Application of surface pretreatment technology for acid etching aluminum alloy
1. Process introduction

In recent years, the production and sales of aluminum alloy matt profiles are increasing, with a market share of over 80%. At present, the alkali etching method is widely used in the production of matt profiles. The aluminum solution loss is between 3% and 5%, which causes huge waste of resources, and greatly increases the difficulty of environmental protection and increases the burden of “three wastes”. The acid etching technology is developed for the deficiency of the alkali etching method, and is completely different from the alkali etching matte sand surface treatment in principle. The process mainly uses a unique high-density, high-density molecular pitting corrosion on the surface of the aluminum profile to improve the surface structure of the profile and eliminate the surface defects caused by the metallurgical process (such as eliminating the squeeze). Indentation, cover welding wire, and alleviate defects such as coarse grains, so that the surface of the aluminum profile is smooth and fine, and the metallic luster is soft, which greatly improves the surface quality of the aluminum profile. Using acid etching matt process, the aluminum solution loss is between 0.5% and 0.8%, which reduces the production cost to less than one-half of the traditional matt process, and the production efficiency is greatly increased. It has great practicality. Economic Value.

2, acid etching mechanism

Acid etching is chemical etching under specific conditions. The aluminum profile contacts the etchant and reacts rapidly to form a complex in a short time. The reaction equation is as follows:




The resulting complex has a certain viscosity and is easily attached to the surface of the aluminum substrate to form a protective film. Extrusion profiles generally have many slight mold marks, which are peak-shaped under a high power microscope. The protective film adhered to the aluminum matrix is thicker in the concave portion, the convex portion is thinner, and the chance of contact with the acid etching solution is larger. The dissolution reaction can be continued relatively quickly; the thick portion is shielded by the adherent, and the reaction speed of the dope is slowed down, and the dissolution rate of the concave and convex portions is different, thereby achieving the flat surface and eliminating the crush marks (sand surface). Light effect. Relieving the coarse crystal and the weld line is directly related to the specific performance of the acid etching. In the aluminum profile production, the coarse crystal and the weld line have a great relationship with the mold, the extrusion process and the homogenization of the cast rod. The deeper the alkali corrosion, the more severe the coarse crystals and weld lines. Since the acid etching speed is fast and the time is short, the reaction is finished without serious coarse crystals and weld lines, so the acid etching process can effectively alleviate the coarse crystals and the weld line.

The main effects of the acid etching additive in the acid etching process are as follows: (1) Corrosion inhibition: Since F- reacts with natural oxide film and aluminum matrix quickly, and the reaction is severe, coarse crystal and weld line are prone to occur. Such surface quality defects. In order to maintain a certain reaction speed without causing quality defects, an organic compound such as sodium gluconate, sodium citrate or glycerin is added to suppress the acid etching reaction rate, thereby achieving a corrosion inhibition effect. (2) Leveling action: There are many slight mold marks on the extruded profile. It is observed as a mountain peak under high power microscope. Adding compounds such as trisodium phosphate, diatomaceous earth and polyol can protect the profile bottom of the profile. Smoothing to achieve leveling. (3) Promoting effect: adding the accelerator can fully diffuse the acid etching additive into the bath liquid, and effectively enter the inner layer of the surface of the aluminum substrate covered by the complex, so that the reaction can be continued, so as to avoid uneven corrosion of the profile. (4) Surfactant effect; adding surfactants such as sodium dodecylbenzenesulfonate and sodium lauryl sulfate can further enhance the degreasing ability, and generate a layer of foam on the surface of the bath to suppress heat loss and grooves. The liquid volatilization, more importantly, is to increase the stability of the bath. This is because the negatively charged surfactant can adsorb around the particles, causing the particles to generate electrostatic reaction forces, so that the particles can be well suspended in the bath and not easily precipitated. . (5) Metal ion action: adding metal ions such as Cu2+ and Ag+ can prevent over-corrosion, making the sand surface effect more delicate, uniform and clear.

3. Test

3.1 Test purpose

Through the acid etching and alkali etching process tests on the aluminum profiles, the surface quality of the profiles obtained by the two processes and the amount of dissolved aluminum in the production process were compared.

3.2 Test materials

Take the same batch of curtain wall profiles of the same model size as test materials. Curtain wall profiles, especially those with large geometrical dimensions and thick wall thickness, are prone to defects such as coarse crystals and weld lines after conventional alkali etching, and are comparatively representative of acid etching and alkali etching processes.

3.3 Test content

3.3.1 Alkali etching process test

Two samples were taken for alkali etching in the alkali etching solution, and other processes were operated according to a conventional oxidation process. The alkali etching process test requirements are shown in Table 1.

Table 1 Alkaline etching process test requirements

3.3.2 Acid etching process test

Two samples were taken for acid etching in the acid etching solution. The profile is acid-etched, washed with water, and alkali-washed in an alkali solution for 1 min. The other processes are operated according to a conventional oxidation process.

3.4 Test results and analysis

Observe the surface state of the 1# and 2# samples prepared by the alkali etching process. 1#The surface of the sample is fine and has slight extrusion marks. The coarse crystal and the welded wire are not obvious. The surface of the 2# sample is clear and the extrusion marks are not visible, but there are slight coarse crystal and weld line exposure. . It shows that with the increase of alkali corrosion time, the surface extrusion marks of the profile are flattened, the sand surface is finer and more uniform, but the defects such as coarse crystal and weld line are easily exposed.

Observe the surface state of the 3# and 4# samples prepared by the acid etching process. The surface quality of the two samples is almost the same, both of which show that the sand surface is fine and soft, and there is almost no existence of extrusion marks, coarse crystals and weld lines. It shows that the acid etching process can obtain the quality of the sand surface in a few minutes, the extrusion marks are eliminated, the welding line can be effectively covered, the coarse crystal phenomenon can be alleviated, and the surface of the profile is not easy with the increase of the etching time. Produced over corrosion.

The wall thickness before measurement of 1# and 2# samples was 3.0mm, the wall thickness measured by 1# sample after oxidation was 2.96mm, the wall thickness loss was 0.04mm; the wall thickness measured by 2# sample was 2.92mm, wall thickness The loss is 0.08mm. It indicates that the alkali etching time increases, the corrosion amount of the profile increases, and the matte profile of the sand surface is produced, and the amount of alkali-dissolved aluminum is more than 3%.

The wall thickness before measurement of 3# and 4# samples was 3.0mm, and the wall thickness measured by 3# sample after oxidation was not less than 2.99mm, the wall thickness loss was not more than 0.01mm; the wall thickness measured by 4# sample was 2.99. Mm, the wall thickness loss does not exceed 0.01mm. It shows that the acid etching time increases, the aluminum content of the profile increases little, and the sand surface matt material is produced, and the amount of acid etching aluminum is above 0.5%.

3.5 Test conclusion

From the above test results and analysis, it can be concluded that the oxidation treatment of aluminum profiles is feasible by acid etching. Compared with alkali corrosion, the loss of aluminum material of the matt sand surface profile is greatly reduced after acid etching, and the surface quality of the profile is improved, the pretreatment time is shortened, the production efficiency is improved, and the comprehensive cost is lowered.

4, production applications

4.1 Process

Degreasing-washing-etching-washing-alkali washing-water washing-water washing-neutralization-water washing-conventional anodizing

(Circular filtration when etching)

The aluminum profile enters the acid etching process after degreasing and water rinsing. The acid was etched and rinsed with water and then alkali washed for 30 s to 1 min to remove the surface-adhered complex. After water washing and neutralization, anodizing (coloring) and sealing treatment are performed in accordance with a normal oxidation process.

The filtering device can effectively ensure the slag removal effect of the bath liquid.

Some manufacturers' circulating filter devices are just a plate and frame filter press. With the increase of filter residue, the flow rate of the bath liquid is not enough, resulting in poor slag removal and affecting the use of the bath liquid.

4.3 common faults and countermeasures

(1) The surface is slightly spotted

Local spot spots appear on the surface of the profile, indicating that the slag discharge is poor, and the acid etch tank has aluminum slag. It is necessary to improve the liquid discharge method and increase the amount of aluminum slag recovery.

(2) matte surface

The surface of the produced profile is rough and dull, indicating that the amount of accelerator in the acid etching additive is insufficient, and the accelerator content is required.

(3) The surface has mechanical grain

The surface of the produced profile has mechanical grain, indicating that the etching time is not enough, or the content of the acid etching additive in the bath is low, and the ammonium hydrogen fluoride or the additive content needs to be added.

(4) Surface over-corrosion

Excessive corrosion on the surface of the profile indicates that the etching time is too long, or the corrosion inhibitor and metal ion content in the acid etching additive are insufficient. The acid etching time needs to be adjusted to supplement the acid etching additive content in the bath.

5, the conclusion

The application of acid etching surface pretreatment technology has received good surface quality effects and considerable economic benefits. In the improvement of product surface quality, reduce production costs, open up new ways for high-grade, intensive production.

专栏:Industry information
作者: 佚名
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