I
E. J.
TUTHILL and R. F. DOMISH
Brookhaven National laboratory, Upton, N. Y.
Corrosion Studies
Calcining Aqueous Radioactive Wastes The potentially serious biological hazards of radioactive waste disposal must be seriously considered as we look forward to the widespread use of nuclear energy for power generation. The formation of chemically stable solids by calcination of wastes containing zirconium and aluminum salts gives mixtures of hydrogen fluoride and nitrogen oxides which are corrosive to process equipment. This study was undertaken to determine which alloys are most suitable for construction of calciners for these radioactive wastes.
a t 325' C. with the evolution of nitrogen oxides (to produce the stable crystalline oxide, 550' C. is required). Calcination of aluminum-zirconium wastes in a single vessel a t 700' C. was believed to cause serious corrosion. Therefore, a n attempt was made to separate the nitrogen oxides from the hydrogen fluoride in the off-gases by using two kilns in series, the first operated a t 325" C. for denitration and the second a t 700' C. for defluorination. These gases could not be completely separated : some hydrogen fluoride is contained in the off-gases from the first kiln and some oxides of nitrogen in the off-gases from the second. Any condensation on metal parts caused severe corrosion. No advantage with respect to kiln corrosion was realized by using the two-stage apparatus. At least four alloys are probably suitable as construction materials for either single-kiln or doublekiln calciner vessels: Illium G, Inconel X, Haynes 25, and Ni-o-nel.
Double-Kiln Calciner
spent zirconium-uranium fuel elements contain fission products, with zirconium, aluminum, chromium, and tin. O n e effective means of preparing these wastes for safe disposal involves calcination to insoluble oxides of aluminum and zirconium, and oxides of fission products, some of which are highly soluble. T h e soluble radioactive oxides are not totally sealed within the structure of the calcined product, and several methods for permanent fixation are possible. O n e involves leaching and fixing the radioactivity on clay ( 3 ); another involves adding a fluxing agent and, by heating to elevated temperatures, converting the mixture to a n inert ceramic mass. I n the work described, corrosion was studied for a number of metals during calcination of simulated aluminum-zirconium waste solutions in rotary kilns. When steam is added, pyrohydrolysis of zirconium fluoride proceeds efficiently a t 700' C., evolving hydrogen fluoride; however, aluminum nitrate can be converted to the largely amorphous oxide
I n the double-kiln apparatus operated at Brookhaven the solution was fed by a constant displacement p u m p through a n air-cooled solution feeder into a rotary ball kiln operated a t 300' C. T h e concentration of hydrogen fluoride was very low; therefore a wide range of materials with resistance to nitric acid and nitrogen oxides could be used. T h e powder formed in the first kiln was fed through a star feeder and screw feeder into a second kiln operated a t 700' C. T h e kilns were constructed of 12-inch sections of 4-inch seamless pipe with a wall 0.237 inch thick. About 6 pounds of 3/,-inch metal balls were placed in each kiln to prevent cake formation. Consequently there were some abrasion and peening of the corrosion test specimens in the rotating kilns. Both kilns were heated by external electric heaters, and the temperature was measured by thermocouples in contact with the exteriors of the rotating kilns. A4slight negative pressure was maintained in the system, and the gases from the apparatus were drawn to plastic adsorption towers cooled internally by a flow of water. A sample of the gas from each kiln was drawn through a heated filter to a Lucite vessel in which condensation and absorption took place.
Specimen Design. Specimens had a n exposed portion inch in diameter and 1 inch long. They were rigidly supported in a ring mount resembling a squirrel cage, so that rotation of the entire assembly would cause the specimen surfaces nearest the axis of the kiln to be abraded by the balls and powder in the same way as the kiln wall. T h e specimens were made as a precision ground shell and a solid cylindrical core (2) of the same material, except where hardness did not permit. T h e shell and core, machined to a n accuracy of = t O . O O O l inch, were slipped together after the core was chilled in liquid nitrogen and the shell heated in boiling water, T h e shell-and-core type was desirable principally because the core, never in contact with the corrosive conditions and therefore not affected by them, provided a circle of reference in the specimen cross section. T h e reference circle gave a standard of size and position from which the depth of penetration could be accurately determined. Planimeter measurements wcre made of the areas of the shell and core on photographs cf cross sections of the specimens. Comparison of the area of the core on the photograph with the actual area give the exact magnification. Before the shell-and-core type was decided upon, consideration was given to the weight-loss method of measuring corrosion quantitatively. However, because the high temperature corrosion of this process causes formation of considerable scale which adheres more tenaciously in somc cases than in others, the weightloss method could not be used unless the specimens could be cleaned perfectly without removing any unaffected metal. A sodium hydride bath, electrolytic descaling, and chemical cleaning methods were considered, but none was thought suitable because of the varied nature of the materials tested. Corrosion Test. The specimens were exposed for 500 hours under conditions almost identical with those of a commercial plant. Each specimen was exposed to exactly the same conditions because continuous rotation of the kiln repeatedly raired a specimen into the gas stream and plunged it down under the bed of balls and abrasive powder. After exposure, they were sectioned with a cutoff wheel having a special jig to insure cutting each specimen a t the midVOL. 51, NO. 12
*
DECEMBER 1959
1471
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A double-kiln calciner was used to form nitrogen oxides separate from hydrogen fluoride point. They were then mounted in Bakelite, ground, sanded, polished, and etched according to standard metallurgical procedures. T h e specimens were photographed on 4 X 5 inch plates, and 8l/* X 11 inch enlargements were made on Resist0 Rapid paper. This paper has a waterresistant film on both sides to minimize dimensional changes. Prints showed the
entire cross-sectional area of the specimen at about 26X and were used for detailed measurements to determ:ne average and maximum depth of penetration. Portions of each specimen were also photographed at 1OOX on 5 X 7 inch plates. Contact prints from these plates were examined for intergranular corrosion and cracks in the metal.
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Corrosion test specimen design
1472
INDUSTRIAL AND ENGINEERING CHEMISTRY
Determination of Average Depth of Penetration. Average penetration depth was determined from planimeter measurements of the area cf the core and shell as shown on the 81/2 X 11 inch enlargements. The magnification factor (actual core area divided by core area measured on photograph) was determined independently for each photograph, so that stretching or shrinkage of the photographic paper caused no error. The difference between the shell area before and after exposure divided by the circumference at the periphery of the original specimen was taken as the average penetration value. For the solid specimens (no reference circle) the magnification was determined by photographing a calibrated disk (Whipple) with the grid in contact with the specimen. Maximum Penetration. Maximum penetration for the shell-and-core type was determined by direct measurement
CALCINING RADIOACTIVE WASTES
Here i s an inside view of the specimen cage after exposure under almost commercial conditions
at the thinnest part of the nonpeened portion of the shell by using the 26X enlargements and subtracting this value from the original thickness of the shell. Maximum penetration for solid specimens can only be estimated, because the original center of tha specimen cannot be located. Peening. Peening of the shells by the balls gave a somewhat qualitative indication of the resistance of the material to displacement under high temperature operating conditions. Abrasion. T h e effect of abrasion could not be determined by measuring thickness of the shell. because many shells were also thinned by peening. Therefore indicaticn of the effect of abrasion was obtained by comparing the area of the inside half-shell which was exposed to greater abrasion with the relatively nonabraded outer half. T h e shell of the specimen as shown on the photograph was divided into tlvo halves by extending a diameter of the core. T h e area of each
half was measured with the planimeter and the areas were compared. I n some cases the area of the peened half-shell was substantially smaller, which showed that the abrasive effect of the aluminum oxide rubbed against the specimen by the balls or the chipping awa) of scale by the hammering action of the balls was causing a n increased rata of attack. I n other cases the area of the cross section of the half-shell exposed to abrasion was the same as or slightly larger than that of the nonabraded half, which indicated that the thinning of the shell was primarily due to the peening action of the balls. Accuracy. A dimensionally unaltered core was essential to measurement of its area and to the geometrical construction of a diameter which, u h e n extended, divided the shell into two halves (peened and nonpeened sections). Several diameters constructed on a photograph of one specimen, the shell of bhich was badly deformed by peening, showed that the core had not been deformed. T h e depth of etching during metallographic preparation of the specimen was studied to determine 1%hether removal of metal by etching would cause significant errcr by decreasing the cross-sectional area of shells and cores. A portion of a specimen was covered with heavy grease and the uncovered portion was etched to a depth greater than normal. T h e specimen was cleaned and examined \vith a metallograph a t high magnification to acquire a small depth of focus. With the line of demarcation between the etched and unetched portions passing through the center of the field of view, these areas were alternately brought into sharp focus. Koting the movement of the calibrations of the fine focu3ing knGb made it possible to measure the difference in elevation of the etched and unetched areas. It was decided that normal etching procedures would remove far less metal than the tolerance alloned in machining the specimens.
Test specimens were rigidly supported in a ring mount resembling a squirrel cage
Table I. Materials in Order of Corrosion Resistance The double-kiln test showed that high-nickel high-chromium alloys were best
.I\*. Max. Corrosion Penetratioii Rate, during I n r h / Y e a i Test, Iiich
Material
.
Inconel X Armco 314 Illium G Ni-o-ne1 Haynes 25 Carpenter 20 440C 310 Duranickel Hastelloy C 430F 316 Incoloy 804 316 404 303 316 Incoloy 347 17-4-PH S nickel 302B 440B 309 430F 446 308 321 Nickel 67, Si 430 303 410 W orthit e 416 Hastelloy B 414 Inconel 403 403 405 Croloy 5 Si 22-4-9 Monel Colov 5 Nickkl, low-carbon High tensile bronze Ampco 16 Mild steel A1203 coating ZrO? coating Tantalum a
(
0.057 0.029 0.031 0.037 0.041 0.047 0.048 O.OS9 0.060 0.062 0.063 0.068 0.069 0.075 0.079 0.080 0.082 0.086 0.089 0.089 0.091 0.094 0.097 0.098 0.099 0.109 0.109 0.112 0.112 0.115 0.116 0.122 0.123 0.125 0.127 0.139 0.143 0.152 0.161 0.174 0.296 0.376 0.404 0.471 0.618 0.620 0.622 0.670
... e . .
... ...
0.002
...
...
0.006 0.006 0.005
...
0.004 0.005 0.005
...
0.007 0.006 0.008 0.007 0.006 0.006
...
...
0.007 0.009 0.009 0.007 0.009 0.014 0.008
...
0.008 0.008 0.012
...
0.011 0.011 0.010 0.012 0.011 0.011 0.013 0.025
... ...
b b
... Shell corroded away. * Entire specimen b
corroded away.
Rounding of the edge of the specimens during polishing was also given attention as a possible source of error. However. the photographs used for detailed measurements were enlargements from 4 X 5 inch plates made with a 12 X lens systam. This system has considerable depth of focus, so that a slight rounding of the edge of the specimen by polishing could be tolerated without significant error. T h e Whipple disk compared favorably with the core as a standard of size. Several specimens were measured with both standards to prove this. VOL. 51,
NO.
12
DECEMBER 1959
1473
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Flow diagram of single-kiln calciner
Flow diagram of double-kiln calciner
This appuratus differed from the double-kiln apparatus in removal of the second kiln and raising of temperature to 700" C.
Results. Rates of average penetration ranged from 0.027 to 0.67 inch per year, except for three specimens which were entirely destroyed (Table I). T h e best materials proved to be high-nickel, high-chromium alloys. Inconel X, Armco 314, Illium G, Xi-o-nel, and Haynes 25 were finally selected as best, with average penetration of
