Study On Charactorization And Corrosion Resistance Of Rare Earth Coatings For Surface Of Cf/Al Composites

In recent years, carbon fibers reinforced aluminum matrix (Cf/Al) composites are applied in some fields such as aerial material and space technology. It is important to improve corrosion resistance for this composite. For the purpose of improving corrosion resistance of Cf/Al composites, two coatings were prepared by rare earth conversion coating and Ce-rich complex films. The fabrication processes, microstructure and corrosion properties of two coatings were examined, their influencing factors were discussed, the deposition and corrosion resistance mechanics were studied in this paper.It was found that the Ce conversion coating covered the whole surface of Cf/Al composites as oxidized islands, but some micro-cracks for the coating. The Ce conversion coating was island-like, and it was piled up by spherical Ce-riched particles. The size of Ce-riched particles was about 4nm~8nm, the multiple Ce-rich compounds were mixing in Ce-rich particles, and particles clustering. The thickness of Ce conversion coating is about 3μm, Ce oxides and Ce hydroxides are main composition of Ce conversion coating, which containing Ce3+ and Ce4+ compounds. The little Al compounds in this coating, it is indicated that Al dissolved during processes of formation films.Uniform Ni-P coating on the surface of Cf/Al composites can be realized by electroless deposition, the catalytic active points for chemical deposition on the Al matrix promoted Ni-P deposition; chemical and physical absorption on the surface of carbon fibers were main reasons for catalytic active points in order to deposit Ni-P alloys. Improvement of corrosion resistance for Ni-P coatings on multi-phases surfaces of Cf/Al composites came from amorphous composition which has insensitive to corrosion reaction in Ni-P coatings, because there were less grain boundary in amorphous films. Moreover, Ni-P coatings can avoid direct touching between corrosion medium and surface of Cf/Al composites. Synergistic effect of them can improve corrosion resistance on Cf/Al composites.Mechanism of rare earth conversion coating on multi-phases surface of Cf/Al composites is stated that"Mechanism of micro-cathodic deposition"was main factor on the Al matrix, on the other hand, chemical adsorption or physical adsorption was main formed Ce-rich coatings on surface of carbon fibers, therefore, the interactivity"Dangling bonds"on the surface of carbon fibers and"Surplus chemical bonds"belonged to rare earth elements had effect of depositing Ce-rich coating. The behavior of Ce compounds deposition occurred near defects of carbon surface and this characteristic could heal the scratch on carbon surface; this phenomenon can be named"self-healing"on the carbon surface.The Ce-rich complex films are high condensing coatings which could obtain by Ni-P coatings with rare earth compounds sealing. There are few cracks in Ce-rich complex films and conforming layers. Image of potential-pH combined with XPS analysis can be indicated that the Ce4+ species were main components of Ce compounds in Ce-rich complex films. Microstructure characteristic of complex films is obvious a nano-crystal structure, additionally, the Ce compounds deposit on the sites of micro-porous on the Ni-P coatings,"sealing-effects"is main causes for improving corrosion resistance of Cf/Al composites.The contrasts of microstructure characterization after immersion testing at the same conditions showed that the corrosion degree of complex films were slightly, the microstructure of coatings changed a little; the Ce conversion coatings had slight loss of color, some evidence supported the existence of coatings exfoliation; the more erosion products adhesive on the electroless plating Ni-P coatings; the carbon fibers had obvious bareness on uncoating samples. The corrosion potential (Ecorr) of complex films has improved more than 400mV as contrast on uncoating samples, the corrosion current density (icorr) of complex films have decreased 1.32μA·cm-2 than uncoating samples. The anodic and cathodic branches of polarization curves of Ce conversion coating were both decreased obviously, the corrosion reaction from anode and cathode were restrained at the same time.The results of electrochemistry impedance spectra (EIS) indicated that the corrosion resistance of different coatings listed as follows: Complex films > Ni-P coatings > Ce conversion coatings > uncoatings. From EIS simply analysis, the impedance values of polarization resistance for complex films was 23051Ω, which improved 8 times than uncoating samples.The main reasons for inhibition corrosion came from contents of amorphous structure in Ni-P coatings and rare earth conversion coatings, which structure is insensitive to corrosion reaction; in addition, the coatings inhibit electrochemical reaction. Ce-rich complex films have the best corrosion resistance because it has formed"sealing effects". Moreover, Ce4+ species deposit surface of Ni-P coatings, there are better oxidation resistance of Ce4+ species than single Ce-rich coating and Ni-P coating.