There is little difference between steel and iron. Pure iron is as soft as copper. By adding up to 2 percent carbon into it, an incredible strength, hardness and mechanical properties are obtained, which is now called steel. Because of the abundance of iron, easy and cheap to obtain, the use of steel is very common. However, there is a weak point in steel, also in iron. Corrosion, in other words oxidation.
Millions of dollars are wasted in the world every year as a result of the rusting of the items used in our daily life. Most of this loss is due to rusting of iron and steel. We can briefly define rusting as the combination of iron with oxygen in the air. Actually, this is an electrochemical reaction. For this reason, rust that starts in one part of the material can pass under the paint and appear in another place.
Not only iron and steel but also other metals rust. For example, aluminum, brass, bronze. However, the very thin layer formed by the combination of material and oxygen in them plays a protective role by cutting the contact of the material with air as soon as it is formed, preventing the progression of corrosion. This layer is so thin that the color of the material is almost unchanged. The characteristic of rusting in iron is that there is no solid bonding on the surface due to the big difference in its and oxygen atoms, the penetration of rust into the material, not only the appearance but also the strength.
Humidity in the air also has a great effect on rusting. The amount of water in the reaction determines the color of the rust. For this reason, the color of rust can be black or very dark brown or yellowish. Another factor that increases the speed of rusting is salt. It increases the rate of this electro-chemical reaction. In winter, places where salt is poured on the roads and seasides due to snow, rusting is faster.
Before stainless steel, the material was painted or galvanized to prevent rusting. These solutions created other problems, especially in the health and food industry. Harry Brearley discovered the first stainless steel by accident in 1913. While experimenting by combining various metals for rifle barrels, he found that some of them were rust resistant. As with any great invention, he also struggled to get it accepted by the industrialists.
Some metals, such as chromium, combine with oxygen very easily and quickly due to their close atomic sizes. They form a very thin but very solid layer, with a thickness of a few atoms. There is no further reaction. Even if this layer is damaged, it forms again. The same thing happens if chrome is added to steel at a certain rate, the steel will no longer rust.
There is 10-30 percent chromium in stainless steel. Depending on this ratio and additions of nickel, titanium, aluminum, copper, sulfur, phosphorus and similar elements, the place of use varies.