New vapor deposited zinc alloy coatings, which are Zn-Mg, Zn-Al, Zn-Cr, Zn-Ni and Zn-Ti coatings, have been developed by vapor deposition pilot-line.
Morphology of surface structure, depth profile of alloy composition and crystal structure have been studied for these coatings.
Zn-Mg alloy coated steel sheet showed the best corrosion resistance. The corrosion behavior of this coating has been investigted. The coating surface is densely covered with a corrosion product layer composed of ZnC12·4Zn(OH)2, which could play a role as a protective barrier film. The corrosion potential remains active enough to give the substrate galvanic protection for a long time in a corrosive environment.
Attention has been directed to corrosion resistance for automobile body panels in recent years and several types of zinc coated steel sheets has been developed for automotive usage. Especially, zinc alloy coated steel sheets have been developed to increase the life time of the coating layer with galvanic protection for the substrate. Zn-Ni alloy and Zn-Fe alloy electroplated steel sheets, Zn-A1 alloy and galvannealed steel sheets by hot-dipped method are used as commercial corrosion resistant steel sheets. Moreover, the zinc-base alloy coatings in which Mg (1)*, Cr (2), Cd (3), Pb (3), Cu (4), Sb (5) and Mn (6) were added have been investigated.
With regard to the electroplating method, generally, there is the limitation of metals to be deposited in aqueous solution because of hydrogen evolution, and there are many difficulties to co-deposit both zinc and metals whose standard electrode potentials are far from that of zinc. As for the hot-dipped coating method, available metals which can be added to the molten zinc bath are restricted due to the increase of melting point and/or viscosity.
Recently, a new coating method, vapor deposition process, has been of great interest (7)-(8). Vapor deposited pure zinc coating as a corrosion resistant steel sheet has been applied to industrial mass production (9). The study of zinc-base alloy coating by vapor deposition has been limited.
We have investigated zinc alloy coated steel sheets by application of vapor deposition process with electron-beam (hereinafter called EB), which has good possibilities for developing new corrosion resistant steel sheets.
In this paper, an outline of a vapor deposition pilot-line and zinc alloy coating process are described. Properties of zinc alloy coatings, espeically high corrosion resistant Zn-Mg alloy coating, are also discussed.