How Corrosion Resistant is Stainless Steel? 

Stainless steel is one of the most widely used materials in industries ranging from construction to cookware, thanks to its remarkable resistance to corrosion – but what makes it so resilient? The answer lies in its unique chemical composition, specifically chromium, an element that forms a protective oxide layer that shields the metal from damage, also known as the passivation layer. This blog post explores the chemistry behind stainless steel’s corrosion resistance and how it makes stainless steel such a common material.

The primary reason stainless steel is corrosion-resistant is due to the element chromium. Typically, stainless steel contains at least 10.50% chromium by weight, which is key to its protective qualities. When chromium is exposed to oxygen from air or water, it forms a thin, passive film of chromium oxide on the steel's surface.

This chromium oxide layer is a barrier between the underlying metal and the environment. Unlike iron oxide (rust), which flakes away and exposes more of the metal to corrosion, the chromium oxide layer adheres tightly to the steel’s surface, preventing further oxidation and damage.

One of the most impressive aspects of stainless steel is its ability to restore. If the surface is scratched or damaged, the chromium in the steel reacts with oxygen in the environment to immediately reform the protective oxide layer. This self-repairing property makes stainless steel exceptionally durable in harsh environments where surface damage may occur.

While stainless steel is highly resistant to corrosion, it's important to note that no alloy is entirely immune. However, stainless steel performs exceptionally well against the following types of corrosion:

• Uniform Corrosion: This type of corrosion occurs across the surface of the metal and is the most common type. It’s also referred to as general corrosion.
• Pitting Corrosion: This is a more localized type of corrosion that is more unpredictable. The rust creates pits at the metal surface which develop into cavities. It typically affects just a small amount of surface material; however, the inner structural damage of the component is often severe.
• Crevice Corrosion: Another form of localized corrosion that will appear in crevices or confined areas. It often occurs when there is not enough oxygen to shield the metal plus a buildup of corrosive fluid.
• Stress Corrosion Cracking (SCC): Also known as corrosion fatigue, caused by tensile stress applied to a material in a corrosive environment. The initial sign of this type of corrosion is the presence of fine cracks on the metal’s surface.

While stainless steel is highly resistant to corrosion, there are some circumstances where it will corrode. We recommend that you closely monitor any stainless steel exposed to substances such as acid or salt water.

Here are a few cases where stainless steel is likely to experience detrimental corrosion.

• Small spaces: In tight spots where there isn’t a lot of oxygen, stainless steel may experience crevice corrosion.
• Extreme temperature: Even if temperatures don’t get high enough for stainless steel to melt, they can reach a point that causes them to rust.
• Exposure to strong acid: While stainless steel is resistant to many acids, strong acids such as hydrochloric acid will cause corrosion and should be avoided.
• Exposure to strong bases: Strong bases such as sodium hypochlorite will cause cracking or etching corrosion. This category also includes other chloride solutions such as seawater, where it is not recommended for stainless steel to be submerged.

Stainless steel is known for being more corrosion-resistant than most alloys due to the high chromium content. Stainless steel 304 has up to 19.50% chromium, while stainless steel 316 has up to 18% chromium and 3% molybdenum, another corrosion-resistant element.

The corrosion resistance of aluminum A356 is comparable to stainless steel due to its aluminum oxide layer. However, stainless steel is considered more corrosion-resistant than aluminum due to the added chromium in the stainless steel.

Carbon steel alloys aren't very corrosion resistant but is commonly used for its strength. The more carbon content in an alloy, the more it is prone to rust and corrosion. The carbon content in carbon steel can range from 0.05-2.10% depending on the alloy.

Overall, if corrosion resistance is the goal for your metal component, stainless steel is the best choice.

Because of its corrosion resistance, stainless steel is ideal for investment casting, a process for making small metal components with tight tolerances. Stainless steel investment castings are used in some of the world’s most critical industries such as food processing, petrochemical, mining, waste management, and many other applications.

If you need stainless steel investment castings, look no further than Milwaukee Precision Casting, Inc. We create investment castings with materials 100% sourced in the U.S.

Contact us today to learn more about our process!