Cathodic hydrogenation of very high strength steels - Influence of arsenic as a hydrogenation promoter
Résumé
ydrogen embrittlement mechanisms of two very high strength steels (VHSS) with ultimate tensile strength reaching 1000 MPa were studied in severe hydrogen charging conditions. These VHSS are a TRIP steel (A) and a Cr-V enriched high strength steel (B). They were submitted to cathodic charging in a sulfuric acid 0.1 N solution with an addition of 5 mg L-1 of As2O3, as a promoter of hydrogen absorption. Electrochemical permeation technique and subsequent hydrogen dosage by thermal desorption tests provided values for hydrogen diffusion characteristics. A and B present diffusivity of respectively 2.6 10-11 m2 s-1 and 2.2 10-10 m2 s-1. Blistering occurred at high charging currents, but residual hydrogen content appeared to be very low for both steels. The results are attributed to the fine homogeneous microstructures with very few lattice defects like inclusions, precipitations, impurities or microcracks that are the most potential sites for H trapping. Otherwise, ordinary tensile tests performed on hydrogenated steel show higher loss of ductility in the presence of arsenic. Fracture surface observations suggest brittle fractures: transgranular for A and intergranular for B. Diffusible hydrogen is to be incriminated in the loss of plasticity since diffusion to crack tips is easy in these steels. All the results highlight As2O3 role in enhancing the steels susceptibility to hydrogen embrittlement.