What is stainless steel?
What is stainless steel? Stainless steel, often abbreviated as stainless steel, is a popular type of steel known for its excellent corrosion resistance. Despite the fact that it is often referred to as ‘stainless steel’, that is actually incorrect. Stainless steel can indeed rust, especially if the protective layer is damaged or if the material is exposed to aggressive conditions such as chlorine or acids.
Stainless steel is an alloy that consists mainly of iron, chromium and nickel. By adding at least 11 to 12% chromium, a thin, invisible layer of chromium oxide forms on the surface. This oxide layer protects the underlying metal from further oxidation. This protection also makes it more difficult to chemically blacken stainless steel: the oxide layer must first be removed with a suitable acid, such as Verzinkshop Metaal Activator or a solution of hydrochloric acid, before the metal reacts with black oxide liquid.
Types of stainless steel and their properties
There are different types of stainless steel, each with its own composition and properties. The most commonly used groups are:
- Austenitic stainless steel (200 and 300 series): such as AISI 304, 316, 316L and 316Ti. These grades contain a relatively high amount of nickel (6 to 26%), are not magnetic when delivered, are easily malleable and have a high corrosion resistance. Ideal for applications where formability and resistance to rust are important. Because they are so rust-resistant, these steels require a powerful activation with hydrochloric acid before you can blacken them.
- Ferritic stainless steel (400 series): like AISI 430. These types contain less nickel, are magnetic and more resistant to light rust, but less deformable. They are slightly easier to blacken than austenitic species.
- Martensitic stainless steel: such as AISI 410 and 420. These steels are hardenable by heat treatment and are always magnetic. They are easy to work with, but less rust-resistant.
- Duplex stainless steel: a mix of austenitic and ferritic structures. Strong, fairly corrosion resistant, and magnetic. Suitable for heavier applications.
Known stainless steel types
- 304 (1.4301): contains 18% chromium and 8% nickel. Commonly used for general applications. Difficult to oxidize black without proper activation.
- 316 (1.4401): with 16% chromium, 10% nickel and 2% molybdenum. Extra resistant to salt and chlorine. Very suitable for marine and chemical environments.
- 316L (1.4404): low carbon content (‘L’ stands for low carbon), more weldable and less susceptible to inter-crystalline corrosion.
- 316Ti (1.4571): similar to 316L, but alloyed with titanium. The grinding pattern is therefore slightly different, which can affect visible work.
Magnetic or not?
Whether stainless steel is magnetic depends on its structure. Austenitic stainless steel is in principle not magnetic, but can become so partly through cold forming (such as bending or pressing). Ferritic and martensitic species are always magnetic. When choosing stainless steel for decorative black oxidation, for example, this is something to take into account, because the process sometimes adheres slightly better to magnetic alloys.
Corrosion resistance and sensitivity
Stainless steel is highly resistant to rust, but in contact with chlorine (e.g. in swimming pool water, bleach or city water) even AISI 304 can corrode. This causes so-called pitting corrosion: small holes or pits in the surface. Variants with molybdenum, such as 316 and 316L, are more resistant to this.
At the same time, these protective properties make stainless steel difficult to blacken. The oxide layer blocks the chemical reaction with black oxide liquids. That is why pre-activation with an acid such as Verzinkshop Metaal Activator is essential. For stainless steel from the 300 series, even a stronger acid, such as dilute hydrochloric acid, is needed to open the surface sufficiently.
Hardness, strength and machinability
The hardness of stainless steel is partly determined by its carbon content. Martensitic stainless steel grades contain more carbon and are therefore harder, but also less corrosion resistant. Austenitic species are softer, more malleable and not hardenable by heat. However, they can be strengthened by cold forming.
The modulus of elasticity of stainless steel is slightly lower than that of ordinary steel (195,000 MPa vs 210,000 MPa). This means that stainless steel is just a little more ‘stretchy’ under load. Tensile strength and yield strength are important metrics for high-strength or flexural load applications.
Stainless versus stainless
People often speak of ‘stainless steel’, but technically speaking, ‘stainless steel’ is more correct. Stainless steel is not immune to rust: it is only more resistant to oxidation due to the passive chromium oxide skin. If this layer is damaged or contaminated (for example by iron particles from tools), the steel can still rust.
That is why it is important to clean stainless steel thoroughly after processing. This is often done via chemical pickling: a process in which surface residues, contamination and iron particles are chemically removed to restore corrosion resistance.
Stainless steel blacks: what is important?
Stainless steel black oxidation is possible, but requires an adapted approach. Due to the high corrosion resistance, the surface is less reactive. That makes a powerful activation step essential. Without this activation, the black oxide liquid will hardly adhere and you will get a blotchy or gray result.
At Verzinkshop we recommend Verzinkshop Metal Activator for stainless steel from the 400 series. For 300 series stainless steel, a mixture of dilute hydrochloric acid is more effective. Note: always rinse well after activation, and prevent the surface from drying before sealing. Preferably use our Stainless Steel Black Oxide for blackening stainless steel – our standard products for steel do not work on this.