S. Evers,1 T. Hickel,2 M. Koyama,3 R. Nazarov,2 M. Rohwerder,1 J. Neugebauer,2 D. Raabe,3 M. Stratmann 1
1 Department of Interface Chemistry and Surface Science
2 Department of Computational Materials Design
3 Department of Microstructure Physics and Alloy Design
Hydrogen atoms, which can be absorbed into steel during production and service, often have a detrimental embrittling effect on the mechanical properties of iron and steels. It is meanwhile known that hydrogen embrittlement (HE) is also affected by the microstructure of the material. Consequently, previous indications that hydrogen atoms are trapped by vacancies, dislocations, and grain boundaries led at MPIE to investigations of superabundant vacancy formation, hydrogen-enhanced local plasticity (HELP), and hydrogen-enhanced decohesion (HEDE). Despite these efforts, any proof of a HE mechanism to be active in a given steel sample has so far been a formidable task, which cannot be achieved by a single method. A direct experimental observation of hydrogen impurities is difficult due to the low solubility and high mobility of hydrogen in steels, whereas pure theoretical investigations are challenged by the complexity and diversity of microstructural features present in steels.
We therefore follow a multidisciplinary strategy to derive a deeper understanding of HE in steels.
This strategy combines novel potentiometric methods based on the Kelvin probe technique to detect the local hydrogen content in materials (GO department), ab initio determination of the same quantities including the local behaviour at grain boundaries (CM department), and characterization of hydrogen induced materials failure (MA department) using orientation-optimized electron channelling contrast imaging (ECCI). Selected findings of these investigations and their relevance for austenitic steels are summarized in the following:
The crucial idea for the new hydrogen detection method is the observation that hydrogen dissolved in a palladium matrix leads to the formation of a hydrogen electrode on the palladium surface, even in dry atmospheres. The origin is the presence of a nanoscopic water layer adsorbed on the surface, enabling the formation of a corresponding electrochemical double layer [1, 2]. As the electrode potential for the hydrogen electrode depends logarithmically on the activity of H in Pd, this potentiometric method is extremely sensitive especially at low activities.
The idea can be employed for the investigation of steels (and various other materials) by evaporating a thin film of Pd on their surface. Since the chemical potential of H in iron-based materials is much higher than in Pd, H diffuses into the Pd film. Time dependent measurements of this accumulation can be used to perform extremely sensitive and laterally resolved measurements of H permeation through and its presence in materials. In the latter case an effective “activity” of H is measured, providing information about depth and density of traps sites. Main challenges of this method are the need for an exact calibration of the potential-concentration correlation for H in the evaporated Pd films, the precise calibration of the Kelvin probe tip in the dry nitrogen measurement atmosphere, as well as its long term stability.