Investigation Of Preparation And Properties Of Trivalent Chromium Coatings On Al 6063 Alloy

Pure aluminum has a thin surface oxide layer which is generally stable in air and aqueous solution,but it lacks some necessary mechanical and physical properties,such as high specific strength,specific modulus of elasticity,creep strength,fatigue strength,hardness,wear resistance and low thermal expansion.Therefore pure aluminum is often alloyed with other elements for engineering applications.The protection imparted by the surface oxide to the Al substrate tends to decrease when Al is alloyed with other elements.This leads to further advancements in the development of effective and economic use of surface modification technologies with the aim of improving the corrosion resistance.Traditionally,chromate(Cr⁺⁶) conversion coatings have been successfully used on Al alloys and other metals such as Zn and steel to improve the corrosion resistance.However,Cr⁺⁶ species are toxic,and European Union has prohibited them in the electrical and electronic industries.Therefore the need for more environmentally friendly processes has led to the development of various Cr⁺⁶-free candidates. Recent research covers a wide variety of new chemical conversion coatings,including Ti,Mo,W,Co,Sn,Zn compounds and rare earth metal salts.All these elements are thought of low toxicity and are relatively abundant in nature.Recently,patents for trivalent chromium coatings on Al alloys have been reported.However,further study on the mechanism has rarely been reported.Considering the above situation,the following investigations are carried out in details:Before the conversion,the influence of the additives(i.e.Al³⁺ ion, H₃BO₃,NiSO₄,inhibitor A and CoSO₄) on the gloss and appearance was investigated systemically using the orthogonal and single factor experiments in three acids(HNO₃,H₂SO₄ and H₃PO₄).The suitable concentrations of Al³⁺ ion,H₃BO₃,NiSO₄,inhibitor A and CoSO₄ were 6 -10gL^-1,0.9-1.2gL^-1,0.8-1.1 gL^-1,0.5-0.8gL^-1,0.8-1.2gL^-1, respectively.The optimal quantities of the additives were as follows:Al³⁺ ion 6 g L^-1,H₃BO₃ 1.2 g L^-1,NiSO₄ 1.1 g L^-1,inhibitor A 0.5 g L^-1, CoSO₄ 1.2 g L^-1.SEM was used to analyse the untreated and treated Al alloy surfaces. A new process for trivalent chromium conversion coatings on Al 6063 alloy has been developed by us,and the influence of five variables(i.e. deposition temperature,time,bath pH,and concentrations of Cr³⁺ compound(KCr(SO₄)₂) and H₃PO₄) on the corrosion resistance of Cr³⁺ coatings on Al 6063 alloy was investigated using the orthogonal and single factor experiments.The optimal conditions and the suitable ranges of the above variables were determined.The suitable conditions were obtained as following:temperature was 30 - 40℃,deposition time 9 min, the pH was adjusted to 2.0 - 3.0,the KCr(SO4)2 and H3PO4 concentrations were controlled within 15 - 25 g L^-1 and 10 - 20 g L^-1, respectively.The optimal conditions were as follows:temperature 40℃, Bath pH 2,KCr(SO₄)₂ 20 g L^-1,H₃PO₄ 20 g L^-1.The morphologies and composition and valence state of the coatings were analyzed by scanning electron microscopy(SEM) and energy dispersive spectrometry(EDS) and X-ray photoelectron spectroscopy(XPS),respectively.The result indicates that the trivalent chromium chemical conversion coatings were made on aluminum alloy surface.Electrochemical properties of trivalent chromium coatings on 6063 aluminium alloy were studied.The polarization result showed that the coatings had more positive corrosion potential(Ecorr) and pitting corrosion potential(Epit),and lower corrosion current density(icorr), indicating that the coatings formed under the optimum conditions had better corrosion resistance.To explain the electrochemical properties of the coatings,A simple surface model was derived and experimentally tested in terms of an equivalent circuit.Good agreement was found between the model predictions and the experimental results. Electrochemical parameters of EIS,such as the surface resistance(Rcoat) and capacitance(Qcoat),the charge transfer resistance(Rct) and double layer capacitance(Qdl),were obtained by fitting the EIS plots.The morphologies of the coated and uncoated electrodes were examined by scanning electron microscopy(SEM),and the results also support the proposed surface model.The corrosion behavior of the coatings was studied by polarization curves and electrochemical impedance spectroscopy at the different immersion times and temperatures.The different equivalent circuit models were also designed and explained according to the different corrosion periods and their properties and carried out simulating.SEM and EDS were used to confirm the mechanism.In order to improve the electrochemical performance of trivalent-chrome coatings,urea,thiourea,monethanolamine(MEA), diethanolamine(DEA) and triethanolamine(TEA) inhibitors were added into the deposition bath.The electrochemical behaviors of coatings formed with different concentrations of urea,thiourea, monethanolamine(MEA),diethanolamine(DEA) and triethanolamine (TEA) were investigated in 3.5wt%NaCl solution at 25 oC,using potentiodynamic polarization curves and EIS.The corrosion resistance of coatings is improved greatly by adding a small amount of inhibitors, whereas the excessive addition deteriorates the corrosion resistance. Thiourea addition presents better effect than urea,TEA better than DEA, DEA better than MEA.To explain the EIS results of the coatings,a simple equivalent circuit was applied.The EIS parameters were obtained by fitting the EIS plots.The results of the polarization curves and EIS show that the inhibitor-containing coatings present better corrosion resistance than the coating without inhibitor.The morphology and composition and valence state of the conversion coatings were examined by SEM and EDS and XPS,respectively.The results indicated that the trivalent chromium coating was developed on Al 6063 alloy,inhibitors were also deposited on the substrates,respectively.A noticeable chemical shift was also observed.The corrosion behaviors of the inhibitor-containing(urea and thiourea) coatings were studied by polarization curves and electrochemical impedance spectroscopy at the different immersion times,the results show that the inhibitor-containing coatings remained unchanged after a longer immersion.To deeply understand and improve performance of the coatings,three complex coatings were prepared.The electrochemical behaviors of the complex coatings formed with different concentrations of NiSO4, CoSO4,Zr(SO4)2 were also investigated.To better explain the EIS results of the coatings,the EIS parameters were also obtained by fitting the EIS plots.The corrosion resistance of Cr-Zr complex coating is larger than that of Cr-Co,the Cr-Co larger than Cr-Ni,and Cr-Ni larger than the basic coating.A mechanism was also proposed and compared with the inhibition mechanism of organic inhibitor-containing coatings,the mechanism is obviousely different.The morphology and composition of the conversion coatings were examined by SEM and EDS,respectively. The results showed that the three complex coatings were prepared.At last,a deposition process model for trivalent chromium conversion coatings was proposed,the XPS was used to confirm the model.In theory, the critical pH of Al,and Cr deposition was calculated using the distributing coefficient and the method of simple iteration.