【Abstract of the Chinese Aluminum Industry Network】Abstract: The non-corrosive area fraction of the 6063 aluminum alloy as the substrate was measured in the conversion slag consisting of fluorosilicate and ammonium fluoride. The influence of the composition and processing conditions of the conversion liquid on the corrosion resistance of the fluorosilicate conversion coating was investigated. The optimized fluorosilicate conversion process parameters are: Na2SiF63-5g/L, NH4F5-7g/L, pH5.5-6.5, temperature 25-35. C, smells 12 to 16 minutes during conversion. After the fluorosilicate treatment, a dense chromium-free conversion film consisting of F, Al, Na, O, and Si was obtained on the surface of the aluminum alloy, and the corrosion potential of the aluminum alloy was significantly positively shifted, and the corrosion resistance was improved.

Key words: aluminum alloys; fluorosilicates: conversion coatings; resistance properties CLC: TQ153.6 Document designation code;: A

Article ID: 1004-227X(2012)05-0037-04

1 Introduction China is a big country in the production and use of aluminum alloys. Aluminum alloys are widely used in aerospace, automotive electronics, and machinery and food industries. Since the standard electrode potential of aluminum alloy is -1.67V, the chemical properties are lively, and corrosion occurs under acidic or alkaline conditions, which affects the normal use of aluminum alloys. Therefore, industrial applications often require corrosion protection of aluminum alloys. Commonly used processing technologies include anodization, micro-arc oxidation, chemical conversion, and coating techniques [1-3]. Due to its economical and effective advantages, chromate conversion has been widely used for a long time. However, due to the carcinogenic toxicity of hexavalent chromium, countries and regions have successively banned the use of chromate metal surface treatment technology. Trivalent chromium conversion technology is currently widely used as a transitional technology to replace chromate conversion. However, trivalent chromium is still toxic and there is a danger of being oxidized to hexavalent chromium. Therefore, it is very urgent to study the new technology of environmentally friendly chromium-free chemical conversion treatment with good corrosion resistance and stable process operation.

Shi et al. [4-7] studied the rare earth conversion coating on aluminum alloy surface and achieved positive results, but the process stability needs to be improved. Wang et al [8-9] studied the passivation effect of molybdate and manganate on Altai gold, but the corrosion resistance of the molybdate conversion film and the stability of the manganate need to be strengthened. MA, Smit et al [11-13] added organics to fluorotitanic acid or fluozirconic acid to treat aluminum alloys to obtain conversion films with good corrosion resistance, but this technology requires high pretreatment of aluminum alloys and is only applicable to Ordinary aluminum alloy. US Air Force Materials Preparation Institute NN. Voevodin et al. [14] used sol-gel method to produce a highly corrosion resistant Si02 coating on the surface of 2024 aluminum alloy. Domestic studies and reports on fluorosilicate chemical conversion coatings are rare. In this paper, the preliminary study on the film formation of fluorosilicate was conducted.

2 Experiment 2.1 The experimental material used a 6063 aluminum film with 3.5cmx1.5cmxO.1cm as the substrate. The main components (mass fraction) were: Si0.20%-0.60%, Fe0.35%.CuO.10%, Mn

L.00%.Mg0.45% to 0.90%, CrO.10%, TiO.10%, Z front row 0%, Al balance.

2.2 Process Degreasing - Washing - Alkali Washing - Washing - Debris Removal - Washing - Passivation - Washing and Drying - Aging.

2.3 Formulations and processes (1) Degreasing: Turc04215NC-LT degreasing fluid (Turco Products Co., Ltd., USA) 50g/L, 60°C, 5min.

(2) Alkaline washing: NaOH solution with a mass fraction of 5%, room temperature, 60-90S.

(3) Despeckling and light emission: SmutGoNC light-emitting solution (manufactured by Turco, USA), room temperature, 5 min.

(4) passivation: Na2SiF62.5 ~ 5.0g / L, NH4F5 ~ 7g / L, 25 ~ 35 °C, pH5.5 ~ 6.5, 12 ~ 16min.

(5) Aging: Place at room temperature for 24 hours.

2.4 Determination of Free Fluoride Ion Concentration Using Fluoride Ion Selective Electrode Method: The concentration of free fluoride ion in the conversion solution was analyzed by a standard curve method using a PFS-SO type fluoride ion concentration meter (Shanghai Dapu Instrument Co., Ltd.). The working electrode is a fluoride ion selective electrode using a cesium fluoride single crystal as a sensing membrane, and the reference electrode is a 232 saturated calomel electrode. The magnetic ion stirring method is used to prepare the total ionic strength adjustment buffer: 58 g NaCl and 57 mL ice, respectively. Acetic acid and 0.30 g of C6H507Na3•2H20 were dissolved in 500 mL of water, adjusted to pH 5.0-5.5 with NaOH, and diluted with water to 1 L.

2.5 Determination of properties (1) Morphology and composition: The Oxford INCA-type spectrometer was used to observe and analyze the morphology and composition of the film in a vacuum environment.

(2) Corrosion resistance: The corrosion resistance of the conversion film was evaluated by a neutral salt spray test and a polarization curve. The salt spray test was conducted in a SY/Q-750 neutral salt spray box (Shanghai Maijie Experimental Equipment Co., Ltd.). The specific operation was in accordance with the ASTM B117 standard: the sample was exposed to a constant temperature of 35°C and a relative humidity of 100%. The corrosive medium was a 5% (mass fraction) Na0 solution, the pH was 6.5-7.2, the test time was 168 hours, and the test piece was 150-300 with the vertical direction. Polarization curve test using PARSTAT2273 electrochemical workstation (Ametek, USA) with saturated calomel electrode (SCE) as the reference electrode, platinum electrode as the auxiliary electrode, and aluminum alloy as the working electrode (the exposed part is a circle with a diameter of 1cm2) The scan rate is lmV/s. The corrosive medium is a 3.5% (mass fraction) NaCl solution.

3 Results and discussion 3.1 Effect of conversion fluid composition on the corrosion resistance of conversion film 3.1016 Mass concentration of Na2SiF6 when converted to 6063 aluminum sheet at NH4F 6.0g/L, pH=5.5, 25°C for 12 min. The effect of the concentration on the corrosion resistance of the conversion film is shown in Figure 1. Fig. 2 is a graph showing the change of the free fluoride ion concentration in the conversion solution with the sodium fluorosilicate mass concentration. The mass concentration of NH4F in the transitional fertility was 6 g/L, ie, the mass concentration of F element rhythm was 3.0 g/L. Since NH4F is a weak electrolyte, the mass concentration of free fluoride ions (F-) in the solution when Na2SiF6 is not added is lower than 3.0 g/L. With the increase of Na2SiF6 concentration, the F-mass concentration in the solution increased, and quickly exceeded 3. Og/L. Figure 2 shows that Na2SiF6 can be hydrolyzed to F- after adding the conversion solution. Figure 1 and Figure 2 show that when the mass concentration of Na2SiF6 is too low (<2g/L), the mass concentration of F in the conversion solution is too low, and the etching effect of the cerium cannot be sufficiently formed on the surface of the aluminum base material, and the film formation is incomplete. Metallographic observations revealed that the film-forming particles were dispersed and no signs of film formation were found. The salt fog test quickly began to corrode, showing dark spots and a rapid corrosion rate; when the mass concentration of Na2SiF6 was high (>6 g/L) The mass concentration of F- in the solution is too high, and over-etching causes ash on the surface of the aluminum, which reduces corrosion resistance. In summary, as the main film-forming substance, the concentration of Na2SiF6 in the conversion solution has a very important influence on the corrosion resistance of the conversion film. When the mass concentration of Na2SiF6 in the conversion liquid was 3—5 g/L, the corrosion resistance of the conversion film was good. After the salt spray test for 168 h, there was no obvious black streaks, dark spots or pitting on the surface of the test piece.

3.1.2 The mass concentration of NH4F F- can activate the surface of the aluminum alloy and promote the formation of the film. Too much or too little F- is not conducive to the conversion reaction. Under the conditions of Na2SiF64.0g/L, pH=5.5 and 25°C, the 6063 aluminum flakes were transformed for 12min. The influence of the concentration of NH4F in the conversion liquid on the corrosion resistance of the conversion coating was shown in Fig.3. When the mass concentration of NH4F is lower than 4g/L, the etching effect on the surface of the aluminum alloy substrate is insufficient, the filming reaction is not carried out, and the film formation is incomplete, which is consistent with the results of Chen Dongchu et al. [15]; NH4F When the mass concentration is higher than 7g/L, a conversion film with good corrosion resistance cannot be obtained on the surface of Altai gold due to excessive etching; when the mass concentration of NH4F is 5-7 g/L, the aluminum alloy matrix can be satisfied. The etching will not hinder the smooth formation of the conversion film due to excessive etching. Compared with NaF, NH4F is used in this process. On the one hand, the effect of NH4F is better than NaF. On the other hand, the solubility of Na2SiF6 is not high (only 6.52g/dm3 at 25°C [16]). The presence of NH4+ can promote the dissolution of Na2SiF6. , improve the solubility of Na2SiF6.

3.1.3 pH

Under the conditions of Na2SiF64.0g/L, NH4F6.0g/L and 25°C, the 6063 aluminum flakes were transformed for 12min. The influence of different pH on the corrosion resistance of the conversion coating was shown in Fig. 4. When the pH is less than 40, the dissolution rate of H- and F- on the surface of the aluminum matrix is ​​too fast to form a complete conversion film on the surface of the matrix. As the pH increases, the corrosion resistance of the conversion film is improved first and then deteriorated. The trend is; pH> 7.0, the conversion solution is not stable, SiF62- easy dissociation precipitation, NH4F will decompose, conversion liquid failure. Therefore, pH is an important factor affecting the passivation process of the conversion liquid, and the preferred pH is 5.5-6.0.

3.1.4 Temperature Under the conditions of Na2SiF64.0g/L, NH4F6.0g/L, pH=5.5, the surface of the aluminum alloy was transformed for 12 minutes, and the influence of the transformation temperature on the corrosion resistance of the conversion film was investigated. The temperature increase has no significant effect on the film formation, probably because F- is added to the conversion solution and the conversion rate is very fast, so the temperature has little effect on the film formation [15]. In comparison, the corrosion resistance of the conversion film obtained at 25 to 45° C. is better than that at 15 to 20° C., but when the temperature is higher than 40° C., the stability of the conversion solution is affected to a certain extent, and the resulting conversion film performance is not improved. good. Taking into account the process stability, the preferred temperature is determined to be 25-35°C.

When the 315 was transformed, the effect of transformation time on the corrosion resistance of the conversion film was investigated under the conditions of Na2SiF64.0g/L, NH4F6.0g/L, pH5.5, and 25°C.

As can be seen from Figure 6, the corrosion resistance of the conversion coating has been greatly improved after 2 minutes of conversion, which also indicates that the conversion reaction rate is very fast. This process is mainly F-etched aluminum to form a fluoroaluminate conversion film. The subsequent conversion reaction is relatively slow, but it plays a crucial role in improving the corrosion resistance of the conversion film. After 168h salt spray test, the un-corroded area fraction of the conversion film obtained after 12 to 20 minutes of treatment was higher than 85%, and the corrosion resistance was good. Therefore, the conversion time is preferably 12 to 16 minutes.

3.2 The surface morphology of conversion film Hard composition analysis Na2SiF64.0g/L, NH4F60g/L, pH=5.5, 25°CF- transformation of aluminum Taiwan gold 12min, the resulting fluorosilicate conversion film surface morphology and micro-A The composition analysis is shown in Figure 7 and Table 1, respectively.

The results of the previous study of the comparative research group found that [17] the crystal structure of the sodium fluorosilicate conversion film is similar to that of the sodium fluoroaluminate conversion film, but the O and Si components are increased in the conversion film, and the surface of the particles and the gaps between the particles are The content of O and Si is basically no difference. In the conversion liquid, F- strongly etched aluminum and coordinated with aluminum ions to form AlF63-, and then reacted with AlF63-+3Na+→Na3AlF6 ↓ (normal temperature Na3AlF6 solubility product was 4x10-10) to produce sodium fluoroaluminate precipitation conversion film [ 17-18]. The SiO2 colloids hydrolyzed by SiF62- in aqueous solution deposit on the surface of the conversion film particles to enhance the corrosion resistance of the conversion film [19-20].

3.3 Polarization Curve Figure 8 shows the polarization curves of 6063 Altai gold before and after fluorosilicate treatment in 3.5% NaCl solution. It can be seen from Fig. 8 that after the fluorosilicate treatment, the corrosion potential of the 6063 aluminum alloy significantly increases from -0.91 V to -0.79 V, and the corrosion current density slightly decreases. The positive shift of the corrosion potential and the decrease of the corrosion current density indicate that the anode process is hindered, that is, the dissolution corrosion of the aluminum alloy is more difficult to occur [21].

4 Conclusions (l) Fluorosilicate conversion of 6063 aluminum alloy, the best process conditions: Na2SiF63 ~ 5g / L, NH4F5 ~ 7g / L, pH5.5-6.25 ~ 35 °C, 12 ~ 16min. Under the optimal process conditions, a chromium-free conversion coating with good corrosion resistance composed of F, Al, Na, O, and Si was successfully prepared on the surface of 6063 aluminum alloy.

(2) During the conversion process, the F-pre-etched Al-Taijin Au surface wells in the conversion liquid form a dense conversion film layer of sodium fluoroaluminate crystal particles, and then the SiO2 colloids produced by hydrolysis of fluorosilicate are deposited on the surface of the conversion film particles. The conversion film has enhanced 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 chemical conversion of environment-friendly aluminum alloys [J]. Research and Application of Materials. 2009, 3(1): 1-4

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