Why Does Stainless Steel Rust?
Stainless steel is a ubiquitous material with a wide variety of applications–from use in medical devices, to automotive parts, to jewelry and cooking utensils. Much of the “magic” of this metallic material is that it is stainless, in theory, it doesn’t rust. However, if you have ever owned or used a stainless steel product it is likely that you have noticed rust (corrosion) and you may have even questioned if its name is a misnomer. Why does a material touted as “stainless” rust? Rust
Most people are familiar with metals, to include stainless steel, corroding when it’s exposed to environments such as seawater. Often, without understanding the exact science of what is occurring, people accept that exposing a metal product to seawater has a damaging effect. The science behind corrosion from seawater is that the water contains chlorine, which is corrosive to metals, including stainless steel. However, corrosion of stainless steel can also occur without producing any corrosion products to analyze (other than rust), and when an obvious corrosive environment is unable to be detected.
To understand what makes stainless steel rust it is first important to understand the science that typically prevents it from rusting. Steel is made of iron and carbon, and stainless steel contains iron, carbon, and anywhere from 12-30% chromium. Stainless steel can contain other elements such as nickel and manganese, but chromium is the key element which makes it rust resistant. When the surface of normal steel is exposed to oxygen, it usually forms ferric oxide (Fe2O3) which has the well-known red rust color. Ferric oxide doesn’t form a continuous layer on the steel because the oxide molecule has a larger volume than the underlying iron atoms, and eventually spalls off leaving fresh steel exposed which then starts a deleterious rusting cycle. When stainless steel is exposed to oxygen, chromium oxide is created on the surface of the steel because chromium has a very strong affinity for oxygen. The chromium oxide is a very thin layer which doesn’t spall off, and it prevents further oxidation of the stainless steel. Even if stainless steel is scratched and the chromium oxide layer is removed, a new chromium oxide layer will form and protect the rest of the stainless steel beneath it. As long as there is sufficient chromium present, the chromium oxide layer will continue to protect the stainless steel and prevent it from rusting.
Have you ever used a steel wire wheel or steel wool to clean off a stainless steel tool, and then the stainless tool rusted in the same spot which was brushed clean? Or have you seen a stainless steel container or sink rust? Stainless corroding in the absence of a corrosive element (such as chlorine) is usually from very tiny steel particles touching the stainless steel surface. Chromium can protect stainless steel if the localized concentration is in excess of 12%, but if you cover the stainless surface with sufficient steel particles, then the localized concentration of chromium can fall below the 12% threshold and the chromium oxide layer fails to protect the stainless steel from oxygen attack. If this type of corrosion happens to stainless steel, it is fixable by: (A) Cleaning off all the rust, and then (B) removing the tiny steel particles by thoroughly cleaning the stainless steel part, usually with a solvent. These two steps should allow the chromium oxide layer to protect the stainless from further oxidation.
A less common form of rusting in stainless steel is after the stainless has been exposed to very high temperatures, often in the 750-1550°F range (400-850°C)1. This type of corrosion is often seen in welding applications in which stainless is heated and then cooled. If this happens, “sensitization” can occur which is where the carbon and the chromium bond together in the stainless steel and form carbides. These carbides situate themselves at the stainless steel grain boundaries, and the grain boundaries become deficient of chromium. With lower chromium concentrations at the grain boundaries, the chromium oxide protective layer can become discontinuous and rusting becomes possible. “Sensitization” can ruin stainless steel forever; however the damage can sometimes be mitigated with complex heat treating.
Although rust is something most of us see on an everyday basis and appears to be a simple occurrence it can result from a number of complex processes. Understanding the materials science of metals is critical to ensuring correct product performance and fixing issues, such as rust, as they occur. It is also critical to understand the materials science of metals and rust when working on a failure analysis investigation involving corrosion.