W. H. J. VERNON Dr. W. H. J. Vernon in presenting the report emphasised the im-portance of the diffusion factor in atmospheric tarnishing. Thus a speci-men of copper might tarnish readily when suspended freely in a room and yet remain bright indefinitely if enclosed in a bell-jar. Indeed under such conditions it would slowly develop a protective oxide film. He called attention to the following papers which had been inadvertently omitted from the list given on p. I 16. Gaze R. C “Note on temper colours.” J SOL. Chem. Ind. 1924, Musun C. W. “Temper colours.” J physicad Chm. 1924 28, 43 349T. 1233. Turner 2’. “The oxidation of metals.” Met. Ind. (Load.) 1924 25, 546. Mr. J. C. Hudson at the invitation of the President said that the ex-tension of the open air tests to different localities for which purpose he was appointed about two years ago had rendered it necessary to devise methods of measurement that are at once sufficiently wieldy to be applied on a large scale and sufficiently sensitive to detect the changes produced within a reasonably short period.The problem now appeared to have been satisfactorily solved in two distinct ways; firstly by the development of a method in which the change in the electrical resistance of a wire, brought about by corrosion is accurately determined secondly by the ap-plication of the weight-increment method to specimens exposed outdoors, but sheltered from the rain in a Stevenson screen. Weight loss tests might also be carried out although in this case as Dr. Vernon had shown caution would be necessary in interpreting the results.Preliminary investigations on experimental methods had been completed and within a month or so, it was hoped to begin a systematic series of tests by all three methods on fifteen typical non-ferrous materials. Provision had been made for exposur 184 DISCUSSION at five different stations in this country which had been selected as typical of rural suburban urban industrial and marine atmospheres respectively. In the course of this preparatory work several points had emerged which bore on the present Report. For instance there was a great differ-ence in the rate of corrosion indoors and outdoors. Dr. Vernon showed that the corrosion of zinc was fifty times more intense on the roof than in the basement. H e (Mr. Hudson) had observed even greater differences in the case of zinc coils exposed in the basement and in a Stevenson screen at the Royal Botanic Gardens Regent’s Park respectively as the following table of weight-increments after sixty days’ exposure would show :-SURROSION OF ZINC COILS IN DIFFERENT TYPES OF ATMOSPHERE.Atmosphere. Weight Increment (Mgms. per dm2 after 60 days). Period of Exposure. Ratio of the Weight Increments. I. Basement . . j! Jan.-Feb. 1927. 2. Stevenson Screen at the June-July 1926. 3. Royal Botanic Gardens. Nov.-Dec. 1926. 1’4 22’1 264 I 16 I‘% The surrosion of zinc was thus from 16 to 189 times greater outdoors than in the laboratory and also there was a very marked difference in the so 100 160 100 255 SOC DAYS FEBRUARY 1416 JULY JANUARY 1 9 ~ 7 rate of corrosion out-doors during the winter and summer periods respectively.All metals corroded appreciably more dur-ing the winter but the numerical ratio of the rates of attack during the two sea-sons was different for different materials ; in the case of tests on twelve materials at the Royal Botanic Gardens this ratio varied from 5-5 to 13 and as a result a radically different or-der of corrodibility was observed during the two periods. Since climate was not only a function of season but also of locality it followed that similar variations in the relative corrodibility of non-ferrous materials were to be expected in different regions of the earth’s surface; they hoped that the coming field tests might bring some of these to light.Dr. Vernon stated (on p. 142) that in the case of brass specimens ex-posed in the tank room there appears to be definite evidence of accelera-tion of corrosion. The results of exposure tests on 60 40 brass specimens in a Stevenson screen where the conditions were much more drastic had confirmed this conclusion. The accompanying curve in which weight DISCUSSION 185 increment was plotted against time showed distinct evidence of an accelerated rate of attack following a slight decrease in the rate during the summer-period. I t was difficult to decide whether this acceleration was due to a specific property of the corrosion product or to the advent of the more stringent conditions associated with winter ; in all probability the result was due to a combination of both factors.I t was true that very marked con-densation of moisture took place on the specimens in winter owing to the increased humidity but on the other hand this condensation appeared to be selective and to occur more readily on some materials than on others. I t would seem that the relative deliquescence of the corrosion-product had an important influence on corrosion. DISCUSSION. Mr. A. E. Munby as chairman of the Atmospheric Corrosion Research Committee expressed the appreciation of the Committee to the Society for the opportunity of the discussion of Dr. Vernon’s work. H e thought that results of ultimate practical value were emerging. He particularly referred to the films formed on metals at high temperatures which might lead to useful results.Professor H. C. H. Carpenter emphasised the fact that Mr.Vernon in carrying out the work described in the report had had to work out his own methods. I t was noteworthy that these. methods should have given such consistent and satisfactory results. The method of weighing the specimens had been devised in conjunction with Professor Conrady. I t was laborious, but the time spent on it was well spent. As the result of the adoption of those methods Dr. Vernon had been able to establish some very important facts in connection with the formation of films. The most important and, he thought vital of those was that the phenomena could be classified under three main heads. Taking the co-ordinates-weight increments and time the curves ob-tained were either a parabola a straight line or a curve which began as a parabola and then rapidly flattened towards the time axis.Copper gave the first zinc the second and aluminium the third. I n the first case the film formation was regulated by diffusion through a solid envelope; in the second by gaseous diffusion only while in the third it appeared to be de-pendent not on diffusion but on adsorption. Of these he gathered that the aluminium type of film was the most impervious while the zinc film was the least impervious. From the point of view of the object with which this re-search had been undertaken the aluminium type of film was accordingly the best while that of ziac was the worst. So far Dr. Vernon had been examining types of films which were formed ‘‘ naturally.” It would be an interesting development of this work to ascertain whether a film of the non-resistant type could be altered so as to render it more resistant by the particular set of conditions under which it was formed.Could e.g., the zinc film be rendered less pervious ? Experiments in these directions seemed he thought to be called for and if successful would be most valuable. The behaviour of the two brasses was he thought particularly interest-ing. The curve of weight-increment was characteristic of copper in the early stages and of zinc in the later stages. This would seem to indicate from the standpoint of resistance to tarnishing that the brasses were inferio 186 DISCUSSION to copper-and speaking more generally that a pure metal was likely to be more resistant than an alloy.The results obtained with iron stood rather in a class by themselves and were clearly influenced by the fact that the metal passed through two oxidation stages. Dr. Vernon’s Report constituteda most important record of scientific evidence bearing on the practical problems he hoped to solve. He (the speaker) thought the way was now opened for the application of those results to the problems in question. Dr. G. D. Bengough said that to one studying the problem of cor-rosion in which the metal is immersed in water or neutral salt solutions the report was of special interest in showing the widely different types of action which might be included under the the word (‘ corrosion.” Not only might the controlling factors be different Gut the whole mechanism might be different from the start-a fact that was perhaps not yet fully realised by the chemical world in general.I t might be of interest to summarise some of these differences which have an important effect on the choice of experimental methods. I . Dr. Vernon had shown that the nature of the metallic surface, whether abraded ground on emery or treated by certain kinds of chemicals had relatively little effect on the extent of corrosion especially in the case of zinc and good agreement between duplicate results was frequently ob-tained (vide First Report and Table IX. p. 171). In immersed corrosion experiments not only did different types of surface give widely different results but closely agreeing duplicates were difficult to get even with the same type.2 . The close approximation to uniformity of action over the surface (apart from edges) in conditions which included exposure to rain was re-markable and quite different from what occurs in immersed corrosion. In Dr. Vernon’s experiments there was practically no local action or pitting, which was the bugbear of immersion tests. Corrosion was strictly propor-tional to the surface area a relation that did not always obtain in immer-sion tests according to the opinion of such eminent workers as Heyn and Seligman. 3. In the atmospheric corrosion tests of Dr. Vernon the nature of the corrosion product was usually of such a nature that continuous corrosion-time curves could be constructed because the action of weighing does not change its nature ; this was an enormous advantage over immersed corro-sion work in which such curves were urgently required but were much more difficult to obtain.The nature of the corrosion product and particu-larly the rate at which it allowed oxygen and sulphur compounds to reach the metallic surface was the main controlling factor influencing the speed of corrosion in atmospheric corrosion; but in immersed corrosion in con-ductivity water or very dilute salt solutions although oxygen was necessary for corrosion the controlling factor was the conductivity of the liquid. Oxygen did not penetrate to the seat of corrosion at all-a fact of great significance in differentiating the two types of action. 4. The nature of the film of the products of the action might evidently be widely different ; for instance Dr.Vernon showed on page 149 that for zinc a granular film showing diffraction colours might be obtained; on the other hand the cathodic films formed between the pits on immersed zinc were continuous and showed interference colours. 5. The initial portions of the time-corrosion curve for zinc were particu-larly interesting; that shown on page 136 appeared to cut the vertical axi DISCUSSION and probably really indicated a rapid initial action. On the other hand, zinc with immersed corrosion showed a retarded initial action in very dilute salt solutions. 6. A study of the curves for immersed corrosion of zinc specimens had recently been carried out in the Corrosion Research Laboratory of the D.S.I.R. They differed markedly from those published in this Report; they were more complicated and consisted of several branches which probably indicated different controlling factors.I t was clear that if the term “corrosion” were to be used to cover attacks both by the atmosphere and by dilute salt solutions a very broad definition was required; indeed it seemed probable that it could not be narrower than some such phrase as (‘ corrosion is oxidation 77. I t seemed clear that much further work-of a high order of accuracy such as had been reached in this Report-was required into the nature of both kinds of action. Mr. U. R. Evans expressed his appreciation of the important results which had been achieved and of the sound policy by which the research had been guided. I t was necessary to understand a disease before they could with confidence prescribe a cure; they might now fairly claim to understand the disease of atmospheric corrosion and could thus approach the subject of mitigation or prevention.He hoped that this sensible policy would be adhered to by the Committee who gave guidance to the work. With regard to Dr. Vernon’s interpretation of his results he was on most points in complete accord with him. On a few minor issues, alternative explanations suggested themselves. The form of the oxidation-curve of aluminium (p. I ~ z ) for instance could be explained most simply, if they remembered the curious change from the pervious to the impervious form of oxide which occurred when aluminium was heated in air. Pilling and Bedworth,l working at 600’ C.found that this change occurred quite suddenly so that the oxide film suddenly ceased to thicken. Evidently at that high temperature the conversion when once started at one point, speedily extend over the whole surface. But at ordinary temperature the extension would be slow and oxidation would come to an end gradually, giving just the type of curve which Dr. Vernon had actually obtained (Fig. 19). No doubt Dr. Vernon was right in attributing the “subsidiary increases ’’ (shown in Fig. 2 0 ) to cracking of the “primary film ’’ ; he had recently obtained evidence that the invisible oxide-film on iron also tends to thicken at ordinary temperature mainly by gas passing through cracks. On page 133 Dr. Vernon had used the weight-increment of heated copper to calculate the thickness of the film produced; he had rightly designated the numbers so obtained by the word “approximate,” since some oxide must have been present on the copper when the first weighing was performed; the metal had been ground in air and although this original oxide was too discontinuous to be protective its weight could not be neglected.He was inclined to think that the true thickness might considerably exceed the numbers given ; optical considerations pointed in the same direction. Dr. Vernon’s work on sulphide and oxide films on copper was of great interest. He had often been puzzled by the fact that specimens of copper, originally tinted through exposure to hydrogen sulphide continued to change in hue even when exposed to air containing practically no hydrogen r.Inst. Met. 1923 3 529 ; especially pp. 573 590 I 88 DISCUSSION sulphide. Probably the oxide formed below a sulphide-film was crystal-lographically continuous with the latter and was of the extended pervious form which could not protect the copper from further attack; a similar pervious form of oxide occurred when hydrogen-charged copper was exposed to dissolved oxygen the whole sequence of interference-tints due to oxide films being then obtained at ordinary temperatures.2 On the other hand when ordinary hydrogen-free copper was exposed to dry air free from hydrogen sulphide the oxide film produced was much less pervious and before “ visible ” thickness had been reached it was able to protect the copper below not only from oxygen but also from hydrogen sulphide-should that gas subsequently gain access to the metal.A mixture of oxygen and hydrogen sulphide acting on freshly abraded copper would cause a film sufficiently pervious to reach interference-tint thickness firstly because the film would be partly sulphide which was naturally pervious and secondly because the removal of the hydrogen of the hydrogen sulphide by the oxygen gave just the conditions needed to ensure that the oxide also should be of the pervious variety. H e did not think that Dr. Vernon need apologise if his curves did not always conform to parabolic or linear forms. Any theory which considered both the physical and chemical stages of the film thickening process in-dicated that curves of these two types could only be expected if one of the two stages proceeded much less readily than the other.A rather more general equation (which however still neglected several complicating factors), was recently suggested by the ~peaker,~ to connect the film-thickness y , with the time t ; it might be written : where KO was a physical constant and R a chemical constant. This reduced to parabolic form when KO was small compared with yk, and to Zinear form whenyk was small compared with KO ; but they might not infrequently expect to meet with curves which were neither linear nor parabolic. Under con-ditions where KO and k were of the same order of magnitude the resultant curve would be approximately h e a r when the thickness was smaM and para-boZic when it was iargz. This was the state of affairs shown in the curve of Fig.2 (page 125) and whilst he did not deny the importance of the edge-effect ernphasised by Dr. Vernon he thought the main explanation of this type of curve was to be found in the way just suggested. Professor E. Wilson said that as the result of Dr. Vernon’s open-air exposure tests high conductivity copper was much more seriously affected by corrosion than copper alloyed with small amounts of arsenic nickel or tin. It was significant that exposure tests made on the roof of King’s College London’4 showed a corresponding result in the case of aluminium. The specimens were in the form of wires 0.126 inch (0.32 cm.) diameter, and were tested at intervals for electrical resistance. I f R, R were the electrical resistances at the commencement and at time t during exposure respectively p, pi the corresponding specific resistances a, a t the corre-sponding cross-sectional areas it followed that RJR = pt/po.x ao/aq. In order to draw a comparison between a specimen of high-purity aluminium (25) and an aluminium alloy (7) the following figures were given :-Chew. S O ~ . 1925 127 2484, 3 ‘‘ Corrosion of Metals,” 1916 p. 14. ”. Inst. Elect. Erig. 1925 63 1108 ; Proc. Physical Soc. 1926 39 15. (Arnold. DISCUSSION High-purity Aluminium Aluminum 25 alloy . . 7 I Time Analysis. of Ex. posure t . si. 1 Fe. 1 Cu. 1 Ni. 1 Al. I I 1.061 1‘127 1’193 1.091 1.025 1.119 { 1.093 1’160 1.269 I t would be seen from the ratio ao/at that for a given time t and length of specimen the loss of metal due to corrosion was much greater in the case of the high purity aluminium.(The British Aluminium Co. Ltd.) that “ an explanation of this discrepancy could be sought only in the particular specimen of pure aluminium which was not typical of the metal now used for electrical purposes. I t would appear that present day high-purity aluminium had much greater immunity from the effects of atmospheric exposure; it would be of great scientific interest to know the reason of this change.” I t was of interest to learn that Dr. Vernon had found sulphate in his corrosion products as this confirmed the analysis of the corrosion on the King’s College specimens; a similar remark applied to the entry of iron. The skin or membrane next to the metal was sufficiently compact to retain even soluble salts and the iron undoubtedly came from an outside source.As illustrating the different types of corrosion Fig. I had been prepared by I t was pointed out by Mr. E. T. Painton FIG. I.-High-purity aluminium FIG. 2.-Duralumin FIG. 3.-Aluminium-copper ( x 30). ( x 30). ( x 30). Mr. P. H. Busbridge from the high-purity aluminium after 23 years’ ex-posure. The corrosion surrounded the aluminium in a more or less even layer. Fig. 2 was taken from a specimen of duralumin the surface corrosion having been removed and showed that the corrosion penetrates the metal almost radially in places; this presented a difficulty in estimating the total corrosion. The type shown in Fig. 3 which was obtained from another alloy containing 1-86 per cent.of copper again differed in that there were circumferential deposits within the metal itself. As showing the effect of rain water it might be stated that the radial thickness of corrosion on the high-purity aluminium was the same after 23 years’ exposure as it was after 8 years although during this period there was a further loss of metal. The magnetic susceptibility of the corrosion product after 2 3 years was 1.6 fold the susceptibility after 8 years’ exposure, which pointed to greater concentration. Proc. Physical SOC. 1926 39 24 communicated remarks = 90 DISCUSSION He suggested that the effects of corrosion upon the mechanical pro-perties of the materials tested might with advantage be investigated. I t was found for example that duralumin became exceedingly brittle on exposure.The effects of exposure upon structure was another important matter which might be considered. Professor T. Turner said that the study of surface films was of the greatest importance in connection with corrosion. By taking a sheet of copper foil and heating it in air the metal became transparent. The thick-ness of the foil might be in the neighbourhood of & of an inch and such as could readily be purchased. The temperature of 15o0-2oo0 was suitable varying with the time occupied. At first the colour was a light emerald green and it gradually darkened in colour passing through reds and purple. In such thin metal the two films one on each side met in the middle. Metals also became transparent when passing into solution as shown in many metallic salts.These transparent films from copper were structureless when viewed under the microscope and were solid solutions of oxide of copper in copper. Probably cupric oxide was formed and dis-solving in copper gave compositions of varying proportions including the percentage composition of cuprous oxide. Oxide of iron (ferric oxide) appeared also to go into solution in iron to form a transparent film. I n such cases the film increased in thickness with time and temperature. With aluminium there was no solid solution of oxide of aluminium in the metal and only a suface oxidation could be produced. Dr. R. S. Hutton said that the report embodied the results of a splendidly sustained effort to increase our knowledge of the fundamental aspects of the subject ; there was no doubt whatever that as its evidence became known not only would it have great influence on the further scientific study of corrosion problems but practical applications of import-ance would arise.Professor Carpenter had mentioned that in undertaking the general supervision of this investigation his chief object had been to secure the greatest possible freedom for Dr. Vernon in his work. The British Non-Ferrous Metals Research Association had fully appreciated this view and had tried to limit the interference in the work to a minimum. Probably without such safe-guarding it would have been impossible to maintain the work for so many years when there had necessarily been no immediate advantage to the industry which gave it the chief support.Emphasis should be laid upon the fact that the report really recorded the discoveries of one man. Dr. Vernon had certainly the advantage of the stimulus of contact with Dr. G. D. Bengough and other workers in Professor Carpenter’s laboratory and had shown exceptional keenness in studying the vast literature of corrosion; but the patient and persistent research and the delicate and original methods of experimenting were entirely his own. The careful study of the initial tarnish films on metals and the discovery of the protective effect of some of these we owed very largely to Dr. Vernon ; his work fitted in most logically with that of other investigators in rather different fields and to-day we appeared to be provided with a much sounder knowledge of the cause and of the progress of atmospheric corrosion than was available beforehand.The prospective application of this know-ledge was most hopeful as with causes and progress made clear the control and prevention could be studied with much better chances of success. I n this connection he asked Dr. Vernon if he considered other mean DISCUSSION 191 of strengthening or providing protective films likely to be of use in the application of non-ferrous metals to constructional purposes. I n the first place S. E. Sheppard 6 described the treatment of copper and its alloys by alkaline persulphate solutions ; although the passage was ambiguous it appeared that the protection might be more easy with copper alloys than with copper itself the reverse of what was found with heat treatment ; this if confirmed was probably important.The exceptional merits of aluminium oxide films for protective purposes were particularly promising where the composition of the alloy allowed for auto-regeneration of an oxide film at points where by accidental abrasion or other means the original oxide film had been broken down. The delicacy of (( sprayed ” or ‘L calorised ” surfaces due to scaling could perhaps be got over if alloys were used in which the aluminium formed one of the constituents. Some years ago in America ‘( Calite ” was suggested with this object and in a different range of alloys both Dr. J. S. Dunn and Mr. R. Genders working for the British Non-Ferrous Metals Research Association had given evidence of the very low oxidations of brass contain-ing a quite small percentage of aluminium.I n such cases the breaking away of the surface film might be autogenously repaired by the oxidation of a further amount of the aluminium in the alloy. Dr. J. S. Owens remarked that one very important thing had so far not been discussed uiz. the connection established between the deposit of dust and the rusting of iron. Protection had been given to exposed plates by passing the air through a cotton wool filter and by suspending the plates in a muslin screen. I t was however somewhat difficult to visualise the action of the muslin screen as the open meshes were of the order of 2 0 0 microns in diameter while the particles of London haze were nearly all under I micron. Why therefore were they unable to pass through such large openings? The commencement of corrosion of exposed iron at specific foci also pointed towards the importance of dust deposition.H e stated that the protection of the metal from dust might be approached in two different ways either the metal might be covered to prevent access of the dust or the air might be purified of the corrosivedust. He drew attention to a statement on the top of page 163 in which Dr. Vernon remarked with regard to the rate of deposit of solid particles upon a surface ‘( I t seems probable indeed that they are not precipitated so fast when the humidity is high.” Dr. Owens asked for the basis of this state-ment as he was of the opinion that the opposite was true; the viscosity of water vapour which governed the rate of settlement was approximately half that of air and therefore the terminal velocity of dust in water vapour would be higher.Also in the case of hygroscopic particles condensation upon them would increase their effective mass and therefore their rate of settlement. He pointed out that while in the neighbourhood of the sea coast large numbers of salt crystals could be found in the air this was not the case further inland and he attributed this to the deliquescence of the particles when the temperature fell and the relative humidity rose at night. He thought that the question of a critical humidity might depend either upon the metal or upon the nature of the suspended matter as different salts had different vapour pressures of deliquescence but if the critical humidity were constant for different metals it would point towards the suspended matter as the governing factor whereas if it varied with differen I 92 DISCUSSION metals in the same atmosphere it would point towards the nature of the metal as the governing factor.As bearing upon the effect of deposited dust on the metal he suggested the exposure of two plates fixed back to back and exposed in a horizontal position so that the upper surface of the top plate received the deposited dust whereas the lower surface of the bottom plate was more or less protected. I t might be found in such a case that the weight increment of the two plates differed. H e also drew attention to the necessity for keep-ing in mind changes in the degree of atmospheric pollution when corrosion experiments were made 'over long periods and instanced the case of London where the deposit of sulphates had practically halved since I g I 5 , which must have a considerable effect on the rate of corrosion.The President remarked that the behaviour of metals of high purity in regard to corrosion was quite different from that of metals containing small quantities of impurity. The difference between a 99.2 per cent. and 99.6 per cent. metal was much less important than the difference between a 99.8 per cent. metal and one containing only faint traces of foreign elements. The spectrographically pure zinc prepared by the New Jersey Zinc Company, and the 99-95 per cent. aluminium which W ~ S also manufactured were extraordinarily inert towards mineral acids being scarcely attacked after a week's immersion in hydrochloric acid.These metals remained very bright and untarnished on exposure to air and it would be of interest to know how their behaviour compared quantitatively with that of the metals which had been examined so far. I t would be interesting to know what evidence Dr. Vernon had for the suggestion that the surface layer had the space lattice of copper oxide. According to the experiments of Langmuir and of Volmer the first thin layers formed at the surface of a metal were of irregular orientation and it was not until a definite thiGkness had been reached that rearrangement to form a space lattice occurred. Professor R. C. Gale suggested that the anomalous behaviour of lead when exposed to the vapours given off by drying paint might be due to the combined action of water vapour carbonic acid gas and volatile fatty acids formed by the oxidation of the linseed oil in the paint.The presence of formic and acetic acids in the products of atmospheric oxidation of linseed oil had been shown by Mulder and others. The type of corrosion suggested was applied in an accentuated form in the process of converting lead to white lead by the so-called Dutch stack process. The photomicrograph facing page 141 showed thegreat difference in the behaviour of the a&ha and beta phases in 60/40 brass when exposed to air. I t would be interesting to compare the behaviour of this alloy with that of a brass consisting entirely of the beta phase since the appearance of the corrosion product wastinot unlike that formed on zinc and possibly might give a weight-increment/time curve of similar type.The inhibitory effect of a muslin screen on the " pitting " of steel might be due to the same cause 8s that found by Pasteur many years ago ie., that solid particles such as bacteria floating in the air will not pass through small capillary channels. This fact was made use of unconsciously by the Sheffield cutler when he prevented the corrosion of his cutlery by wrap-ping it in paper which prevented the access of solid or liquid corrosive particles. Professor A. W. Porter said that the protecting action of muslin was no doubt due to the prevention of air currents. A plate exposed in an ordinary room was subject to constant currents of air arising from difference DISCUSSION I 93 of temperature and these brought with them fresh supplies of sulphur or other active materials.The muslin stopped these streams; the thin film of air clinging to the plate was rapidly exhausted and its sulphur content could only be replenished by diffusion. I t did not seem likely that muslin of the mesh used would very materially reduce the chance of dust falling on the plate. In regard to other matters in the report the most surprising fact was that consistent results could be obtained at all from experiments lasting over both summer and winter periods. The precise part which the sulphur played appeared to require further elucidation. Investigations with ordinary atmospheric air were of great importance in regard to the protection of metals from corrosion under ordinary conditions of exposure; but the scientific elucidation of the results obtained required experiments made with artificially '' doped " air the character of which could be maintained as uniform as possible throughout any one investigation.The basis of the square-law for weight increment was rather difficult to understand. The equation suggested by Mr. Evans was still more obscure in its derivation. I t might be mentioned that the curve in Fig. 2 was quite well fitted by W = A log (a + t). The reason for such an equation was as obscure as for the two others; it was however an equation which turned up in connection with many phenomena. Mr. A. Wolf said it would be interesting to study the behaviour of a copper surface prepared by the familiar chemical reducing process of passing methyl alcohol over the strongly heated metal.H e would expect such surfaces to be peculiarly susceptible to atmospheric corrosion in contradis-tinction to those which are protected by a film of oxide. With regard to the experiments on the protective power of lanoline he pointed out decrease in reflectivity on prolonged exposure might be partly due to darkening of the protective film (owing to atmospheric oxidation) and to the adhesion of suspended matter in the air to the sticky surface ; the same factors would also increase the weight increment. I n his opinion the removal of as much as possible of the lanoline by draining at 100' C. and wiping with cotton wool rendered the test too severe.The usual com-mercial procedure when using lanoline as a protective agent was to dip the article to be coated in a 10 per cent. solution of lanoline in a non-in-flammable solvent such as trichlorethylene. The above method had been used with great success for the protection of aeroplane engines from atmospheric corrosion during transport and storage ; it had been found that bright metallic surfaces treated in this manner would even resist corrosion when wholly or partly immersed in sea water during transport in the holds of ships. The increase of fatty acid corrosion of brass with increase in zinc content on exposure of specimens to the atmosphere of a domestic kitchen was in accord with the experience that zinc was among the most easily corroded metals used in bearing metal alloys where the lubricant contained a large proportion of free fatty acid.Professor W. H. Merrett said that whereas on the one hand corrosion films afforded protection to the metal on the other hand they actually aided corrosion. Kolled copper sheets strip and wire frequently become coated with a fine green patina of basic copper carbonate especially when exposed to air near the sea. Some portions of the Dockyard roofs in this country which were covered with copper were to a great extent protected from further corrosion by this coating. Concerning the use of lanoline for the protection of brass exposed to I I94 DISCUSSION “ dew-point ” conditions Professor Merrett said that until the war linseed oil alone or linseed oil and white lead were generally used as a temporary measure for protecting machinery from corrosion ; they were not efficient, since as the oil dried the layer which was left cracked considerably so that air and moisture could readily find their way through and corrode the metal beneath.Aero and other engines recovered from torpedoed vessels were found to be intact when they had been previously treated with lanoline while those covered with other substances were corroded beyond redemption. Professor Merrett said he would like to see more work done upon the corrosion of iron. There seemed to be considerable evidence to show that wrought iron was superior to mild steel against atmospheric and other types of corrosion. Dr. Vernon dealt with the protection afforded by a film obtained by exposing copper to temperatures above the normal in air and both Mr.Munby and Professor Desch had referred to the “ red ” copper produced by the Japanese. This could be obtained by casting copper on canvas under water and then exposing the cake to the action of steam. Such colouring lasted indefinitely ; in fact some specimens at the Royal School of Mines which were at least fifty years old still retained their fine red colour. Japanese experts were also able by suitable treatment to produce pro-tective patinas on copper which might be made to show tints ranging from a rich golden sheen to chocolate. Professor Merrett stated that he had been able to reproduce some of these patinas by suitable chemical treatment of copper. Mr. S . Field and Mr. W. S. Patterson (communicated) The re-sults of the experiments upon zinc are of particular interest to us.The straight line relationship is now definitely established for indoor atmospheric exposure where the deposit accumulates. I t will be of great interest to see if this is maintained in out-door exposure where the surrosion product is removed at irregular intervals by weathering. I t would appear from Fig. 27 where the erosion (approx. go per cent. of the total corrosion) is plotted for out-door exposure that the corrosion of the zinc is accelerating, although over the first year the attack follows a straight line. I t is curious that over three and a half years’ indoor exposure the surrosion curve for zinc shows no seasonal fluctuations. Unquestionably there is considerable variation during this period in the acid sulphur content of the air and there can be no doubt that the surrosion deposit will contain sulphates.Is the attack of air upon zinc totally uninfluenced by sulphur compounds or does the deposit once established prevent the access of such sulphur compounds to the metal whilst permitting the access of oxygen? We suggest tentatively that the deposit which is of a porous granular nature may be acting as a filter; this appears to throw light on Dr. Vernon’s results. I f the scale functions in this way it is not neutral but really protective in that it eliminates a corrosive agent from the air. Dr. Vernon’s suggestion of a critical humidity below which variations in water content do not appear to cause much effect upon the corrosion of zinc is a very important one and our own work confirms the suggestion.Dry air does not produce any measurable effect upon zinc increasing the humidity to almost saturation has not increased the attack very considerably, but at complete saturation there is a large increase in the corrosion. Dr. Vernon has shown page 138 that in an unsaturated atmosphere the difference in the corrosion of highly polished and enieried zinc plates is practically nil. Dew is Lanoline however did not dry and crack. In a saturated atmosphere there is a difference DISCUSSION I95 deposited in small drops on the polished specimen and the usual pitting accompanying differential aeration occurs. I n the emeried plates the attack takes place along the groves cut by the emery and these are quickly made visible by the incrustation of deposit along them.Dr. L. H. Callendar {communicated) The section dealing with aluminium interests me as I have been engaged for the last six years on the study of the corrosion of this metal and its alloys. I have had some of this specially pure American metal supplied to me by Dr. Vernon, analysed both spectroscopically and chemically. The spectroscopic analysis showed on& Iron Silicon Copper and Sodium. The chemical analysis gave Iron 0.017 per cent. Silicon 0.014 per cent. Copper 0.014 per cent., making the total impurities (with Sodium about 0.002 per cent.) less than 0.050 per cent. Referring to the curves for the rate of oxidation of aluminium in air given on page 152 in some work I am shortly publishing I have obtained some similar curves using a totally different method for measuring oxida-tion.This method which is an adaptation and improvement on that de-scribed in a previous paper7 and measures changes in potential of the aluminium surface and increases in the electrical resistance appears to in-dicate both the rate of oxidation of the surface and the thickening of the oxide film. It is more sensitive than the weight-increment method as it indicates surface oxidation in a few minutes instead of only after several hours. Using the weight-increment method as Dr. Vernon himself ob-serves the differences obtained are really due to differences in the relative freedom from oxide of the aluminium surface. That is to say Dr. Vernon’s curves for aluminium probably only represent the rate of thicken-ing of the oxide film the initial combination of oxygen with the metal being missed as this process will be mainly completed during the time taken (about half an hour) for the initial weighings.Potential curves I have obtained by the electrical method fall steeply at first showing the rapid covering of tne metal with a molecular oxide film. Some of the most interesting experiments in the paper are those in Section VII. in which Dr. Vernon shows that screening the air from dust prevents corrosion of iron. I t is evident that the dust sodium chloride, etc. settling on the metal tend to interfere with the formation of the normal oxide film and may even break it down where it has already formed. My experiments on Aluminium tend to indicate that in commerce substances deposited on the metal from the air probably determine the location of pits when the metal comes in contact with water.That this has not been generally recognised by previous investigators is because in their researches they have removed these anodic dust spots by cleaning or polishing or scraping the surface of their metal. This dust retained by the oxide film while it is thickening forms hundreds of anodic spots the oxide film itself acting initially as the cathode of the corrosion cell and under suitable con-ditions any of these spots may develop into pits on the metal. I n recent experiments I have found that minute emery particles embedded in aluminium increase the corrosion frequently though not always forming starting points for pits on the metal.Anything that interferes with the even formation of the oxide film may thus be a cause of corrosion of the metal. Professor C. Benedicks (communicated) The fact that zinc specimens gave the same weight-increment whether they possessed a dull emeried 7 (. Passification and Scale Resistance in Relation to the Corrosion of Aluminium Alloys,” J. Inst. Met. 1925 3 57 1 96 DISCUSSION surface or a brightly polished one is interesting. Actually in the first case the true surface area must be much greater than in the other and, hence a quicker rate of weight-increment would be expected. The ex-planation seems to be one of the two following. Either we may assume that the reaction takes place only on the ridges of the specimens.This would constitute a special case of the “edge effect ” (p. 124) reduced to much smaller dimensions than otherwise. Or it may be that the zinc surface is attacked only at discrete points (pp. 139-140). The “point-surrosion ,’ of a zinc surface in this respect seems to offer an analogy to the well-known oxidation of an aluminium surface in the presence of traces of mercury. Consequently the surrosion on zinc would seem to be of a definite electrolytic character due to some impurities while e.g. the “ surrosion ” of copper would appear as a more purely chemical process. The conclusion as to the pre-determining influence exerted by an initially-formed film of sulphide or oxide are extremely interesting. The initial though very thin layer of sulphide may be said to exert precisely the same action as in ordinary solutions a “ crystallisation centre ” ; the expression ‘‘ germative layer ” used by ‘the author seems a very appropriate one.The observations on lead (p. 158) lend valuable support to the con-ception of this selective action. I t is very interesting to find that under conditions permitting water condensation on the surface corrosion is most severe on the purest copper. H e (Professor Benedicks) recently had to deal with a case of severe pitting on a copper plate covered with a thick oxide layer. The conclusion he reached was that the corrosion was to a great extent due to the remarkably pure character of the electrolytic copper the electrolytic potential being displaced in a iess noble direction with increasing purity-at least with respect to cuprous oxide.Mr. Arthur J. (i. Smout (communicated) While the experimental work which has been carried out by Dr. Vernon on the problem of atmos-pheric corrosion is most interesting and of the highest scientific order I hope that the practical aspect of this important subject will not be lost sight of especially by those professions and branches of industry which are in daily contact with the problem. Dr. Vernon has during the past five years or so devoted the whole of his time to this problem arid it would therefore be exceedingly helpful to industrial workers if the author could at some near date summarise those aspects of the whole of his researches which have perhaps a more practical bearing on tarnishing and atmospheric corrosion.At the same time I hold the view that the onus of application lies not so much with the research worker as some seem to think but with the scientific and managerial staff of the works and industry concerned. Industry will benefit by this long and patient research only to the extent to which its scientific workers are prepared to apply the results to the betterment of their products. I n this connection I congratulate Dr. Vernon not only on having studied the fundamental principles underlying the phenomena of tarnishing and atmospheric corrosion in such a manner that his work will stand the test of time but on putting forward in this report many of his results in such a manner that they can be understood by persons industrially interested in the problems studied.Dr. Vernon has given definite leads in certain directions and has made certain definite statements all based on sound experimental work; it now remains for Industry to apply his suggestions. The author’s observation contained in Part II. of the report that modification of composition may result in greatly reduced corrosion whe DISCUSSION 197 various metals and alloys are exposed to the weather under ordinary con-ditions can be fully substantiated by works experience. The writer is connected wtth a firm of copper manufacturers who supply large quantities of copper for roofing and similar purposes in all parts of the world. Long experience has shown that for this purpose arsenical copper (c.f. British Engineering Standards Specification) containing 0.3 to 0-5 per cent.arsenic is the best form of copper to employ for this purpose and is much more suitable than H.C. copper of over 99.9 per cent. purity. The reason appears to be that a protective film is formed on this arsenical copper in the early days of its life by the action of the rain water. Cases are by no means uncommon where roofing contractors, especially in countries where rain is more or less infrequent deliberately cause this film to be formed by washing down by some suitable means in order to preserve their work from possible general deterioration in the course of time. I t is well known that the presence of arsenic in copper has a marked influence on the cuprous oxide eutectic causing a “balling up” of the particles of oxide; the suggestion is made that this “balling up” of the oxide in arsenical copper may be the cause of the improved results obtained.The writer has reason (based on many works’ observations) to believe that the cuprous oxide which is present in manufactured copper has a large bearing on the question of tarnish and atmospheric corrosion. Special methods are used by certain copper manufacturers to cast the cakes, which are subsequently rolled into sheet form in such a manner that the cuprous oxide will neither be excessive nor unduly segregated or concen-trated in any portion of the cake which will form part of the finished sheet or sheets. Methods are employed whereby that portioh of the castings in which segregation of cuprous oxide is likely to take place can be subsequently removed as process scrap in the course of manufacture.Such material is technically known as ‘( bloom copper.” This concentration of cuprous oxide was noted by F. Johnson in 1925 in MetaZ Industry. The writer’s works experience shows also that all corrosion and tarnish-ing problems are materially affected by comparatively small modifications of composition of the material under observation and that impurities play an important part sometimes acting as a retarder and sometimes as an accelerat-ing agent. In this connection it is now well known that ultra-chemically pure aluminium and zinc have properties totally different from those of the same metals as employed industrially even when compared with the so-called high-grade varieties. I t is therefore unfortunate that the author has not given detailed analyses of the various coppers brasses and types of zinc which he employed ; this omission can be made good by supplying as an appendix to the paper full figures including oxygen in the case of the coppers ; the value of the report to subsequent workers will be considerably enhanced.Dr. J. Newton Friend (communicated) Dr. Vernon has explained the breaks in his aluminium curves by suggesting (p. 154) that when the primary film of oxide or other material on the surface of the metal exposed to corrosion has practically ceased to thicken any further appreciable change takes place through the occurrence of cracks or fissures in the primary film. If such is the case one might perhaps expect a certain amount of pitting or localised corrosion to occur in the neighbourhood of the cracks just as one finds pitting to occur in steel at cracks in the millscale when the metal has been exposed to corrosion without first de-scaling.It would be interestin DISCUSSION to learn if Dr. Vernon has observed any such tendency. In my own ex-periments an aluminium bar exposed to the sea for four years is deeply pitted at various places a typical pit measured with a spherometer being 2.35 mm. deep. The lead zinc tin copper and copper-alloy bars on the other hand are quite different. Dr. Vernon’s observation on the influence of dust upon the corrosion of iron in air at temperatures above the dew point is distinctly interesting, although quite in accordance with what one might expect.His statement that such small variations in the purity and composition as occurred between his specimens of iron and steel exerted a negligible influence upon his results is entirely in harmony with my own observations. Professor C. 0. Bannister (communicated) The report covers a very wide field of enquiry an indication of the many lines of investigation it is necessary to follow up. Many of the results have a direct bearing on work in which the writer is personally interested. The fact that in the case of zinc the relationship between weight-increment and time when exposed to the ‘‘ unsaturated ” type of atmosphere was found to be consistently linear, even for long periods is surprising since similar tests carried out by the writer on galvanised sheets showed a decided acceleration of corrosion after some months of exposure.The different principles involved in the atmospheric corrosion of iron and of copper and bronze are of particular interest to all engaged in the examination of ancient specimens of metal objects etc. ; the new light now thrown by Dr. Vernon’s reports on the methods of progress of corrosion in the case of the different metals will‘ prove to be a most valuable aid in the correct interpretation of the changes which have been taking place during centuries. I n a recent book by the writer and H. Garland on Ancient Egyptian Metallurgy it is suggested that iron and iron tools were known at much earlier periods than those generally accepted. I t is interesting to note that the few very ancient specimens of iron tools which have resisted total decay have been those found in sheltered positions protected largely from atmospheric dust as well as from moisture.H. Sutton and J. W. W. Willstrop (Royal Aircraft Establishment) (communicated) The section on aluminium is closely related to some work they have been doing in their laboratory and in which they have been isolating the oxide films on normal and treated aluminium sheets. The writers’ experiments indicate that the film present on a normal sample of commercial aluminium sheet is of the order of 2 0 0 U thickness calcu-lated from its weight and assuming a density of 4. This result is in a fair agreement with Dr. Vernon’s calculated thickness of the film produced by atmospheric oxidation. The thickness of the natural film may be expected to vary according to the nature of the surface.Dr. W. H. J. Vernon (in rep& to tAe disczcssian) expressed his gratification both with the way in which the report had been received and with the valuable discussion to which it had given rise. The difficulty of responding adequately to individual contributions would be appreciated, but he would endeavour to deal as far as was possible or necessary with the various points which had been raised. He appreciated the opening remarks of Mr. Munby as Chairman of the Atmospheric Corrosion Research Committee. Professor Carpenter had very usefully surveyed the three main types of film that might be developed upon metal surfaces. H e had pointed out the most serviceable of these and had cogently enquired whether such DISCUSSION I99 type might not be stimulated artificially upon metals which normally gave rise to the less protective variety.With suitable modification of treatment, probably supplemented to some extent by modification of composition the author saw no reason why that desideratum should not be attained and he agreed that it was along such lines that future progress was likely to be made. He heartily agreed with Dr. Bengough as to the widely different types of action which came under the heading of corrosion. The results obtained according as one was dealing with the metal-gas or the metal-liquid interface certainly provided marked contrasts some of which were probably not unconnected with the molecular and ionic nature of the reactions in the two cases respectively.A more useful conception however in particular cases was that of the “controlling factor,” which Dr. Bengough had mentioned. A good example of this was afforded by the consideration of duplicate results in the tarnishing of copper and the rusting of iron respectively. In the first part of the Report tables cf results corresponding to the various curves had been omitted through considerations of space. I n the case of copper it could be said that duplicate experiments had always shown extremely close agreement, certainly much closer than the bulk of those given in Table IX. (Open Air Tests) to which Dr. Bengough had alluded. Here however the process was controlled entirely by the properties of the initially-formed film which, in turn depended upon the atmospheric (gaseous) conditions prevailing at the time of the first exposure; hence the agreement of duplicates was readily understood.In the case of iron on the other hand the degree of rusting of a given specimen depended upon the extent to which particles were precipitated upon the surface during the course of exposure evidently a much more capricious factor; in this case duplicate results almost invariably shewed- considerable divergences. The differences between results obtained in atmospheric and immersed corrosion respectively appeared to have very useful light thrown upon them by the contribution of Dr. Callendar in the present discussion. Thus considering the case of aluminium in the formation of the primary film it was evident that a large number of slight discontinuities or pin-holes in the film would affect the weight-increment but very slightly whereas on immersion in a liquid the distribution of those pin-holes might determine the whole course of the attack.Under such circumstances it was evident that no two specimens could be expected to yield similar quantitative results. The author fully agreed with Dr. Bengough as to the advantage afforded in atmospheric corrosion work by the great extent to which the effects could be followed by direct weighing. The high order of accuracy which Dr. Bengough and his associates were obtaining in their investiga-tions into the genesis of immersed corrosion was attended with much greater difficulties but their results would undoubtedly prove of far-reaching importance.The ‘ alternative explanations ’ brought forward by Mr. Evans naturally called for serious consideration. Anything which would throw light on the oxidation curve obtained for aluminium and for lead was greatly to be welcomed since there was real difficulty in interpreting this curve. The suggestion however that the oxide changed from a pervious to an im-pervious form during oxidation of the metal seemed only to raise the question as to what was the mechanism of this change; the answer ap-peared to be equally obscure under ordinary temperature conditions. O 2 00 DISCUSSION greater help was Mr. Evans’ view that oxidation might start from ‘L points,” the flattening of the curve then marking the stage at which the whole sur-fact was covered with oxide.The author was particularly interested to note that his suggested explanation for the subsequent ” increments was confirmed by Mr. Evans’ work on oxide films on iron. The figures given on page I 33 for the “ approximate ” thickness of oxide films upon copper were really only intended for comparing the thickness of the respective films which purpose was served even though the individual figures were admittedly very approximate. It seemed however, most significant that in practice one found the metal showing a most marked predilection to give either a straight line or a parabola ; in the course of the present work truly intermediate types had not been encountered in the case of pure metals. The initial straight line given by copper speci-mens in the early stages of tarnishing he believed was to be explained on the lines suggested in the text; otherwise the “edge effect” would still have to be accounted for whereas a consideration of this eKect would lead one to expect exactly the sort of relationship which had been realised experimentally.I t might be noted that an ‘‘ intermediate ” curve appeared to obtain in the case of the brasses but here the work seemed strongly to suggest that the constituent metals were actually behaving as separate entities. The results which Professor Wilson had quoted from his investigations on the electrical properties of wires as affected by prolonged atmospheric exposure were interesting and he would await with interest any further development as to the effect of impurities on.the atmospheric corrosion of aluminium. Of considerable interest was the evidence adduced by Mr. Busbridge as to the penetration of corrosion into the interior in the case of the aluminium alloy. I n reply to Professor Wilson’s suggestion that the mechanical properties of the materials should have been tested the author would point out that in the case of non-ferrous metals mechanical test.s were not sufficiently sensitive to detect changes due to atmospheric cor-rosion within a reasonably short period of time. Such experiments as those of Professor Turner on transparent films demonstrated directly what could only be inferred indirectly from interfer-ence colours as to the transparent nature of the oxide films produced at the temperature stated.Touching the question however as to the particular oxide which was formed he would point out that since the whole layer became transparent including the intermediate film of unoxidised metal there would appear to be a very close similarity between the crystalline structure of the oxide and that of the metal a condition which, it would seem could only be fulfilled in the case of cuprous oxide. With regard to the work of Sheppard (mentioned by Dr. Hutton) on the action of potassium persulphate solutions on copper or brass as originally described 9 the experiments showed that whereas on immersing a strip of the metal quickly in the solution it blackened readily and completely if it were lowered slowly blackening either did not take place or was very patchy and imperfect generally not more than a tarnish.From his own (admittedly limited) experience in repeating the experiment the author could confirm the “ very patchy and imperfect ” nature of the tarnish film so produced. Sheppard showed that the reactions involved were probably complex. zinc. Mr. Evans’ general equation was of great interest. 8A similar suggestion had been made by Professor Benedicks in connection with Nature 1925 116 608 DISCUSSION 2 0 I Scientifically the observation quoted was undoubtedly of great interest but it had not seemed to him to be sufficiently cognate to call for inclusion in the present report. The great disparity between the size of the muslin mesh and the dimensions which Dr. Owens’ important work had established for the solid particles in the atmosphere was certainly puzzling.The author thought it was helpful to approach the matter in two ways firstly considering the solid particles themselves by regarding them as not stationary but in a state of oscillation as presumably they would be in the air whence the dimensions of the mesh necessary to trap them would be equivalent not to the actual diameter of the particles but to the distance representing the extent of their lateral movement ; secondly as pointed out by Mr. Wolf, by considering the muslin meshwork not as clean-cut holes but as probably including small branch fibres projecting toward the centre of the mesh. I n addition the probability of a certain amount of capillary attraction between the solid particle and the material of the mesh should also be borne in mind.With regard to the influence of increasing humidity upon the rate of precipitation of solid particles the author gladly accepted Dr. Owens’ correction. This rendered the lesser intrinsic corrosivity of the more humid atmosphere all the more remarkable and caused even greater importance to attach to the specific nature of the solid particles themselves during the summer and winter periods respectively. The critical humidity mentioned by Dr. Owens certainly differed very greatly for different metals probably most largely on account of differences in the vapour pressures of the respective corrosion products. At the same time the author fully agreed that hygroscopic particles settling from the air must play some part in determining the actual value of the critical humidity.The President had raised a point of great interest in respect to the effect of impurities a matter to which Mr. Smout had also referred. When one was dealing with the metal-gas interface impurities in the metal appeared to play an unimportant part. Thus in the experiments with aluminium the author had employed the 99.95 per cent. metal l o referred to by the President but with respect to the rate of film formation this had not differed appreciably from the metal of ordinary purity. H e agreed however that when one came to deal with the metal-liquid interface-and more particularly with corrosion in acid solutions-the question of the last traces of impurities might become of great importance. Zinc afforded a good example of this because whei eas the spectroscopically pure metal displayed the excessively high resistance which the President had described there was evidence that between this material and metal of ordinary purity there was a critical composition, equivalat to the merest trace of impurity at which the metal dissolved with extreme rapidity.ll With regard to the space lattice of oxide films Pilling and Bedworth12 had shown in the case of thick films that these were definitely crystalline; they had published photo-micrographs of such films which bore close re-semblance to ordinary metallic microstructures.It was evident that the crystalline structure must also hold true for much thinner films which would 10 See p. 150 also p. 195 contribution by Dr.Callendar. 11 On the authority of Mr. Gilbert Kigg late of the New Jersey Zinc Company ; see See also T. D. Lynch and F. G. H. C. Lancaster Trans. Faruduy Soc. 1924 19 920. Breyer Pvoc. Amer. Soc. Test Mat. 1924 24 762. l2 J. Inst. Metals 1923 29 558 2 0 2 DISCUSSION therefore be expected to possess a definite space lattice; in the case of copper (as noted in reply to Professor Turner) there were good reasons for believing that this was the space lattice of cuprous oxide. With progressive thinning of the film a stage must clearly be reached when the unit lattice of the oxide was not completed for the whole of the surface ie. a state of irregular orientation in harmony with the results of Langmuir and Volmer. This state of affairs had been envisaged by the author (p.132) and it would appear that the metal was susceptible to tarnishing influences just when it was in this condition. When once there was a definite oxide space lattice over the whole surface the surface would naturally take on the properties of the oxide as distinct from those of the metal and one would expect a more or less well-defined break in properties as one approached the completely oxidised condition; it would be recalled that this sudden break had been experimentally realised. The author had been greatly interested in Professor Gale’s suggestion, endorsed by Mr. Wolf with regard to the anomalous behaviour of lead. The possibility of traces of formic acid giving rise to the observed effects had been considered at the time ; to test this lead strips had been suspended over formic acid solution.Only slight effects were produced in this way, however less than those obtained by substituting turpentine l3 for formic acid, and very considerably less than those obtained in actual practice in the presence of vapours from drying paint (characterised by the “smell of paint”). Evidently there were other factors at work which so far had not been elucidated. The author could not agree with Professor Porter that the inhibitory effect of a muslin screen upon rusting was due simply to the prevention of air currents. If that were so the tarnishing of copper should also be pre-vented. I t was truethat the rate of tarnishing was somewhat reduced by screening behind muslin but tarnishing still went on and copper so exposed passed through the usual sequence of colour changes.On the other hand, iron specimens similarly exposed might be kept apparently indefinitely, without rusting. Moreover the rusting of iron could also be stopped by filtering the air irrespective of the rate of flow ; under such conditions how-ever the tarnishing of copper was not affected and indeed proceeded at a rate which increased with increasing rate of flow of the air. In connection with the tarnishing of copper Professor Porter had ex-pressed his surprise that consistent results should be possible over both winter and summer periods ; Professor Porter’s surprise however could not exceed that of the author when this remarkable fact was first observed. I t was one which certainly called for explanation but at the same time it indicated fairly clearly that the explanation was to be found in a specific property of the initial film.Professor Porter like Mr. Evans differed from the author in the inter-pretation of the curve shown in Fig. 2 and in each case a different mathe-matical solution was proposed. While the author was glad that both equations had been included in the discussion he could not refrain from deprecating undue reliance being placed upon an equation simply because it happened to fit a given curve. H e considered that the test should be whether the equation agreed with the experimental facts of which after all, the weight-increment/time curve was only one. The simple parabolic 13 Levy and Defries (“ Action of Turpentine on Iron,” 7. SOC. Chem.Inn. 1923 42, 4pT) point out the probability of acids of much higher molecular weight than formic or acetic acid being formed during the oxidation of turpentine in presence of moisture DISCUSSION 203 equation (W2 = K t ) was capable of perfectly definite physical interpretation (see p. I 24) 14 and within the author’s experience was in agreement with the whole of the observed facts. H e hardly thought that the method of covering with lanoline described in the report rendered the test too severe for the particular purpose in view as suggested by Mr. Wolf; probably indeed the conditions of exposure were not sufficiently drastic to bring out the full protective effect which had been induced. Although very little excess lanoline remained the heat-treatment would undoubtedly result in its penetrating into the metal to some extent ; in addition a certain amount of chemical change would pro-bably take place at the surface resulting in a still more resistant layer.The treatment happened to be in fact very similar to that devised by J. J. Manley,15 for the protection of brass weights except that Manley used linseed oil combined with much more drastic heating. The author be-lieved that more favourable figures would have been obtained in the present tests if the excess of lanoline had been removed more completely. This did not affect Mr. Wolff’s remarks concerning complete immersion conditions. The author fully agreed with Professor Merrett as to the importance of “ metal-colouring ” in connection with atmospheric corrosion.He thought, however that the work had demonstrated the great difference in the pro-tection afforded by oxide and sulphide films respectively notwithstanding that the initial appearance might be very similar. The results suggested that in selecting from the innumerable products of the metal-colourer’s art, preference should be given from the point of view of protection to those films having an oxide as distinct from a sulphide base. The permanence of the Japanese specimens described by Professor Merrett seemed to afford a good illustration of that principle ; it was interesting to note that in those cases the film of oxide was obtained by the decomposition of water. In reply to Mr. Field and Mr. Patterson concerning the comparative absence of seasonal fluctuations in the surrosion curve for zinc he stated that under unsaturated conditions of humidity wide variations in the atmospheric sulphur content had relatively little effect on the surrosion of freshly cleaned metal; this was brought out by reference to Fig.8, where curves A and D represented periods of maximum and minimum sulphur content respectively (& Fig. I ) . This did not mean however, that some such mechanism as that suggested did not assist in maintaining the linear relationship ; the suggestion was worthy of serious consideration. The author was glad to have confirmation from Messrs. Field and Patterson as to the idea of “ critical humidity ” ; his colleague Mr. J. C. Hudson, had obtained a number of results bearing upon the greatly increased rate of attack upon zinc in the neighbourhood of the dew-point.Dr. Vernon said he had already referred to Dr. Callendar’s communi-cation and while he would look forward with interest to the forthcoming paper in which Dr. Callandar’s results would be recorded more fully he was glad that some account of such important work had been included in the present discussion. Of the two explanations which Professor Benedicks had put forward as to the reason for the agreement in weight-increment of zinc specimens having very different surface conditions the author preferred the second ; the conception of “ point-surrosion ” was a helpful one and appeared to throw light on the initial disposition of the weight-increment/timz curve. l4 See also J. S. Dunn Proc. Roy. SOC. 1926 (A) 111 210.‘5 Phil. Mag. 1922 4 948 204 DISCUSSION H e could not see however that the evidence favoured the view that the action in the case of zinc was electrolytic in character. H e admitted that his own work upon zinc was very incomplete so much time having been taken up in connection with the tarnishing of copper so that the initial mechanism of zinc surrosion was still obscure to him. H e noted with interest Professor Benedicks’ confirmation of the curiously unfavour-able behaviour of very pure copper towards atmospheric corrosion which he (Professor Benedicks) connected with the cuprous oxide content of the copper. The high resistance displayed by other grades of copper must clearly be attributed to the development of resistant films through the presence of specific elements in the metal but hitherto he had regarded the H.C.copper as having suffered merely by comparison. Professor Benedicks’ explanation as to the effect of cuprous oxide had not occurred to him. H e was impressed by the fact that Professor Benedicks’ conclusion was also reached by Mr. Smout. The specific information Mr. Smout had given concerning the influence of cuprous oxide and of arsenic together with other cognate matters was valuable. H e regretted he could not give the detailed analyses for which Mr. Smout had asked. The copper and zinc employed in “Series A ” (p. I 66) were of high conductivity and high-grade re-distilled quality respectively (the brasses were also of special purity); an analysis of the zinc appeared on p. 933 of the First Report.I n the case of copper he suggested that the vital effects of small amounts of added elements having been definitely established the further question of very small traces might well be made the subject of investigation. Dr. Friend’s communication pointed to some of the difficulties peculiar t o corrosion research. The publication of the results of Dr. Friend’s ex-tensive series of field tests would be anticipated with interest. Professor Bannister has raised a point of great interest in connection with the relative behaviour of iron and the non-ferrous metals after the lapse of many centuries. Considering for example iron and copper the corrosion curves of which were represented by parabolas about the vertical and horizontal axes respectively (6 Fig. 2 3 ~ and Fig. 12 of the First Report) and bearing in mind the friable and compact nature of the re-spective corrosion products it was not difficult to realise that copper specimens would remain intact after similar specimens of iron had long since disappeared. Later work had shown the important part played by hygroscopic solid particles in the atmospheric rusting of iron leading to marked acceleration in the rate of attack even under conditions well below the dew-point. I t was interesting therefore to note that the atmosphere of Egypt highly charged with hygroscopic particles as it was would provide ideal conditions for that type of attack from which copper and bronze were relatively immune ; it was significant also as Professor Bannister had pointed out that the ancient iron specimens which had survived had been protected largely from atmospheric dust as well as from moisture. I n conclusion Dr. Vernon again expressed his appreciation of the assistance he had received during the prosecution of the work and of the action of the Research Association in permitting the publication of the Report. The President in thanking Dr. Vernon for reading the paper and the speakers for their contributions expressed as had all the speakers his appreciation of the sustained accuracy which characterised the work and of the patienceand skill with which it had been carried through. ABERDEEN THE UNIVERSITY PRES