Will Anodized Aluminum Rust

Due to the anodizing process, anodized aluminum parts exhibit properties like high durability, corrosion resistance, and wear resistance. However, a common question regarding anodized aluminum is whether it will still rust under conditions that promote rusting.

Knowing whether anodized aluminum parts can rust is a question of aesthetics. Rust can also affect their functions. Hence, while this article will answer the question, it will also answer other questions about the parts’ aesthetics and functions.

What is Anodized Aluminum?

Anodized aluminum is aluminum that undergoes the anodizing process, an electrochemical process that adds an oxide layer to the surface.  Anodizing has three types: Type I, II, or III, each with different outcomes due to the type of electrolyte and amount of current used. 

Type I aluminum anodizing uses chromic acid at a moderate current, leading to a part with a thin coating that serves more of an aesthetic function. Type II and III anodizing uses sulfuric acid, but Type III anodizing occurs at a higher current. 

A higher current results in a thick and hard coat of about 25 micrometers (Type I and II ranges from 1.8-25 micrometers), resistant to corrosion and wear. As a result, Type III anodizing (the Hard Coating process) provides the strongest protection compared to Type II and I. 

 

Generally, the anodizing process involves submerging the aluminum part in an electrolyte bath, with the aluminum part as the anode and stainless steel as the cathode. When current is applied to the bath, the aluminum surface reacts with oxygen from the electrolyte, forming aluminum oxide.

Will Anodized Aluminum Rust?

No, anodized aluminum sheet metal parts do not rust. Since rust is a type of corrosion and aluminum anodizing is corrosion resistant, it is expected to get this type of question. Yet, to understand the answer to the question, you must understand the rust formation process. 

Rust is a type of corrosion peculiar to iron and characterized by the formation of a red or brown red colored flaky iron oxide in the presence of oxygen and water. Rust is a destructive form of corrosion.

Anodized aluminum containing only aluminum cannot rust because it does not contain iron. However, anodized aluminum alloys containing iron, such as 7075 alloy, can rust if the anodization wears off, incomplete anodization or mechanical damage occurs to the layer. 

Nevertheless, anodized aluminum, irrespective of the presence of iron, is susceptible to corrosion through processes like pitting and galvanic corrosion.

How to Maintain the Anodized Aluminum Color

After undergoing an expert-vetted anodizing process, you can maintain the anodized aluminum color using the process highlighted below. However, if you suspect the anodizing procedure is low quality, you should do it again. 

Clean the anodized aluminum sheet metal part

Use mild soap and water to clean the anodized aluminum sheet metal part. While cleaning, avoid using abrasive scrubbers that can scratch the anodized layer.

Apply Protective Coating

Apply a protective wax or sealant such as teflon on the anodized part surface. This will seal the color, allow you to maintain the color and shine, and improve the part’s resistance to corrosion and wear. However, regular application is required to promote the longevity of the color.

Avoid Harsh Chemicals

Maintaining the color of aluminum depends much more on its operational environment. You should ensure that harsh chemicals like acids or alkaline, which can degrade the anodized layer, are absent.

How to Remove the Aluminum Anodized Color

Whether it is for repairs or aesthetic changes, use the methods highlighted below to remove aluminum anodizing color using the following methods:

Chemical Stripping

This method involves using sodium hydroxide (lye) to dissolve the anodized layer. However, careful preparation and adherence to procedures are important to prevent damage to aluminum. Below are necessary things about using chemical stripping to remove the aluminum anodized color:

Materials Needed

Materials needed include sodium hydroxide (lye), protective gloves and eyewear, plastic or glass containers, water, and soft brushes or sponges. 

Steps

Once you have the material, follow the steps below on how to remove the color from the part: 

Step 1: Preparation

Protect yourself using protective gloves, eyewear, and clothing from the sodium hydroxide. Prepare 5-10% sodium hydroxide solution (ensure you pour the sodium hydroxide into the water in a plastic or glass container. 

Step 2: Application

Submerge the anodized aluminum sheet metal part in the sodium hydroxide solution and allow it to soak for a few minutes to hours, depending on the thickness of the anodized layer and the strength of the solution.

Step 3: Monitoring

Watch the aluminum part. When stripping, use a soft brush or sponge to scrub the surface. Scrub gently to avoid damaging the underlying aluminum substrate. 

Step 4: Wash Away the Sodium Hydroxide

Once the solution removes the anodized layer, rinse the part with water to remove sodium hydroxide.

Step 5: Finishing

Dry the stripped aluminum part and apply other surface finishing to achieve the desired appearance or smoothness.

Mechanical Removal

This method involves mechanical processes like sandblasting or grinding to remove the anodized layer physically. While effective, it can leave a rough surface that requires further finishing.

Materials Needed

Materials needed include sheet metal fabrication tools like a sandblaster or grinding tool, Protective gloves and eyewear, sandpaper (various grits), and polishing compound (optional)

Steps

Once you have the material, follow the steps below on how to remove the color from the part using mechanical removal: 

Step 1: Preparation

Protect yourself from debris and dust using gloves, eyewear, and clothing. Afterward, set up the sandblaster or grinding tool according to the manufacturer's instructions.

Step 2: Removal

Use the sandblaster or grinding tool to remove the anodized layer from the aluminum surface evenly. Maintain a consistent motion to avoid creating uneven spots.

Step 3: Finishing

After the anodized layer is completely removed, use finer grits of sandpaper to smooth the surface. You can also apply a polishing compound to achieve a smoother and shinier finish.

Step 4: Cleaning

Wipe the aluminum part with a soft, clean cloth to remove dust or debris.

Electrochemical Stripping

You can also remove the anodized layer by reversing the anodizing process. This electrochemical process involves submerging the anodized aluminum in an electrolyte bath and applying a reverse current.

Materials Needed

The materials needed are an electrolyte solution (e.g., sulfuric acid or sodium hydroxide), a power supply with adjustable current, protective gloves and eyewear, a plastic or glass container, and water.

Steps

Once you have the material, follow the steps below on how to remove the color from the part using electrochemical stripping: 

Step 1: Preparation

Protect yourself from debris and dust using protective gloves, eyewear, and clothing. Prepare the electrolyte solution in a plastic or glass container.

Step 2: Setup

Connect the aluminum workpiece to the negative terminal (cathode) and the stainless steel to the anode. 

Step 3: Stripping

Submerge the anodized aluminum sheet metal part in the electrolyte solution and apply current. The anodized layer will begin to dissolve. Monitor the process closely.

Step 4: Neutralizing

After the removal, rinse the aluminum piece using water to remove the remaining electrolyte solution.

Step 4: Finishing

Dry the aluminum piece and apply other surface finishing depending on the desired result.

Conclusion

Rust is a type of corrosion, and anodized aluminum is insusceptible to corrosion. Hence, people ask the question, “Will anodized aluminum rust?” The answer is no, and this article discusses why and other issues related to anodizing aluminum parts and their aesthetics. By understanding anodized aluminum and its maintenance, manufacturers and consumers can make informed decisions about its use.