Why Does Stainless Steel Rust on Occasions?

While the name ‘stainless steel’ implies that it cannot stain or rust in any way, the truth of the matter is that stainless steel does rust in certain conditions, although not as badly or as quickly as conventional steel.

Stainless steel comes with a built-in resistance to corrosion which is dependent upon the amount of chromium present. Without a reasonable amount of chrome near the top surface of the stainless steel, it renders it impossible to form a new layer of chromium oxide when the top surface is scratched off. This leaves the material vulnerable to various types of corrosion.

What makes stainless steel rust-resistant?

In order to understand what makes stainless steel rust-resistant, you need to understand how it varies from other steels.

Stainless steel contains at least 10.5% chromium. This quickly reacts with oxygen surrounding it and forms a thin layer of oxide on the surface of the steel. Whereas iron oxide is often flaky and corrosive rust, chromium oxide, on the other hand, hugs the steel to form a protective barrier. Known as a passive film, the chromium oxide seals the iron in the alloy, keeping it away from air and water in the environment. It’s this film which gives stainless steel it’s rust resistance.

Stainless steel is low in maintenance and because of its rust-resistant properties, it makes it an ideal material for many applications.

Types of stainless steel corrosion

There are 6 types of stainless steel corrosion, namely:

• General – This is considered the safest form of corrosion since it is manageable, predictable, and even preventable.
• Galvanic – This refers to corrosion caused by two dissimilar metals coming into contact with one another in an electrolyte where the more active metal corrodes, and the noble metal is protected
• Pitting corrosion – This is small cavities or holes that occur when stainless steel is exposed to chlorides contained in an environment
• Intergranular corrosion – This is where the boundaries of crystallites are more likely to corrode than an inside surface
• Stress corrosion – this is where tensile stresses combine with corrosive environmental conditions causing cracks to form
• Crevice corrosion – This is localised corrosion forming in the crevice between two surfaces that are joined together

Stainless steel rust prevention

Steps should be taken to prevent corrosion throughout the lifecycle of the stainless steel. Appropriate practices in the design and fabrication stage, as well as routine maintenance, will prolong the appearance and performance of stainless steel.

Design

Crevices and cavities should be limited, and drainage holes should be provided wherever possible. The design should also allow for air to circulate freely through the application.

Fabrication

It’s important to avoid steel coming into contact with iron or ordinary steel during the fabrication process. Any dust particles from carbon steel permitted to land on the stainless steel during fabrication, for instance, can contaminate the surface increasing the risk potential for corrosion.

Maintenance

Another way of protecting stainless steel from rust and corrosion and limiting the progression of existing rust is regular maintenance. It’s critical to remove any rust by chemical or mechanical means. It should then be cleaned down with warm soapy water and a rust-resistant coating applied.

For more information on steel or to contact us for a quote for a custom project please give Metro Steel a call on 07 3204 1000 or drop by our facility at 109 Kabi Circuit, Deception Bay, Qld 4508.