Depassivation Process Research Articles

Laser-Ellipsometric and Photoelectrometric (EPEM) Video Microprobe

An automatic computerized system simultaneously measuring the thickness of the surface layer and the semiconducting characteristics of oxide and other surface layers on an electrode surface during the corrosion or any other electrochemical studies is described. The automatic recording of ellipsometric parameters and a photopotential at sharply focusing the probing laser beam at a chosen surface site provides the extremely high sensitivity to the microscopically localized passivation and depassivation processes. The phase-modulation principle of recording the ellipsometric parameters with a laser source, which is most promising for the local corrosion–electrochemical investigation, is naturally combined with the technique of measuring photo-emf with the modulation of not only the phase, but also the intensity of the incident radiation. The character of the intensity change with time (the type of modulation) can be arbitrarily set up to the modulation frequencies of hundreds megahertz, which enables one to study photoelectrochemical processes with substantially different characteristic times (such as the lifetime of electron or hole carriers in the surface oxide).

Non-protective alumina growth in sulfur-doped NiAl(Zr)

The 1,200°C oxidation behavior of NiAl was examined at various levels of sulfur and zirconium dopants to test the possibility of a critical S/Zr ratio required for adhesion. Cyclic furnace testing for 200 1-h cycles and interrupted testing for 500 h were used as screening tests. Pure NiAl and NiAl(Zr) with 0.14 at. % Zr were chosen as model base compositions; they exhibited normal, slow-growing scales (3 mg/cm2) with excellent adhesion for the Zr-doped alloys. NiAl with about 120 ppma S exhibited a substantial weight loss (−20 mg/cm2) in cyclic tests and a very large weight gain (+60 mg/cm2) in interrupted tests. The major surface phase remained as α-Al2O3. Sulfur doping the NiAl(Zr) alloy caused massive weight gains of 80–100 mg/cm2, swelling, cracking, and nearly complete conversion into NiAl2O4 and α-Al2O3. The initial objective of determining critical S/Zr ratios for adhesion was therefore unattainable. Initiation of the catastrophic attack was examined after a 10 h exposure, revealing a few sites of broad, raised, and cracked ridges. In cross-section, the ridges appeared as nodular intrusions, with a complex, fractal, oxide–metal interface. They were primarily alumina (with occasional entrapped islands of NiAl2O4 or pure Ni metal). They possessed a unique microstructure consisting of 0.3 μm lamellae, separated by 0.1 μm open channels. This allowed for rapid growth controlled by gaseous diffusion. The microstructure is discussed in terms of SO2 evolution and a sulfur-driven depassivation process.

In situ spectroscopic ellipsometric investigation of vacuum annealed and oxidized porous silicon layers

Vacuum annealed and oxidized porous silicon layers (PSL) were investigated by in situ spectroscopic ellipsometry (SE). The nominal porosity of the layers was between 60 and 77% and the nominal thickness was 500 nm. The annealing was performed by direct ohmic heating (R.T.<450°C) in 5×10 −10 Torr vacuum. The oxidation was performed in two steps, the first step at 5×10 −5 Torr, the second at 10 Torr. Two optically different types of silicon compounds, a bulk-type silicon (c-Si) and a fine-grain polycrystalline silicon with enhanced absorption due to extensive grain-boundary regions (p-Si) were mixed with voids in the appropriate ratio to fit the spectra of as-prepared PSL. For the annealed PSL amorphous silicon (a-Si) was needed in conjunction with p-Si. The oxidized PSL could be fitted with a reduced a-Si content. We can interpret the annealing effect as a depassivation process of the inner surfaces of the PSL (a-Si fraction). At the same time, oxidation leads to a repassivation process.

An experimental study of the quasi-two-dimensional corrosion of aluminum foils and a comparison with two-dimensional computer simulations

The propagation of a (1 + 1)-dimensional corrosion front into a quasi-two-dimensional aluminum foil confined in an electrochemically inert thin cell was studied experimentally. The corrosion experiments were carried out under controlled electrochemical and flow conditions leading to corrosion fronts consisting of isolated and coalesced pits. The geometry of digitized images of these disorderly corrosion fronts was analyzed quantitatively. A two-dimensional computer model these disorderly corrosion fronts was analyzed quantitatively. A two-dimensional computer model that includes passivation and depassivation processes was used to simulate the corrosion fronts and the simulated fronts were compared quantitatively and qualitatively with the experimental results. Unlike earlier passivation/depassivation models, this model includes the increasing electrolyte concentration in corrosion pits during the corrosion process, leading to the development of a more corrosive environment in the pits.

Electron-stimulated desorption of hydrogen from the Si(111) surface by scanning tunneling microscopy

Preparation of suitable silicon (111) wafers in weakly alkaline HF solutions results in the formation of atomically flat, hydrogen-terminated surfaces. Under high-vacuum conditions, the scanning tunneling microscope has been employed to selectively desorb the passivating hydrogen from nanometer-sized surface regions. The hydrogen depassivation process is studied as a function of current and applied bias voltage, voltage polarity, and exposure time to incident electrons either on individual surface locations or by varying the speed of tip motion to control the electron dose. The experimental findings are interpreted in terms of two distinct desorption mechanisms and the respective desorption yields are specified.

Depassivation and repassivation of AISI321 stainless steel surface during solid particle impact in 10% H 2SO 4 solution

Erosion-corrosion experiments on AISI321 stainless steel in 10 wt.% H 2SO 4 + 15 wt.% corundum sand were carried out by means of a rotating system with the specimen on the edge of a disc. Dynamic polarization curves during erosion-corrosion were measured at different rotating rates and the weight-loss kinetics was studied at different applied potentials and rotating rates. The straining electrode technique was used to simulate the depassivation and repassivation processes during erosion-corrosion. The experimental results showed that in the whole passive region the apparent passive current density and the weight loss during erosion-corrosion were constant at the fixed rotating rate and increased quickly with the rotating rate in the range of flow rate from 2.5 to 10 m s −1. The dissolution behaviour of the straining electrode at different applied potentials and straining rates was found to correlate with the dynamic polarization current of specimens during erosion corrosion, indicating that the electrochemical corrosion in the erosion-corrosion process was controlled by both the depassivation caused by the solid particle impact and the regrowth of protective film on the fresh specimen surface.

Zinc deposition and passivated hydrogen evolution in highly acidic sulphate electrolytes: Depassivation by nickel impurities

In highly acidic sulphate electrolytes used for zinc electrowinning, the steady-state behaviour and the impedance of zinc electrodes have been analyzed under potentiostatic control. It is shown that zinc deposition takes place under conditions where hydrogen evolution remains passivated on the deposit surface. A depassivation of hydrogen evolution leading to the deposit corrosion can be provoked by an electrode excursion near the corrosion potential. The presence of nickel impurities facilitates this depassivation process even though the electrode potential remains cathodic. The depassivation becomes easier with the existence of a scratch on the substrate surface, because of bubbles retained on this defect.

The Effect of Depassivation at Specific Crystal Facets on the State of Activation of Nickel Substrates for Electrodeposition

Changes in the surface structure of nickel substrates at open circuit in after an anodic etch treatment have been investigated by electrode potential measurements as a function of time, by voltammetric repassivation in neutral sulfate and phosphoric acid solutions, and by microscopic examination. It was found that partial depassivation occurs at specific crystal facets within a few hundred seconds. Studies involving single crystals indicated that the {100} and {110} orientations depassivate within this time frame, whereas the passive film on the {111} surface is more stable. The depassivation process is accelerated by iron impurities in the substrate and this impurity effect is enhanced by the presence of dissolved oxygen, which also enhances the etching of depassivated areas. These data suggest that poor activation of nickel substrates for electrodeposition can result from partial depassivation to form bare spots on the surface, through which appreciable cathodic current can pass without reducing the remaining oxide layer.