**Abstract**
The non-corrosive area fraction of 6063 aluminum alloy was analyzed in a conversion slag containing fluorosilicate and ammonium fluoride. The study investigated the effects of the composition and processing conditions of the conversion solution on the corrosion resistance of the fluorosilicate conversion coating. The optimized fluorosilicate conversion process parameters were determined as follows: Na₂SiF₆ at 3–5 g/L, NH₄F at 5–7 g/L, pH between 5.5 and 6.5, temperature ranging from 25 to 35°C, and a conversion time of 12 to 16 minutes. After treatment, a dense, chromium-free conversion film composed of F, Al, Na, O, and Si was formed on the surface of the aluminum alloy. This significantly improved the corrosion resistance by shifting the corrosion potential positively.
**Keywords**: Aluminum alloys; Fluorosilicates; Conversion coatings; Corrosion resistance; CLC: TQ153.6; Document code: A; Article ID: 1004-227X(2012)05-0037-04
**Introduction**
China is a major producer and consumer of aluminum alloys, which are widely used in aerospace, automotive electronics, machinery, and food industries. Due to its low standard electrode potential (-1.67 V), aluminum alloys are prone to corrosion under acidic or alkaline conditions, limiting their application. To prevent this, various surface treatments such as anodization, micro-arc oxidation, chemical conversion, and coatings have been developed. Chromate conversion has been traditionally used due to its cost-effectiveness, but it contains hexavalent chromium, which is carcinogenic. As a result, trivalent chromium conversion is now more common, though it still poses environmental risks. Therefore, there is an urgent need for eco-friendly, chromium-free conversion technologies that offer good corrosion resistance and stable performance.
Recent studies have explored alternatives like rare earth conversion coatings, molybdate passivation, and organic-modified fluorotitanic acid treatments. However, these methods often require complex pre-treatment or suffer from limited stability. In this paper, we present a preliminary investigation into the formation of fluorosilicate conversion coatings on 6063 aluminum alloy.
**Experiment**
The experimental material used was 6063 aluminum alloy with dimensions of 3.5 cm × 1.5 cm × 0.1 cm. The main components (mass fraction) were: Si 0.20%–0.60%, Fe 0.35%, Cu 0.10%, Mn < 1.00%, Mg 0.45%–0.90%, Cr 0.10%, Ti 0.10%, Zn 0%, and Al as the balance. The process included degreasing, washing, alkali washing, descaling, passivation, drying, and aging. The passivation step involved a solution of Na₂SiF₆ (2.5–5.0 g/L) and NH₄F (5–7 g/L), with a pH of 5.5–6.5, at 25–35°C for 12–16 minutes.
To determine free fluoride ion concentration, a fluoride ion selective electrode method was employed using a PFS-SO type meter. For film characterization, an Oxford INCA spectrometer was used to analyze morphology and composition. Corrosion resistance was evaluated via neutral salt spray testing and polarization curves.
**Results and Discussion**
The effect of Na₂SiF₆ concentration on corrosion resistance was studied at 6.0 g/L NH₄F, pH 5.5, and 25°C for 12 minutes. Results showed that at lower concentrations (<2 g/L), the conversion film was incomplete, leading to rapid corrosion. At higher concentrations (>6 g/L), excessive etching occurred, reducing corrosion resistance. The optimal range was found to be 3–5 g/L.
NH₄F concentration also had a significant impact. Too little resulted in poor etching and incomplete film formation, while too much caused over-etching. The optimal range was 5–7 g/L. pH was another critical factor, with 5.5–6.0 being ideal. Temperature had minimal effect on film formation, but 25–35°C was preferred for process stability. Conversion time of 12–16 minutes was optimal, resulting in a high non-corrosive area fraction after 168 hours of salt spray testing.
Surface analysis revealed a dense film composed of sodium fluoroaluminate and SiOâ‚‚ colloids, enhancing corrosion resistance. Polarization curve tests showed a positive shift in corrosion potential, indicating reduced dissolution rate.
**Conclusion**
(1) The optimal fluorosilicate conversion process for 6063 aluminum alloy is: Na₂SiF₆ 3–5 g/L, NH₄F 5–7 g/L, pH 5.5–6.5, temperature 25–35°C, and 12–16 minutes. This produces a chromium-free conversion coating with excellent corrosion resistance.
(2) During the process, Fâ» etches the aluminum surface, forming a dense layer of sodium fluoroaluminate, followed by deposition of SiOâ‚‚ colloids, further improving corrosion resistance.
**References**
[1] Gao Cheng, Yu Jinyong, Ye Yiyong, et al. General situation of research on micro-arc oxidation process of aluminum alloy gold[J]. Plating and Finishing, 2009, 28(2): 22–25.
[2] Gong Weihui, Zhen Dongchu, Li Wenfang, et al. Development of surface treatment technology for environment-friendly aluminum alloys[J]. Research and Application of Materials, 2009, 3(1): 1–4.
Chemical Anchor
Chemical Anchor
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