| The heterogeneous microstructure of aluminum alloys is a key factor for high strength to weight ratios,making it widely employed in aerospace applications.However,the effect of structural heterogeneity favors different electrochemical reactions that contribute to localized corrosion.For last 30 years,hexavalent chromium salts have been used frequently for corrosion protection and conversion surface treatments of aluminum and its alloys.This method has been restricted by the international regulation authorities due to expose of carcinogenic hexavalent chromium to the environment.Therefore,various alternative technologies have been developed that can substitute the chromate conversion coatings.In the present work,we designed a new active protective coating on the 2024T3 aluminum alloy surface to protect it against localized corrosion.Firstly,the corrosion inhibition effects of lithium carbonate(Li2CO3),2-MBT,8HQ,AMT and tannic acid on AA2024-T3 were evaluated and compared.Electrochemical impedance spectroscopy(EIS)and surface analysis showed that Li2CO3 could provide long-term corrosion inhibition without desorption.Various concave anodic aluminum oxide(AAOs)layers were formed on the 2024-T3 aluminum alloy,and were immersed in lithium carbonate solution to obtain a lithium based protective layer(Li-AAOs).To explore the active protective response,an epoxy coating was further applied onto Li-AAOs and was scratched and measured the response in a 3.5 wt.%NaCl solution.The protection properties of AAO,Li-AAO,scratched AAO/epoxy coating and scratched Li-AAO/epoxy coating were studied and compared.The formation,growth mechanism and leaching of lithium from the lithium based protective layer were explored by EIS and time of flight secondary ion mass spectroscopy(ToF-SIMS).The EIS data showed the lithium leached from the lithium based protective layer and covered the scratched area over the immersion time.The AAO and scratched AAO/epoxy coating systems showed decrease in the corrosion resistance over the time interval.The SEM and TEM morphologies showed that the growth of the lithium protective layer consisted of three regions,columnar,porous and barrier.This layer provided adequate active protection over the long immersion test.The 3D images from ToFSIMS results provided the information about the growth and leaching of lithium over the immersion time at the scratched area.The mass spectra gave a detailed image of the generated compounds,including lithium layered double hydroxide(LiLDH),lithium with pseudoboehmite(Li-PB)and pseudoboehmite(PB).Different voltages of 25 V,30 V and 32 V have been applied on the 2024-T3 aluminum alloy to produce different AAOs layers.The SEM and TEM images of 25 V and 30 V Li-AAOs displayed very similar morphologies,while 32 V Li-AAOs showed different morphology.To explore the active protective response,an epoxy coating was further applied onto Li-AAOs and was scratched and exposed to NSS environment for different time intervals.The electrochemical measurements of 32 V Li-AAO/epoxy coating showed the higher active protective response than 25 V and 30 V Li-AAOs/epoxy coatings.The low impedance value of 32 V LiAAO/epoxy coating at |Z|0.01Hz changed from 0.9 MQ cm2(1 day)to 5.0 MQ cm2(8 day),indicating the active corrosion protection property.Furthermore,the potentiodynamic polarization of 32 V Li-AAO/epoxy coating displayed the lowest corrosion current as compared to 25 V and 30 V Li-AAOs/epoxy coatings.In 32 V Li-AAO,the lithium leached from the complex network of columnar region into the scratched area,whereas 25 V and 30 V Li-AAOs did not show any columnar region.So,the 32 V Li-AAO/epoxy coating system showed the best lithium based protective layer in terms of corrosion resistance on the AA2024-T3.The X-ray photoelectron spectroscopy(XPS)results showed that before lithium inhibitors exposure,the maximum enrichment of the copper on AAO surface promoted the formation of the lithium protective layer. |
