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A 16Cr5NiMo supermartensitic stainless steel was subjected to different tempering treatments and analyzed by means of permeation tests and slow strain rate tests to investigate the effect of different amounts of retained austenite on its hydrogen embrittlement susceptibility. The 16Cr5NiMo steel class is characterized by a very low carbon content. It is the new variant of 13Cr4Ni. These steels are used in many applications, for example, compressors for sour environments, offshore piping, naval propellers, aircraft components and subsea applications. The typical microstructure is a soft-tempered martensite very close to a body-centered cubic, with a retained austenite fraction and limited δ ferrite phase. Supermartensitic stainless steels have high mechanical properties, together with good weldability and corrosion resistance. The amount of retained austenite is useful to increase low temperature toughness and stress corrosion cracking resistance. Experimental techniques allowed us to evaluate diffusion coefficients and the mechanical behaviour of metals in stress corrosion cracking (SCC) conditions.

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The European Green Deal aims to achieve the climate neutrality in Europe by 2050; furthermore, an intermediate target fixes a first reduction of greenhouse gas emissions by minimum 55% within 2030, with reference to 1990 levels. In these contexts, and in particular in hard-to-abate processes as steelmaking, hydrogen could be a key instrument to achieve the above-mentioned sustainability targets. It can be produced from renewables and does not produce direct CO2 emission when used. On the other side, one of the main issues related to hydrogen use is the fact that it involves a certain risk linked to potential susceptibility of steels to hydrogen embrittlement phenomenon that could also lead to catastrophic failures. Suitable materials should be used in these applications by performing a prior evaluation of material susceptibility. The characterization consists in determining the behaviour of materials when exposed to hydrogen atmosphere, in order to determine a hydrogen threshold value for failure. To approach this theme, a first experimental was carried out on steels typically applied in oil and gas industry. Experimental included hydrogen diffusivity, solubility and mechanical tests. Moreover, electrochemical charging was compared to gaseous charging, confirming the first methodology is much more severe and thus not very significant for high-pressure gas applications. Furthermore, a continuous monitoring technique for hydrogen embrittlement risk assessment is under development. The methodology is based on high-resolution gas sensors used to measure the hydrogen flux permeating through the metal wall of transport and storage components. Relative signal can be correlated with the material susceptibility to hydrogen embrittlement in order to assess related failure risk. The device was tried on a pressure vessel containing pure hydrogen to assess the system’s sensitivity.

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