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Duplex (DSS) and austenitic stainless steels (ASS) are frequently used in many energy related applications. The duplex grade is considered to have outstanding mechanical properties as well as good corrosion resistance. The austenitic phase combines high ductility, even at low temperatures, with sufficient strength, and therefore such materials are applied in storage and transport of high-pressure hydrogen. During service in acidic environments large amounts of hydrogen can ingress into the microstructure and induce many changes in the mechanical properties of the steel. Embrittlement of steels by hydrogen remains unclear even though this topic has been intensively studied for several decades. The reason for that lies in the inability to validate the proposed theoretical models in the sub-micron scale. Among the very few available methods nowadays, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) enables a highly accurate mapping of hydrogen in the microstructure in a spatial resolution below 100 nm. In the present work ToF-SIMS was used as a main tool in order to investigate the effect of deuterium on a duplex microstructure of lean and standard DSSs during and after the electrochemical charging process. Electrochemical charging simulates the service of a component in acidic environments under conditions of cathodic protection that are commonly applied to prevent corrosion reactions. ToF-SIMS after multivariate data analysis (MVA) was combined with high resolution topographic images and electron back-scattered diffraction (EBSD) data to characterize the structural changes. It was observed that the ferritic phase was affected almost identical in all steels whereas in the austenitic phase significant differences were obtained in the lean duplex in comparison to the standard DSS. The obtained results have been compared to similar investigations on a AISI 304L austenitic stainless steel. The advantage of the combined techniques is reflected by the ability to correlate the hydrogen distribution in the microstructure and the resulted phase transformation.
Corrosion fatigue (CF) of duplex stainless steel (DSS) X2CrNiMoN22-5-3 was investigated in in-situ geothermal conditions of the Northern German Basin (NGB). The influence of the load type on CF behavior of the alloy was examined by applying push/pull and rotating bending cyclic load to failure. The hour-glass shaped specimens with both polished and precisely turned surface finish were tested in corrosion chambers allowing for the permanent circulation of Northern German Basin electrolyte heated to 369 K. Additionally, the influence of a protective cathodic potential on the CF life expectancy was investigated by employing a potentiostat. The rotating bending cyclic load led to a higher CF life expectancy, which decreased with increasing duration of the experiment due to a higher deterioration rate of the material. The application of a cathodic potential caused a significant increase of the CF lifespan from 4.7 x 105 (open-circuit potential (OCP)) to 107 cycles (preset threshold cycle number) for the potential range from USHE = – 450 to – 900 mV. Microstructural analysis of the CF damage revealed horizontal grain attack within corrosion pit cavities, multiple fatigue cracks and preferable deterioration of the austenitic phase.
High resolution ToF-SIMS imaging of deuterium permeation and cracking in duplex stainless steels
(2017)
Fundamental understanding and elucidation of hydrogen assisted degradation and trapping mechanisms is dependent on sufficient imaging techniques for respective hydrogen interactions, in particular with multi-phase metallic microstructures. The present work shows the progress in elucidating the deuterium behavior in austenitic-ferritic duplex stainless steels under the consideration that deuterium behaves in many ways similarly to hydrogen. A novel combination of deuterium permeation and in-situ Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) imaging technique is compared with post charging ToF-SIMS imaging experiments. As a step beyond state-of-the-art, integration of chemo-metric and high resolution structural characterization techniques with computational multivariate data analysis (MVA) and data fusion is presented.