[en] The resistance to sulfate attack of mixtures accelerated with alkali-free and alkaline accelerators was found to be mainly influenced by the Al³⁺ and SO₄^2- added via the admixtures. Microstructural observations showed decalcification and disintegration of the CSH gel, which acted as an additional Ca²⁺ supplier as compared to the CH for ettringite formation. The CSH decalcification was mainly observed with a homogeneous distribution of the alkali-free admixture. The disintegration of the CSH gel increased the porosity and allowed more sulfate solution to penetrate into the specimens. This process promoted the swelling of the specimens and directly contributed to the expansion, explaining the lack of a direct relationship between the ettringite formation and the expansion. Moreover, the CSH gel disintegration, typical for MgSO₄ attack, also occurred with Na₂SO₄ solutions and depending on the aluminate-sulfate distribution and the extent of the CSH gel disintegration, different damage types were detected. At higher temperatures (65 deg. C) the damage was mainly controlled by the growth, the rearrangement and the thermal stability of ettringite

[en] A new experimental method for the investigation of stress corrosion cracking mechanisms in thin sheets is presented: Using the foil penetration technique, the growth kinetics of the intergranular corrosion of age-hardened Al-Cu alloys have been measured in the unstressed condition and under various constant uniaxial tensile stresses. A pure binary Al-4wt%Cu alloy and a commercial AA 2024 alloy, both tempered to maximum susceptibility to intergranular corrosion, have been tested in aqueous chloride solutions under potentiostatic control. All measurements have been carried out on various sheet thicknesses between 0.2 and 1.0 mm under tensile stresses ranging from 0 to 88% of the 0.2% proof stress. A significant reduction of the penetration times by factors between 2 and 10, compared to the results for unstressed specimens, has been observed for all applied stress levels in systems where only small numbers of cracks are growing simultaneously. In systems with large numbers of cracks or complete crack networks, no influence of stress on the crack growth kinetics was found. The discussion of these results shows that all, even the highest observed mean crack growth rates can be explained by pure anodic dissolution of the grain boundary regions at the crack tips. The increase of the dissolution current density at the crack tips by tensile stresses is due to the widening of the crack, which reduces the integral ohmic resistance of the system and improves the mass transport conditions between the crack and the bulk electrolyte. The widening of the cracks depends on the stress distribution in the whole specimen cross section and not on the stress intensity at the crack tips. 23 refs., 14 figs., 1 tab

[en] A computer aided transient technique was developed to monitor and to quantify metastable pitting. It will be shown that these metastable pitting events may inititalize crevice corrosion. Probabilistic calculations, using a Poisson distribution, are carried out in order to estimate the susceptibility to crevice corrosion of a given metal/electrolyte system. Since metastable pitting events are stochastic processes, critical potentials for localized corrosion are only statistical values. A general model based on the ohmic potential drop as a stability criteria for localized corrosion describes and defines metastable pitting, two different types of stable pitting corrosion as well as crevice corrosion. (author) 20 refs., 12 figs

[en] Titanium nitride (TiN) coatings, produced by physical (PVD) or chemical (CVD) vapor deposition techniques are used routinely to improve the wear and corrosion resistance of a surface. The main problem in using TiN as a protective coating in aggressive environements are pores and pinholes in the coating where the substrate is exposed to the electrolyte. In this work, the electrochemical and corrosion behaviour of TiN films on quartz glass, carbon steel, 304 and 316 stainless steel is studied by polarization curves and electrochemical impedance spectroscopy (EIS) in hydrochloric acid. It is shown that the TiN coating can be used successfully only on substrates that passivate easily. On mild steel rapid corrosion takes place at pores in the coating due to the very noble steady state potential of the TiN coating. The interaction of the metallic substrate with the TiN coating is discussed for the two limiting cases mild steel (active) and 316SS (passive). It is shown that the determination of the coating porosity is possible for the active substrate only. On the passive substrate the occurence of an additional time constant in the high frequency region of the spectrum qualitatively indicates the presence of pores. A quality control of the coatings based on this fact might be possible. (author) 15 refs., 6 figs., 2 tabs

[en] A new corrosion testcell concept to study macroelement activity on concrete covered steel rebars is presented. This test cell makes it possible for the first time to divide an active corroding central area from its passive surrounding with definite variable area proportions. With help of this testcell practical and theoretical aspects of steel rebar corrosion in concrete were examined in a more detailed way: a) The possibility and limitations of concrete-building inspection-service with frequency response analysis (FRA) b) Macroelement impedance data and resulting physical models should be brought in a reasonable connection with the existing theory. Those under laboratory conditions performed FRA showed a strong dependence upon the area proportion between the corroding area and its passive surrounding within the corrosion macroelement (CME). This area-ratio is throughout presented as an unknown quantity in the practice of concrete-building inspection-service. Impedance spectra of such different active/passive area-ratios generally showed at the first view 2 time constants. These two time constants cannot necessarily be directly assigned to the basic electrochemical reaction such as the metal dissolution reaction and the oxygen reduction reaction. This has been discussed for the dynamical system analysis of an appropriate electrical simulation-circuit, mainly consisting of two parallel RC-circuits. (author) 20 refs., 9 figs

[en] Localized corrosion processes such as pitting and crevice corrosion cause major practical problems, affecting the performance of generally corrosion-resistant passive metals. Adequate knowledge on the mechanisms involved in localized corrosion are not only of scientific but also of great practical importance. The mechanisms proposed until now are discussed, including contributions from new investigations on chloride induced metastable pitting as well as photoelectrochemical and surface analytical studies. Based on these results a deeper insight into the mechanistical aspects of pit initiation, the early growth stage and the transition to stable growth is attained. In order to improve the corrosion-resistance of passive metals the ionic as well as the electronic properties of the passive films are equally important

[en] The mechanism of the passivation and the effect of metalloids on the stability of the passive films of amorphous Fe-Cr-(B,P,C) alloys has been investigated by polarization measurements, impedance spectroscopy and potential decay measurements. The results show that phosphorus facilitates the active/passive-transition by forming a porous iron-phosphate pre-passive layer on the alloy surface in the active range of the dissolution. This layer blocks the active sites of the surface and accelerates the cathodic H₂-evolution reaction. The formation of the passivating chromium oxide layer takes place in the pores of this layer. In the passive range of the alloys oxidized phosphorus gets incorporated in the outer layer of the passive film. The presence of oxidized phosphorus as PO₄^3- anions in the passive film increases the localized corrosion resistance in Cl-containing solutions. The effect of the incorporated phosphates in the passive film is discussed with respect to the bipolar fixed-charge induced passivity model. The phosphates make the outer layer of the passive film cation-selective and thus hinder the penetration of the chlorides into the film. The oxidized boron species cannot change the ion-selectivity of the film; instead of this they negatively affect the stability of the passive film. (author) 18 refs., 9 figs., 3 tabs

[en] The interrupter technique presented, using a computer assisted data acquisition and calculation method, allows the determination of the ohmic potential drop (IR-drop) in potentiostatic experiments. The results obtained for the passivation of a 304 stainless steel in low conductivity ethanol/water solutions containing hydrogen chloride show the importance of the accurate correction of the IR-drop to calculate the correct kinetic parameters. Impedance measurements give the same values for the ohmic resistance as the computer-assisted interrupter technique described in this paper. The complex plane plots of the impedance show three capacitive loops before the current density reaches its maximum. (author)

[en] Recently developed microelectrochemical methods are powerful techniques to study localized processes in the micrometer range. The investigations confirm the key role of inclusions, predominantly manganese sulfides, in being initiation sites for localized corrosion. However, localized corrosion is also affected by the temperature. Many investigations use the temperature as a pitting criterion, i.e. in critical pitting temperature tests. This work presents the investigation of two commercial DIN 1.4301 stainless steels with different sulfur levels in Na₂SO₄ and NaCl solutions at various temperatures. In addition to conventional electrochemical techniques and critical pitting temperature tests, microelectrochemical measurements are presented which allow to study localized corrosion in the micrometer scale at temperatures up to 90 C. Results from locally resolved measurements show that higher current densities in large scale experiments originate from many single activation and repassivation processes. The effect of the temperature on these processes is compared with the results from large scale experiments in the temperature range of 0 C to 90 C. (orig.)

[en] Maintenance and planning of the restoration work on reinforced concrete structures need a rapid, non-destructive inspection technique that detects corrosion at an early stage. In this paper the field experience with electrochemical techniques are reported. The results of potential surveys on bridge decks of the Swiss highways, realized with a new computer controlled eight-wheel measurement system, clearly show that a fixed potential value (as proposed in ASTM C 876-87) for the identification and location of active corrosion of steel in concrete does not exist. A statistical elaboration of the potential data allows to identify the potential value for corroding and passive rebar for each structure individually. A rapid new technique using galvanostatic pulse measurements was tested successfully on site. It gives clear, unambiguous results on the corrosion state of the rebars when half-cell potentials are uncertain. In addition, the ohmic resistance and the apparent polarization resistance are determined from the evaluation of the transients. From these values the concrete resistivity and the actual corrosion rate of the rebars may be estimated