SYNTHETIC CORROSION PITS AND T H E ANALYSIS OF THEIR CONTENTS' E. D . PARSONS, H . H. CUDD,
AND
H . L. LOCHTE
Department of Chemistry, University of Texas, Auatin, Terns Received June 9 , 1041
While theories regarding the mechanism of the most dangerous type of corrosion, known as pitting, have been well developed, especially by U. R. Evans and coworkers, this has been done largely without the aid of chemical analysis of the solutions enclosed within the pit. Baylis (1) found that tubercles on the inside of water pipes contain solutions with a pH of about 6, regardless of the pH of the water in the pipe, and also reported over 1 per cent of chloride and sulfate in the layer of corrosion product enclosing the pit. He came to the conclusion (2) that the pH of solutions in dormant pits lies between 6.4 and 6.8, while that of active pits is lower, apparently near 6.0. In another paper (3) he reported that in some cases the amount of ferrous sulfate is over 2 per cent of the total weight of the tubercle, including the liquid held within the pores of the solid mass, and expressed the opinion that in some instances the liquid probably contains as much as 5 per cent of ferrous sulfate. He stated that the results obtained indicate that the pH of the solution inside the tubercle is between 5.8 and 6.6, although it is very difficult to obtain samples without contact with air and consequent change in pH. Dr. Baylis also reported that he had analyzed the contents of a number of very large tubercles covering one or more pits, but did not attempt the analysis of the contents of small single pits (4). Aside from the work of Baylis, little seems to have been done, presumably because of difficulties in obtaining samples and in analyzing the small quantities involved. The development of micro methods, especially the elegant micropotentiometric method of Karl Schwarz (9), has made the analysis feasible. There remains, however, the difficulty of obtaining unchanged and uncontaminated samples for analysis. The senior author found in 1930 that it is possible to prepare what might be called synthetic pits by using the exposed end of an iron wire inside a glass tube of suitable dimensions as anode or pit bottom and a flat coil of bare copper wire above 1 This article is baaed upon theses submitted by H . H . Cudd in 1936 and by E.D. Parsons in 1939 to the Faculty of the University of Texas in partial fulfillment of the requirements for the degree of Master of Arts. 1339
1340
E. D. PARSONS, N.
n.
CUDD, AND H. L.
Locnm
the glass tube as cathode: as shown in figure 1. If the dimensions of all parts, especially the area of the copper cathode, are correct and the short-circuited assembly is placed in water or a mixture containing the correct ratio of corroding and protecting anions to produce pitting-a solution of chloride at a pH above 8, for instance-some of the cells will yield a yellow tough film across the tube a t some point between the anode and cathode. This film tends to bulge upwards, break, and permit formation of another and perhaps a third film before a stable film finally forms.
A
FIQ.1 FIG.2 FIG.1. P i t assembly. A, iron wire; B, copper-wire cathode; C, flat spiral of cathode; D, paraffin insulation; E, exposed end of iron anode; F, cork; G , glass tubing. FIG.2. Titration assembly. A, chromel-wire support, spring, and conductor; B, potassium chloride-agar bridge to calomel electrode; C, buret tip; D, magnet; E, electrode titration loop; F,part of microscope stand; G , binding post; H, iron-wire armature; I, wire to potentiometer.
When this happens the dark green flaky precipitate that first forms near the anode and most of any previous film material dissolve to yield a clear 2 A reviewer of this paper thinks that a copper-wire,cathode yields a higher cell potential than is produced in a tubercle. The authors do not think that this is necessarily true except for a short time, since polarization, cathodic as well as anodic, and internal resistance due to the membrane reduce the available potential to a small value. Since the effects produced vary extensively with the length of the copper wire used (area of the cathode), it is evident that cathodic polarization must be large. Our experiments show that stable films are encountered only a t certain definite current densities which are obtained through the use of a definite area of cathode.
SYNTHETIC CORROSION PITS
1341
solution which may be pipetted out, after pouring out supernatant liquid and gently rinsing the tough film with distilled water. Unfortunately, the conditions under which such stable films form are highly critical, so that most of the tubes finally have corrosion products overflowing to the bottom of the beaker. The rate of corrosion at the anode is evidently limited by cathode polarization. This in turn is governed by the amount of oxygen reaching the copper cathode, which depends on the area of cathode exposed, the separation of the wires in the coil, the depth of immersion, the concentration of the electrolyte, the temperature, etc. The overflowing of corrosion products to the bottom of the beaker is partly eliminated by placing the flat spiral of cathode almost in contact with the top of the tube. Under these conditions corrosion products soon seal the tube, and the whole solution between this seal and the anode clears up as before and may be analyzed, even though the seal is sometimes broken and some material overflows. The results reported in this paper are submitted to show a technique that may be employed in the study of corrosion pits, rather than to make a contribution to the study of corrosion. The results presented do show, however, that the pH of solutions in corrosion pits containing chloride ions is 6.0 or less; also, that this pH as well as the concentration of other ions present are governed partly by diffusion through the membrane, since the concentrations determined vary, as would be expected where several films form between anode and cathode. The fact that a high sodium-ion concentration is usually accompanied by low chloride- and ferrous-ion concentrations seems to indicate that spontaneous rupture of the membrane and partial equalization of concentrations may occur a t any time in the life of a corrosion pit. Visually, this increase in pressure and rupture of the membrane are often observed in the case of membranes formed within the glass tube. EXPERIMENTAL
Corrosion pit assembly
The “sjkthetic pits” were prepared as indicated by figure 1. A is a piece of common baling wire with a diameter of 1.6 mm. and the composition reported previously (7). The exposed end, E, was ground, polished, and inserted in the cork, F, until about 1 mm. of wire protruded through the cork. The wire was then bent as shown, and the cork was inserted in the length of glass tubing, G. The KO.22 bare copper-wire spiral, C, was then connected by twisting it around the freshly polished iron wire as shown, and was adjusted as desired with respect to the glass tubing. This assembly was then dipped into melted paraffin in such a way as to
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E. D. PARSONS, H. H. CUDD, A N D H. L. LOCHTE
coat all the exposed iron wire while leaving the upper part of the glass tubing clear. Very few of these assemblies thus protected later showed corrosion except where desired. The assembly was then immersed in a beaker containing 9 parts of 0.1 N sodium chloride and 1 part by volume of 0.1 N sodium hydroxide to yield an initial pH of approximately 12. Distilled water was added daily to keep the volume of solution constant,
AQE
1
TABLE 1 Analysia o j cell contents PH
1
CONCENTRATION, I N MQ.-EOUIY. PER CUBIC CENTIMETER
Fer+
1
CI-
1
Na+
&YE
28 30 31 33 40 46
5.8 6.0 6.0 6.2 6.2 6.0
1
0.058
0.162 0.156 0.154 0.161 0.169 0.178
~
0.085
0.105 0.113
0.057 0.062 0.076 0.065 0.065
Series A: Cathode, 9-cm. copper wire; cell, 7 x 25 mm. 24
1
6.0
1
0.172
I
0.240
1
0.068
Series B: Cathode, 15-cm. copper wire; cell, 7 x 25 mm. 31 32 35 36 38 39 40 41
5.8 5.8 5.8 5.8 5.8 5.8 6.1 6.3
0.108 0.246 0.230 0.192 0.159 0.115 0.158
0.062
0.170
0.036 0.043 0.067 0.047
0.236
0.226 0.216
I
0.058
4.065 0.048 0.040
but it was not feasible to exclude carbon dioxide from the air and the pH dropped in time to a buffer mixture with a p H of 8.85-8.86. Greenish black corrosion products began to form in a few minutes and within 2 days filled between 25 and 50 per cent of the tube. Meanwhile a yellowish membrane tended to form a t some point between the electrodes in the tube. I n most cases this membrane bulged upward and soon rup-
SYXTHETIC CORROSI9N PITS
1343
tured, only to form another membrane a few millimeters higher up in the tube, In many cases this process continued until the tube overflowed and red products accumulated in the bottom of the beaker. This was due to too high a current and could be remedied by using a shorter copperwire spiral or by immersing the assembly deeper in the solution to cut down the amount of oxygen available for corrosion. It proved impossible to get more than 10 per cent of stable films out of apparently identical set-ups. In view of this difficulty, the copper-wire spiral was then placed almost in contact with the tube, and it was found that corrosion products usually sealed the tube contents with red products between the copper spiral and the tube. This film was less tough and stable but could be rinsed gently and punctured to obtain the sample for analysis. The results reported in table 1 were all obtained by this method. In most cases a small amount of dark green precipitate remained undissolved in contact with the anode, but there was sufficient clear solution for the analyses. A large number of identical cells were set up; a t various time intervals one of these was selected for analysis and the pH, the iron concentration, and the chloride concentration were determined. Single small droplets of the solution were placed on a strip of treated filter paper, and the color produced was compared with that produced by similar drops of buffers covering the pH range involved. The pH values of the standard buffers as well as of the solutions in the beakers mere checked by means of glass electrodes. For pH values below 5.8, bromocresol green was used as indicator; for values above 5.8 a mixture of 250 mg. of methyl red and 105 mg. of methylene blue dissolved in 200 cc. of 90 per cent ethanol was used in treating the filter paper. The buffers were prepared according to McIlvaine’s citric acid-sodium phosphate mixtures (8). The presence of iron salts limited the accuracy of the determinations to between 0.1 and 0.2 pH unit. The 0.05 N ceric sulfate solution was standardized potentiometrically against standard iron wire according to the method of Willard and Young (10). In most titrations air was excluded by means of a slow stream of carbon dioxide entering the funnel below the titration assembly. Silverfoil electrodes and 0.1 N silver nitrate solution were used in the chloride titrations. Potentiometric apparatus Schwartz in his original apparatus used a small loop of platinum wire as titration vessel, stirrer, and electrode for suitable titrations. The apparatu’s was modified as shown in figure 2. The use of a Ford spark coil, D, using 60-cycle current from a small variable voltage transformer to vibrate the titration loop E through the iron wire, H, proved convenient. To avoid violent jerks on turning the current on or off, the vibrating system
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E . D. PARSONS, ,H. H. CUDD, AND H. L. LOCHTE
was controlled by means of a fine thread (not shown) connecting the binding post, G, and the frame, so as to limit the amplitude of vibration. A Leeds and Northrup student-type potentiometer with a vacuum-tube galvanometer constructed by L. D. Goodhue (6) was used in determining potentials and in the pH determination with glass electrodes. The microburet of Schwartz was used with minor modifications, although it was found in the iron titrations that it was more convenient to raise the funnel through the microscope stand, F, until it touched the wire loop and raised it into contact with the buret tip, than to lower the tip to make contact with the drop. The buret was calibrated by weighing water delivered over various intervals of the buret. The total calibrated volume was 0.1613 cc., and each unit of the scale represented an average value of 0.001613 cc. The magnifier used permitted the estimation of 0.1 unit TABLE 2 Analysis of multi-film p i t s AQE
NO. OF FILM8
CHAMREB ANALYZED
PH
CONCENTRATION, I N YQ.-EqUIV PER CUBIC CENTIMETER
Fe++
c1-
N a+
0.490 0.405
0.560 0.2.32 0.488
0.070 0.087 0.047
&lid
11 13 28
29
Lower Upper Lower Upper Middle Lower Upper Middle Lower
5.9 6.0 5.8 5.8 5.6 5.2 5.6 5.4 5.2
0.441 0.242 0.415 0.550
0.244 0.420 0.517
or 0.00016 cc. The volume of standard solution used ranged from 0.025 to 0.1 cc., depending on the size of the sample. If too large a volume of sample was employed, the drop tended to fall out toward the end of the titration, while too small a volume would not adhere at the start. We found that the platinum loop retained a larger drop if the wire was flattened before forming the loop. The sample was sucked into a micropipet which had been filled with carbon dioxide or hydrogen gas. The amount used in any titration was determined by the difference in weight. Chloride determinations proved troublesome at first because of delay in obtaining a steady potential, but the difficulty was cured by resort to Clark’s nitric acid treatment of the electrode (5). This permitted readings within 1 or 2 min. except near the equivalence point, where 5 to 6 min. were usually required to obtain equilibrium readings.
SYNTHETIC CORROSION PITS
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RESULTS
The results obtained are presented in tables 1 and 2. The sodium concentration was obtained by difference, since only iron and sodium salts were present. While considerable concentrations of bicarbonate ion may have been present a t the pH range encountered, this anion was not determined and the sodium-ion concentration calculated is low by this amount. Table 2 shows the results obtained with chance assemblies in which several distinct films formed so that several regions, each separated from the others by a film, were available for analysis. The upper section in each case is the one nearest the cathode, and the lower is the one surrounding the anode. In each case there is an increase in H+, Fe++, and C1- as the anode is approached, as would be predicted if the concentrations encountered were governed by diffusion; hence it is possible that the solution a t a sufficiently protected anode would show a pH considerably below 5 , although it would probably be prevented, by hydrogen evolution, from reaching a value of 3, which was obtained with a ferrous chloride solution containing 0.5 per cent ferrous iron. SUMMARY
An apparatus is described which permits the study of pitting and tubercle formation with separated anode and cathode areas. A modification of the apparatus and technique involved in the micropotentiometric titration method of Schwartz was used in the analysis of the contents of the artificial corrosion pits and is suggested for the study of actual corrosion pits and tubercles. The results obtained indicate that the concentration of ions in such tubercles is governed by both diffusion and electric transport, so that for any one tubercle and condition there is an equilibrium between ions passing through the membrane by diffusion and by electric transport. REFERENCES (1) BAYLIS,J. R.:Chem. & Met. Eng. 32, 874 (1925). (2) BAYLIS, J. R . : J . .4m. Water Works Assoc. 16, 606 (1926). (3) BAYLIS,J. R . : Ind. Eng. Chem. 18, 378 (1926). (4) BAYLIS,J. R . : Private communication (1941)., (5) CLARK,W.: J . Chem. SOC.1928, 749. L . D.:Iowa State Coll. J . Sci. 10,7 (1935). (6) GOODHUE, (7) LOCHTE, H.L . , AND PAUL,R . E . : Trans. Am. Electrochem. Soc. 64, 155 (1933). (8) MCILVAIXE, T.C . : J. Biol. Chem. 49,183 (1921). (9) SCHWARTZ, KARL:Mikrochemie 13, 1, 6 (1933). (10) WILLARD,H.H., AND YOVSG, PHILENA: J . Am. Chem. SOC.60,1323 (1928).
