stack of black anodized aluminum extrusion bars, connector, joint, screw , slot nut and angle bracketsAnodized aluminum products are everywhere. From the carabiners on your backpack to the iPhone in your pocket, this electrolytic anodic process is a practical method for manufacturers looking to make their aluminum stronger so it lasts longer in high-pressure situations. The anodized layer of protection resists corrosion and minimizes the impact of blunt forces and scratches.

Another metal that’s manufactured to last longer is stainless steel. This versatile metal thrives in hygienic environments. Medical equipment and cookware need to be durable but easily sterilized so it’s safe to use repeatedly. That’s why manufacturers often turn to stainless steel as their material of choice.

Both anodized aluminum and stainless steel are great options when corrosion-resistance is paramount. But they both offer different qualities depending on the part you’re creating. Anodized aluminum is a third of the weight as stainless steel making it great for aircraft. However, the strength and thermal properties of stainless steel make it the preferred metal for modern buildings.

One of the main differences between the metals is how they corrode, which is an important consideration to make before manufacturing any part.

How Aluminum and Stainless Steel Corrode Differently

While stainless steel is known for being corrosion-resistant, when it’s paired with other elements, it can respond differently when exposed to corrosive environments. That’s why stainless steel comes in different grades to protect itself given the application. Here are the ways stainless steel can be threatened by corrosion.

  • • Pitting corrosion: This localized corrosion occurs when the environment contains chlorides.
  • • Crevice corrosion: This occurs when there is a low amount of oxygen and a buildup of chlorides.
  • • Galvanic corrosion: This type of corrosion happens when dissimilar metals are in contact with the same erosive force such as rain or condensation.
  • • Stress corrosion cracking (SCC): Mainly an issue for austenitic stainless steel, SCC is when a crack grows on the surface of the steel.
  • • Intergranular corrosion: Also known as an intergranular attack (IGA), this corrosion occurs when the steel has been continuously exposed to heat between 800 and 1000 degrees Fahrenheit. This type of corrosion begins inside the steel.

Aluminum alone does corrode, but it doesn’t rust like steel. Aluminum is most susceptible to galvanic and pitting corrosion.

Galvanic corrosion occurs when aluminum comes in contact with a precious metal such as steel and copper. Instead of creating rust, the aluminum will break down resulting in a dull appearance.

Pitting corrosion also impacts the appearance of aluminum more than its functionality. This type of corrosion happens when chlorides are present in the environment. Pitting can also happen when the aluminum is exposed to environments beyond the 4 to 9 pH range. The more basic or acidic the environment, the faster aluminum will break down. Concrete, for example, has a pH value of around 12.5 and 13.5. This creates extreme pitting corrosion on aluminum.

Anodizing aluminum will help protect the metal from wearing down and corroding quickly in these difficult environments where exposure to harmful elements are imminent.

How to Anodize Aluminum

Aluminum anodizing is a controlled electrochemical process in which an oxide (anodic) layer is chemically built on the surface of the metal. This oxide layer acts as an insulator and can be dyed in a wide variety of colors.

Anodizing provides surface corrosion protection along with an excellent substrate for decorative finishes. The hardness of the aluminum is increased to a hardness of 48 to 55 on the Rockwell C scale. Unanodized aluminum has a Rockwell hardness of 38 to 44.

Hardcoat anodizing is a highly abrasion-resistant, non-conductive aluminum oxide (Al2O3-xH2O) coating that makes an aluminum surface harder than tool steel due to greater thickness and weight than conventional anodic coatings. Anodic coatings form an excellent base for dry film lubricants, Teflon, paint, and adhesives. The hardness of the aluminum is increased to a hardness of 60 to 70 on the Rockwell C scale.

How to Stress Relieve Aluminum

The stress-relieving temperature is normally between 550 and 650°C for aluminum parts. Soaking time is about one to two hours. After the soaking time, the components should be cooled down slowly in the furnace or in air. A slow cooling speed is important to avoid tensions caused by temperature differences in the material, this is especially important when stress relieving larger components.

If necessary, stress relieving can be performed in a furnace with protective gas to protect surfaces from oxidation. In extreme conditions, vacuum furnaces can be used.

Can You Anodize Stainless Steel?

The oxide layer added to aluminum during the anodizing process has the inverse effect on steel and ultimately creates rust. While you can anodize steel, it would do more harm than good.

What you can do to stainless steel to make it more corrosion resistant is use a process called plating or conversion coating. This type of metal finishing helps protect the steel in corrosive environments so it holds together during regular, high-impact use.

How Does Corrosion Occur Between Anodized Aluminum and Stainless Steel?

When joined together in a similar environment, corrosion can occur between anodized aluminum and stainless steel. Localized corrosion occurs because the oxidation of aluminum during the anodizing process makes it passive. The oxidation can dissolve when it contacts strong acid or alkaline solutions. The removal of this oxide film starts the corrosion of the aluminum.

Another risk is when the anodized aluminum and stainless steel are in contact. Alone, aluminum is an unnoble metal and if it comes in contact with a noble metal, such as stainless steel, galvanic corrosion can occur. Theoretically, this will happen if the anodized layer cracks after continuous pressure. The “unlayered” aluminum will be directly exposed to a large area of stainless steel, resulting in rapid corrosion.

While anodized aluminum and stainless steel offer stronger corrosion-resistance than most metals, they can still be exposed over time. And when put together, corrosion can still occur. If you have any questions about anodizing or how you should finish your metal part, our team is more than happy to help you through the process. If you now know what you need, our team is ready to start the anodizing or other metal finishing process.