May, 1952
67 1
COLLOIDAL FERRIC OXIDEBY CORROSION INHIBITOE IONS
PRECIPITATION OF COLLOIDAL FERRIC OXIDE BY CORROSION INHIBITOR IONS BY W. D. ROBERTSON Hammond Metallurgical Laboratory, Yale University, NEWHaven, Connecticut Received Julv i7, 1961
The relative precipitating power with respect to a ferric oxide sol of anions employed as corrosion inhibitors has been determined. It is concluded that the colloidal precipitating power of anions is not a significant factor in the corrosion inhibition mechanism.
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Introduction Several proposals have been made regarding a possible connection between the relative precipitating power of various anions with respect to the positive ferric oxide sol and the pronounced effect of the same ions on the corrosion rate of iron.lS2 However, sufficiently extensive data for an adequate evaluation of the proposed correlation has not been available, particularly with regard to ions that markedly inhibit corrosion . For the present purpose, the ions may be divided into three general classes in accordance with their effect on the corrosion rate: the accelerators, of which chloride is typical; sulfate and nitrate which have a less pronounced accelerating effect; and the inhibitors typified by chromate. Recently, nitrite3 and molybdate and tungstate4 have been added to the class of inhibitors. Thus, there is available a reasonably large group of anions of different charge whose general effect on corrosion rate is known and, since the effects of the various classes of ions are so greatly divergent, a correlation between their relative precipitating power and’ their effect on corrosion rate should be readily apparent. For example, chloride and nitrite are, respectively, an accelerator and an inhibitor but both are univalent anions; similarly, sulfate is a moderate accelerator whereas other divalent anions, chromate, molybdate and tungstate, are efficient inhibitors. If the rate of corrosion in solutions of these ions is directly related to their precipitating power, it may be anticipated that nitrite would fall in the class of divalent ions and presumably the mechanism of inhibition would then involve t h e precipitation of hydrated ferric oxide in contact with the reacting iron surface, inhibiting subsequent reaction. Alternatively, if the precipitation values of nitrite and sulfate are similar to other univalent and divalent anions, respectively, it may be concluded that colloidal considerations are not a predominant part of the corrosion or inhibition mechanism. The following experiments were undertaken to obtain the necessary data with which to evaluate the proposed hypothesis. Experimental Procedure In view of the fact that only relative precipitating power of the ions was required for direct comparison, a oommer(1) J. Newton Friend, Carnegie Scholarship Memoirs, 21, 125 (1922). (2) W. D. Robertson, “Encyclopedia, of Chemical Technology,” 4, 487 (1949). (3) A. Wachter and S. S. Smith, Ind. Eng. Chem., 35, 358 (1943); A. Wachter. ibid., 37, 749 (1945). (4) W. D. Robertson, J . (and Trans.) Electrochem. SOC.,98, 94 (1951).
cially avdilable, dialyzed 5% ferric oxide sol was used (Eimer and Amend So-D-12). A stock solution containing 2 g. per liter of FenOawas prepared by dilution and the pH adjusted to 5.0 f 0.1 by dropwise addition of 0.1 M sodium hydroxide. The initial pH of the solution, 2.90, was increased to limit polymerization of the molybdate and tungstate ions and, to maintain the relative character of the experiment, all precipitations were carried out a t constant pH as measured by the glass electrode. Subsequent experiments to determine the effect of the hydroxyl ion alone indicated that precipitation does not occur below pH 6.5. Twenty milliliters of the stock solution and 20 ml. of a solution of the sodium salt of the Drecipitating ion were mixed in a reproducible manner in a containerwhich was patterned after Weiser6 but more conveniently made entirely of glass from standard taper (45/50) Pyrex joints. Following inversion of the container, the resulting solution was poured into a test-tube and allowed to stand for 24 hours. Successive tests with bracketing concentrations resulted in a narrow range of ion concentration above and below the precipitation values given in Table. I. In thc event of questionable completeness of precipitation, the apparently clear supernatant liquid was pipetted out and tested with a n excess of sodium sulfate. The criterion of precipitation adopted was the appearancc of a narrow band of clear solution (less than 1 mm.) a t thc top of the tube in 24 hours. In a series of ten tubes containing increasing concentrations of the precipitating ion, the distinction between (by definition) incomplete precipitation indicated by flocculation without clarification .at the t>opof the tube and definite clarification was readily observed and verified by successive bracketing of concentrations for the purpose of narrowing the precipitation range.
Results The values obtained are presented in Table I, together with those of Weiser and Middleton6 for comparison. It should be noted that the TABLE I FesOa S O L , 1 G. AT pH 5.0
PRECIPITATION VALUES FOR
I’WR LITEIZ,
Preciiitntion valiic (inilliriioles/liter)
Ions
This work
Po:-
0.23
wo: -
.33 .36
cr0:CrZO:MOO:-
s0:-
(OH-) 108NOzCI ClOa-
closNos-
\\’cker and
LIiddlcton
... ...
...
0.650 ,375
.38 ,40 (.73) pH 6 . 5
...
...
...
.437 0.900
...
15.5 16.5 102.0
103.0
...
115.6 131.2
...
...
(5) H. B. Weiser and E. B. Middleton, THIS JOURNAL, 2 4 , 30 (1920).
672
BOOKREVIEW
comparison of the two sets of data, obtained with SOIS prepared in a different manner, is only indicative of similar trends; however, with the exception of the chloride ion, the correspondence between the two sets of data is good. The variation obtainable between univalent ions and the clear distinction between univalent and divalent ions is again demonstrated. e Conclusions It is apparent from the data that nitrite, molybdate, tungstate and chromate fall in their respective class of univalent and divalent ions with respect to precipitating power; in other words, there is nothing distinctive about the inhibitor ions that
VOl. 56
would indicate that their colloidal properties are involved in the inhibitor mechanism. The fact that sulfate, a moderate accelerator, and nitrite, an efficient inhibitor, have precipitation values characteristic of their charge apparently means that precipitation of colloidally dispersed corrosion products is not directly involved, otherwise chloride and sulfate should both be inhibitors or, alternatively, nitrite should be an accelerator. Acknowledgment.-This work was done while the author was associated with the Institute for the Study of Metals, University of Chicago, and was in part supported by Army Air Force Contract #AF 33 (038) 6534.
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. BOOK REVIEW College Chemistry. By LINUSPAULING, Professor of Chemislry in the California Institute of Technology W . , H. Freeman and Company, San Francisco, Calif. 1950. x -I- 705 pp. 213 figs. 37 tables. 16 X 24 cm. Price, $4.50. In this book-one might almost say edition-the illuminating and unconventional approach of Pauling’s “General Chemistry” is still there, although in a more diluted form. The treatment of fundamentals is clcar and adequate, although the occasional obscure sentences of “General Chemistry’’ have been not quite completely eliminated. A t first glance the two books appear almost identical, but in “College Chemistry” the order of chapters has been changed-for cxample the treatment of g a m and gas laws is now considered much earlier-and material shifted around between chapters; the contents have been expanded by addition of more factual matter on metallurgy, manufacturing processes, chemistry of the common elements, biochemistry and photography including color photography. The printing of the book and also the illustrations have been improved and there are very few misprints. Thc main reason given for publishing “College Chemis-
try” is, it seems, the need for a freshman text “written in a more slowly paced and less mathematical form.” The attempt to achieve a slower pace seems to have been made in the main, by the omission of paragraphs on more sophisticated topics, except in one or two chapters, such as the onc on gases and gas laws, where a certain amount of rewriting has been done; many of the chapters, however, including the first five or so, have virtually the same wording as i n “General Chemistry.” The writing in a less mathematical form appears to consist in omitting the use of calculus in rate equations, thus bringing the book into line with most conventional freshman texts. A pity. I n this reviewer’s opinion it is questionable whether the average or subaverage freshman, to whom it seems popular to cater these days, will find the present book so much easier to understand than “General Chemistry” that its existence is justified. There is nb doubt, however, that Prof. Pauling’s “College Chcmistry” is one of the best freshman texts available a t the present time. DEPARTMENT OF CHEMISTRY HARVARD UNIVERSITY GEOFFREY WILKINSON CAMBRXDGE 38, MASSACHUSETTS
