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 as 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?
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.
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. Check out our extensive line of stainless steel drums.