A Guide to Anodizing: Process, Types and Applications

What is Anodizing?

Anodizing is a surface treatment method that produces a thin film with a thickness ranging from 0.5 to 150 µm. The thin film enhances the corrosion resistance, wear resistance, strength, and surface hardness of non-ferrous materials like aluminum.

The anodized layer or oxide layer is formed electrolytically. It differs from conventional electroplating, which deposits another metal on the substrate. Instead, anodizing creates a thin coat that is a part of the metal surface.

When anodizing parts, the anodized layer thickness can increase over time due to further oxidation. The type of anodizing, like Type I, Type II, and Type III, also produces different thicknesses.

Which Materials Can Be Anodized?

The anodizing process is primarily for metals. However, not all metals are compatible with it. Aluminum is the most common of all the anodizing materials, hence the term anodizing aluminum. Titanium and magnesium are also very popular in several industries.

Metals like copper and iron are incompatible with anodizing because the oxides formed are unstable and will flake off, exposing the substrate surface to further corrosion.

In addition, plastics are not compatible with the anodizing process. While some surface finish enthusiasts claim that conductive plastics like Polyetheretherketone with conductive fillers, Polyaniline, and Polypyrrole are compatible with anodizing, they are not. They are subjected to surface coating techniques different from the true anodizing process.

The table below shows common materials that can be anodized, their properties, common uses, and why they are anodized.

Why is Aluminum the Most Used for Anodizing?

Aluminum is the most used metal for anodizing because its inherent properties support this surface treatment process and make it popular in many industries.

The primary reason for the compatibility of aluminum with anodizing is its strong natural tendency to form an oxide layer on exposure to air. Anodizing takes advantage of this behavior, thickening the oxide layer in a controlled and uniform manner.

Another reason aluminum is the most used with anodizing is that the aluminum oxide layer is porous, encouraging the absorption of dyes and sealants. After anodizing aluminum, sealing the pores locks the dyes, a feature not seen in many other anodized metals.

What’s more, aluminum has good electrical conductivity, which allows for the effective passage of electric current during anodizing. As a result, the anodizing process becomes more efficient, creating a uniform oxide layer on the surface.

Different Parameters Involved in the Anodizing Process

The anodizing process is governed by several parameters that will affect anodized metal’s properties, like thickness, duration, color, hardness, and porosity when altered. This section will introduce each parameter and its effects.

Time

The duration of the anodizing process will alter the film thickness and pore formation. A longer duration will result in deeper pore formation and a thicker coat. For example, Type I anodizing has a short duration and fewer pores but a coat with a thickness ranging from 8 to 16 μm, compared to Type III, which has more pores and a thickness of 35 to 50 μm.

Voltage

The voltage used during anodization determines the speed of forming the anodizing layer. Operating at a high voltage leads to faster oxide layer formation and porosity. As a result, lower voltages are recommended for decorative anodizing, while higher voltages are suitable for wear-resistant surfaces.

Electrolyte Composition

The choice of electrolyte influences oxidation and porosity. For example, sulfuric acid creates a fine, dense oxide layer ideal for coloring, while chromic acid creates a thinner, more corrosion-resistant layer.

Current Density

Current density is the amount of electric current per unit area of the workpiece surface, and it controls the oxide layer growth rate. Anodizing at a high current density produces a thick and hard oxide layer. However, it can generate more heat.

As a result, the anodizing manufacturer must invest in management systems to prevent defects like burning or excessive porosity. Lower current densities are slower, but the oxide layer has controlled growth.

Temperature

The operating temperature can alter the anodizing process speed and the oxide layer characteristics. Hard coat anodizing aluminum or other metals at lower temperatures (0-5°C) and creating a thick and hard layer while operating at higher temperatures (20-25°C) produces thinner and porous layers.

Anodizing Processes

The anodizing process refers to the three types: Type I, Type II, and Type III. Each process operates at a unique time, voltage, and electrolyte, tailored to different applications. For example, Type II anodizing is for decorative applications, while Type III is for parts with industrial applications.

Pre-treatment

Surface preparation before anodizing significantly affects the final result. Pre-treatments like cleaning, degreasing, and etching ensure the metal surface is free from contaminants and ready for uniform oxide formation. The surface roughness created by pre-treatment can influence the adhesion of the oxide layer and the overall appearance of the anodized part.

Anodizing Process: How Does it Work?

The anodizing process is an electrolytic with metals like aluminum as the substrate. The substrate is connected to the positive terminal (anode), and a highly conductive metal like stainless steel or aluminum is connected to the negative terminal (cathode).

Chromium acid CrO₃(3-10% by weight) or Sulfuric acid H₂SO₄ (15-20% by weight) are electrolytes used for anodizing. Supplying current to the acid bath allows the anodic metal to undergo oxidation, forming an ion that reacts with oxygen ions to form the oxide layer.

The steps involved in the anodizing process are highlighted below:

Preparation and Cleaning

Before anodizing a part, it must have a smooth and uniform surface. Mechanical surface treatment techniques like sanding, polishing, grinding, and bead blasting remove irregularities. Chemical cleaning techniques like alkaline or acid cleaning can remove contaminants like grease and oils. In addition, rinse with deionized water can remove residual cleaning agents.

Electrochemical Process

Make the workpiece the anode and a highly conductive metal like stainless steel or aluminum the cathode. Both are immersed in the electrolytic bath containing sulfuric or chromic acid and pass electricity through the electrolysis setup to cause the anode to oxidize and lose electrons.

Oxidation;

Metal (W)→ W³⁺

Next, the metal ion (W³⁺) reacts with oxygen ions generated by the dissociations of the electrolytes to form the oxide layer.

2W³⁺ + 3O²⁻ → W₂O₃

Anodizing Bath and Reaction

Electrolytes used in anodizing can incorporate color into metal parts via dyeing, electrolytic coloring, and integral coloring.

In terms of dyeing, after anodizing, the oxide layer absorbs dyes. In electrolytic coloring, metal salts are deposited into the oxide layer’s pores through an electrochemical process, producing fade-resistant colors. Lastly, the integral coloring color is formed directly in the oxide layer, typically resulting in darker shades like bronze and black anodizing.

Sealing

You can seal the anodized layer using cold sealing, mid-temperature sealing, or hot sealing to reduce the formation of corrosion, scratches, and stains.

Cold sealing is the immersion of the metal parts in a solution containing nickel-fluoride at room temperature. It creates a sealed layer of fluoro-aluminate. Mid-temperature sealing involves immersing the unsealed anodized part in a metal salt at 60-80°C and sealing the pores with the metal salts.

Hot sealing involves immersing the metal salt near-boiling deionized water at 95 to 100°C. The pores swell and close, forming a dense layer sealing the part. If the sealing is poor or absent, the porous metal oxide layer accumulates dust and debris.

Main Factors to Affect Anodized Finish

The quality of the anodizing depends on several factors, which include:

Base Metal Composition

The base metal composition affects the quality of the anodized finish. For example, comparing aluminum alloys subjected to anodizing shows that pure aluminum of the 1100 series has a smooth, uniform finish. In contrast, aluminum alloys containing silicon have darker, uneven finishes.

Surface Preparation

Cleaning and polishing to remove contaminants will lead to a better fusion of the anodized layer with polished surfaces, giving different textures, like matte or bright finishes. In contrast, poor preparation can cause streaks or uneven color.

Electrolyte

The electrolyte solution can determine the anodized film thickness and appearance. High bath temperatures produce thinner coatings, while lower temperatures create thicker, more durable layers.

Electrical Current and Voltage

High-voltage anodizing produces a thick but rough film while anodizing at a low voltage produces a thin and smooth film. Additionally, current density impacts the adsorption of dyes.

Time in the Bath

A longer anodizing time will lead to thicker films, which improve corrosion resistance, hardness, wear resistance, etc. Moreover, it can cause increased roughness observed in hard anodizing.

Technical Considerations in Anodizing

A successful anodization is only achievable by understanding the different technical considerations associated with the process. These include the anodizing equipment and processing parameters.

Anodizing Tank Setup

The anodizing tank is a container usually made with polypropylene, PVC, or coated stainless steel that houses the electrolyte. A rack mechanism holds the anode, and a separate fixture holds the cathode. Both electrodes are connected to an electrical busbar for a uniform and controlled power supply.

During the anodizing tank setup, you must consider the tank size, busbar capacity, secure connection, cathode-to-anode area ratio( 1:1 or 1:3), filtration for electrolyte impurities, etc.

Control of Acidic Solutions

You should also consider the acidic solution temperature and concentration as they will alter the film thickness and quality. Concentrated acids increase oxide layer formation and cause rougher surfaces or burns.

Managing Anodizing Layers and Thickness

Temperature, current density, acidic concentration, and time will affect the concentration of the metallic and oxygen ions. In turn, they will alter the oxide layer growth and thickness since the anodizing layer forms when oxygen ions from the electrolyte migrate to the metal surface and react with the metal atoms.

Anodization Bath Maintenance

Impurities will affect the efficiency of anodizing. Hence, when the bath solution accumulates impurities like metal salt residues and dissolved chemicals and the metal ion concentration increases above standard level(<20 g/L is normal), you should filter the impurities and maintain the PH level, proper agitation, and ion concentration.

Types of Anodizing

There are three main types of anodizing processes: Type I, Type II, and Type III anodizing. This section will summarize each type and the properties of the resulting anodized part.

Chromic Acid Anodize (Type I)

Type I or chromic acid anodizing creates a thin film (0.00002”- 0.0001”). As a result, they are more suitable for decorative and functional purposes. After sealing, it mimics the performance of type II and III thin films.

Type I anodizing process has a limited color option, limited to grey or dark gray. It is suitable for thin coating and fatigue strength with applications in making aircraft components, military equipment, and precision instruments.

Sulfuric Acid Anodize (Type II)

Type II is the most common type of anodizing process. It uses sulphuric acid as the electrolyte, creating a thicker layer than type I (0.0001” – 0.001”). Additionally, this anodized finish has better corrosion resistance and wear resistance.

Type II anodizing process has numerous or limitless color options. It is suitable for aesthetics and functional purposes with applications in consumer electronics, automotive parts, etc.

Hard Anodize (Type III)

Type III produced an anodized finish with a thickness between 0.0005″ to 0.006″. It is the densest and strongest type of anodizing, making it suitable for parts used in harsh environments. Type III anodization method can use chromic, sulphuric, or oxalic acids as electrolytes.

The type III anodizing process also has limited color options, including darker shades of grey to black. It is suitable for heavy-duty wear and corrosion resistance and has applications in hydraulic cylinders, military vehicles, and marine hardware.

Benefits of Using an Anodizing Process

Anodizing creates a hard corrosion-resistant film and offers several benefits compared to other surface treatment techniques.

Corrosion Resistance

Anodizing creates parts with high corrosion resistance due to the formation of the thin film. The thin film is impervious to corrosive factors like moisture, and chemicals like acids and bases. Additionally, exposing an anodized part to environmental moisture further increases its layer thickness making it more corrosion-resistant.

Enhanced Aesthetic

Another benefit of anodizing a part is its enhanced aesthetic properties. This can lead to the introduction of better color options, different surface textures, and customization options with high longevity.

Electrical Insulation

Anodizing a part should insulate it. However, you can control the electrical conductivity by controlling the film thickness using different anodizing processes. Nevertheless, the base metal still has its inherent electrical conductivity.

Limitations of Anodizing

Anodizing also has limitations, which you must know and understand before incorporating into your project. Here are a few:

Material Restrictions

Anodizing is primarily used on metals like aluminum, titanium, and magnesium. It is incompatible with copper, iron, and plastics.

Surface Imperfections

Unlike some surface treatment techniques, anodizing does not mask surface defects. This means that scratches, dents, or irregularities present on the base metal before anodizing will remain visible after the process.

Color Range

Only Type II anodizing has several color options. Chromic acid and hard anodizing have limited color options, restricted to darker colors like grey and black.

Practical Tips When Choosing the Anodizing Process for Your Parts

To choose an anodizing process for your projects or parts, you need to consider the following practical tips:

Consider the Application

The operating environment or function of the parts will largely determine the type of anodizing properties. When working with parts that will be used in harsh environments, such as marine or outdoor use, hard anodizing (Type III) offers better durability and corrosion resistance. For aesthetic purposes, a better option is Type I or II anodizing.

Prioritize Aesthetics

If the part’s aesthetic appearance is important, Type II or sulfuric acid anodizing allows for a wide variety of dye colors to enhance the look of your parts.

Evaluate Alloy Composition

Pure aluminum works best for anodizing. Alloys with high silicon or copper content may have uneven, darker finishes.

Assess Thickness Requirements

For thicker anodized metal, you can increase the duration of the anodizing process. This will also improve wear and corrosion resistance. However, it will make the surfaces rougher.

Plan for Sealing

Ensure the anodized layer is sealed properly, especially for parts exposed to weathering, to improve durability and prevent fading or discoloration.

Applications of Anodized Parts

The anodizing process widely applies to parts manufactured using CNC machining, sheet metal, extrusion, etc. Below are the applications of anodized metals with specific examples across industries.

Anodized Aluminum Parts

The anodized aluminum parts are applicable in the following ways:

• Making automotive components like wheel covers, fuel caps, and engine covers;
• Aerospace parts like skin panels, fasteners, and interior cabin items;
• Home appliances and kitchenware items;
• Electronic and electrical enclosures;
• Medical equipment housing, scalpel handles, sterilization tray handles, etc.

Anodized Magnesium

Magnesium is a light metal with a high strength-to-weight ratio compatible with anodization, especially hard coating anodization. Application of anodized magnesium include:

• Bicycle frame components;
• Vehicle battery housing;
• High-performance tools and hardware;
• Drone, satellite, and aircraft components.

Anodized Zinc Parts

Zinc has inherent wear and is corrosion-resistant; anodizing a zinc part will increase its properties. Applications include:

• Automotive fasteners;
• Anodized screws and fuel system parts aircraft;
• Nut, bolts, plumbing fittings, and lighting fixtures;
• Tools handles, electronic casings, tool handles, furniture hardware, etc.

Anodized Titanium

Titanium is an engineering material widely applied in the aerospace, medical, and defense industries. It is compatible with Type II and III anodizing techniques, and you can make vibrant and iridescent colors with titanium without dyes. Applications of anodized titanium include:

• Aerospace fasteners, hydraulic system components, structural housing, etc;
• Golf clubs, bicycle components;
• Medical implants and surgical instruments;
• Watch casings, bracelets, etc.

Anodizing Finish vs. Other Surface Finishes

Compared to other coating techniques like electroplating or powder coating, anodizing provides a thinner layer of protection which can affect its application. This section compares anodization to the two processes.

Anodizing vs. Powder Coating

Powder coating is a dry coating process that involves spraying charged power on the surface of the part and curing it in an oven to facilitate bonding between the coat and the workpiece.

The table below summarizes the anodizing vs powder coating;

Anodizing vs. Electroplating

Anodizing is similar to electroplating because they both use electrolysis. However, the key difference between both processes is that anodization forms a protective oxide layer in the alloy while electroplating deposits a secondary material on the workpiece/substrate.