Mastering the Art of Anodizing with Pre-treatment Excellence
Anodising is an electrolytic passivation process used to enhance the surface properties of metal parts, predominantly aluminium and its alloys. This process forms a thick, protective oxide layer on the metal surface, which improves corrosion resistance, surface hardness, wear resistance, and aesthetic appearance. Anodising involves using the metal part as the anode in an electrolytic cell, where an electric current is passed through an acid electrolyte, typically sulfuric acid. The current causes oxygen ions from the electrolyte to bond with aluminium atoms at the surface of the part, creating the oxide layer.
Importance of Surface Prepration
Surface preparation is crucial in the anodising process because it significantly affects the quality and characteristics of the final anodised layer. Proper surface preparation ensures that the anodised coating adheres uniformly and firmly to the metal surface, providing the desired properties and performance.
The preparation involves several stages, including mechanical and chemical pre-treatments:
Step One: Mechanical Pre-Treatment (Surface Preparation)
- Polishing: Polishing involves using abrasives to smooth and brighten the metal surface. This process can produce a highly reflective, glossy finish, making it ideal for decorative applications. The process typically uses a rotating cloth or wheel with polishing compounds that contain fine abrasive particles like alumina (Al2O3).
Its purpose is to give a brilliant shine to the metal by removing macroscopic reliefs be mechanically “coating” the ridges. Two processes are commonly used:
- Lamination, obtained by special rollers (High gloss, glossy finish, industrial gloss).
- Mechanical, by polishing machines.
Mechanical polishing is a surface finishing process aimed at removing surface imperfections and creating a smooth, reflective finish on metal parts. The primary objectives are to enhance the aesthetic appearance of the metal surface, improve its surface smoothness, and prepare it for subsequent treatments such as anodising. This process is essential for applications where a high-quality, mirror-like finish is desired.
Process Description:
This step involves using rotating wheels or belts coated with fine abrasives to smooth and brighten the metal surface. The process can produce a highly reflective, glossy finish, making it ideal for decorative applications. The equipment used includes polishing machines, which range from handheld tools to large, automated systems, depending on the size and complexity of the parts being polished. Various abrasive compounds, including aluminium oxide, silicon carbide, and diamond paste, are selected based on the required finish and material being polished. It is crucial to control the process to avoid excessive surface deformation, which can affect the final appearance after anodizing.
Advantages of Mechanical Polishing:
- Enhanced Aesthetic Appeal: Produces a bright, reflective finish that is visually appealing.
- Improved Surface Smoothness: Reduces surface roughness, leading to smoother surfaces that are easier to clean and maintain.
- Preparation for Anodising: Provides a clean, smooth surface that enhances the quality and adhesion of the anodised layer.
- Customization: Allows for customized finishes based on specific requirements, whether a high-gloss mirror finish or a fine matte surface is needed.
- Control Over Process: Provides operators with direct control over the polishing process, enabling them to achieve desired surface characteristics and meet stringent quality standards.
Limitations Mechanical Polishing:
- Time-Consuming: Mechanical polishing is labor-intensive and can be time-consuming, especially for complex shapes and large surfaces.
- Cost: The process can be costly due to the need for specialized equipment and skilled labor.
- Surface Deformation: Excessive polishing can lead to surface deformation, creating a thin layer of altered material that may affect the final properties of the anodized coating.
- Size and Shape Restrictions: It is not easily adaptable for any size or shape, particularly for intricate or irregular geometries, making it challenging to achieve uniform polishing on all surfaces.
Brushing: This involves using abrasive brushes to remove surface imperfections and create a uniform, matte finish. Brushing is often used to achieve specific textures and patterns on the metal surface.
Blasting: In this process, the metal surface is bombarded with abrasive particles, such as sand or glass beads, to clean and roughen the surface. Blasting is effective for removing oxides, scale, and other contaminants.
Step Two: Chemical Pre-Treatment
Cleaning: Removes oils, greases, and other contaminants from the metal surface using detergents or solvents. It ensures that the surface is chemically clean and ready for subsequent treatments.
Etching: The metal is immersed in an acidic or alkaline bath to remove a thin layer from the surface, creating a uniform texture. Etching can produce a matte finish or prepare the surface for brightening (Chemical polishing), which produces a high gloss finish suitable for decorative applications.
Desmutting: This step removes any remaining oxides or smut (residues from etching) using a neutralizing solution, ensuring the surface is clean and ready for anodising.
Proper surface preparation is essential as it affects the adhesion, appearance, and durability of the anodised coating. Inadequate cleaning can result in poor adhesion, leading to defects such as flaking or peeling, while improper etching can leave uneven surfaces, impacting the uniformity and protective properties of the anodised layer.
Brightening:
As its name suggests, it consist, by levelling the microscopic reliefs, of increasing the reflectivity of the treated surface, i.e. its shine. These reliefs must not exceed 20 Micron in order to obtain a good reflective quality, which is why prior mechanic polishing is often necessary. It is therefore a question of carrying out a dissolving action preferably on the reliefs rather than in the hollows.
Chemical polishing:
Based on the selectivity of the dissolution of the metal, this bath containing a mixture of strong acids and oxidants operates by a succession of oxidation reations( action of nitric acid which forms a viscous aluminim oxide layer) and dissolution(by phosphoric and sulphuric acids). This viscous laye, which adheres strongly to the part being treated, is thin and consists of a mixture of acid saturated with alumina. It is preferentially deposited in the hollows and locally limits the speed of dissolution of the metal.
Salts of different metals (copper, nickel, cobalt, chromium,iron,zinc,etc.) are added to the bath to control this speed (formation of an oxide film).
The presence of this viscous layer therefore contributes to increasing the selectivity of the bath by protecting the hollows against the dissolving action with much greater efficiency than for edges or convex reliefs.
Advantages of Chemical Brightening:
Enhanced Aesthetic Appeal: Chemical brightening provides a high specular reflective finish, making the treated surface visually appealing. This is particularly beneficial for applications requiring a mirror-like shine, such as lighting reflectors and decorative trims for household applications.
Better Adhesion: The smooth and clean surface achieved through chemical brightening allows for better adhesion of subsequent coatings, such as anodizing or painting. This results in higher quality finishes and improved durability of the coated products
Enhanced Reflectivity: Significantly increases the reflectivity of treated surfaces, beneficial for applications requiring high reflectance.
Time Efficiency: Generally faster than mechanical polishing, as it can treat multiple parts simultaneously and does not require labor-intensive manual work.
Limitations of Chemical Brightening:
Chemical Handling: Requires careful handling and disposal of chemicals, which can be hazardous to health and the environment if not managed properly.
Cost: Involves the use of specific chemicals that can be expensive, and there may be additional costs associated with proper waste disposal and safety measures.
Process Control: Requires precise control of process parameters (e.g., temperature, concentration, time) to achieve the desired finish, demanding skilled operators and careful monitoring
Environmental Impact: Despite modern advancements, there can still be environmental concerns related to the disposal of used chemicals and effluents, requiring strict compliance with environmental regulations.
