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2018

Pretreatment System for Painting

Author：

Chuangzhi Coating

Before painting/coating, it is necessary to eliminate the oil stains (such as: lubricating oil, emulsion, grease, perspiration, etc.) and dust on the surface of the coated object, so the cleaning is an indispensable process in the pretreatment system before painting/coating. The chemical treatment (conversion film treatment) or mechanical processing (such as shot blasting) for the coated object surface is selected according to the substrate, surface conditions of the coated object or the coating process requirements.

Typical Pretreatment Process:

Degreasing
Rinsing
Surface Conditioning
Phosphating
Rinsing
Passivating
Deionized water rinsing

Degreasing & Derusting

Degreasing and derusting are two basic processes before phosphating. A good phosphating film can only be formed on the workpiece surface where oil, rust and oxide scale are completely removed, because these surface contaminants will seriously hinder the growth of the phosphating film, and will also affect the adhesion, drying performance, decorative properties, and corrosion resistance of the coating.

Rinsing

The rinsing process after removing oil and rust is an auxiliary process before painting/coating. After removing oil and rust, some nonionic surfactant and CL- are easily attached to the surface of the workpiece. If these residues are not thoroughly cleaned, they may cause thinning of the phosphating film, resulting in linear defects.

Phosphating

Phosphating is a method in which a metal workpiece is treated with an acidic solution containing dihydrogen phosphate and a chemical reaction occurs to generate a stable insoluble phosphate film layer on the surface.

There are many ways to phosphating, according to the phosphating temperature, it can be divided into high-temp phosphating (90-98℃), medium-temp phosphating(60-75℃), low-temp phosphating(35-55℃) and normal temp phosphating.

Passivating

The phosphate film is thin, generally in the range of 1-4g/m2, the maximum is not more than 10g/m2. Its free pore area is large, and its corrosion resistance is limited. After the phosphating, a passivation treatment is performed to fill and oxidize the phosphating film and stabilize it in the atmosphere.

Phosphating film drying

Drying the phosphate film can play two roles. On the one hand, it removes the moisture from the surface of the film and prepares for the next process of painting. On the other hand, it improves the corrosion resistance of the film after painting.

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