Effect Of Pretreatment On Corrosion Behavior Of Al₁₈B₄O₃₃w/6061 Composite Coated By Rare Earth Conversion Coating

The Microstructures and the corrossion properties of the coated composite were studied in detail. The Microstructure of the rare earth conversion coating was investigated by scanning electron microscope (SEM) and transsion electron microscope (TEM). X-ray photoelectron spetrum (XPS) was employed to investigate the elements composition and elements valency of the conversion coating. The corrosion behavior of the composite with conversion coating was evaluated by dynamicpolarization curves and electrochemical impedance spectroscopy (EIS).SEM observation indicated that the morphologies of convesion coating on the composite were different when various processes of pretreatement were selected. A particle-like conversion coating was obtained by pretreatment with HF or NaOH for a short time while a dry-mud-like conversion coating was produced by pretreatment for a long time. When boiling water (BW) pretreatment was used, an opposite phenomenon was observed.TEM observation revealed that both particle-like and dry-mud-like conversion coating had a fine-crystalline microstructure, but the latter was composed of amorphous at a certain degree.XPS analysis indicated that the conversion coatings were composed of cerium oxide and hydroxide. The cerium in the conversion coating exists in the form of: Ce³⁺ and Ce⁴⁺.The high corrosion resistance of coated composite was obtained by pretreated with HF (NaOH) for short time or with BW for long time. The corrosion resistance of the coated composite can be improved significantly by suitable pretreatment.Analysis of EIS revealed that the particle-like conversion coating on the composite can protect the composite effectively, compared with that of dry-mud-like conversion coating.