CORROSION PROTECTION OF COLD-ROLLED STEEL BY LOW-TEMPERATURE PLASMA INTERFACE ENGINEERING .2. EFFECTS OF OXIDES ON CORROSION PERFORMANCE OFE-COATED STEELS

The effects of oxides on the interfacial chemistry and on the corrosio n performance of cathodic E-coat/cathodic plasma polymer of trimethyls ilane (TMS)/cathodic plasma treated cold-rolled steel (CRS) systems we re 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 abse nce of oxides. The cathodic plasma treatment of CRS surface by (Ar + H -2) plasma removes oxides, and the subsequent in situ cathodic polymer ization of TMS provides the plasma polymer of TMS/(oxide-free) steel s ystem, in which Fe-C bonds, but no Fe-O bond, are found. The depositio n 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 moiet ies which exist in the surface state of CRS (including plasma polymer layer and oxides, etc.) are subjected to cathodic reduction. The catho dic reduction could create a weak boundary or defective spots in the i nterface, which adversely affect the corrosion protecting ability of t he coated system. The cathodic E-coat applied directly on the oxide-re moved surface of CRS (without Zn phosphate or plasma polymer) showed a better corrosion performance than that applied on the Zn phosphate-ch romate CRS, whereas the direct application on the surface with oxides delaminated in the corrosion (GM scab) test. The best result (better t han E-coat/Zn phosphate-chromate/EGS) was obtained for E-coat/cathodic polymer of TMS/oxide-free CRS. The corrosion performance of this syst em was not adversely affected when lead was removed from the E-coat. A cathodic E-coat performs best when the adverse effect of cathodic red uction is minimized. (C) 1997 Elsevier Science S.A.