Stainless steel is one of the most commonly found metals in our lives today, and we encounter this metal throughout every single day, in cutlery, furniture, electrical appliances… it is so common, that we often don’t realise how amazing this metal is, a metal that is resistant to corrosion under almost any environment encountered in our daily lives; and this, for a metal that is largely composed of iron, a metal known to rust in the slightest of moisture, forming a flaky brown rust which corrodes at the structure of the material. How is stainless steel so “stainless” and corrosion resistant? Maybe, we have to first take a look at an interesting discovery made in China.
We all know of the terracotta army in China, part of a vast mausoleum complex built for Qin Shi Huang 2,200 years ago. However, did you know that the intricately sculpted terracotta soldiers used to be armed with real bronze weapons? Archaeologists uncovered many fine examples of bronze weaponry from the Qin dynasty while excavating the mausoleum complex.
A sword unearthed from the Qin Shi Huang Mausoleum, retrieved from http://www.iflscience.com
The immense scale of the mausoleum complex stunned the world when it was first discovered. But now, what truly surprised chemists is the exceptional condition of the blade despite having been buried for more than two thousand years.
But what can explain this? Could it be the quality of the soil, or the condition in which the sword had been buried? Probably not. As can be seen, the bronze handle of the sword oxidized normally, forming a green layer of oxidation. However, the blade remains a shiny grey colour, suggesting a different chemical composition protecting it from the environmental conditions which resulted in the rusting of the handle.
Investigating this further, scientists discovered that the blade had a 10 to 15 thick layer of chromium oxide, and this oxide layer protected the blade from corrosion. While the presence of the chromium oxide layer was verified, there may be some doubts in the thoughts of some: could this just be an “accident”? That for some reason, impurities in the smelting process gave rise to this chromium oxide passivation layer, and was not an intentional addition to the smelting process to give a “stainless” blade?
However, further archaeological evidence suggests that this discovery is not a one-off accident, but instead a technology that has consciously been used by Chinese metallurgists when forging the finest blades to protect them from oxidation.
The Sword of the King of Yue (~500BC), from http://www.lishiquwen.com
Archaeologists in China later unearthed an intricately designed sword inlaid with gold lettering, allowing it to be identified as being forged around 500BC. When unearthed from the ground, the sword was still so sharp as to cut the hand of an archaeologist who held it by the blade; without any treatment, the blade was found to be able to cut through 20 sheets of paper effortlessly, despite being buried for 2500 years. The secret to this blade is yet again a layer of chromium oxide on the blade. It now seems that Chinese blacksmiths had discovered this technology more than 2500 years ago, that has unfortunately been lost until it was rediscovered in the 20th century.
This surprising discovery of an ancient marvel brings us back to one of the marvels of material science engineering in the 20th century that truly impacted the lives of the ordinary person: that of stainless steel. Stainless steel contains more than 13% chromium by mass, resulting in the formation of a passivation layer of chromium (III) oxide when exposed to air. In harsher environments, a thicker oxide layer may be necessary, hence the percentage of chromium in the steel is usually increased.
Of course, we all know that “stainless” steel is not very “stainless” after all. In other words, stainless steel does rust. As you might observe though, the rusting pattern in stainless steel is distinctly different from that of iron. While iron tends to form large patches of rust which flake off, rusting in stainless steel often occur as small dots on the surface, looking almost like some sort of fungal infection. Of course, no fungus is at play here, just corrosion science.’
A rusting stainless steel sink, retrieved from amtecorrosion.co.uk
Such type of corrosion is known as “pitting” corrosion, due to the formation of corrosion “pits” on the surface of the metal. For stainless steel, such corrosion often occurs in saltwater; this is also the reason why metal fixtures are not advisable for outdoor areas that is near to the sea (where the breeze is often laden with salt water droplets).
Pitting Corrosion. Retrieved from: http://www.aperam.com
The main “corrosive agent” in salt water is the chloride ion. The chloride ion attacks and destabilizes the chromium (III) oxide layer, and at areas where the oxide layer is sufficiently compromised, the metal underneath is oxidized by the dissolved oxygen in the water and dissolves into solution, forming pits of corrosion where the metal has been corroded away. The oxide layer can form over this pit (known as “repassivation”), protecting the metal from further oxidation. But if the release of metal ions at the pits is aggressive enough (depending on the environment of corrosion), repassivation may not occur, and pitting corrosion continues.
In even stronger corrosive mediums, such as acids, the layer of chromium (III) oxide breaks down evenly, leading to uniform corrosion instead of pitting corrosion.
In primary school, we learn that an iron nail submerged in water containing a layer of oil floating on it rusts much slower. However, an oxygen-deficient environment can actually accelerate the rusting of stainless steel. This is because in an oxygen-deficient environment such as in small crevices, the protective layer of chromium (III) oxide forms incompletely, making it easier for the steel to be corroded if the water contains other electrolytes that can act as oxidizing agents to corrode the iron in the stainless steel.
One of the most interesting type of corrosion is microbiologically influenced corrosion. Sulfate reducing bacteria are bacteria that are anaerobic, relying on sulfur sources instead of oxygen to drive metabolic function. They obtain energy by oxidizing organic molecules or elemental hydrogen, and reducing sulfate to hydrogen sulfide.
Microbiologically influenced corrosion. Retrieved from: sassda.co.za
The hydrogen sulfide formed reacts with the iron in the steel to form iron sulfide, thus corroding the steel.