Inox, Technically Considered

General

Inox, stainless steel, or RVS (rust-free steel), all synonyms for the same material. But what exactly is inox, and how does its corrosion resistance increase?

The correct definition of stainless steel is "an iron alloy containing at least 10.5% chromium and no more than 1.2% carbon." These values mark the threshold at which steel begins to form a self-healing oxide layer (also called an oxide film or passivation layer), protecting the underlying metal and significantly improving its corrosion resistance. This oxide layer forms spontaneously upon contact with air (oxygen), resulting in a passive layer that enhances corrosion resistance. Thus, "stainless steel" is technically the correct term, not "rust-free."

If the steel surface is damaged, the iron alloy is initially exposed without the oxide film. At this moment, the material is especially vulnerable, but typically the oxide layer spontaneously repairs itself upon renewed contact with air.

There are, of course, many different iron alloys that meet the definition of "stainless steel," some being more corrosion-resistant than others. Besides iron, chromium, and carbon, elements like nickel, molybdenum, and titanium are also added. The most commonly used alloy, RVS304, consists of 18% chromium and 8% nickel.

Historical Context: Stainless steel is an alloy discovered in the early 20th century, originating from the arms industry. A specific alloy proved more resistant to corrosion and offered advantages like good wear resistance and heat resistance, ideal for making gun barrels. However, its breakthrough in everyday use came only after World War II, becoming highly popular due to its numerous advantages.

Yield Strength and Melting Temperature of Metals

The heat resistance of metals is often indicated by their yield strength and, to a lesser extent, their melting temperature.

The yield strength is a material constant describing the point in a stress-strain diagram where a ductile material begins to "flow" or the point where plastic deformation starts. The term melting temperature is self-explanatory. The melting temperature is less critical; a structure will collapse due to reaching its yield strength long before it starts melting.

The yield strengths found in the literature may vary; consider the following list as an indication of the heat resistance of a material relative to another:

  • Al: 50 MPa
  • RVS: 170 MPa
  • Steel: 235 MPa
  • Cu: 258 MPa
  • Ti: 500 MPa

Titanium stands out with a high yield strength. Adding titanium significantly increases the heat resistance of materials. For instance, the qualities RVS321 and RVS316Ti are not considered true heat-resistant grades but are much more heat-resistant than their siblings without titanium, namely RVS304 and RVS316.

For completeness, here are some melting temperatures:

  • Pb: 327°C
  • Zn: 419°C
  • Al: 660°C
  • Cu: 1084°C
  • Ni: 1455°C
  • Ti: 1667°C