Anodizing properties
Anodizing and Food Contact
Aluminum is a widely used material for the production of objects intended for contact with food, such as pans, baking trays, utensils, containers, and components for food processing. To ensure food safety and limit metal migration to food, aluminum is often protected by coating or anodizing.
Among the available treatments, anodizing represents one of the most effective solutions to increase the chemical and mechanical resistance of aluminum in food applications, provided it is properly designed and used.
Aluminum migration to food
Aluminum alloys exhibit good corrosion resistance thanks to the spontaneous formation, in contact with air, of a passivating layer of aluminum oxide. This layer creates a protective barrier against the external environment, but has an extremely low thickness and limited chemical resistance.
The natural passivation layer:
- is stable in neutral or mildly aggressive environments
- does not resist in acidic or alkaline environments
In particular, the natural oxide layer is dissolved:
- in acidic environments with pH < 4.5
- in alkaline environments with pH > 8.5
Under these conditions, aluminum is no longer protected and can dissolve into the food.
Similar behavior is observed in the presence of chloride ions (for example in salted foods), which can attack the oxide layer even at neutral pH, leaving the metal exposed to corrosive action.
Specific Release Limit (SRL)
To ensure food safety, a maximum migration limit has been defined for aluminum, indicated as SRL – Specific Release Limit.
The P-SC-EMB (Committee of Experts on Packaging Materials for Food and Pharmaceutical Products) has established a:
SRL limit for aluminum: 5 mg/kg of food
Objects intended for contact with food must comply with this limit under the intended conditions of use. Verification of compliance with the SRL limit is the responsibility of the economic operator who places the finished product on the market.
Anodized aluminum and contact with food
Unlike the natural passivation layer, the anodizing layer is much thicker, has a more compact structure, and possesses greater surface hardness. These characteristics make anodized aluminum more suitable for contact with food compared to untreated aluminum.
Chemical resistance of the anodic layer
Anodized aluminum shows excellent resistance to neutral saline solutions containing chlorides and good resistance in moderately acidic or alkaline environments.
Accelerated corrosion tests in salt spray show that anodizing, particularly hard anodizing, can exceed 1000 hours without significant corrosive phenomena.
In the presence of strongly acidic or strongly alkaline foods, however, the anodic layer is progressively dissolved. Under these conditions, anodizing does not guarantee unlimited protection over time.
Mechanical resistance and wear
The anodic layer is characterized by high hardness and wear resistance. These properties allow reducing aluminum transfer due to abrasion and improving the durability of the component in contact with food.
Recommended treatment for food applications
For the protection of objects intended for contact with food, the following is recommended:
- Hard anodizing (OX-HS)
- High thickness: 50 ± 10 µm
- Mandatory hot sealing
- 6000 series alloys or other alloys compliant with EN 601 / EN 602
EN 14392 Standard for Food Contact and FCM Regulation (EC) 1935/2004
The European standard EN 14392 – Requirements for anodized products for use in contact with foodstuff defines the requirements for anodized aluminum products intended for contact with food.
Main requirements
Alloy composition The aluminum alloy must comply with the standards:
- EN 601
- EN 602
Since anodizing is a transformation of aluminum, the alloying elements are also present in the anodic oxide layer.
Mandatory sealing The anodic layer has a porous structure. To seal the porosity and increase corrosion resistance, the sealing phase is mandatory, performed by immersion in hot water (typically at temperatures above 95 °C). Sealing reduces the permeability of the layer, improves chemical resistance, and helps limit aluminum migration.
Migration verification and operator responsibility
Compliance with the requirements of EN 14392 standard is not in itself sufficient to guarantee compliance with the SRL limit.
Aluminum migration depends on the aluminum alloy of the base material, the applied surface treatment, and the actual conditions of use of the finished product.
The surface treatment alone does not constitute a finished material. The responsibility for compliance with Regulation (EC) 1935/2004 and the SRL limit lies with the economic operator who places the finished product on the market, understood as a combination of base material and treatment.
Migration verification is therefore necessary, particularly in the presence of strongly acidic or alkaline foods, high temperatures, prolonged contact times, or repeated washing or sanitization cycles.
Compatible foods and critical conditions
Anodizing with sealing is generally suitable for foods with moderate pH, such as water and beverages (pH 5–8), milk and dairy products, meat and fish, salted foods, oils and fats.
However, a specific migration assessment is necessary in the presence of very acidic foods (pH < 3), alkaline foods, prolonged contact at high temperatures, or repeated aggressive washing or sanitization cycles.
Conclusions
Anodizing represents an effective and established solution for the use of aluminum in contact with food, but does not eliminate the need for case-by-case technical and regulatory assessment.
In summary, anodizing significantly improves the chemical and mechanical resistance of aluminum, reduces aluminum migration to food, but does not guarantee unlimited protection under extreme conditions.
Regulatory compliance must always be accompanied by an assessment of actual conditions of use and, when necessary, by specific migration tests.
Available documentation: