(Henry B. Linford Award for Distinguished Teaching) Determination of Local Hydrogen Concentrations in High Performance Alloys Using Local Probe Methods
Authors: Scully John , Schaller Rebecca Filardo , Rincon Troconis Brendy C 
Published: February 2020

Introduction:
Techniques for the measurement of hydrogen in high strength metallic alloys and effective hydrogen diffusivity (DH,eff) lack micrometer scale spatial resolution. This is of great importance since the hydrogen-metal interactions and processes occurring at the micrometer to nanometer length scales control hydrogen embrittlement (HE). This presentation discusses three methods which enable spatial resolution and mapping of absorbed hydrogen at sub-millimeter length scales in ultra-high strength steels (UHSS) and stainless steels.
The scanning Kelvin probe (SKP), scanning Kelvin probe force microscope (SKPFM), and SKP combined with secondary ion mass spectrometry have recently been developed as useful techniques for spatial H detection with depth (using cross-sectional analysis), lateral detection, and mapping.[1-7] In terms of H resolution, the SKP can detect concentrations as low as 0.01 atomic ppm.[8] This presentation explores three novel techniques for the spatial detection of hydrogen concentrations. These include the SKP, the scanning electrochemical microscope (SECM) and re-scaling methods to produce mm scale cracks or crevices that are electrochemically equivalent to small scale occluded sites [5, 9-11].

Experimental and Results:
Two secondary age hardened martensitic steels were examined, UNS K92580 and UNS S46500. The SKP voltage (Φ) dependence on dissolved H concentration was optimized by mapping under conditions producing a low corrosion rate; an essential aspect of successful hydrogen mapping. A calibration curve was developed relating absorbed hydrogen concentration to Φ. High strength steels were pre-charged and exposed to MgCl2 and NaCl droplets, which produced natural acidic pit sites that enabled local hydrogen uptake[12]. The factors controlling calibration of SKP potential and hydrogen concentration as well as those limiting spatial resolution are discussed. Moreover, DH,eff coefficients were determined.[5] Results were corroborated with previous studies of DH,eff.[13] The SECM also was optimized to enable detection of corrosion processes with spatial resolution and utilized to measure spatially a pre-dissolved concentration of hydrogen across pre-charged and uncharged zones [10]. The talk will close with comparison of the length scales of various physical processes occurring during hydrogen embrittlement compared to those accessed by these techniques.

Acknowledgement:

Research was sponsored by the US Air Force Academy under agreement number FA7000-13-2-0020 and ONR under PROJ0007990. The authors would like to acknowledge the Army Research Laboratories, as well as Monash University, in particular Dr. Sebastian Thomas and Prof. Nick Birbilis. The research was partially supported by a 2014 Australia Endeavour Award Research Fellowship with support from Dr. Kishore Venkatesan and Dr. Ivan Cole at CSIRO.

References:

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