Why Doesn’t Stainless Steel Rust?
Stainless steel is one of the most widely used materials in the world, it is the hardy workhorse of the engineering, energy, and medical sectors, and forms the backbone of the cutlery industry. Almost everybody knows what stainless steel is, and what stainless steel does best (the clue is in the name), but few know just why this multi-purpose metal is so resistant to corrosion. The answer is that the element behind stainless steel’s remarkable environmental resistance is also the element which provides its characteristically shiny appearance: chromium.
The Origins of Stainless Steel
Though many English, French, German and Polish inventors have staked a claim to the discovery of stainless steel since the early 1800s, stainless steel is generally understood to have been officially ‘discovered’ by Sheffield steelmaker Harry Brearley in 1912 while researching corrosion-resistant alloys for use in the manufacture of gun barrels. Brearley found that the reduction of carbon and the addition of chromium significantly increased the chemical resistance of his alloy, and noted that it was chromium in particular which was the secret to his alloy’s exceptional resilience against water and household acids. Although many additional elements such as manganese and silicon are added to modern stainless steels, Brearley’s criterion still holds true today, and an alloy must have a minimum of 10.5% chromium to be classified as a ‘stainless steel’.
The presence of chromium within stainless steel alters its properties significantly, granting much-needed oxidisation resistance to steel, a metal which is primarily alloyed with iron - an element with notoriously terrible rust-resistance. Chromium causes stainless steel to react differently with oxygen, forming a thin layer of unreactive chromium oxide on the metal’s surface, rather than a thick layer of rust which eats into the metal indefinitely. Of course, there are also innumerable different grades of stainless steel, each with different chemical compositions and additives which are intended to enhance specific qualities. Stainless Steels with even greater rust and chemical resistance are available - one example is A4 or 316 stainless steel, which contains the additive molybdenum. Molybdenum increases a trait known as lattice strain, tightening the crystalline structure of the metal’s surface on a molecular level, which increases the amount of energy required to penetrate the chromium oxide layer.
How 'Stainless' is Stainless?
A popular misconception is that all grades of stainless steel are completely immune to rust. The truth is that, in actuality; stainless steels are resistant to rust – some more so than others. In certain environments and under certain conditions, even the most resilient grades of stainless steel will fall victim to oxidisation and corrosion, although this can take many years. Despite this, components such as stainless steel screws do still offer a remarkable improvement in environmental resistance when compared to standard steel components.
Stainless steels can lose their chemical resistance in a number of ways, one of which is complete submersion in water, which prevents oxygen from reacting with the metal’s surface, and can prevent a chromium oxide layer from reforming if scratched or damaged. Non-specialised grades of stainless steel can also be damaged at very high temperatures (upwards of 400°C), where chromium atoms bond with carbon, causing chromium deficiencies in the metal’s surface. One of the most common testing procedures for the chemical resistance and quality of stainless steels is a salt spray test. This procedure exposes the material to a highly concentrated chloride environment, which is intended to simulate a long period of heavy use. In a salt spray test, stainless steels do corrode, but the purpose of the test is usually not to determine whether or not the material survives, but how quickly, and with what severity the corrosion occurs.