Electrochemical Behavior of Passive Iron in Acid Medium: I . Impedance Approach

Abstract

Wide frequency range impedance measurements are applied to a kinetic investigation of passive iron in various acidic media. Special care was taken in the electrode preparation to reach stable current densities false(7 μA·cm−2 normalin Fe/1M H2SO4,25°Cfalse) . Impedance data were obtained as a function of potential and temperature in the steady state. An original approach to the nonsteady‐state behavior is introduced by combining transient response to the time dependence of the impedance. Contributions from transport in the film and interfacial impedance are separated. The classical view of high field conduction mechanism is refined with respect to the generality of the description and also to the film properties in various environments leading to the concept of chemically dependent high field ion migration. Low frequency behavior in the millihertz range supported this conclusion by ruling out the idea of diffusion‐controlled passive current. The capacitance observed is attributed to the film‐growth kinetics.

Electrodeposition and corrosion behaviour of a Ni–W–B amorphous alloy

“…The same type of passivation and dissolution process was also observed by Keddam et al [20]. Figure 10 shows the impedance diagrams, which are associated with transpassivation and may be attributed to dissolution of the passive film as also observed by Keddam et al [21]. By the end of the impedance tests on the Co-W-B alloy, almost complete dissolution of the electrodeposited film occurred, exposing the copper substrate.…”

Effects of pH and fluoride concentration on the corrosion of titanium

“…All the features from the theory of Keddam et al25 is observed for these measurements with an estimated charge transfer resistance, R ct , of the corrosion process to 1–3 kΩ cm 2 , Table II. A semiqualitative measure of the film thickness can be estimated from the charge storing capacity of the film.…”

“… At high frequencies a semicircle is observed with a diameter directly correlated to the charge transfer resistance, R CT of an ongoing Faradaic reaction. The larger the diameter, the more sluggish is the corrosion reaction, Table II. In the medium frequency domain an inductive loop is often present which is suggested by Keddam et al25 to be related to the build up of a surface charge and might be related to an intermediate oxidation step within a surface film. At low frequencies, a second capacitive tail is developing that is linked to the ratio between the film forming current to the dissolution current at steady state. In the case of passive iron, a constant low frequency capacitance was obtained that corresponds to the charge storing capacitance of the passive film and therefore can be related to a constant film thickness.…”

“…Some features of the impedance spectra are discussed in the light of an investigation on the electrochemical behavior of passive iron by Keddam et al25 Three frequency regions are distinguished in the complex impedance plane, Figures 4(a) and 5(a). At high frequencies a semicircle is observed with a diameter directly correlated to the charge transfer resistance, R CT of an ongoing Faradaic reaction.…”

The effect of trimethylol propane tetraacrylate (TMPTA) and organoclay loading on the properties of electron beam irradiated ethylene vinyl acetate (EVA)/natural rubber (SMR L)/organoclay nanocomposites

“…In the medium frequency domain an inductive loop is often present which is suggested by Keddam et al25 to be related to the build up of a surface charge and might be related to an intermediate oxidation step within a surface film.…”