Unlike iron or carbon steel, stainless steel does not rust. On paper, it is one of the most durable metals on the market, at least in terms of rusting and corrosion. What makes stainless steel so tough? And… does stainless steel really not rust?
Let’s take a brief look at stainless steel — what it is, why it shouldn’t rust (but can), and how you can help prevent your stainless steel from corroding.
What makes stainless steel “stainless?”
No matter its variety, all steel is an alloy composed primarily of iron with a tiny bit of carbon to give it strength and help it resist fracturing. Some steel also contains a smattering of other elements such as sulphur, oxygen, phosphorus, nickel, silicon, or chromium.
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Chromium is the key to corrosion prevention.
Stainless steel prevents rusting because of its composition. It contains a much higher amount of chromium than carbon steel. By definition, “stainless” steel must contain at least 10.5% chromium and no more than 1.2% carbon and other alloying elements.
As a result, carbon steel, which contains little chromium, is harder, but stainless steel is cleaner.
What is chromium?
Chromium is a mineral found in everything from paint pigment to broccoli. In the 1800s, people used chromium to make products look beautiful since the element can be burnished to a high shine. Manufacturers still put chromium on cars, plumbing, and furniture. Today, 85% of the world’s chromium is used in making metal alloys like stainless steel.
How does chromium prevent rusting?
Chromium and other elements react with air and water to form a thin film on the outside of the steel. This film serves as a barrier that prevents corrosion.
What if it gets damaged? The film repairs itself even if the stainless steel gets scratched or marred.
Stainless steel’s clean, corrosion-resistant surface makes it appropriate for a variety of products, including kitchen sinks, bank vaults, and spoons. Although it basically doesn’t rust, stainless steel is much weaker than carbon steel, making it unusable as a support for buildings or bridges.
What are the other properties of stainless steel?
Stainless steel comes in a variety of surface finishes from matte to bright. It can be brushed, engraved, tinted, or embossed. Strong and hard, stainless steel is also resistant to extreme temperatures and can withstand heat up to 800 C.
Since most detergents do not harm stainless steel, it’s easy to clean and thus popular as a component of kitchenware. Perhaps most importantly, it’s one of the greenest materials available. Fully recyclable, stainless steel has a near-100% actual recovery rate.
When does stainless steel corrode?
Stainless steel is not 100% corrosion resistant, however. In some situations, environmental conditions can lead to corrosion.
Any time stainless steel gets exposed to conditions beyond its grade, corrosion can take place. For example, high concentrations of phosphoric acid or nitric acid can damage stainless steel. Environmental stress may cause pitting or localized corrosion, and elevated temperatures can increase the corrosive power of many chemicals.
Strong chlorides such as salt can also corrode stainless steel. When in constant contact with saltwater or salty environments — on a Navy ship, for instance — grade 304 stainless steel will suffer pit corrosion. Either a specialized coating or upgrading to grade 316 stainless steel will solve this problem.
Welding stainless steel, especially two different varieties, can weaken the alloy’s natural defenses. The weaker stainless steel will start to corrode more quickly. In addition, fastening stainless steel to iron or carbon steel can cause it to deteriorate.
Finally, any extreme environment may damage stainless steel.
How can you prevent stainless steel from corroding?
If you’re using stainless steel for the pots and pans in your home kitchen, corrosion should never be a problem. Industrial uses for stainless steel, however, might occasionally stress the alloy and cause it to wear down.
Here are four ways you can prevent stainless steel from corroding:
Use the right grade. Stainless steel comes in four distinct families — austenitic stainless steel, ferritic stainless steel, duplex stainless steel, and martensitic and precipitation hardening stainless steel. Each family contains several grades of steel. To complicate matters further, the grades differ between national and regional associations. Research the families and grades to find which one is the best choice for your product.
Apply a preventative coating or metal paint. Most steel corrosion happens uniformly over the exposed part of the metal, which is called “uniform attack corrosion.” An oil-based or water-based protective coating can help prevent this from happening. Metal paint can also form a barrier against corroding elements; however, remember that paint chips and must be reapplied regularly.
Employ a steel lubricant. It’s not easy to protect moving parts such as steel hinges, bearings, and joints. If you paint these, the paint will crack and look unsightly. Instead, consider a steel lubricant that allows easy movement and repels moisture.
Talk to a professional about galvanization. Galvanized steel is steel that has had a special protective coating made from zinc applied to the exterior. More chemically active than steel, zinc oxidizes when exposed to air and thereby protects the steel underneath. Since galvanization involves extreme temperatures and powerful chemicals, it’s definitely a job to outsource to professionals.
To return to our original question, does stainless steel rust? Under normal circumstances, no, stainless steel doesn’t rust. The chromium component in the alloy blend will protect the iron from rusting. Like nearly anything else, however, if you expose stainless steel to enough extreme environments, you can damage it.
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Kloeckner Metals is a full-line stainless steel supplier and service center. Kloeckner Metals combines a national footprint with the latest fabrication and processing technologies alongside the most innovative customer service solutions.
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Natalie Spira is Kloeckner's Marketing Communications Manager. Previously, she was the CEO and Founder of Fraction Marketing, a marketing agency catering to ecommerce startups. Natalie holds a MBA from Tel Aviv University with concentrations in entrepreneurship and marketing and a BA in English from UCLA.