Chrome highlighted for resistance properties

With stainless steel considered a “marvel of modern engineering”, owing to its involvement with products ranging from kitchen appliances to surgical instruments, independent corrosion and failure consultant Simon Norton notes that stainless steel’s most celebrated feature is its “remarkable” resistance to corrosion – a property largely owing to the incorporation of chromium.

Norton is an expert in corrosion and failure investigation with a specialist expertise in stainless steel.

Chromium is an element classified as nonferrous metal that is brittle in its natural state and forms a critical additive in making stainless steel corrosion resistant.

Chromium can be used in a chemical coating process known as chrome plating whereby a thin chromium layer is applied to a raw metal substrate.

Chromium ore is frequently mined with other platinum group metal (PGM) ores in the PGMs-rich reefs found in South Africa.

Norton explains that iron – the “backbone” of traditional carbon steel – readily reacts with oxygen and moisture, forming the familiar reddish-brown rust. However, using chromium in the steel mix serves to disrupt this process through a phenomenon called passivation.

“When the chromium content in steel exceeds 10.5%, it reacts with oxygen to form a thin, transparent layer of chromium oxide on the steel surface. This chromium oxide layer, despite its microscopic thickness, acts as a formidable barrier, preventing further access of oxygen and moisture to the underlying iron,” Norton explains.

Used as such, he explains that chromium is deployed as a “fortress wall”, wherein the chromium oxide layer guards the steel, while any intruding oxygen or moisture will not corrode the steel.

“This self-healing property is another key aspect of chromium’s magic,” notes Norton, adding that while minor scratches or abrasions on the surface of stainless steel can disrupt the passivation layer, chromium readily reacts with fresh oxygen to reform the protective shield, ensuring the “remarkable corrosion resistance” of stainless steel.

The amount of chromium, along with other alloying elements such as nickel, molybdenum and nitrogen, determine the specific properties and corrosion resistance of different stainless steel grades. Stainless steel contains more than 10.5% chromium, thus making it stainless.

There are, however, many stainless steel alloy categories using chromium.

The austenitic stainless steels within the 300 series, typified by 304 and 316, contain between 18% and 20% chromium, and between 8% and 10% nickel. This composition, he says, promotes a stable austenitic microstructure, imparting high ductility, formability and weldability to these alloys.

The addition of another useful element called molybdenum at about 2.5% produces the 316 grade of stainless steel, Norton adds.

Renowned for their resistance to general corrosion, even in mildly acidic or alkaline environments, Norton explains that these steels find practical use in applications such as food plants, tubing, piping and process vessels.

Ferritic stainless steels, falling under the 400 series, feature lower chromium content of between 11% and 13%, and lack nickel, possessing magnetic properties.

While these steels exhibit higher strength and lower ductility compared with austenitic grades, their corrosion resistance is notable in oxidising environments, with limitations in acidic or chloride-rich conditions.

Automotive exhaust systems, building materials and appliances constitute common applications for ferritic stainless steels.

Norton points out that martensitic stainless steels, also part of the 400 series, attain elevated strength and hardness through a specific heat treatment process that alters their microstructure.

With varying chromium content of between 12% and 17%, martensitic stainless steels offer good corrosion-resistance in mild environments but may be prone to stress corrosion cracking, Norton says, adding that their robust strength makes them suitable for applications such as use in cutlery, surgical instruments and shafts.

Martensitic stainless steel, specifically, is used widely for surgical instruments.

However, while chromium is key to corrosion resistance, Norton admits that other alloying elements also play crucial supporting roles.

Nickel, for example, in an austenitic steel enhances ductility, weldability and resistance to specific corrosive environments and is known as an austenite former.

Molybdenum is an element that further improves pitting and crevice corrosion-resistance, especially in environments containing chlorides, while nitrogen strengthens the austenitic structure and enhances pitting resistance.

Copper, meanwhile, improves atmospheric corrosion resistance and adds a warm reddish hue to some decorative stainless steel grades.