**Abstract**
The non-corrosive area fraction of 6063 aluminum alloy was measured in a conversion slag composed of fluorosilicate and ammonium fluoride. The study investigated the impact 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: 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 consisting of F, Al, Na, O, and Si was formed on the surface of the aluminum alloy. This significantly increased the corrosion potential and improved the overall corrosion resistance.
**Keywords**: Aluminum alloys; Fluorosilicates; Conversion coatings; Corrosion resistance.
**CLC Classification**: TQ153.6
**Document Code**: A
**Article ID**: 1004-227X(2012)05-0037-04
**Introduction**
China is one of the world's largest producers and users of aluminum alloys, which are widely applied in aerospace, automotive electronics, machinery, and food industries. Due to its low standard electrode potential (-1.67 V), aluminum alloy is chemically reactive and prone to corrosion under acidic or alkaline conditions, which limits its use. To address this, various surface treatments such as anodization, micro-arc oxidation, chemical conversion, and coating techniques have been developed. Chromate conversion has been widely used for its cost-effectiveness, but it contains hexavalent chromium, which is toxic and carcinogenic. As a result, trivalent chromium-based methods have become more common, though they still pose some risks. Therefore, there is a growing need for eco-friendly, chromium-free chemical conversion technologies that offer excellent corrosion resistance and stable performance.
Several studies have explored alternative methods, such as rare earth conversion coatings, molybdate passivation, and organic-modified fluorotitanic acid treatments. However, these approaches often face challenges in stability or applicability. This paper presents a preliminary investigation into fluorosilicate chemical conversion coatings, focusing on their formation and performance.
**Experimental Methods**
The experiment used 6063 aluminum alloy sheets with dimensions of 3.5 cm × 1.5 cm × 0.1 cm as the substrate. The main components (by mass) included 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 (0.00%), and the rest being Al.
The process involved degreasing, washing, alkali cleaning, descaling, passivation, drying, and aging. The passivation step used a solution containing Na₂SiF₆ (2.5–5.0 g/L) and NH₄F (5–7 g/L), with controlled temperature (25–35°C), pH (5.5–6.5), and time (12–16 minutes).
To determine the free fluoride ion concentration, a fluoride ion selective electrode method was employed using a PFS-SO type meter. Surface morphology and composition were analyzed using an Oxford INCA spectrometer. Corrosion resistance was evaluated through neutral salt spray tests and polarization curves.
**Results and Discussion**
The effect of Na₂SiF₆ concentration on the conversion film’s corrosion resistance was studied. When the concentration was below 2 g/L, the film formation was incomplete, leading to poor corrosion resistance. At higher concentrations (>6 g/L), excessive etching reduced the film’s effectiveness. The optimal range was found to be 3–5 g/L.
NH₄F concentration also played a key role. Too low or too high concentrations led to insufficient or excessive etching, respectively. The best results were achieved at 5–7 g/L. pH was another critical factor—too low or too high values caused instability in the conversion solution. The ideal pH range was 5.5–6.0.
Temperature had a minimal effect on film formation due to the fast reaction rate of Fâ». However, temperatures above 40°C affected solution stability. The preferred temperature range was 25–35°C. Conversion time influenced the final performance, with 12–16 minutes yielding the best results.
Surface analysis showed that the fluorosilicate conversion film consisted mainly of sodium fluoroaluminate and SiOâ‚‚ colloids, contributing to enhanced corrosion resistance. Polarization curve tests confirmed a significant increase in corrosion potential after treatment, indicating better protection against corrosion.
**Conclusions**
(1) The optimal fluorosilicate conversion process for 6063 aluminum alloy includes Na₂SiF₆ at 3–5 g/L, NH₄F at 5–7 g/L, pH 5.5–6.5, temperature 25–35°C, and a conversion time of 12–16 minutes. Under these conditions, a dense, chromium-free conversion film composed of F, Al, Na, O, and Si was successfully formed, significantly improving corrosion resistance.
(2) During the conversion process, Fâ» etched the aluminum surface, forming sodium fluoroaluminate crystals. Subsequently, SiOâ‚‚ colloids from hydrolyzed fluorosilicates deposited on the film surface, enhancing its protective properties.
**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.
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