Time： Popularity：0times
In the entire powder coating process, pretreatment is the "foundational project" that determines the coating's adhesion, corrosion resistance, and service life—just like laying a good foundation before building a house. If the pretreatment of the metal surface is not done properly, even the best powder coating will struggle to achieve its optimal performance. Different substrates have vastly different material properties and surface conditions, requiring corresponding "tailored" pretreatment methods. Today, we'll break down the core logic of powder coating pretreatment, specific solutions for different substrates, and key details to pay attention to in practice.
1.1.1 Thorough Impurity Removal
Remove all foreign matter from the metal surface, including iron filings and welding slag generated during processing, grease, cutting fluid, and dirt adhering during use, as well as oxide scale and rust formed in the environment. These impurities directly affect the adhesion between the coating and the substrate.
1.1.2 Surface Optimization
By treating the metal surface to create a uniform, rough microstructure, a good "adhesion foundation" is provided for the powder coating, allowing the coating to firmly "grip" onto the substrate.
1.1.3 Unified Standards
Regardless of the source of the metal substrate (e.g., steel from different steel mills) or the type of surface contaminants, pretreatment ensures that the workpiece surface meets a unified coating standard, avoiding coating quality fluctuations caused by inconsistent surface conditions.
The key to pretreatment is "adaptation"—matching the substrate characteristics while also meeting the final usage requirements of the workpiece. Simple interior decorative parts may only require basic cleaning, while outdoor anti-corrosion parts and high-requirement industrial components require multi-step conversion coating treatments. Another easily overlooked key point is that electrostatic powder coating relies on an electric field; the metal surface must be free of a high-resistance insulating film, otherwise it will limit or even prevent powder deposition. This is a crucial issue that needs to be avoided in pretreatment.
Common metal substrates for powder coating include steel, aluminum, copper, zinc alloys, and galvanized steel. Different substrates have different chemical properties and surface conditions, requiring different pretreatment solutions:
Steel is the most commonly used coating substrate. To achieve optimal corrosion resistance and salt spray resistance, zinc phosphate conversion coating is the preferred solution. Zinc phosphate treatment forms a dense crystalline film on the steel surface, enhancing coating adhesion and effectively isolating moisture and corrosive media, extending the workpiece's service life.
Aluminum substrates are advantageous because they are easy to spray after surface cleaning, and the coating adhesion is already good. However, for outdoor workpieces requiring high weather resistance and corrosion resistance, a proprietary chromate conversion coating is recommended to further enhance performance. Chromate coatings form a protective film on the aluminum surface, significantly improving corrosion resistance and coating durability.
The key to pretreatment of these zinc-containing substrates is to avoid surface oxidation affecting adhesion. A suitable phosphate coating is recommended. The phosphate coating reacts chemically with the zinc surface to form a uniform conversion film, providing a stable adhesion base for powder coatings.
The problem with these surfaces is the tendency to trap air, resulting in "porosity" defects after spraying. In practice, two key points need to be controlled: first, the shape design of the metal workpiece (minimizing deep holes and narrow slits) and the coating thickness (not too thick); second, "preheating treatment" can be tried—preheating the workpiece for several minutes before spraying allows trapped air to escape in advance, effectively improving the porosity problem.
The core pretreatment processes include oxide/iron scale removal, grease removal, and coating conversion treatment. Each step has practical techniques and pitfalls to avoid:
Scale Oxides (rust) and iron scale can severely affect coating adhesion. There are three common removal methods:
3.1.1 Mechanical Friction Method
For small-scale workpieces or localized rust removal, manual friction with a wire brush or sandpaper can be used. It is simple to operate but inefficient.
3.1.2 Sandblasting Method
Suitable for large-area workpieces, it has a good rust removal effect and can form a uniform rough surface. Here's a practical tip: The UK and many European countries have banned the use of sand as an abrasive. Disposable coarse abrasives, recyclable metal abrasives, and ultrafine abrasives (such as 600-mesh fused alumina, walnut shells, and tiny glass beads) are now commonly used.
3.1.3 Abrasive Selection Tips
Ultrafine abrasives (such as glass beads with a diameter <25µm) can achieve a very uniform surface, but the rust removal speed is slower. Excessively coarse abrasives will result in an overly rough surface, limiting powder flow during drying and easily leading to loss of gloss and a rough surface contour. Surface roughness references for different abrasives: The "peaks and valleys" of a typical sandblasted steel surface are approximately 100µm; after treatment with 180/220 grade fused alumina, it is approximately 3-5µm; after treatment with glass beads, it is approximately 1-1.5µm. The choice can be made according to coating requirements.
Grease is the "number one enemy" affecting coating adhesion. Common methods include the following, each with its own advantages, disadvantages, and applicable scenarios:
3.2.1 Solvent Wiping
Wiping the workpiece with a cloth soaked in solvent is suitable for small-scale production, quick and convenient, but it's important to frequently change the cloth and solvent (otherwise, the grease is merely "spread out" without being truly removed). If it's just dust, an adhesive cloth is more suitable. The disadvantages are high labor and material costs, and some solvents pose fire and health risks.
3.2.2 Solvent Immersion
Immersing the workpiece in a solvent tank removes grease after the solvent evaporates. However, as the amount of contaminants in the solvent tank increases, the removal effect decreases. Multiple solvent tanks can be connected in series to improve the effect, but this requires a large space, results in high solvent consumption, and also poses safety risks; therefore, it is not recommended as a priority method.
3.2.3 Solvent Vapor Degreasing
Suspending the workpiece in chlorinated solvent vapor (such as trichloroethylene) in specialized equipment, the vapor condenses on the low-temperature workpiece surface, dissolving the grease before flowing back into the solvent tank. The efficiency is higher than the previous two methods, but solid particles may remain on the surface after degreasing. This can be improved by using a boiling section or ultrasonic stirring. Some solvents require the addition of special additives to enhance their effectiveness.
3.2.4 Cleaning with cleaning agents
Prepare a cleaning agent solution with hot water, spray or immerse the workpiece, then rinse and dry. Suitable for light contamination, but cannot remove aged grease or severe stains.
3.2.5 Emulsion detergents
Mostly kerosene/water emulsions or kerosene-based concentrates. They operate at lower temperatures (some can be used at room temperature), and are effective by spraying or immersion. Suitable for temperature-sensitive substrates.
3.2.6 Alkaline detergents (recommended first choice)
Spray or immerse with a high-temperature aqueous solution (40-90°C), followed by two rinses and drying. Spraying is highly efficient and low-cost (5-60 seconds), while immersion requires 1-5 minutes and a concentrated solution. Alkaline detergents effectively emulsify or separate greases and can even remove severe contaminants. Some contain crystal refiners that optimize the crystal structure of subsequent phosphate coatings. Important reminder: For substrates such as lightweight alloys, zinc, electroplated metals, and aluminum, alkaline detergents must be used under controlled conditions; otherwise, they will be corroded by the alkali. 3.2.7 Pickling: Treatment with suppressive sulfuric acid or hydrochloric acid can thoroughly remove rust and iron filings, but is only suitable for iron/steel surfaces. High-quality rinsing is essential after pickling to avoid acid residue contamination, and rapid drying (or immediate application of a conversion coating) is necessary to prevent re-rusting.
3.3.1 Phosphate Conversion Coating (Mainstream Choice for Steel Substrates)
Core Logic: The heavier the coating, the stronger the corrosion resistance; the lighter the coating, the better the mechanical properties (such as impact and bending resistance). A balance must be struck based on the workpiece requirements. Excessively heavy phosphate coatings may experience crystal destruction under mechanical forces, affecting coating integrity. Standard Reference: BS3189/1959 specifies zinc phosphate as Grade C and ferric phosphate as Grade D; the recommended film weight for zinc phosphate is 1-2 g/m², and for ferric phosphate, 0.3-1 g/m². Spraying or immersion can be used, and chromate passivation is usually unnecessary. Process Flow: Ferric phosphate coating (spraying) generally involves 3-4 steps (including two water washes + drying); zinc phosphate coating (spraying/immersion) involves 5 steps (alkali degreasing → water wash → zinc phosphate → two water washes), and drying must be done as soon as possible after treatment.
3.3.2 Zinc Surface Conversion Coating
Lightweight zinc phosphate coating is recommended. Electrostatic galvanizing has fewer pretreatment issues; the higher the zinc bloom in hot-dip galvanizing, the worse the adhesion, requiring careful control of surface smoothness.
3.3.3 Chromate Conversion Coating (Aluminum and Aluminum Alloys)
The coating can be colorless, chromium trioxide yellow, or chromium phosphate green. A film weight of 0.1-0.5 g/m² is recommended. The process is alkaline degreasing → water washing → chromate conversion → two water washes. High-quality coatings require softened water for the final wash, and the conductivity of the washing tank must be monitored to ensure cleanliness.
3.3.4 Waterless System
Primarily based on chromate, this system eliminates the water washing step. Its advantages include energy saving and environmental friendliness. However, the film properties (conversion coating or dried film) remain controversial. It is suitable for scenarios with high environmental requirements and moderate film performance requirements.
3.3.5 Heavy Metal-Free Pretreatment
With tightening environmental standards, the application of pretreatment containing heavy metals such as chromium has decreased. Early chromium-free pretreatments were ineffective, but recent technologies have met standards. In 1996. Qualicoat approved its use for coating architectural aluminum materials, representing a future development trend.
Pretreatment wastewater must meet local environmental standards, and regulations are becoming increasingly stringent: Ferric phosphate solutions can usually be discharged directly; Zinc phosphate solutions must be diluted to the specified concentration before discharge; Chromate-containing washing solutions are toxic to marine life and require special treatment before discharge; they cannot be discarded directly.
The core of powder coating pretreatment is "tailored to local conditions and targeted solutions"—the characteristics of different substrates determine the choice of pretreatment processes, while the final application requirements of the workpiece (such as interior decoration or outdoor corrosion protection) determine the level of precision required. From removing oxides and grease to the conversion coating, every step must be meticulous: thorough removal of oxides and grease is fundamental; the appropriate selection of the conversion coating is key to improving corrosion resistance and adhesion; and standardized washing, drying, and wastewater treatment are the finishing touches to ensure coating quality and environmental compliance. In practice, in addition to following the above methods, attention must be paid to details: such as avoiding substrate corrosion, ensuring the surface is free of insulating film, and controlling the weight balance of the conversion film layer. Proper pretreatment is essential to fully realize the decorative and protective properties of powder coatings, resulting in workpieces that are both aesthetically pleasing and durable. Hopefully, this guide will help you avoid pitfalls in actual production and create high-quality powder-coated products.
Company Phone
+86-21-6420 0566
Working hours
Monday to Friday
Mobile phone:
13816217984
Email:
info@qinsun-lab.com
