Does Aluminum Rust or Corrode?

One of the most desirable aspects of aluminum is its natural ability to resist corrosion. Its light weight, strength, and corrosion resistance make it a popular choice for equipment, such as refrigerators and ranges; aluminum cookware is also an affordable option for commercial kitchens and foodservice operations.

When it comes to maintaining tools and equipment constructed of aluminum, users often have questions about how to prevent rusting, especially in pots and pans. Simply put, aluminum doesn't rust, but that's not because it doesn't corrode. Corrosion in aluminum forms when the metal contacts water or air, protecting the surface of the metal with an oxidized layer in most cases.

Aluminum: Rust vs Corrosion

Referring to aluminum equipment as rust resistant is technically inaccurate because rust is simply the form of oxidization, or corrosion, that occurs exclusively in iron and its alloys.

Rust appears as reddish-brown flakes and can eat away portions of steel and other metals. Corrosion in aluminum can have a similar effect in some cases.

What Does Aluminum Corrosion Look Like?

Corrosion in aluminum looks much like uncorroded aluminum¹, but unlike rust, the aluminum oxide layer in aluminum presents as gray or powdery white. The effects of aluminum corrosion are visible when the oxidized layer is compromised. These visible effects may range from discoloration to visibly absent chunks of metal.

The following situations represent opportunities for corrosion to remove the oxide layer in aluminum, damaging the metal beneath it:

• Exposure to a more noble metal directly or through an electrolyte, such as salt present in water. This is referred to as galvanic corrosion and can eat away pieces of aluminum equipment. In galvanic corrosion, the more noble metal acts as an anode, drawing electrons from the cathode, or less noble metal. While aluminum is high on the nobility chart, metals containing magnesium and zinc, including galvanized steel, will act as an anode and attack the oxidized surface layer.
• Exposure to liquid solutions or humid atmospheres that are highly acidic or alkaline, or contain high levels of chlorides such as salt. In aluminum cookware, pitting and discoloration are the most common types of corrosion, resulting from atmospheric corrosion. Discoloration, or a darkened surface, will occur first, potentially followed by the appearance of small holes in the surface of the metal. Aluminum cookware affected by these types of corrosion won't only be aesthetically undesirable, but could also cause aluminum to leach into food.
• Corrosion by physical degradation of the surface, such as a jet of water. This type of corrosion, referred to as erosion corrosion or cavitation, occurs in aluminum pipes or tubing that receive a constant flow or impingement of water or other liquids. Damage to the oxidized layer will occur simply from the force of water or other liquid, with the increased effects of corrosion when chlorides, acids, or bases are present.
• Corrosion due to tensile stress, or stress corrosion cracking. This is a type of intragranular corrosion, or corrosion that occurs when electrons are exchanged between the grain and the alloy microstructure. Stress corrosion cracking happens when an environment is humid or wet, a particular aluminum alloy is susceptible to this type of corrosion, and the aluminum is under tension. This could occur in an aluminum part that's under a high amount of pressure.

How to Reduce Corrosion in Aluminum

• Prevent or reduce exposure to acids, bases, and chlorides, as well as dissimilar metals. Don't cook anything salty or acidic in aluminum cookware, especially if that food is liquid-rich. Clean cookware with a gentle detergent and don't place it in a dishwasher.²
• Choose a product with a protective coating. Anodized layers and paint or powder coats contain an extra layer of protection for the aluminum oxide layer that can prevent all types of corrosion.
• Keep aluminum dry when possible. The presence of liquid often speeds corrosion, especially if salt is present. Hand dry cookware immediately after cooking and don't store it next to dissimilar metals. Regularly clean and dry aluminum equipment surfaces, especially those in humid areas.
• Keep dissimilar metals, such as galvanized steel, from contacting unprotected aluminum surfaces.
• Follow correct care procedures for aluminum cookware to reduce the chance of pitting and discoloration.

Rust Resistant vs Rust Proof

While aluminum products are occasionally marketed as "rust resistant," the reality is that this metal doesn't and can't rust. The term "rust" refers to oxidized iron, which means there's no difference between rusting and corrosion when the corrosion is occurring in an iron alloy. The term "corrosion resistant" best conveys that aluminum includes a natural defense against corrosion but is still susceptible to damage.

Sources

1. Aluminum Corrosion & Its Different Types. Fractory. Velling, Andreas. Accessed December 2020.
2. Is Aluminum Cookware Safe? Cooks Illustrated. Accessed December 2020.