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The Ultimate Guide To Aluminum Anodizing

The Ultimate Guide To Aluminum Anodizing

If you want to learn about aluminum anodizing, then read this guide.

Whether you want to learn about classification criteria, benefits, coloring methods or different stages during the process – you will find all information right here.

What Is Aluminum Anodizing?

Aluminum anodizing refers to a passivation method conducted electrochemically to change an aluminum substrate’s surface into a layer of aluminum oxide. The oxide coating offers enhanced resistance to wear and corrosion.

Anodizing Aluminum Process
Anodizing Aluminum Process

Types of Anodizing Aluminum Process

The MIL-A-8625 standard specifies three primary categories of aluminum anodizing procedures with unique characteristics alongside others. These include:

The MIL-A-8625 standard specifies three primary categories of aluminum anodizing procedures with unique characteristics alongside others. These include:

1. Type I: Chromic Acid Anodizing

Chromic Acid Anodizing

Chromic acid is the main catalyst in the oxide layer formation in this process type. The aerospace and aviation sectors make extensive use of Type I derived components which always take a grey façade. This is due to the thinness of the coating which reduces its dye absorption capacity.

It has the following features:

  • The oxide coating in this process is the thinnest with capable resistance to corrosion.
  • This resulting aluminum oxide film has dielectric property with non-conductive capability making it suitable for primer use.
  • The anodizing process is useful when making parts with tight tolerances without altering part dimensions.
  • Aluminum parts anodized by the Type I process offer capable qualities for forming finding use in high stress situations.

However, chromic acid is hazardous making its use an environmental issue. A unique treatment plant for chromic acid wastewater is therefore necessary.

2. Type II: Sulfuric Acid Anodizing

Employs sulfuric acid rather than chromic acid and is the most popular method in aluminum anodizing. When used as an electrolyte solution, sulfuric acid effectively creates porous structures.

These features are synonymous to this anodizing process:

  • Components anodized with Type II technique can readily absorb paint, glue, and dye. They therefore find use for decorative purposes since you can dye them in different colors.
  • Results in thicker coating of aluminum oxide ranging between 0.1 to 1 mils.
  • Type II anodizing coating is non-conductive with dielectric qualities.
  • Type II anodized components are harder than those furnished from Type I.
  • They have decent resistance to corrosion and abrasion.

Type II anodized aluminum components find use in architectural applications, consumer goods such as electronics, cookware, ornamental and military applications. Additionally, compared to Type I anodizing, Type II anodizing is cheaper given reduced energy needs, chemical usage and waste treatment.

3. Type III: Hard Anodizing

This anodizing type also employs sulfuric acid but at a lower temperature and higher voltage and current density. Aluminum components furnished from Type III anodizing find use in the aerospace, aviation, and military sectors.

Some of the products made from hard coating are automotive parts such as pistons and gears, linear guides, and compressor fittings. Hard anodizing results in the following oxide coating qualities:

  • A thicker and porous coating exceeding 1 mil.
  • Although slightly costly than Type II anodizing stemming from necessary process conditions, Type III anodizing is non-hazardous and environmentally safe.
  • By adding PTFE, it reduces the friction coefficient especially advantageous for components under frictional duress.
  • Creates a dark film which can absorb black dye.
  • Improved resistance to wear, first-rate electrical insulation and excellent abrasion.
  • Produces the toughest and longest-lasting coatings.
  • They are also very resistant to corrosion.

However, it can significantly alter the part’s dimensions, making it unsuitable for components with very tight tolerances.

Other aluminum anodizing methods include:

4. Thin-Film Sulfuric Acid Anodizing

While similar to Type II anodizing, with this method, the concentration of sulfuric acid is much lower. The following features are inherent with this anodizing type:

  • They easily take up colour through dyeing.
  • They resistance to corrosion is inferior to Type I and II anodizing.
  • You obtain an oxide coating with a smaller thickness than both Types I and II making them more tolerant to stress application.

5. Clear Anodizing

With clear anodizing, after employing sulfuric acid tin the process, you employ a bath of hot water to seal the anodized area. The procedure results in a consistent and clear layer on the aluminum, augmenting its appearance.

While you can leave the part undyed, the coating thickness influences the colour intensity. This process is common as a finish for frames, extrusion profiles, auto trims, photography plates and railings and sidings.

6. Phosphoric Acid Anodizing

Another alternative to chromic acid anodizing, this method employs phosphoric acid in the oxide layer formation. You can employ this method as a pretreatment for primer application.

The resulting coating from this method endures extreme humidity. It also possesses aberrations making its adherence qualities outstanding, especially with structural adhesives.

7. Boric-Sulfuric Acid Anodizing

The safety and environmental risks associated with Type I anodizing led to the boric-sulfuric acid anodizing as an alternative. It finds extensive use in the manufacture of parts for aviation and aerospace with the following features:

  • Like the Type I method gives superb paint, lubricant, and adhesive adherence.
  • It is useful for items with tight tolerances.
  • Provides dependable resistance to corrosion.

8. Bright Dip Anodizing

With this technique, you undertake a pre-treatment employing sulfuric and phosphoric acid mixture resulting in a shiny appearance. You thereafter perform sulfuric acid anodizing and dyeing before undertaking sealing.

The aluminium alloy grade you employ determines the final appearance. Even so, bright dip anodizing improves the aluminum component’s overall aesthetic appeal.

Classification of Aluminum Anodizing Operation

Anodized Aluminum Sheet

The classification of aluminum anodizing operation is in two:

1. Batch Anodizing

In this process, you frame the components and submerge them in successive pools of the catalytic solution. At the conclusion of the procedure, you gather the pieces. It is a popular process for making machine parts, kitchenware, and castings.

2. Coil Anodizing

Here, you unwound the material, usually pre-rolled, before undertaking a continuous anodizing process. You coil the material upon completion before shipping. This method finds use when making sheets, wires and regularly shaped parts.

Benefits of Anodized Aluminum

Going for anodized aluminum draws the following advantages:


Anodized aluminum has a bright hue of silver appearance which you can dye and polish to your liking. The result is usually a bright or matte finish which is aesthetically appealing.


The high strength to weight ratio of this metal compound makes it cheaper per unit area compared to similarly used metals. Additionally due to its light nature, it is cheaper to ship.


While aluminum is a resilient material by itself, the anodizing process strengthens it further. In fact, anodized aluminum is about three times tougher than regular aluminum.

Environmental Friendly

The by-products resulting from anodization technique are non-hazardous making them environmentally friendly.

Maintaining Color

When you anodize aluminum, its hardness is such that it doesn’t flake or chip. Consequently, when you dye the surface, the colour adheres into the pores making it impossible to scratch out.


Aluminum when anodized weighs around sixty percent less than similarly sized metals of brass, copper and stainless steel. It is then easier to manipulate during design and even makes shipping cheaper.

Options for Adhesive

The structure of the oxide layer is such that it can accommodate adhesion due to its porous nature. You can therefore apply paint, adhesive and even other aluminum components.

Optional Custom Colors

The porosity attained prior to sealing allows you to dye the coating to a suitable color. You can also fashion it to resemble another metal such as bronze, stainless steel, gold, copper or brass.

Protection against Chemical Reactions

The anodized metal surface becomes impervious to chemically induced reactions at any temperature. Anodizing makes the oxide film structure stable and unreactive to chemical imposition.

Application of Anodized Aluminum

The following industries employ anodized aluminum:


The rust free nature and polished look of anodized aluminum makes it a perfect material for use in construction. Additionally, given strength, lightness and availability in different colors and texture, you can employ it for both exteriors and interiors.

Consumer Goods

The attractive finish capable with anodized aluminum and the available colour options makes it a favorite material for consumer goods. Microwaves, washing machines and ovens fashioned from such are durable and offer desired quality for customers.


Anodizing improves the durability of aluminum making it suited for use in vehicles, ships and aircraft. Its resistance to corrosion makes it a necessary material in transportation modes exposed to different elements.

Stages in Anodizing Aluminum


The process for anodizing aluminum encompasses the following stages:

Step 1: Pre-Treatment

Pre-treatment influences the quality of finishing and the material’s ultimate look. It entails eliminating small surface flaws and cleaning impurities from the raw aluminum surface that could impede the process.

You can undertake pre-anodizing treatments mechanically or chemically.

Anodizing Aluminum Process
Anodizing Aluminum Process

a. Chemical Pre-Treatment – Entails utilizing a variety of chemicals (acids or alkali) to eliminate unwanted matter on the surface. You then alter the surface roughness to create a unique appearance through an etching or brightening process.

  • Etching- You carry out etching by dipping the material in alkali or acid such as sodium hydroxide or ammonium fluoride respectively. Results in a matte or dull surface by doing away with a uniform layer that minimizes surface flaws.
  • Brightening- You eliminate the minuscule surface flaws making them decidedly reflective by flattening and smoothening. The shiny finish emanates from soaking in nitric or phosphoric acid solution blended with additives to lessen emissions and enhance brightness.

Step 2: Mechanical Pre-Treatment

Entails exposure techniques including shot peening, sandblasting, and abrasive polishing. These process augment the coating’s hardness, resistance to fatigue and adhesion quality.

  1. Electrolysis- It is the focal step where you immerse the aluminum component in a bath full of positively and negatively charged electrolytes. The aluminum component is the anode to which you connect the power supply’s (DC) positive terminal.
  2. Oxide Film Formation- The chemical make-up of the electrolyte bath determines the oxide film type generated as follows:
Anodizing Process
Anodizing Process

Barrier Oxide Film

Barrier Oxide Film

A barrier oxide layer develops in the presence of a neutral solution that doesn’t dissolve aluminum oxide.  This layer shields the aluminum surface from the elements due to its chemical reaction resistance. The voltage supplied between the electrodes determines the barrier oxide film’s thickness.

Porous Oxide Film

A porous oxide film develops when subjected to a watered down acidic solution to about ten percent concentration. Sulfuric acid finds common use resulting in aluminum oxide molecules accumulation in the bath.

A barrier oxide coating forms on the anode, however, continuous flow of current results in creation of a porous oxide layer. Employing higher voltages and extending the electrolysis process results in thicker oxide layer.

Step 3: Coloring

You undertake the coloring process after removing the chemical bath and drying the part. During coloring, you dye the aluminum oxide layer by filling the pores with pigment. The dye adheres to the oxide layer such that it is not removable by scratching.

Step 4: Sealing

The sealing process secures the adhesive, dye or lubricant on the porous oxide layer preventing corrosion, discoloration, and unwanted absorption. When sealing, you apply a sealing agent to secure the pores immediately after coloring owing to the layer’s sensitivity.

Coloring Methods after Anodizing Aluminum

Anodized Aluminum Color Options
Anodized Aluminum Color Options

The following approaches are useful in coloring:

Dip Coloring

With dip coloring, you submerge the anodized component in a solution containing the color pigments. Dye absorption is on the aluminum oxide layer’s pore cavity but susceptible to UV.

The dye make up defines the produced color. Dip coloring is a low-cost technique that enables manufacturers apply different color hues to the aluminum components.

Integral Coloring

Integral coloring combines the anodizing and coloring processes. In this case, the anodizing solution consists sulfuric and organic acids occasioning a thicker coating with enhanced resistance to abrasion.

Integral coloring is costly given the difficulty in producing an aluminum oxide coating with color. You can also only achieve a narrow color range of brown, yellow strains, gray, bronze and black.

Two-Step Electrolytic Anodizing

Here, you submerge the anodized component in a solution of metallic salts before applying an electrical current. This results in the deposition of metallic ions in the porous oxide electrolytically assigning the part a unique color.

You can employ copper, cobalt, tin and nickel as coloring metals. The metallic ion type and quantity defines the color intensity.

Interference Coloring

In this process, there’s an increase in the metallic ions deposits through an electrolytic process by expanding the porous oxide layer base. The interference of the light waves optically creates different color shades such as yellow, blue, red and green.


With the information in the guide, you can get the best anodizing aluminum finish for your unique and specific project.

When you conduct this process carefully, you will definitely get the best surface finish for your aluminum surface.

More Resources:

What Is Anodizing? – Source: Anodizing

How to Anodize Aluminum – Source: Wiki How

Anodizing – Source: Wikipedia

Anodized Aluminum – Source: IQS Directory


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