Surface Treatment: It is a process that, through mechanical and chemical methods, forms a protective layer on the surface of a product, safeguarding the body. Achieving a stable state in nature, it enhances the product’s corrosion resistance, aesthetics, and consequently elevates its value. The selection of surface treatment types primarily considers the usage environment, service life, aesthetic preferences, and of course, economic value.
The process of surface treatment includes pre-treatment, film formation, post-film treatment, packaging, warehousing, and shipping procedures. Pre-treatment involves mechanical and chemical treatments. Mechanical treatment comprises procedures like sandblasting, shot blasting, grinding, polishing, and waxing, aimed at eliminating surface irregularities and rectifying other surface imperfections. Chemical treatment removes oil stains and rust from the product surface while creating a layer that enhances the bonding of film-forming substances or activates the metal body, ensuring a stable state of the coating, enhancing its adhesion to the protective layer, thereby providing body protection.
Common surface treatment methods for aluminum materials include chemical treatments like chromization, painting, electroplating, anodizing, electrophoresis, and mechanical treatments such as brushing, polishing, sandblasting, and grinding.
Two types of aluminum alloy sheets
Aluminum alloy sheets can be categorized into two major types based on surface treatment methods: non-coated and coated products.
Non-coated products
- This category includes hammer-tone aluminum plates (irregular patterns), embossed plates (regular patterns), and pre-passivated, anodized aluminum surface-treated plates.
- These products do not undergo paint treatment on the surface and have low aesthetic requirements, resulting in lower prices.
Coated products
- Classification: Based on the coating process, they can be divided into spray-coated products and pre-roll-coated products. According to the types of paint, they can be categorized as polyester, polyurethane, polyamide, modified silicone, epoxy resin, fluorocarbon, etc.
- Among multiple coatings, the main performance difference lies in their resistance to sunlight and ultraviolet rays. The most commonly used coating on the front side is fluorocarbon paint (PVDF), known for its strong UV resistance. For the backside, polyester or epoxy resin coatings are chosen as protective paint.
Aluminum alloy surface treatment processes involve a series of methods and techniques aimed at enhancing the appearance, performance, and durability of aluminum alloy products. Here is the fundamental knowledge of aluminum alloy surface treatment:
Top 10 Classifications of Daily Surface Treatment Techniques for Aluminum Alloys
- Anodizing: Utilizes electrochemical principles to generate an Al2O3 oxide film on the surface of aluminum and its alloys. This oxide film provides protective, decorative, insulating, and wear-resistant properties.
- Electrophoretic Coating: Involves immersing aluminum alloys in a paint solution and using an electric field to coat the surface, enhancing corrosion resistance and appearance.
- Micro-Arc Oxidation: Applies high-voltage discharge in an electrolyte solution to create a ceramicized surface layer, increasing hardness and corrosion resistance.
- Physical Vapor Deposition (PVD): Deposition of metallic ceramic decorative coatings using vacuum deposition technology, enhancing wear resistance and appearance.
- Electroplating: Electrochemical process to deposit a metal coating on aluminum alloy surfaces, improving corrosion resistance and aesthetics.
- Powder Coating: Application of powder paint using spray equipment, followed by curing at high temperatures, to enhance weather resistance and appearance.
- Brushed Finish: Creation of linear texture through grinding or abrasion, improving surface decoration and metallic luster.
- Sandblasting: Propels abrasives at high speeds onto aluminum alloy surfaces, increasing surface roughness and cleanliness.
- Polishing: Surface refinement using abrasives or polishing tools to enhance gloss and smoothness.
- Etching: Utilizes photochemical etching to create concave-convex or specific patterns through chemical solutions.
Anodizing
It primarily involves anodic oxidation of aluminum. Utilizing electrochemical principles, it creates an Al2O3 (alumina) oxide film on the surface of aluminum and aluminum alloys. This oxide film possesses protective, decorative, insulating, and wear-resistant properties.
Process:
- Monochrome, gradient colors:
- Polishing/Sandblasting/Brushing → Degreasing → Anodizing → Neutralization → Dyeing → Sealing → Drying
- Bicolor:
- ① Polishing/Sandblasting/Brushing → Degreasing → Masking → Anodizing 1 → Anodizing 2 → Sealing → Drying
- ② Polishing/Sandblasting/Brushing → Degreasing → Anodizing 1 → Laser Engraving → Anodizing 2 → Sealing → Drying
Technical Features:
- Enhanced strength.
- Achieves any color except white.
- Achieves nickel-free sealing, meeting requirements in European and American countries.
Technical Challenges and Improvement Key Points:
The success rate of anodizing affects the final product’s cost. Enhancing the anodizing success rate mainly depends on suitable oxidant usage, appropriate temperature, and current density. This necessitates continuous exploration and breakthroughs by structural component manufacturers during the production process.
Electrophoresis
Employed for stainless steel, aluminum alloys, etc., it imparts various colors to products while maintaining metallic luster, enhancing surface performance, and exhibiting good corrosion resistance.
Process: Pretreatment → Electrophoresis → Drying
Technical Features:
- Advantages:
- Rich color options.No metallic texture, compatible with sandblasting, polishing, brushing, etc.
- Processing in a liquid environment allows treatment of complex surfaces.
- Mature process, suitable for mass production.
- Disadvantages:
- Moderate ability to mask defects; high pretreatment requirements for die-cast parts.
Micro-arc Oxidation
A process generating ceramic surface layers in an electrolyte solution (generally weak alkaline), resulting from the synergy of physical discharge and electrochemical oxidation.
- Process:
- Pretreatment → Hot Water Rinse → Micro-arc Oxidation → Drying
- Technical Features:
- Advantages:
- Ceramic texture, matte appearance, no high gloss, fine touch, fingerprint resistance.
- Suitable for various substrates: Al, Ti, Zn, Zr, Mg, Nb, and their alloys.
- Simple pretreatment; excellent corrosion resistance, weather resistance, and heat dissipation.
- Disadvantages:
- Limited color options currently available; mainly black or gray; high electricity consumption contributes to one of the highest costs among surface treatments.
- Advantages:
PVD Vacuum Coating
Physical Vapor Deposition (PVD) is an industrial process that utilizes physical processes to deposit thin films.
- Process:
- PVD pre-cleaning → Vacuum chamber evacuation → Target washing and ion cleaning → Coating → Coating completion, cooling, removal → Post-processing (polishing, AFP)
- Technical Features:
- PVD (Physical Vapor Deposition) allows the deposition of highly hard, wear-resistant metal-ceramic decorative coatings on metal surfaces.
Electroplating
A technique using electrolysis to attach a metal film to the surface, preventing corrosion, enhancing wear resistance, conductivity, reflectivity, and improving aesthetics.
- Process:
- Pretreatment → Cyanide-free copper → Cyanide-free brass tin → Chromium plating
- Technical Features:
- Advantages:
- High gloss coating, high-quality metallic appearance.
- Substrates include SUS, Al, Zn, Mg, relatively lower cost compared to PVD.
- Disadvantages:
- Poor environmental protection, poses significant pollution risks.
- Advantages:
Powder Coating
Applying powdered coatings using electrostatic spraying equipment to a workpiece’s surface, resulting in an even coating adhering to the workpiece. The powdered coating undergoes high-temperature baking for solidification, offering diverse effects based on different types of powder coatings.
- Process:
- Workpiece preparation → Electrostatic dust removal → Spraying → Low-temperature leveling → Baking
- Technical Features:
- Rich colors, available in high gloss or matte.
- Lower cost, suitable for architectural furniture, heat sink shells, etc.
- High utilization rate, 100% use, environmentally friendly.
- Strong defect masking capability.
- Can replicate wood grain effects.
Brushed Finish
Brushing involves grinding the product’s surface to create linear patterns, serving as a decorative surface treatment method. Depending on the resulting patterns, brushing can be categorized as: straight brushing, random brushing, corrugated brushing, and swirl brushing.
Technical Features: Brushing treatment provides a non-mirror-like metallic luster to the metal surface, simultaneously eliminating minor defects on the metal surface.
Sandblasting
It utilizes compressed air as a driving force to produce a high-speed jet of abrasive material onto the surface of the workpiece, altering the appearance or shape of the workpiece’s outer surface, achieving a certain level of cleanliness and varying roughness.
Technical Features:
- Achieves different levels of glossiness or matte finishes.
- Removes minute burrs on the workpiece surface, making it smoother, eliminating the hazards of burrs, and enhancing the grade of the workpiece.
- Eliminates residual dirt from previous treatments, improving the cleanliness of the workpiece surface, presenting a uniform metal color, enhancing the appearance of the workpiece.
Polishing
It involves the use of flexible polishing tools, abrasive particles, or other polishing media to modify and process the surface of the workpiece. Based on different polishing processes—rough polishing (basic polishing process), intermediate polishing (precision processing process), and fine polishing (finishing process)—selecting appropriate polishing wheels can achieve optimal polishing effects, simultaneously improving polishing efficiency.
Polishing enhances the dimensional accuracy or geometric precision of the workpiece, obtaining a smooth surface or a mirror-like gloss, while also capable of eliminating gloss.
Etching
Etching, also known as photochemical etching, involves removing the protective film from the area to be etched after exposure and development. During etching, it contacts a chemical solution to achieve a dissolution or corrosion effect, forming concave-convex or hollowed patterns.
Process Flow:
Exposure method: Material preparation – Material cleaning – Drying – Applying film or coating – Drying – Exposure – Development – Drying – Etching – Removing film – OK
Screen printing method: Material preparation – Cleaning metal sheets (stainless steel or other metal materials) – Silk-screen printing – Etching – Removing film – OK
Technical Features:
Advantages:
- Enables fine processing of metal surfaces.
- Bestows special effects on metal surfaces.
Disadvantages:
Most etching liquids (acids, alkalis, etc.) used in etching are environmentally hazardous.
