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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.

In order to clarify the effect of design and materials of the hem as well as the climatic factors on perforation corrosion of the automobile doors, field car and laboratory investigation has been carried out Field car investigation revealed that corrosion of the hem can be minimized by using two side galvanized steel plus adhesives. The ratio of wet/dry environment was evaluated in laboratory on hemmed sample, and it was found that the design of the hem in conjunction with the various wet/dry ratio affected the corrosion rate differently.

Water penetration into the paint film of car bodies causes cosmetic corrosion. The circumstances experienced by driven cars can be classified into three groups, corresponding to high, medium and low water migration, respectively. In the first group a lot of water enters into the paint/substrate interface and lifts up the film. The stability of the phosphate layer influences the adhesiveness. In the second group salt water brings about alkaline blister formation. The corrosion resistance of the substrate against alkali in cathodically polarized state determines the paint adhesion. The third group corresponds to the scab corrosion, which can be suppressed well by zinc coating layer.

The corrosion behavior of cold rolled-, galvanized and galvannealed steel sheet was investigated. Unpainted, ED painted and three-coat painted specimens were used. Cataphoretic primer prevents corrosion on the underlying steel more effectively than an anaphoretic primer. Cathodically electropainted cold rolled steel exhibits a good film adhesion in all tests. In alternately wet and dry conditions cathodically electrodeposited zinc plated steels show a better adhesion property than anodically electrodeposited ones. Upon consideration of actual car driving conditions, it was concluded that zinc plated steels, as well as cold rolled steel, are suitable for cathodic painting system.

The corrosion behavior of an automobile body caused by de-icing salt was classified into various corrosion phenomena, of which paint exfoliation and perforation were studied fundamentally. There are 2 types of paint exfoliation. One is paint adhesion, where underfilm corrosion plays a decisive role. Another is wet adhesion, where water immersion through the paint film into the paint/substrate interface is important. Perforation corrosion can be simulated by corrosion test using lapped panel specimens. CCT conditions which should be applied for all exposure tests were determined on the basis of experimental data.

For the accumulation of knowledge of perforation corrosion, three doors of three field vehicles made with one-side galvanized steel on the inside of the outer panels were investigated with some microscopic and analytical methods after 8 or 10 years’ field running in the snow-belt areas of North America. For these doors, perforations were found within the lapped part of the door hems. Outer panel perforation began as zinc layer corrosion of the inside surface of the outer panel at the lapped part and/or as “bent part” corrosion of the outer panel at the door hem. The inside of the outer panel lightly corroded at the open part in spite of the lack of primer on the inside surfaces of the outer and inner panels. The corrosion behavior of a lapped panel specimen test was studied and compared with that of investigated door hem corrosion. In the case of these doors, the test provided a result having a fairly good agreement with the field vehicle doors’ behavior.

Corrosion protection mechanism was investigated on newly developed-metal-finished steel sheets for automobile applications. Such alloying elements to zinc as Ni, Co, Mn, Mg and Al retard oxygen reduction reaction because of their stabilizing effect of Zn(OH)2 films, leading to improved corrosion resistance. Wet adhesion of cathodic primed full painted system is governed by alkali-induced dissolution of phosphate and zinc coatings during electrocoating. Fe in coating layer improves wet adhesion because of its high alkali-resistance. Both Fe and Ni have a pore blocking effect on phosphate films, with resultant a superior paint performance. The factors affecting perforation of jointed panels were also discussed.