W.J. Lorenz

Determination of corrosion rates by electrochemical DC and AC methods

Abstract A brief review is given of DC and AC techniques which can be used to determine corrosion rates. The advantages and disadvantages of the extrapolation method of Tafel lines, polarization resistance measurements and impedance measurements are discussed. In particular it is shown that the intercept of the capacitive impedance loop with the real axis of the complex impedance diagram does not correspond to the charge transfer resistance of complicated corrosion systems exhibiting several time constants in the capacitive and inductive loop(s). Therefore, the correlation between this intercept value and the corrosion rate is not generally valid. Experimental data are presented for two types of iron (Marz and Johnson-Matthey) and 4340 steel in de-aerated and aerated 0.5 M H 2 SO 4 and 1 M HCl in the absence and presence of the following inhibitors: triphenylbenzylphosphonium-chloride (TPBP + ), propargylic alcohol (PA), 2-butyne-1,4 diol (BD) and hexynol (H). Corrosion rates have been determined by applying DC and AC measurements and solution analysis by atomic absorption. The results, in the absence of inhibitors and in the presence of TPBP + , show that the corrosion rate is unequivocally correlated to electrochemical DC data and to the extrpolated value of the inductive loop of AC measurements to zero frequency. However, in systems containing the other inhibitors the corrosion rate cannot be correlated to the polarization resistance because of an irreversible desorption of the inhibitor in the close vicinity of the corrosion potential, which gives an unpolarizable behaviour of the system in the anodic range. The relatively low inhibition efficiencies in the presence of PA and BD can be explained by a superimposed fast electrochemical reduction of the additives. Moreover, it is shown that the inhibition efficiency depends on the hydrodynamic conditions. The results of these investigations show that electrochemical DC and AC techniques can successfully be applied for determination of metal corrosion rates in systems that have simple corrosion kinetics. In more complicated systems knowledge of the details of the mechanism is necessary for interpretation of the experimental data.

AC-impedance measurements on porous aluminium oxide films

Porous aluminium oxide layers, prepared under different technical formation and sealing conditions, have been investigated by AC-impedance measurements in the frequency range 60 mHz ⩽ f ⩽ 65 kHz in 3.5 w% K2SO4 solution. The results can approximately be described by a simple equivalent circuit characterizing the barrier- and porous-layer. It is demonstrated that variation of the formation-current density, as well as the sealing procedure, mainly affects the properties of the porous layer as indicated by characteristic changes of the dielectric constant and the specific resistivity.

A theoretical treatment of the kinetics of iron dissolution and passivation

Abstract The anodic behaviour of highly pure iron in weak acid aqueous solutions which are free of oxygen and surface active compounds takes place in different ranges: active dissolution, transition range, prepassive range and passive state. This is indicated by the experimentally observed current-density potential curves which show a maximum I and a minimum in the transition range, a further increase in the prepassive range and a maximum II before reaching the passive state. Different mechanisms including consecutive and parallel steps are discussed in order to find the kinetic equation describing the complete anodic behaviour. The derived kinetic equations are examined by computer simulations. The simulated cd potential curves were compared with the experimental ones. A dissolution mechanism is postulated which satisfies the experimental results. The active dissolution range is characterized by the consecutive (Bockris-) mechanism including the adsorbed species (FeOH)ads. In the transition range a second intermediate [Fe(OH)2]ads is formed, which acts as a membrane inhibitor on the active dissolution. The prepassive range can be described by further electrochemical parallel steps starting from this second intermediate. The passivation is due to the formation of nonporous oxide layers which arise from [Fe(OH)3]-oxide phase and [Fe(OH)2]ads including Fe2O3 and Fe3O4.

The determination of corrosion rates by electrochemical d.c. and a.c. methods — II. Systems with discontinuous steady state polarization behavior

Abstract Corrosion systems exhibiting discontinuous anodic polarization curves have been investigated by steady state polarization, linear sweep polarization, large and small signal pulse polarization, a.c. impedance measurements and solution analysis. It is shown that the true corrosion current density can only be determined if the non-linearity of the system is avoided. This is not possible from analysis of a.c. impedance results without model assumptions.

The Kinetics of iron dissolution and passivation

Abstract The anodic behaviour of pure, recrystallized iron in acid sulphate and perchlorate solutions free of oxygen and surface active substance was investigated in the potential range between the active and passive state. The measurements were carried out using an iron rotating disk electrode by cyclic voltammetry under quasi-steady-state conditions with compensation of the ohmic drop. The determined kinetic data confirm mainly the previously proposed dissolution mechanism. The shortened form of this mechanism is The iron dissolution kinetics was found to be independent of the anion concentration in the overall potential range. New results with regard to the passivation and reactivation potentials of the metal are discussed.

Electrochemical behaviour of iron in alkaline sulphate solutions

The electrochemical behaviour of pure iron in alkaline sulphate solutions was studied using cyclic voltammetry atT = 298 K. It has been found that the results depend on the polarization pre-treatment of the electrode and the activation state of its surface. At starting potentials in the range of hydrogen evolution and at maximum activation conditions, the voltammograms show three anodic current maxima and two cathodic ones. A correlation between these different maxima is given. The electrode processes, which may take place at these maxima, are discussed in terms of possible iron dissolution mechanisms.

Zur anodischen auflösung von reineisen im bereich zwischen aktivem und passivem verhalten

The kinetics of anodic iron dissolution in sulphate ions containing solutions, which are free of oxygen, are investigated in a over potential range between active and passive states of the metal. The measurements were carried out on a rotating disk electrode in the pH-range between 3·5 and 5·5 using the potentiostatic triangular voltage sweep method under quasi-steady-state conditions. The measured current density-potential curves show two current maxima. At relatively low anodic overvoltages, i.e. before the maxima I is reached, the used recrystallized iron dissolves in the active state corresponding to the consecutive (Bockris-) mechanism. At high anodic overvoltages the passive state arises after passing the maxima II. In the transition range, i.e. in the potential range of the maximum I, a second mechanism of iron dissolution is discussed. This mechanism takes place parallel to the consecutive one. An intermediate [Fe (OH)2]ads will be formed, which dissolves in a following rate-determining chemical step or is changed in an oxide phase respectively.

Influence of lead and thallium underpotential adsorbates at silver single crystal surfaces on different redox reactions

The influence of underpotential Pb- and Tl-adsorbates at rotating disc silver single crystal surfaces (111), (100), and (110) on the kinetics of ‘outer-sphere’ and ‘inner-sphere’ redox reactions was studied in 0.5 M NaClO4 and 0.5 M Na2SO4 solutions (2 < pH < 4) containing Fe3+, Ce4+ or NO3− ions. While the reduction of Fe3+ and Ce4+ under limiting diffusion conditions was not affected by the metal adsorbates, a strong inhibition effect correlated to the degree of Pb- or Tl-adsorbate coverages was observed in the case of the reduction process in the presence of NO3− ions. The results are interpreted in terms of a strong chemical interaction between the reactant or a reaction intermediate and the silver substrate, taking into account the previously proposed superlattice structures of the metal adsorbates, depending on the orientation of the silver single crystal surfaces and on the underpotential range.

The kinetics of iron dissolution and passivation depending on temperature and ionic strength

Abstract The anodic behaviour of pure, recrystallized iron in weak acid sulphate solutions in the potential range between the active and the passive state of the metal was investigated. The influences of the temperature and the ionic strength on the kinetic data were studied. The results establish the iron dissolution and passivation mechanism previously proposed from measurements in sulphate and perchlorate solutions of high ionic strength and at room temperature. This mechanism is characterized by the formation of different intermediates [Fe(OH) n ] ads with n = 1, 2, 3 depending on the potential.

Galvanostatic and potentiostatic measurements on iron dissolution in the range between active and passive state

Abstract The kinetics of anodic iron dissolution in sulphate ions containing solutions, which are free of oxygen, were investigated in the range between active and passive states of the metal at pH ⋍ 5. The measurements were carried out at fixed electrodes under mechanical stirring of the electrolyte at T = 298K using galvanostatic as well as potentiostatic pulse technique. In the prepassive range the obtained transients indicate transition states, which can be attributed to parallel reactions to the active metal dissolution. The mechanism of the metal dissolution in the prepassive range is discussed.