Chemicoat Co., Ltd.　Basic knowledge

Usually, surface of metal products is not metallic substrate but paint film for protection and aesthetics. To keep the paint beautiful for a long time, surface treatment is necessary procedure.

The first step of surface treatment is to remove oil and garbage on the surface of metal . If there is any oil or garbage on the surface ｗhile painting, the painting film will come off soon.

Generally, removing oil is called "degreasing" and the used agents are called "degreasing agent". Because degreasing agents are alkaline, in order to remove agents left, it is necessary to be washed by water after degreasing. After degreasing and washing, if left for a while, the metal will get rusty soon. So between degreasing and painting, it is necessary to do rust proof treatment.

"chemical coating treatment" is the process that enhances air tightness of painting film for rust proof , the used agent are called "chemical coating agent". Because chemical coating agents are acidic, in order to remove agents left, it is necessary to be washed by water after chemical coating treatment.

Example of general surface treatment process (zinc phosphate treatment)

Degreasing

The purpose of degreasing is to remove the oil (such as rust proof oil and processed oil) and dirt attached on the surface of metal. Generally degreasing contains "solvent degreasing" and "alkaline degreasing".

Generally, solvent degreasing is carrrid out with undiluted solution at room temperature. The materials can be painted just after degreasing. So it is fit for a small amount of processing.

Alkaline degreasing has advantages in cost and workability for a great amont of processing.

Alkaline degreasing needs treatment equipment, but the concentration of agents used is lower than the one in solvent degreasing (usually 2-5%). So it fits for dipping and spraying.

Rust removal

The purpose of rust removal is to remove the oxide and rust on the material with chemical method. Rust removal agents contains various acids. (Phosphoric acid, sulfuric acid, hydrochloric acid, etc.)

Immersing in rust removal agent, rust on the surface of material reacted with acid and is removed. Rust removal agent blended with other acid inhibitor is used to reduce the excess etching process.

(Inhibitor is adsorbed on the treated material to protect it and suppress etching. But oxide film of rust does not adsorb inhibitor and is removed by acid.)

Surface adjustment

The purpose of surface adjustment is enhancement of performance and reduction of treatment time in next process.

Crystals become dense by surface adjustment. In order to achieve high resistance especially in electrodeposition paint, the coating agent is required to form especially fine crystal and uniform thin film on the surface of work.

In addition, the time of chemical coating process is reduced after surface adjustment.

Chemical coating for spray usually costs only 1 to 2 minutes So the surface adjustment process is generally essential.

Chemical coating

The purpose of chemical coating is to make the metal proof against rust before painting, and to improve corrosion resistance and adhesion of painting.

Coating is generally falls into "zinc phosphate coating" and "iron phosphate coating" when treats SPC (steel).

The management of iron phosphate coating is easier while the corrosion resistance of zinc phosphate coating is better. Chemical coating process is that agents react with metal then generate water-insoluble metallic compound on the surfaces.

The metallic compound as chemical coating protects metal surfaces from rust, and improves paint adhesion and corrosion resistance.

Chemicals management

Chemicals of treatment are determined with material, surface condition and paint standards of material to be processed.

Selected chemicals are managed by three items (concentration, temperature and time).

By controlling the three items precisely, the state of surface treatment is maintained during construction of agent bath, paint appearance and film performance can be maintained well.

Water washing management

The water washing process after chemical treatment is also important.

If other agents are used, there are so many different pH (degreasing agent=alkaline, chemical coating agent=acidic) that insufficient water washing after chemical treatment will lead to the fact that some agents are brought to the next step. The component may be off balance and surface treatment may fail.

Also, if the final water washing is polluted, it may cause blisters or adhesion failure after painting even if the condition of treated surface is good.

Equipment management

Besides the two points above, equipment management is also important factor of spray line.

Even if the two points above are managed precisely, if spray nozzle is clogged, problems of degreasing, chemical coating and water washing will occur. It may cause the occurrence of defect in paint appearance and coating performance.

On dipping line, it is necessary to manage the stirring equipment for working properly.

Now being prepared.