Laser Pretreatment Method Before Metal Coating-ShangHai RunQia Electronics Technology Co.,Ltd.

Laser Pretreatment Method Before Metal Coating

In industrial production, workpieces are often contaminated with rust, scale, loose old paint films, oil stains and other dirt during processing, transportation and storage. Such contaminants exert adverse effects on metal coating.

(1) The presence of iron rust reduces the contact area between the coating and the substrate surface, lowering the adhesion of the paint film to the steel surface. Meanwhile, impurities such as ferrous sulfate and sodium chloride contained in rust products will further accelerate metal corrosion. Therefore, rust removal is usually the primary task of metal surface pretreatment. If the rusted surface contains saline impurities, water cleaning shall be conducted prior to rust removal.

(2) Scale is a dense oxide layer formed at high temperature during hot rolling or high-temperature forging of steel. It is usually firmly attached yet relatively brittle compared with the steel substrate, and has a smaller coefficient of thermal expansion. It tends to crack and peel off under thermal cycling. Electrochemically, the electrode potential of scale is 0.15～0.20 V more positive than that of iron. In corrosive media, scale acts as the cathode while iron corrodes preferentially. Especially when a large area of scale remains, the extensive scale forms a large cathode and the exposed iron forms a small anode, which accelerates localized corrosion of iron. In humid environments, the large cathode area of residual scale and the small anode area of bare steel where scale has fallen off induce localized corrosion on exposed zones. Numerous micro gaps exist between scale and the steel surface, allowing moisture and corrosive media to penetrate beneath loose scale and causing crevice corrosion and concentration cell corrosion. Consequently, scale must be completely removed before coating. If painting is performed without thorough scale removal, poor coating quality and greatly shortened service life will result.

(3) Oil contamination on the steel surface reduces the surface free energy (dropping from 0.004 N·cm to 0.0002 N·cm), even lower than that of the coating material. This prevents the coating from wetting the steel surface properly, seriously weakening wetting adhesion and even causing shrinkage cavities in the paint film.

Surveys show that over 50% of coating defects are caused by improper pretreatment. Hence, metal materials generally require pretreatment before coating. Otherwise, not only the bonding strength and corrosion resistance of the coating will be impaired, but the substrate metal may still corrode under the coating, leading to premature peeling of the paint film. In summary, proper surface pretreatment before coating is an essential guarantee for obtaining high-quality protective coatings and extending product service life.

Appropriate pretreatment of workpieces can achieve the following purposes:

(1) Improve the adhesion of paint films on coated surfaces. Although adhesion is related to coating type and quality, surface treatment is also a critical factor. If the workpiece surface is unclean with moisture, oil, dust, scale, rust or other contaminants, or carries poorly bonded old paint films, the newly applied coating will adhere weakly, resulting in blistering, cracking, peeling and other defects. This exposes the metal to harmful media and causes corrosion and damage. Surface pretreatment aims to eliminate such harmful substances. In addition, a proper surface roughness is required to enhance coating adhesion. Surface roughness greatly affects coating quality. Appropriate roughness improves mechanical interlocking between the coating and substrate and enhances bonding strength. However, excessive roughness leads to uneven coating distribution; particularly at wave crests of rough surfaces, the coating tends to be too thin and induces early rusting. Practical experience shows that a surface roughness of 40～75 μm is optimal for steel. Moreover, a porous conversion film formed during pretreatment allows coating penetration into micropores to form an anchoring effect, further increasing bonding strength.

(2) Enhance surface corrosion resistance and extend paint film service life. Residual moisture, oil, rust and other contaminants between the coating and substrate will corrode the metal surface and damage the paint film. Thorough surface treatment meeting technical requirements greatly improves corrosion resistance and coating durability. Furthermore, a non-conductive and insoluble phosphate or oxide conversion film formed by pretreatment inhibits the formation of corrosion galvanic cells and significantly slows down coating corrosion. It is generally acknowledged that phosphating can increase coating corrosion resistance by 2 to 3 times. For this reason, phosphating treatment has been highly valued across industries, and 100% phosphating is widely adopted in automotive coating production.

(3) Avoid coating defects and ensure aesthetic appearance. Incomplete or inadequate surface treatment causes various coating defects. Oil stains lead to fish eyes and peeling; wax causes slow drying, tackiness and pinholes; rust and scale result in blistering, impairing appearance and undermining the decorative and protective functions of coatings. Surface roughness also directly affects coating gloss. For instance, untreated rough cast iron usually results in dull and matte coatings after direct painting.

Basic requirements for workpiece surfaces before coating:

1.Free of oil stains and moisture;

2.Free of rust and oxides;

3.Free of adhesive impurities;

4.Free of acid, alkali and other residues;

5.Possess appropriate surface roughness.

With the rapid development of science and technology, laser technology has been increasingly applied in various fields of production and daily life, such as supermarket barcode scanning, laser printers, laser cosmetology and myopia treatment. Laser equipment is also widely known for industrial cutting, drilling and welding. However, its application in the cleaning industry remains less familiar.

Laser cleaning technology is a new cleaning process developed rapidly in the past decade. With unique advantages and irreplaceability, it has gradually replaced traditional cleaning methods in many fields. Shanghai Runqia Electronic Technology Co., Ltd. takes a leading position in domestic research, development and application of laser cleaning technology, with numerous successful application cases and substantial benefits for customers.

Principle of Laser Surface Cleaning

Pulsed laser cleaning relies on the characteristics of laser pulses generated by the laser source. It is based on the photophysical reaction induced by the interaction between high-intensity short-pulse laser beams and contamination layers. The physical mechanism is summarized as follows:a) The laser beam emitted by the laser is absorbed by the contamination layer on the target surface.b) Intense energy absorption generates rapidly expanding plasma (highly ionized unstable gas) and induces shock waves.c) Shock waves fragment and remove contaminants from the surface.d) The laser pulse width must be sufficiently short to avoid heat accumulation that may damage the substrate.e) Experiments indicate that plasma generates directly on the metal surface when oxides are present.

Plasma is only produced when the laser energy density exceeds a threshold value, which depends on the type of contamination or oxide layer to be removed. This threshold effect is crucial for effective cleaning while ensuring substrate integrity. There also exists a second threshold; exceeding it will cause damage to the base material. To achieve efficient cleaning without substrate damage, laser parameters must be precisely adjusted to keep the pulse energy density between the two thresholds.

Advantages of Laser Surface Cleaning

Compared with traditional cleaning methods such as mechanical friction cleaning, chemical corrosion cleaning, liquid-solid high-impact cleaning and high-frequency ultrasonic cleaning, laser cleaning possesses prominent superiorities:

3.1 Laser cleaning is an eco-friendly green cleaning method. No chemical agents or cleaning liquids are required. Removed contaminants are mostly solid powder with small volume, easy storage and recyclability, fundamentally solving environmental pollution caused by chemical cleaning.

3.2 Most traditional cleaning methods are contact-based, exerting mechanical force on the workpiece surface that may cause damage. Residual cleaning media may also adhere to surfaces and cause secondary pollution. The non-abrasive and non-contact nature of laser cleaning completely avoids such problems.

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