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.’Ī rusting stainless steel sink, retrieved from .uk 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. The Sword of the King of Yue (~500BC), from Īrchaeologists 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.
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. 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? 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.īut 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. 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.Ī sword unearthed from the Qin Shi Huang Mausoleum, retrieved from 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.
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