The Preparation And Characteristics Of A Rare Earth Coating On Aluminum Alloy By Brush Plating

Various surface techniques are developed and applied on aluminumalloys in order to improve the corrosion resistance. The rare earthconversion coatings as environmentally friendly alternatives have beeninvestigated by many authors, to replace the chromate conversion coatingsfor the protection of aluminum alloys. However, the rare earth conversioncoatings are generally prepared by chemical conversion orelectrodeposition processes, which are diffcult to be applied on largesurface areas of aluminum alloys in the industrial field. This is because thechemical conversion or electrodeposition processes takes long time and thesolution is unstable and hard to be recycled. Therefore, the aims of thisstudy are to develop a new method to prepare the cerium based coating andthe nano-TiO₂modified rare earth composite coating on aluminum alloy.A cerium based rare earth coating was prepared on LY12aluminumalloy by brush plating. The obtained Ce coating showed layered structure,adhered well on the substrate with a3⁴μm thickness. The surface of thecoating was mainly composed of Ce（OH）₃. The influences of Ce salt concentration and brush plating voltage on the coating anti-corrosionproperty in NaCl solution were investigated, and the best corrosionresistance was obtained under the condition of20g/L Ce concentration and7V brush plating voltage. The corrosion current density decreased by oneorder of magnitude and the impedance value of the low frequency（0.01⁰.1Hz） increased by about30times compared with the values for theoriginal LY12Al alloy. After480h of neutral salt spray testing, the brushplated coating for the protection of aluminum alloy was estimated to beabove8grade. During the bursh plating process, the developed platingsolution is more stable without containing the oxidants and may be easilyrecycled. In addition, the brush plating technology may be applied on largeareas. Therefore this technology provides a possibility to protect the surfaceof large structures such as aluminum alloy ships and vehicles.A nano-TiO₂modified rare earth composite coating was prepared onthe surface of LY12aluminum alloy by brush plating in order to furtherimprove the self-adhesive strength and the corrosion resistance of the rareearth coating. The composite coating obtained under the condition of20g/L Ce concentration,5V plating voltage and4g/L TiO₂concentration wasuniform and compact, and showed good adhesive strength to the substratewith the3.2μm thickness. The grain size of the coating was only2.6nm.The composite coating was composed of mainly Ce（OH）₃and smallamount of Ce（OH）₄, CeO₂and TiO₂（2%）. Besides, the addition of nano-TiO₂particles results in the decrease of Ce⁴⁺ content and the increaseof Ce³⁺ content in the coating. The Ce（OH）3content of the compositecoating increase to almost90%. When the nano-TiO₂content in the platingsolution increase from2to8g/L, the TiO₂content in the composite coatingincrease from1.22to4.13wt.%.The addition of nano-TiO₂results in refined coating grains andenhances the self-adhesive strength of the rare earth coating. Themicro-hardness of the coating increases and the crack percentage and theporosity of the rare earth coating decreases due to the effects of adding thenano particles. After480h of neutral salt spray testing, no obviouscorrosion could be observed on the plated coating surface, and thenano-TiO₂modified rare earth composite coating for the protection ofaluminum alloy was estimated to be10grade. But, the corrosion was foundin the surface of the pure rare earth coating after240h of neutral salt spraytesting. And the corrosion current density decreased by two orders ofmagnitude compared with the values for the pure rare earth coating.