Abstract
The effects of hydrogen in metals are a pressing issue causing severe economic losses due to material deterioration by hydrogen embrittlement. A crucial understanding of the interactions of hydrogen with different microstructure features can be reached by nanoindentation due to the small volumes probed. Even more, in situ testing while charging the sample with hydrogen prevents the formation of concentration gradients due to hydrogen desorption. Two custom electrochemical cells for in situ testing were built in-house to charge the sample with hydrogen during nanoindentation: “front-side” charging with the sample and the indenter tip immersed into the electrolyte, and “back-side” charging where the analyzed region is never in contact with the solution. During front-side charging, surface degradation often occurs which also negatively influences analyses after hydrogen charging. The back-side charging approach proposed in this work is a promising technique for studying in situ the effects of hydrogen in alloys under mechanical loads, while completely excluding the influence of the electrolyte on the nanoindented surface. Hydrogen diffusion from the charged back-side toward the testing surface is here demonstrated by Kelvin probe measurements in ferritic FeCr alloys, used as a case study due to the high mobility of hydrogen in the bcc lattice. During nanoindentation, a reduction on the shear stress necessary for dislocations nucleation due to hydrogen was observed using both setups; however, the quantitative data differs and a contradictory behavior was found in hardness measurements. Finally, some guidelines for the use of both approaches and a summary of their advantages and disadvantages are presented.Graphical abstract
Highlights
Hydrogen represents a genuine opportunity for the generation of low-carbon-emission energy with the development of fuel cells [1, 2]
The indented region can be considered dislocation-free in the analyzed volume, as the dislocation density of the annealed Fe–Cr, revealed by chemical etching, is lower than 1012 m-2
Note that dislocation nucleation can occur under the dislocation-free volume in the material below the indenter tip at the location of highest shear stress or as well at surface asperities, where local stress concentrations assist in dislocation nucleation
Summary
Hydrogen represents a genuine opportunity for the generation of low-carbon-emission energy with the development of fuel cells [1, 2]. In these processes, numerous factors are involved (material characteristics, mechanical state, environmental conditions), and defining a single failure mechanism is not possible. Numerous factors are involved (material characteristics, mechanical state, environmental conditions), and defining a single failure mechanism is not possible This is why often collaborative, complementary, competing or contradictory mechanisms have been proposed [6,7,8,9,10]
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