Abstract
Hydrogen in combination with mechanical stress can lead to rapid degradation of high-strength steels through environmentally assisted cracking mechanisms. The scanning Kelvin probe (SKP) was applied to automotive martensitic steel grade MS1500 in order to detect local reactivity of the surface after hydrogen uptake and tensile deformation. Hydrogen and stress distribution in microstructures can be characterized by SKP indirectly measuring the potential drop in the surface oxide. Thus, the links between electron work function, oxide condition, and subsurface accumulation of hydrogen and stress have to be investigated. It was shown that plastic strain can mechanically break down the oxide film creating active (low potential) locations. Hydrogen effusion from the steel bulk, after cathodic charging in aqueous electrolyte, reduced the surface oxide and also decreased potential. It was shown that surface re-oxidation was delayed as a function of the current density and duration of cathodic hydrogen pre-charging. Thus, potential evolution during exposure in air can characterize the relative amount of subsurface hydrogen. SKP mapping of martensitic microstructure with locally developed residual stress and accumulated hydrogen displayed the lowest potential.
Highlights
Hydrogen directly affects metallic materials by degrading their mechanical properties.Hydrogen-assisted cracking (HAC) limits the application of high-strength steels (HSS) in different branches of industry, such as the transport industry, because their sensitivity to cracking increases proportionally with their strength [1,2,3]
High-strength steel fasteners or automotive high-strength steel sheets may fail due to hydrogen-delayed fracture mechanisms
The results showed that in this case, there was no significant hydrogen amount effused distribution
Summary
Hydrogen directly affects metallic materials by degrading their mechanical properties. Hydrogen-assisted cracking (HAC) limits the application of high-strength steels (HSS) in different branches of industry, such as the transport industry, because their sensitivity to cracking increases proportionally with their strength [1,2,3]. High-strength steel fasteners or automotive high-strength steel sheets may fail due to hydrogen-delayed fracture mechanisms. With the application of tensile stress with a stress intensity factor exceeding a specific threshold, the hydrogen induces subcritical cracks, whose growth leads to failure [4,5,6]. It is well established that to induce HAC, critical diffusible hydrogen content in the HSS, which is a function of local stress, must be reached [7,8,9,10]. The locations which are prone to cracking are close to the corroding surface containing the highest hydrogen concentration (Figure 1) [8]
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