102 PERKINS: THE POROSITY OF IRON.

XII1.-- The Poi-osity of I r o n .

By WILLIAM HUGHES PERKINS.

IRON which has been immersed for some time in a solution of alkali hydroxide and then thoroughly washed with water is stated by Dunstan and Hill (T., 1911, 99, 1853) to exhibit a certain ‘‘ passivity ” towards nitric acid or copper sulphate and towards atmospheric corrosion. Friend (T., 1912, 101, 50) believes this phenomenon to be different from ordinary passivity, and asserts that it is caused by the retention in the pores of the metal of a small quantity of alkali, which is sufficient to prevent corrosion. He does not explain why it also renders the iron inactive towards copper salts and nitric acid. The statement that alkali is absorbed is criticised by H. B. Baker (Annual Reports, 1911, 8, 31), who

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PERKINS: THE POROSITY OF IRON. 103

failed to confirm E’riend’s results, using rather more dilute solutions of the alkali hydroxides. Before Baker’s criticism appeared the present author had been led to repeat Friend’s experiments, on account of what appeared to him to be a grave defect in the method used to extract the retained alkali from the metal. After soaking Kahlbaum’s iron foil in concentrated ( 6 N ) sodium or potassium hydroxides and then thoroughly washing in a stream of distilled water, Friend placed his metal in a shallow porcelain dish contain- ing distilled water. It is difficult to see how this procedure could lead, in the case of sodium a t any rate, to satisfactory blank teste. To obtain distilled water which is free from sodium is no easy matter, and contact with the glaze of a porcelain dish will always produce a distinct flame reaction in quite a short time. It was felt desirable, therefore, that the experiments should be repeated in platinum dishes, and that an attempt should be made to obtain more conclusive evidence, if possible, of a roughly quantitative nature.

The first problem was a careful repetition of Friend’s experiments with sodium hydroxide. His procedure was followed in every detail, except that platinum dishes were used for the extraction in place of porcelain. It was possible in most cases to distinguish between the test and the blank, but the difference was not a t all striking. A satisfactory blank experiment was not obtained, even when the distilled water was collected directly from the tin worm of the condenser in the platinum basin. I n the case of potassium the flame test and spectroscopic test are much less delicate under ordinary conditions, but it was found that, on carefully concentrating the solution to about 0.1 c.c., there was a distinct indication of potassium. It might be possible t o arrive at a more definite solution of the problem as far as it concerns sodium and potassium hydroxides by the use of more highly refined methods and more complicated apparatus, but it appeared more profitable at this &age to extend the inquiry to other substances. Baker (Zoc. cit.) mentions the use of barium hydroxide, and this alkali was chosen for the next experiments. The iron (Kahlbaum’s iron foil) was immersed for three months in nearly saturated baryta water in an atmosphere free from carbon dioxide. It wae then well washed with distilled water until after soaking for five minutes, the solution gave no turbidity with sulphuric acid and no flame coloration. The metal was then immersed in dilute hydre chloric acid for an hour, the liquid being then poured off through a filter and tested for barium with sulphuric acid and by the flame coloration. A distinct cloudiness and a green flame coloration were always obtained from the test and none from the blank

The result was very uncertain.

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104 PERKTNR: THE POROSITY O F IRON.

solution. It is possible, however, that the barium may have been retained on the surface of the iron as insoluble carbonate formed from the traces of carbon dioxide which could not be asmmed to be absent from the metal. Further experiments were therefore carried out xi th lithium hydroxide. A saturated solution of this alkali was mixed with about one-fifth its volume of boiled distilled water. Pieces of iron 5 x 4 cm. in area were immersed in it for periods varying from three weeks to six months, the vessel being kept well stoppered. When the metal was removed it was well washed, first under the tap and then in a stream of distilled water, being subjected at the same time to vigorous rubbing either with the fingers or with cotton-wool. This process occupied from ten to twenty minutes in each case. After a final thorough rinsing it was placed in a platinum dish containing about 5 C.C. of distilled water and left for about twenty-four hours. The water was then poured off into a clean platinum crucible, evaporated down to about one-fifth of a c.c., and then tested by the flame test on a clean wire. There was in all caaes a distinct coloration, which was not obtained in any of the blank tests. For the blank tests a piece of the same iron was treated in exactly the same way, except that it was not immersed in the lithium hydroxide. It is, of course, possible, on account of the relative insolubility of lithium carbonate, to advance against these results arguments similar to those used in the case of barium. The solubility of lit’hium carbonate, however, is so distinct (more than 1 per cent. a t 1 5 O ) that i t is not likely to have been retained as a surface deposit during such thorough washing. The substitution of electrolytic iron (Schuchardt) for the iron foil did not modify the results obtained. A gold crucible which had contained lithium hydroxide for some months, after washing well for five or ten minutes with running water, required a daily change of water for more than four weeks before the spectroscopic test for lithium failed to show its presence in the water. It is clear, therefore, that traces of alkalis, and presumably therefore other solutions, are retained by metals in such a way that their extraction is a slow process of simple diffusion, and cannot be hastened by shaking or even by gentle rubbing. Whether this is due to actual porosity or to the formation of a surface layer is not quite clear, but the “ absorption ” is obviously very slight. To obtain some estimate of the absolute quantity an attempt was made to obtain approximate figures, using ammonium hydroxide as the alkali and Nessler’s reagent as a quantitative indicator. A large piece of iron foil about 500 sq. cm. in area (reckoning both sides) was well polished and cleaned, and then immersed for six weeks in concentrated ammonia solution. The washing was carried out first with distilled water, and $hen at the end wikh four changes of

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PERKINS: THE POROSITY OF IRON. 105

ammonia-free water (500 C.C. contain less than 0.000002 milligram of ammonia). As a rule, the washing, after three or four prelimin- ary rinsings, required about fifteen minutes and about ten changes of water, the last four each remaining in contact with the metal for two minutes. The metal was then covered with 500 C.C.

ammonia-free water in another vessel and left for three days. A t the end of this time all the water was distilled through an ammonia-free condenser, and the distillate tested. Four experi- ments gave quantities of ammonia varying from 0*00002 t o 0*00003 gram for 500 sq. cm. of metal. A quantity of this order of magnitude may or may not be sufficient to account for the anoma- lous behaviour of the iron which retains it, but it is doubtful whether one is justified in assuming that it is retained by actual crpores’7 in the metal. I n connexion with ammonia one difficulty was kindly pointed out to the author by Professor Smithells. It was a t one time believed that ammonia was formed during the rusting of iron, and this view has not, to the knowledge of the writer, ever been confuted. The German edition of Berzelius’s “ Lehrbuch der Chemie ” (1834) contains the following paragraph (Vol. III., p. 427):

“ I n trockner Luft oxydirt sich das Eisen nicht, um so rascher aber in feuchter und besonders bei Gegenwart von vie1 Kohlen- saure. Es entsteht hierdurch der sogenannte Rost, welcher ein Gemenge von kohlensaurem Eisenoxydul mit Eisenoxydhydrat ist. Das Eisen oxydirt sich dabei nicht bloss auf ICosten der Luft, sondern zugleich wird auch Wasser zersetzt, dessen Wasserstoff sich im Entstehungszustande mit Stickstoff aus der Luft zu Ammoniak verbindet. I n dieser Reaction besteht zwar nicht hauptsachlich die Oxydations-Erscheinung, sie findet aber doch stets so unverkenn- bar staat, dass ein schwach gerothetes Lackmuspapier, welchm man in eine verkorkte Flasche aufgehangt hat, auf deren Boden sich mit Wmser angefeuchtete Eisenfeilspahne befinden, nach wenigen Stunden geblaut wird. Ein Theil des sich bildenden Ammoniaks verbindet sich rnit dem Eisenoxyd, und so enthalt auch sonderbarer Weise alles mineralisch vorkommende Eisenoxyd, 60WOhl das aus den Urgebirgen, als das aus jungeren Formationen, Spuren von Ammoniak, welches in Destillations-gefiissen ausgetrieben werden kann.* Diese Verhaltnisse sind zuerst von Chevallier beobachtet worden.”

It appears, therefore, if this view is correct, that a quantity of ammonia may be produced in the period during which the iron is immersed in the distilled water when it undergoes a, good deal of corrosion. Carefully conducted experiments have shown

This statemeut is repeated by MeiidelBev (Principles, 1891, Vol. II., 318).

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106 SEGALLER THE RELATIVE ACTIVITIES OF

that the ammonia produced during the rusting of large pieces of iron foil, which lost from 0.4 to 0.6 gram in weight in the process, was less than 0-000002 gra.m, as determined by Nessler’s reaction. The experiment with litmus paper as indicator could not be repeated, whilst the geological evidence, bearing in mind the nature of iron oxide, is not very surprising.

It may be remarked, in conclusion, that in all the author’s experi- ments the iron which had been soaked in alkali and then well washed, rusted much more irregularly, and was more liable to pitting than iron which had not been so treated.

THE UNIVERSITY, LISEDS.

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