Stainless Steel, Aluminum and its alloy elements. Metals in the industry

Stainless Steel ⁽¹⁾

Information obtained from the Handbook of Stainless Steel by Outokumpu.

Stainless steels are ferrous alloys based on iron, chromium (with a minimum of 11%), carbon and other elements, mainly nickel, molybdenum, manganese, silicon, titanium, etc., which give them a particular resistance to some types of corrosion. The element that gives them their corrosion resistance is chromium, through the formation of a thin invisible and transparent layer of chromium oxide, which is self-repairing.

The addition of other elements improves their properties, as required in the final application and in the operating environment in question, as described below.

Aluminum ⁽²⁾

In significant amounts, Aluminum increases resistance to oxidation and that's why it is used in certain heat-resistant grades.

Chromium ⁽²⁾

Chromium (Cr) is the element that gives stainless steels their corrosion resistance: as the chromium content in the alloy increases, the corrosion resistance increases even at high temperatures. Chromium promotes ferritic (magnetic) microstructures.

Nickel ⁽²⁾

The addition of Nickel (Ni) increases the ductility and hardness of stainless steels. It reduces the rate of corrosion and is very useful in acidic environments. Nickel promotes austenitic (non-magnetic) microstructures.

Carbon ⁽²⁾

Carbon (C), significantly increases mechanical resistance, but when combined with chromium dissolved in the alloy it forms chromium carbides, which reduces resistance to intergranular corrosion. Therefore, the carbon content should be kept as low as possible. Carbon promotes austenitic microstructures. In ferritic stainless steels, carbon reduces hardness and corrosion resistance. In martensitic stainless steels, carbon increases hardness and resistance but reduces toughness.

Manganese ⁽²⁾

At low temperatures, Manganese (Mn) promotes austenitic (non-magnetic) microstructures and is used to replace certain nickel contents in austenitic stainless steels, thus giving rise to chromium-manganese steels (200 series). However, at high temperatures it stabilizes the ferrite phase. Manganese increases the solubility of Nitrogen and is therefore used to achieve high contents of this element in duplex and austenitic stainless steels.

Copper ⁽²⁾

The addition of Copper (Cu) increases formability and machinability. It increases corrosion resistance in certain acidic environments and promotes austenitic microstructures.

Molybdenum ⁽²⁾

Together with Chromium, Molybdenum (Mo) significantly increases corrosion resistance and mechanical resistance.

Titanium ⁽²⁾

Titanium (Ti) is a strong former of ferrite and carbides, thus reducing the effective carbon content. In austenitic steels with high carbon content, it is added to increase resistance to intergranular corrosion (stabilized grades), also increasing mechanical properties at high temperatures. In ferritic stainless steels, the addition of Titanium increases toughness, formability and corrosion resistance.

Niobium ⁽²⁾

Niobium (Nb) is a strong former of carbides and promoter of the ferritic structure. In ferritic stainless steels (stabilized grades), the addition of Niobium and/or Titanium increases hardness and minimizes the risk of intergranular corrosion, by combining with excess carbon thus preventing the formation of chromium carbides and the consequent dilution of this critical element in the solution.

Nitrogen ⁽²⁾

Nitrogen (N) is a strong former of austenite and significantly increases mechanical resistance. It increases resistance to localized corrosion, especially if combined with Molybdenum.

Silicon ⁽²⁾

Silicon (Si) increases resistance to oxidation at high temperatures and also in very oxidizing solutions at lower temperatures. It promotes the ferritic microstructure and increases resistance.

Cobalt ⁽²⁾

Cobalt (Co) is used in martensitic steels to increase hardness and resistance to hardening.

Vanadium ⁽²⁾

Vanadium (V) is used only in hardenable stainless steels. It forms carbides and nitrides at low temperatures and promotes the formation of ferrite in the microstructure, increasing toughness.

Sulfur ⁽²⁾

The addition of Sulfur (S) increases the machinability of stainless steels. It reduces corrosion resistance, ductility and formability. In small quantities, it improves the weldability of stainless steels.

Aluminum ⁽³⁾

Aluminum (Al) is a lightweight, ductile and malleable silver-colored metal. Although aluminum is an active metal, a thin layer of invisible and transparent aluminum oxide gives it stability and corrosion resistance.

Commercial Aluminum with 99% purity is characterized by its ductility, good weldability, electrical conductivity and excellent corrosion resistance; its commercial applications include food containers, decorative packaging, electrical wires and cables, profiles for sales and doors, foil for food preservation and for packaging of medicines, etc.

Aluminum forms a series of excellent alloys with a wide range of properties and applications when combined with alloy elements such as Copper (Cu), Manganese (Mn), Magnesium (Mg), Silicon (Si) and Zinc (Zn).

Applications in the medical sector ⁽³⁾

Titanium

Titanium is a metal with wide use in medical applications, particularly in internal applications in the human body because it is physiologically inert compared to other metals. Osseointegration is a unique phenomenon in which the body's natural bone and tissue binds to the titanium implant, firmly securing the implant in place.

Titanium is used as a shield for implanted devices that control cardiac function; products that dispense medication and perform neurostimulation; such as rods, pins and orthopedic plates. In addition to the use of implants, titanium is the ideal choice for surgical instruments, such as drills, forceps, retractors, scissors, needle holders and eye surgery equipment. The metal does not interfere with medical tests that require magnetic resonances or scans

Niobium

There is a growing interest in niobium and its alloys for use in medical equipment. It is often found in devices such as pacemakers, as it is physiologically inert. Niobium treated with sodium hydroxide forms a porous layer that aids osseointegration, which also makes it an attractive alternative for internal applications.

Copper

Copper can be used to transmit signals to small implants and tools effectively, this being one of the main reasons for the growing interest in this material in the medical sector for internal devices.

Copper is a ductile metal with a high level of thermal and electrical conductivity. Pure copper is relatively soft and malleable, it is easy to work with, and the ease with which it can be transformed into wire, added to its excellent electrical properties, make copper a useful material for electrical medical devices when properly protected. Due to its high conductivity, it is possible to embed small copper wires in devices to emit or receive signals, or transport electrical charges to perform tasks within the body.

Copper ions are soluble in water, where they function at low concentration as bacteriostatic substances, fungicides and wood preservatives. For this reason, copper can be used as a surface that repels germs, which increases antibacterial and antimicrobial characteristics in buildings such as hospitals.

To learn more about the application of metals in the medical industry, we invite you to read our article: Specialty metals make the creation of unmatched medical equipment possible.

Bibliography

(1) Di Caprio, Gabrielle. “Stainless Steels”, Ed. Ebrisa, S.A. (1987), Barcelona.

(2) Outokumpu Oyj “Handbook of Stainless Steel” (2013), Pages. 13-15

(3) Considine, Douglas M., “Chemical and process technology encyclopedia”, McGraw-Hill Book Company, Page. 76.

(4) Ulbrinox. “Specialty metals make the creation of unmatched medical equipment possible” (2021, February) https://www.ulbrinox.com.mx/blog/metales-de-especialidad-hacen-posible-la-creacion-de-equipo-medico-sin-igual