Anodising is a widely used surface treatment on metallic materials such as aluminum that uses an electrochemical process to form a protective oxide layer. Although the basic principle is the same, there are two main types of anodising that differ significantly in their characteristics, applications, and purposes: hard anodising and decorative anodising (also known as natural or aesthetic).
Decorative anodising is probably the best known and most widely used, as it is primarily used to give aluminum objects a decorative, smooth, corrosion-resistant finish, albeit with relatively little protection compared to the hard version. This type is generally chosen for architectural components such as windows, doors, decorative panels, and household items such as appliances or electronic devices. The main reason lies in the fact that it allows for a wide range of bright, uniform coloration due to the special porous structure of the oxide layer formed, which is capable of effectively absorbing and retaining pigments.
From a technical point of view, decorative anodising involves moderate operating conditions: in fact, the treatment takes place at room temperature or at least close to 20°C, with a relatively low voltage and current density. This leads to the formation of a thin layer, typically between 5 and 25 microns, which maintains good adhesion to the base material and a fair corrosion resistance capability, provided that proper final sealing is performed. It is important to note that this type of anodising, precisely because of its light weight and limited thickness, is not particularly suitable for applications in extremely aggressive environments or subject to significant mechanical or abrasive stresses.
Also known as natural or type II anodising (according to ISO 7599 / MIL-A-8625 Type II standards), it produces thin (5-25 µm) and porous layers.
Larger pores (50-100 nm), ideal for absorbing dyes;
Smooth and transparent finish, suitable for light and metallic colours.
Standard corrosion and wear resistance: sufficient for non-aggressive environments.
Advantages
Good degree of protection under normal conditions;
Low cost.
Limitations
High porosity;
Inadequate for extreme conditions or high wear.
In contrast, hard anodising, also known as hard anodising or type III anodising, is an enhanced version of the process, developed specifically to give the treated material exceptional mechanical and chemical properties. This type of treatment is distinguished first of all by the hardness and thickness of the protective layer generated, which can reach typical values between 30 and 80 microns, even reaching over 100 microns in the most extreme cases.
However, these high-performance characteristics result in higher treatment costs compared to decorative anodising, due to the specialised equipment required, the high energy consumption needed to maintain low temperatures, and the use of specific electrolytes.
This type of anodising is mainly oriented toward purely technical and functional applications, such as mechanical components, parts subject to heavy wear and abrasion, industrial equipment, automotive, marine and aeronautical.
Technical Characteristics.
Known as Type III according to ISO 10074 / MIL-A-8625 Type III, these have thicknesses between 30-80 µm, with peaks up to 150 µm.
Thickness: typically 40-60 µm, sometimes 80 µm;
Current/intensity: higher density (1.5-5 A/dm²; voltages up to 120 V);
Small pores (20-40 nm), almost closed.
Oxide hardness of up to 500–600 HV;
Significantly increased resistance to wear and abrasion compared to untreated material;
Improved electrical insulation compared to standard aluminium (breakdown voltage up to 2,000 V);
Corrosion resistance: enhanced protection, even in aggressive environments.
Reduced fatigue strength: 30-60% loss;
High cost: complex process, energy, cooling.
The choice between decorative and hard anodising therefore depends strictly on the intended end application of the product. If a primarily decorative solution is sought, with good corrosion resistance performance and low cost, natural anodising is undoubtedly the most suitable choice. Conversely, when operating conditions are very aggressive, both mechanically and chemically, and technical performance becomes a top priority, hard anodising stands out as the ideal solution, despite higher costs.
Another relevant aspect when comparing these two types of treatment concerns the reference standards: while decorative anodising generally follows ISO 7599, UNI 10681 and MIL-A-8625 type II standards, hard anodising refers to more stringent standards such as ISO 10074, UNI 7796 and MIL-A-8625 type III. This detail is crucial when choosing a treatment, as the technical specifications and standards required by individual industries can significantly influence the final decision.
Decorative anodising
Bodywork and device shells (computers, smartphones);
Objects, household appliances;
Domestic and commercial applications under moderate conditions.
Hard anodising
Aerospace, automotive, military, marine sectors;
Food equipment components and scratch-resistant surfaces.
Hard anodising: ISO 10074, UNI 7796, MIL-A-8625 Type III
| Feature | Decorative Anodising (II) | Hard Anodising (III) |
| Layer thickness | 5-25 µm | 30-80 µm |
| Pore size | 50-100 nm (very porous) | 20-40 nm (few pores) |
| Hardness/Resistance | Moderate (150-300 HV) | High (300-600 HV) |
| Cost | Relatively inexpensive | Significant |
In conclusion, both anodising processes have definite areas of excellence. A thorough understanding of them enables informed decisions to be made in the design and production of components and structures, thereby optimising durability, cost, and overall performance. Knowing how to choose the treatment best suited to one's needs is indeed crucial to ensure that the finished products fully meet the requirements of the specific applications, balancing aesthetics, performance and economic sustainability in the best possible way.