H. Böhni

A new microelectrochemical method to study pit initiation on stainless steels

Abstract Microelectrochemical techniques using microcapillaries as microcells are powerful methods to study local phenomena on passive metal surfaces. The enhanced current resolution to pA and fA allows us to study local processes in the nm-range. The results obtained from stainless steels indicate that MnS-inclusions, acting as potential nucleation sites for pitting corrosion, are dissolved even in chloride-free solutions, whereas stable pit growth only occurs in the presence of chlorides. The microelectrochemical technique easily allows us to distinguish between active and inactive inclusions. Noise analysis of potentiostatic measurements revealed that the RMS current noise, increasing linearly with the exposed area, is largely caused by the dissolution of small inclusions. Furthermore, the results allowed us to correlate pitting potentials with the inclusion size, indicating that the inclusion size has to be kept well below 1 μm to improve substantially the pitting resistance of stainless steels.

Micro- and nanotechniques to study localized corrosion

Microelectrochemical methods and scanning tunneling microscopy (STM) are powerful techniques to study localized corrosion processes on passive metals in the μm- and nm-range. On commercial stainless steels pit nuclei are initiated due to the oxidation and dissolution of inclusions, even in chloride-free environments. Microelectrochemical techniques allow to perform in situ studies on the electrochemical behaviour of single inclusions. In presence of chlorides additional nucleation sites are activated. It is presently assumed that most of these pit nuclei also originate from small inclusions. Chlorides also accelerate and stabilize pit growth.

Corrosion resistance of super duplex stainless steels in chloride ion containing environments: Investigations by means of a new microelectrochemical method. I. Precipitation-free states

Abstract A new mircoelectrochemical method was applied to perform potentiodynamic polarisation experiments on areas in the range of 10 μm. For the first time, the individual corrosion behaviour of both single phases in super duplex stainless steels was determined. The results show a good correlation with the empirical pitting resistance equivalent number (PREN) of the corresponding single phase. The microelectrochemical experiments have revealed two different kinds of interactions between the ferrite and the austenite phase at the phase boundary, namely a superposition or a separation of the two polarisation curves of the single phases. Potentiodynamic polarisation of large areas with representative amounts of both phases are performed in hydrochloric acid electrolyte in order to compare the corrosion behaviour of the single phases with the corrosion behaviour of the entire alloy. Both, the pitting potentials, evaluated by means of macroelectrochemical experiments in pH-neutral lithium chloride electrolyte, and the critical crevice corrosion temperatures show a good correlation with the PREN of the weaker phase. Part I of this paper deals primarily with solution annealed materials. Solution annealing of the samples was performed at temperatures where minimal amounts of precipitates were formed and where the element partitioning determines the corrosion resistance of the single phases. Part II deals with the influence of precipitations on the corrosion behaviour.

Microelectrodes for studies of localized corrosion processes

Electrochemical techniques using microcapillaries as electrochemical cells allow the study of local processes on passive metal surfaces. Due to the enhanced current resolution down to pA and fA, processes in the micro- and nanometer range can be studied. The results obtained on high-molybdenum containing stainless steel indicate that pit initiation is caused by active MnS inclusions. Molybdenum as alloying element has only a minor effect on the initiation process. The superior corrosion resistance of the high-molybdenum containing alloy has to be attributed to a considerably improved repassivation tendency. Noise measurements revealed that the current noise is largely caused by the dissolution of small inclusions. Furthermore microelectrochemical noise measurements allow evaluation of the experimental data with respect to their statistical relevance.

Microelectrodes for corrosion studies in microsystems

Abstract Electrochemical techniques using microcapillaries as electrochemical cells allow to study local processes on metal surfaces. Since the usual large-scale electrochemical techniques provide only average data integrating over a large surface area (mm 2 –cm 2 ), they are inadequate to investigate local corrosion processes. Due to the enhanced current resolution of the microelectrochemical methods down to pA and fA, processes in the micro- and nanometer range can easily be studied. This method allows to perform local electrochemical investigations of single areas of special interest, such as grain boundaries, second-phase precipitates or inclusions. Therefore, the microcapillary technique is well suited to study the corrosion behavior of microsystems. Results obtained from studies of localized corrosion processes of stainless steels are presented and discussed.

Comparison of the Semiconductive Properties of Sputter‐Deposited Iron Oxides with the Passive Film on Iron

The semiconducting properties of sputtered magnetite (Fe{sub 3}O{sub 4}) and oxidized magnetite layers with different Fe(II) contents were compared with the passive film on iron. Electrochemical impedance spectroscopy and photoelectrochemical experiments were carried out in borate buffer, pH 8.4. An evaluation of the impedance data according to the Mott-Schottky concept showed that the capacitance of all films is linked with their doping concentrations rather than their thicknesses and that the doping species of the passive film on iron is Fe(II). For the passive film a potential-dependent doping concentration was found. Photoelectrochemical investigation of the passive film and the sputtered oxide layers showed that for low doping concentrations, the photocurrent increases with doping, whereas for high doping concentrations, an increase in doping leads to a decrease of the photocurrent. Possible causes for this effect are discussed. Further, the combination of a light reflectance technique with photocurrent measurements allowed consideration of light absorption effects in the data treatment and separation of photocurrent contributions from the space-charge layer from film-thickness effects.

Illumination effects on the stability of the passive film on iron

Abstract The effect of illumination on passivity and the initiation of pitting corrosion on pure iron in borate buffer pH 8.4 is studied. It is observed that a monochromatic (325 nm) illumination of the sample leads to a very strong increase in its pitting corrosion resistance—apparent from pitting potential and incubation time measurements in chloride containing electrolytes. This effect is strongly dependent on the light intensity. The cause for the decreased pitting susceptibility is not based on a change in the field distribution within the passive film but is much more associated with light induced changes in the passive film properties during passivation. These changes can neither be attributed to a film thickening nor to an altered temperature caused by illumination. The electronic properties of the passive films were studied by Mott-Schottky analysis and photocurrent transient measurements. The results indicate that illumination during passivation leads to modifications in the electronic properties of the passive films—specifically to an increase of the bulk doping—and the surface state density. Further it can be concluded that the electronic properties of the passive film on iron can rather be described in terms of a semiconductor model than by a dielectric approach.

Migrating Corrosion Inhibitor Blend for Reinforced Concrete: Part 1—Prevention of Corrosion

Abstract The efficiency of a migrating corrosion inhibitor in preventing corrosion of mild steel was investigated in saturated calcium hydroxide (Ca[OH]2) solutions and in mortar. The protective ef...

Effects of Temperature and Chloride Concentration on Pit Initiation and Early Pit Growth of Stainless Steel

The influences of temperature and chloride concentration on pitting corrosion of stainless steel AlSI 304 were investigated. Conventional large-scale measurements (sample diameter, d = 0.8 cm) were conducted to study the effect of temperature on pit initiation, metastable and stable pitting in 0.1 M Na 2 SO 4 , and in 0.1 M NaCl solutions at 0, 30, 60, and 90°C. Microelectrochemical measurements (d = 60-80 μm) were performed to investigate pit initiation events at room temperature, 60, and 90°C in Na 2 SO 4 , in NaCl, and in LiCl solutions of various concentrations (0.01-10 M Cl - ). It was found that the MnS inclusions dissolved at lower potentials when the temperature was increased. In the presence of chlorides, an increase in temperature influenced pit growth and decreased the pitting potential. The temperature was found to aid in stabilizing pit growth and hindering repassivation. Pitting corrosion at single MnS inclusions was initiated by conducting potential sweeps with a microelectrochemical cell until the pitting potential was reached. In the presence of chlorides, pits initiated at the interface of the MnS inclusions and the metal matrix. The extent and morphology of the attack at the interface between the inclusion and the metal matrix varied with chloride concentration and temperature. At high temperatures and high chloride concentrations, numerous additional initiation processes or the passive surface around the larger inclusions are taking place, leading to an etching-type corrosion of this area.

Migrating Corrosion Inhibitor Blend for Reinforced Concrete: Part 2— Inhibitor as Repair Strategy

Abstract The efficiency of a migrating corrosion inhibitor in stopping or reducing corrosion of steel has been investigated in saturated (sat.) calcium hydroxide (Ca[OH]2) solutions and mortar. In solution, a slight decrease of the corrosion rate was found when the inhibitor blend (10%) was added after the onset of pitting corrosion. Investigation of the diffusion rate in mortar demonstrated the high mobility of the hydroxyalkylamine molecule in porous cement paste. Thus, it was assumed that it is, in principle, possible to obtain high inhibitor concentrations at the rebar to influence corrosion rate. However, on reinforced mortar samples showing chloride-induced pitting corrosion, no reduction of the corrosion rate could be detected after inhibitor application. The discrepancy between the observed high diffusion rate and the lack of corrosion mitigation was rationalized by the fact that only the diffusion of the volatile component of the inhibitor was measured. Migration of the nonvolatile component (car...