# What is stainless steel? # stainless # acid-resistant
Stainless steel is not a uniform material—it is a group of steel alloys that, thanks to an appropriate chromium content (at least 10.5%), are fairly resistant to corrosion, to a greater or lesser extent depending on the composition. Colloquially referred to as "stainless," it finds wide application in both industry and everyday life—from kitchen equipment to industrial or hydraulic installations. Depending on the chemical composition and properties, various grades of stainless steel are distinguished, characterized by different features and prices, even though they may look similar.
This is a common misconception among laypeople. As mentioned, stainless steel varies—there are different types, and depending on the chemical composition and level of protection (so-called passivation), it can still rust—i.e., corrode—under certain conditions, though these are typically very harsh environments, such as exposure to aggressive media, prolonged contact with mineral salts or chlorides, etc. The key is selecting the right type of stainless steel with an appropriate level of passivation for the specific application and potential environmental aggressiveness.
Certainly, we all know that, for example, car wheel arches tend to rust over time—or that road salt spread by road crews in winter can damage car bodies, even though they are made of stainless steel. Stainless steel can also rust under specific conditions, and the term "stainless" is not unconditional.
Below, we will briefly review the types of stainless steel and examine their chemical composition, considering how their content affects their corrosion resistance. Which one is a good choice, why a particular type is used in specific applications, and, finally, how this translates into economics.
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Stainless steel 304 or S304 is one of the most commonly used grades of stainless steel worldwide. It belongs to the austenitic steel group and contains about 18% chromium and 8% nickel. Grade 304 steel is:
This is stainless steel, but not fully acid-resistant, lacking full protection—it can corrode in environments containing chlorides (e.g., saltwater). However, it is often used to produce popular accessories or components for heating or sanitary technology, such as floor drains, heating manifolds, filters, or heating system components. Treated water, such as that found in the network, is no problem for S304 steel. However, it is not suitable for groundwater, underground water, or untreated water, as these may often contain mineral salts that can cause corrosion over time (though this also depends on the water's composition).
Stainless steel 316 is an improved version of 304, with the addition of molybdenum (min. 2–3%), which significantly increases its corrosion resistance in aggressive environments, such as the chemical industry. Because molybdenum enhances its resistance to acidic environments:
Acid-resistant steel 316 can also corrode when exposed to very aggressive media—but it is also possible to increase its molybdenum content, which automatically improves its resistance, e.g., to saltwater.
In the case of larger blocks of 316 steel, it is essentially resistant to water with limited salt concentration—if it is steel with high molybdenum content, it can even be used in the shipbuilding industry, i.e., for boats sailing in saltwater.
➡️ However, the resistance of acid-proof steel to salt, chlorides, and mineral salts differs in the case of thin steel sheets—here, the addition of salt can cause damage, and it would indeed require a high molybdenum content and special protection. An example of this is plate heat exchangers—devices in heating technology used as boiler components or heat pumps. They consist of plates only 0.3mm thick—so even when made of 316 stainless steel, they should not be used with saltwater or groundwater of unknown composition.
Beyond 304 and 316 steel, there are others—but these are more specialized grades, so they are less commonly heard of. Examples include:
Additionally, there are grades of stainless steel with increased corrosion resistance—with higher molybdenum content, which we will discuss later in the article.
These terms do not always overlap. Not all stainless steel is acid-resistant, but all acid-resistant steel is stainless.
Why is molybdenum added? The answer is simple. Molybdenum is an element that increases stainless steel's resistance to corrosion because, as a chemical element, it exhibits very high resistance. Adding molybdenum is an additional cost in steel production, but some applications require it.
Molybdenum (Mo) in steel improves:
In standard 304 stainless steel, molybdenum content is usually trace, while in acid-resistant 316 steel, it is a minimum of 2–3%, but some alloys contain up to 10% molybdenum. Of course, it’s still important to remember that molybdenum in stainless steel is not always the ultimate solution for ensuring resistance. Additionally, steel has another drawback—it is heavy. That’s why, in many applications, titanium—considered maximally resistant and lightweight—is used. Titanium (Ti) is, however, a rare material—and thus much more expensive than steel. Below is a comparison of pool heat exchangers made of steel and titanium, illustrating the price difference.
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A higher molybdenum content technically results in a slightly different classification, so to make our article complete, below are examples of other stainless steel grades. You may encounter symbols such as:
Although steel was known in ancient times, its widespread use was impossible because no method existed for its mass and relatively inexpensive production. It was rare—for example, in the Middle Ages, steel swords were so valuable that knights rarely owned them, often using iron weapons instead, contrary to myths and popular movies.
The breakthrough came in the 19th century—in 1856, when Henry Bessemer in the UK developed the first effective industrial process for producing steel from pig iron—allowing impurities to be removed by blowing air through it. This enabled the construction of the world as we know it—only then did the rapid development of railways become possible, eventually connecting even distant cities and making the modern world feasible.
Stainless steel, however, was invented later—only in 1913, also in England, as the country prepared for war with Germany, which broke out a year later as World War I.
Check out our other technical articles on the Nordic Tec blog
Undersizing of a heat exchanger - what does it mean in practice?
Oversizing the heat exchanger - what is it for?
What is the production process of brazed plate heat exchangers?
How to choose a heat exchanger for a gas boiler?
How to properly connect a Nordic Tec plate heat exchanger? Can I reverse the flow directions?
History of plate heat exchangers
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