Some of the greatest inventions in the history of planet Earth were results of pure accidents and coincidences. Something similar led to the birth of stainless steel. It was in the year 1913, that English metallurgist Harry Brearly, accidentally found out about chromium’s ability to make steel stain free.
What “Stainless” Really Means
Many people do not truly understand the meaning of being stainless. The general perception is that stainless steels do not ever rust. This is not true. Corrosion or rusting is a very big problem in the stainless steel industry. Then why is the word stainless used? This is because chromium has the property to add to steel’s resistance to stain. As in, when combined with chromium, steel is able to resist corrosion much better. Therefore, chromium does not make steel stain free permanently. Instead, it increases the corrosion resistance of the steel to the point where it becomes known as stainless steel.
Elements Required for Corrosion Resistance
The corrosion resistance of modern day stainless steels is not only dependent on chromium. Other elements such as nickel, niobium, molybdenum, and titanium are instrumental in creating and maintaining the corrosion resistance of stainless steel. All these materials enhance the “stainless” property of the steel to the extent where limited exposure to air and moisture does not result in rust formation.
The Significance of Chromium
A common misconception among people is that stainless steel requires a huge amount of chromium. This is far from the truth. Only 12% chromium is needed to make any steel stainless. Once this seemingly insignificant amount of chromium is added, the steel is able to resist rust and stain much less than other variants of the same metal.
Once the chromium is mixed with the stainless steel to produce an complex alloy, oxygen from the atmosphere combines with the chromium. This causes the formation of an invisible, thin layer of chromium containing oxide referred to as the passive film. There is not a lot of difference between the size of the chromium and oxygen atoms, therefore they are joined together harmoniously on the surface of the metal. The end result is a stable layer of oxide that is a few atoms thick.
Regeneration of Chromium Oxide
If the metal is scratched, this passive layer of oxide is rattled and removed slightly. The area that has been left exposed, or in other words the area that is void of the oxide layer, is covered by an additional oxide layer that is regenerated as soon as the initial layer is removed. As a result, oxidative corrosion is prevented. The reason why iron does not have this corrosion resistance property has to do with the size of the iron atoms which is smaller than the size of its oxide. As a result, the layer that is form is very loosely packed and does not have the stability to last for a considerable amount of time.
Factors That Compromise Corrosion Resistance
The passive layer atop the stainless steel is always in need of oxygen to self-repair. Therefore in areas that have low amounts of oxygen, corrosion resistance of the stainless steel is compromised significantly. The same effect takes place in areas that have very poor circulation. Another factor that affects corrosion resistance is the chloride content in water. Generally, sea water contains chloride salts that damages and removes the passive film at a very high rate. It is in fact more detrimental to the corrosion resistance of stainless steel than environments that are low on oxygen.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)