Corrosion protection of automotive steel by plasma polymer-electrocoating combined coating system

Plasma polymers were used as adhesion promoters for corrosion prevention. The corrosion performance of plasma polymer-electrocoating (Ecoat) combined coating system is one order of magnitude better than the conventional phosphate-Ecoat system on cold rolled steel (CRS). A scribe creep of 0.3 mm for the plasma polymer Ecoat combination was observed in a four week corrosion test, compared to 3.0 mm for the phosphate Ecoat system on CRS for the same testing period.;An oxygen plasma or an argon-hydrogen mixture plasma pretreatment is essential for adequate adhesion. Both plasma polymerized methane and the plasma polymerized tri-methyl silane (TMS) can be used for corrosion protection. However, plasma polymerized methane is more sensitive to the post-treatment and air exposure.;Superior corrosion resistance for the plasma polymer-Ecoat combined coating on an argon-hydrogen plasma pretreated CRS was achieved through (1) enhancement of Ecoat adhesion by plasma polymerizaton in a DC discharge, (2) formation of highly corrosion resistant species in the polymer-steel interface, and (3) suppression of cathodic polarization during the Ecoat process.;The critical W/FM value for DC plasma polymerized methane and organosilanes to render water insensitive adhesion and satisfactory corrosion performances are ;The chemical composition of the plasma polymer-steel interface on a CRS panel pretreated with an argon-hydrogen mixture plasma did not change during the cathodic Ecoat process. On the other hand, the chemical composition of the plasma polymer-steel interface was changed considerably after the deposition of cathodic Ecoat on an oxygen plasma pretreated CRS panel. Peak oxygen content decreased by about 25% and interfacial iron oxide decreased by about 50% after the cathodic Ecoat process.