The durability of weathering bridge steel depends on the development of a rust layer which should protect the steel from chloride transport and ensure good resistance of the steel-rust interface simultaneously. Mechanically induced perturbations affect the function of the rust layer in different ways. Namely, the rust layer is affected differently by local shallow damage, local deep damage, broad shallow damage, and broad severe damage in the presence of the same concentration of chlorides. Here two types of weathering bridge steels (Q420qDZ25 and Q420qDNH) are compared in terms of stage-resolved assessment of rust layer resistance, charge transfer resistance, corrosion potential, corrosion current density, phase composition, and distribution depth of chlorides. There are differences between the two steels both in chemical composition and structure: Q420qDNH has 0.63 wt.\% Cr, 0.36 wt.\% Ni, 0.350 wt.\% Cu, and carbide-free bainitic structure, whereas Q420qDZ25 has 0.35 wt.\% Cr, 0.14 wt.\% Ni, 0.012 wt.\% Cu, and ferrite-pearlite structure. Unperturbed rust layer on Q420qDNH provides \Rr{} = 72.81 \ohmcm{} and \Rct{} = 80.59 \ohmcm{}, whereas for Q420qDZ25 \Rr{} = 45.85 \ohmcm{} and \Rct{} = 53.14 \ohmcm{}. Local shallow damage to the rust layer leads to \Rr{} reduction by 51.7\% for Q420qDNH, although \Rct{} of the former exceeds \Rct{} of the latter. Extensive corrosion changes the ranking: corrosion potential of Q420qDZ25 decreases down to \De{} = 8.381 and \Rct{} = 4.34 \ohmcm{} but that of Q420qDNH remains at the level of \Rct{} = 22.73 \ohmcm{} and \De{} = 5.158. This means that Q420qDNH quickly loses its contributions of the outer-barrier layer whereas Q420qDZ25 is controlled by activation of the steel-rust interface since the rust of the latter contains lots of chlorides and porosity and, therefore, does not provide charge transfer resistance.

Q420 bridge steel,rust-layer damage,chloride wet-dry cycling,weathering steel,electrochemical impedance,charge-transfer resistance,FeOOH