Surface finishing is an essential step in sheet metal fabrication. Even if a part is cut, formed, and welded perfectly, its performance and look still depend on the surface treatment. Finishing protects the metal from rust, enhances its durability, and imparts the appearance required for its intended use.
In industries such as electronics, medical devices, and automotive manufacturing, different finishes serve distinct purposes. Some finishes enhance electrical conductivity, while others prioritize strength, color, or environmental protection. Choosing the right finish ensures the part not only looks good but also functions well in its intended setting.
This article looks at standard finishing options for sheet metal parts. We will cover plating, anodizing, sealing, powder coating, painting, and passivation. Each method has its own benefits, depending on the metal, the part’s design, and its intended purpose.
Покрытие
Plating involves depositing a thin layer of metal onto a substrate, often through an electrochemical or chemical process. The goal is to enhance surface properties without changing the base material’s structure.
Plating thickness usually ranges from 0.1 to 25 microns. Thicker layers provide stronger protection but are more expensive and require longer application time.
Никелирование
Никелирование is widely used in the fabrication of sheet metal. It balances corrosion resistance, wear resistance, and visual appeal. Nickel coatings are hard, smooth, and bright, which helps reduce friction while providing a polished appearance. Nickel plating can be done with electroplating (bright nickel) or electroless plating, each suited for different applications.
Bright nickel uses electricity to create a shiny, reflective surface. Additives improve smoothness and luster. This finish is standard on panels, handles, and decorative parts. It resists corrosion and highlights the precision of fabricated parts. For stronger protection, bright nickel is often paired with chromium or copper layers. It also improves surface hardness, reducing scratches or deformation.
Electroless nickel plating applies a nickel-phosphorus or nickel-boron alloy chemically, without the use of electricity. This ensures even coverage on edges, corners, and cavities where electroplating might be uneven. It’s ideal for complex sheet metal parts with cutouts or detailed shapes. Electroless nickel provides excellent corrosion and wear resistance while maintaining precise dimensions.
Zinc, Tin, Gold, Silver, and Other Plating Options
Other metals are plated depending on the needs for conductivity, corrosion protection, or cost.
Цинкование is a cost-effective method that protects steel by forming a protective layer, thereby extending the part’s life. Coatings are typically gray or bluish, but can be passivated to add color or provide extra protection. Zinc is ideal for indoor or mild environments.
Tin plating resists corrosion and improves soldering. It has a smooth, soft finish that enhances conductivity and reduces friction. Tin works best for non-wearing parts, especially in electronics.
Gold and silver plating are used for high conductivity and corrosion resistance. Gold is highly conductive, tarnish-resistant, and common in connectors and terminals. Silver is slightly cheaper and very conductive, but it may tarnish without protection. Both are widely used in aerospace, telecommunications, and precision electronics.
Anodizing (Electrochemical Conversion Process)
Anodizing is a process that uses an electrical current to form an oxide layer on a metal surface. The part acts as the anode in an electrolytic cell, which is where the name comes from. Oxygen ions from the electrolyte combine with metal atoms on the surface, creating a porous oxide film.
The process typically consists of three steps: cleaning, anodizing, and sealing. Cleaning removes oils and dirt. Anodizing forms the oxide layer. Sealing closes the pores to protect against corrosion. The porous coating can also absorb dyes, allowing for decorative coloring.
Anodized surfaces are more complex than the base metal. This improves wear resistance and protects against corrosion. The layer thickness typically ranges from 5 to 100 microns, depending on the application.
Анодирование алюминия
Aluminum anodizing is the most common type of anodizing. It strengthens aluminum’s natural oxide layer, making it more durable and decorative. Different acids create different film thicknesses and properties.
There are three main types used in industry: Type I (chromic acid), Type II (sulfuric acid), and Type III (hardcoat). Each type has its advantages, depending on the part’s intended use.
Type I – Chromic Acid
Type I uses chromic acid as the electrolyte. It creates a thin oxide layer, usually 0.5 to 2.5 microns thick. This type offers good corrosion resistance and maintains dimensions mostly unchanged, which is ideal for tight-tolerance parts.
The thin coating maintains strong fatigue resistance. It is often used in aerospace and defense, where parts need both precision and corrosion protection.
The downside is that chromic acid is expensive and not very environmentally friendly. Many industries are replacing it with safer alternatives.
Type II – Sulfuric Acid (Decorative)
Type II uses sulfuric acid and produces a thicker, more porous layer, typically 5 to 25 microns. The porous structure can absorb dyes, making it perfect for decorative or branded parts.
This type balances corrosion protection, hardness, and appearance. It is common in consumer products, electronics, and architectural components. After dyeing, sealing locks in the color for long-lasting durability.
Since the layer is thicker, the dimensions change slightly. Designers usually account for this when working with tight tolerances.
Type III – Hardcoat
Type III, or hardcoat anodizing, creates a very thick and dense layer, up to 100 microns. It uses lower temperatures and higher current than other types.
Hardcoat is ideal for parts that need maximum wear resistance in harsh environments. The surface hardness can reach levels comparable to those of hardened steel. It can also reduce friction and provide electrical insulation.
Applications include machinery, aerospace hardware, and military equipment. Hardcoat is less suited for dyeing, but it offers unmatched durability and corrosion resistance.
| Тип | Электролит | Typical Thickness (µm) | Варианты цвета | Общие приложения | Примечания |
|---|---|---|---|---|---|
| Type I - Chromic Acid | Chromic acid | 0.5 – 2.5 | Limited (gray) | Aerospace parts, precision components | Thin layer, minimal dimensional change, good for tight tolerances |
| Type II - Sulfuric Acid (Decorative) | Sulfuric acid | 5 – 25 | Wide range (can be dyed) | Consumer products, architectural parts, electronics | Excellent color absorption, widely used decorative finish |
| Type III - Hardcoat | Sulfuric acid (low temp, high current) | 25 – 100 | Limited (dark gray to black) | Industrial machinery, aerospace, defense components | Thick, dense, wear-resistant layer; ideal for harsh environments |
Анодирование Титана
Titanium anodizing works similarly to aluminum anodizing, but it produces color naturally. The color is determined by the oxide thickness, which varies with voltage. This creates a range of colors, from gold and blue to purple and green.
It improves corrosion resistance in medical, marine, and aerospace applications. Titanium anodizing also enhances biocompatibility, making it an ideal material for implants and surgical tools. The surface becomes harder, smoother, and more wear-resistant.
Titanium anodizing adheres to established standards to ensure consistent quality. Common standards include:
- AMS 2487: Titanium anodizing for corrosion and wear protection.
- AMS 2488: Titanium oxide films for color and surface improvement.
Sealing (Post-Anodizing)
After anodizing, the oxide layer is still porous. These pores can absorb dyes or trap dirt if left open. Sealing is a post-anodizing step that seals the pores, enhancing corrosion resistance and maintaining the surface’s appearance over time.
Sealing usually involves soaking the anodized parts in a heated solution. This hydrates the oxide layer and closes the pores. The primary sealing methods are:
- Hot Water Sealing: The part is placed in boiling deionized water (around 96–100°C). The aluminum oxide changes into aluminum hydroxide, which swells and seals the pores. This is the simplest and most common method.
- Nickel Acetate Sealing: Often used for dyed parts or when extra corrosion resistance is needed. The nickel salt reacts with the oxide to create a stronger, more durable seal.
- Cold Sealing: Done at lower temperatures with chemicals like nickel fluoride. It is faster and saves energy, making it ideal for high-volume production.
Порошковое покрытие
Порошковое покрытие is a dry finishing method that adds a protective and decorative layer to metal parts. Instead of liquid paint, it uses electrostatically charged powder that sticks to the metal surface. The part is then baked in an oven, where the powder melts and forms a smooth, durable coating.
This process gives excellent resistance to wear, corrosion, and UV damage. It’s a popular choice for sheet metal parts in both industrial and consumer products.
The powder coating process has several key steps to ensure a strong, even finish:
- Surface Preparation: The metal part is cleaned and, in some cases, treated with chemicals or subjected to sandblasting to remove grease, oil, or rust. A clean surface helps the powder stick better.
- Powder Application: The powder—often polyester, epoxy, or polyurethane—is sprayed using an electrostatic gun. The charged particles cling to the grounded metal part.
- Curing: The coated part is baked in an oven at around 160–220°C (320–430°F). The heat melts the powder and fuses it into a uniform, hard film.
- Cooling and Inspection: After curing, the part cools and is checked for even coverage, gloss, and any surface defects.
Рисование
Рисование is a standard finishing method for sheet metal parts. It applies a liquid coating that adds color, protects against corrosion, and gives a smooth surface. Unlike powder coating, painting cures at lower temperatures, making it suitable for a broader range of materials.
This method is ideal when specific colors, glossy finishes, or cost-effective protection are needed. It is widely used in the automotive, electronics, and general manufacturing industries.
The painting process involves several steps to ensure a durable, high-quality finish:
- Surface Preparation: The metal surface is cleaned to remove oils, dirt, and rust. Pretreatments, such as phosphating or priming, can enhance paint adhesion and corrosion resistance.
- Primer Application: A primer coat helps the paint stick to the metal and provides a uniform base. It also adds an extra layer of protection against corrosion.
- Topcoat Application: The main paint layer is applied by spraying, brushing, or dipping. Common paint types include acrylic, polyurethane, or epoxy-based coatings.
- Curing or Drying: Depending on the paint, the part either air-dries or is baked in an oven to harden the surface.
Passivation and Conversion Coatings
Пассивация and conversion coatings protect metal surfaces from corrosion. Passivation removes surface contaminants and forms a thin, stable oxide layer on the surface. Conversion coatings chemically react with the metal to create a protective film that boosts corrosion resistance and improves paint adhesion.
For stainless steel, passivation removes free iron and restores the natural chromium oxide layer. This strengthens corrosion resistance without changing the metal’s appearance or dimensions. It is commonly used in food processing, medical devices, and aerospace components.
Conversion coatings are applied to aluminum, zinc, and steel. On aluminum, chromate or phosphate coatings enhance corrosion resistance and improve paint adhesion. On steel, phosphate coatings increase lubricity during forming and create a more substantial base for painting or powder coating.
If you’re planning a sheet metal project and want the best finishing results, send us your drawings today. Our team will review your design and suggest the best materials, processes, and finishes. Start your quote now and make your parts stronger, more durable, and visually perfect.
Привет, я Кевин Ли
Последние 10 лет я занимался различными формами изготовления листового металла и делился здесь интересными идеями из своего опыта работы в различных мастерских.
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Кевин Ли
У меня более десяти лет профессионального опыта в производстве листового металла, специализирующегося на лазерной резке, гибке, сварке и методах обработки поверхности. Как технический директор Shengen, я стремлюсь решать сложные производственные задачи и внедрять инновации и качество в каждом проекте.
