Investigation Of Preparation And Formation Mechanism Of Zirconium-based Conversion Coating On Aluminum Alloy

Aluminum alloy is always treated by a chemical conversion coating to fulfill many industrial requirements due to the poor corrosion resistance in aggressive environments and the poor adhesion of the organic coating to the aluminum alloy surface provided by the natural oxide layer formed in air.Even though chromate conversion coating could dramatically improve the corrosion resistance of aluminum alloy,the toxicity and carcinogenicity of Cr?VI?drive scientists to find out an non-chromate conversion process.In the present work,the zirconium-based conversion coating as the alternative to chromate conversion coating composes of the major salts?K2Zr F6 and Na H2PO4?,the accelerant?Na F?and the p H modifier?H3PO4?.The alkaline washing and acid pickling pretreatments will be applied on aluminum alloy sample before the coating formation.A chromium-free zirconium-based conversion coating was prepared on AA5083.The Zr CC improved the corrosion resistance of AA5083 and the adhesion of epoxy resin significantly,the performance of which was much better than that of the commercial chromium-free treatment and close to that of commercial chromate conversion coating treatment.The zirconium-based conversion coating displayed a three-layer structure with a thickness of approximately 1.5 ?m,including an inner layer at the alloy/coating interface,a thicker middle layer with nodular particles and an outer layer on the top.Formation of the zirconium-based conversion coating proceeded via the stacking of nodular particles layer upon layer,the composition of which was mainly amorphous Na3 Al F6,Zr?HPO4?2·H2O,Zr O2 and Al PO4.The zirconium-based conversion coating with the special three-layer structure provided good protection despite the absence of a “self-healing” ability.The effect of different pretreatments to the corrosion resistance of zirconium-based conversion coating was detailedly investigated on AA6061.Firstly,the effect of alkaline washing was confirmed by altering the treatment time.The results revealed that the alkaline washing pretreatment was necessary and the shorter treatment time was in favour of enhancing the corrosion resistance of zirconium-based conversion coating.Secondly,the nitric acid pickling pretreatment was carried out after different alkaline washing time,which could markedly improve the corrosion resistance of zirconium-based conversion coating.Moreover,a higher concentration nitric acid pretreatment contributed to the coating formation with a better corrosion resistance,resulting from the investigation results of the constant alkaline washing time and different concertrations niric acid pickling.Finally,the pretreatments including hydrofluoric acid would sharply reduce the corrosion resistance of zirconium-based conversion coating.In the present work,the relatively optimal pretreatment process was confirmed,including alkaline washing by 1 mol/L Na OH for 20 s and acid pickling by 50 m L/L HNO₃ for 600 s.The influence on the distribution of surface elements,roughness and potential was further studied after different pretreatments,combaining with chemical and electrochemical reactions,reaction rate,the variation of electrochemical behavior and evolution of microscopic morphologies during the coating formation process.On this basis,the pretreatment mechanism model was established.With the alkaline washing time increasing,a corrosion product coating gradually formed on the surface of aluminum alloy,and the height and potential differences between intermetallics and matrix phase gradually increased and decreased,respectively.However,the corrosion resistance of zirconium-based conversion coating was reduced due to the increasing defect number.Additionally,the employment of the nitric acid pickling pretreatment enhanced the potential differences between intermetallics and matrix phase.The potential differences and thickness of passive film gradually increased with the nitric acid concentration increasing.A compact and uniform zirconium-based conversion coating with a better corrosion resistance would form on aluminum alloy with a bigger particle size after nitric acid pickling pretreatment.However,the surface feature of aluminum alloy was observably altered after the pretreatment including hydrofluoric,including the increase of the surface roughness,the decrease of potential difference,or the separation of Cu,and the corrosion resistance of the corresponding zirconium-based conversion coating with a higher formation reaction rate was reduced due to the existence of many defects.