July, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
671
Corrosion of Monel Metal in Photographic Solutions’” By J. I. Crabtree and G. E. Matthews EASTMAN KODAKCo., ROCHESTER, h-.Y.
the coating of metallic silITTLE of the data For photographic processing apparatus receiving intermittent ver, especially if deposited available in the literuse, monel metal is quite satisfactory, but if the alloy is exposed unevenly, tend to set up ature on the corrocontinuously to strong acid fixing solutions, especially at high sion of monel metal in variinternal electrical currents temperatures and in electrical connection with other metals, serious within the alloy which ous alkalies, acids, and other corrosion takes place. The corrosion appears as pitting and not as locally accelerate the rate corrosive solutions3 is of a uniform dissolution of the metal. Only temporary protection of corrosion. value in predicting the effect from corrosion is given by surface deposits of metallic sulfides due of photographic solutions on to the presence of colloidal sulfur in the solution, or by silver deRESISTIVITYOF MONEL this alloy, and in this conposited electrolytically f r o m an exhausted fixing bath. nection the authors agree Physical strains produced by maltreating the alloy do not cause APPARATUS IK P R a C T I C E with Hatfield,4 as well as segregation of the corrosion in the area thus partially fatigued. Hamlin and Turner,5that it From many years of exMore rapid corrosion occurs at the air line than below the surface is necessary to make tests perience with monel apof the solution; this is largely due to conuection currents which under actual practical conparatus in photographic produce a more complete renewal of the solutio’n in contact with the ditions in order to deterdark-room practice, it has alloy in this air-line region and to incrustations which occur at and mine the suitability of a been found that homoaboue the air surface. particular metal or alloy for geneous articles constructed anv mecific x)urDose. bv stamDine: and without The unusual resistivity of monel met’alto corrosive solutions welding or soldering have a much longei lire than nonhohasled to its adoption for constructingphotographic apparatus, mogeneous articles, such as tanks, which are soldered or such as film clips, hangers, small tanks, and other equipment welded. Homogeneous articles used intermittently usually receiving intermittent use, for which purpose it is quite fail to develop signs of corrosion within a year, although the satisfactory, but when exposed to the various photographic surface of the metal rapidly blackens. In the case of solsolutions for prolonged periods, especially at high tempera- dered tanks continuously filled with a fixing bath, it was tures, arid when in contact with a,nother metal or alloy, it is observed that corrosion pits formed rapidly and increased in corroded, as outlined in a recent paper by the authors and area and depth until holes appeared extending entirely H. A. Hartt.6 In the present paper a more complete de- through the alloy, especially if the tank was placed in a leadscription of the type of corrosion observed and of the con- lined sink. ditions l,o which the alloy was subjected will be given. It In view of the fact that the rate of corrosion of the different was previously shown’ that photographic developers were articles varied widely under apparently the same conditions, not affected by monel metal and in turn did not corrode the a more careful study of the various factors affecting the coralloy, so that the discussion herein will be confined exclusively rosion was made as outlined below, although the tests were to fixing baths. made under conditions far more severe than those occurring in common practice in order to determine as far as possible COMPOSITION OF PHOTOGRAPHIC FIXING BATHS the limiting resistivity of the metal.
L
“
&
A photographic fixing bath is a comparatively complex solution containing sodium thiosulfate (hypo), acetic acid, potassium aluminium alum, and sodium sulfite, and with use the bath accumulates silver salts (since hypo dissolves the unexposed silver halide in the emulsion), developer oxidation products, and more or less free sulfur as a result of the decomposition of the thiosulfate (especially a t high temperatures). The acetic acid is undoubtedly the most corrosive of the constituents, although the sulfur tends to form sulfides on metallic surfaces. Metallic silver is also deposited on the alloy if the fixing bath contains silver salts. Both the sulfides and 1 Presented as a part of the Intersectional Symposium on Metals a t the joint meeting of the Eastern New York, Syracuse, Cornell, Rochester, and Western New York Sections of the American Chemical Society, Syracuse, N. Y., February 1 and 2, 1924. Communication No. 202 from the Research Laboratory of the Eastman Kodak Company. 3 Croasdale, Bull. A m . I n s t . M i n . Eng., 92, 1916 (1914); Metal I n d . , 13, 313 (1915); Eng. News, 73, 890 (1915); Williams, M e t . W o ~ k92, , 93 (191.9); Power, 50, 14 (1919); Merica, Chem. Met. E n g . , 24, 291 (1921); McKay, T r a n s . Am. Electrochem. Soc., 41, 201 (1922); Thompson and McKay, THISJOURNAL, 15, 1114 (1923); and Arnott, Trans. Faraday Soc., 19, Part 1, 196 (1923). 4 Trans. Faraday Soc., 19, Part 1, 159 (1923). 6 Hamlin and Turner, “The Chemical Resistance of Engineering Materials,” 1923, p. 12. The Chemical Catalog Co., Inc. 6 Crabtree, H a r t t , and Matthews, THISJOURNAL, 16, 13 (1924). 7 Crabtree and Matthews, Ibid., 15, 666 (1923).
EXPERIMENTAL DETAILS Strips of monel metal (20-gage in thickness) were placed in 200 cc. of the various solutions contained in 250-cc. Pyrex beakers and held in a vertical position by inserting between slots in a sheet of cardboard resting across the top of the beaker. The volume of solution was maintained constant by adding water a t intervals. It was thus possible to study the corrosion of single strips or of metallic couples; in the latter case the two metals contained in the same beaker were externally connected by means of a wire. The experiments were usually continued for 8 weeks. Tests were made with the following fixing baths:
+ +
(A) 30 per cent hypo 5 per cent acid hardener a (normal fixing solution) (B) 30 per cent hypo 20 per cent acid hardener (C) Exhausted hypo (Bath A dissolved silver halides) a
+
An acid hardener of the following composition was used. Sodium sulfite Acetic acid (28 per cent) Potassium aluminium alum Water t o
90 grams 500 cc. 60 grams 1 liter
The acid hardener is added to the hypo solution in order to neutralize the alkali of the developer carried into the
I.VDUSTRIAL A K D E.VTGI.VEERING CHEMISTRY
672
To1 16, No. 7
although it is difiicult to sliow t.he effect of this fa.ctor with comparative photographs. Only a vcry diglit etching was noted on strips of nioncl immersed in a normal fixing bath (A) at 21' C. (70' F.) for several months, whereas serious corrosion was observed i n a few weeks when the tests were 1x0 TIIE RATEOF C ~ R K ~ S I O V mntle at 43" C. (110' F.), especially at the "air line" if the Practical tests indicated that, the rate of corrosion of mom4 alloy v a s not ent.irely immersed. When the alloy is in elecmetal was determined by t.he following factors: trical eoniicctioii with another metal, the effeet of temperaTIMEOF C o s r . 4 WITH ~ ~ THE Soi.umox--The progress of cor- ture is very much greater (see "Effect of Electrolysis," rosion with time is shown by Figs. 1 to 48 (enlargement below). C O ~ ~ O S I T OH I O FIXIXL. .~ B.crH--l'lain hypo solutions of about 3 diameters). The monel metal shiivn in Figs. I and 2 (early stages of corrosion) was immersed in Solution 8 ; various concentrations (without acid hardener) corrode in Figs. 3 and 4 (ad- moncl vcry slo~vly,so that im trouble from corrosive effects is e ~ e experienced r with socli solutions. Wlien acid hardener is added and tlic hatb contains the following constitticntsnamely, hypo, acid, sulfit,c, and aloin--the rate of corrosion is only materially :8fceted by increasing the acid concentration. A bath containing 20 per cent acid hardener, for inst,ance, corrodes tliree t,o four times as rapidly as one eontairiing 5 per cent hnrdenrr. The developer oxidation products carried into the fixing bat,h and to t.aii OT harden the gelat,in. The sulfite acts as a prescrrative in preventing t.he decompositiqn of the hypo by the acetic acid.
Pro 4
grayish hrown spots on
the surface film depositedonttiealloy(Fig. I). \Then the surface film wits soraped awiy, discolored areas vere observed ahere pitting subseqiiently occurred. At the end of 3 or 4 wecks scnles arid bribblcci spots were formed imdor wliirli ext,en.ire corrosion ivas found (Vig. 2). After 5 to 6 weeks the moriel was badly pitted (Fig. 3 ) . $fter 8 weeks the original surface layer of the alloy as destroyed and the rough surfaee was corercd xitli small craters (Fig. 4). These figures repremit irnly relabive rate-: of corrosion aiid sliould be regarded 5s indicative of the average result noted from many serics of experinimts. In Fig. 5 is shown a typical corrosion pit which was formed in 2 weeks on an uiiconiiected strip of metal under tlie ahove conditions. Similar corrosion of monel in fixing hatlis has been observed by Hubbard.s TEMPERATDIE OF 8or;cTrox-Tcmperature is undoubtedly t.he most imporbatit factor xhich affects the rate of corrosion, I/ Thc suthors are indebted to L. Is. Jewcll of thin laboratory for as%ip. tan
