Introduction to types of surface treatment in CNC machining

Chemical finishing processes in CNC machining involve the use of chemical agents to alter the surface of the machined component. Chemical part machining can produce complex parts with tight tolerances. Unlike mechanical methods that physically remove or deform surface layers, chemical techniques alter the surface through chemical reactions, which has a range of different advantages and applications.

1.Chemical Milling

Chemical milling, also known as chemical milling or chemical etching, is a process that uses controlled chemical reactions to remove material from the surface of a part.

1. Technical Features: The process involves using masking agents to protect certain areas of the part while exposing other areas to chemical agents to dissolve the unprotected metal. The depth and rate of material removal can be precisely controlled, allowing the creation of complex shapes and features that may be difficult or impossible to achieve using traditional machining methods.
2. Materials and Applications: Chemical milling can be used on a variety of metals, including aluminum, steel, copper, and titanium. It is particularly beneficial for aerospace components such as fuselage structures, where weight reduction without compromising structural integrity is critical.
3. Advantages and Disadvantages: The main advantage of chemical milling is its ability to produce complex, precise features without inducing stress or mechanical deformation. However, the process requires careful handling of hazardous chemicals and produces waste that must be properly managed.

2.Passivation

Passivation is a chemical process that enhances the corrosion resistance of metals, especially stainless steel, by removing free iron from the surface and promoting the formation of a thin inert oxide layer.

1. Technical features: The process typically involves immersing the part in a passivating acid bath, such as nitric or citric acid, to remove surface contaminants and promote the formation of a protective oxide layer.
2. Materials and applications: Passivation is critical for medical devices, food processing equipment, and any stainless steel parts exposed to corrosive environments. It ensures the life and reliability of the parts by significantly reducing the corrosion susceptibility of the parts.
3. Advantages and disadvantages: Passivation can significantly improve corrosion resistance and enhance the natural oxide layer without changing the appearance of the part. However, it is only effective for certain materials, such as stainless steel, and requires precise control of the chemical process to avoid damaging the parts.

3.Chemical conversion coating

Chemical conversion coating is used to deposit a protective coating on metal parts through chemical reactions, improving their corrosion resistance, enhancing paint adhesion, and providing electrical insulation.

1. Technical features: The method involves immersing the part in a solution containing chemicals that react with the metal surface to form a protective film. Common types of conversion coatings include chromate coatings on aluminum and phosphate coatings on steel.
2. Materials and Applications: Conversion coatings are widely used in the automotive, aerospace, and electronics industries for parts that require enhanced corrosion protection and paint adhesion. They are suitable for aluminum, zinc, cadmium, magnesium, and steel surfaces.
3. Advantages and Disadvantages: Chemical conversion coatings provide excellent corrosion resistance and surface preparation properties, ready for subsequent finishing processes. Major disadvantages include the need for precise chemical control and environmental issues associated with certain chemicals, such as hexavalent chromium in chromate coatings.

4.Powder Coatings

Powder coating is a dry coating process in which powdered materials are electrostatically sprayed onto a surface and then cured under heat to form a hard protective layer.

1. Technical Features: The outstanding feature of this process is its ability to produce thick, uniform coatings that will not sag or droop, with excellent durability and resistance to chipping, scratching, and corrosion. A wide range of colors and finishes are available, from matte to high gloss, with significant aesthetic diversity.
2. Materials and Applications: Powder coatings are primarily used on metal surfaces, including steel, aluminum, and brass, making them a popular choice for automotive parts, outdoor furniture, and industrial equipment. Their rugged properties ensure that components can withstand harsh environments and frequent use.
3. Pros and Cons: Advantages of powder coatings include environmental friendliness, fewer volatile organic compounds (VOCs) released than liquid coatings, and durability. However, their application requires specialized equipment and they cannot be easily applied to non-conductive materials.

5.Galvanizing/Galvanizing

Zinc coatings are typically achieved through a galvanizing process, where a protective layer of zinc is applied to steel or iron to prevent rust.

1. Technical Features: Galvanizing can be done through a hot-dip galvanizing process, where parts are immersed in molten zinc, or an electrogalvanizing process, where zinc is applied through an electrochemical process. Both methods offer excellent corrosion protection and extend the life of the parts.
2. Materials and Applications: Zinc coatings are primarily used on steel components and are essential in the construction, automotive, and marine industries, where parts are exposed to corrosive environments.
3. Advantages and Disadvantages: While zinc coatings are effective in preventing corrosion, they are susceptible to white rust if exposed to moisture and if passivation is inadequate. In addition, coating thickness needs to be carefully controlled to avoid brittle parts.

6.Alodine

Alodine is a brand name often used synonymously with chromate conversion coating, which is a chemical treatment of aluminum that enhances aluminum’s corrosion resistance and provides a good primer for paint adhesion.

1. Technical Features: The process uses a chromic acid-based chemical solution to treat aluminum parts, leaving a protective chromate layer on the surface. It also improves electrical conductivity, making it suitable for electronic applications.
2. Materials and Applications: Aluminum oxidation treatment is widely used in the aerospace and electronics industries for parts that require a lightweight and durable surface. It is particularly beneficial for aluminum alloys that are susceptible to corrosion.
3. Advantages and Disadvantages: The advantage of Alodine treatment is that it improves corrosion resistance and paint adhesion without adding weight or changing part size. However, the use of chromic acid creates environmental and health issues, so strict handling and disposal measures must be taken.

7.Black Oxide

Black oxide is a conversion coating for ferrous metals, stainless steel, copper and copper-based alloys that provides a matte black surface, slight corrosion resistance and minimal dimensional change.

1. Technical Features: The process involves a chemical reaction between iron on the metal surface and oxidizing salts in a black oxide solution, forming a magnetite layer on the part.
2. Materials and Applications: Common applications include firearms, automotive parts and tools, which require a non-reflective, corrosion-resistant surface. It is also used for aesthetic purposes in construction and consumer products.
3. Advantages and Disadvantages: Black oxide provides a beautiful and durable surface that keeps the surface lubricated and enhances wear resistance. However, its anti-corrosion properties are limited compared to other coatings and usually require supplemental oil or wax treatment.