Interface-Engineered Parylene C Coating for Corrosion Protection of Cold-Rolled Steel

Vacuum-deposited poly(chloro-p-xylylene), or parylene C, can be used for corrosion protection of cold-rolled steel (CRS) through interface engineering. Interface engineering replaces galvanizing, zinc phosphating, and cathodic electrocoating (E-coating). Parylene C polymer has excellent bulk properties, including its moisture barrier, toughness, and electrical insulation characteristics, but its poor adhesion to most smooth or nonporous substrates has restricted its application. Parylene C coating applied directly to a CRS surface does not provide corrosion protection because there is no adhesion between the film and the substrate. It was shown that adhesion could be improved to the extent parylene-coated CRS panels performed as good as, if not better than, a control (E-coat/zinc phosphate/electrogalvanized steel [EGS]) under conditions of a commercial scab corrosion test. Improvement was achieved by placing an interlayer of plasma polymer to provide interfacial bonding to the CRS surface and the parylene C film. The interface engineering involved in-situ consecutive vacuum processes consisting of: the removal of oxides by (argon + hydrogen) plasma, the direct current (DC) cathodic plasma polymerization of trimethylsilane (TMS), the in-situ cathodic polymerization of methane on the plasma polymer of TMS (in the parylene reactor), and the deposition of parylene C.