Failure of structures and of many types of automatic equipment is often due to crevice corrosion. Differential aeration was advanced some time ago as a major cause of such attack. The present investigation was made to demonstrate other factors in the mechanism of crevice corrosion. Electrochemical methods were used to determine effects and relationships of polarization potentials and corrosion current densities in the main volume of electrolyte (which has free access to the open surface of the metal tested) and in the metal's crevices and clearances. Special laboratory apparatus was used with which crevice corrosion conditions could be simulated and in which “crevice” width was adjustable. Iron, aluminum and stainless steels were tested in 0.5N NaCl. Iron was also exposed to a mixture of Na2SO4, NaCl and NaNO2, to 8.6N HNO3 and to 0.2N H2SO4. Observations were also made of linear selective type crevice corrosion at the metal-dielectric interface perimeter in acid and of comparative corrosion rates above, at, and below the water line of partly-immersed iron. (In the latter case, a “crevice” is considered to exist near the liquid's meniscus.) It was concluded that much destruction of metal in clearances is due to peculiar electrochemical behavior which results in acceleration of the anodic metal ionization reaction and deceleration of the cathodic reaction. Insignificant differences in potential between metal on the open surface and that in the crevices initiate operation of corrosion cells. Linear-selective dissolution of iron in acids, on the phase boundaries of the metal-dielectric-acid system, proceeds according to the crevice corrosion mechanism. In water-line zone corrosion, the crevice corrosion mechanism is also in evidence, rate depending on whether the electrolyte is neutral, acid or alkaline. In inhibited media, metal potential near the water line becomes negative, due to difficulty of access of inhibitor, so that macrocells are activated.

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Brasses are α , ( α + β ) , and β solid solutions of copper-zinc system containing up to 50 at. % Zn. Articles of brasses commonly operate under the action of natural water or humid atmosphere. The specific feature of brass corrosion behavior is their dezincing. Thereby, zinc predominantly passes into the corrosive medium, and copper appears on the brass surface in the form of fine-grained porous layer. Dezincing can occur throughout the article surface or can be localized at some areas forming so-called plugs. In the case of uniform dezincing, the corrosion rate in sea water does not exceed 1 mm/year, whereas in the case of local attack, it reaches 4-5 mm/year. The dezincing is most probable in stagnant hot water with an increased content of chlorides. The attack is most intense in the slots and gaps in structures, under the layings and various deposits [1].The brass corrosion rate is commonly estimated by the sample weight loss; however, in doing so, copper metal formed during dezincing, is ignored as the corrosion product. Therefore, the corrosion rate appears to be underestimated. To estimate correctly the corrosion resistance of brasses, the dezincing should be characterized quantitatively. To do this, the dezincing coefficient is used [2]:where ( C Zn / C Cu ) sol and ( C Zn / C Cu ) alloy are the ratios between the concentrations of zinc and copper in the solution and in the alloy, respectively.Another peculiarity of copper and brass corrosion is the acceleration of their damage due to the accumulation of oxidized copper in the corrosive medium. The effect is most pronounced in aerated acidic chloride solutions. Univalent copper, which forms in the anodic process, reaches the outer boundary of diffusion layerand is readily oxidized to bivalent copper. Then, bivalent copper diffuses to the alloy surface and acts as a specific depolarizer reducing to univalent copper. Then, univalent copper passes again into the bulk solution and is oxidized with oxygen. The repetition of the process significantly increases the corrosion rate (an autocatalytic effect) [3]. Thus, the corrosion rate and the dezincification of brass are interrelated. Dezincing reduces the corrosion resistance of brasses at the same corrosion current due to the formation of copper metal layer; however, in the absence of dezincing, the copper corrosion products are accumulated in the corrosive medium and cause the autocatalytic effect, which, in closed systems, can be considerably stronger than the dezincing effect. Therefore, undertakings preventing the brass dezincification should be performed taking into account the properties of the corrosive environment and the technical system, where the brass is operated. In connection with this, in this work, we study the corrosion resistance and dezincification of brasses of various chemical and phase compositions in unstirred and running aerated water media. TEST PROCEDUREBrasses were prepared in an electric resistance furnace by directly fusing copper (99.99 at. %) and zinc (99.97 at. %) together in qu...

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