Degreasing of Aluminum Alloy Anodizing Process

There are oily substances on the surface of aluminum after rolling, extrusion or machining, and this dirt is still attached to the surface. In addition, there will be a natural oxide film on the surface of aluminum. Moreover, after the surface of aluminum is mechanically finished, the surface of aluminum will be attached with waste powder and other residues produced by mechanical polishing. These foreign substances must be completely removed to present a clean aluminum surface. The surface cleaning and purification process described above is called degreasing.

​ Degreasing is the first process of chemical pretreatment of aluminum. After this process is done well, the subsequent alkaline washing can be more uniform to improve the quality of the anodized film. The quality of degreasing can be checked by the surface wettability effect. After degreasing and water washing, the surface of aluminum should be completely and evenly wetted with water, and the continuity of the water film should be maintained for at least 30s to be qualified.

The degreasing of aluminum includes acid degreasing, alkaline degreasing, organic solvent degreasing, electrolytic degreasing and ultrasonic degreasing.

Acid degreasing

​Acid degreasing is mostly carried out at room temperature. It is easy to control production and has low cost. It has replaced traditional alkaline degreasing and has become the main process for degreasing aluminum (especially aluminum profiles). In an acid degreasing solution based on sulfuric acid, phosphoric acid or nitric acid, oil will undergo hydrolysis to generate glycerol and corresponding higher fatty acids. Adding a small amount of wetting agent and emulsifier to the solution is conducive to the softening, freeing, dissolving and emulsification of oil, and significantly improves the degreasing effect. When the sulfuric acid content in the acid degreasing solution is insufficient, it can also be supplemented with "waste sulfuric acid" in the anodizing tank. This can ensure the sulfuric acid concentration required for the degreasing tank and effectively reduce the aluminum ion concentration in the anodizing tank at the same time, without increasing the production cost of degreasing and the burden of wastewater treatment.

Acid degreasing is a degreasing method that does not corrode or damage the surface of aluminum. For aluminum alloys, acid degreasing causes less damage to the surface than alkaline degreasing.

​Alkaline degreasing

The use of alkaline degreasing in aluminum surface pretreatment is a traditional process. Aluminum is an amphoteric metal and is easily corroded by alkaline solutions. Therefore, a solution with a lower alkalinity can be used for surface degreasing and cleaning. A degreasing solution with too strong alkalinity may cause uneven corrosion of the aluminum surface, because it tends to corrode the clean surface of the aluminum faster, while the corrosion rate of the surface with grease is slower, which may cause cleaning marks on the aluminum surface.

The working principle of alkaline degreasing is: the alkali first reacts with the grease to saponify, generating soluble soap, and the saponification reaction is used to eliminate the combination of grease and the aluminum surface. The saponification reaction is a chemical reaction between grease (mainly stearic acid) and alkaline substances (such as sodium hydroxide) when heated together to generate soap and glycerin. Then the surfactant in the alkaline degreasing reduces the surface tension, and due to the excellent emulsification, dispersion and penetration, the purpose of degreasing is achieved.

Generally speaking, alkaline degreasing solution should meet the following requirements:

① All salt substances should be completely soluble, stable in performance, and easy to wash off;

② The alkalinity is suitable for the base metal aluminum, and the pH value is preferably between 9 and 11;

③ The buffering property of alkalinity is good, which is conducive to maintaining stable activity;

④ It has good wetting ability, and the cleaning time generally does not exceed 15 minutes;

⑤ It has high emulsification ability, can effectively dissolve and dissipate grease, and form soluble saponification with it;

⑥ It has the ability to disperse and condense other pollutants such as dust, and dissipates the polluted particles in the solution by colloid action;

⑦ It has an inhibitory effect on the erosion of the aluminum substrate;

⑧ Environmental protection and hygiene requirements are non-irritating to the skin, preferably non-toxic, and convenient wastewater treatment;

⑨ When using hard water for preparation, the salt in the hard water will not be deposited on the aluminum surface.

Of course, production costs must also be considered. In the process selection of chemical pretreatment, production costs are a factor that must be considered. The biggest problem with alkaline degreasing is that the solution needs to be heated, which not only increases production costs, but also brings certain difficulties to production control. The alkaline degreasing solution is generally controlled at a temperature of 45-65°C. The temperature often drops during the production process, and it has to be heated frequently. The manufacturer should strictly control the temperature range specified by the degreasing agent supplier. Generally, if the temperature is too low or too high, it will have a serious impact on the degreasing effect and may even cause the product to be scrapped. For this reason, it is best for manufacturers with conditions to install temperature automatic control devices. In addition, the concentration range of alkaline degreasing agents is much narrower than that of acidic degreasing agents, and attention must be paid to controlling concentration changes during production.

Organic solvent degreasing

Organic solvent degreasing is a degreasing method that uses the characteristic that oils and fats are easily soluble in organic solvents. Organic solvents can dissolve both saponified oils and non-saponified oils, have strong degreasing ability, and are fast and non-corrosive to aluminum substrates. However, organic solvents are flammable and toxic. Unless they are really needed, they should be avoided as much as possible. The organic solvents available for use are: carbon tetrachloride, trichloroethylene, benzene, gasoline, kerosene, alcohol and acetone, etc.

After degreasing with organic solvents, it is generally not possible to obtain a surface as clean as acid degreasing or alkaline degreasing. There is often a residual film on the treated surface, which requires further post-processing (alkaline washing). In addition, organic solvents are highly volatile, easy to ignite, and have high costs. Therefore, they are very limited in their promotion and use. They are mainly used for small batches, small workpieces, or extremely dirty aluminum products. For example, cleaning the residues on the surface of mechanically polished aluminum products requires a long cleaning time because of the acid degreasing or alkaline degreasing treatment, which will cause serious gloss loss. However, there is no corrosion problem or gloss loss problem with organic solvent degreasing. After degreasing with organic solvents, only a short post-processing is required to complete the cleaning, thereby effectively maintaining the brightness obtained by the original polishing. For some special aluminum workpieces whose blank surfaces were originally protected with lanolin or some kind of paint, organic solvents are often used to pre-remove the protective film. Trichloroethylene is commonly used for bath degreasing. The boiling point of trichloroethylene is 87.19℃, and the density of its vapor is about 4.5 times that of air, so it is easy to escape. However, trichloroethylene is toxic, so condensation equipment must be installed.

Electrolytic degreasing

Degreasing is carried out by relying on the large amount of gas discharged by the electrolytic reaction. This is a degreasing method that uses physical action. When complete degreasing is required, the electrolytic degreasing method is used. It is used for brightening treatment or bright chrome plating.

In the electrolytic degreasing method, the representative electrolyte components are trisodium phosphate, sodium phosphate, and surfactant. The electrolyte concentration of the electrolytic degreasing tank is 15-30g/L, the temperature is 60-70, and the voltage is 6V. The object to be treated is used as the cathode for 20-60S of cathode treatment, and then 10min of anodic degreasing.

Ultrasonic degreasing

Ultrasonic degreasing cleaning is a method that uses the interaction of ultrasonic waves and cleaning agents to achieve the purpose of cleaning. The frequency of the ultrasonic cleaning machine is a high-frequency sound wave above 16kHz. When ultrasonic waves of considerable power are emitted into the degreasing tank, the tank liquid vibrates, the pressure inside the liquid suddenly decreases at a certain moment, and then the pressure suddenly increases at a moment, and this is repeated continuously. When the pressure suddenly decreases, many small vacuum cavities will be generated in the solution, and the gas dissolved in the solution will be sucked into the cavities to form bubbles. At the moment of bubble formation, the pressure suddenly increases, the bubbles are crushed, and shock waves are generated. Under the action of the shock wave, the moment the bubbles burst, instantaneous high temperature and high pressure will also be generated, which will accelerate the convection in the tank liquid, quickly remove the oil stains from the metal surface, and achieve the degreasing effect. Even without adding any degreasing agent, ultrasonic waves have the effect of degreasing and oil removal.

Ultrasonic waves use continuous instantaneous shock waves to peel off the oil stains. The cavities, high temperature and high pressure caused by ultrasonic waves have a strong mixing effect on the parts, which strengthens the dissolution, saponification and emulsification effects of the degreasing agent and improves the degreasing efficiency.

The management of the degreasing tank liquid should be regularly analyzed according to the concentration of the main components, and the insufficient parts should be replenished in place. During the continuous degreasing process, the contamination status of the degreasing tank and the degreasing effect are determined so that the tank liquid can be updated in a timely manner.