Preparation of a self-developed chrome-free colored coating based on a titanium conversion coating on 6063 Al alloy

Abstract

A new conversion coating improved via the combination of a self-developed Mannich base and hexafluorotitanic acid has been studied and compared with chromate conversion coatings and coatings without the Mannich base. The Mannich base was synthesized from polyether amine D230, formaldehyde and catechol easily at 35 °C. The structure of the self-developed Mannich base was determined by Fourier transform infrared spectroscopy (FTIR) and the properties of the conversion coatings were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), a polarization curve, a K₂Cr₂O₇ dropping test and a saltwater immersion test, respectively. The results of SEM, the K₂Cr₂O₇ dropping test and the saltwater immersion test demonstrated that the Ti-Mannich base conversion coating had a better surface morphology and corrosion resistance than those of titanium conversion coatings. The results of FTIR and XPS showed that the main components of the conversion coating were TiO_x, Al₂O₃ and organics. A formation mechanism of the coating was proposed, in which an orange-red chelate compound formed by a chelation reaction of phenolic hydroxyl groups and Ti(IV) could bind to an Al surface later, and the coating appeared yellow because of the chelate. This new conversion coating, which has better performance, is a potential alternative to chromate conversion coatings.

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1. Introduction

Recent studies of chromate-free conversion coatings have identified several potential alternatives to chromate conversion coatings (CCC). Among the various reported alternatives, such as conversion coatings based on cerium, vanadates, rare earth elements, cobalt-rich oxide layers, phosphates and permanganates,^1–5 zirconium-based and titanium-based treatments, are the most promising alternatives, particularly for Al alloys, and many researchers have reported treating conversion coatings that could effectively inhibit the corrosion of Al alloys^6–9. However, these coatings are flawed with respect to sufficient anticorrosion performance and achromatic color.

To improve the performance of chrome-free coatings, various polymers have been used in conversion treatments. For example, some coatings were improved by hexafluorotitanic acid or hexafluorozirconic acid combined with aminotrimethylenephosphonic acid,¹⁰ nabutan,¹¹ poly(acrylic acid),^12,13 acetylacetonate,¹⁴ methacrylate,¹⁵ tannic acid,¹⁶ epoxysilane¹⁷ and epoxyacrylate resin.^18,19 Many researchers have reported conversion coatings modified by organic admixtures, in which carbon chains are mainly located in the outer layer of the coatings.^19–21 Even though some coatings are colored, their corrosion resistance and adhesion to an Al substrate can still be improved.

The Mannich reaction is a three-component condensation reaction that involves an active hydrogen-containing compound, formaldehyde and a primary or secondary amine. Metal complexes of Mannich bases have been studied extensively owing to the selectivity and sensitivity of the ligands towards various metal ions.^22,23 A Mannich base as an organic corrosion inhibitor has been applied in the surface treatment of Al alloys.^24–27 In this paper, an inhibitor was prepared by the Mannich reaction at 35 °C to improve an inorganic titanium conversion coating (TCC). A Mannich base, owing to its phenolic hydroxyl groups, can chelate Ti(IV) to form a chelate compound. Ti(IV) can passivate a metal substrate, which is transformed into the structure Al–O–Ti. Although organic–inorganic coatings have been reported on Al alloys, conversion coatings modified by a Mannich base inhibitor on the surface of an Al alloy are rare.

In this paper, a Mannich base was prepared as a corrosion inhibitor that was subjected to conversion treatment to obtain a colored Mannich base-titanium conversion coating (TMCC) with superior corrosion resistance. Moreover, the structure and mechanism of formation of the coating are discussed. Its potential as an alternative to chromate conversion coatings is proposed.

2. Experimental

2.1. Preparation of chrome-free conversion coating on Al alloy

Al alloy (6063, wt%: Al 98.2, Zn 0.10, Mn 0.1, Fe 0.35, Si 0.40, Cu 0.10, Mg 0.55, Cr 0.10, Ti 0.10) was used as received. The Al alloy sample was cut into sections with a size of 10 × 10 × 2 mm for later tests.

A variety of Mannich bases were prepared via the Mannich reaction. We studied the effects on different Mannich bases of the factors of amines (polyether amines, cholamine, ethanediamine), aldehydes (formaldehyde, paraformaldehyde), acids (catechol, resorcinol, pyrogallol), temperature, and reaction time. Finally, we found that the best synthesis process was as follows: 300 g polyether amine D230 (PEA) was reacted with 30.03 g formaldehyde (PA) for 1.5 hours at 25 °C with 24.6 g ethyl alcohol as a solvent, the pH of the mixture was kept in the range of 7–9 by adding KOH solution, and then 66 g catechol (CH) was added to continue the reaction for 6 hours at 35 °C and the unreacted PEA was removed by steam distillation. The Mannich base (CH–PEA–PA) that was obtained was a brown oil (Fig. 1a) insoluble in pure water (Fig. 1b), which would turn orange-red when mixed with H₂TiF₆ solution (Fig. 1c). A possible reaction equation of the formation of CH–PEA–PA is shown in Fig. 2.

Fig. 1 Image of CH–PEA–PA (a), dissolved in pure water (b) and in H₂TiF₆ solution (c).

Fig. 2 Reaction equation of formation of CH–PEA–PA.

The surface of an Al alloy was polished by emery papers of 600–1200 grit, ultrasonically degreased in acetone for 3–5 min, and then rinsed with deionized water.

For comparison of different performances, a chromate conversion coating (CCC) was also prepared according to a commercial process from Foshan Kefoo Technology Co., Ltd. Samples were immersed in a chromate-based bath containing 4 g L⁻¹ CrO₃, 0.3 mL L⁻¹ H₂SO₄, 3 mL L⁻¹ HNO₃ and 1 g L⁻¹ HF for 120 s at a temperature of 32 °C. Table 1 shows information for different coatings on the Al alloy.

Table 1 Different types of conversion coating on Al alloy

Components of conversion solution	Abbreviation for coating	Color of conversion coating	Weight of conversion coating
H2TiF6, NaF	TCC	Colorless	75 mg m^−2
H2TiF6, NaF, CH–PEA–PA	TMCC	Yellow	107 mg m^−2
H2ZrF6, NaF, CH–PEA–PA	ZMCC	Colorless	—
CrO3, H2SO4, HNO3, HF	CCC	Orange-yellow	100 mg m^−2

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TMCC was formed by immersing Al alloy samples in the conversion bath for 7 min at 30 °C. The surface of TMCC appeared yellow, unlike the colorless TCC. HNO₃ was used to adjust the pH value of the solution to 3.5.

2.2. Testing

X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the components of the Mannich base (CH–PEA–PA) and conversion coatings. XPS is a method used to obtain chemical information by the analysis of surface energy spectra. XPS was carried out with a PerkinElmer PHI 5000C ESCA system (USA) using Al Kα radiation (1486.6 eV) with a power of 250 W. The pass energy was set at 93.9 eV and the binding energies were calibrated by using contaminant carbon at a binding energy of 284.6 eV.

A scanning electron microscope (SEM, Philips XL30, Japan) was employed to observe the surface morphology of conversion coatings on the 6063 Al alloy.

Electrochemical measurements were performed on an electrochemical workstation (CHI606D, Shanghai Chenhua Instruments Inc., China) at room temperature. Electrochemical tests were used to evaluate the corrosion resistance of coated samples in 3.5% NaCl solution. A saturated calomel electrode (SCE) was used as a reference electrode. A naked or coated Al alloy electrode was used as the working electrode.

The weight of the coating was tested according to GB/T 9792-1988. A boiling water adhesion test was carried out according to GB/T 5237.5-2000. A saltwater immersion test was conducted in 3.5% NaCl solution for three days and the appearance of the surface was observed according to GB/T 1766-2008. A K₂Cr₂O₇ dropping test was carried out according to the standard SJ 1276-77. The dropping solution consisted of 25 mL HCl solution (d = 1.19), 3 g K₂Cr₂O₇ and 75 mL distilled water for every 100 mL dropping solution. A drop of the K₂Cr₂O₇ dropping solution was put on the surface of the coating and would turn to green from orange because of the reduction of Cr⁶⁺. The time until drop discoloration was related to the corrosion resistance of the conversion coating, which could be recorded for later comparison of different coatings.

3. Results and discussion

3.1. SEM analysis

Fig. 3 shows SEM micrographs of an Al pickling sample, TCC and TMCC, respectively. The pickling sample shows the surface of the Al alloy (Fig. 3a). After treatment with hexafluorotitanic acid, an uneven and perforated coating was formed (Fig. 3b). However, on treatment with CH–PEA–PA the coating changed to become compact (Fig. 3c), which may lead to better corrosion resistance. The structure of TMCC seemed to become thicker relatively, because the weight of the conversion coating increased from 75 to 107 mg m⁻² (Table 1). It could be speculated that Ti(IV) played a bridging role between CH–PEA–PA and the Al alloy.

Fig. 3 (a) SEM micrograph of Al pickling sample. (b) SEM micrograph of TCC. (c) SEM micrograph of TMCC.

3.2. XPS and FTIR analysis

Fig. 4 shows the FTIR spectra of the CH–PEA–PA and the Ti–CH–PEA–PA composite. In the FTIR spectrum of CH–PEA–PA, the strong and broad absorption peaks at 3596–3300 cm⁻¹ prove the existence of strong –OH absorption. The peaks at 3000–2800 cm⁻¹ suggest the existence of CH₂ and CH₃ on the aromatic ring. The peaks at 1610–1600 cm⁻¹ and 1520–1500 cm⁻¹ were due to the structure of C=C in the aromatic ring. The peak at 1260 cm⁻¹ was due to the structure of C–O, and the peak at 1103 cm⁻¹ was due to the structure of C–N. The peak at 767 cm⁻¹ shows that the benzene ring was triply substituted at the 1, 2, and 3 positions. There were no absorption peaks at 1715 cm⁻¹, which indicates that the formaldehyde did not react.

Fig. 4 FTIR spectra of CH–PEA–PA and Ti–CH–PEA–PA composite.

In the FTIR spectrum of the Ti–CH–PEA–PA composite, the –OH absorption peak was narrower and weaker than that of CH–PEA–PA alone, which means that some Ph-OH groups were combined with Ti(IV) or the Al substrate. The peaks at 1609 cm⁻¹, 1475 cm⁻¹ and 751 cm⁻¹ prove that positional substitution did not occur after adding CH–PEA–PA. An inference from these results is that Ti(IV) made the benzene ring asymmetric and the inductive effect made the characteristic frequency shift to higher wavenumbers. In the fingerprint region (400–1300 cm⁻¹) the IR spectrum had changed substantially, which indicated that the functional groups in the composite had changed, and the peaks at 650 cm⁻¹ and 566 cm⁻¹ indicate the presence of Ti–O in the Ti–CH–PEA–PA composite.

Fig. 5a and b show the results of the general XPS spectra of TCC and TMCC. C, O and Ti were detected as the major elements in TCC, and C, N, O, F and Ti were detected as the major elements in TMCC. Fig. 6 shows the high-resolution XPS peaks.

Fig. 5 (a) General XPS spectrum of TCC. (b) General XPS spectrum of TMCC.

Fig. 6 (a) High-resolution XPS peaks of Ti 2p. (b) High-resolution XPS peaks of O 1s.

Fig. 6a shows the high-resolution XPS peaks of Ti 2p. In TCC, the Ti 2p₃ peak at 464.39 eV was attributed to TiO₂ and the Ti 2p₃ peak at 458.66 eV was attributed to TiO₂–Al₂O₃. In TMCC, the Ti 2p₃ peak at 458.14 eV was attributed to Ti–O (458.2 eV) or TiO₂ (458.1 eV).

Fig. 6b shows the high-resolution XPS peaks of O 1s. In TCC, the O 1s peak at 531.83 eV was attributed to passivated Al₂O₃ and AlOOH. In TMCC, the O 1s peak at 531 eV was attributed to Al₂O₃, C–O and Ti–O.

Owing to the above results, it could be speculated that the main components of TCC were TiO₂–Al₂O₃, AlOOH and TiO_x, which was consistent with the literature.^21,28,29 In TMCC, the main components were TiO_x, Al₂O₃ and O–C. The results suggested that organic compounds had been incorporated in the coating.

3.3. Corrosion resistance properties of coatings

3.3.1. Electrochemical analysis. Fig. 7 shows the polarization curves of the pickling sample, TCC, CCC and TMCC, respectively. The result shows that the corrosion current densities of the pickling sample, TCC, CCC and TMCC were 4.47 × 10⁻⁸ A cm⁻², 1.07 × 10⁻¹⁰ A cm⁻², 6.17 × 10⁻¹¹ A cm⁻² and 2.51 × 10⁻¹¹ A cm⁻², respectively. The potentiodynamic polarization of the pickling sample did not exhibit an obvious passivation region. However, a rather low current density and a high potential could be observed in the polarization curves of CCC and TMCC. CCC had a lower corrosion current density than those of TCC and the pickling sample. The corrosion current density of TMCC was one-quarter of that of TCC and TMCC had the highest corrosion potential, which means that TMCC had the best corrosion resistance among these coatings. The excellent anticorrosion property of TMCC may provide an alternative way of substituting for CCC.

Fig. 7 Tafel polarization curves of different coatings.

3.3.2. Corrosion tests on coatings. Table 2 shows the test results for different coatings. A long time for the K₂Cr₂O₇ dropping test indicates good corrosion resistance. It was shown that the corrosion resistance of TCC was relatively poor, whereas that of TMCC was excellent. The result of the K₂Cr₂O₇ dropping test showed that the corrosion resistance of TMCC was much better than that of TCC. The above results indicate that CH–PEA–PA improved the corrosion resistance of TCC alone.

Table 2 Corrosion resistance performance of different conversion coatings

Type of conversion coating	Saltwater tolerance	K2Cr2O7 dropping test (s)
TCC	Shedding and turning white	20
TMCC	No shedding or color change	83
CCC	No shedding or color change	82

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4. Mechanism analysis

As an organic corrosion inhibitor, corrosion inhibition by the self-developed Mannich base (CH–PEA–PA) occurs as follows:

R₂NH + H⁺ → R₂NH₂⁺

The type of cation in CH–PEA–PA could repel H⁺ in corrosive media.

Ti + R₂NH → Ti ← NHR₂

Al + R₂NH → Al ← NHR₂

CH–PEA–PA could chelate metal ions by the above chelation reactions. Considering Fig. 1b and c, it is more likely that phenolic hydroxyl groups in CH–PEA–PA would chelate Ti(IV) (Fig. 8), which makes insoluble CH–PEA–PA dissolve in water. It can also be inferred that the color of the Ti–CH–PEA–PA compound and the conversion coating is due to the chelation reaction.

Fig. 8 Chelation reaction between Ti(IV) and CH–PEA–PA.

Based on the results of XPS and FTIR, the process and mechanism of formation of TMCC have been discussed. When the Al alloy samples were immersed in the solution of H₂TiF₆, the conversion coating was formed according to the following reactions:³⁰

2Al + 4H₂O → 2AlOOH + 3H₂↑ → Al₂O₃ + H₂O

2Al + 6H⁺ + 3TiF₆²⁻ + 5H₂O → 2AlOF·3TiOF₂ + 10HF + 3H₂↑

When CH–PEA–PA was added, a chelation reaction occurred between phenolic hydroxyl groups and Ti(IV). An Al–O–CH–PEA–PA composite could be formed on the Al alloy surface. As a colorant, CH–PEA–PA made the coating yellow. The inorganic–organic system made the conversion coating denser and thicker, and amino groups improved the corrosion resistance of the coating. At the same time, CH–PEA–PA was crosslinked in the coating through functional groups, which could promote adhesion and slow down the corrosion rate. The structure of the conversion coating was inferred to be as follows (Fig. 9).

Fig. 9 Coating structure of TMCC.

5. Conclusions

A composite conversion coating that was mainly composed of Ti(IV) and a self-developed Mannich base (CH–PEA–PA) was prepared by simple immersion into a conversion solution at room temperature. According to SEM, XPS and FTIR analyses, the process and mechanism of formation of TMCC were proposed. Phenolic hydroxyl groups in CH–PEA–PA combined with Ti(IV) by a chelation reaction, and then Ti(IV) formed a structure of Al–O–Ti–CH–PEA–PA via chemical bonding. As a colorant, CH–PEA–PA made the coating yellow. The results of electrochemical analysis and corrosion tests indicated that CH–PEA–PA improved the corrosion resistance of TCC alone. The corrosion resistance of TMCC was much greater than that of CCC. Therefore, TMCC was found to be promising for replacing toxic CCC.

Acknowledgements

The authors would like to thank the support of Shunde District Science and Technology Bureau of Guangdong Province, China (Grant No. 2013CXY14).

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