Does Stainless Steel Rust?

Does Stainless Steel Rust?

PEMCO

Will Stainless Steel Rust?

Stainless steel is armed with built-in corrosion resistance but it can and will rust in certain conditions—although not as quickly or severely as conventional steels. Stainless steels corrode when exposed to damaging chemicals, saline, grease, moisture, or heat for prolonged periods of time.

 

Stainless steel’s protection against corrosion is largely dependent on the amount of chromium present. If there is not enough chromium content near the surface of the stainless steel, a new chromium oxide layer cannot be formed when the top layer is scratched off. This leaves the material very vulnerable to several types of corrosion.

Stainless Steel Basics

To understand why stainless steel is rust-resistant, and how this resistance breaks down, it is helpful to understand how these alloys are different than other steels.

 

Stainless contains a minimum of 10.5% chromium. This chromium reacts quickly with surrounding oxygen to form a thin oxide layer on the steel’s surface. Unlike iron oxide, which often is in the form of flaky and corrosive rust, the chromium oxide clings to the steel. It therefore acts as a protective barrier. The chromium oxide is known as a passive film which seals the iron in the alloy away from the air and water in the environment. This film gives stainless its rust-resistance. 

 

Stainless steel is low maintenance, and its resistance to oxidation and staining makes it an ideal material for many applications. 

 

There are four main types of stainless steels: austenitic, ferritic, martensitic, and duplex. Austenitic stainless steel is dominant in industry and comprises over 70% of total stainless steel production. Its properties include a maximum of 0.15% carbon and a minimum of 16% chromium, yielding very strong protection against rust. Ferritic stainless steel has reduced corrosion resistance compared to austenitic grades, but fares better than martensitic stainless steel. Duplex stainless steels have high resistance to localized corrosion particularly pitting, crevice corrosion, and stress corrosion cracking.

Stainless Steel Rust Prevention

Steps to prevent stainless steel corrosion should be undertaken throughout the stainless steel lifecycle. Best practices during the design and production stage, as well as routine maintenance, will prolong the performance and appearance of the metal.

1. Design

Taking a proactive stance with stainless steel design pays off in the long term. Proper planning in the design stage of stainless steel applications will minimize water penetration and reduce the potential for surface damage. Drainage holes for water should be used when possible, and cavities and crevices limited. Air flow is critical, and the design should encourage air to circulate freely throughout the application.

2. Fabrication

During the fabrication stage, it is important to prevent stainless steel from contacting iron or ordinary steel. This requires vigilance in surveying the surrounding environment including work tables, tools, storage units, steel turning rolls, and chains. Any carbon steel dust particles settling onto the stainless steel during fabrication can contaminate its surface, increasing the potential for rust formation. Furthermore, cleaning and grinding tools that have been used with carbon or low alloy steel must be kept separate from stainless steels.

3. Maintenance

Regular maintenance plays a key role in stainless steel rust prevention, as well as limiting the progression of existing rust. It is essential to remove any rust that has formed using mechanical or chemical means. The resulting grime can then be cleared away using warm water and soap. After cleaning, a rust-resistant coating should be applied.

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