What Is Corrosion In Metals?
Corrosion is the process through which materials deteriorate when electrochemical, chemical or other reactions take place. Rusting happens as a result of a chemical reaction between water and oxygen forming an orange patch on the metal. There are few metals that can neither corrode nor rust easily as discussed below.
Stainless steel metal is formed through a mixture of elements like iron which can easily be oxidized to form rust.
Another element is the chromium which forms at least 18% of the stainless-steel content. Chromium is very reactive, however, the oxide layer formed on the surface of chromium does not allow rusting to take place. The oxide layer formed through the reaction of chromium does not allow oxygen to penetrate the metal.
Stainless steel also contains nickel and molybdenum in its content; these enhance its corrosion resistance properties.
These metals have less or no iron in their chemical composition and therefore they cannot rust. However, they can form oxides on their surfaces, for instance, copper can oxidize gradually forming some green patina.
The green patina acts as a layer of protection against corrosive elements. Bronze being the mixture of tin and copper and other elements is more corrosion resistant compared to copper.
Brass is made up of zinc, copper, and other elements and it also offers high resistance to corrosion and cannot rust.
Aluminium is lightweight and can resist corrosion and rust and that is why it is popular for making aircraft parts.
Almost all aluminium alloys do not contain iron and because of this, the metal cannot rust even when exposed. In the presence of water, aluminium oxide forms on the surface of the metal. The aluminium oxide layer formed on the surface of the metal acts as the protective layer against corrosion.
Many industries prefer to use aluminium in their manufacturing processes since it is also cost-effective.
Galvanized steel is obtained by coating any carbon steel using a thin layer of zinc.
It is highly resistant to rust and corrosion. However, after a very long period of time say 50 years, the material may give in to corrosion. The zinc coating can protect the steel metal underneath through cathodic action and also through the formation of a zinc oxide layer.
Additionally, zinc oxide formed on the surface of the metal happens because zinc is very reactive to oxygen when exposed to moisture.
Titanium in its pure form is highly resistant to corrosion from chemicals or acids or saltwater.
In the presence of oxygen, it can form a titanium oxide on its surface. The titanium oxide formed on the surface is resistant to corrosion and prevents it from rusting. Pure titanium, however, when exposed to very severe conditions, can not resist corrosion for a very long time.
Applications such as implants made with titanium can last for 20 years or more.
Applications Of Corrosion Resistant Metals
Stainless steel is used in industries such as food and catering, seawater desalination plants, chemical, construction, medical, and civil engineering.
Aluminum is used in power lines, window frames, electronics, household appliances, ships, aircraft, and many more. Copper is used in making power transmission lines, cookware, spark plugs, electrical cables, and heat exchangers. Titanium is used for making jewelry, tennis rackets, surgical tools, scissors, bicycle frames, and many others.
Bronze is used in bushings, bearings, and nautical applications.
Improving Corrosion Resistance Of Sheet Metal
The corrosion resistance properties of sheet metals can be improved through the surface finishes described below.
Spray Paint Processing
The part that has been spray painted can be temporarily resistant to rust. Such parts may include pipes and electrical enclosures.
The formation of metal oxides on the surfaces of the metals aids in corrosion resistance and prevents rusting. Metal anodizing is popular in metal fabrication process.
The use of another metal which is resistant to corrosion to coat a sheet metal which is less or not resistant to corrosion.
This is described as the coating of a carbon steel using a thin layer of zinc metal through hot-dip process.
Criteria For Choosing Corrosion Resistant Metals
Corrosion-resistant metals are supposed to display the following properties. The chosen metal should not dissolve in aggressive solutions. Again, the metal should display high resistance to all types of local attacks such as deep penetration in local pitting.
A corrosion-resistant metal should also withstand enhanced corrosion from applied stress or fluctuating stress. The metal must be able to stand the pressure inserted between two loads in contact and slip over each other. Corrosion-resistant material should be able to resist a combination of two varying corrosion sources. The metal should be able to resist accelerated local corrosion. It should be able to resist selective dissolution like in the dezincification of brass.