MORPHOLOGY OF PHOSPHATE LAYER INVESTIGATED BY VACUUM DEPOSITING A PARYLENE-C FILM

Poly(monochloro-para-xylylene), or parylene C, was used to investigate the morphology of phosphates deposited on cold-rolled steel (CRS) and galvanized cold-rolled steel (GCRS). Vacuum deposition polymerization of parylene coating enables the polymer to penetrate deep into the po rous structure if such spaces are available in the substrate where par ylene C is deposited. The morphological difference between zinc phosph ate (Zn-2[PO4](2)) deposited on CRS and that deposited on GCRS was inv estigated by subjecting a parylene C-coated phosphated steel to the Ge neral Motors scab test and examining the delaminated surface of the pa rylene C film (which becomes a negative replica of the structure of th e phosphate layer that is removed). Zn-3(PO4)(2) on GCRS was porous an d provided an excellent anchoring base for adhesion of a parylene C la yer. Because of excellent adhesion, parylene C-coated GCRS showed exce llent corrosion protection. Zn-3(PO4)(2) on CRS provided a rough surfa ce, but the layer was not porous and did nor provide good adhesion of She parylene C layer, Depositing a thin layer of plasma polymer on She surface of Zn-3(PO4)(2) on CRS improved adhesion of parylene C and yi elded excellent corrosion protection. Three major steps were necessary : cleaning of the phosphate surface by oxygen plasma, deposition of a plasma polymer with good adhesion to the Zn-3(PO4)(2) surface to hinde r dissolution of the Zn-3(PO4)(2) layer and deposition of a second pla sma polymer that provided free radical binding sites for parylene C ad hesion.