Study Of Self-healing Layered Double Hydroxide Conversion Coatings On The Surface Of A1 Alloys

The traditional processing on aluminum and its alloy was chromate conversion processing and chromate conversion coating had excellent corrosion resistance,good self-healing performance,but the process of processing had a great impact on the environment and human body.Existing chromium-free processing with poor corrosion resistance,such as zirconium titanium ceramic processing,silane treatment,manganese acid salt and rare earth processing conversion coating,cannot replace chromate treatment.Therefore it is necessary to explore new sace treatments.This paper was mainly about the study of the in situ growth of hydrotalcite-like film on aluminum alloy.It could be devided into two parts.(1)LiAl-LDHs conversion coating modified by Aspartic acid(Asp-LiAl-LDHs)was formed on aluminum alloy in situ growth and corrosion resistance,self-healing were studied in this paper.The Scanning electron microscopy(SEM)images showed that conversion film with blade-shapped structure was uniform and compact.The appearance of the characteristic peaks of C-H,-COOH in infrared spectroscopy(FI-IR),the shift of characteristic peak(003)of LDHs to the small angle and the slight increase of the layer spacing improved the successful intercalation of aspartic acid into LiAl-LDHs.The corrosion current density and AC impedance value of aspartic acid intercalated LiAl-LDHs conversion coating were 2.485×10-8 A·cm-2 and 2.213×105 ?·cm-2,respectively.Aspartic acid intercalated LiAl-LDHs conversion coating can complete 270 h of detection in neutral salt spray test.X-ray photoelectron spectroscopy(XPS)results demonstrated that Cl-anions exchange partial Asp anions according to the change content of element on conversion coating.From the above results,the possible mechanism via exchange/self-assembly was proposed to illustrate the phenomenon of self-healing,some smaller crystal platelets,similar to the original Asp-Lilac-LDHs conversion coating,have formed in the scratch area.With increasing immersion time,these platelets grew larger,leading to further healing and filling the cracks of the conversion coating.Simultaneously,it also gave the strong support about forming film in the low temperature(25-35O?).(2)The difference of Asp-LiAl-LDHs conversion coating formed by dipping and spray was discussed.K2Cr2O7 dripping experiment,Tafel polarization curve,AC impedance spectroscopy and neutral salt spray(NSS)were applied for corrosion resistance of conversion coatings.The addition of accelerator promoter could make bigger in size and thicker in thickness and improve corrosion resistance.The conversion coating after the pretreatments using hot water also had better corrosion resistance.The corrosion current density and AC impedance value of conversion coating by spray method were 1.632×10-7 A·cm-2 and 5.417×104 ?·cm-2,respectively.Asp-LiAl-LDHs conversion coating could complete 270 h of detection in neutral salt spray test.The conversion coating formed by both dipping and spray could protect Al alloy form corrosion.However,the adhesion,the resistance of acid and alkali solution and the pollution resistance test of conversion coating formed by dipping were better than pray.