Surface Treatment Of Aluminum And Aluminum Alloys

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Introduction to Aluminum Alloys and Surface Treatments

Aluminum alloys are classified into two main categories: wrought and casting alloys. Each of these categories is further differentiated based on their property development mechanisms. Many aluminum alloys undergo thermal treatments that depend on phase solubility, such as solution heat treatment, quenching, precipitation, or age hardening.

In order to improve surface properties of aluminum products, such as wear resistance, corrosion resistance, and reflectivity, various surface treatments have been developed. These treatments are typically grouped into three categories: electrochemical treatments, chemical treatments, and coatings. This article provides an explanation of these methods and their associated terms and definitions.

Electrochemical Treatment Methods for Aluminum Alloys

Electrochemical Brightening: Electrochemical treatment used to enhance the optical reflectivity of a surface.

Electropolishing: The process of polishing a metal surface by making it anodic in an appropriate electrolyte.

Anodized Metal: Metal that undergoes an electrolytic oxidation process, creating an oxide coating with protective, decorative, or functional properties.

Clear Anodized Metal: Metal with a colorless, translucent anodic oxidation coating.

Color Anodized Metal: Metal that is anodized and colored, either during the anodizing process or through subsequent coloring methods.

Integral Color Anodized Metal: Metal anodized using an electrolyte, usually based on organic acids, which produces a colored coating during anodizing.

Electrolytically Colored Anodized Metal: Metal with an anodic oxidation coating, colored by the electrolytic deposition of a metal or metal oxide into the pore structure.

Dyed Anodized Metal: Metal with an anodic oxidation coating that has absorbed dye or pigments.

Combination Color Anodized Metal: Metal anodized with a combination of electrolytic coloring and subsequent dye absorption.

Interference Color Anodized Metal: Metal with an anodic oxidation coating colored by optical interference effects.

Bright Anodized Metal: Anodized metal characterized by high specular reflectance.

Protective Anodizing: Anodizing primarily for corrosion or wear protection, where appearance is secondary.

Decorative Anodizing: Anodizing focused on achieving a decorative, aesthetically pleasing appearance.

Architectural Anodizing: Anodizing for use in permanent, exterior, and static applications where appearance and long life are crucial.

Hard Anodized Metal: Metal anodized for wear and abrasion resistance.

Sealing: Post-anodizing treatment to reduce porosity and absorption capacity through hydrothermal processes.

Cold Impregnation: A chemical process carried out at low temperatures to reduce the absorption capacity of the anodic oxidation coating.

Significant Surface: The portion of the product to be coated, where the coating is essential for either serviceability or appearance.

Chemical Treatments for Aluminum Surface Enhancement

Chemical Brightening: Chemical treatment to improve the optical reflectivity of a surface.

Chemical Polishing: Polishing of a metal surface by immersion in a chemical reagent solution.

Degreasing: The removal of oil or grease from a metal surface using organic solvents or aqueous detergents.

Etching: The process of roughening a metal surface through selective dissolution using acids or caustic media.

Pickling: The removal of a thin metal layer using chemical action, typically through caustic solutions.

Coating Methods for Aluminum Alloys

Coating (Organic): The application of coating materials on a metallic substrate, including liquid or powder coatings that are cured, or the lamination of plastic films.

Coil Coating: Continuous coating of a metal strip.

Backing Coat: A single coating, typically applied to the reverse side of a product, with minimal requirements for appearance or corrosion protection.

Chemical Conversion Coating: Chemical treatment using solutions to build an oxide film, often containing chromates or phosphates.

Priming: The application of a primer, often pigmented with corrosion inhibitors like zinc chromate.

Pretreatment Priming: The application of a solution containing resin, chromate, and acid to enhance adhesion for subsequent painting.

Single Coat System: A single coating, with specific requirements for appearance, malleability, or corrosion protection.

Multiple Coat System: A system consisting of a base coat, intermediate coats, and a top coat to meet specific functional and aesthetic requirements.

Organic Coating: A dry film coating, such as paint or a metal laminate.

Film Coating: A thin organic film applied to a substrate after priming and adhesive applications.

Lacquering: The application of a transparent coating that dries through solvent evaporation.

Painting: The application of a pigmented formulation to create a non-transparent finish