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Chrysanthemin, the main constituent of the colouring matter of Victoria plums and raspberries, has been found to act as a cathodic depolarizer for the corrosion of tin by citric and malic acids. About 5 moles of chrysanthemin are required for the dissolution of 1 mole of tin but it is provable that this equivalence varies with the concentrations of anthocyanin and its tin complex. Rate constants have been determined for the reaction of tin with a malic acid solution of chrysanthemin and with the anaerobic syrup from bottled yellow plums, Victoria plums, and raspberries. The amount of tin dissolved per unit volume of the syrups increased in the order in which they have been mentioned and is greater than would be expected from their anthocyanin contents, indicating that other depolarizers are present in the fruit syrups.

Most forms of corrosion are the result of electrochemical processes; therefore, the effectiveness of corrosion inhibitors is largely determined by the electrochemical potential of the metal that is to be protected. This potential, together with the pH and the electrochemical reactivity of any component added to the corrosive environment, will determine whether oxidation—reduction reactions will occur on the metal surface. If difficultly-soluble compounds form on the metal as a result of such reactions, then the metal can become passivated and the additive is potentially a corrosion inhibitor.(1,2) If, on the other hand, there is no electrochemical transformation of the additive, then the role of the potential will be to affect the free energy of adsorption of the components of the solution. The surface concentration of the adsorbate will therefore depend on the potential and, as described by the Bronsted—Polanyi—Semenov principle, on the activation energy of adsorption. In this connection, one of the fundamental concepts of electrochemistry introduced by Frumkin in 1927, the potential of zero charge of a metal E q=0, is of considerable importance.

In the preceding chapters, we have focused on the complex-forming properties of components of the medium. The consideration of these and other properties—principally hydrophobicity—allows a better understanding of the aggressive or inhibitive effectiveness of chemical compounds or ions to be reached. This is not surprising since complex formation plays a large role even in the elementary act of metal dissolution. Thus, metal cations passing into an electrolyte are always associated with solvent molecules, counter ions, or other components having free electron pairs. The aquoand hydroxyl-containing complexes of technologically important metals are thermodynamically quite stable and are largely responsible for the importance of water in the corrosion and passivation of metals. Various compounds present in water will be adsorbed by a metal, essentially by the substitution of a solvent molecule by the adsorbate; such processes will include cases involving coordination and chemical bonding with the metal.

Many technically important metals and alloys begin to corrode in neutral solutions from the passive state, or, at least, from a condition in which a metal surface has been partially passivated by an oxide. Hence, in the majority of cases, the initial stages of corrosion are localized at defects in the primary oxide film, which is normally formed on contact of the metal with air. It is then reasonable to expect that the ability of components of a solution to initiate or inhibit these events will depend to a marked extent on the adsorbability of either the actual components, or the products of their interactions with cations of the corroding metals.*

Despite experience over many years with the use of corrosion inhibitors in industrial cooling systems, the problem of the rational treatment of water is still very real in the context of the anti-corrosion treatment of pipelines and equipment. There are various reasons for this, but the most important are associated with ecological requirements and water scarcity. The intensification of technological regimes is also important, for example, the increase in temperature of heat exchange surfaces can stimulate not only corrosion but also scale formation.

The role of pH is important because it characterizes the activity of the dissociation products of the most widely occurring solvent of all corrosive media, water, and consequently, to a first approximation, its chemical composition. A knowledge of the potential, E, of the metal and of the pH of the environment may often be sufficient for assessing the aggressiveness of the environment. Thus, by changing the pH with the value of E held constant it is possible to suppress corrosion by means of either passivation or cathodic protection. This can be seen from Pourbaix diagrams which show the pH-E dependence of the electrochemical reactions that cause corrosion. As a rule, the kinetics of these reactions depends on pH since H3O+ and OH− either participate directly in them or are their products. According to Antropov(108) the rate of corrosion of a metal, K,in acid is associated with pH by the equation: (2.1) in which b is a constant dependent on the nature of the metal. Analysis of the values of b has shown that for transition metals, such as iron, it is possible to inhibit corrosion by the screening that results from both adsorption of an additive as well as from changes in the bonding energy of hydrogen with the metal Inhibitors of the cationic type are even capable of affecting the nature of the controlling stage of the cathodic reaction. For non-transition metals, the choice of inhibitors is restricted because many anions accelerate the discharge of H3O+. Grigor’ev and Ekilik(33) used Eq. (2.1) to explain the reduction in the role of the chemical structure of a series of organic inhibitors with decrease in pH of hydrochloric acid solutions. However, for the corrosion of metals in neutral media, Eq. (2.1) is, as a rule, not applicable because of the diffusion limitations of the cathodic reaction, among other reasons.

In the manufacture or repair of various types of machines, mechanisms, or components and devices, there is usually a need for careful cleaning of the metal surface since contaminants may not only adversely affect the appearance of the item, but may also interfere with subsequent technical treatments. For example, it is not possible to deposit high-quality metallic or paint coatings on a surface that has not been degreased. Furthermore, the operations of soldering, cold-working, or finishing often require the removal of not only preservative oils and greases, but also oxide films It is also often necessary to remove metal working compounds, polishing liquids or pastes, metallic dusts, abrasives, etc. from a metal surface.

Novel test rigs are described for the study of the corrosion of metal specimens under controlled heat fluxes. In the corrosion of stainless steels in nitric acid, tests at various heat fluxes with steel surface temperature kept constant have shown that the cooler acid present at the surface under higher heat fluxes leads to slightly smaller corrosion rates than under isothermal conditions. Crevice corrosion can develop under the gasket sealing the stainless steel specimen to the test cell. This crevice corrosion can produce enhanced corrosion rates (by factors up to 100), not only on surfaces within the crevice, but also on those external to the crevice. The factors influencing the development of crevice corrosion are discussed.

A model of the electrochemical conditions in a static crack has been developed for steel in the active state based on the steady‐state mass transport of species by diffusion and ion migration. The main reactions considered were anodic dissolution, hydrolysis of ferrous ions, and cathodic reduction of hydrogen ions and water with the assumption that the electrode reactions were taking place both at the tip and on the walls of the crack. The reduction of oxygen in the crack was not included since it was demonstrated in a separate analysis that this reaction is insignificant in a static crack at steady state. The pH and potential drop in the crack have been evaluated as a function of external potential, crack dimensions, and other parameters assuming a bulk solution of 3.5% . The effect of dissolution and hydrolysis of alloying elements and also the buffer reactions of seawater on the pH within a crack were also assessed, but mass transport by ion migration was neglected in these cases. The theoretical predictions of the potential drop and pH were in reasonable agreement with values determined experimentally in an artificial crevice of the structural steel BS 4360 50D in 3.5% and in artificial seawater.

A study has been made of the effect of sulphide inclusions on the corrosion fatigue of mild steel in 0.6 M NaCl solutions. The results indicate that a sulphur-enriched band of ferrite exists around the inclusions and that corrosion occurs in this contaminated band, both in the presence and absence of applied stress. Cyclic stress enhances the formation and coalescence of micropits at and around the inclusions, leading to the nucleation of cracks.

When iron is passivated in chromate solutions in the pH range 7·5–1·0, an oxide film forms on the surface and thickens logarithmically with time. At pH ≤ 1·6, dissolution of iron takes place at a measurable rate that is considerably higher than the rate of oxide film growth at the same pH. The amount of iron in solution varies in a parabolic manner with time and the spontaneous dissolution current is given by log idiss = –0·84 pH + constant. The dissolution current of active iron in sulphuric acid solutions is related to the pH of the solution by the expression log idiss = –0·36 pH + constant.These relationships are interpreted in terms of electrochemical reactions occurring at the surface of the iron, when it is active in sulphuric acid solutions, or at the oxide/solution interface in chromic acid solutions, when the iron is passivated and so covered with a surface film of oxide.

The mechanisms suggested previously for the formation and growth of nuclei in the decomposition of barium azide have been examined in detail. It is concluded that these cannot adequately account for the experimental results available on the thermal decomposition, and photo and ionic conductivity of this salt.

The adsorption of phosphate and ο-iodohippurate ions on iron surfaces has been measured at varying potential during anodic passivation in near neutral solution. Considerable adsorption of both anions occurs in the active potential region, this adsorption being potential dependent. The phosphate adsorption is about three times that of ο-iodohippurate at the same potential. The adsorption of phosphate ions increases with pH in the range 7–9 and with increasing concentration in the range 4 × 10−5 to 4 × 10−4 M. When the iron surface is converted from the active to the passive state, the surface coverage with adsorbed anions is an appreciable fraction of a monolayer. The larger the anion adsorption on the active surface, the more easily does passivation occur. Considerable uptake of phosphate ions occurs into the passive film on the iron surface, indicating that passivation is associated with a three-dimensional film.

Radioaktive Isotope sind die Grundlase der empfindlichsten Methoden zur Bestimmung der Stoffaufnahme an Feststoffoberflachen. Diese Methoden konnen in drei Gruppen eingeteilt werden: 1. Auswertung nach Herausnahme der Probe aus der radioaktiven Losung; hierbei wird die irreversible Stoffaufnahme gefunden; 2. Verarmungstechnik, wobei die Konzentrationsanderung der radioaktiven Losung gemessen wird; 3. Auswertung an der eingetauchten Probe. Die hierzu verwendeten Gerate werden beschrieben. Ebenso ermoglichen die Tracer – durch Auswerten ausgewahlter Oberflachenteile oder autoradiographisch – die Ermittlung der Aktivitatsverteilung uber grosere Oberflachen. Radiotracer methods of corrosion testing Radioactive isotopes provide the most sensitive methods for the determination of the uptake of material by solid surfaces. These methods fall into three groups: 1. Post-immersion counting, where the irreversible uptake is determined by counting the specimen after removal from the solution; 2. Depletion techniques where the change in concentration of the radioactive solution is measured; (3) immersed counting techniques. The equipment used for these measurements is described. The tracers further enable the activity distribution across larger surfaces to be determined by counting selected surface districts or by autoradiography.

Results obtained on the rate of photolysis of barium azide and its variation with temperature and light intensity are reported. These are not in accord with Mott’s mechanism and an alternative approach involving the production and trapping of excitons is proposed.

The anodic passivation of iron in aqueous carbonate solutions, pH 11·3, at room temperature gives potentiostatic polarisation curves with two well defined current peaks. the potential of the first oxidation pea. −0·58 v., indicates that passivation is primarily due to the formation of an oxide film, probably mamly magnetite. the iron surface also becomes covered with a film containing ferrous carbonate. The second oxidation peak, at −0·22 v and less than half the size (coulombic equivalent) of the first peak, is due to the oxidation of the ferrous carbonate film to form hydrated γ-ferric oxide. The sizes of the two current peaks depend on the carbonate concentration and on pH. Dissolved oxygen in the solution has no direct effect on the passivation processes in 0·1 M sodium carbonate.

phosphate ions were foud by radiotracer methods to be firmly bound by oxide films on mild steel after immersion in 0·1 M sodium phosphate over the pH range 7–13. Under inhibitive conditions, autoradiography showed.that the major part of the phosphate uptake was distributed uniformly over the surface, but localy increased uptake occurred to a variable extent at points and areas of the surface. At pH 8, approximately a monolayer of phosphate ions was taken up initially by the oxide. Thereafter the phosphate ion uptake increased linearly with the logarithm of time of immersion, this uptake being decreased by icreasing time of exposure to air before immersion. Increasing the pH decreased the amount of imtialy uptake of phosphate ions. The results are consistent with ion exchange of phosphate ions initially with the surfce layer of the air-formed oxide film and continuing during the thickening of the oxide film in the solution.

The anodic behaviour of copper in air-saturated solutions at 25°, 60° and 90°c has been determined by stepwise potentiortatic polarisation. In 0·01 M NaHCO3, the critical breakdown potential of the protective oxide layer is displaced to considerably more positive potentials as the temperature increases. In 0·01 M NaHCO3 containing 0·01 M NaCl, the critical breakdown potential is displiiced tornore negative values with increasing temperature, but the corrosion potential–breakdown potential difference undergoes little change. In 0·01 M NaCl, the surface film on copper is essentially non-protective, and linear Tafelplots are observed. A change in the pH of 0·01 M NaHCO3, with or without 0·01 M NaCl, from 8·6 to 7·3 has little effect on the valuesof the breakdown potentials. The significance of the results in relation to the initiation of pitting corrosion of copper in natural waters is discussed.

The thermal decomposition of un-irradiated azide and the effect of short-period preirradiation on this process has been studied. Nucleus formation is considered to be the production of F -centres which, by interaction with vacant anion sites, acquire mobility and so aggregate to form more stable double F -centres, or nuclei. Nuclear growth involves decomposition at the metal-salt interface, and the nitrogen produced diffuses out through the crystal. The activation energy for growth of small nuclei is shown to be higher than that for growth of nuclei of visible size; this concept requires the introduction of a slow growth correction, and the pressure of nitrogen expressed as a function of time of heating t is then given by a relation of the formula p = const. ( t — y ) 6 . The effects of pre-irradiation in promoting nucleus formation and the processes occurring in the ageing of freshly prepared azide are also discussed.

The anodic passivation of iron in air-saturated and de-aerated solutions of a number of inhibitive anions has been studied by the determination of potentiostatic polarisation curves. The critical passivation potential, critical passivation current density and leakage currents through the passive films have been found to depend on pH and on the nature of the anion present in the solution. The critical passivation potential (N.H.E. scale) in solutions of anions, where passivation occurs relatively easily, varies with pH in the range 6–12 according to Ep = + 0.09 – 0.06 pH volt For the anions studied, the critical passivation current densities in de-aerated 0·1 M-solution increase (and the difficulty of passivation increases) in the order: nitrite < hydroxide < chromate < borate < phosphate < carbonate < benzoate < bicarbonate < nitrate.

The formation, breakdown and composition of surface films on copper in air-saturated solutions at 25°c has been studied by potentiokinetic polarisation. In 0·01 M NaHCO3, the protective oxide layer breaks down at a critical potential togive pitting corrosion. Increasing the concentration of hydrogen carbonate ions decreases the protectiveness of the oxide film towards general corrosion, but makes the filmmore resistant to breakdown. In 0·01 M NaCl, the oxide film formed is initiallyfairly protective and exhibits a breakdown potential on rapid polarisation. As the chloride concentration is increased, the oxide film becomes less protective and the breakdown potential becomes displaced to more negative potentials and is more poorly defined.

IN carrying out research in these laboratories on the corrosion of tin in anaerobic fruit acids, information was required on the thickness of the natural oxide film on tin. Since there appeared to be no published information about the thickness of tin oxide films, it was necessary to measure it, and for this purpose electrometric reduction has been found suitable. This method1,2 is generally considered to be applicable only to metals more electronegative than hydrogen ; but it succeeds for tin because of its high hydrogen overpotential. At room temperature the oxide film on tin probably consists of stannous oxide3,4, and we have calculated our results on this basis.