A novel method is described for measuring the thickness of a barrier typ~ anodic oxide coating or the barrier layer portion of a porous type anodic oxide coating. This method is used to follow the evolution of the barrier layer during the early stages of the formation of a porous type coating on aluminum and to establish certain dimensions of the fundamental oxide cells which comprise this type of coating.
corrosion temperature is very close to the maximum temperature of the cycle. CONCLUSIONSThe linear corrosion rate of Zircaloy-2 which occurs at very long exposure times at low temperatures can be determined in a relatively short time by measurement of the corrosion rate at the low temperature in question after sufficient previous exposure at a higher temperature to reach the linear portion of the weight gain-time curve. This technique should be applicable to other alloys of the Zircaloy type.The transient effects which persist for a time after a change of temperature has been effected are discussed in terms of several aspects of corrosion mechanism. No completely satisfactory explanation of this behavior is offered.
the dissolution rate, R, of the film at the pore base is proportional to current density, C, according to: R = 310C.They base this on the assumption that dissolution rate at the barrier layer must balance film growth at this point because barrier thickness remains constant. From this the authors calculate that, in anodic oxidation of aluminum in 15% sulfuric acid, the acid concentration and temperatare in the pore base must approximate 51% H..,S04 at bp of 128~ in order to account for the high dissolution rate.There is, in fact, no reason to assume a linear relation between current density and dissolution rate. Barrier thickness remains a more or less constant function of voltage, not because its growth is balanced by dissolution at the pore base, but because growth is balanced by breakthrough and conversion into porous type coatings.The dissolution rate suggested by Hunter and Fowle as required to balance growth is, in fact, the dissolution rate not only of the barrier layer but also of the total coating (barrier plus porous coating) and even this only when the limiting total fihn thickness has been reacted. Of this total dissolution much the greater part will, of course, take place at or near the surface of the porous layer.In view of the linfited solubility of aluminum in 50% sulfuric acid and the necessarily very slow diffusion of A1 ions from the pore base through the coating into the electrolyte, the very high rate of dissolution at the pore base could not be maintained at any rate, while, on the other hand, conditions for equilibrium between growth and rate of dissolution at the barrier layer only apply when no porous type of coating is formed as, e.g., in boric acid or ammonium tartrate electrolytes. It should be stated that this does not detract from the important part played by local rise in temperature inside the coating due to the dissipation of electric energy (with the heat of formation of alumimm~ oxide as a contributory factor). This temperatare differential between electrolytes in the film and the bulk of the solution has been somewhat surprisingly ignored in many of the previous theories on anodic fihn growth.M. S. HUNTER AND P. FOWLE: The linear relationship between solution rate and current density during the formation of the porous type of anodic oxide coatings may appear to be illogical because oxide formation, which is a function of era'rent density, is an electrochemical process; whereas, solution of oxide is primarily a chemical process. Our measurements show, however, that the thickness of oxide between the pore base and the metM (barrier thickness) remains constant once current density has reached a steady value. Since the total thickness of oxide must equal barrier thickness plus the depth of the pores, and since barrier thickness is constant, the thickness of oxide formed in any period of time must equal the amount by which the depth of the pores is increased during this same period. Thus, the linear relationship between solution rate and current density is an established fact, unreas...
The effect of variations in electrolyte and forming conditions on the formation of porous type anodic oxide coatings on aluminum are discussed, with particular reference to the manner in which these variables control oxide formation, pore development, and the thickness of the barrier layer. Formation and solution rate data are applied to show that, during the formation of a porous type coating, conditions at pore bases are vastly different from those existing in the main body of the electrolyte.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.