Corrosion protection of cold-rolled steel by low temperature plasma interface engineering .2. Effects of oxides on corrosion performance of E-coated steels

The effects of oxides on the interfacial chemistry and on the corrosion performance of cathodic E-coat/cathodic plasma polymer of trimethylsilane (TMS)/cathodic plasma treated cold-rolled steel (CRS) systems were investigated. The depth profile analysis by sputtered neutral mass spectroscopy (SNMS) revealed that the nature of the chemical bonds in the plasma polymer/steel interface depends on the presence or the absence of oxides. The cathodic plasma treatment of CRS surface by (Ar + H-2) plasma removes oxides, and the subsequent in situ cathodic polymerization of TMS provides the plasma polymer of TMS/(oxide-free) steel system, in which Fe-C bonds, but no Fe-O bond, are found. The deposition of the same plasma polymer on O-2 plasma pretreated CRS provides the plasma polymer/oxides/steel system, in which Fe-O, but no Fe-C, bonds are found. During the cathodic E-coat deposition, the reducible moieties which exist in the surface state of CRS (including plasma polymer layer and oxides, etc.) are subjected to cathodic reduction. The cathodic reduction could create a weak boundary or defective spots in the interface, which adversely affect the corrosion protecting ability of the coated system. The cathodic E-coat applied directly on the oxide-removed surface of CRS (without Zn phosphate or plasma polymer) showed a better corrosion performance than that applied on the Zn phosphate-chromate CRS, whereas the direct application on the surface with oxides delaminated in the corrosion (GM scab) test. The best result (better than E-coat/Zn phosphate-chromate/EGS) was obtained for E-coat/cathodic polymer of TMS/oxide-free CRS. The corrosion performance of this system was not adversely affected when lead was removed from the E-coat. A cathodic E-coat performs best when the adverse effect of cathodic reduction is minimized. (C) 1997 Elsevier Science S.A.