Spatial determination of diffusible hydrogen concentrations proximate to pits in a Fe–Cr–Ni–Mo steel using the Scanning Kelvin Probe

• SKP combined with CH,diff calibration is a novel method for spatial H determination.
• SKP can map CH,diff with local spatial resolution in samples containing pits.
• Local spatial H measurement concept is expandable to a broad range of materials.
• Local technique is suitable for development for application to sub-μm length scale.
• SKP H detection method applied with front side detection not permeation mode.

The diffusible hydrogen concentration (CH,diff) was mapped spatially, proximate to acidic corrosion pits formed in MgCl2 droplets, on an ultra-high strength stainless steel using a Scanning Kelvin Probe (SKP). A secondary hardened martensitic stainless steel (Fe–11Ni–12Cr–1Mo–0.005C–0.02Mn wt%) was evaluated. Predetermined, uniform CH,diff levels quantified by electrochemical extraction after homogeneous cathodic charging, were calibrated versus the SKP potential at 57% RH (0.16 kPa H2O, 20.26 kPa O2, 79.03 kPa N2, and 0.55 kPa residual gas species) [1]. Calibration enabled estimation of CH,diff near pitted surfaces. Cross-sections of corrosion damage sites after pitting were also examined to determine the distribution of the CH,diff perpendicular to the exposure surface.

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