May, 1927
ILVDUSTRIBL A S D ENGI,VEERI,VG CHEMISTRY
Conclusion Experimental fiyork is still in progress a t Indianapolis, for the sump into which the Fvater and grease from the digesters is decanted contains, after skimming off the grease, a water carrying all the water-soluble materials of the garbage. These include sugars, starches, salt, etc., and experiments so far conducted indicate that it is well worth drying for its food values. With the new plant in operation and the difficulties always attendant upon the breaking in of a new plant ironed Out, this further measure of conservation will doubtless be taken. The visitor to the garbage reduction plant of the Board of Sanitary Commissioners of the City of Indianapolis is impressed with the engineering which characterizes the place and
613
the soundness of the work upon which the present process is installed, and feels that he can recominend to others interested in the problem a careful inspection of this installation. One surprise is the fact that the income from the ~ ~ 6 0 , 0 0bond 0 issue authorized for the Purpose has Prolvedmore than enough t o construct the plant, which has been built a t a figure less than the appropriation. The operation of the plant should answer conclusively the question with which this description W R S introduced, for i t seems to have been demonstrated that when properly processed American city garbage is a material well worth working foq its values and that this step in conservation can be widely recommended.
The Intercrystalline Corrosion of Metals’,’ By Henry S. Rawdon h-ATIONAL
B U R E A UOF
STANDARDS, U’ASHINGTON,
D.
c.
Intercrystalline corrosive attack of metals is by no means rare, nor is it confined to any particular metal. I t occurs in the soft, weak metals, such as tin and lead, as well as the stronger and harder ones, iron and nickel. When i t is due to corrosion alone, i t can usually be related to some structural features peculiar to the metal under observation and to its composition. In a few cases, however, such as the corrosion embrittlement of pure lead, a relationship of this kind has not been established, and in the embrittlement of duralumin the evidence is largely indirect. The corrosion of a metal while under tensile stress is a common cause of intercrystalline brittleness, although even here it is only by means of certain corrosive solutions, which are different for the different metals, t h a t the result is accomplished. No generalization as to the essential characteristics of the corrosive solution necessary for causing intercrystalline attack of stressed metals appears to be warranted from the evidence so far available.
In general, any practical remedy for the trouble m u s t be along one of two lines. The stress acting on the metal, whether internal or externally applied, may be reduced considerably below the yield point of the metal. Most of the short-time laboratory tests have shown t h a t in order to produce failure within a reasonable time in the laboratory the metal m u s t be stressed close to its yield point. The practical solution of the problem of corrosion cracking in wrought brasses and other copper alloys has been along this line. The other method is to reduce the corrosive attack either by protective coatings, as in the case of duralumin, or by preventing so far as possible the formation and accumulation of the corrosive solution, as in the treatment for the prevention of “caustic embrittlement” of boiler plate. There appears also to be a possible third solution applicable in certain cases, which depends upon a change in the structural conditions in the alloy, particularly as related to the grain boundaries, by suitable heat treatment or possibly other processes. . . . . . ., . . . . . .
HE gradual change of a metal from a condition of high strength and often of high ductility to one of relatively
T
low strength and high brittleness, the substance retaining its original appearance in most other essential respects, is very disconcerting.. The “spontaneous” cracking, which may be so severe as to be better called “disintegration,” of a metal under what seem to be very mild conditions, such as storage, is another perplexing phenomenon. These and similar cases may conveniently be grouped under the term ”intercrystalline brittleness.” The susceptibility of many metals to intercrystalline corrosion under certain conditions may properly be considered as one phase of the more general subject of intercrystalline brittleness of metals. The failure of metals by intercrystalline fracture is by no means rare. The oft-cited example of the extremely high brittleness of copper or gold when alloyed with an apparently insignificant amount of bismuth (0.01 per cent or less) is very familiar. The bismuth, which is readily soluble in the molten metal, is practically insoluble in the solid, and as the molten alloy solidifies the bismuth forms Presented before t h e Division of Water, Sewage, and Sanitation a t the 73rd Meeting of t h e American Chemical Society, Richmond, V a . , April 1 1 t o 16, 1927. ? Published by permission of t h e Director of the X;ational Bureau of Standards.
a thin film enveloping the crystals throughout the mass of the metal, to which the high degree of brittleness of the metal is to be attributed. Numerous other such examples might be cited. This phase of intercrystalline brittleness will not be discussed further, however, since it can readily be shown that, from its structure, such an alloy is initially brittle and this property does not develop as a result of any subsequent attack, corrosive or otherwise. The present discussion will be confined to those cases in which a metal or alloy, initially ductile, is rendered weak and brittle as a result of the corrosive conditions to which it is subjected during usage. The hypothesis has been advanced and very skilfully defended3 that under proper conditions of stress any metal should be expected to exhibit intercrystalline brittleness; that is, fracture of the metal would occur by the separation of the constituent grains from one another and no indication of ductility would be shown by the specimen as a whole. This theory involves considerable speculation concerning the ult’imate structure of metals, particularly the conditions at the grain boundaries, and will not be considered further here since it has been well established by experimental evidence that corrosion has a pronounced accelerating effect upon the
*
Rosenhain and Archbutt, Proc. R o y . SOC.( L o n d o n ) , 96, 53 (1919) Rosenhain, Trans. Faredoy Soc., 17, 1 (1921).
lliiic disiiitegration of metals. Hence, in any of the actual service conditions this feature is so iinporiant as to constitute the major part of the problem. It is to this peculiar feature oi the corrosive attack of certain motals under certain conditions that this paper is devoted. Intercrystalline Corrosion as Related to Microstructural Features of Metal
In uurncrous cases of the embrittling of metals as a result of intercrystoili~ie corrosion, the attack can be definitely shown to be related to the structure of the metal. Tix---l'nre tin shows very little t e n d e n c y t o w a r d intercrystdiine corrosion. Spcciincns corroded for 90 days in a solutinii of hydrocliloric acid and staiiiious chloride .;howcd a somewhat greater attack along tlie grain boundaries tlinn elsewhere.' ?;o evidcrice of brit,tleness was exhibited, however, u p o n sovcrely bending such corroded tin specimens. If tin contains a sinall amount of alnniinum, however, it x z y readily becomes embrittled upon very slight corrosion. Foil rolled from such tin in the coiirse of a few weeks or months heconios so brittle that it can he readily crumbled in the fingers.& If the corrosive attack is accelerated by irnniersing the tin in a solution of sodium chloride, the embrit.tling action occurs very readily and microscopic examination of tlie corroded tin shows that the attack is confined almost entirely to the micro-constituelit resulting from the presence of the w r y sinal1 aniount of aluminum in the tin. Figure 1 shows tlie aparance of suclr ernbrittled tin foil. ".".. if leit, "errin also siiows a tendency to iniw ag &iro:'j.,u5 ternal oxidation when exposed to tlre ~ , e ~ i mso e britiie aiirr e a c t i o n of stearn. Uurgesa ;md lew inonihs by airnos,,ileriC coira;ioe, it Merica' have shown that contamina~ ~ ~ ~ , b ~ j ~ ~ tion ; : ofethe~ t,in " with ~ ~ small ~ p ~amounts ~ ~ of tion of t i n : c u 0.06, Pb other metals, p a r t i c u l a r l y zinc, 0.24. lie less than 0.05. SI, 0.04, AI 0.51. z,, renders the tin susceptible to t,his *ctectd. (diff.1 99.10 type of corrosive attack. TI& be*er cent. havior of tin at times is a matter of serious consequences, for example, when impure tin is 1 as the filling for safety fusible boiler plugs. The oxidation of the tin under the action of the steam progresses tlirougliout the body of the tin filling along the network of tlre constituents formed by the metal impurities, and if the plug is not removed from service, the tin is finally transformed into a hard refractory inass of stannic oxide. haD--Lead, even of a high degree of purity (99.993 per cent), when corroded by means of nitric acid-lead acctate solution7 has been found t,o develop intercrystalline brittleiiess as a result of corrosive attack localized along the grain boundaries. (Figure 2) With very coarsely crystalline l e d this can he very readily demonstrated without the aid of the microscope. The attack of the metal between the grains is 80 much inore rapid than oirer the surface as a whole that relat,ively.deep "ditches" are formed between the pains.
:;::
Results of this kind have been interpreted by some investigators as indicating the existence of an allotropic form of lead, but more recent researches have shown that an assumed allotropy is not necessary in explaining the phenomenon. Theil* concluded fronr his work that the presence of a small amonnt of nitrite in tlie corroding solution is necessary for this type of corrosion on Icad. He also reported that a single crystal specimen of lend was, as might be predicted, immune to corrosive attack of this type. The susceptibility of lead to intercryst,alline corrosive attack at times bccoines a rather serious industrial problem. especially in the case of underground lesd-covered telephone mid telegraph cables. As will be shoivn later, in aerial iiistallations the probleiii is further inrolved because of tlie stresses resultirig fwio the weight of the suspended cables. Figure 3 slio\vs liow rcrioiidy lcad cable sheath may be delcrioratcd from int,ercrystalline corrosion. According to Anderegg nricl Acliato:s wlio have publislred tlie remits of rather extended csporimeiits and field observations on this subject, telephone cable shentlis of comrucrcially pure lend :ire somewhat less resistmt to inrrasi\~iCattack by the various
+";
I
.
.
s R:iwdon. Krynitskv. . . arid Berliner. Chcm. B' Mel. Enr . 26. 109. 154. ?I2 (1922). llcyn and Wetrel, Miil. I(aisrr-lvlihrinrinr1. Memliforicn., 1, 4 (1922). 6 Bur. Slandnrds, Tcch. Po$" 63 (1915). 7 Rrwdon, [bid., Sci. Poprr 577 (1020).
Pi4ure 2 -Lead EmhrillleJ
by Infer~ryilallilieCorrosion
corroding agents present in soils than the commoiily uscd lead-tin alloy (1 to 3 per cent tin) but more so than the antimonial lead sheathing (approximately 1 per cent antimony). According to Hachnel'O the addition of 3 per cent of tin increases the life of underground lead cable sheaths to seven or ten times that of pure lead sheaths. 8
R m ,63, 1052 (1020). University Zne. Erpt. Stil., Buil. I8 (1924).
I Purdue
iiibliogiaphy.
coolrins
/I
~ood
May, 1927
INDUSTRIAL A N D ENGINEERING CHEMISTRY
line corrosion that it may well he m e n t i o n e d here briefly. '1 piece of sheet copper alloyed with m e r c u r y exhibits a very high degree of brittleness upon bending. Microscopic examination bas shown that the fracture of such a piece of amalgamated sheet c o p p e r is intercrystalline in character and that the mercury shows a decided preference for t h e grain houndaries in dissemin a t i n g itself Figure 3-Commercial Lead Cable Sheafh t h I' h the (Underground) Embriffled i n Service a8 a Result of Corrosion. Lead Contained One Per Cent Of cross sectioll of -:_ l'LL (0) Appc~innceoi corroded material. x the p per ( 6 ) IO some poitions of ( 0 ) c ~ n s i i t u c n tgrains oi strip. Certnin l u ~ dcould readily be crumbled apart as shown. x 8 (6) C T seciioll ~ ~ ~ Eiiawiiig mod. composit.ions of e m t c c ~ r r o s i o n Attack is i,itemrysiallinc; specimen brass are also is unriched. X 50 readily disintcgrated imto their constituent graiirs wlreti attackeh by mermry; 1)es~li~~ has used this method for Separating hrhss into its cmstitiient g r : h for study, and a t the Bureau of Sta~iilards it lins been used for obtaining single grains of zinc,.'$ r i ~ u n r im--~I'urc s almriinuni does not appear to be 111)preciably susceplibie to int,ercrystallirie corrosion. The cases that 11nm been reported relate either to impure alii~ n i n n m ' ~ otor an alloy of the duralumin type particularly wlien in tire forrir of s l ~ e e t . ~Figure 4 is typical of such a change ilr sheet duralumin. The tensile properties of duralumin sheet are very materially affect,ed, the greatest change being the very pronounced loaering of the elongation. The maxiinurn tensile strerigt,lr is not reduced to the same degree as tire ductility. Figure 5 shows bhe characteristic appearance of teated tension specimens of duralumin sheet in the usual condition and also after emhrittlement. The incre'asing use of sheot duraliiniin as a structural material in aircraft as vi-ell as for numerous other purposes makes this change of tensile properties by intercrystalline
615
corrosion a nratter for very serious coiisidcration. An iiivestigation of the subject as related t,o aircraft material" is now in progress a t the Bureau of Standards, t.he results of which will appear quite reassuring and point to a successful solution of the problem. The susceptibility of duralumin to this type of corrosive attack appears to be intimately related to tire structural condition obtaining a t the grain boundaries as a restilt of certain lieat-treatment proccdiire and mechanical treatment.. By corroding diiralumin with a chloride solution, preferably one to wliich an oxidizing reagent such as hydrogen peroxide, as described by Mylius,'J hris been added, the variation in t,lre tendency of different duralnmins t,o intercrystalline corrosion embrittlcment. em be readily demonstrated. (Figiire ti) The practical means for preventing this deterioration of dur;ilumin consists, first, in increasing the rcsistarrce of the mat,erial toward intercrystalline corrosiun by tire proper heat treatment of the dur:ilurrriii, and secoiid, in protecting the surface against corrosive attack by a suitable coating. The type of coating to be used depends upon the severity of the service conditions. Anodic oxidation followed by greasing, as described b y Bengough rand Sutton,l6 promises to give adequate protection for ordinary atmospheric exposure conditions. DIE CASTINGSAnother i m p o r t a n t group of commercial alloys susceptible to t2rk type of deterioration is the class of zinc-base diecasting alloys. As a result of such an attack, which a s a rule is most severe in hot, moist climates, a die casting often becomes liard and brittle and severely warped. The t e n d e n c y of siicli alloys to deteriorate can be readily sliown by Ireating a small c a s t i n g of rcgiilar shape, such as a cylinder, in steam together with a comparison spccimen i n "dry heat." (Figure 7)
l i y microscopic oxaniinatioii it can readily be dcmoiistrated that the swelling and distortion of the diecastiug alloy in {.he "moist heat" is the result of a prefereutial corrosive attack oxidation of certain structural constituents of the alloy. According to tire results of an extensive investigation by lirauer and Pcirce," the presence of small amounts of certain metals in the i.inc-aluminiim alloy used for die castiug is responsible for t,he undesired behavior of the niaterial. Of tile coritarninating metal.;, which need be present only to the cxtent of a fen, huiidredtlis of one per cent, lead has been classed by these inyestipators as the ioost powerful accelerntor, with t,iiiand cadmium as close seoorlds.
by the attack of sulfurous gases uii nickel while uuder stress, as illustrated by the behavior of nickel termiiials of certain spark plugs used in internal-combustion eiigiues.'s This
Combined Effect of Stress and Corrosion in Producing Intercrystalline Brittleness By far the most striking cases of intercrystallirie corrosioI1 are those which result when a metal is subjected to a corrosive solution while it is s i m u l t a n e o u s l y st.ressed in t e n s i o n . Many of the cases of int.ercrystalline brittleness which constitute serious problems from a n i n d u s t r i a l standpoint fall into this class.
Piguse 6-Sheot Duralumin Embdtfled by Corrosion in Lahorafory Sheet duralumin was heat-frcsted by quenchins Erom 500" 10 610- C . in medium indicated and then af-inf at r m m tempcrature (or 6 days. Specimens were then corroded lor 2 days by intermittent repeated immersion at 1.5-minute interval% in B normal solution oi sodium chloride t o which hydrogen neioxide had been added (!I pant? chloride solution ,to 1 pait peroxide). Note difference m s u s c e ~ f i bility of material according t o quenchinz medium used. Tensile properties in thice cases weessentiallv the same: uitinste tensile strength 00.000 lbs. per qq. I D , (4225 kg. per sq. cm.). elongation ih 2 inches (6 cm.), 21 per cent.
Flnure 5-Tension Specimens of Sheof Duralumin. x 1 (01 Emhiittied by intercrystalline corrosi~n,ultimate tensile strength
48,800 ibs. p e r sq. in. (3423 kg. per sq. ~m.1,eloogation in 2 inches (6 E=.), 8.6 per cent. Initislly the properties of this material mere essentially the s ~ m as e those of @I. ( b ) Ordinary condition heat-treated, ultimste tensile strength 65,800 lbr. pei sq. in. (3936kg. per sq. em.), elongation in 2 inches, 16 per cent.
effect has been observed in spark plugs for gasoline engiues, but is liable to be more serious in engines using heavy oils, the sulfur content of which is usually much higher than that HIOX-TEMPERATURE of gasoline. Figure 9 shows the character of the cracks INTEXCRY~TALLINE AT- formed in the nickel terminals of a spark plug of a somewhat ~ ~ ~ x - K u m e r o uins unusual type. vestigators have found The action of a molten metal upon another (solid) metal that when a tension while st,ressed in tension frequently results in a11 intercrystaltest of a metal is made line attack of the second metal. This effect can be classed at a high t.emperatnre, as "corrosion" only by a liberal interpretation of the term. t h e f r a c t u r e occnra It may he briefly considered, however, since in all essential normally in an inter- respects the effectsupon the properties of the metal attacked crystalline manner. I n are identical with those of true intercrystalline corrosion. order to show this best, however, the temperature must be not far below the melting point of t h e m e t a l . With some metals--nickel, for example--a tension test made at a temperature several hundred degrees (C.) below tlrc melting point shows a pronounced intercryst,alliue fracture, as is Fho%-n i n F i g u r e 8. Thismaybe attributed, at least in large measFikure 7 -Swelling a n d I>fsfortion of Die Casringa by Inferure, to oxidation during crYsfalline corrosion the progress of the test. (0) E k t i i E 1ocom"tive f"%.b,,X COYer from Canal zone. x L/. Mornish., 7 . 1 (I\ovceilxi, 1928~. js i ' r r i ..im. .sor. resting .iiiilinioii, a6, ~ t 11, . zis (ioitii
New Method for Identifying Rayon
X'iCure 14 Mir'rUtfrUcture of Braas Showine azi Infercrystalline Artack ufrer Being Immersed in a Solution of a
A n e w nicthod of distinguishing cuprammonium from other kinds of rayon has been found by the Burcau of Staiidards. In the manufacturc of ciiorummotiiiim raven. cotton. usuallv in the form of cotton liiitcrs, is dissolved in ammoniacal copper oxide, arid the resultant solution forced through spinnerets into LI solution of acid or caustic alkali. The thread of reyon is thcii mssed throueh a second bath containine acid in which most of the copper 'Is dissolved. As all of th; copim is not removed. a test for it affords a means of distinguishing this type of rayon from the other three. If copper is found *_check tcst is made.
