Process control for surface conditioning and phosphating treatment of aluminum before spraying

Surface Adjustment in Phosphating Process

Surface conditioners can even out the surface irregularities caused by degreasing with alkaline solution or rust removal by acid pickling, creating a large number of extremely fine crystallization centers on the aluminum surface. This accelerates the reaction speed of the phosphating process and is beneficial for the formation of the phosphating process film.

(1) Impact of Water Quality

If the water used for the tank solution contains severe rust or a high content of calcium and magnesium ions, it will affect the stability of the surface treatment solution. When preparing the tank solution, a water softener can be added in advance to mitigate the impact of water quality on the surface treatment solution.

(2) Usage Time

Generally, colloidal titanium salts are used for leveling. They possess colloidal activity, and when used for a long time or when containing more impurity ions, the colloidal activity will be lost. At this time, the colloidal stability is destroyed, the bath solution precipitates and separates into layers, forming a flocculent state. The bath solution must be replaced at this point.

Phosphating Process

The phosphating process is a chemical and electrochemical reaction that forms a phosphate chemical conversion film, and the film formed is called a phosphating process film. In bus painting, low-temperature zinc-based phosphating process solutions are commonly used. The main purpose of the phosphating process is to provide protection for the base aluminum material and to a certain extent prevent aluminum corrosion; it is used as a primer before painting to improve the adhesion and corrosion resistance of the paint layer. The phosphating process is a very important part of the entire pretreatment process, with a complex reaction mechanism and many influencing factors. Therefore, the production process control of phosphating solution is much more complex compared to other solution baths.

(1) Acid ratio (the ratio of total acidity to free acidity)

Increasing the acid ratio can speed up the reaction rate of the phosphating process, resulting in a thinner and finer phosphating film. However, if the acid ratio is too high, the film layer may become excessively thin, which can lead to gray deposits on the phosphated workpiece. If the acid ratio is too low, the phosphating reaction is slow, the crystals in the phosphating film are coarse and porous, corrosion resistance is low, and the phosphated workpiece is prone to yellow rust. Generally, the required acid ratio varies depending on the phosphating bath system or formula.

(2) Temperature

Properly increasing the bath temperature accelerates film formation. However, if the temperature is too high, it can affect changes in the acid ratio, thereby impacting the stability of the bath. At the same time, the nuclei of the film layer become coarse, and the amount of sludge produced in the bath increases.

(3) Sludge Amount

As the phosphating process continues, the amount of sludge in the bath gradually increases. Excessive sludge can affect the interfacial reaction on the workpiece surface, causing the phosphating film to become uneven, dusty, or even fail to form. Therefore, the bath must be emptied and cleaned in a timely manner according to the quantity of workpieces being processed and the usage time to remove sludge and precipitates.

(4) Nitrite NO-2 (Activator Concentration)

NO-2 can accelerate the phosphating reaction, improving the density and corrosion resistance of the phosphating film. If the concentration is too high, white spots or iridescence may appear on the film; if too low, the film forms slowly, and the phosphating layer is prone to yellow rust.

(5) Sulfate ion SO2-4

If the concentration of the pickling solution is too high or the rinsing is not well controlled, it can easily lead to an increase of sulfate ions in the phosphating process solution. Excessive sulfate ions will slow down the reaction rate of the phosphating process, making the phosphating film grains coarse and porous, causing severe gray deposits, and reducing the corrosion resistance of the phosphating film.

(6) Ferrous ions Fe2+:

When the amount of ferrous ions in the phosphating solution is too high, it can reduce the corrosion resistance of room temperature phosphating films; cause medium temperature phosphating film grains to be coarse, surfaces to have white powder deposits, and reduce corrosion resistance; and increase sediment formation in high temperature phosphating solutions, making the solution turbid while also increasing the free acidity.