These metals which we use in daily life are malleable, strong and ductile. While we use them, they come in contact with environment. After a time, they lose their shine and a colored layer comes upon them.
Whereas some metals lose their strength and become so breakable. Because of this a chemical reaction takes place between metals and its environment. Therefore, the destruction of metals by interaction with the environment is called corrosion of metals.
It is generally found on the outer surface of the metal. In the process of corrosion, the upper layer of the metal becomes unprotected, and in addition, the metal’s inner surface also. Therefore, corrosion tends to take place all way up to the depth of the metal.
Let us discuss the types of Corrosion below.
Types of Corrosion
The following are the common types of corrosion that usually occurs to metals:
- Galvanic corrosion
- Uniform corrosion
- Stress corrosion cracking
- Crevice corrosion
- Pitting corrosion
- Hydrogen grooving
- High-temperature corrosion
- Graphite corrosion
- Microbial Corrosion
- Metal Dusting
#1. Galvanic Corrosion.
It is a type of electrochemical corrosion in which one metal is attacked more readily when in contact with another, particularly in the presence of an electrolyte. In this form of corrosion, deep pits and grooves are generally made on the upper surface of the metal.
Good examples of this sort of corrosion are when copper touches steel in saltwater.
Another good example is that when aluminum and carbon steel is combined and immersed in seawater, the aluminum corrodes quickly leaving the steel in a safe state.
#2. Uniform Corrosion.
These common types of corrosion results in a uniform loss of metal over the uncovered part of the surface due to the attack of the whole surface of the metal exposed; it becomes thinner and eventually fails.
One good thing about it is that its effect on the performance of the material is usually slight.
This type is often caused by rubbing with high-speed fluid, whether abrasive or not. For instance, zinc and steel dipped in dilute sulphuric acid, dissolve uniformly over their surfaces.
#3. Stress Corrosion Cracking (SCC).
Quite simply, SCC is cracking of the surface of a material in corrosive conditions. This can lead to sudden failure of ductile metal alloys under tensile stresses, especially at elevated temperatures.
It is highly chemically specific, in that there are certain alloys which will likely suffer from SCC only on exposure to chemical environments.
For a given alloy, it will often occur in a mildly corrosive environment. Metal parts found to have severe SCC may show signs of shininess while being filled with tiny cracks.
A fine example of this corrosion is the stress corrosion cracking of austenitic stainless steel in a chloride solution.
#4. Crevice Corrosion.
Crevice corrosion refers to corrosion that occurs in a space such as that found in gaps, where a stable solution becomes trapped and is not renewed. These types of spaces are referred to generically as crevices. This corrosion generally occurs where bolts and nuts form lap joints which may lead to cracks.
Examples of areas you may find crevice corrosion are gaskets, washer bottom surfaces and bolt heads.
These are aluminium alloys and stainless steel and also affected by crevice corrosion, due to the generation of a differential aeration cell which results in the formation of rust in the cracks.
#5 Pitting Corrosion.
Pitting corrosion is extreme corrosion, which results in the totally random development of lots of little pores in the metal.
It is localized at a discrete point in the metal, but moves outwards as the corrosion cell is formed surrounded by the metal surface.
Once started, this is continually growing, taking any number of shapes large or small. Brought about by failure of structure due to those influences, the pit penetrates the metal vertically from the surface. The main reason this is caused is that the metal is in stagnant or slow-moving fluids.
Think of a drop of water on a finish of steel. The pit would be initiated in the centre of that (anodic site)
#6 Hydrogen Grooving.
Bubbles of hydrogen separate the iron sulphate film from the pipe wall when carbon steel is subject to concentrated sulphuric acid. This is basically most to be observed as hydrogen grooving.
This type of corrosion manifests itself, in piping, as grooves caused by the action thereon of a corrosive agent and the interaction of fabricating pipe components and bubbled hydrogen which perish that coating on that steel immediately on direct contact with it.
#7. High-temperature Corrosion.
As the name implies, this type of corrosion occurs on almost all metals from overheating. High-temperature corrosion occurs when gas turbines, diesel engines, furnaces or other machinery come into contact with hot gas containing certain contaminants.
A hot atmosphere which contains mainly gases of oxygen, sulfur, and other compounds can attack the metal chemically.
The compounds will readily oxidise the metals. Car engines as an illustration, use metals that have to withstand continuos periods of high temperatures in a gas which also contains corrosive products from the combustion.
#8. Graphitic corrosion.
In graphitic corrosion, the metal component of grey cast iron pipes is leached away or converted into corrosion products, leaving behind the graphite.
Some cast iron can also be consumed by simply being in salt water or acid too long, in which case the graphite remains in situ, softening the metal surrounding it.
#9. Microbial corrosion.
Corrosion resulting from the presence or activities of micro-organisms on the biofilm on the surface of the corrosive material, whether metallic, as steel, or non-metallic, as concrete.
Microbial corrosion may occur in metallic and non-metallic materials, whether with or without oxygen present.
#10. Metal dusting.
A highly destructive types of corrosion which had been occurring in susceptible materials exposed to high carbon activities/environment, including synthesis gas. Rust is defined as the disappearing of metal powder from bulk metal.
The corrosion occurs when carbon monoxide (CO) in the form of a graphite dew layer deposits on the surface of the metals; as the layer grows, it has a tendency to form meta-stable M3C species (M is the metal), away from the metal surface, thus destabilizing the surface.
Corrosion Prevention Methods
The following are the corrosion prevention methods used to prevent corrosion:
- Tinning
- Painting
- Primer
- Galvanizing
- Electroplating
- Chrome plating
- Cladding
- Silver Plating
- Gold Plating
- Nickel Plating
- Copper Plating
- Rhodium Plating
- Zinc Plating
- Cadmium Plating
- Bluing
- Calorizing
#1. Tinning.
Tinning. The process of coating of the iron plate into tin or coating of a soldering iron is called tinning. Any kind of metal especially of sheet metal becomes anti corrosive by means of tinning. These are washed then they are put in acids of water and salt.
Having done this, these are dipped in the solution of melted tin. In case of sheets, these are passed over rollers to shed of the extra tin.
For the other jobs, tin is split blows are made. For drying of the sheets, and other jobs and also for bringing shinning over of them, wooden dust is rubbed by moving drums. Thus, metals and sheets are made beautiful.
#2. Painting.
Of all kinds of corrosion prevention methods, painting is the most Favorite. The metal job is cleaned properly in the first. Then varnish mixed paint or enamel by means of a brush or spray is applied.
This paint or enamel actually forms a lay over the job and helps in providing the anti-corrosive treatment to a great extent. This usually adds the beauty of the job. Red lead also provides the anti-corrosive treatment.
#3. Primer.
To avoid further oxidation of the metal surface, Primer is required to be applied at once after preparing and cleaning the paint surface. Obviously, the primer process tends to increase the adherence of the coating to the surface of the steel.
Most of the primers used on steel are red lead, red oxide or zinc chromate. The primed steel does not rust easily, thereby improving the life of the materials. Citing the example of etch primer or zinc chromate; these are of benefit to metals like aluminum and galvanized iron.
#4. Galvanizing.
The process of covering the iron with zinc is known as galvanizing. It is not an electrical process and consists of merely dipping the iron in molten zinc. Beforehand the job is cleaned with the acid of salt.
It is then washed with water and dipped in a solution of molted zinc, out of it is taken. To shed the extra zin, a wire brush or wire rollers are used generally. Such kinds of sheets and jobs are termed G.I. sheets, G.I. bolts, etc.
#5. Electroplating.
The process of depositing a metal by electric means is termed electroplating. To clear the definition, we may say that the process of depositing a thin layer of a metal over a job made of another metal is called electroplating.
Before electroplating, the job is first cleaned; it is then cleaned thoroughly by buffing. Sufficient of the adequate solution is then filled in a tank for the purpose of pasting it on to a metal. Thus, if we want to do a copper plating on brass, we will fill copper sulphate in the tank.
For different metals, there are different solutions of the metallic salts. Two copper plates are hung in this tank; the brass job too, is hung in it. Treating these copper plates as anode and brass plates as cathode, we shall connect them on to the DC supply and give a current of 2 to 3 volts.
By doing this, the copper will start melting from the copper plates and starts depositing on the brass plates. The thickness of the metal layer depends upon the supply of current and the length of time the job has been dipped into the solution.
After being electroplated, the job is washed with bare water and put in wooden dust. This process brings shining to the job. For the purpose of shinning buffing is done again.
#6. Chrome Plating.
Chrome plating is an electroplating process in which chrome acid, called hexavalent chromium, is used. It is the process of electroplating a thin layer of chromium on a part consisting of metal.
The chromed surface has been used for a variety of applications, including decorative effects, corrosion resistance, for ease of cleaning, and for giving hardness of surface. For mere purpose of appearance there are chrome imitators too which can be very cheap.
#7. Cladding.
A method of preventing corrosion of dissimilar metals is cladding or joining together without fusing or gluing.
This is accomplished by bringing together by forcing home two sheets, one on either side, by pressure, and at the same time forcing through a die.
The U. S. Mint applies this principle of cladding to securing at coin composed of more than one metal, so that the cheaper metal may serve as a filling.
#8. Silver Plating.
This consists in depositing upon metal surface preventing rust forming, and giving glossy appearance thereto. This process is used, among other things, for the purpose of preventing rust, assisting the chance of ditto, hardening, imparting tone and adhesion to colour paint, etc.
Matte silver plating, bright silver plating, semi bright silver plating, are instances of silver plating obtained. Several methods are employed for obtaining the same; one, which is used, consists in covering a solid surface with a sheet of metal, such a sheet then being applied to the surface by fusing into it with heat and pressure.
The principal use of silver plating is for the purpose of decorating to be used in lithographing, plates, cups, etc., and for medals.
#9. Gold Plating.
A method of preventing corrosion, it ranges so depositing a thin layer of gold upon the surface of another metal copper, silver, etc; by electro chemical plating.
The beautiful colour, electrical conducting power, and freedom from oxidation renders gold a most desirable metal of which the above is an application in several fields.
For Jewellers, the art of so-called gold plaiting, occurs to the point of view, as a means of improving the conductance, commonly in that of Electrical agency, but apart from that, many parts are employed in the process of gold plaiting for preventing their becoming tarnished, as in the case of circuit boards.
#10. Nickel Plating.
Nickel Plating means applying a thin layer of Nickel, by electroplating, to a metal surface. Nickel is served as a well-known method of protection against corrosion; it is also electroplated for good appearance and resistance to corrosion of the base metal.
The Nickel surface may be hard and wear resisting, or may be intended for the recovery of valuable metal, applied to parts that are poorly or undersized.
Applications of Nickel Plating are found in many household units, such as door hinges, knives and the like, tap shower fittings.
#11. Copper Plating.
Copper Plating also means electroplating a deposit of copper over a metal surface, another type of metal plating that does not call for expense where high conductivity is the primary requisite. Although usually serving as an undercoat, Copper is often used as pure metal.
Laying down as pure metal in class A, Copper, is used for decorative effects, imperviousness to corrosion, electrical and thermal conductivity, or merely for the purpose of serving as a foundation for additional deposits.
The process is used for circuiting electronic elements of all types, in the form of printed circuits.
#12. Rhodium Plating.
Rhodium coating is primarily a process for the prevention of corrosion by coating the metal with a thin film of Rhodium which gives added strength and lustre, and is also somewhat scratch resisting of the finished surface.
Jewelry that is plated with atomic coating of Rhodium is more lustrous and should resist tarnish better than other metals; it is a most useful product in the production of jewelry, where white gold plating is wanted. The primary benefits of the process are, that it does not dent or tarnish, and retains its lustre.
#13. Zinc Plating.
Zinc plating is the electroplating process of a thin coating of zinc alloy on to the surface of a metal object.
This forms a physical barrier, preventing rust from reaching the surface of the underlying metal. It is often used in the automotive industry to resist oxidation and corrosion of the metal.
The zinc plating process is used in the application of vehicle boards and, also, other metal parts such as nuts and bolts, fasteners and automotive parts.
#14. Cadmium Plating.
Cadmium plating is a sacrificial coating; that is, it will corrode and break down before the underlying metal thus, protecting against corrosion. Cadmium has many properties, including high resistance to saltwater corrosion.
In addition, it also improves adhesion, lubricity and corrosion resistance. Even, though, the cadmium plating is very thin, nevertheless it provides adequate protection against wear. Almost all conductive metals can be plated with cadmium.
#15. Bluing.
The process of blueing consists in covering steel with a black oxide coating for preventing it from rusting. We called it blueing after this blue-black process.
Steel is heated by nitrate of potash (salt-peter) to about 315°C, then immersed in a molten solution. The steel remains in the solution for several minutes depending upon the colour and depth.
This process results in a blue/blue-black colouring of the outer surface of the metal depending upon the process specified. The process of blueing is used by nearly all gun manufacturers and gunsmiths and many gun owners using expensive firearms.
#16. Calorizing.
This is also a corrosion prevention method of making aluminum coatings. In this case, aluminum powder is used for preventing corrosion from the metal surface and the coating of aluminum is attached to iron or steel by heating it in an aluminum and alumina mixture.