Local Probe Techniques for Hydrogen Measurement in Ultra-High Strength Steels

Hydrogen embrittlement (HE) is a concern specifically in ultra-high strength steels (UHSS) prevalent in key structural and vehicle components. Hydrogen production and uptake in high strength materials is well documented in many full immersion environments such as during cathodic protection. However, there is a lack of information regarding hydrogen production in atmospheric exposure conditions. Marine aerosols, industrial pollutants, and other environmental factors, such as UV and relative humidity, greatly affect atmospheric corrosion damage, where oxygen reduction (ORR) is the primary cathodic reaction. However, the hydrogen evolution reaction (HER) can produce atomic hydrogen. Hydrogen production and uptake that occurs on a metal surface is likely affected by many of the same environmental factors that cause high environmental severity since anodic and cathodic reactions are coupled during naturally occurring corrosion. An understanding of the effects of these various exposure conditions on hydrogen production and uptake in UHSS is necessary to develop new alloys with improved corrosion resistance as well as to anticipate and manage the effects of environment severity on embrittlement susceptibility of currently employed UHSS.

This thesis aims to obtain a better understanding of the corrosion processes generating hydrogen electrochemically and the factors controlling hydrogen production, uptake, and diffusible concentrations, at both the meso- and micrometer length scales during the atmospheric exposures of selected high performance alloys. Hydrogen production and uptake was investigated in three ferrous alloys, two ultra-high strength steels (UNS S46500 and UNS K92580), and one carbon steel (UNS G10180). Included in this study are both traditional analysis of meso-scale H uptake under atmospheric exposure conditions using thermal desorption spectroscopy as well as pioneering local detection methods. The objective was to develop and demonstrate the utility and efficacy of the novel local hydrogen probes. The applications of the Scanning Kelvin Probe (SKP) and the Scanning Electrochemical Microscope (SECM) were explored. The detection of spatial distribution of the diffusible H concentration was demonstrated on pre-exposed metallic surfaces with the SKP and SECM. The application of these local scale techniques is shown to provide local and spatial information on H concentration pertinent to atmospheric corrosion and previously unattainable with meso-scale techniques. An understanding of the H severity at this local scale can be expanded to the meso-scale and provide significant information on severity of atmospheric exposure environments in terms of H production, uptake, and transport.

The SKP was explored as one viable option. Factors that may limit the feasibility to measure diffusible H concentrations using this method were assessed. An explanation for the measurement concept for H detection in the SKP and the controlling factors was developed. Moreover, a relationship between the measured SKP potential and the diffusible hydrogen concentration (CH,Diff) was determined. Factors such as RH, size of charging area, and time were also explored. Finally, simulations of steady state H and H+ diffusion verified experimental determination of local H concentrations produced from exposures.

A second novel technique for spatial, local H determination was explored; the SECM. The explanation of the measurement concept for H detection in the SECM and controlling factors were rationalized. Optimization of solution, potential applied to the SECM microelectrode tip, the tip to sample height selection, scan rate, and scan size were presented. A correlation between H pre-charged materials and the effect on interrogating current measured was determined and a quantitative relationship between the current measured at the SECM tip and CH,Diff was presented. The effects of solution phase transport of species used in the redox competition mode were compared to solid state diffusion simulations to assess spatial resolution.

Both the SKP and SECM techniques were then extended towards the detection of local hydrogen concentrations sites on samples pre-exposed under atmospheric conditions (in MgCl2). Electrochemical analysis of conditions present in atmospheric drops was investigated to better understand the basic roles of various environmental parameters such as pH, Cl- type, and concentration. SKP and SECM scans of samples from controlled atmospheric pre-exposures are presented to demonstrate the application of these techniques to local detection of naturally absorbed H due to production and uptake from atmospheric corrosion processes.

The scientific contribution of this thesis is that it established novel local measurements of hydrogen concentrations during atmospheric exposures using local probes. Methods for not only hydrogen detection but accurate quantification on steel surfaces were established. The scientific basis for the SKP and SECM measurements, detection, resolution, and quantitative relationships between probe signal and H concentration, were developed. Explanations for the effects of hydrogen concentrations on these measurement techniques, methods for application, as well as suggestions for future work are given to enhance the field of hydrogen embrittlement research through localized H detection. From the engineering perspective, a new method to quantify environmental severity and alloy susceptibility was developed.