January,. 1923
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Problems of Corrosion’ By B. D. Saklatwallaz
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VANADIUM CORPORATION OF AMERICA, BRIDGEVILLE, PA.
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copper to pure iron, however, instead of promoting corrosion posed in service to manifold agencies and forces renders it noncorrosive. It is more or less self-evident that corrosion of a metallic causing their deterioration, which we collectively call corrosion. We are concerned with the final result of alloy submerged in a gas or liquid is the chemical interacthese deteriorating agencies, and consequently our interest tion between the solid and gas or liquid phase. The phefrom an engineering standpoint in analyzing these forces nomena in such cases are rather complex, as the varying individually has been remote. We have, therefore, up to physical factors influencing the reaction are numerous. Conthe present time attacked the problem of corrosion experi- sequently, the resultant reaction velocity, or, in other words, mentally in a purely practical way, and our efforts toward the practical rate of corrosion, will be the algebraical sum of counteracting corrosion in metals have consisted, more or all the physical influences and the resultant chemical effects. less, of empirical elimination of certain impurities or additions . .We can thus see that, while the addition of another foreign of different elements in homeopathic or allopathic doses to the element to a pure metal may set up a greater galvanic action, known engineering alloys. This method of working has it might produce other physical results in the characteristics not only made progress slow but has led to a conglomeration of the aggregate particles themselves, lowering their inherent of indefinite and even apparently contradictory information. chemical activity so that the algebraic sum of chemical actions might be on the side of lower corrosion. Hence, we ENERGY CHANGES IN FORMATION OF ALLOYS see the importance of the study of the physical nature of the The application of mathematical physics to metallurgical constituent aggregates and the forces governing their reacproblems through development of metallography, in recent tion activity. Such study will undoubtedly enable us to times, has revealed to us, not only the complex structure of discern and control the components constituting the resulthe engineering alloys, but also the equally complex energy tant corrosion. changes taking place during the solidification or “birth” of PHENOMENA OF SOLIDIFICATION these alloys. These energy changes and the conditions under which they occur have a bearing on the physical propIn order to grasp the significance of these “prenatal” inerties of the individual structural particles forming the so- fluences for the study of corrosion, an insight into the phelidified alloy. As examples of the phenomena influenced by nomena occurring during the process of solidification is necthese changes may be mentioned-the growth of crystals, the essary. According to a theory first propounded by Quincke, distortion of crystal faces, the orientation of their surfaces, crystallization starts by the separation out of the fluid magma the formation or absence of an intercrystalline “cement” of a small quantity of an immiscible liquid phase. This medium, internal crystal cohesion, surface adhesion between small quantity of ‘Loily,l’immiscible liquid serves to form the adjacent crystals, and surface tension of the crystal skin. cell walls or bubbles for the other liquid phase present in a These influences are exerted by energy changes prior to solidi- much larger quantity. We have thus a structure designated fication of the metal, and may therefore be designated as by Quincke as “foam formation.” As in the case of all foam being of “prenatal” origin. phenomena, surface tension of the liquid phases plays a great part a t this instant, determining the number, size, PHYSICAL PROPERTIES IMPORTANT shape, etc., of the crystayine particles when subsequent soOur studies on the subject of alloy corrosion have been lidification takes place. In this foam stage, as the temperamore or less along morphological rather than histological ture further decreases, several nuclei or points of crystallizalines. No doubt a study of the alloy as a whole is of impor- tion, constituting crystalline skeletons for the building up of tance, but the study of the inherent physical properties of the the final solid crystals, are formed. The number and arrangeindividu:tl aggregate constituents is undoubtedly much more ment of these nuclei determine the form and dimensions of important. The morphological study has led to the exploi- the growing crystals. The growth continues until checked tation ol’ our existing theories-the electrolytic theory, the by the presence of neighboring crystals, the mother liquor oxide theory, and the colloidal theory. The accelerative continually diminishing by solidification and being discorrosivc influence of a galvanic current produced by two het- placed by the growing crystals, until the last traces of it erogeneous elements in the presence of an electrolyte, the cor- solidify between the crystal surfaces, forming an amorphous rosive action ascribed to chemical activity of the necessary intercrystalline “cement.” Here again we can readily see presence of oxygen, the protective action furnished by certain that in the filling of capillary spaces by an amorphous films of oxide, the dependence of corrosion on the catalytic “cement” surface tension will play a prominent part. action of colloids formed during the reaction, are all pheThe size and geometric form of the final solidified crystals nomena of a more or less secondary origin. They un- and their free surface-energy properties, such as adhesion, doubtedly play a great part in corrosion action, but their in- solution pressure, adsorption, etc., are dependent on this fluence can be masked by influences of a more inherent nature. characteristic of surface tension. The property of surface Our experience with development of noncorrosive alloys has tension between two liquids or a liquid and a solid is a maniproved that such is the case. For instance, the electrolytic festation of the degree of stability of the free molecular theory demands that the more homogeneous and pure the energy equilibrium. Very slight changes in the physicometal is the more noncorrosive it should be. The addition of chemical reactions taking place during solidification, such as can be ascribed to the addition of another element to the I Received October 6, 1922. Genwal Superintendent. rnolten magma, or the presence of a foreign phase as a non-
A’TERIALS of construction, such as metals, are ex-
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metallic inclusion, w ill cause a disturbance in this equilibrium, tending to produce different physical properties in the crystal aggregates finally solidified. A study, therefore, of the surface tension phenomena of the crystal aggregates might lead to valuabIe explanation of the inherent noncorrodibility of the known alloys and to the systematic discovery of newer ones. CONCLUSION The purpose of the foregoing remarks is by no means to set up the property of surface tension as the “philosopher’s stone” of corrosion. Undoubtedly galvanic action, oxidation, protective films, etc., play an equally important part. The aim in these remarks is to direct attention to a channel of research hitherto neglected from the corrosion standpoint. For a methodical study, the problems of corrosion may be systemized into two main groups-morphological and histological. Our previous work has all been preponderantly
Vol. 15, N o . 1
morphological. We might also base our division on charrtcteristics of “prenatal” and of “postnatal” origin. The characteristics of the first nature will be such as the effect produced by the formation of definite chemical compounds by proportioning the constituents of the alloying elements, and structural changes produced through addition of small quantities of alloying or purifying agents. As characteristics of “postnatal” origin we may mention change in corrodibility produced by adequate heat treatment, mechanical work, surface polishing, etc. I n the preceding remarks one phase of the corrosion problem has not been touched-namely, the study of protective coatings, such as paints. As this method of protection in no way has a bearing on the properties of the metal t o be protected, it does not come within the scope of the metallurgical aspect of the problem. The study of protective coatings is, no doubt, a fertile and extensive field of research in its own way.
Control of Industrial Processes by Light-Sensitive Means‘ By Lloyd Logan 1000 HAMLINF AvE., ST PAUL, MI“.
application to the control LTHOUGH the literConsiderations of advancing labor costs and of the difficulty of of manufacturing processes, ature on the subject obtaining skilled operators in chemical and industrial works have especially in chemical inof photo-electricity forced more and more strongly on manufacturers the necessity and dustry, appears to have comprises over seven hundesirability of replacing human agencies, wherever possible, by been practically neglected. dred papers published since automatic means. The ready success of the thermostatic control the discovery by Hertz in of the temperature of kilns, furnaces, and buiIdings indicates the TYPESOF PHOTOSENSI1887 of the action of light trend toward the elimination of manual control. Variations in TIVE CELLS on the discharge of elecelectrical conductioity and in the specific gravity of solutions are A brief reference to the tricity and a perhaps even other agencies which have been employed to effect automatic control various types of photosengreater number of articles of industrial processes. sitive cells will indicate how on photo-resistance effects, I t is suggested that another agency may be made a powerful means their properties may be the research in this field of solving many of the most difficult problems in the control of inutilized. These distinctive has been devoted largely dustrial processes. This agency is the extremely remarkable properties consist in all to the important bearing of property of light of producing electrical effects, such as changes of cases of variations of elecphoto-electric action on resistance or electrification of certain substances when it strikes trical characteristics when theories of radiation and them. I t is the purpose of this paper to attempt to outline the possible subjected to light. While subatomic phenomena. field of application of this agency to the control of industrial operano cell yet devised exactly Applied photo-electricity tions and processes and to suggest means for the accomplishment duplicates the color sensimust still be considered a of such control. tiveness of the human eye, comparatively neglected or can be used, without subject. While scientific applications of light-sensitive cells to stellar photometry,2 special means, for the direct measurement of color, neverand-among other applications-the photophone of Bell, theless a rather wide choice of characteristics is available. the automatic lighting of buoys, the electrical transmis- These cells are broadly classified as selective or nonselective, sion of photographs, the control of torpedoes, and the according as they respond only to certain ranges of wave ingenious optophone of Fournier d’Albe for enabling the lengths or to the total amount of radiation. Of the latter type important examples are the bolometer, blind to read, have reached varying degrees of practicability, only relatirely few industrial applications have been pro- the radiomicrometer, and the thermopile, none of which shows posed. While also a few attempts to effect control in increased sensitivity to radiation of a particular region of the certain directions are evidenced by proposed applica- spectrum. tions to the regulation of the luminosity of vacuum-tube Of the selective class one of the oldest types is the selenium lamps13 the regulation of voltage,4 and the control of cell, in which the conductivity of a thin layer of .the leadultraviolet lamps in the sterilization of water,6 the field of gray modification of selenium, formed by slow cooling a t 217” C., is increased by exposure to light. I n one form this 1 Presented before the Division of Industrial and Engineering Chemistry cell consists of a coating about 0.0001 in. thick forming a a t the 63rd Meeting of the American Chemical Society, Birmingham, Ala., high-resistance conductor between two series of metal plates, April 3 to 7, 1922. Chemical and other applications of the methods referred to in this paper are subjects of patents pending. usually German silver, each series being clamped together as 2 Kunz and Stehbins, Phys. Rev., [2] 7 (1916), 62. shown in Fig. 1, to form an electrode of the cell. The cell 8 Moore, U. S. Patent S56,483 (1917). is heat-treated to drive out all moisture and occluded gases. 4 Moskowitz, U. S. Patent 1,150,953 (1915) These precautions are necessary on account of the suscepti5 Pratt, U. S. Patent 1,208,830 (1916).
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