Acid etching degreasing process before aluminum alloy coating

The sprayed aluminum alloy profile has excellent corrosion resistance and weather resistance, sturdiness and durability, good appearance decoration, simpler anodizing than the production process, environmental protection and safety, and is widely used in construction, decoration, transportation and other industries. Aluminum alloy profiles must be chemically cleaned and etched in the pre-spray process to remove contaminants such as grease, natural oxide film, and dust that adhere to the surface, leaving the aluminum alloy substrate exposed and forming a uniform activated surface. This is the guarantee of chromium. The key to quality and spray quality of acid conversion coating. The acid etching and degreasing process studied in this paper combines the three processes of degreasing, alkali etching, and decontamination and light extraction of traditional processes into one process, which reduces costs and saves energy. The treated aluminum alloy surface is smooth and smooth, maintaining the original luster of the aluminum alloy. , Aluminum dissolution loss and sedimentation of the tank liquid, and chromate conversion processing supporting performance, especially for aluminum alloy surface treatment before spraying.
First, aluminum alloy coating spray treatment process

Process flow: Workpiece mounting â†’ acid etching degreasing â†’ water washing â†’ water washing â†’ chromate conversion treatment â†’ water washing â†’ pure water washing â†’ draining â†’ drying â†’ spraying â†’ curing.

Aluminum spray pretreatment production line is divided into automatic spraying vertical line and intermittent full-impregnated horizontal line. The vertical line is suitable for large batches and simple shapes. The production efficiency is high and the processing quality is stable. The disadvantage is that the equipment is complex and the investment is large; the horizontal line is suitable for processing complex-shaped workpieces with low investment and wide application. The disadvantage is that the processing speed is higher. Slow, low productivity.

Acid degreased process formulation and parameters: sulfuric acid (97%), 30g/L-50 g/L; ammonium bifluoride, 3 g/L-8 g/L; citric acid, 2 g/L-4 g/L; nitric acid Sodium, 1 g/L-3 g/L; Ferric sulfate, 0.5 g/L-1.5 g/L; Dipropylene glycol, 1 g/L-2 g/L; Degreasing agent, 0.1 g/L-0.5 g/L Temperature, 5Â°C-40Â°C; Time, 3min-5min impregnation, 1min-3min spray; Spray pressure, 0.1MPa-0.2MPa.

Second, the composition and process parameters

1. sulfuric acid

Diluted sulfuric acid is a kind of non-oxidizing inorganic acid, which has slight chemical etching effect on aluminum alloy profiles, lower use cost and less volatile acid mist. Sulfuric acid dissolves and strips away the natural oxide film formed on the surface of aluminum and aluminum alloys; the exposed aluminum alloy matrix reacts with sulfuric acid to activate aluminum surfaces. When the concentration of sulfuric acid in the bath is less than 30%, the chemical dissolution rate of aluminum slowly increases with the increase of the sulfuric acid concentration, and the loss of aluminum is less. When the concentration of sulfuric acid in the bath exceeds 30%, the corrosion rate of aluminum is rapid. Accelerated, easy to produce excessive corrosion, light brightness decreased. In general, the concentration of sulfuric acid in the bath should not exceed 15%.

2. Ammonium hydrogen fluoride

The fluoride ion is used as an accelerator to remove the oxide film in the acid etching degreasing solution, which can accelerate the dissolution of the oxide film on the aluminum surface, eliminate or reduce the extrusion marks and other defects, and make the surface of the aluminum profile smooth, uniform and uniform. Fluorine is a halogen element with small volume and strong electronegativity. It can strongly oxidize aluminum and have certain ability to complex with impurities. It can dissolve alloy elements such as manganese, copper and silicon on the surface of aluminum alloy. Impurities, inhibiting and removing black ash on aluminum, give a brighter metal surface. Ammonium hydrogen fluoride is more chemically stable than hydrofluoric acid and has less harmful effects on human skin than hydrofluoric acid. Therefore, the use of ammonium bifluoride instead of hydrofluoric acid provides fluoride ions for the solution, which is safer in production and can be reduced. Fouling of stainless steel equipment. The corrosion rate of stripped oxide film increases with the increase of ammonium bifluoride concentration. The content of ammonium hydrogen fluoride should not be too high. Otherwise, the reaction may cause excessive corrosion and reduce the luster of aluminum surface, which is not conducive to subsequent chromate conversion treatment.

3. Citric acid

The citric acid mainly acts as a corrosion inhibitor in the acid etching degreasing solution, and can effectively inhibit the corrosion of aluminum in the acidic medium. Citric acid is a ternary organic acid that can physically and chemically adsorb on the surface of aluminum substrates, change the double layer structure at the metal/solution interface, and increase the activation energy of the metal ionization process, thereby slowing the corrosion rate and reducing the Loss amount to avoid excessive corrosion. Citric acid is a complexing agent for metal ions and can form stable complexes with aluminum, magnesium, iron and other ions dissolved from the etching solution, reducing the content of aluminum ions in the bath and masking the interference of impurities in the bath. Stabilizes the solution and reduces sedimentation, thereby prolonging the life of the acid degreased grease. The addition of citric acid has a certain effect on reducing aluminum loss, reducing the consumption of chemical materials, reducing bath sediment, and improving operating conditions.

4. Sodium nitrate

Sodium nitrate is an oxidizing agent in the acid degreaser and has a good light and leveling effect on aluminum alloys. The role of sodium nitrate is to cause oxidation reaction on the surface of the aluminum material. The formed aluminum oxide passivation film suppresses the acidic preferential etching of the depression on the surface of the aluminum, so that the corrosion surface of the aluminum profile is smooth and smooth.

5. Iron sulfate

Trivalent iron ions can accelerate the corrosion of the aluminum surface and remove the oxide film. Fe3+ can undergo displacement reaction on the surface of the aluminum material to form a micro battery, which generates a trace amount of hydrogen on the aluminum surface and takes the oil stain out of the aluminum surface. The reaction process is AL+Fe3. ++2H+â†’AL3++Fe2++H2â†‘. The oxidant in the bath can reoxidize the Fe2+ produced by the above reaction to Fe3+, reduce the accumulation of Fe2+, and stabilize the bath. Therefore, Fe3+ does not consume itself during the reaction, which mainly promotes the decontamination of aluminum. And finish.

6. Dipropylene glycol

In the acid etching degreasing solution, dipropylene glycol and other organic low-molecular-weight solubilizers that are easily dissolved in water and micro-toxicity can be added, which can greatly reduce the surface tension of the solution, and significantly improve the cleaning ability and solution stability of the acid etching degreasing solution. The molecular structure of the oil has a relatively long hydrocarbon group, which is similar to that of an organic solvent and can be mutually dissolved. Therefore, the use of an organic solvent having a strong dissolving power for the oil can improve the degreasing effect of the acid etching degreasing liquid.

7. Degreasing agent

The oils and fats on the surface of aluminum materials can be generally divided into two types: mineral oils, fats and oils, and animal and vegetable fats. Surfactants have good wetting and emulsifying abilities to the above greases. During the process of acid degreasing, surfactants are adsorbed on oil and solution. At the interface, the hydrophilic gene points to the solution, the oleophilic gene points to the oil stain, and it is arranged in an orientation, so that the interfacial tension of the oil and the solution is greatly reduced, the adhesion of the oil on the metal surface is weakened, and the mechanical peeling of the hydrogen gas is produced in the acid corrosion of the metal. Under the oil loosening, it is easy to disperse into tiny oil droplets suspended in the solution from the metal surface. The solubilization of the surfactant can prevent the removed oil from adhering to the surface of the aluminum material. The key to improving the deoiling efficiency at room temperature is the choice of surfactants and the optimization of the ratio. Various surfactants should be selected according to the surface tension, cloud point, critical micelle concentration, and HLB value of the surfactant. Good adsorptivity, wettability and dispersibility, especially strong penetrating and emulsifying power, low foam, good biodegradability, anionic and non-ionic surfactants with stable performance in acidic solution, compounding Adjust the HLB value to 12-16 to suit the range of solution cleaning. The synergistic effect of the two components can achieve mutual compensation and synergistic effects, reduce the surface tension and critical micelle concentration of the solution, increase the cloud point, increase the surface activity, and significantly improve the wetting performance and cleansing effect. improve. In the case of acid degreased by spraying, a large amount of foam will be generated due to the action of mechanical force, and the solution will overflow from the tank in the form of foam. Therefore, an appropriate defoamer should be added to suppress the generation of foam. Tests have shown that choosing sulfonate anionic surfactants, alkylphenol polyoxyethylene ethers, polyether nonionic surfactants for compounding can achieve good degreasing effect.

8. temperature

The test results show that the removal rate of oil-removing oxide film increases with increasing temperature. The increase in temperature can reduce the viscosity and interfacial tension of oil stain, improve the etchability of aluminum material to aluminum, and enhance the emulsification of surfactant and oil. effect. When the temperature is lower than 5Â°C, the speed of deoiling and removing the oxide film is slowed down, which affects the production progress; when the temperature is higher than 40Â°C, the components of the acid degreasing and degreasing liquid are volatile, resulting in an acid mist, an increase in acid loss, and energy consumption. Increased, aluminum is prone to over-corrosion and black ash. Therefore, the use temperature of acid degreased fluid should be controlled within the range of 5Â°C-40Â°C.

9. time

The treatment time is too short, the surface of the workpiece oil, natural oxide film is not completely removed, will hinder the chromate conversion filming reaction to normal; processing time is too long, will increase the loss of aluminum, the workpiece surface roughness, gloss Decline, lower productivity. The processing time is closely related to the composition of aluminum, the amount of oil on the surface, the thickness of the natural oxide film, the processing temperature and the method, etc. It is generally taken from 1 minute to 5 minutes. The principle of determining the workpiece is to remove the oil and natural oxide film completely. Acid degreasing time.

Third, the application of acid etching degreasing process

1. Bath preparation

First add 1/2 volume of tap water to the tank, slowly add the calculated amount of concentrated sulfuric acid under stirring, and cool to about 40Â°C; then add the calculated amount of other ammonium hydrogencarbonate, citric acid and other chemicals in order, and stir until completely dissolved; Then add water to the specified volume, stir well, and determine the acidity before putting it into use.

2. Bath solution management

With the continuous increase in the amount of aluminum alloy processing, acid degreased liquid is continuously consumed and brought out during use, the acidity and degreasing effect are reduced, and various effective components are gradually reduced. Therefore, regular analysis of bath solution should be conducted regularly to make timely Sulfuric acid, ammonium bifluoride and other chemicals are added to keep the bath process parameters within the specified range. The oil on the surface of the workpiece is displaced, dispersed, and then agglutinated and suspended on the surface of the acid degreased fluid. The oil on the surface of the fluid must be spilled to prevent the workpiece from being stained with oil for the second time. After long-term use, when the accumulation of aluminum ions in the bath solution is too high, the bath solution should be partially or completely replaced.

3. Quality inspection

Visual inspection, after the treatment of acid degreased degreasing, the surface of the aluminum alloy profile exposed metal color, smooth, bright, oil and natural oxide film in addition to the net, after washing, the water film is continuous, complete, the surface is completely wet, do not hang water, no Black ash and excessive corrosion.

Fourth, the conclusion

1. The acid etching and degreasing process completes the processes of degreasing, alkaline etching, decontamination, and light extraction at one time. Compared with the separation process, the process is simplified, the area and equipment investment are reduced, and chemical raw materials and cleaning water are saved.

2. When used at room temperature, the bath does not need to be heated to save energy, no evaporation of the acid mist, no pollution to the working environment, corrosion of the stainless steel equipment and plants by the acidic substances, and prolonged service life of the cleaning equipment and the plant.

3. Strong oil removal ability, quick removal of oxide film, high corrosion inhibition efficiency, low aluminum melting loss, no over-corrosion and black ashing. After processing, the workpiece is silver white, bright and shiny, and the surface quality is good.

4. The solution has stable performance, less sediment, convenient management and maintenance, and the bath can be continuously recycled for a long period of time. Each tank can be used for more than one year.