a decreased corrosion rateand a more
cathodic corrosion potential for the metAl (12).
WADC TR 55-109 -12-
-
A study of the effects of fluoride additions on the potential of
aluminumin fuming nitric acid indicates that in some cases a
definite cathodic shiftwas obtained, while in others fluoride had
little effect. A factor which mayexplain the above discrepancy is
the ability of aluminum to form an oxide film.With complete or
heavy oxide film formation, i.e., after izmersion in fumingnitric
acid, alulhinum would be expected to be under conditions of anodic
con-trol, and fluoride would not be expected to affect the
potential further.However under conditions of incomplete oxide film
formation, fluoride would beexpected to produce a cathodic shift,
as was obtained in some of the rufts.The condition of the oxide
film on the aluminum may also explain lack ofreproducibility
obtained with this electrode.
It has been noted that the effect of fluoride is to shift the
potentialof stainless steel anodically, which is inconsistent with
the normally ex-pected effect of an anodic inhibitor. However, it
is possible that the in-soluble fluoride film produced, limits the
cathodic area to the extent thata high cathodic current density is
obtained, resulting in a high degree ofcathodic polarization. It is
also plausible that the presence of a fluoridefilm would disturb
the auto-catalytic cathodic reaction cycle which is obtainedon
nobler metals in nitric acids. (13, 15, 16). These considerations
leadto the conclusion that the observed potential behavior is not
necessarily in-consistent with the assumption that fluoride
functions primarily at the anodicareas.
3. EFFECT OF WATER ADDITIONS:
The effect of water additions on the potential of aluminum and
stainlesssteel in an acid containing 5% NO2 is shown in Figure 24.
It can be seen thatthere is a definite anodic shift of the
potential of both metals with 5% ormore added water. Figure 24 also
indicates an anodic shift at the 3% addedwater level which is not
as pronounced as that with 5% added water.
The more anodic potential and reduced corrosion rate obtained on
stainlesssteel indicates that the effect of water additions on
stainless steel is pri-marily one of polarization of local
cathodes. This may be due to the factthat water represses the
reaction which account for the referenced autocauaytictype cathodic
reaction obtained in nitric acids. The dependence of the
self-ionization of nitric acid to nitronium ions and nitric ions in
nitric acid onthe water content (14) may also be a factor. *
The more anodic potential and the reported increase in corrosion
rate ofaluminum with added water lead one to believe that the
effect water additionshave on this electrode is one of anodic
depolarization. This corresponds withthe results of a previous
investigation of the polarization characteristics ofaluminum
electrode in fuming nitric acid at 1600F. Here it was shown
thataluminum was anodically polarized more easily at low and
intermediate watercontents (2 and 5% water) than at 9% total water
content (17).
WADC TE 55-109 -13-
-
4. EFFECT OF NITROGEN DIOXIDE:
An indication of the effect of NO2 on the potential of the
electrodes atroom temperature obtained is shown by comparing
Figures 15, 19, 23 and 24 (runswith acids containing 1 to 5% NO2 ),
with Figures 17 and 20 (runs with acids con-taining about 15% NO2
), and with Figures 22 (rur with acid containing 30% NO2 ).In runs
where additions were made only the comparison cells should be
con-sidered.
The results contained in the above mentioned figures indicate
that overthe time tested, NO2 concentration did not affect the
observed potential ofeither stainless steel or aluminum to a
significant degree.
With respect to stainless steel, the insensitivity of the
observed po-tential to NO2 content may be explained if one
envisions that as a result ofthe cathodic reaction process, a very
high N02 content exists at the surfaceof the metal. Under these
conditions, changes in bulk NO2 content would notgreatly change the
high NO2 region next to the electrode, and hence would notbe
expected to change the potential. The above considerations are
partiallyconfirmed by the results of corrosion tests on stainless
steels under flowingconditions (18). As the velocity of flow is
increased, a linear reduction incorrosion rate is obtained,
presumably due to a progressive sweeping agay ofthe lower oxides of
nitrogen next to the surface, hence breaking the autocata-lytic
cathodic reaction cycle. It may be that the reduction in corrosion
rateat high NO2 content reported in reference (8) is due to an N02
induced passivecondition (19), which was not obtained in these
experiments.
The effect of N02 content on the aluminum electrode is not clear
becausereportedly the corrosion rate of aluminum increases both
with NO2 content (1),and with increased turbulence (18), and yet no
significant effect on potentialwas noted as the NO2 concentration
was increased. More closely controlled andcoincident experiments
are required to establish the effect of NO2 on aluminumcorrosion in
fuming nitric acid.
5. EFFECT OF AGITATION:
During the course of the above work, several experiments were
conducted inthe various acids to determine if agitation produced by
a small magnetic stirrerhad any effect on the observed potentials.
It was found that the agitationproduced by the stirrer used had
little effect on the observed potentials.
6. i0roucipnIT OF NEASUMMTS:
In these experiments, stainless steel was found more
reproducible as anelectrode than aluminum. All readings on
stainless steel (comparison cells)fell within 40 millivolt of 1120
millivolts. Readings on the alumimm electrodesvaried from 720 to
460 millivolts, with the majority of readings falling in the700-750
millivolts range. This variation in the aluminum' half cell
potentialmay be accounted for by the condition of the oxide film as
mentioned previously.
WADC TR 55-109 -14-
-
7. RSLT I__H THE PLATINUM ELECTODE:
During the course of investigation of the aluminum vs calomel
and stainlesssteel vs calomel cells, observations were made on the
following cells existingin the apparatus shown in Figure 12:
PLATINUM VS. STAINLESS STEELSTAINLESS STEEL VS. ALUMINUMPLATINUM
VS. ALUMINUMPLATINUM VS. SATURATED CALOMEL
In all measurements it was possible to calculate the potential
observed in thecell, stainless steel vs aluminum, from the aluminum
vs calomel and stainlesssteel vs calomel couples as well as from
the platinum vs aluminum and platinumvs stainless steel couples. On
this evidence, a platinum electrode could beemployed in work of
this type instead of the saturated calomel electrode, whichwould
eliminate the use of an aqueous-fuming nitric acid Junction.
D. SUNAM•t
The addition of fluoride shifted the observed potential of type
347 stainlesssteel in the anodic direction. Fluoride additions
tended to shift the potentialof aluminum in the cathodic direction.
The effect was more pronounced in thecase of stainless steel.
The NO2 concentration did not affect the observed potentials to
a significantdegree.
The water concentration of the fuming nitric acid electrolyte
affected theobserved potential of both the aluminum and stainless
steel electrodes attotal water concentrations from 5% to 10%. An
anodic shift of potential wasobtained with both electrodes.
An attempt was made to correlate in a qualitative manner, the
observed effectof fluoride, NO2 and water on the potential with the
previously reported effectsof these materials on the corrosion rate
of stainless steel and aluminum infuming nitric acids.
More refined techniques and coincident experiments may be able
to elucidacefurther the effects of the various factors.
WADC TR 55-109 -15-
-
BIBLOGRAPHY
1. Mason, D. M., Taylor, L. L. and Keller, H. F., Storability of
Fuming NitricAcid. Jet Propulsion Laboratory Report No. 20-72,
California Institute ofTechnology, Pasadena, California, December
1953
2. Scheer, M. D. (U) Symposium on the Practical Factors
Affecting the Applicationof Nitric Acid and Mixed Oxides of
Nitrogen as Liauid Rocket Propellants.Research and Development
Board Report No.FRO 200/12, The Department ofDefense. March 1952.
pp 238. (CONFIDENTIAL)
3. Materials for Rockets Using Nitric Acid. Aerojet General
Corporation Tech-nical Memorandium No. 172. Aerojet General
Corporation, Azusa, California.March 1951.
4. Corrosion Test Program No. 1. Bell Aircraft Company., Peport
No. BLR 51-106Bell Aircraft Corp., Buffalo, New York. August
1952.
5. Fontana, M. G., Material for Handling Fuming Nitric Acid.
Part 1, 2, and 3,United States Air Force Technical Report 6519,
United States Air Force,Wright Air Development Center. January
1954.
6. 6000 Pound Thrust Jet Propulsion Unit Part II - Materials.
United StatesAir Force Technical Report 6519, United States Air
Force, M. V•. Kellogg Co.,
"*Jersey City, New Jersey. March 1948.
7. Fontana, M. G., Materials for Handling Fuming Nitric Acid,
Part 3. UnitedStates A4 " Force Technical Report 6519, United
States Air Force, Wri~htAir Development Center. January 1954.
8. Quarterly Projects Report. United States Navel Air Rocket
Test StationReport No. 30. United States Navy. April 1953.
9. Feiler, C. E. and Morrell, G. (U) Investigation of Effect of
Fluoride onCorrosion of 2S-0 Alum:mimm and 3_7 Stainless Steel in
Fuminx Nitric Acid at170F. Research Memorandum E53L17b. National
Advisory. Committee forAeronautics, Lewis Flight Propulsion
Laboratory, Cleveland, Ohio. February1954. pp 8 (CONFIDENTIAL)
10. Binger, W. W., (U) SM sium on the Practical Factor
affecti*the Auul-cation of Nitric Acid and Mixed Oxides of Nitrosen
as Liouid Rocket ProDellanta.Research and Development Board Report
No. FRO 200/12, The Department ofDefense. March 1952. pp 316
(CONFIDENTIAL)
WADC TR 55-109 -36-
-
11. Evans, U. R., Metallic Corrosion Passivity and Protection.
2nd Edition.Longmans, London. 1946. pp 759.
12. Mears, R. B. and Brown, R. H. A Unified Mechanism of
Passivity and In-hibition Part II. Journal of the Electrochemical
Society. Volume 97.March 1950. pp 75-82.
13. Evans, U. R. Metallic Corrosion Passivity and Prbtection.
2nd Edition.Longmans, London. 1946. pp 224.
14. Ladnayi, D. J., Miller, M. 0., Karo, W. and Feiler, C. E.
(U) Some Funda-mental Aspects of Nitric Acid Oxidants for Rocket
Applications. ResearchMemorandum E52JO1. National Advisory
Committee for Aeronautics, LewisFlight Propulsion Laboratory,
Cleveland, Ohio. January 1953. pp 17.(CONFIDENTIAL)
15. Ellingham, H. J. T., Alternative Electrode Reactions. Part
I. Reactions ata Platinum Cathode in Nitric Acid Solutions. Journal
of the ChemicalSociety, 1932. pp 1565.
16. Hedges, E. S. The Action of Nitric Acid on Somes Metals.
Journal of theChemical Society, 1930. pp 561.
17. Fontana, M. G., Materials for Handling Fuming Nitric Acid.
Part 3.United States Air Force Technical Report 6519, United States
Air Force,Wright Air Developmen1 Center. January 1954. pp 9.
18. Fontana, M. G., Materials for Handling Fuming Nitric Acid.
Part 4. UnitedStates Air Force Technical Report 6519, United States
Air Force, WrightAir Development Center. March 1954. pp 18-20.
19. Evans, U. R., Corrosion by Nitric Acid. Transactions of the
Faraday Society,Volume 40, 1944. pp 120.
WADC TH 55-109 -17-
-
FIGU,''? 1 TflDT APPAYAI'TU"' FOT' COiRRO2IoI Tl';2.
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FIGURE 2 CORROSION BEHAVIOR OF ARMCO17-7 PH SS IN UNINHIBITED
RFNA (12 % NOz1
WADC TR 55-109 -19-
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FIGURE 3- CORROSION BEHAVIOR OF ARMCO17-7 PH SS IN UNINHIBITED
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WIADO TR 55-109 -26-
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FIG 10 PLATE304M
MAGN X2 50 DIAM ETO4 NoneREMARKS:
Surface of specimen exposed to RFNA at 160OF for 4 days
FIG 11 PLATE 3LY780 MmAG X500 DIA ETCH Electrolytic, 10% Chromic
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WADC Th 55-109 -27-
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