| Popis: |
Thin protective zirconium oxide coatings prepared via sol-gel with zirconium propoxide and acetyl acetone (acac) or hydroxypropyl cellulose as stabilizers have been successfully deposited onto iron substrates at low temperature. Electrochemical current density vs. potential scans showed a reduction of the iron dissolution current density and coating porosity in comparison to uncoated iron, with a strong decrease of these parameters around 5 orders of magnitude for a HPC concentration = 0.004 g/l. This result is expected as the steric effect of HPC. A mixed oxide layer of zirconium and iron was observed in SIMS depth profiles. The mixed oxide layer thickness showed a dependence on the acac molar ratio. Increases of acac shorted the overall film thickness, while the mixed oxide layer was thicker. ZrO2-HPC coatings displayed however, a thicker mixed oxide layer and additional increases of the zirconium intensity direct onto substrate surface. So a higher interdiffusion of zirconium into deeper layers and of iron towards coating surface was reached. XRD revealed that the mixed oxide layer is localized between the substrate and coating surface and possess crystalline structure. At a HPC concentration = 0.004 g/l no ZrO2 reflexes could be detected. ZrO2 crystals of 70 nm size were identified by TEM in ZrO2-acac coatings. However, ZrO2-HPC films showed an amorphous structure constituted of ZrO2 and iron oxide containing nanocrystalline zirconium oxide particles immersed in the mixed oxide layer. The crystals (monoclinic 112) size was between 5 and 10 nm. This result explains why there were not zirconium oxide reflexes visible in the XRD analysis. Obviously the ZrO2 crystalline particles are too small (less than 10 nm) to be detected by XRD so they appear as amorphous material [83]. The particles are immersed in a compact and amorphous mixed oxide matrix. The correlation between growth of the mixed oxide phase and decrease of dissolution current density suggests that it is the mixed oxide phase with zirconium nanoparticles that is responsible for the reduction of film porosity and the good corrosion protection properties of the films. |
