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Hydrogen embrittlement (HE) of 1.4614 and 1.4543 maraging stainless steels is characterized with fine structural TEM-ASTAR, hydrogen electrochemical permeation, thermodesorption and tensile testing (with slow strain rate during H cathodic charging) experiments. Both solutions annealed and 550 °C aged states are studied. When aged, 1.4614 grade contains nanometric hexagonal Ni3Ti precipitates whereas 1.4543 grade contains three families of nanometric precipitates, namely, hexagonal Ni3Ti, non fcc-rich Cu and G-phase rich in Si, Ti and Ni with cube-cube relations with martensite. From the permeation and thermodesorption results, it appears that austenite and nanometric precipitates seem to constitute deep traps for hydrogen in both grades. From tensile tests with in situ H charging, 1.4614 and 1.4543 maraging grades show a high susceptibility to HE with relative reduction of area (RRA) strictly superior to 80% for both metallurgical states. The presence of retained austenite does not seem to bring any HE resistance because of its transformation into martensite during deformation. [ABSTRACT FROM AUTHOR]

Ferritic stainless steel (AISI 441) oxidation was studied at 900 °C in synthetic air. At short time of oxidation (3 min), the oxide scale is comprised of chromia including some MnCr2O4 nodules (of a few tens of nanometers in diameter) and exhibits n-type conduction. Later, after 10 h, a trilayered morphology was observed with an internal equiaxed chromia, an intermediate columnar chromia and a top covering spinel layer. A model for oxide scale growth has been developed in this paper, taking into account all experimental observations and earlier studies. [ABSTRACT FROM AUTHOR]

• Substrate expansion during cooling (ferrite transformation) increased oxide cells. • TEM-ASTAR and Raman allowed to identify a discontinuous Cr 2 O 3 layer. • Mn 2 O 3 and MnCr 2 O 4 spinel form since the beginning of high temperature exposure. • Tetragonal spinel (Mn,Cr) 3 O 4 forms not as a continuous layer, close to cubic spinel. • Silicon oxide formation was not observed after 1 h at 800 °C. FeMnSiCrNi alloys form Mn-rich oxide and Mn/Cr spinel at high temperature. However, studies on the oxidation mechanism are still incipient. Thus, in order to understand the initial oxidation stages of the alloy Fe-17Mn-5Si-10Cr-4Ni-VC at 800 °C, up to 24 h, the oxide scale has been characterized by XR-Diffraction, Raman spectroscopy and TEM-ASTAR. Ferrite expansion during cooling was detected and a new oxidation mechanism was proposed, with simultaneous Mn 2 O 3 (externally) and Mn/Cr spinel (internally) formation since the beginning of oxidation, as also discontinuous Cr 2 O 3 surrounded by spinel. The formation of tetragonal-(Mn,Cr) 3 O 4 was also reinterpreted, highlighting the importance of Mn-diffusion on MnCr 2 O 4. [ABSTRACT FROM AUTHOR]

AISI 441 sheets were oxidized from 4 to 24 h at 800 °C in wet atmosphere. Micrometric oxide nodules containing Ti, Nb, Cr and Mn were observed to form on the passive scale. The internal microstructure of these objects was investigated using STEM–EDX and FIB-SEM tomography. Experimental results reveal a complex microstructure linked with the presence of Si and Nb and their competition for interfacial oxidation. Eventually, the chemical compositions of the nodule and of the oxidation affected zone in the nodule vicinity are discussed in relation with thermodynamic calculations investigating the stability of the different oxides. [ABSTRACT FROM AUTHOR]

The corrosion of titanium alloys in pressurised water at 350 °C and 17.5 MPa was studied in a loop made of stainless steel. The corrosion mechanism involved simultaneous TiO 2 formation and dissolution, which led to surface precipitation of TiO 2 anatase and FeTiO 3 ilmenite crystallites. Based on image analyses made on cross-sections, a minimum corrosion rate, for all tested titanium alloys, was estimated at 4 µm year−1 at 350 °C. Comparison with a previous study indicated an increase of the corrosion rate by an approximate factor of 4 between 300 °C and 350 °C. • Three titanium alloys were exposed to water at 350 °C and 17.5 MPa. • A continuous nanometric TiO 2 layer was formed. • Micrometric anatase and ilmenite crystallites precipitated. • The corrosion rate was superior to 4 µm year−1. [ABSTRACT FROM AUTHOR]

• The nanoscale TiO 2 layer was covered by anatase and ilmenite crystallites. • The thin TiO 2 layer evolution was modelled by a Loriers-Haycock model. • Titanium and ilmenite precipitation ratii were defined and measured. • A phenomenological model was proposed to link mass variation to corrosion rate. • Corrosion rates of all studied materials stand between 2 and 6.6 μm/year at 300 °C. The corrosion of titanium alloys in primary water at 300 °C led to a global weight gain due to oxidation and surface precipitation, despite some dissolution. A model was proposed, taking into account the weight contribution of each oxide type to the global mass evolution with exposure time. Based on this model, on mass variation and GD-OES results, a corrosion rate ranging between 2 and 6.6 μm.year-1 at 300 °C for all tested materials was estimated. Due to the simultaneous corrosion of the used stainless steel reactors, a protective ilmenite layer finally precipitated at the surface of the samples. [ABSTRACT FROM AUTHOR]