C.-O. A. Olsson

Passivation of Fe-Cr alloys studied with ICP-AES and EQCM

The response of sputter deposited Fe–15Cr and Fe–25Cr alloys to a rapid potential increase in the passive region was studied using two in situ methods: electrochemical quartz crystal microbalance and inductive coupled plasma-atomic emission spectroscopy (ICP-AES). Both methods indicated an increase in the dissolution rate of iron following the potential change; the total amount of material dissolved being higher for the Fe–15Cr alloy. For both alloys, about 10% of the total dissolved material was Cr. This result compared well with experiments on a 430 stainless steel. ICP-AES is shown to be useful for the in situ determination of partial currents of metal dissolution using solution analysis in a flow cell. 2002 Elsevier Science Ltd. All rights reserved.

Effect of chromium content and sweep rate on passive film growth on iron-chromium alloys studied by EQCM and XPS

Abstract Growth of passive films on Cr and Fe–Cr alloys with Cr concentrations ranging from 15% to 54% was studied in situ with the electrochemical quartz crystal microbalance (EQCM). Mass change and current transients were measured as the response to a potential change in the passive region from 0 to 800 mV (SHE) in a pH 1.5 sulfate electrolyte. From these measurements, the thickness change as well as integral and differential growth fractions were calculated. The growth fraction expresses the fraction of oxidized metal that remains in the film. It was found that higher Cr contents gave thicker films and an increase in the growth fraction. The thickness change curves were compared to film growth models assuming rate control at either a film interface or in the film itself through high field conductivity. The integral growth fraction for the potential change experiment was found to vary approximately linearly with the Cr bulk composition. The fraction of dissolved Cr calculated from EQCM data matched well with previous solution analysis results.

Atmospheric oxidation of a Nb–Zr alloy studied with XPS

Abstract The long term atmospheric oxidation of a Nb–Zr alloy was studied by XPS. Curve fit parameters were defined and strictly constrained to obtain a consistent evaluation of film thickness and chemistry. A formalism was introduced to distinguish between the water, hydroxide and oxide contributions to the surface oxide. Ageing experiments were carried out simultaneously exposing one lot of samples to a laboratory atmosphere for 10 months and a second lot to a dry atmosphere in a desiccator.

Evaluation of Passive Film Growth Models with the Electrochemical Quartz Crystal Microbalance on PVD Deposited Cr

With the electrochemical quartz crystal microbalance, it is possible to obtain time-resolved in situ estimates of thickness changes of passive films. Thickness changes recorded for physical vapor deposited pure Cr during potential scans in the passive region in acidic media were compared to two different theories for passive film growth: the potential change is governing the growth rate through a reaction at either of the oxide film interfaces (interface model), or it controls the ion conduction current through the film (high field model). The two models proved equally successful in predicting the film growth curves for varying potential-step amplitudes and sweep rates. However, for experiments with different initial passive film thicknesses, the interface model was more successful in predicting film growth.

Film Growth during Anodic Polarization in the Passive Region on 304 Stainless Steels with Cr, Mo, or W Additions Studied with EQCM and XPS

Anodic film growth in the passive region was studied using the electrochemical quartz crystal microbalance (EQCM) technique. Samples were produced using physical vapor deposition with the compositions of a 304L stainless steel with additions of 20 atom % Cr, Mo, or W. By simultaneous in situ recording of mass and current, it was possible to calculate the film thickness change and growth fractions in real time. These quantities were then used to interpret changes in film composition in real time. Whereas addition of Cr gave lower mass loss and smaller thickness changes, addition of either Mo or W increased the amount of material dissolved and also resulted in thicker passive films. These thickness changes were found to be consistent with growth characteristics suggested by growth control at either of the metal/film or film/electrolyte interfaces.

EQCM Study of Anodic Film Growth on Valve Metals

A comparative study of anodic film growth on niobium, zirconium, and tungsten in 0.4 M Na 2 SO 4 + 0.1 M H 2 SO 4 was performed using the rotating electrochemical quartz crystal microbalance (EQCM). Potential sweep as well as potential step and hold experiments were performed in the range between open circuit and 8 V SHE . Significant differences in the mass transients between the three metals were observed. During the formation of anodic films on niobium and zirconium, the dissolution was negligible and the fraction of the current serving for film growth was always close to unity. On tungsten, anodic dissolution in combination with film growth led to a growth fraction that diminished with time. The results obtained by the double potential step technique were compared with numerical simulations based on models considering either high field conduction or an interface reaction as limiting the kinetics of film growth. The growth of anodic films on niobium and zirconium was found limited by high field conduction, whereas film growth on tungsten was interface controlled. Film growth on tungsten was accompanied by important anodic dissolution, while dissolution of niobium and zirconium was negligible.

Electrochemical Quartz Crystal Microbalance Studies of the Passive Behavior of Cr in a Sulfuric Acid Solution

The passive properties of chromium in acidic solutions are of fundamental importance to a vast number of engineering materials, e.g., stainless steels, that owe their corrosion resistance to the formation of a thin oxide film strongly enriched with chromium. In this study, the focus is set on analyzing changes in the passive film during a potential sweep in the passive region on Cr. Chromium is well suited for this type of experiments, since it has a wide passive region in acidic media, about 1 V. The use of physical vapor deposition (PVD) to produce the Cr electrodes offer some advantages to conventional bulk metals; one benefit is the minimization of the size and number of inclusions. Chromium deposits are readily produced by magnetron sputtering. The surface states of chromium in the active region and after passivation have been described by, e.g., Bjornkvist and Olefjord, 1 who suggested a reaction path where the anodic active dissolution proceeds via a formation of adsorbed Cr(OH)2 as the rate-determining step. By rotating disk electrode (RDE) experiments, it has been concluded that the active peak is dominated by dissolution of Cr 21

Flow Cell for EQCM Adsorption Studies Application to Iodide Adsorption on Gold

A flow cell was designed for electrochemical quartz crystal microbalance (EQCM) measurements where the inlet solution can be rapidly changed without interruption of the electrolyte flow. This experimental setup is particularly well suited for adsorption studies, since it allows an introduction and removal of adsorbates from the electrochemical cell, without disturbing the continuity in the mass signal. To account for changes in viscous loading when introducing the adsorbate, the quartz crystal resistance and frequency were recorded simultaneously. The response of the quartz crystal to different parameters such as electrolyte, temperature, and quartz roughness in the flow cell was evaluated. To illustrate the usefulness of the setup, adsorption of iodides on a gold surface was investigated. A negative frequency shift was detected when the iodide-containing solution was fed through the cell at 0.2 V S S E . After correction for viscous loading, this frequency shift could be attributed to iodide adsorption. Experiments with Nal, KI, and LiI showed the same behavior. The results matched well with the response expected for the formation of an iodide adsorption layer on a hcp configuration. The adsorption reaction was irreversible for the experimental conditions used.