WHAT MAKES STAINLESS STEEL "STAINLESS"

Stainless steel, far from being a material that "doesn't rust," is actually an alloy that oxidizes in a controlled and beneficial way. This apparent contradiction is resolved by understanding the phenomenon of passivation: a process by which certain metals, when reacting with ambient oxygen, form a superficial oxide layer that not only doesn't detach but also protects the underlying metal from future chemical aggressions. This behavior is characteristic of metals such as titanium, aluminum, vanadium, tantalum, or niobium, and of course, chromium, a key element in the composition of stainless steel.

When chromium (Cr) is in an alloy with iron (Fe), as in stainless steel, its high reactivity with oxygen allows for the spontaneous formation of a chromium oxide (Cr₂O₃) film on the metal's surface. This layer, barely a few nanometers thick—invisible to the human eye and perceptible only through electron microscopy—acts as a chemical barrier, preventing oxygen, water (H₂O), or any other corrosive agent from penetrating and affecting the steel's internal structure. Unlike iron oxide, which is bulky, porous, and easily detaches, chromium oxide is compact, adherent, and chemically stable, ensuring durable protection without the need for additional coatings.

Iron, by itself, is a highly reactive metal whose oxidation generates compounds such as FeO, Fe₂O₃, and Fe₃O₄. These oxides, commonly known as rust, display colors ranging from yellow to intense red, passing through brown and metallic black, depending on their hydration state and chemical composition. However, none of them have the ability to protect the base metal, as they tend to flake off, continuously exposing new layers of iron to environmental attack. Therefore, conventional steel requires treatments such as chroming, nickel plating, or galvanizing to prevent its degradation.

In contrast, stainless steel benefits from the presence of chromium in proportions greater than 10.5% by mass. This amount is sufficient for the aforementioned Cr₂O₃ layer to form upon contact with atmospheric oxygen, passivating the surface and stopping the corrosive process. It is important to note that this initial oxidation is not a defect, but a virtue: stainless steel is "oxidized" from the very first moment, but in a way that protects it instead of destroying it. This is why objects made with this material—from cutlery to architectural structures—retain their appearance and functionality for decades without intensive maintenance.

The confusion generated by the term "stainless" lies in its literal interpretation. For the general public, it may seem that this type of steel is immune to oxidation, when in reality what happens is that it oxidizes in a controlled and beneficial way. This passive oxidation is what makes it an exceptionally corrosion-resistant material, even in humid, saline, or acidic environments. Thus, stainless steel is not a metallurgical miracle, but the result of a deep understanding of metal chemistry and how to leverage its properties to create durable, safe, and aesthetically appealing materials.

To illustrate the functioning of stainless steel in an accessible way, I will use an analogy with the rocky planets of the Solar System. As is well known, the first four planets in order of proximity to the Sun—Mercury, Venus, Earth, and Mars—share a similar structure: they are spherical bodies orbiting the Sun, and their surface, when reached, offers a solid base, composed of minerals and silicates. In all of them, the crust is formed by complex aluminosilicates, while their cores contain an iron and nickel alloy, in liquid or solid state depending on the planet, which generates their magnetic field and contributes to their gravitational force.

Mercury, for example, has a proportionally larger metallic core than Earth, but lacks an atmosphere. This absence makes it a direct target for asteroids, which are attracted by its gravity and collide unimpeded. If Mercury is observed with a high-powered telescope, a surface riddled with craters will be seen, the result of impacts that, over millions of years, have perforated its crust as if projectiles were being fired at a stony sphere. The lack of a protective barrier means its surface is exposed to constant cosmic aggression.

Mars, the so-called red planet, suffers a similar situation. Its atmosphere is extremely tenuous, practically non-existent in terms of protection. Therefore, it also exhibits large craters, visible even with modest telescopes. The Moon, although not a planet but a natural satellite, represents another clear example: its surface is marked by asteroid impacts that, without an atmosphere to slow them down, have left indelible traces throughout its history. It is enough to observe it with binoculars—or prismáticos, as they are called in some countries—to notice the presence of rounded craters that evidence its vulnerability.

In contrast, Venus and Earth have dense atmospheres that act as shields. Although both planets attract asteroids with greater gravitational force, their atmosphere progressively disintegrates them before they reach the surface. In Earth's case, this protection is largely due to the ozone layer (O₃), while Venus has an atmosphere composed mainly of carbon dioxide (CO₂), a greenhouse gas that keeps its surface temperature constant and extremely high. Curiously, although Venus is further from the Sun than Mercury, its atmosphere makes it the hottest planet in the solar system. Life would be impossible there: the scorching heat and the toxicity of the air make it resemble a Dantesque vision of hell.

The analogy makes sense when this planetary protection is compared to the behavior of stainless steel. In the world of metallurgy, carbon steel balls, upon contact with water or moist air, suffer corrosion that compromises their integrity. In contrast, stainless steel balls are coated by a passive layer of chromium oxide (Cr₂O₃), which prevents the penetration of diatomic oxygen (O₂), just as the Earth's atmosphere prevents asteroids from reaching the surface. It is as if each oxygen atom were an asteroid, and the chromium oxide layer acted as a protective atmosphere that preserves the internal structure of the metallic sphere.

This comparison, though seemingly charming, is profoundly revealing. The ozone layer protects life on Earth, just as the passive layer of chromium oxide protects the useful life of stainless steel. If we continue to deteriorate our atmosphere, we could transition from the beneficial presence of ozone to the predominance of carbon dioxide, transforming our planet into a replica of Venus. Therefore, this analogy not only seeks to explain a metallurgical phenomenon but also to raise awareness: caring for our atmosphere is protecting our existence.

Although this topic has been addressed before, it deserves to be revisited with special emphasis, as the terminology we use to refer to stainless steel can lead to a mistaken interpretation of its real properties. In Spanish-speaking countries, as well as in Portugal, France, Italy, and other regions whose language derives from Latin, the term "acero inoxidable" (stainless steel) suggests, by its very linguistic construction, an absolute quality: the impossibility of oxidizing. However, this idea is technically incorrect. Stainless steel is not invulnerable to oxidation, but rather possesses exceptional resistance to corrosion, which is a fundamental distinction.

In English, this alloy is called Stainless Steel, a name that, without intending to provoke linguistic jealousy, is more precise. "Stainless" literally means "without stains," and refers to the material's ability to resist the formation of visible rust, that reddish stain that characterizes iron or common steel when it oxidizes. It does not claim that the steel does not oxidize, but that it does not stain, which is a more honest and technical way of describing its behavior against corrosive agents.

In German, the term used is Edelstahl, composed of edel, meaning "noble," and stahl, meaning "steel." This nomenclature also does not promise absolute immunity, but it does highlight the nobility of the material compared to its less resistant counterparts. Noble steel differs from ordinary steel in its ability to maintain its structural and aesthetic integrity under adverse conditions, without succumbing to the surface corrosion that affects other metals.

Of course, it is not about changing the name of stainless steel in Spanish-speaking countries. It would be absurd to attempt a terminological reform of such magnitude. What can—and should—be done is to foster a more precise understanding of what "stainless-ness" actually implies. Stainless steel does oxidize, but it does so in a controlled and superficial way, generating a passive layer of chromium oxide (Cr₂O₃) that protects the material from deeper oxidation. This layer is so thin that it is invisible to the human eye, giving the impression that the metal does not oxidize at all.

To illustrate this idea, let's think about oxygen. Can you see it? No. Can you touch it? Neither. But we know it's there, because we breathe it, because it inflates a basketball, because it allows a ball to bounce on the court. We don't see the air, but its presence is evident by its effects. Similarly, the oxide that covers stainless steel is present, even if we don't perceive it visually. It's like the air inside the ball: invisible, but essential. Thus, stainless steel is not a promise of eternity without rust, but an intelligent solution that uses chemistry to protect itself. And that, far from being a weakness, is a display of technical sophistication.

I hope this reflection helps to better understand the nature of this material, and contributes to a fairer and more precise appreciation of its true value.

A common — and surprisingly widespread — confusion is the inability of many people to distinguish between chrome-plated steel and stainless steel. Although aesthetically both can present a similar shine, the structural difference between them is profound and significant. Just observe an everyday example: car rims. What causes that shiny finish? Are they made of common steel, stainless steel, or simply chrome-plated? In low-end or mid-range vehicles, it is common to find aluminum or chrome-plated steel rims, while in mid-to-high-end models, solid stainless steel might be used, although commercially they are called “alloy wheels.” This term, more marketing than technical, suggests superior quality, although the “alloy” in question can be either aluminum or stainless steel.

For a metallurgy or mechanics professional, the difference between aluminum and steel is evident at first glance. For the general public, a rudimentary method is to approach a magnet. However, this trick is not always conclusive, as ferritic and martensitic stainless steels are ferromagnetic, just like pure iron, allowing the magnet to adhere. Therefore, differentiating between a chrome plating and a stainless steel alloy requires more than a simple magnetic test. The essential distinction lies in the composition: chrome-plated steel consists of a carbon steel base superficially coated with metallic chromium, while stainless steel is a homogeneous alloy in which chromium is integrated into the iron matrix. To illustrate, consider a one-ton car body made of chrome-plated steel: it will contain, at most, 1 kg of chromium. In contrast, a car body of the same weight made of stainless steel can incorporate up to 150 kg of chromium into its structure. The difference is not only quantitative but qualitative.

Stainless steel is perfectly viable for small objects, but when it comes to large-scale industrial production, its cost becomes a limiting factor. For example, although there is no technical impediment to manufacturing firearms from stainless steel, the price makes it impractical. Think of any pistol you imagine: you probably visualize it in matte black. That hue is not accidental but the result of controlled oxidation or phosphatization processes applied to carbon steel, which, in addition to protecting the surface, give it that characteristic finish.

Another revealing example is found in automobiles from past decades. In the 1970s and 80s, most car bodies were made of iron or carbon steel, which explains their vulnerability to corrosion. Very few models were built with stainless steel, the DMC DeLorean being the most emblematic case. This vehicle, immortalized by the Back to the Future film saga, features a stainless steel body that does not require paint for its preservation. In fact, applying paint could damage its passive protective layer. The DeLorean, though iconic, did not consolidate commercially and was soon surpassed by aluminum bodies and even more modern materials like carbon fiber.

çIn short, stainless steel is not a simple coating that beautifies the surface: it is a solid alloy, designed to resist corrosion from its core. Its presence in industry is synonymous with durability, although its cost reserves it for applications where resistance and aesthetics are fully justified.

Contrary to popular belief, stainless steel is not an indestructible material. In fact, there are more chemicals capable of damaging it than those that are innocuous. Its defense mechanism is simple but effective: as long as the superficial layer of chromium oxide (Cr₂O₃) remains intact, the metal remains protected against corrosion. However, this barrier can be compromised under certain environmental or chemical conditions, exposing the steel to degradation processes.

To understand the nature of stainless steel, it is useful to imagine it as common steel that has already been oxidized in a controlled manner. In dry or humid environments, even the most resistant steels that are not stainless develop a dark patina that thickens over time, which is undesirable. In contrast, in stainless steel, the opposite occurs: contact with humid air or fresh water favors the formation and regeneration of its protective layer, thanks to a constant supply of oxygen. This capacity for “self-healing” manifests, for example, when a stainless steel knife is scratched. If a rusted conventional steel knife were sanded, a grayish surface similar to pure iron would be obtained. But when a stainless steel knife is sanded, its protective oxide is temporarily removed, and it regenerates spontaneously in the presence of oxygen. This property makes stainless steel an exceptional, almost “living” material that defends itself through an automatic chemical reaction.

However, this defense has its limits. Being an oxide layer, it can be destroyed if exposed to aggressive agents such as salt water. The chlorine dissolved in seawater slowly attacks the Cr₂O₃, exposing the underlying metal, which then begins to corrode. This type of deterioration is known as "pitting," a term without a direct translation into Spanish, which describes localized corrosion in the form of small cavities. Pitting is just one of many ways stainless steel can be affected. Other dangerous substances include hot alkaline salts, halogens and their compounds, lyes, and reducing acids. Although stainless steel is highly resistant, certain compounds present even in food can compromise its integrity.

To ensure the durability of a stainless steel part, it is essential that it is exposed to a source of oxygen, not simply an oxidizing agent. This distinction is crucial: elemental fluorine, for example, is a powerful oxidant but does not provide oxygen, and its contact with stainless steel can be catastrophic. A true source of oxygen is one that facilitates the formation of protective oxides, such as water (H₂O), which is abundant, economical, and effective. Stainless steel shows excellent resistance to fresh water, which actively contributes to maintaining its passive barrier intact.

The close relationship between iron (Fe) and chromium (Cr) is not coincidental but a direct consequence of their shared physical properties. Both elements are close in the periodic table, have similar atomic radii, possess comparable hardness, share a body-centered cubic (BCC) crystal structure, and exhibit high melting and boiling points. This structural affinity facilitates their total miscibility: they can combine in any proportion, from as little as 0.01% to 99.99%, without generating incompatible phases. This characteristic makes chromium an ideal ally for modifying the properties of iron without compromising the homogeneity of the alloy.

At first glance, it might seem that the higher the chromium content, the more beneficial the resulting alloy will be. However, this assumption deserves deeper reflection. Iron is a tough, ductile, and malleable metal, qualities that make it suitable for manufacturing wires, thin sheets, and structural components. Chromium, on the other hand, is rigid and brittle, which limits its use in applications requiring plastic deformation. Therefore, although chromium provides corrosion resistance and surface hardness, its excess can compromise the workability of the material.

For a steel to be considered stainless, it must contain at least 10.5% chromium. This minimum proportion allows the formation of a passive layer of chromium oxide (Cr₂O₃) that protects the underlying metal from oxidation. In certain special grades of stainless steel, the chromium content can reach up to 25%, although these cases are less common and are usually intended for very specific applications. In most commercial stainless steels, the chromium content ranges between 12% and 18%, an interval in which an optimal balance between corrosion resistance, formability, and production cost is achieved. While it is true that a steel with 18% chromium will be more resistant than one with 12%, the difference is not linear or absolute, as other alloying elements intervene, modifying the behavior of the alloy.

One of these key elements is carbon (C), whose presence decisively influences the hardness, mechanical resistance, and weldability of steel. The interaction between chromium and carbon can lead to the formation of carbides, which affect both the microstructure and intergranular corrosion resistance. Therefore, the design of a stainless steel cannot be limited to adjusting the percentage of chromium but must consider the set of present elements and their metallurgical synergy.

As anticipated at the beginning of this text, the interaction between iron (Fe) and chromium (Cr) has been discussed so far without considering the carbon (C) content, precisely to avoid confusion for the reader. The objective of this work is to offer an accessible, clear, and understandable reading for anyone, regardless of their age or technical background. However, the time has come to introduce carbon into this metallurgical equation, as its influence on the properties of stainless steel is as decisive as it is complex.

The iron-chromium alloy, by itself, presents good malleability and certain ductility. However, as in all types of steel, the mass percentage of carbon largely determines the mechanical and thermal characteristics of the material. In stainless steels with low carbon content, hardness and rigidity are reduced, while in those with high carbon content, both properties increase notably, especially after subjecting the material to heat treatments. This balance between carbon and metal must be carefully adjusted according to the requirements of each application, keeping in mind that carbon, although improving hardness, reduces corrosion resistance.

This phenomenon is explained by carbon's tendency to form carbides with chromium, such as Cr₇C₃ or Cr₂₃C₆, which limits the availability of chromium atoms to generate the protective oxide layer (Cr₂O₃). In other words, carbon "sequesters" chromium, preventing it from fulfilling its passivating function. Thus, as carbon content increases, the ability of stainless steel to resist corrosion decreases, even if the chromium percentage also increases.

An illustrative example is found when comparing two grades of stainless steel: AISI 420A, with approximately 12% chromium and 0.25% carbon, and AISI 440C, which contains between 18% chromium and 0.95–1.2% carbon. At first glance, it might be assumed that the second is much more corrosion resistant due to its higher chromium content. However, this difference is offset by the high carbon content, which reduces the effectiveness of chromium as a protective agent. Furthermore, both steels present different crystalline structures: AISI 420A adopts a ferritic structure, which is the most stable form of iron and also of chromium, while AISI 440C presents a martensitic structure, generated by rapid cooling after heat treatment.

Ferrite is soft, tough, and flexible, making it ideal for applications that do not require great hardness, such as cutlery, decorative pieces, or cutting tools not subjected to extreme stresses. In contrast, martensite is extremely hard and rigid but less tough, making it the preferred choice for components that must withstand intense abrasion, such as industrial blades, bearings, or surgical instruments.