CHARLES L. PETERSON, PAUL D. MILLER, and EARL
L.
WHITE
Corrosion Research Division, Battelle Memorial Institute, Columbus 1, Ohio WALTER E. CLARK Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
Materials
of Construction for Head-End Processes
Aqueous Reprocessing of Nuclear Fuels laboratory studies of gasand liquid-phase reactions show.
..
b Tantalum, titanium, molybdenum, nickel, the Hastelloys, Inconel, Haynes 25, Type S816, and lllium R are promising, depending on the route chosen for the Zircex process
b
Unalloyed titanium may fill all construction requirements in the Darex process
the volatile zirconium tetrachloride is distilled off (Zircex process), and the uranium-containing residue is dissolved in nitric acid. The solutions resulting from each of these head-end treatments after chloride stripping are handled by the usual methods of solvent extraction for the recovery of uranium. These and other processes for the recovery of spent nuclear fuel have been described ( 4 ) . The corrosion evaluation of materials of construction for equipment for these processes is being carried out by Battelle Memorial Institute as part of a nuclear fuel reprocessing program under way a t the Chemical Technology Division of Oak Ridge National Laboratory.
The Zircex Process
OKE
of the greatest cost factors in the production of nuclear power is the reprocessing of spent fuel to recover uranium and reduce fission product wastes t o small volumes suitable for storage. In the ideal case, all fuel types would be handled by one reprocessing scheme. I n practice, no single existing process will handle all the types of fuels now in use; it is advantageous to employ headend treatments which produce solutions suitable for processing by existing separation and purification procedures. One proposed scheme is shown schematically in Figure 1. Stainless steel fuel elements are dissolved in mixed nitrichydrochloric acid (Darex process), while zirconium fuels are subjected to dry hydrogen chloride a t 400' to 600" C.,
Dissolution of the metal or metallic chloride residue, following gas-phase hydrochlorination, should be done in the same reaction vessel. Therefore, with hydrochlorination requirements in mind, several materials were screened by exposure to boiling solutions (Figures 2 and 3) with compositions simulating those that could occur a t various stages during the dissolution. Specimens were exposed beneath the boiling liquid, a t the interface, and in the vapor in units consisting of Erlenmeyer flasks topped by water-cooled Allihn condensers operated with continuous refluxing. T h e specimens were removed a t weekly intervals, washed, scrubbed, dried, and weighed. T h e loss in weight was then converted to a corrosion rate expressed as average
OFF GAS (OXIDES OF NITROGEN-NOCI,C1~1
DAREX
L
-________-___
1
LIRCEX
Figure 1. Zircex and Darex head-end processes can be used to give uranium solutions ready for recovery b y solvent extraction
32
INDUSTRIAL
AND ENGINEERING
CHEMISTRY
penetration in mils per month. Usually the acid solution was renewed a t the beginning of each exposure period. Figures 2 and 3 show the extent of corrosion measured by weight change following 4 weeks of exposure. A limiting value of 3 mils per month was arbitrarily chosen as the maximum allowable corrosion rate before study of a material was discontinued. Table I gives the compositions of the materials evaluated and others. In addition, candidate materials were tested in boiling solutions of 15M nitric acid-0.4,M uranyl nitrate through which hydrogen chloride gas was bubbled; only tantalum, titanium, Carpenter 20, and rolled Haynes 21 performed satisfactorily, and all suffered excessive attack in hydrochlorination tests or in subsequent cyclic tests. Both tantalum and titanium showed excellent resistance to all the initial solution conditions. Tantalum showed almost no measurable attack under all test conditions, while titanium and a titanium-6% aluminum-4y0 vanadium alloy showed maximum rates of only a few tenths of a mil per month. After the nitric acid concentration of the solution had been depleted by reaction with the hydrogen chloride bubbled through, titanuim underwent a catastrophic attack. The amount of nitric acid required to prevent this type of attack was not accurately determined but appeared to be very low; no nitrate was ever found in the solutions after such attack was observed. Oxidizing agents such as iron(II1) and, to a lesser extent, LO2+,+also inhibited the attack of hydrogen chloride on titanium. The color of the solutions in which accelerated attack was observed indicakd that there may be a correlation between the oxidation potential necessary for the passivation of titanium and that required for oxidation of uranium to the sexivalent state. This, however has not been substantiated. I t was concluded that unless adequate oxidizing conditions are provided to maintain the metal in a passive state, catastrophic failure may occur. The corrosive or passivating action of various gases on titanium in azeotropic hydrochloric acid solutions was also examined. Sitrous oxide, nitric oxide, nitrogen dioxide, nitrosyl chloride, and chlorine were passed singly through a
Cortosion R o t e , m i l s permonth
Corrosion R a t e , mils per mont+
,
20
3.0
40
,
5.0
Tantalum Titanium
Tilonium Vitollium Hoynes
Interface
25
F i g u r e 2. Six materials showed l o w corrosion rates in 0 . 5 M Zircex solutions (left). Eight materials showed l o w corrosion rates in 3M Zircex solutions (right)
Type 5-816 -62 Excessive aExcessive
m c
g Z S o t 331 hr
c"
Type S-590 Corpcnter 20 Corpcnler 2 0 otobilized c
C ) I E ~ ~ I 506hr
8
s-
12-14 01 161h i Excessive
c
2'
Eicerrwe
Exposed for 4 weeks unless noted otherwise Cond~lionr B o i l i " q s o l u l ~ o n s3 0 M H * , 0 4 M U C I , . 2 8 MNO;
C o n ditions -
B o i l i n g solutions
co 0 5 MHKO,,
co 0 4 M UCI,
system containing boiling azeotropic ( 6 . l M ) hydrochloric acid. I n one experiment, pieces of 75A titanium were placed a t the vapor, interface, and liquid positions in the flask and the gas was bubbled through the solution. I n the other experiments, the specimens were exposed in the vapor only and the gas was passed above the solution. The corrosion rates were high (up to 1 inch per month) and about the same for specimens exposed to nitrous and nitric oxide and to azeotropic hydrochloric acid alone. The corrosion of vapor-phase speci-
mens was inhibited almost completely by nitrogen dioxide and chlorine. Nitrogen dioxide imparted a considerable amount of inhibition to the liquid-phase specimens, but chlorine gave no inhibition. Other studies showed that uranium in azeotropic hydrochloric acid will inhibit the corrosion of titanium specimens submerged in the boiling liquid. This would account for the resistance of the specimens to the liquid phase during catastrophic attack of the specimens exposed in the vapor phase. The hydrochlorination of certain fuels,
such as those with a uranium dioxide core, may leave a low-chloride residue after volatilization of the zirconium tetrachloride. Materials for such service were evaluated in boiling solutions of 5M nitric acid-0.4M uranyl nitrate containing 500 p.p.m. of chloride. Tantalum, titanium, Vitallium, Haynes 25, Type S-816, Type S-590, Carpenter 20, Carpenter 20 stabilized, and Types 347 and 304 ELC stainless steel showed rates of less than 1 mil per month after exposure for about 4000 hours. Another proposed dissolution step following hydrochlorinationwould use water as the solvent, ending with a solution of uranium trichloride. Elimination ex-
Table 1. Metal Tantalum Titanium Vitallium Haynes 21 Haynes 23 Haynes 25 Haynes 30 Haynes 36 Type 5-816 Type S-590 Carpenter 20 Carpenter 20 (stabilized) Type 304 stainless Hastelloy X Hastelloy F Hastelloy C Illium R Inconel
A Nickel Monel Nichrome V Hastelloy B Hastelloy W
co
... ..
62.2 62.0 64.6 50.4 50.6 52.8 44.7 20.0
... ... ...
...
2.5
2.5max.
...
... ... ...
... ... ...
Composition of Candidates Considered for Materials of Construction Nominal Composition, % ' by Weight Cr Ni Fe Mn Mo Cu Si W
...
... ...
... ...
...
27.4 28.0 26.0 20.0 26.0 18.5 19.0 19.7
2.8 2.5 1.5 max. 10.0 15.0 10.0 20.0 20.0
0.7 2.0 2.0 2.0 2.0 2.0 3.0 26.0
20.0
29.0
20.0
29.0
19.0 22.0 22.0 15.5 22.0
15.5
... ... 19.0 ... 6.0
... ...
0.66
...... ...... 5.5 ... 5.2 ...
...
... 0.53
1.3 0.6 2.0
... ...... ... . . . . . . 1.0 rnax. 6.0 ... ... . . . . . . 0.50 4.0 ... 0.3 3.7 ... 1.Omax.
44.2
0.75
2.0
3.0
1.0
43.6
0.75
2.0
3.0
1.0
9.5
68.4
2.0
45.0 45.0 54.6 63.8 76.2
23.9 21.0 5.5 2.5 7.5
1.55 1.0 1.0 0.25
99.f 67.2 75.8 65.0 61.5
max. max. max. max. mctx.
...
... ... 1.5 ...
...
....
1.5
1 .o
&.Omax.
2.5max.
5.0 5.5
1.00 1.0
...... 9.0 ... 6.0 ... 16.0 ... 6.5
...
4.0 0.2
...... .!.
1.0 1.0
0.1
..
1.5 1.0 1.0
...
I
... ...
0.25
30.0
......
28.0 25.0
1.0
...
... ...
...
C
... ...
...
0.22 0.30 0.40 0.15 max. 0.40 0.40 0.40 0.45
...
0.07
...
5.5 15.0
14.5 4.0 3.75
...
... ...
0.08
...
0.15 max.
1 .o
3.8
...
... ...
0.08 0.20 0.08
... ... ... ... ...
0.15 0.25 max.
VOL. 51, NO. 1
...
... ...
Nb
__ B
... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 0.03 4.0 ... ... 3.75 ... ... 0.7
...
... ... ... ...
...
*..
... ... ... ... ...
...
... ... ...
... ...
JANUARY 1959
... ... ...
...
33
($zht) bizht) 0
IO
20
30
,
14
C o r r o s i o n R a t e , m i l s per m o n t h 40
50
I
I
f
Tan t a IYm
-840t
160 hr
S 6 - 2 8 a l IWhr I60 hr
-77at
E 2 7 - 2 9 at 332 h
-
-
z
0
Figure 4.
Condittons Boiling Solution 5 0 M H + , O 4 M U C I , ,
'
10
20
80
tration was occurring
Exposed for 4 weeks unless noted otherwise
34
70
Corrosion rates appeared to remain low in 3 M
Figure 3. Five materials showed low corrosion rates in 5M Zircex solutions
The unit, mils per 100 cycles, is more meaningful from a processing standpoint than the familiar mils-per-month unit.
60
HN03, 0.4M UC13 solutions but sectioning showed high pene-
4 6 M NO,.
periments were run in boiling solutions of 0.4M uranium trichloride. None of the five materials considered for such a system (Carpenter 20, Haynes 21, and Type S-816 alloys, and Type 304 ELC and Type 316 stainless steels) withstood the attack for even 1 week. Serious stresscorrosion cracking was shown by the Type 316 stainless and Type S-816 specimens at cut edges and stencil marks. T o evaluate the performance of materials under conditions expected in actual operation, a system of cyclic exposures was set up. Specimens were placed in a large tube furnace, which was first flushed with argon and then brought to 600' c. under an approximately equimolar mixture of hydrogen chloride and hydrogen. Zirconium tetrachloride was provided by means of a small piece of zirconium in the entrance end of the furnace. After 11 hours a t temperature, the hydrogen chloride and hydrogen were shut off, and the furnace was thoroughly flushed and allowed to cool under argon. The specimens were removed, rinsed in solutions of 3.0, 5.0, or 15.0M nitric acid, and then placed in reflux units containing similar acid solutions to which uranium and chloride had been added. The specimens were exposed to the refluxing solutions fcr 2 hours, removed, rinsed with tap and distilled water, and quickly airdried. Following this, they were either returned to the furnace for another cycle or, if it were the conclusion of a multiple of five cycles, were weighed and corrosion rates, expressed as penetration in mils per 100 cycles, were calculated from the weight losses measured.
30 40 50 Exposure Time, c y c l e s
If 25 cycles per month are estimated as the average, this unit is converted to mils per month by dividing by 4. As preliminary experiments showed that titanium and tantalum were unsatisfactory materials for construction of the hydrochlorinator because of excessive corrosion rates in the hydrogen chloridehydrogen phase, the cyclic exposures were confined to alloys high in chromium, cobalt, nickel, etc. The following materials have been investigated in the twostep cycle: Type S-816, Type s-590, Haynes 21, 23, 25, 30, and 36 alloys, Carpenter 20, Carpenter 20 stabilized, and Type 304 ELC stainless steel. Corrosion and tensile specimens of these materials and of Inconel, nickel, and Hastelloys B and C \vere exposed only to the hydrochlorination step. Only two alloys, Haynes 25 and Type S-816, showed good resistance in the twostep cyclic exposures, as measured by weight-loss data. They consistently gave corrosion rates of 2 mils per 100 cycles, or less, after 75 cycles from the high-temperature hydrogen chloride-hydrogen zirconium tetrachloride atmosphere, to solutions which were 3M nitric acid0.4M uranium trichloride. 5M nitric acid-O.O1.M uranium trichloride, or 5M nitric acid-0.4M uranium trichloride. Cycling specimens of these alloys into l 5 M nitric acid with 0.01M uranium trichloride added at the start of each boiling period resulted in much higher rates, which gradually increased to around 15 mils per 100 cycles after 75 cycles. Metallographic sectioning and inspection of the specimens of Haynes 25 and Type S-816 exposed for 35, 50, and 75 cycles revealed that, in all cases, the
INDUSTRIAL AND ENGlNEERlNG CHEMISTRY
specimens cycled into the boiling acid solutions underwent severe scaling and an intergranular type of attack. Corrosion rates based on measurements of the thickness of the unattacked metal are several times greater than those calculated from weight-loss data in the 3 and 5M nitric acid solutions. Corrosion rates determined from both weight-loss measurements and metallographic examinations are presented in Figures 4 and 5. The attack on both metals increases hith increases in the concentrations of both chloride and acid, the effect of increased concentrations being much greater on Type S-816 than on Haynes 25. Inconel, Haynes 25, Type S-816, Illium R, nickel, and Hastelloys B and C showed good resistance to the hot step. Rates measured by weight loss and by metallographic examination \$ere, in dll cases, on the order of 0.1 to 0.2 mil per month or less. Tensile specimens were exposed to the hot step, subsequently tested, and the results compared with those from specimens in the as-received condition and from specimens which had been subjected to the same temperature (600' C.) under argon. Inconel. Haynes 25, Type S-816, and Illium R uere not affected by the hydrochlorination. Hastelloy C increased in tensile strength and hardness and decreased in elongation. Hastellov B and nickel were not investigated. Several materials are available for the two-vessel hydrochlorination process. T h e optimum materials of construction for the dissolver in such a process would appear to be titanium or tantalum. hTo metal or alloy examined is entirely satisfactory for a combined hydrochlorinatordissolver in the Zircex process as now envisioned, although Haynes 25 may be considered marginal for this application. Its use would require a design of sufficient
NUCLEAR TECHNOLOGY
10
30 40 SO Exposure Time, Cycle8
20
60
70
80
Exposure Time. cycles
Table II. Twenty-three Materials Were Screened for Zircex LiquidPhase Hydrochlorination (Corrosion rates of materials are based on 24 hours of exposure at 4 2 5 O C. in boiling AICla.NHaC1 gaseous HCI) Corrosion Rate, Mils per Month Vapor Liquid Specimen phase phase Platinum 0.00 0.00 Gold 0.01 0.01 Tungsten 0.1 0.3 0.6 0.4 Molybdenum Molybdenum 0.7 Nickel 6.4 0.2 Hastelloy X 8.2 0.5 8.2 Hastelloy C 0.9 12.2 Hastelloy C 1.6 Hastelloy C 13.0 1.3 Hastelloy C stabilized 10.2 1.0 Inconel X 0.2 10.4 Illium R 0.7 10.5 2.0 11.4 Hastelloy B Monel 0.9 11.9 Type 5-590 13.6 0.7 Inconel 0.1 14.0 Hastelloy F 0.6 16.1 Type 5-816 0.2 16.5 Carpenter 20 stabilized 0.7 17.6 Hastelloy W 2.4 18.2 40.2 4.2 Haynes 36 Haynes 25 43.9 0.9 Haynes 30 50.6 0.2 66.5 2.4 Haynes 23 Haynes 21 82.9 3.8
+
...
Corrosion Rate, 7-Day Test,
Specimen Molybdenum Nickel Hastelloy X Hastelloy C Nichrome V
Mils per Month Liquid phase Vapor phase 0.04 1.19 2.73 8.39 6.24
0.02 0.08
0.07 1.59 0.13
thickness to allow for an expected corrosion rate of about 3 mils per month. An alternative method of hydrochlorinating zirconium fuel elements would involve the use of a low-melting fused
Figure 5. Slightly lower corrosion rates were exhibited in
12
0.01M
10
UCl3 solutions but sectioning showed substantial penetration (upper left). A higher chloride content in the 5M HNO3, 0.4M UC13 solutions produced more rapid a t t a c k on T y p e S-816 (upper righf). With higher nitric acid concentrations
8
5M
“08,
(15M “ 0 8 , 0.01M UCl3) both weightloss measurements and sectioning indicate excessive penetration (right)
6 4
2
T l l l l l l l l l l l l l
0 12
10 8 6
4
2 0
10
salt which contains hydrogen chloride. Figure 6 % a schematic flowsheet for such a process, using the double salt aluminum chloride-ammonium chloride at the reflux temperature of 425’ C. Limited corrosion investigations have been carried out in this system, care being taken to prevent access of air which would unduly complicate the evaluation of data. The results obtained from 24-hour exposures are listed in Table 11. Some of the materials investigated and Nichrome V were exposed for 7 days in this same environment. The corrosion rates with all but Hastelloy C were considerably lower than those measured during the 24-hour exposure. Metallographic examination of these specimens following exposure revealed that nickel and Hastelloy X had undergone a very slight intergranular penetration a t the surface. Intergranular attack was not observed on molybdenum, Hastelloy C, or Nichrome V. O n the basis of these very limited
20
30
40
50
60
70
80
Exposure Time, cycler
studies, molybdenum appears to be the favored material of constuction, if the technology is sufficiently developed to allow fabrication of the equipment without difficulty. Rapid advances have been reported recently (3, 5). Because of the leveling off observed in the corrosion rate8 of nickel and some nickelchromium alloys, these materials merit further consideration. Cost rules out platinum and gold, both of which showed superior corrosion resistance.
The Darex Process The proposed solvent for stainless steel fuel elements in the Darex process consists of hot 2M hydrochloric acid-5M nitric acid. Elimination-type studies were run in this solution in the presence of slight contaminations of metal ions and in solutions corresponding approximately to those expected a t the midpoint and final stages of dissolution of Type 304 stainless steel. Available hydrogen ion VOL. 51, NO, 1
JANUARY 1959
35
Figure 6. A l i q u i d phase reaction could be used as an alternative to the gas-phase Zircex head-end process
Zr CI,
AICI, NH,CI AICI, 'NH,CI AICI, NH,
Distillation HCI
t HNO,
4 - 7
concentrations in these solutions were 7.0 4.0, and l . O M , respectively. All studies \cere conducted a t reflux temperature (ca. l l O o C.). The beginning solution was the most corrosive of the three. This probably results from the higher acid concentration and the near absence of uranium and other metal ions which, in some cases, are corrosion inhibitors. I n this solution, the order of merit was from tantalum, which was unattacked, through titanium, Haynes 21, and zirconium to Type S-816, \+hich showed rates as high as 6 mils per month. In the middle and final compositions, the Haynes 21 and Type S-816 specimens showed rates varying from about 0.1 to 1.5 mils per month. Tantalum, titanium, and zirconium were virtually unattacked. Zircaloy 2 and the 676 aluminum-4Yo vanadium alloy of titanium were also evaluated in solutions representing final concentrations and found to be resistant. To some of the solutions, 50 to 100 p.p.in, of ruthenium was added to represent one of the possible fission products which has been reported ( 7 ) to promote corrosion in other systems; the presence of ruthenium produced no deleterious effects corrosionwise and appeared to decrease the corrosion of liquid and interface specimens of titanium. All of the titanium and tantalum specimens from these studies were examined for possible embrittlement by means of the bend test, but no loss in ductility was indicated. Tantalum and titanium appeared the most promising container materials a t this stage, but both are subject to hydrogen embrittlement. If either is to be used as a container for the Darex dissolution, it may act as a cathode whenever the dissolving stainless or uranium fuel elements contact the container walls. Such conditions could foster hydrogen pickup and embrittlement. Consequently, an experimental program was undertaken to evaluate the possible harmful aspects of such galvanic action.
36
Chloride
Dissolver
stripper
Adjustment
Solvent extraction
1
Incidental to this program, the potentials, half-cell potentials, and cell currents between the proposed container materials, titanium and tantalum, and dissolving stainless steel and uranium were measured. Polarization curves based on these studies have been reported (2). Analyses of the gases evolved during dissolutions were also attempted (2). Electrolytic cells were assembled using either tantalum or titanium cathodes and carbon rods for anodes. In each cell, the metal in question was made cathodic, by means of an impressed e.m.f., to that value which it reaches while in contact with dissolving uranium in boiling Darex acid. Operation was continuous in the refluxing acid. At intervals, strips cut from the cathodes were given a bend test and then analyzed for hydrogen. A noticeable embrittlement of tantalum was observed after 1500 hours of exposure but the titanium cathode remained ductile throughout the experiment. The hydrogen content of tantalum increased from 180 to 350 p.p.m. in 700 hours, but showed little further increase up to 1500 hours. N o significant increase in hydrogen content was noted for titanium. The titanium cathode did scale to an extent corresponding to a metal loss of 1.7 mils per month. This is a much higher rate than was measured in subsequent exposures in the presence of dissolving stainless steel, where less difference in potential exists and dissolution products were allowed to accumulate. The evidence gained from these studies seemed to warrant abandoning consideration of tantalum as a material for the dissolver. The dissolution process could be operated either batchwise or continuously. Both operations were simulated in laboratory corrosion studies. In an assembly termed a flowing dissolver, fresh Darex acid was continuously fed to a borosilicate glass reaction vessel, heated externally to maintain the acid at its boiling point. A rod of stainless step1 \yas gravity-fed continually through
INDUSTRIAL AND ENGINEERING CHEMISTRY
a closed tube, so that only a small portion of the tip emerged into the boiling acid. Dissolution of this rod was reasonably uniform, once the temperature became sufficiently high to prevent the stainless from going "passive." The rod (5/16 inch in diameter) dissolved at approximately 1 inch per hour to form a conical, very sharply pointed tip. The feed to and overflow from the flask were adjusted to give a retention time within the flask which would use as much of the acid value of the entering stream as possible, without allowing the stainless dissolution to stop from passivation. Another assembly was constructed in much the same manner, exccpt that no provisions for acid flocv \vere made. I n this batch dissolver: the dissolution of the stainless rod was allowed to continue until no more would dissolve, and then the spent acid \vas replaced. I n both dissolvers, a circular piece of titanium was positioned directly below the point of the stainless rod, so that galvanic coupling would occur between these two metals. Specimens of other metals under investigation were arranged throughout the apparatus. Some were immersed, some positioned at the interface, and some placed at various heights in the vapor. The data from some of the first dissolver studies are listed in Table 111. Cnalloyed titanium (75.1) and tantalum were virtually unattacked after approximately a 3-month exposure. (At the time of these studies the einbrittlement of tantalum had not been proved.) The titanium alloy was some\chat less resistant. Haynes 21 and Type S-816 showed rates of less than 1 mil per month. Crystal-bar zirconium was severely attacked and pitted. This attack, which occurs in the presence of the gaseous decomposition products resulting from the actual dissolution of stainless steel, is obviously more severe than that recorded during the elimination-type studies, where temperatures. acid strengths, and corrosion products were similar, but the acid solution decomposed to a much smaller estent. Gas samples taken during one hatch dissolution were analyzed by mass spectrograph. Appreciable quantities of hydrogen chloride, chlorine. nitrogen dioxide, and nitrous oxide !vel-e found. Undoubtedly some nitric acid \vas also present. At intervals specimens of unalloyed titanium (75A) \cere removed, analyzed for hydrogen by hot extraction, and inspected for embrittlement by bend tests. Neither hydropen absorption nor embrittlement could be detected. This was also true for pieces cut from time to time from the circular titanium disks \vhich were in constant contact with the dissolving stainless steel rod. Titanium performed so well that all further examinations \vere concentrated
NUCLEAR TECHNOLOGY on this material. Other dissolver experiments were made using the flowing assembly. Specimens of unalloyed titanium containing weldments made by field methods were exposed. A variety of shapes were conceived which resulted in butt welds and lap welds with one or two beads. The lap-welded specimens presented the opportunity to study possible crevice attack. Other specimens were stressed with keepers which held them a t a known stress level, including levels in excess of their elastic limit. Other specimens were welded, transversely or longitudinally, and then placed in further stress by appropriate jigs. Most of these specimens were exposed for 2000 hours with no indication of any corrosion problems. Metallographic sectioning through weldments and the bottoms of crevices has revealed no indication of stress cracking or intergranular attack. Corrosion rates measured by weight losses on some of the more conveniently shaped specimens have shown good agreement with those presented in Table 111. One operational difficdty has been observed with titanium during dissolvertype experiments : the tendency for a thick sludge or scale to form on the titanium and on the walls of the dissolver during dissolution. A study of this material by x-ray diffraction techniques showed it to be largely amorphous, with small amounts of spinels and metallic silicon. A ,short treatment with boiling 10% aqueous sodium hydroxide removed this sludge completely and had little effect on the titanium itself. Further Table 111.
studies with a flowing dissolver have shown that a weekly cleaning regimen consisting of 3 hours of treatment with boiling sodium hydroxide solution should not have a deleterious effect on the protective film on titanium. T h e titanium remains completely passive during subsequent acid dissolutions. There is always the queqtion of what will happen to titanium if its protective oxide film is injured and the environment is not sufficiently oxidizing to effect immediate repair. Titanium specimens placed high in the reflux system above a continuous dissolver have been mechanically abused daily for 2 months in an effort to answer this question. So far their loss in weight has been negligible and there has been no evidence of failure of the protective coating to heal. Conclusions
A feasible corrosion-resistant container material for the single-unit Zircex hydrochlorinator-dissolver is not apparent at present. Of the materials studied, the best choice is Haynes 25. Its use would require that penetration rates of approximately 3 mils per month be tolerated. T h e process should be run in two steps. Several materials, such as Inconel, Haynes 25, Type S-816, Illium R, and nickel, appear promising for construction of the hydrochlorinator. The cold residue would then be dissolved by transferring it to a container made from titanium or tantalum. Preliminary experiments in the alternate
Corrosion Rates of Metals Exposed to Active Darex Systems Were Measured Unalloyed titanium and tantalum w e r e not attacked
Special Material
Specimen Location
Corrosion Rate, Mils per M o n t h Batch dissolver Flowing dissolver 1839 Hr.
Titanium (75A) Tantalum
Haynes 21 Type S-816
Vapor Interface Vapor Interface
0.01-0.07
2483 Hr 0.02-0-03
Gain
Gain
Vapor Interface Liquid Vapor Interface
0.24-0.43
Liquid
Ti-6 % AI-4 % V
Crystal-bar zirconium
Vapor Interface
0.01 .A.
...
0.35 0.52-0.69 0.73 0.81
... ...
0.00
Gain 2207 Hr. 0.09 0.38
...
...
... 1000 Hr. 0.15-0.19
Gain 45 Hr.
... ...
Vapor 67-69 Interface 59-68 Note. Gain indicates a gain in weight of 100 mg. or less on a 1 X 2 inch specimen.
liquid-phase Zircex hydrochlorination system employing fused aluminum chloride-ammonium chloride indicate that nickel and the Hastelloys may be satisfactory for construction of the hydrochlorinator. Molybdenum appears even better from a corrosion standpoint. I n the Darex system unalloyed titanium appears highly satisfactory for any part of the apparatus, with expected maximum corrosion of about 0.1 mil per month. No deleterious effects are expected around weldments or a t points of high stress levels. T h e siliceous scale which forms in the dissolver can be removed by treatment with boiling 10% sodium hydroxide with no adverse effect on the corrosion resistance of the titanium. Corrosion rates for tantalum are negligible in all cases, but this metal is subject to hydrogen embrittlement when employed as the dissolver material. Other possible materials of construction are Haynes 21, with a corrosion rate of less than 0.5 mil per month, and Type S-816, with less than 1 mil per month. These results are derived from laboratory scale experiments. Pilot-plant evaluations on a larger scale are recommended prior to any plant construction based on these data. Acknowledgment
Several Battelle staff members were closely associated with these investigations: H. A. Pray, R. S. Peoples, F. W. Fink, E. B. Friedl, 0. M. Stewart, J. D. Jackson, W. D. Beasley, and T. E. Snoddy. C. D. Watson, O a k r R i d g e National Laboratory, originally made the suggestions which resulted in the design of the cyclic experiments, while M. G. Fontana, The Ohio State University, served as a consultant to the project. literature Cited (1) Griess, J. C., Oak Ridge National Laboratory, private communication. (2) Miller, P. D., Peterson, C. L., White, E. L., Fink, F. W., “Evaluation of Container Materials for Zircex and Darex Nuclear Fuel-Recovery Processes,’’ BMI1242 (Dec. 11, 1957) (unclassified AEC report). (3) Rothschild, G. R., Air Reduction Go., private communication. (4) Savolainen, J. E., Blanco, R. E., Chem. Eng. Progr. 53, 78-81-P (1957). (5) Yancy, R. w., Matertah in Design Eng. 46, No. 2, 112 (1957).
RECEIVED for review April 7, 1958 ACCEPTED October 24, 1958 Division of Industrial and Engineering Chemistry, Symposium on Reprocessing Chemistry for Irradiated Fuel, Aqueous Methods, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Work done for the U. S. Atomic Energy Commission by Battelle Memorial Institute under agreement with the Union Carbide Nuclear Co. under contract W-7405-eng-92. VOL. 31, NO. 1
JANUARY 1959
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