Hot and Cold Sealing of Anodizing

When discussing aluminum anodizing, one of the most important, and often underestimated, aspects is the sealing phase of the anodic layer. It is precisely this phase that determines the corrosion resistance, color stability, and treatment durability.

After anodizing, the part’s surface consists of a network of microscopic pores, perfectly ordered, that open outward like tiny interstices. It is thanks to these pores that the component can be colored, but they are also a potential entry point for external agents and chemical substances. If the pores are not properly sealed, the corrosion protection of the anodized part is reduced. For this reason, in parts where corrosion resistance is fundamental, the final sealing phase becomes indispensable.

Types of Sealing

There are two main sealing methods:

• Hot sealing
• Cold sealing with nickel salts

With hot sealing, the thermal energy of near-boiling water is used to react the aluminum oxide and transform it into a more stable form: boehmite. This change causes a swelling of the oxide, which physically closes the pores. The result is a surface that is more compact, less permeable, and better protected. It is a well-established process still widely used in industrial applications. However, it requires heated tanks, energy, and careful control of immersion time to avoid undesired effects such as matting or staining.

Cold sealing, on the other hand, follows a completely different principle. It does not use heat to swell the oxide, but a chemical solution, usually based on nickel salts, which chemically occludes the pores. Nickel ions penetrate the anodic film and react with the oxide structure, “inactivating” it. The advantage is that the process occurs at room temperature or slightly above. The disadvantage is the presence of nickel residues, which can be problematic in sensitive applications (for example in the food industry or medical fields), given the toxicity of nickel salts.

Both methods pursue the same objective, sealing the anodized surface, but are based on different principles:

• hot sealing is a chemical-physical process, based on thermal and structural transformations of the oxide;
• cold sealing is a chemical process, based on the interaction between salts and the anodic structure.

This difference in approach directly affects performance, costs, and application areas.

Hot Sealing

The deionized water used in hot sealing reaches approximately 95-98°C. At this temperature, the aluminum oxide reacts forming the aforementioned boehmite, a crystalline phase that closes the pores of the anodic film.

Advantages:

• No metallic residue or heavy metal contamination.
• Excellent corrosion resistance, ideal for outdoor applications.
• Stable, proven, and easily controllable process.

Disadvantages:

• High energy consumption.
• Slight reduction (5-10%) in abrasion resistance.
• If not properly managed, can generate opaque halos or reduce surface brightness.

Cold Sealing with Nickel Salts

Process temperatures are lower (20-30°C). Nickel penetrates the anodizing pores and chemically occludes them, without swelling. However, a final immersion phase in hot water (≈60°C) is necessary to complete pore closure.

Advantages:

• Energy savings: no boiling tanks and lower operating costs.
• Reduced process times (even just 2-3 minutes).

Disadvantages:

• Presence of nickel ions not acceptable in many sectors (food industry, medical, “green”).
• Need for constant chemical monitoring of the solution.
• Possible surface deposits or chromatic variations if the process is not well controlled.

How to Choose

The choice between hot and cold sealing is never absolute, but depends on:

• component’s intended use,
• operating environment,
• required performance level.

For components intended for outdoor environments or subject to humidity, salt spray, or chemical agents, hot sealing remains the most reliable and durable solution. Conversely, for rapid and less demanding treatments, where optimizing energy costs is desired, cold sealing can be a valid alternative. However, it must be considered that the use of nickel salts involves environmental and safety criticalities, making it less suitable for those seeking a “green” treatment free of heavy metals.

Conclusions

Sealing represents a crucial step that determines the final performance of anodizing. The choice between hot and cold sealing must be based on the operating environment, required durability, and desired aesthetic characteristics. Both methods can guarantee effective treatment if correctly applied, but the final decision must always be guided by the technical specifications of the component and the operating conditions to which it will be exposed.