Protective mechanisms occurring on zinc coated steel cut-edges in immersion conditions

Electrochemical processes occurring on the cut-edge of a galvanized steel immersed in NaCl solutions were studied using numerical simulations, and in situ current and pH profiles measured over the cut-edge. These results clearly demonstrate that only the steel surface remote from the zinc coating is cathodically active, oxygen reduction being strongly inhibited in the vicinity of zinc. This trend was confirmed by local polarization curves recorded on these distinct areas. Ex-situ AES and SEM analysis and cathodic polarization curves in solutions containing Zn2+ ions led to conclude that this cathodic inhibition was related to the fast nucleation of a dense Zn(OH)2 film on the steel surface. After a long term exposure, a new galvanic coupling takes place between the Zn(OH)2 covered area, showing an anodic activity, and the remaining steel surface covered by bulky white zinc corrosion products.