Nanoscratching characterization of austenitic stainless steel modified by cathodic hydrogenation

The austenitic stainless steels find a great variety of applications in hydrogen atmospheres, which can be in the gaseous atmospheres at high pressure or in liquid atmospheres like solutions with clorets and sulfuric acid. The AISI 304 austenitic stainless steels are the most commonly used for these applications. When these materials are cathodically charged at room temperature, some surface effects are observed: numerous cracks are nucleated on the surface of the material, and martensitic phase transformations occur during hydrogenation and outgassing process. The depth of the surface modified by hydrogen (H) is of the order of a few microns. The objective of this work is to analyze the effects induced by H in an austenitic stainless steel using nanoscratch testing. It is an important technique because it permits one to evaluate thin regions in the range from micrometers to nanometers. Nanoscratch testing is used to verify whether H alters the plastic and elastic behavior, to compare the scratch morphologies in regions with many cracks and regions without cracks, and to give an estimate of the thickness of the affected region by H through the observation of the cracks into the scratches. The results show that the scratch morphology depends strongly on crack distribution. A lateral deviation in the scratch occurs in the grains with many elongated cracks parallel to the scratch direction. This is correlated with the grain orientation, which indicates the necessity to analyze the microstructural aspects in order to understand the fracture behavior induced by hydrogen in these materials. This technique can contribute to enhance the understanding of the H embrittlement problem in austenitic stainless steels and can be extended to analyze the failure mechanisms of the other materials.