Steel is a durable and a cost-effective metal. But when exposed to the atmosphere, it starts corroding. A huge amount of resources are wasted annually in metal industries due to metal corrosion. The phenomenon of corrosion adversely affects and causes deterioration in metals through oxidisation. The corrosion of steel is an electrochemical process, and the coinciding existence of moisture and oxygen in the atmosphere leads to corrosion. The reaction starts when the oxygen in the atmosphere oxidises iron in the presence of water.
Also, the emissions from human beings in the atmosphere like carbon dioxide (CO2), carbon monoxide (CO), sulphur dioxide (SO2), nitrous oxide (NO2) and many other chemicals can accelerate the corrosion process. Additionally, if any two metals with different potential come in contact with each other, current will flow, and the more reactive metal will corrode in comparison to the less reactive metal.
The rate at which the corrosion will progress depends upon the climatic condition around the steel. The high level of moisture/wetness and pollution in the atmosphere would cause corrosion at a much faster rate and vice versa. Therefore the rate of corrosion is different in different places due to variations in atmospheric conditions. And, hence it is essential to do Corrosion Protection for Steel. Below are some protection systems which can stop or slow the corrosion process.
Here the steel is coated with a protective layer which works as a barrier between the metal and oxygen, water or ions from the atmosphere. Lower permeability of water by the coating system will ensure better protection. The coating can be Paint coating which gives improved durability to steel. Or it can be Metallic coating. The four commonly used methods of applying a metal coating to steelare hot-dip galvanising, thermal spraying, electroplating and sherardizing.
Moreover, Zinc had high corrosion resistant properties and used as a protective coating on many products and under many conditions.The quality of the coating, its selection, its application and handling all are important factors in deciding the kind of protection offered.
In this process, fabricated steel or iron is immersed in a kettle of molten zinc. The steel reacts with the zinc to form an alloy coating. The process provides a barrier and cathodic protection resulting in Corrosion Protection for Steellasting decades.
This system consists of providing paint or powder coating over hot-dip galvanised steel. This provides enhanced corrosion protection than either of the individual methods.
Different steels like stainless and weathering have special chemistry and elements. They provide corrosion resistance by taking advantage of these unique properties.
Stainless steel contains iron and chromium, and presence of chromium ensures excellent stain and rust protection.
Weathering steel contains copper, develops a natural iron oxide patina that once formed is stable. The steel is allowed to rust after which it creates a protective layer or patina. A layer protecting the surface is developed.
Compared to carbon steel, weathering steel offers increased corrosion protection.
Sacrificial Protection (Cathodic Protection)
The reaction between two dissimilar metals can be used to protect against corrosion. Zinc is the widely used metal for steel protection. Protection is offered through preferential oxidation by placing zinc in direct contact with the steel. Zinc corrodes against steel and effectively stops the oxidation reactions on the steel. Also, zinc normally corrodes at a slower rate.