Corrosion Protection of Galvanized Steel Sheet-Corrosion Investigation of Field Vehicle

Three doors from three field vehicles made with one-side galvanized steel on the inside of the outer panels were investigated for perforation corrosion using microscopic and analytical methods after 8 or 10 years' field exposure in the snow-belt areas of North America. Perforation of these doors occurred within the lapped part of the door hems. Outer panel perforation began at the zinc layer on the inside surface of the outer panel at the lapped part and/or at the “bent part” of the outer panel of the door hems. Details of the micro corrosion behavior for the inside surface of the outer panel were made clear using EPMA analyses. The corrosion products on the inside surface of the outer panel were identified with X-ray diffraction.

Initially, zinc corrosion begins at the zinc coating layer of the upper side at the wax-free zone of the lapped part. Eventually zinc corrosion occurred beneath the wax layer as “Under-film corrosion”, proceeding gradually from the bottom to the top of the lapped part. The zinc is transformed into ZnO from the protective ZnCl₂·4Zn(OH)₂ initial product. Eventually, the steel substrate corrodes after the zinc coating loses its sacrificial corrosion protection. The steel is transformed into Fe₃O₄ and α, γ-FeOOH. It was found that this corrosion behavior was a common phenomena for the observed door hems.