December, 1929
INDUSTRIAL ALVDE,VGINEERISG CHEMISTRY
--
1283
Properties of Nickel in Caustic Evaporation' Robert J. McKay INTERKATIONAL NICKEL COMPANY,
INC., NEW
YoRK,
N. Y
In selecting the one of many possible alloys which tubes. I n many cases nickel should be used for a given piece of apparatus, such as tubes or tubes and sheets are uses for them play a a caustic evaporator, the alloy should first be shown attached to cast-iron or steel useful part in the deto have the desirable resistance to the corrosive conparts, and then there is alvelopment of chemical and ditions or to be so constituted that its corrosion can be ways a tendency for galvanic other industries. An advance controlled within given limits. The alloy showing the corrosion, which must be alalong this line which has cerlowest loss in weight in corrosion tests is seldom the lowed for in design. tain unique features is the one selected, but after the foregoing requirement is CIRCULATION-Difference introduction of nickel into satisfied the alloy having the best composition and in circulation rate produces evaporation equipment f o r mechanical properties and greatest ease of working difference in corrosion by its caustic soda. I n entering this and availability should be used. effect on protective films, on field nickel replaces metals In caustic evaporation the corrosion rate of nickel in electrolytic con c e n t r a t i on that are cheaper in some cases caustic solution is so low that it is negligible. In steam, cells, and on the supply of by as much as 8 or 10 to 1. corrosion may occur with high carbon dioxide content. low-concentration corrosives Keither the apparatus nor the With control of the carbon dioxide content the corrosion to the metal surface. There m e t a l i s new. Nickel has is minute. Potentials as high as 0.5 volt are obtained is usually little practical effect long been used as a laboraby contact between nickel and cast iron or steel, and of varying c i r c u l a t i o n on tory material for caustic fuamperages indicating rapid corrosion are obtained in corrosion in caustic solutions, sions and the fact that it is strong hot solutions. Serious corrosion will occur even though the rate differs h a r d l y acted on a t all by only where the type of contact and areas exposed favor materially in different proccaustic solutions or by steam it. esses. I n the steam it has an a t high temperatures has been Hard-drawn nickel tubing has good strength and effect in supplying c a r b on known for many years. But hardness characteristics for evaporator tubing. Hotd i o x i d e and oxygen to the it is a long step economically rolled plate should be used for tube sheets. Methods corroding surface. from this laboratory experiFrLais-Protective or acof forming and welding nickel linings are described. ence to the manufacture of celerating films have not been Arc welding is probably the most useful process in this an evaporator w h i c h m a y type of manufacture. found to be vital to the corroweigh 20 tons. Two major sion resistance of evaporators. considerations have led to this step-first, the growing requirement for high-purity caustic, Films of various sorts are usually present on the caustic side and are probably protective. Protective films are commonly and second, the length of life of equipment. absent from the steam side. Corrosion as Affected by Processes and Evaporator Types T E M P E R A T U R E - T ~ ~ usual increase of corrosion with temperature is of practical importance in caustic evaporation. Although a complete discussion of the several caustic procCorrosion both by steam and by caustic is much greater as esses and evaporator types is unnecessary to an understanding of the use of nickel in them, certain factors typical of the temperature increases. The use of nickel may sometimes be advisable in high-temperature but unnecessary in lowcertain processes should be mentioned in passing. There are temperature parts of the same process. two separate sets of corrosive conditions in all evaporatorsin the caustic solutions and in the steam. It n-111 be conCorrosion by Caustic venient to discuss these under headings of the six practical factors which the author has mentioned elsewhere as useful From time to time sample lots of nickel tubes have been for generalization on corrosion (2, 3 ) . installed in caustic evaporators a t various temperatures and HYDROGES-IOA C O ~ C E S T R . ~ T I hydrogen-ion O ~ - ~ ~ ~ con- concentrations which were determined by the convenience of centrations in caustic solutions are so low that this factor installation. Such samples, after exposure for 1 to 2 years, becomes invisible in its effects. In the steam an increase in have usually shown no determinable action by the rough hydrogen-ion concentration increases the corrosion. The methods of visual inspection arid measurement available for presence of carbon dioxide in the steam has this dfect to a their examination. I n a few cases fairly definite quantitapractically important extent. tive results have been obtained. dkTMOSPHERIC O S Y G E N AXD OYIDIZIXG LkGENTS--.OWing t o A welded nickel tube 15-as inserted in an evaporator a t an the method of handling, caustic solutions themselves are arerage temperature of 95' C., delivering caustic a t 50 per unlikely to contain air or oxidizing agents. Air may be cent concentration for a period of 21 months. I t was weighed present in the steam and where it is the corrosion is in- before and after insertion, the first w i g h t being 3.63 kg. creased. (8 pounds) and that after removal 3.57 kg. (7 pounds 14 ELECTROLYTIC EFFECTS-oJTing to the continuous move- ounces). This weight difference was within the limits of ment of both steam and caustic solution, intense loc''11 concen- accuracy of the weighings. Tlle tube x a s cut in half and tration cell:: are seldom present. There may be Concentration examined and its thickness taken n-ith a micrometer along the cell effects on the tube sheets and in cases where improper whole length. This thickness was 2.11 mm. (0.083 inch) construction has left crevices, as, for instance, in rolling in * 0.025 mm. (0.001 inch), which mas the thickness specified on the original tube. At a point near one tube sheet the 1 Presented before the Division of Industrial and Engineering Chemthickness averaged 2.08 mm. (0.082 inch). This 0.03-mm. istry a t the 78th Meeting of the American Chemical Society, hlinneapolis, M ~ n n . September , 9 t o 13, 1929. (0.001-inch) thinning was apparently from the external or
N
EW metals and new
steam side, siiice tlic migiml surface was intact on tlie caustic side as detennined by visual inspection. Iii auotlier case a ivelded iiickel tube was u s d iii an evaporator liaiidling caustic a t approximatcly 90" C. (194' IT.) at a steam temperature of about 121° C . (250" F.) for 4 years and 5 mont.Iis. This tube yins removed arid carcfiilly examined in the laboratory. It liad been aiuiealcd aiid drawn after wcldiiig. The metal was of intermediate Iiariliiess, having been naither annealed nor normalized after t l i e last drawing.
tciidiiig below the surface of concentrated sulfuric acid. The outlet tube from this flask was connected to a calcium chloride drying tower, wbich, in turn, was connected to a Cenco Hyvac pump. A mercury manometer was also connected to the inlet tube to the drying flask. This flask was immcrscd in ice water during the test. The steel test pieces were sheets 2.6 by 5.0 by 0.150 cm. All other test pieces were of the usid disk type, 2.5 cm. in diameter by Q.625 cm. thick. The caustic soda was in the form of a solution which, in commercial practice, forms the feed to multiple-effect evaporators for concentration to proper strength for fiiial concentration in
substance disappeared and thc solution became a"&ar, light hlue, which remained when the solution was cooled. The solution as received was found by titration to contain 32 per cent sodium hydroxide and to have a density of 40" BC. (sp gr., 1.38). I n mnrriilg a test, 1000 cc. of the caustic solution was placed in the filter flask, which was heated on a sand bath to 93' C. (200" F.). (With the concentration used the solution steamed slightly at that temperature under atmospheric pressure.) The stopper carrying the thermometer was then removed and 3 wi&, each carrying a test piece at its lower end, were lowered into the flask. The upper euds of the wires were sccurely held in lace between the StODDeT and the neck of the Aask. Conn&bn was then made td ihhe vacuum pump and d l connections were scaled with de Khotitisky cement. The temperature and vacuum used were sufficient to came brisk boiling of the solution. When the solution had concentrated down t o a predetermined mark on the flask the vacuum pump was disconnected
The quarititative data are given in Table I. Each test is the average of three specimeris labfe I
MS.PW so. dm. per 3.5 9.3
Figure 1-Apparatue
6.3
1,z 8.3
Mrn. 0.0051 0.083
for Determining Corrosion by Cauatic
The-grain was reriiarkahly large and metal unusually free from inclusions. There was no iiidication of excessive corrosion on tlie iiiner surface. The line of the weld was marked by a slightly elevated longitudinal hand about 5 inin. wide. On examination under the binocular microscope it was seen to carry a thin, dark olive tarnish and numerous tiny moist globules of adhering caustic. Corrosion had heen slight. The surface was a series of extremely shallow depressions, separated by narrow ridges, giving tlie impression of a network. This tubc had thinned somewhat certain places. The maximum thinning was 0.23 m u . (0.009 inch), or 0.063 min. (0.0025 inch) per year. This thiuiiiiig was mainly from the external or steam side, although the exact proportion due to st.eam and to caustic could not be determined. The foregoing tests niade in actual plant apparatus are enlightening, but somewhat inconclusive, owiiig to the absence of proper reference points for measurement and to other difficulties of control. The amount of galvanic protection, Sor instance, afforded these tubes is not defined. It is therefore wnrth while to cornpare these results with data from laboratory and seiu-servicc tests. Four rcpreseiitative iiietals were tested in the laboratory by D. E. Ackerinan as follows:
.
day 0.0
A 1-liter filter flask was fitted with a rubber stopper through which a thermomcter projected. To the side neck was connected the inlet tube of a sulfuric acid drying flask, the inlet tube ex-
In addition to this laboratory test, a test was made in an operating evaporator with the use of the spopl type specimen holder (5). Roughly, this apparatus consists of a central metal supportiiig bar witli two eiid disks held together by four tie-rods. On the central supporting bar, which is first covered with a suitable insulatiiig material, are strong disksof tlie metals to be tested, separated from one another by short tubes of the insulating material. The wliole is clamped together by nuts on the ends of the supporting bars aud tierods (Figure 1). This apparatus is 60 designed RS to eonhine easy operation, standard specimens, flexibility in number of specimens, permanent or easily reylaceablc parts, and compactness for carrying and testing. Tahle I1
--,
380-384 Nifkei 3m-384 Copper-nickleiincsiioy 880-384 steci 380-384 Monrl metal
89 82
0.71
0.9 0.48
82 83
., ..
.. ..
1 4
1.1 0.59
0,00018
2.7
0,0021 0.011 0,00043 o 063
13
0.0045
0.00009 0.0024
Iii the present test this spool type specimen holder was suspended in the liquor of tlie downtake of a caustic evapora-
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1929
tor, concentrating caustic soda from 30 to 50 per cent a t an average temperature of 82" C. (179" F.). The quantitative data obtained are given in Table 11. The quantitative rates obtained in the laboratory test, in the semi-plant test, and in the tubes are not in close agreement. This is to be expected, as the tests are not made with close control of the six practical factors mentioned earlier.
Figure 2
Anode
1285
impurities in it. As mentioned in the paragraphs on corrosion by caustic, in one case a penetration of 0.025 mm. in two years has been noted, and in another, a penetration of 0.23 mm. in 4l/? years. The latter case n.as a welded tube and there was rather serious corrosion at weak points of the weld. An abstract of the laboratory report by Fraser on the corrosion of the tube apart from the weld is as follows: The outer or steam contact side of the nickel tube was corroded appreciably in an area extending down from the top tube sheet a minimum of 20 em. (8.5 inches) on one side, increasing t o a maximum of 61 em. (24 inches) almost opposite. The heaviest corrosion extended around the tube near the tube sheet and this tapered off gradually to the region of light corrosion. For one inch from the tube sheet the metal was quite rough. Pits were roughly 3 mm. in diameter and fairly deep. The grains stood out sharply in the deep portions. Between pits the surface mas only slightly attacked, showing traces of the grinding ridges. The balance of the corroded region was more generally corroded with many hummocks whose tops showed grinding scratches and in some cases were covered by blue-green corrosion products. Between these the surface was a mass of tiny pits similar to those on the slightly corroded surfaces below this corroded area. The maximum penetration of corrosion was 0.23 mm. (0.009 inch). The following tests illustrate the possibility of corrosioq and give an idea of the rate of the corrosion reaction.
Casthon
Figure 3
Not only are the results in disagreement, but in several cases the metals are not even in the same order as t o their resistance. It is obvious that no one of these tests can be relied on for too definite conclusions. By a careful examination of all of them, however, certain useful conclusions can be drawn. The most important is that the corrosion rate of any of the three non-ferrous alloys is low enough to give many years service in caustic evaporators. There is a rather definite advantage of monel metal and nickel over the copper-nickelzinc alloy, and, as far as a comparison of monel metal and nickel is concerned, the value of having caustic free from copper makes nickel the practical metal, regardless of variations in such tests as these. Corrosion by Steam
Probably the widest experience with nickel in contact with steam has been in its use as steam turbine blading. In this service nickel has been subjected to the action of steam a t various temperatures and pressures, wet and dry, usually a t high rates of movement. Few quantitative data on rates of action are available, because the rate is so slow as to make measurement practically impossible. However, it is possible for impurities which will produce some corrosion to exist in steam and the study of corrosion by steam thus resolves itself into a study of these impurities. The constituent that most merits consideration is carbon dioxide. By increasing the hydrogen-ion concentration, carbon dioxide in solution makes possible the usual corrosion reaction between metal, acid, and air. There is evidence that a small amount of corrosion may occur on nickel evaporators from the steam, or rather from
Figure 4
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INDUSTRIAL AAiD ENGINEERING CHEMISTRY
1286
Vol. 21, s o . 12
the conclusion that nickel will have a negligible corrosion rate in steam under the proper conditions and that unusual concentrations of carbon dioxide are dangerous to nickel and other metals. Galvanic Corrosion
CORROSION PRODUCT
METAL
LOSS
1
2
3
Mg.per Monel metal Monel metal Nickel Nickel
Copper Monelmetal Nickel Steel a
N o t removed Removed every test N o t removed Removed every test
I I
16 50 21
19 85 5
20 20 3
42 30 41 55
18 45 76 45
4
5
Av.
per day 67 51 75 68 S5 212 98 120 128 84
sp. dm.
91 92 75 71
35 97
Mg. per sq. dm. P e r day 31 41 40 14 -38 30 35 35 31 49 5 2" 24 2 a Za Z a 0 5 77 57 2 a 0 o o 2 0 2 5 0 3 35 z Greater than 6000
Gain.
I n another series of tests run under much the same conditions the average rates were as follows: copper 48; monel metal 66; nickel, 10. Again, as was the case in tests on caustic corrosion, the results are not in close agreement and incorrect deductions might be made if any one set were solely relied on. Taking t h e results of all the tests together, however, we may draw
The galvanic relations of various metals in caustic evaporations are of interest, as in many other types of apparatus. Galvanic corrosion relations between metals are not simple relations inherent in the metal themselves. They are closely associated with the effect of the corroding solutions on potentials and on polarization. Metals vary in relative potentials under different corroding conditions and exchange relative positions as anode or cathode, depending on these conditions. The factors determining these changes have not been elucidated. Therefore no brief statement covering caustic evaporation can be made and space is not available here t o discuss the general subject. The curves showing the results of tests of pure nickel with certain other metals (Figures 2 to 7) illustrate the relations which exist. These tests were made above and close to the top tube sheets of operating caustic evaporators. Jleasurements were made on both open and closed circuit. The electrodes used had the following areas: nickel, 4.6; copper, 3.65; cast iron, 7.3; commercial pure iron, 4.0 sq. cm. The general conclusions to be drawn from these tests a t present are that corrosion of practical importance may occur on either cast iron or steel due to contact with either nickel or copper and that the tendency is greater in caustic of higher temperatures and concentrations. It should be clearly understood, however, that the actual amount of this corrosion is entirely dependent on such features of design as relative areas of metal exposed and type of contact between metals. In other words, the potentials and amperages shown in these tests are not those existing in operation apparatus. Composition and Mechanical Properties of Nickel
It may seem contradictory t o say that corrosion resistance is not the principal criterion in determining the best material for a corrosion resistant use. Yet a moment's thought shows this to be true. The actual corrosion rates of monel metal
Figure 7
in the various tests here shown are less than those of nickel and yet there can be no question of the greater suitability of nickel, owing to other factors. It is well known that steel is used economically under corrosive conditions where other metals are more resistant. Thus, to understand why nickel is the most suitable material for this purpose, we must have in mind certain properties other than corrosion resistance which also are vital in determining the choice of metal. Possibly the most outstanding of these is the purity of the metal. Commercial nickel is over 99 per cent pure nickel
INDUSTRIAL AND ENGINEERIlYG CHEMISTRY
December, 1929
1287
and cobalt. It contains less than 0.5 per cent iron and less The hot-rolled plate is much more amenable to machining than 0.25 per cent copper. Both these metals are undesirable and when the tools are ground to proper high clearance a s impurities in caustic and for many uses freedom from angles and sharp cutting angles no undue difficulty is excopper is particularly desirable. Therefore the freedom of perienced. It is, of course, true that more power must be nickel from these metals and any others which are deleterious used and greater care taken to insure proper condition? of is a primary determining factor in its use. tools, lubrication, and cooling than with the weaker steels The eraporator tubing is, of course, an item of much and cast iron. interest. The seamless drawn nickel tubing which is comNickel Bodies monly used is manufactured by hot-piercing a billet, hotUp to the present time bodies of solid nickel plate have rolling the blank to a convenient intermediate size, and cold-drawing through chromium-plated dies to the finished not found great favor on account of the large amount of size, with intermediate annealing and pickling. The finished expensive material tied up in them. As a temporary expetube is straightened and tested under a hydraulic pressure, dient, evaporators of steel or cast iron have been lined 11-ith light gage nickel sheets. Such lining is feasible because which is determined by the size of the tube. There is a large amount of abrasion in the evaporation of the working properties of these nickel sheets. They will process. To resist this and to give the desirable rigidity the qtand all the necessary bending and shaping operations and tubing should be hard and tough. Nickel tubing will always there is apparently no danger of failure of deformed portions have a tensile strength of over 4200 kg. per sq. em. (60,000 from such delayed corrosive effects as corrosion cracking or pounds per square inch) in the metal and a hardness of 80 corrosion fatigue. The favored method of making joints is to 100 Brinell. I n evaporator work it is specified hard drawn by welding. Welding may be done on nickel sheets by any with stress relief anneal only. This insures properties well of the ordinary welding methods. On light gage sheets above those mentioned. The stress relief anneal prevents the oxyacetylene weld produces a sound-appearing closechange froin uniform shape or decrease of strength in long grained metal in the n-eld. Owing to the mechanics of perservice under corrosive conditions. The bursting strengths forming the welding operations in the interior of evaporator of nickel tubing given in Table V are calculated from data bodies, however, welding electrically with the metallic or cnrbon arc and reforming and closing the pores in the meld by on some typical sizes. air-hammering has been the most successful method. Table V Several types of seams may be used. Three of special inS E A J ~ L E S SDR.4W.N T U D E BURSTIXG PRESVRE terest niay be mentioned: ( a ) The edge or flange weld, made Kg. per sp. cm. L b s . per sq. an. by “breaking up” the edges of the sheet and then melting l/n-inch irdn pipe size 1108 15,800 down the contact line of the two bent edges until they are 1-inch iron pipe size 850 12,100 274 3,900 ?-inch 16 Ga. almost flush with the sheet; ( b ) the butt weld made by melting 21/e-inch 14 Ga. 281 4,000 3-inch 12 Ca. 309 4,400 a filler rod along the line of two edges in contact in the same plane; (c) the lap weld made by melting a filler rod along the Actual proof by test of these figures is experimentally edge of one sheet laid over another. Pilling and Kihlgren ( i ) have described the strength of difficult, oTying to the high pressures involved, but a number of tests made indicate that they are approximately correct. nickel welds. Strengths of standard weld specimens are made by butt welding the 7.6-cm. (3-inch) sides of 7.6 by Nickel T u b e Sheets 15.2 cm. (3 by 6 inch) specimens 3.2 mm. (0.125 inch) thick In most cases d i e r e it is economical to use nickel tubing it and then milling off the edges to leave a section 5.1 cm. (2 is also advisable to use nickel tube sheets. Ga1.i-aniccorrosion inches) wide and about 12.7 em. (5 inches) long. The results of cast-iron or steel tube sheets may occur. The areas con- are summarized in Table VI. cerned and the type of contact ordinarily used often make Table \.I this corrosion serious. The nickel plate used in tube sheets is hot-rolled directly from cast ingots of special shape, which ELONGATION lIETHoD TENSILE STRENGTH O F NICKELWELDS I N 5.1 CM. CIENCY depends on the shape and size of the desired plate. Thesr plates will have a tensile strength of 4900-5600 kg. per sq. KR _ . _aer sa.- cm. Lbs. 9er sa. i n . Per cenf Per cent cm. (70,000-80,000 pounds per square inch) and yield point 32.1 98 Xletallic arc 5180 74,000 of 2100-2800 kg. per sq. em. (30,000-40,000 pounds per square Carbon arc 30.0 90 4750 67,800 34,s 96 Oxy-acetylene 5050 72.000 inch). They are so rigid and durable that they can be made appreciably lighter in section than rolled steel or iron. Literature Cited Plates as light as ‘/zinch thick have been successfully used. The temper of these plates is appreciably harder and (1) Fraser, Ackerman, and Sands, IND.EXG.C H E X 19, 332 (1917). stronger than that of full annealed metal, and this temper is ( 2 ) McKay, Chem. M e t . Eng., 36, 546 (1929). Fraser, and Searle, .4m.Inst. Jlining .Vel. Eng., Tech. Pub. more suitable for the necessary machining operations. Owing (3) McKay, i g a (1929). t o the toughness of the metal, soft annealed nickel is likely (4) Pilling and Kihlgren, J . A m . W e l d i n g Soc., April, 1929. t o cauqe rapid dulling and deterioration of machining tools. ( 5 ) Thompson and McKay, IND.ESG. CHEM.,15, 1114 (1923).
.
1 ~
1
Correction ’ A. P. I.). In the second table on page 940 the “material inA few corrections should be made in our article entitled “Re- soluble in carbon tetrachloride but soluble in carbon disulfide” should be changed t o read “material soluble in carbon tetralationship between Calorific Value and Other Characteristics of Residual Fuel Oils and Cracked Residuums,” which appears on chloride and carbon disulfide.” In the last paragraph of conclusion 12, on page 941, the “33.8 per cent of carbon tetrachloride AND ENGINEERING page 933 of the October issue of INDUSTRIAL insoluble asphaltenes” should read “33.8 per cent of carbon CHEMISTRY. J. C. In the “box” the formulas for calculating the calorific value tetrachloride soluble aspha1tenes.”-W. F. FARAGHER, (90 X ’ A. P. I.) and 17,645 (54 X MORRELL, A N D J. L. ESSEX should read: 17,010
+
+
