The corrosion behaviour of the martensitic T91 steel and the austenitic AISI 316L steel was analysed. The steels were immersed in stagnant molten Pb–55.2wt%Bi alloy at 823 K for different exposure times (t=550–2000 h). The corrosion tests were carried out both under Ar and under Ar–5%H 2 mixture. Under the oxidising conditions (PO 2 =6×10 −3 Pa), the formation of oxide layers was observed which prevent the penetration of the liquid alloy into the matrix, while under the Ar–5%H 2 mixture (PO 2 =3.2×10 −23 Pa), two phenomena occurred: a ‘reactive penetration’ at the liquid alloy/steel interface and the competition between oxidation and penetration.