January, 1924
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
18
Effect of Electrolvsis on t h e Rate of Corrosion of Metals in Photographic Solutions’” By J. I. Crabtree, H.A. Hartt, and G. E. Matthews EASTMAN KODAKCo.,
ROCHESTER,N.
Y.
data in the present paper selecting The resistioity of a metal or alloy towards any photographic on the basis of the elecmaterials for the solution is impaired if it is in contact with a second metal which is trolytic theory. Other corconstruction of more electropositioe in the solution used. A photographic electrorosion theories are less firmly photographic apparatus, motioe series is gioen wherein the rate of corrosion of the metal or alloy established and are genersuch as trays, tanks, clamps, is roughly proportidnal to the distance of separation of the metals ally applicable only to and pipes for conveying as listed. Metals plated with siloer. either chemically from an specific cases. photographic solutions, proexhausted fixing bath or by electroplating, are more resistant to deIf two dissimilar metals viding their resistivity to veloping solutdons according to the homogeneity of the d o e r coating, in electrical contact are imcorrosion is satisfactory, but their resistivity towards fixing solutions is only slightly greater mersed in a photographic metals and alloys are usualthan that of the unplafedmetals. The presence of copper or its alloys developer or fixing bath, an ly to be preferred because of in electricalcontact with a more electropositive metal in a photographic electrolytic cell is formed their mechanical properties, deoeloper materially shortens the life of the developer. Tin and alloys analogous to, say, a Lesuch as ductility, nonfracontaining tin tend to produce high initial fog in a photographic Clanch6 cell, which consists gility, and general workdeoeloper. but this f o g later decreases to a normal value when the essentially of a zinc and carability. solution possesses better dealoping powers than a blank, indicating bon rod immersed in a soluI n a previous paper3 it the possibility of using a tin alloy or metallic couple which will tion of ammonium chloride was shown that the suitgradually discharge tin into the deoeloper thereby acting as a precommonly indicated as ability of a metal for the conseroatioe of the solution. follows: struction of photographic For the practical worker, the relative suitability of various metallic apparatus depends on (1) couples for use with developing and fixing solutions is discussed. Zinc/arnmoniurn chloride the resistivity of the metal solution/carbon to the most corrosive liquid with which it will come in contact, (2) the effect of the The zinc tends to go into solution owing to its electrolytic metal on the photographic properties of the solution, (3) solution tension, and in order to become ionized in solution the tim.e during which the solution will be in contact with the $he zinc particles must become positively charged according metal, (4)the cost of the metal, and ( 5 ) the adaptability of to the equation Zn - 2- = Zn++. When the zinc goes into the metal for construction purposes. Only the rate of cor- solution it loses two electrons, thus leaving the electrode rosion of single metals was considered and the resultant data negatively charged. If the zinc and carbon rods are now were only applicable to the case of homogeneous metal externally connected, a flow of current will take place articles. However, in the construction of most metal ap- through the electrolyte (ammonium chloride) of the cell paratuii it is desirable to use a second metal or alloy in the and the external circuit because of the difference in form of solder to render the joints or seams free from leaks, potential between the zinc and carbon electrode. The while in the construction of pipe lines for the conveyance of negative charge of the zinc electrode will then be neutralized photographic solutions, including distilled water, it is fre- by the positive charge of the carbon electrode, thus allowing quently not possible to employ faucets or fittings of the same more zinc to become ionized, with the result that there will be material as the pipe line. The importance of metallic con- a continuous flow of current until all the zinc has become tact between two dissimilar metals or alloys in hastening the ionized or dissolved. rate of corrosion was pointed out. A tank made of an alloy, Several factors influence the rate and quantity of the flow “X,” with soldered seams showed peculiar properties-namely, of current even in a single cell containing a single electrolyte. a developer in such a tank gave very bad fog and with When several substances are present in a solution, as is the fixing baths the alloy developed bad corrosion in a rel- case with a photographic developer or fixing bath, and when atively short time, A further study was therefore made, the electrodes are composed of an alloy, as are many types of as outlined in the present paper, of the effect of electrolysis photographic apparatus, the problem presents many combetween various couples (pairs of metals) as may exist in plications and it becomes difficult to predict the course and different types of photographic apparatus with relation to (a) velocity of a reaction. the effect of electrolysis on the rate of corrosion of the metals, If two metals or alloys, one of which is more.electronegaand (b) the effect of the metallic couples on the photographic tive than the other, are immersed in a developer which is alproperties of the solution. kaline or in a fixing bath which is acid, and if the metals are in electrical contact, the more electronegative metal will THEORETICAL DISCUSSION tend to dissolve (corrode) a t a greater rate than if the metals Although various theories of corrosion have been proposed are not connected. Thus, in the case of a tank made of copfrom time to time,4a5?6Jit is possible to correlate all the per, or lead, or of an alloy such as Monel, brass, etc., which is 1 Presented before the Division of Industrial and Engineering Chemsoldered with a second metal or alloy, or in the case of piping istry at the 66th Meeting of the American Chemical Society, Milwaukee, installation in which the fittings and faucets are of a different Wis., September 10 to 14, 1923. metal or alloy than the pipe itself, an electrolytic cell or 2 Communication No. 191 from the Research Laboratory of the Eastman battery is set up wherein the metals constitute the electrodes Kodak Company. * Crabtree and Matthews, THISJOURNAL, 16, 666 (1923). and the photographic solution is the electrolyte. Under such 4 Whitney, J . A m . Chem. SOL,26, 394 (1903). conditions it would be expected that the normal rate of cor6 Moody, J . Chem. SOC.(London), 89, 720T (1906). rosion of the more electronegative metal or alloy would be 6 Friend, Ibid., 119, 9321‘ (1921). accelerated. 7 Wijson, THIS JOURNAL, 15, 127 (1923).
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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PRELIMINARY INVESTIGATION From preliminary experiments? it was found that a developer prepared in a tank of alloy “X” (copper-nickel) soldered with 50 : 50 lead-tin solder gave excessive fog. The following observations were made from experiments carried out to determine the cause of this abnormal fog: A developer was allowed to stand in the tank for 24 hours, when fog tests were made. The developer was then kept in the tank for 1 month and fog tests were made at various intervals. The results of these experiments showed that (a) at the end of 24 hours with normal development excessive fog was produced; (b)a t the end of 1month the developer appeared to be less oxidized than a developer kept in an enamel-lined tank-that is, it was less discolored from the usual oxidation products formed in the developer after standing over a period of 1 month; (c) the fog diminished with the age of the developer, but the developing power of the solution was the same after keeping 1 month in the alloy tank as in an enameled tank. The solder with which the seams of the tank were soldered appeared to be slightly etched, and the original luster of the metal had disappeared and was replaced by a more or less dark; grainy deposit. The alloy itself was unaffected as far as could be observed from its physical appearance. I n order to test the theory that the fog was produced by an electrochemical reaction, the conditions that existed in the tank were duplicated by immersing a strip of the alloy “X” and a strip of solder in a similar developer contained in a glass beaker. I n one set of experiments the metals were electrically connected while in another the metals were unconnected. It was found that the results were strictly in accordance with those obtained in the case of the tanks, and when the metals were connected the resulting fog was materially greater than when the metals were unconnected. When the metals were not connected the fog produced could only be a result of the dissolution of the tin, since it was found that neither alloy “X” nor lead produced fog when alone in developers, while tin produced excessive fog. Since the fog in the case of the unconnected cell was much less than in the case of the connected cell, electrolysis must have taken place in the connected cell with the result that a larger quantity of tin ions was discharged into the solution. The data from which this theory was developed are given in Table I.
paired in such combinations as might be used for ( a ) tanks, or ( b ) piping and accessories (faucets, etc.). In each case a comparison was made between the metals when electrically connected and when unconnected. The following observations were made:
EFFECTOF SOLUTION ON THE METAL-If the metal showed only general attack, such as loss of polish, slight roughening of the surface and discoloration, it was considered “etched.” If local areas were dissolved away leaving pitted effects and extensive surface roughening, it was described as “corroded.” POTENTIAL DIFFERENCEBETWEEN THE ELECTRODES-This was measured by means of a pH pyrovolter (an instrument working on the potentiometer principle). Note-At the outset it was considered that the potential difference between the electrodes might be roughly proportional to the rate of corrosion, but this in turn is dependent both on the normal solubility of the metal in the solution and the accelerating effect of the electrolysis on this rate of solution. The measurements obtained were unreliable, varying as much as 200 per cent from day to day. In some cases tGe polarity of the electrodes became reversed during the course of the experiments, illustrating the disturbing effects produced by surface deposition, polarization, and the overvoltage of the electrodes.
CHANGEIN WEIGHTOF Fh,EcTRoDEs-~he measurements were rounded off to the nearest 0.1 gram. EFFECT OF METALLIC COUPLES ON PHOTOGRAPHIC PROPERTIES OF SOLUTION-The various solutions after b’eing in contact with the metals were tested as follows: 1- Water. (a)The tendency to precipitate organic developing agents when used in compounding a developer formula was observed. ( b ) The fogging tendency and developing power of a developing solution compounded with the water in question were compared with a developer compounded with distilled water. %--Developers. The fogging tendency and the developing power of the developer after b&g in contact with the various metals were determined. The fog was taken as the density produced by a given developer in a given time [7 minutes a t 18’ C. (68’ F.)] on unexposed Eastman positive film. The relative developing power of the solution was measured by the density produced by the development of a known exposure giving a density which under normal conditions of development would lie midway between the toe and shoulder of the characteristic curve of the emulsion.8 Note-Developer formula: elon, 2.3 grams; hydroquinone, 9.2 grams, sodium carbonate (dry), 48 grams, sodium sulfite (dry), 96 grams; potassium bromide, 2 grams; water t o 1 liter. From the earlier experiments it was found that results obtained with this developer were analogous to those obtained with other developers and for the experiments in this paper this representative formula was chosen.
3-Fixing Baths. The rate of fixation and the general usefulness of the bath were recorded. EXPERIhlENTAL PROCEDURE
TABLE I -Fog Density-Unconnected Connected Lead, Alloy “X” 0.23 0.22 Tin, Alloy “X” 0 30 4.00 Solder,a Alloy “ X ” 0.59 4.00 Blank 0.23 a50 per cent lead and 50 per cent tin.
Vol. 16, No. 1
Electrode Potential Volts 0.29 0.43 0.33
These experiments proved conclusively that electrolytic effects resulting from two or more metals in contact in photographic solutions mere of more importance than was previously supposed.
GENERAL INVESTIGATION Experiments were made with combinations of various metals used in photographic practice in the following solutions: (a) water, ( b ) developers, and (c) fixing baths. The question of suitable metals for use with hypo-alum and other toning baths was not considered, because it was shown in the earlier paper3 that metal containers for such solutions are to be avoided, since no metals are known which will not be corroded by these solutions. Metals and alloys were tested which had shown up most favorably for the construction of photographic apparatus as indicated by the results of the previous work. Thesewere
Strips of the metals 5 x 15 cm. ( 2 x 6 inches), electrically insulated, were half immersed in 300 cc. of solution contained in 400-cc. Pyrex beakers. One set was observed in which the metals were externally connected by means of a copper wire; another was unconnected. The water and developer tests were kept a t a temperature of 21” C. (70” F.) for a period of 1 or 2 weeks, as indicated in Table 111, and then placed in a hot room a t 43” C . (110” F.) for the remaining period in order to accelerate the effect of time, and especially to observe the effect on the properties of the solution with regard to color, incrustation on the electrodes, and corrosion of the metals. The fixing bath tests were kept at a temperature of 43” C. (110” F.) throughout the period of the test. METALLIC COUPLES IN CONTACT WITH WATER The following combinations of metals were studied in distilled water in order to test their suitability for containing and piping pure water: silver-copper, tin-silver, and tin-copper. Silver was used against copper and tin in order to favor corrosion as much as possible. The tests were car8 Ferguson, “Photographic Researches of Hurter and Driffield,” Royal Photographic Society, London, 1920.
I S D U S T R I A L A N D ENGINEERING CHEMISTRY
January, 1924
ried out a t a temperature of 21 O C. (70’ F.) for 5 weeks and then placed in the hot room 43’ C. (110’ F.) for 8 weeks. Although water is a more or less inactive chemical with regard to the corrosion of the more electropositive (noble) metals, a marked etching of the surface was very apparent in the case of copper, especially if the copper was electrically connected with a more positive metal (silver), when it became badly etched and tarnished. I n the case of the copper-silver couple the solution (water) was not noticeably changed. With the tin-silver couple both metals were unTIN NICKEL
- BRASS
76 Y.
- SOLDER [lS
-AD
+5
