In situ spectroscopic and corrosion studies of ultra-thin gradient plasma polymer layers on zinc

By means of an audio frequency plasma polymerisation ultra-thin gradient plasma polymer layers were deposited on zinc and zinc-coated iron. The aim was to generate an interfacial polymeric layer which bonds to an oxidised metal as well as to a subsequently applied organic coating and acts as an interfacial barrier layer for ions and water. Surface modifications were done in an in situ plasma cell with infrared reflection absorption spectroscopy (IRRAS). The zinc surface was first activated by an oxygen plasma to provide a freshly oxidised and contamination free oxide surface. The intermediate stages of the surface reactions could be revealed. Carbon dioxide molecules as oxidation products adsorbed on the growing zinc oxide and were desorbed at a later stage. An organosilicon plasma polymer was deposited directly on top of the oxide layer from a hexamethyldisilane (HMDS) plasma. Afterwards a cyclohexene (CHEX)/hexamethyldisilane co-plasma polymer was deposited. The top plasma polymer layer was deposited from a pure cyclohexene plasma. The resulting polyethylene-like surface film was subsequently plasma oxidised. This led to a surface with incorporated oxygen containing polar groups. Complementary X-ray photoelectron spectroscopy (XPS) measurements confirmed the vertical gradient in the chemical composition of the layer. The overall thickness of the ultra-thin gradient layers was less than 10 nm. The stability of the gradient layer in corrosive environments was studied by means of the Scanning Kelvin Probe (SKP). A model organic polymer was deposited onto the plasma modified zinc surface and the delamination kinetics were measured as a function of time. The ultra-thin plasma polymer layer led to a significant increase of the corrosion resistance.