Process Control for Surface Treatment of Aluminum Before Coating

Surface Adjustment in Phosphating Process

Surface conditioners can even out the surface irregularities caused by degreasing in alkaline solutions or rust removal in acid pickling, allowing a large number of extremely fine crystallization centers to form on the aluminum surface. This accelerates the reaction rate of the phosphating process and facilitates the formation of the phosphating film.

(1) Impact of Water Quality

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

(2) Usage Duration

Generally, surface conditioners use colloidal titanium salts, which possess colloidal activity. If used for a long time or if the solution contains many impurity ions, the colloidal activity will be lost. At this point, the colloidal stability is destroyed, the tank solution will precipitate and layer, forming flocs. The solution must be replaced at this stage.

Phosphating Process

The phosphating process is a procedure in which chemical and electrochemical reactions form a phosphate chemical conversion coating. The resulting chemical conversion coating is referred to as a phosphating film. For bus painting, a low-temperature zinc-based phosphating solution is commonly used. The main purpose of the phosphating process is to provide protection to the aluminum substrate, to some extent preventing aluminum from corrosion; it also serves as a primer before painting, enhancing the adhesion of the paint layer and its corrosion resistance. The phosphating process is a crucial part of the overall pretreatment process. Its reaction mechanism is complex, and there are many influencing factors, making the process control of the phosphating bath more complicated than that of other baths.

(1) Acid Ratio

Increasing the acid ratio can accelerate the reaction speed of the phosphating process, resulting in a thin and delicate phosphating film. However, if the acid ratio is too high, the film layer may become too thin, making the workpiece prone to gray deposits during phosphating. If the acid ratio is too low, the reaction speed of the phosphating process slows down, the phosphate crystals become coarse and porous, corrosion resistance is reduced, and the workpiece is prone to yellow rust. Generally, the required acid ratio varies depending on the phosphating solution system or formulation.

(2) Temperature

Properly increasing the bath temperature can speed up the film formation, but if the temperature is too high, it will affect the acid ratio, which in turn affects the stability of the bath solution. Additionally, the film nuclei become coarse and the amount of sludge in the bath increases.

(3) Sludge Amount

As the phosphating reaction progresses, the amount of sludge in the bath gradually increases. Excessive sludge can interfere with the interfacial reaction on the workpiece surface, leading to uneven film formation, heavy gray deposits, or even failure to form a film. Therefore, the bath must be refreshed and cleaned in time according to the amount of workpieces and the duration of use.