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A model is developed of the mechanism of passivation of metals by oxide films. It is based on the assumption of the formation of a passivating film caused by the reaction of OH− ions with the metal surface. With increased potential the film‐free area decreases exponentially and with a higher exponent than does the density of the anodic dissolution current. Thus in the conversion from the active to the passive state, the current density is reduced exponentially to the constant and very low value characteristic of the passive state. This mechanism of passivation is explained by equations which describe the density of the anodic dissolution current as a function of the pH and the potential difference between the metal and the electrolyte. Good agreement with the theory is shown by published data for iron, nickel, chromium, gold, and silver. A simple method is given to derive all the constants required for the mathematical formulation of the anodic dissolution current from experimental data. Flade's equation relating to the boundary potential between the active and the passive state of iron is discussed. The principles and formulas, although derived specifically for formation of oxide films, are considered also to be applicable to the electrochemical passivation by other films.

This study of the analysis of polarization curves is basecl on the identilicatio~i of the partial currents of the single electrochemical reactions involved. The relationship between the gradient of polarization curves and the corrosion rate in uninhibited corrosion processes is investigated. The analysis of polarization curves is explained and two methods of using polarization curves in corrosion studies are compared. The conditions for stability of the active and the passive states are specified for application in anodic and cathodic protection. I. INTRODUCTIONIn the course of studies of the corrosion of steel in the presence of hot alkaline solutions (as in the ltraft process of pulping of wood), some basic features of the electrochemical theory of corrosion were defined, and two methods for recording polarization curves were developed (1). The theory and experin~ental methods are expected to apply generally to the electrocheinical reactions of any inetal with any electrolyte. In this paper the practical corrosion problem, as studied previously, is considered only as an example of the general application of the theory and methods.The practical problein concerned carbon-steel digesters attacked by alltaline pulping liquors in kraft mills. T h e cooking liquors contain 40 to 75y0 mill white liquor, the remainder being black liquor. The constitue~lts of white liquor are about 100 g/liter sodium hydroxide and 35 g/liter sodium sulphide and minor quantities of sodium thiosulphate and polysulphides. Blaclt liquor is the product of the reaction between the liquor mixture and wood (2).The initial laboratory studies measured corrosion rates as a function of the liquor composition. Ruus and Stoclcman (3) fouild by laboratory experiments that sodium hydroxide, sodium sulphide, and sodium thiosulphate are main factors determining the corrosion rate in cooking liquor. This finding was corroborated by a Canadian field study (4). The role of polysulphide, thiosulphate, and dimethyldisulphide a s depolarizers has also been described (5, 6, 7). However, two independent studies (3, 8) reported that the corrosion rates were not always reproducible even under presumably identical conditions. For instance, in an extreme case negligibly small corrosion rates of 10 or 20 M.P.Y. (milli-inches per year) were found alternating with rates of 650 and 750 M.P.Y. I t appeared that the conditions were close to the border line between the stable active and passive states. Several series of potential-time curves revealed that steel immersed in white liquor a t room temperature assuines either the active state a t a potential of about -1.05 volt or the passive state a t about -0.7 to -0.8 volt in reference to a saturated calomel cell (1) depending only on minor differences in the pretreatment of the steel. Another problem was provided by the observation that, during the heating period of the cook, the corrosion rate is high and falls u~lexpectedly during the later phase of the cook (3).These observations, together with the probleins co...

Based on Ohm's and Kirchhoff's laws, a differential equation is derived relating the changing potential slope in the length direction of a pipe to the current density at the inner pipe surface. The extension of the pipe that can be passivated or maintained in the passive state by supplying current with one cathode is formulated for linear as well as exponential types of polarization curves and for pipes of finite and infinite length. Completely passivated pipes can be kept in the passive state over a much greater distance from the cathode than partly passive pipes. Agreement is found with reported experimental data on the throwing power of anodie protection. The same basic equations are valid with respect to the throwing power of cathodic protection, which is much smaller than that of anodic protection at comparable corrosiveness. On the application of anodic protection redox reactions of substances produced on cathodes, anodes on corroding metals and alloys can strongly influence the behavior.For the application of anodic protection (1-6) in any lengthy vessel the throwing power (4) of the current is of primary importance. Evidently passivity is reached very rapidly in a vessel when the current density on its whole surface surpasses the maximum value of the polarization curve. However, even at lower average current densities, formation and expension of passivity occur when the current density locally surpasses the maximum value of the polarization curve. After the area of best current ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 141.211.4.224 Downloaded on 2015-06-29 to IP

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