Growth of passive oxide films on iron and titanium under non-stationary state

Growth of passive oxides on iron and titanium under the non-stationary state was studied during constant current density (CD) oxidation and potential sweep oxidation. Increase and decrease of the thickness of the passive oxide film were determined from the relation between dissolution and growth CDs. By using rotating Fe disc-Pt ring electrode, the CDs of the dissolution from the oxide to solution and the growth of the passive oxide on iron were simultaneously measured. From the ratio of the growth CD to the dissolution CD and the pH dependence of the growth CD, a possible mechanism was discussed. Next, the growth of passive oxides on iron in pH 8.4 borate solution and on titanium in 0.1 m sulfuric acid solution during the potential sweep oxidation was examined by using three-parameter ellipsometry. Oxide growth was initially delayed when the sweep in the positive direction was started, and then the oxide film linearly grew with the potential increase. It was found that the oxide growth rate influenced the properties of the oxide film. The electric field in the oxide film during the non-stationary growth was discussed, concerned with the CD of the oxide film formation.

2017 W.R. Whitney Award: Perspectives on Chloride Interactions with Passive Oxides and Oxide Film Breakdown

The nature of passivity and its breakdown have garnered great interest before and since Schonbein used the term “passivity” in 1836 to describe the “altered state” of iron. There has been a large body of experimental work, and a number of theories describing passivity and its breakdown leading to pitting corrosion have been proposed. However, there continues to be debate on this topic, which includes the discussion as to whether pit initiation is controlled by oxide film breakdown or by the pit growth kinetics. This communication will focus on oxide film breakdown without drawing any conclusions on the rate controlling step. As all currently proposed mechanisms require Cl− interactions for oxide film breakdown in Cl-containing environments, the question becomes what is the nature of the interaction of Cl− with the passive film, adsorption and/or incorporation, or neither? The interaction of Cl− with the passive film on pure aluminum and Type 316 stainless steel will be reviewed and summarized using available experimental data concerning Cl− interactions both from prior work at the Naval Research Laboratory and work reported in the literature. A point will be made that choosing the appropriate experimental procedure and data analysis is of great importance for getting high-fidelity data.

Kinetics of Corrosion Processes of Alloy on the Intermetallic Phase Matrix FeAl in High Temperature in Air Atmosphere

The aim of this paper was to determine the resistance to high-temperature corrosion in atmosphere of air for alloy Fe-40Al-5Cr-0.2Ti-0.2B. Corrosion tests were conducted in temperatures from 600 to 900°C in time from 2 to 64 hours. Conducted tests have shown a slight increase of weight of samples in periods of time which followed. Increase of weight is connected with corrosion products in the form of passive oxides which form on the surface of the alloy. Kinetics of corrosion processes has parabolic course in tested temperature range which proves the formation of passive layers of corrosion products on the surface of samples. Heat resistance of the alloy on intermetallic phase matrix FeAl brings about potential possibilities to apply this alloy as a material meant for work in elevated and high temperatures in the environment which includes oxygen.

Cold-neutron depth profiling as a research tool for the study of surface oxides on metalsSpecial Issue on Neutron Scattering in Canada.

A recent experiment at NIST has demonstrated that neutron depth profiling (NDP) based on the (n, α) reaction could be developed into a tool that could be routinely used for the study of passive oxides on metals. Whereas most metals are not (n, α) active, oxides grown with 17O, the only (n, α) active oxygen isotope, can be observed and tracked by this technique. Problems due to contamination of the samples by boron were encountered, but were shown to be surmountable. For our samples, the NDP facility at NIST, as it exists today, has enough flux and energy resolution to separate the α particles emitted by 17O from those emitted by 10B. Substantial improvement in the data collection rate, easily achievable with arrays of additional detectors, will make NDP a useful tool in the study of passive oxides.