Composition and Performance of Nanostructured Zirconium Titanium Conversion Coating on Aluminum-Magnesium Alloys

4 1.0 g/L, and tetra-n-butyl titanate (TBT) 0.08 g ⋅L. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrum (FT-IR) were used to characterize the composition and structure of the obtained conversion coating. The morphology of the conversion coating was obtained by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Results exhibit that the zirconium titanium salt conversion coating of Al-Mg alloy contains Ti, Zr, Al, F, O, Mg, C, Na, and so on. The conversion coating with nm level thickness is smooth, uniform, and compact. Corrosion resistance of conversion coating was evaluated in the 3.5 wt.% NaCl electrolyte through polarization curves and electrochemical impedance spectrum (EIS). Self-corrosion current density on the nanostructured conversion coating of Al-Mg alloy is 9.7×10A ⋅cm, which is only 2% of that on the untreated aluminum-magnesium alloy.This result indicates that the corrosion resistance of the conversion coating is improved markedly after chemical conversion treatment.


Introduction
Due to the high strength/weight ratio, easy processing, excellent physical, and chemical properties, aluminum-magnesium-based alloys have been widely used for the materials of automotive, mobile phone, and aircraft [1,2].However, low hardness, poor wear resistance, and susceptibility to intergranular corrosion of the aluminum-magnesium alloys hinder their practical application [3].Under natural conditions, the thin oxide film formed on the surface of aluminum alloy is easily damaged, especially in the acid (alkaline) conditions.The dissolution of this film greatly reduces the corrosion resistance of aluminum and its alloys.Therefore, it is important to improve the corrosion resistance of aluminum alloy through suitable methods.Surface treatment is one of these methods.
Anode oxidation, microarc oxidation, chemical conversion coating, plating, and so on have been widely performed to protect aluminum and aluminum alloys from corrosion [4][5][6][7].Among them, chemical conversion treatment has been attracting more attention due to its facile operation and low cost.Chemical conversion film is formed to oxidize the surface layer into the metal oxide layer [8].Chromate treatment, including phosphate-chromate method, acid chromate method, and alkaline chromate method, is the concernful treatment method for chemical conversion [9][10][11][12].Acid chromate treatment is more widely used in the actual application [13]; due to that, the films formed via alkaline chromate treatment are soft, loose, and vulnerable to scratches.Although the passive films have the advantage of simple process, low production cost, and good corrosion resistance, hexavalent chromium is carcinogen which can cause serious harm to humans and environment [14][15][16][17][18].According to EU scrappage instruction announced on July 1, 2002, less than 2 g hexavalent chromium can be used for the production of every car [19].Simultaneously, the "Restriction  [20].Therefore, chromefree treatment process has been one of the key technologies for the surface treatment of the aluminum-magnesium alloys.
In this paper, zirconium and titanium salt were used as the main salt to form zirconium titanium conversion coating on the aluminum-magnesium alloy surface.XRD, SEM, XPS, FT-IR and AFM were applied to characterize the composition, morphology and structure of chemical conversion film coated aluminum-magnesium alloy.Electrochemical measurements were performed to evaluate the corrosion resistance of the chemical conversion film.

Materials and Methods
Aluminum-magnesium alloys were chosen as the raw materials.A smooth and uniform surface was obtained after pretreatment such as alkaline cleaning, pickling, and activating.For the comparison, all the sizes of samples are 40 mm × 30 mm × 2 mm.The surface process is as follows: sample → polishing → removing oil → alkaline cleaning → pickling → rinsing → chemical conversion treatment → rinsing → drying.The composition of conversion solution is shown in Table 1.
X-ray diffraction (XRD) was carried out to characterize crystalline structure and composition of the conversion coating.X-ray photoelectron spectroscopy (XPS) was performed to examine the surface composition and element valence state of conversion coating.Meanwhile, Fourier transform infrared absorption spectra (FT-IR) were picked to identify compound structure of titanium salt conversion coating.SEM and AFM were selected to analyze the surface morphology of zirconium titanium conversion coating.
CHI660A electrochemical workstation (CH Instrument Corp., Shanghai, China) was used to conduct polarization test.The corrosion test was carried out in 3.5 wt.% NaCl solution at 25 ∘ C with standard three-electrode system.Sample, saturated calomel electrode (SCE), and platinum sheet were used as working electrode, reference electrode, and auxiliary electrode, respectively.

XRD Analysis. XRD is used to analyze the composition of zirconium titanium salt conversion coating. As shown in
Figure 1, the film layer obviously contains large amounts of aluminum due to that aluminum is the substrate.Diffraction peaks attributed to ZrO 2 , TiO 2 , Al 2 O 3 and Na 3 AlF 6 phases are also observed.These substances are generated from the surface conversion of aluminum-magnesium alloy substrate.Because the conversion coating is thinner, powder scraping from aluminum-magnesium alloy contains less conversion coating.As observed, diffraction peaks of the aluminum alloy substrate are very strong, while the diffraction peaks of other phases are very weak.Therefore, it is proposed that some materials in the conversion coating have not been detected.Binding energy (ev) Binding energy (ev) Zr, Al, F, O, C, Na, Mg, and other elements were contained in zirconium titanium salt conversion coating.Most of the elements in the solution are observed in the conversion coating.This result indicates that the conversion coating is derived from the cooperation of all the compositions of the conversion solution.The H element cannot be detected by XPS.Therefore, it cannot rule out the existence of H. Figure 3 is the narrow area scan spectra of Ti2p, Zr3d, Al2p, F1s, O1s, and C1s.Spectra peaks of each element were    To sum up, Zr and Ti elements in the conversion film exist as Ti 4+ and Zr 4+ , respectively.Al element mainly exists as oxides and complexes, and C element may be in the form of organic polymers.4 is the Fourier transform infrared spectrum of zirconium titanium salt conversion coating and tannic acid, respectively.The peak located at 3445 cm −1 can be assigned to hydroxy benzoic acid.Compared to the benzoic hydroxy of tannic acid (3375 cm −1 ), the higher wave number shift (3445 cm −1 ) can be attributed to the lower complexation extent of benzoic hydroxyl.The peaks at 2920 cm −1 and 2851 cm −1 should be attributed to the symmetric and asymmetric vibration of the methylene, respectively, indicating the transformation reaction of tetran-butyl titanate in the process of conversion coating.The characteristic peak at 1715 cm −1 can be ascribed to the ketonic group of tannic acid, which shifts to 1646 cm −1 after chemical conversion.The shift is corresponding to the hydrolyzation of tannic acid.The peaks at 1600∼1450 cm −1 and 1250∼700 cm −1  of conversion coating are attributed to the characteristic adsorption of benzoic group, which are clearly different with that of Tannic acid.The changes of benzoic groups are mainly due to two reasons: the change of the benzoic groups' number and the reaction of benzoic group with other groups.In this study, the change is ascribed to the complexation between the metal ion and the trihydroxy-benzoic acid derived from hydrolyzation of tannic acid.

FT-IR Analysis. Figure
From the previous analysis, we can conclude that the metals on the surface transform to metal compounds after conversion treatment.Hydroxide, ketonic, and benzoic groups are formed on the surface of the aluminum-magnesium alloy.These groups will be propitious to improve the contact between the substrate and the oil paint for the consequent coating.

SEM Analysis.
Figure 5 shows the SEM images of the conversion coating at various magnifications.The conversion coating film layer is uniform, smooth, dense, continuous, no damage, and cracking behavior.Almost no defects in the zirconium titanium salt conversion coating are observed.The particles are homogeneous spherical grains.

AFM Analysis.
The three-dimensional morphology of the film was observed by atomic force microscope (AFM).Figure 6 is the images with the scanning area of 5 m × 5 m, respectively.As shown, the conversion coating surface is smooth and uniform.The size of the surface particle is narrowly distributed.The part of graph with bright color represents the region with high level position, while the part with dim color is the lower level position.For analysis, the height difference is less than 50 nm, which indicates that the thickness of the conversion film is nanoscale.

Polarization Curve Analysis.
Tafel polarization curves of aluminum-magnesium alloy before and after conversion process, performed in 3.5% NaCl solution, are shown in Figure 7. Electrochemical parameter calculated by polarization curve can be seen in Table 2.The corrosion current density of the blank sample and zirconium titanium conversion coating is 4.701 × 10 −6 A⋅cm −2 and 9.7 × 10 −8 A⋅cm −2 , respectively.The corrosion current density of conversion coating is reduced by two orders of magnitude, about 2% of that on the blank sample.conversion process is very different.The Ac impedance spectroscopy of untreated aluminum-magnesium alloy is composed of high frequency capacitive impedance arc and low frequency inductive impedance arc.However, the low frequency inductive impedance arc is not observed in the Ac impedance spectroscopy of the aluminum-magnesium alloy after being treated.The resistance and capacitive impedance of the film can be investigated through comparing the diameter of the high frequency capacitive reactance arc.As shown, the resistance and capacitive impedance of aluminum-magnesium alloy specimen after chemical conversion processing is greater than that of aluminum-magnesium alloy specimen without the conversion process.It indicates that polarization resistance is increasing when the corrosion reaction occurred.Conversion coating can effectively slow down the electron transfer within the coating layer and protect aluminum-magnesium alloy matrix from erosion caused by the corrosive medium.These effects will improve the corrosion resistance of aluminum-magnesium alloy.

EIS Analysis
Inductive impedance arc in the low frequency, corresponding to the dissolution of conversion coating on the surface of aluminum-magnesium alloy (mainly alumina) in corrosive Cl − medium, is observed in the EIS spectroscopy of aluminum magnesium alloy without conversion process.The rate of dissolution increases continuously, leading to conversion film being thinner, and then causes holey corrosion.On the contrary, inductive reactance arc in the low frequency is absent in the EIS spectroscopy of aluminummagnesium alloy through transformation processing.This is due to the fact that conversion coating on the surface of the aluminum-magnesium alloy effectively inhibits dissolving penetration of corrosive medium into conversion coating.Aluminum-magnesium alloy conversion coating greatly improves the corrosion resistance of aluminum-magnesium alloy and effectively protects the alloy matrix.
Figures 8(b) and 8(c) are the Bode diagram and phase angle Bode diagram of the AC impedance spectroscopy of aluminum-magnesium alloy before and after the conversion process.The impedance of the aluminum-magnesium alloy after conversion is an order of magnitude greater than that without the conversion process.It implies that aluminum-magnesium alloy after conversion treatment has better corrosion resistance.The phase angle Bode diagram of untreated aluminum-magnesium alloy has a time constant at the phase angle of 70 degree.Meanwhile, aluminum-magnesium alloy after conversion treatment displays a time-constant value around 85 degrees at the frequency of 1-10 Hz.These results imply that the conversion coating is uniform.

Conclusions
Uniform, continuous Ti-Zr conversion coating can be successfully prepared using NaF, TBT, K 2 ZrF 6 , Tannic acid, and MgSO 4 as film-forming substances under the condition of 25 ∘ C.
Electrochemical tests show that corrosion current density of conversion coating on titanium zirconium conversion coating is 9.7 × 10 −8 A⋅cm −2 , which is only about 2% of that of the aluminum-magnesium alloy substrate.Impedance value of the conversion coating is about an order of magnitude larger than that of the untreated aluminum-magnesium alloy.Therefore, the corrosion resistance of aluminum-magnesium alloy is significantly enhanced after surface conversion coating.
AFM and SEM results exhibited that the dense and uniform Ti-Zr conversion coating is made of small spherical particles with uniform size and contains Ti, Zr, Al, Mg, OH, and so on.
Figure 2  shows the XPS of zirconium titanium salts conversion coatings on the Al-Mg alloy.Ti,
fitted and analyzed according to XPS database, established by the national standard technology association (NIST X-ray Photoelectron Spectroscopy Database), through XPS Peak 4.1 software.Ti2p spectrum (Figure3(a)) is composed of two fitted peaks at 458.66 eV and 464.41 eV, corresponding to the Ti2p3/2 and Ti2p1/2 of TiO 2 from XPS database, respectively.These results indicate that Ti exists as the TiO 2 state.

Figure 4 :
Figure 4: Fourier transform infrared spectrum of zirconium titanium salt conversion coating and tannic acid: (a) zirconium titanium salt conversion coating, (b) the tannic acid.

Figure 3 (
c) shows F elements in conversion coating surface.It mainly exists in the form of AlF 3 ⋅3H 2 O and Na 3 AlF 6 compounds.As seen in Figure 3(d), Al element in conversion coating is mainly based on AlN, Al 2 O 3 /Al, Al 2 O 3 , AlO  /Al, AlF 3 ⋅3H 2 O, and Na 3 AlF 6 compound form.The peaks of C and O can be attributed to organic functional groups (Figures 3(e) and 3(f)), revealing that the C and O mainly exist as complex compound.

Figure 7 :
Figure 7: Aluminum-magnesium alloy polarization curves before and after conversion treatment.

Figure 8 :
Figure 8: Aluminum-magnesium alloy AC impedance curves before and after treatment: (a) Nyquist diagram; (b) impedance Bode diagram; (c) the phase angle Bode diagram.

Table 1 :
Composition of conversion solution.