Metal Materials for Handling Aqueous Hvdrofluoric Acid J
RSORTIMER SCHUSSLER Metallurgy Department, Carbide & Carbon Chemicals Co., Oak Ridge, Tenn.
A
KUMBER of chemical processes, pitrticularly in the
newer industrial fields, require the handling of aqueous hydrofluoric acid. 9 considerable background of information has been accumulated on the corrosion resistance to aqueous hydrofluoric acid of various materials of construction. Essentially all of the industrial experience and corrosion test data on metals or alloys which are reported in the literature deal with observations on single materials tested or exposed separately or on ideal test specimens of a metal or alloy (2-7). Very little has been reported on the corrosion resistance of joints of the materials commonly used for handling hydrofluoric acid. Many materials have useful resistance to aqueous hydrogen fluoride when completely immersed in the liquid, but when partially immersed suffer a very severe attack a t the liquid level line or a t areas where condensation of vapor occurs. An extensive test program is in progress to study such effects, but has not included study of the corrosion resistance of welded or silverbrazed materials such a s Monel or copper-nickel. Experience has shon n t h a t leaks often develop in joints during service in the handling of aqueous hydrofluoric acid. The purpose of the investigation described was to study the corrosion resistance of such joints when exposed to aqueous hydrofluoric acid a t concentrations of 48 and 5% under conditions selected to represent those probable in service. Additional corrosion tests in hydrofluoric acid are being run on Monel !veld joints made by the metal arc, shielded inert gas, and oxyacetylene welding processes. The purpose of these tests is to determine the susceptibility toward stress corrosion cracking of as-welded, stress-relieved, and annealed hlonel welds made by the three procedures. During the period when this investigation was in progress, the International Kickel Co. was conducting extensive studies on the stress corrosion cracking of htonel and other nickel alloys in hydrofluoric acid vapor (1).
points for evaluating the extent of the corrosion attack. The face side of the welds was not disturbed except to remove welding slag. Some of the weld samples were left in the as-welded condition, while the remainder were given selected heat treatments. One group was given a stress-relieving treatment for 15 minutes at 1150 'F.; another group was annealed for 15 minutes a t 1700 O F.; still another group was annealed for 2 hours at 2100' F. The latter high temperature anneal was a n attempt t o eliminate by diffusion the cast, dendritic structure occurring in the weld metal. All heat treatments were conducted in a dry 90% nitrogen-10% hydrogen atmosphere. T h e samples were then ground lightly on the root side of the welds with a belt sander, using KO. 180 grit paper. This provided flat sample faces of uniform surface finish for the tests. The measurements of Teight and surface area were the last procedures in sample preparation prior to the corrosion tests. The copper-nickel welds were made by the Revere Copper and Brass Co., Rome, N. P., using a practice similar t o that used in making the Monel welds. The sample preparation was identical except for the appropriate changes in the heat treatment temperatures for copper-nickel. The stress-relieving treatment was 15 minutes a t 900" F., the annealing treatment was 15 minutes a t 1500" F., while the high temperature, diffusion anneal was for 2 hours a t 1700" F. Silver-brazed lap joints of None1 were made by overlapping 1 X 2 X l / 8 inch annealed Monel plates for a n approximate '/c inch. The nominal compositions of the brazing alloys w e d in making the specimen joints are given in Table I. Handy flux (it fluoride type) was used in all cases and complete filling of the joints was obtained by uniform heating of the samples with a gas torch. The surfaces of the brazed samples were prepared by belt grinding, and the samples viere weighed and measured prior to corrosion testing. Separate corrosion tests were also conducted on wire or sheet samples of the brazing alloys.
EXPERIMENTAL PROCEDURE
X~TERIALS TESTED. The base materials used in the corrosion tests \yere: cold-rolled Monel and annealed Monel, cold-rolled copper-nickel and annealed copper-nickel (70% copper, 30% nickel), Illium R, commercially pure lead, and silver (99.999% pure). A11 these materials were available in plate or sheet form. The specimens were cut to have about 100 sq. cm. of surface area in each specimen ( 2 x 2 inches square or slightly greater). PREPARATION OF SA?mms. T h e Monel welds used in the corrosion test8 were prepared by the metal arc process, using Inco 130X filler metal. The m.elds were made using a copper back-up plate t o enswe complete root penetration. The plates were clamped tightly before welding to minimize distortion during welding. Both of the Ti-beveled, 4 X 12 X l / 4 inch htonel plates comprising the joint were in the annealed Condition, except for composite joints which were made b y joining a n annealed plate to a cold-n-orked plate. One joint was made n i t h the plates preheated to 400" F. prior t o welding. The weldments were then sectioned into 2 x 2 inch samples, with the weld running longitudinally through the center. The root side of the welds waE ground flush with the base metal in order t o have known reference
TABLE I. NOMINAL COMPOSITIONS OF SILVER-BRAZING ALLOYS USEDIN HYDROFLUORIC ACIDCORROSION TESTS
Alloy Designation
Ag
Chemical Composition, Per Cent Zn Cd Sn In
Cu
h-i
The Illium R, in I/*-inch sheet, was tested in the as-received condition. The Illium R brazed samples were made using only the ASTM Grade 7 brazing alloys. Separate corrosion tests were conducted on lead and silver samples. The lead was avaiIable in '/g-inch sheet and the silver in 0.020-inch sheet. Surface preparation on these samples consisted only of solvent degreasing prior t o weight measurements before the corrosion tests were started.
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T A B L E 11. C O R R o S I o N PENETRATION RATES (Based on weight loss measurements at 150° P.) Corrosion Penetration, Inch/l'ear Saniple Monel and Monel weids ( 3 2 d a y exposure) None1 None1 weld
None1
Condition Bnnealed, base metal As-welded Stress-relieved Preheated Annealed High temperature annealed Annealed, base metal Cold-worked, base metal
hIonel welds (duplicate test, 31-day exposure) None1 weld Copper-nickel a n d coppernickel welds (29-dav . " exposure) Copper-nickel Copper-nickel weld
Copper-nickel hlonel welds and coppernickel welds (in same b a t h , 31-day exposure) Monel weld CoDDer-nickel weld _. Silver-brazed Monel joints (40-day exposure) Monel
As received As-welded Stress-relieved Annealed High temperature annealed Annealed, base metal
0.002 0.003 0,004
0 . ois' 0.034 0.054
0,004
0.039 0.038 0.030 0.047 0.033
0.005 0.002
...
--___.
0.001 0.004 0,004 0.004 0,004
... o.ooi
,..
0,039'
0.136 0.126
0,026 0.032
...
0,076 0.088 0.091 0.080 0.108 0.066
0,005 0.007 0.007 0.006 0.007 0,004
0 029 0,034 0.033 0.027 0,032 0.020
0.003 0.004 0.006 0.004 0.004 0,002
0.094 0.177
0.017 0.029
0 102 0.116
0.008 0.014
0,004 0,083
0.016 0.014 0.013 0.014 0,017 0.014 0,016 0.016 0.015 0.022
0.016 0 030 0.028 0.030 0.033 0.027 0.027 0.026 0.026 0.032
0.004 0,002 0.001 0,001 0.023 0,002
As received Base metal Brazing alloy Brazing alloy Brazing alloy Brazing alloy Brazing alloy Brazing alloy Brazing alloy Base metal Rase metal
0.207 0.039 0.046 0 026 0.042 0.033 0.053 0,123 0.104 0.114
Silver (31-day exposure) Base metal a
0,049 0.069 0,056
0.073 0.080 0,059 0.074 0.079 0,081 0.082 0.047
Illium R Monel
Illium R Silver-brazing alloys (32-day exposure) Copper-nickel Easy Flo 4 ETX Ready Flo ABTM Grade 7 RT-SN ASTM Grade 3 Allstate 172 Monel Illium R Lead (31-day exposure) Base metal
c _____ 6% HF Totally immersed Partially immersed Totally immersed
Partially immersed
0.026 0,039 0.038 0.027 0.038 0,040 0.067
0.054
...
...
0.003 0.002 0.001
0.022
0.022 0.038
0.033 0.043 0.099 0.019 0,024 0.048 0.060
0.001 0.010 0,006 0.006 0.009 0.006 0.015 0,006 0,004 0.012
0.070 0.063
0.056 0.060
0.O W 0 . 02an
0.015 0.023
0.000012
0.000024 0.000025
0.000009 0,000009
0.000021 0.000018
0.036 0.030
0,049
0.000020
Corner of sample missing swing t o corrosion attack.
The niaterials and types of specimens and TESTPROCEDURE. the conditions used in the corrosion tests are presented in Table 11. Each test was made on groups of duplicate samples; one group of samples in each test was exposed at the liquid level line-Le., with the sample supported vertically and one half of the sample above the liquid level line-while the other group was completely immersed in the bath. The samples were supported on fluorothene racks, with free access of hydrofluoric acid permitted to all surfaces of the samples. The racks also provided electrical insulation, so that the samples vere not in metal-tometal contact. The tests were conducted in 3-liter fluorothene beakers fitted with groove-type lids t o minimize escape of the vapors and to cause the condensed vapors to drip onto the upper group of samples. The hydrofluoric acid baths mere not agitated during the tests. As the lids on the baths were intentionally not
TABLE111. CORROSION PRODVCTS FORMED ON MONELAND COPPER-NICKEL ABOVE LIQUID LEVELLINE (Chemical composition) Acid Ni-Cu Ratio Conon., % Material HF Copper, 7% Nickel, % In deposit In base metal Monel 5 9.94 24.9 2.5 2.3 48 11.8 24.3 2.1 2.3 Copper-nickel 5 32.6 18.7 0.57 0.39 43.4 8.19 0.19 0.39 48
made leak-tight, the test conditions provided for the condensation of oxygen-saturated hydrofluoric acid on the upper samples. However, the only oxygen present in the liquid was that due to diffusion from the vapor. The corrosion tests were made using concentrations of 48 and 57, hydrofluoric acid by weight. The bath temperature w a s maintained a t 150" i 10" F. for all tests by thermostatically controlled water baths. The test period was about 30 days in each case. No obeervations were made during the test period, except for routine control checks of the teEt temperature and the hydrofluoric acid and water levels. After exposure, the samples were photographed to iecord the appearance of the corrosion products formed during the tests. Some of the corrosion product was collected from various samples and submitted for chemical analysis (see Table I11 for results). Follow-ing this, the samples were neutralized in sodium carbonate solution and the corrosion products were removed by mild abrasion in hot water to permit evaluation of the extent of the corrosion attack. DATA A h D DISCUSSION
The rate and type of attack were determined both by measurements of weight loss (Table 11) and by optical examination. I n the calculations of weight loss, the surface areas were determined by geometric measurements. The values for density of the materials under study were taken from the literature. The optical observations included a macrostudy of the corrosion attack,
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Surface of Partially I m m e r s e d M o n e l Weld Samples Exposed t o 5 % hydrofluoric acid a t 150' F for 32 days
F
Cold-worked base metal control
supplemented by a microstudy of sections selected to determine particular types 6f corrosion damage to the samples. A series of photographs was made to obtain a visual record of the attack experienced on selected, representative samples. Photomicrographs were made of cross sections taken to illustrate the types of attack in greater detail. Silver. Silver exhibited very low corrosion rates under all test conditions. The samples were essentially unaffected when either totally or partially immersed in the hydrofluoric acid baths. Silver \vas the most resistant of the materials tested. Lead. Lead did not appear to be usefully resistant to aqueous hydrofluoric acid under the test conditions. Heavy lead fluoride deposits were built up over the exposed surfaces. The lead sheets became laminated by the corrosive attack and corner pieces of the samples dropped off during the test procedure. Illium R. Deep corrosion pits developed in the Illium R samples which were completely immersed in the test baths. The samples were severely attacked a t the liquid level line. Furthermore, Illium R developed stress corrosion cracks during silver brazing. Because of this behavior, Illium R did not appear to warrant further study. Monel, Totally Immersed. SEVERITYOF ATTACK. Monel exhibited good resistance to aqueous hydrofluoric acid when totally immersed in the baths. The lower, totally immersed portions of the welded samples shown in Figure 1 and of the silver-
brazed samples shown in Figure 2 illustrate that only slight corrosion occurred. The portions of samples above the liquid level which are essentially free from corrosion were protected by corrosion products and hence were similar t o totally immersed areas. Therefore, both welded and silver-brazed joints appeared to be satisfactory under these conditions. The experimental evidence indicates that when the joints are completely immersed the welds can be used in the as-welded condition-i.e., without stressrelieving or other heat treatment. The choice of brazing alloy for silver-brazed joints does not appear to be critical; however, the brazing alloys of higher silver content, such as ASTM Grade 7 , appear to be preferable. Monel apparently can be used satisfactorily in either the annealed or cold-worked condition when it is completely immersed. The corrosion rates based upon measurements of weight loss can be applied with reasonable confidence to the various Monel samples which were tested totally immersed, as the attack was slight and fairly uniform in all cases. The values of corrosion penetration were found to be low and to vary by as much as factors of 2 to 5 between runs under similar conditions. They were consistent within a run, however, and the values obtained fall within the ranges reported in the literature. The variation in corrosion rates between runs may well have been due to the variable oxygen content. The lack of control of the oxygen concentration makes the quantitative penetration rates less significant,
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fluoric acid, the cracking was even more severe; the cracks oi,iginated at, the grain boundaries but propagated entirely transgranularly, as illustrated in t8hecross section in Figure 4. The weld metal, 17-hich had a cast, dendritic structure in tho samples, showed more attack in essentially all cases than did the base metal. The most severe cracking occurred on as-welded Roldnic;its, and none was observed on samples annealed after welding (Figure 1). The cracking occurred in bot,h the weld metal and base metal. Vhile these particular stress-relieved samples in these groups did not shoT1- cracking, limited evidence indicates that stress-relieving may not be a complete cure against such clacking in metal arc i d d joints. Severe cracking was otisrrvrd in both as-welded and stress-relieved, longitudinal seam, metal arc ix-cldcd joints of Xonel pipe eamplcs which vere partially inimerscd in 5 % hydrofluoric acid a t 150" F. for 20 days. Hon-ever., similar joint,s annealed after nelding and exposed similarl>. w e r ~free from cracks. Preliminary data from tests now in progress intlicat,e that Monel welds made bj- the shielded inert gas prow susceptible to such cracking than are welds made by eit,hoi, the metal arc or the oxyacetylene process. The shielded inert gas welds are not, completely immune to cracking, but, it appc.ars that shielded inert gas 11-elding is definitely the preferred method for making Monel joint,s. Stress corrosion attack is believed to be the cause of the cracking. It has been suggested that hydrogen embrittlcnient, may play a part in the cracking of &Ionel exposed to aqueous hydrofluoric acid vapors ( 1 ) . This investigat,ion indicates that the stress corrosion cracking is motivated by the condensation o l oxygen-saturated vapor on the samples and by internal stresses in the material.
Figure 2.
Face View of Partially Immersed Silver-Brazed Samples
Exposcd t o 6 R hydrofluoric acid a t 1503 F. f o r 40 days A . Annealed Monel base metal control B . None1 lap joint brazed with E a s y Flo 4 alloy C . Illium R brazed with ASTRZ Grade 7 alloy D . Rlonel lap joint brazed with Ready Flo alloy E . .\Ionel l a p joint brazed with ASTAI Grade 7 alloy F. 3 l o n e l lap joint brazed with Allstate 172 alloy
but a t the same time demonstiates that oxygen is very inipoi tant in the corrosion mechanism. Monel, Partially Immersed. SEVERITYOF ATTACK. llonel suffered a severe attack a t and above the liquid level line. When weight loss measurements xere used as a rough indev of the severity of attack, the rate of corrosion on portions of the samples a t and above the liquid level line \vas consistently 20 to 100 times that experienced on the totally immersed samples. Hon ever, weight loss measurements can be applied as indexes of corrosion penetration only when the attack is uniform. The attack a t and above the liquid line definitely Ras not uniform. This was observed in the microstructure studies made on blonel base metal, None1 welds, and silver-brazed lap joints of Rlonel. Recent work has shown that materials such a s -\Ionel and copper-nickel suffer a severe attack a t and above the liquid line when the vapor contains oxygen ( 5 , 6). The severe attack did not occur when a purified nitrogen purge was used to exclude all oxygen from the system. The Alone1 base metal suffered a n MOSELWELDSAMPLES. intergranular attack, with some intergranular cracking, on samples partially immersed in 48% hydrofluoric acid. The cross section shown in Figure 3 illustrates this type of attack and also s h o m transgranular cracks propagating from the regions of intergranuIar attack. On the samples partially immersed in 5% hydro-
Figure 3.
Longitudinal Section from Edge of Partially Immersed Monel Specimen
Exposed t o 48% h>drofluoric acid f o r 30 days at 150' F. Carapella's etch
lOOX,
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Figure 5. Photomicrograph of Extensive Transgranular Stress Corrosion Cracking of a Monel U-Bend Specimen Exposed t o 5% hydrofluoric acid t o t h e liquid level for 72 hours a t 150° F. I O O X , Carapella's etch
Figure 4.
Longitudinal Section from Edge of Partially Immersed Monel Specimen
Exposed to 54: hydrofluoric acid for 30 days at l B O o F. etch
2,5OX, Carapella'a
EFFECTOF INTERNAL STRESS. Several additional tests confirmed the belief that the severe attack suffered by Monel when partially immersed was due to stress corrosion cracking induced by residual stresses, and that such cracking occurred only a t or above the liquid level line. Two strip samples were cut from an annealed Monel plate and bent into U shapes. One strip was left in this condition, while the other was clamped in a fixture to maintain the stress induced by bending. The samples were partially immersed in 5 % hydrofluoric acid a t 150" F. Cracks due to stress corrosion attack were noted on the tension side a t and above the liquid level line on both samples after only 3-day exposure. The transgranular cracks found in the clamped sample are shown in the cross section in Figure 5. The sample that was bent but not clamped showed similar but less severe cracke. No cracks were observed a t areas removed from the position of maximum stress on the portions of the sample that were partially immersed. Nor did any areas of the portions of the samples which were totally immersed in the bath show cracks, even at the position of maximum bending stress. The stress corrosion cracking which occurs in Monel exposed to aqueous hydrofluoric vapors apparently occurs a t very low orders of internal stresses. The level of internal stress below which such cracking would not occur was not determined. SILVER-BRAZED SAMPLES. In silver-brazed joints exposed totally immersed, especially those made with an alloy of high silver content such as ASTM Grade 7, the brazing alloy functioned much as an inert silver gasket and was relatively unaffected except for slight depletion of copper, zinc, and other elements from the braze. This depletion of alloying elements resulted in some weakening at the extreme, exposed edges of the braze. The dark areas at the brazing alloy fillets shown in Figure 6 illustrate the depth of attack on the traverse of the filler metal. The cross sections shown in Figure 6 were taken at 45" at the liquid level in order t o exaggerate the attack.
At and above the liquid level line, the base metal adjacent to the brazing alloy tends to be undermined by a selective attack (Figures 6 and 2). This attack probably is galvanic in nature, owing to metal ion type of cell action. However, the evidence indicates that, with a sufficient overlap, it would take longer for the base metal t o be undermined along the braze interface than for the corrosion attack to perforate the base metal wall thickness. The base metal in the silver-brazed samples that were partially immersed in 48% hydrofluoric acid suffered a n attack similar to that noted on welded samples. The base metal a t and above the liquid level in the silver-brazed samples that were partially immersed in 5 % hydrofluoric acid had severe stress corrosion cracks that initiated at the sheared edges on the samples (Figure 2). The path of propagation of the cracks, like that on the weld samples, was found to be transgranular (Figure 6). SERVICEEXPERIENCE. I n attempting to apply these data t o service conditions, the welded samples were not SO severely stressed as some welded areas encountered in plant applications. On the other hand, silver-brazed joints made with couplings probably result in a lower order of internal stress in the Monel than the lap-type, plate samples employed in the tests. Two similar plant facilities have been used which involved handling concentrations of hydrofluoric acid ranging from the maximum boiling point azeotrope (38% hydrofluoric acid) to anhydrous hydrogen fluoride. The first facility employed metal arc weld joints in the as-welded condition. A number of failures similar t o that shown in the Figure 7 cross-section occurred in the weld areas in the system. Shielded inert gas welds, stressrelieved after fabrication, were used in the second facility and no failures have been reported. Unless proper techniques are used, butt-type weld joints often have irregular, sometimes jagged, projections a t the root of the weld. These irregularities tend t o form pockets in which acid collects and remains stagnant, resulting in a localized attack at the liquid level line, as described previously. Lap-type joints (couplings), silver-brazed with ASTM Grade 7 alloy, have been employed in applications where welding was impractical. Where proper techniques were used t o obtain brazed joints that were nonporous, completely filled, and of long lap length, the service life has been acceptable. Poorly made
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the edges of the samples, due t o the c o n d e n s a t i o n of vapors containing oxygen. IIowever, the principal wuse Exuosed a t liouid level t o anlieoils hvdrofluoric acid a t l:Oo 17. for 40 days 2 X I rnacroetched for the excessive corroeion apimrcd to -. 9. Exposed t o 48% €IF B . Expo.3cd t o 6°C HI' be a concwiiralion cell type of att;rclc. Tliia statement is bawd on tlic follon-ing ohservs,tions: the attack occurs only nt and a h v e the liquid level line whcre condensation of oxygen-sut,urated vapor occurs. thc attack is charactcrizcd by deep grooves or pits running vertically on the samplw, the at,taclr a t the liquid level line is most severe whcrc the spacers in the sample holders would direct the Row of condensate on the samples, and the attack did not o c c ~ ~ atr locations ~ h c r o hcavg deposits of corrosion products formed and protected the siu.fat:c. COPPER-SICKEL ~ ~ E I ,S.rxrr,e3. D On the unrestrained weld sainplc~that, w ~ r ot , e s t d : the licxit 1 ttmerits appeared t o offer little advantage over the a s - d d c d condition. Nor did tiic use of annealed Isasci metal offer a particular advantage over coldFigure 7 . Typical Failure in Monel Weld $reas i n 4qiaeoiis worked material. The JTeld nictal shoncd a greater attacalc, in Hydrofluoric Acid Pliant kpplieation general, than did the base metal. A
B
Figure 6 .
Sections of 3loriel Lap Joints
3 X n>acioetchod
brazed joints-i.e., joint,s tlitLt were porous or did not have sufficient penetration-have given poor service. T h i e is a definite limitation of brazed joints, as the quality of thc joint cannot be determined reliably by inspection; hovever, welded joints do not suffer this disadvantage, as a weld ran be seen readily as it is being made. Copper-Nickel, Totally Immersed. SEVERITYOF ATTACK. Copper-nickel, like Monel, exhibited low corrosion rates ivhen totally immersed in tmhcbathe. The lower, tot,ally iminerscd portions and also areas above the liquid level protected by corrosion products on the copper-nickel samples shown in Figure 8 had negligible attack. Welded joint's apparently can be used in this service in the as-welded condition. While silver-brazed joints in copper-nickel were not tested, such joints would be erpec+d to have suitable resistance to corroeion under these conditions. Since the attack was uniform, the corrosion rates based on weight loss measurements can be applied t,o the colilx?r-niclrc~l samples -which were totally immersed. Copper-Nickel, Partially Immersed. SEVERITY A N D TYPE08' ATTACK. Copper-nickel suffered a severe att,aclr at and above the liquid level line (see Figure 8). Deep grooves developed in the samples exposed t,o 5% hydrofluoric acid, and severe t,liiniiiiig occurred a t the liquid level in the samples exposed to 43% hydrofluoric acid. The grooving attack resulted from preferential corrosion along bands in the metal. The bands mag be due t o residual coring in the alloy--that is, the bands differ slightly in -----f
Figure 8.
Edge of Partially Immersed Copper-Kickel Weld Samples
Exposed t o 48 and 6% hydrofluoric acid a t 160° F. for 2P days Top samples. Exposed to 5 % HF Bottom samples. Exposed t o 48% H F A . -4s-received base metal control B . Annealed base metal control C. As-welded weldment D . Stress-relieved weldnient E . Annealed weldment F . High temperature weldm(~nt
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Oorrosion Products on Monel and Copper-Nickel. The analyses of the corrosion product deposits formed above the liquid line on Monel and copper-nickel are given in Table 111. The deposits on the copper-nickel samples exposed to 48% hydrofluoric acid were blue in color like copper fluoride, while the deposits on the samples exposed to 5% hydrofluoric acid were green in color like nickel fluoride. The differences in the ratios of nickel to copper in the corrosion deposits do not necessarily indicate a preferential attack on the base metal. Such differences may result from the relative solubility of the corrosion products which form and then gradually leach away. Thus in 5% hydrofluoric acid relatively more copper fluoride than nickel fluoride went into solution in the acid or washed away; therefore, the remaining insoluble corrosion products became relatively rich in nickel. Silver-Brazing Alloys. The most informative way to evaluate or compare silver-brazing alloys is in the form of brazed joints. Such joints have been considered in the discussion of Monel. However, some tests mere made on the brazing alloys in mire or sheet form in order to determine order of magnitude differences between alloys. All of the silver-brazing alloys tested in this manner were weakened to varying extents as a result of exposure to hydrofluoric acid and were not resistant to bending. This may be due to a selective solution of alloying elements such as zinc, cadmium, tin, and copper, ivhich results in a porous, fibrous silver skeleton remaining. However, the mechanism of corrosion may involve dissolution of the alloy and replating of thp silver. The brazing alloys after exposure were changed in appearance to the soft matte gray color of silver. The attack occurred on totally immersed samples as well as on partially immersed samples. The extent of loss of ductility, as well as the rate of attack, decreased as the silver content of the brazing alloy increased The brazing alloys of higher silver content, such as ASTM Grade 7 , did not suffer much greater attack a t or above the liquid level line than when totally immersed in 48% hydrofluoric acid. Its corrosion rate in 5% hydrofluoric acid was similar to that for Monel. In silver-brazed joints of Monel made with the ASTM Grade 7 alloy, the attack of the brazing alloy appeared to be confined to exposed surfaces and penetration into the brazed joint was dependent on the depth of undermining which first occurred on the base metal (Figure 6). Therefore it appears that such embrittlement of the brazing alloy in hydrofluoric acid is not as serious a problem as is the higher corrosion rates of the material it joins. The brazing alloys of higher silver content definitely are to be preferred for silver-brazed joints to be used in hydrofluoric acid service. SUMMARY
Monel and copper-nickel showed little attack when completely immersed in the aqueous hydrofluoric acid baths. Either welded or silver-brazed joints in these materials should give good service under these ideal conditions. Furthermore the welded joint8 could be used in the as-welded condition and, if silver-brazed joints are used, the selection of brazing alloy is not critical. The evidence obtained in this study indicates t,hat the life of Monel and copper-nickel in hydrofluoric acid service will be considerably decreased when a liquid level line is involved, or when alternate evaporation-condensation of vapor containing oxygen occurs. The susceptibility of these materials to severe attack under these conditions is the most important’ consideration in their service life. Furthermore, hlonel (and perhaps copper-nickel also) can suffer stress corrosion cracking under these conditions, particularly in welded areas. Silver-brazed joints of Monel show undermining of the brazing alloy and can
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show stress corrosion cracking under these conditions if the Monel has sheared edges. These severe types of attack on Monel or copper-nickel may be avoided by keeping the metal surface continuously immersed in the liquid (by keeping the system full a t all times), or purging the vapor with purified (oxygen-free) nitrogen, or maintaining the pressure in the system above atmospheric pressure, so that oxygen is excluded. If any or all of these conditions can be incorporated in the design, either Monel or copper-nickel should prove to be a suitable long-life material of construction when the process conditions involve acid concentrations and temperatures similar to those studied here. Some systems cannot be kept full of liquid and it is generally beyond the limits of practicality to exclude oxygen completely. It is doubtful whether any industrial hydrofluoric acid application can be regarded as involving the ideal exposure conditions of a quiet bath and t’otnl immersion. Therefore it appears desirable to incorporate certain precautionary meaaures in Monel Eystems for hydrofluoric acid: using only fully annealed Monel for fabrication] using welding techniques which avoid ragged projections or crevices, using the shielded inert gas process for welding, and stress-relieving the weld areas or, better st’ill, fullannealing followed by slow cooling. Silver-brazed joints should have long lap lengths (suggested as a minimum of three to five times the base metal thickness) and should rigidly conform to proper joint clearances to obtain high strength and freedom from crevices. Only brazing alloys of high silver content such as ASTM B73-29, alloy 7 , should be used, and sheared or severely cold-worked edges on the base metal should be avoided. I t mould also be safe practice to apply the above precautions to systems fabricated from copper-nickel. When alternate condensation-evaporation of aqueous hydrofluoric acid vapors containing oxygen cannot be avoided, the syfitem should be so designed that these severe conditions are confined to localized areas of the system. Among the metal materials tested, only silver offered superior resistance to attack a.t and above the liquid level line. I n t,his type of service pure silver or, if the temperature is low enough, certain plastics or impervious graphite may offer the lowest cost materials of construction, considering their long expected service life. Neither Illium nor lead appeared to have useful resistance to corrosion by aqueous hydrofluoric acid under the test conditions. Even when tot,ally immersed, Illium R was perforated under some test conditions. Lead did not appear to resist the general chemical att.ack. ACKNOWLEDGMENT
The assistance of A. G. Dobbins and D. S. Kapolitan, who did most of the work in sample preparation and test operation in this investigation, is gratefully acknowledged. LITERATURE CITED
(1) Copson, 1%. R., and Cheng, C. F., International Nickel Co.,
Bayonne, N. J.. unpublished data. (2) Friend, W. Z., and Teeple, H. O., Oil Gas J., 44, Eo; 45, 87-101 (1946). (3) Holmberg, M. E., and Prange, 5’. A , , IND.ENG.CHEM.,37, 1030-3 (1945). (4) Phillips Petroleum Co., “Hydrofluoric Acid Alkylation,” (1946). ( 5 ) Pray, 13. A., Fink, F. W., Friedl, B. E., and Brann, W.J., “Corrosion-Resistant Materials for Hydrofluoric Acid,” Battelle Memorial Institute, Progress Report, BMI-268 (June 15, 1953). (6) Pray, H. A., Fink, F. W., Friedl, B., Ericson, G . L., and Peoples, R. S., “Corrosion Studies Connected with the Refining of Uranium,’’ Battelle Memorial Institute, Progress Report, BMI-255 (Oct. 31, 1951). (7) Whitaker, G. C., Corrosion, 6, 283-5 (1950) : 9, 74-6 (1953). RECEIVED for review April 27, 1954.
.4CCEPTED August 17, 1964. Based on work performed for the Atomic Energy Commission by Union Carbide & Carbon Corp. at Oak Ridge, Tenn.
