In this article, we will discuss the Corrosion – Basics, Mechanism, Types of corrosions commonly encounter in industries. Now let we start from basics
What is corrosion?
Corrosion is a natural way of deterioration of material
What is the necessary for understanding about corrosion?
The annual cost due to corrosion of materials and of protection against corrosion will increase all over the world. These are countries will spend money for corrosion protection
Countries Estimated Money
United States 13 Billion dollars
China 9 Billion dollars
Gulf countries 11 Billion dollars
India 8.7 Billion dollars
This tremendous cost is less surprising when we consider that the corrosion occurs. For example, one large chemical plant will spend around 2 million dollars a year for painting steel to prevent from rusting.
Worldwide Corrosion in automobile fuel systems alone cost around 100 million dollars per year. Auto radiators account for about 52 million dollars. Normally in automobiles, the cost of painting comes around 1/3 of the vehicle cost. If we reduce corrosion or preventing corrosion will drastically change world economy.
- 1 Corrosion Mechanism (Electrochemical Aspects)
- 2 Polarization:
- 3 Types of Corrosion:
Corrosion Mechanism (Electrochemical Aspects)
Before we are going see the various types of corrosion, it is essential to now the mechanics of corrosion.
The electrochemical nature of corrosion can be illustrated by the attack of zinc by hydrochloric acid. When zinc is placed in dilute hydrochloric acid a vigorous reaction occurs; hydrogen gas is evolved and the zinc is dissolves forming a solution of zinc chloride. The reaction is as follows
Zn + 2HCl ——-> Zncl2 + H2
From the above equation, it is found that the chloride ion is not involved in the reaction. Hence the above equation can be rewritten as follows
Zn + 2H+ ——-> Zn +2 + H2
From this equation when the zinc ion reacts with the hydrogen ions in the acid solution to form zinc ions and hydrogen gas. During the reaction, zinc is oxidized to zinc ions and hydrogen ion is reduced to hydrogen. The reaction can be divided into two reactions.
In the above reaction oxidation (Anodic reaction)
Zn ——> Zn+2 + 2e
The reduction reaction (Cathodic reaction)
2H+ + 2e ——> H2
An oxidation or anodic reaction is indicated by an increase in valence or a production of an electron. A decrease in valence charge or the consumption of electrons signifies a reduction or cathodic reaction. During metallic corrosion, the rate of oxidation will equal to the rate of reduction.
The above concept is illustrated in the figure below. Here the zinc atom has been transformed into Zinc ion and two electrons. These electrons which remain in the metal are immediately consumed during the reduction of hydrogen ions. These two reactions will occur simultaneously at the same point on the surface of the metal.
Similar as zinc, both iron and aluminum are also rapidly corroded by hydrochloric acid. The following reaction takes place.
Fe +2HCl —–> FeCl2 + H2
2AL + 6HCl —–> 2AlCl3 + 3H2
Corrosion Mechanism of Iron oxide (Rust) Formation
Now consider when an iron is immersed in water or sea water which is exposed to the atmosphere. Now corrosion will occur due to the anodic reaction is
Fe —–> Fe+2 + 2e
And also the medium is exposed to the atmosphere, it contains dissolved oxygen. Both water and sea water are nearly neutral, thus the cathodic reaction takes place as follows
O2 + 2H2O + 4e —–> 4OH–
Now, remember that the sodium and chloride in the sea water do not participate in the reaction. The reaction is only between the iron and water. The reaction can be rewritten as follows
2Fe(OH)2 + H2O + ½ O2 —–> 2Fe(OH)2
Now the final product is familiarly called as “Rust”.
Normally the acid solution containing dissolved oxygen will be more corrosive than air free acids. Oxygen reduction simply provides a new means of “electron disposal”. The same effect is observed if any oxidizer is present in acid solution.
Since the anodic and cathodic reactions occur during corrosion are mutually dependent, it is possible to reduce the corrosion by reducing the rate of either reaction.
The concept of polarization is very important in understanding the behavior of corrosion and corrosion reaction. The rate of electrochemical reaction is limited by various physical and chemical factors. Hence an electrochemical reaction is said to be polarized or retarded by these environmental factors. Normally polarization is divided into two types
- Activation polarization
- Concentration polarization
It refers an electrochemical process which is controlled by the reaction sequence at the metal-electrolyte interface. Now consider that the hydrogen evolution reaction on zinc during corrosion in acid solution. With reference to the figure shown below the hydrogen reduction on the zinc, the surface will follow four steps as follows and these steps are common for all the materials. The steps are shown by numbers.
Step 1:(Refer above figure the steps are marked in box)
The hydrogen ions must be absorbed or attached on the surface of the metal (Zinc) before the reaction can proceed.
Following step 1 the electron transfer will occur resulting in a reduction of the metal
The hydrogen ions then combine with the electron to form hydrogen molecules.
These hydrogen molecules then combine to form hydrogen gas.
The speed of reduction of hydrogen ions will be controlled by the slowest of these steps.
It refers an electrochemical reaction which is controlled by the diffusion in the electrolyte. This is shown in the below figure. In this case, a number of hydrogen ions in solution (diluted acids) is quite small and the reduction rate is controlled by the diffusion of hydrogen ions to the metal surface. Note that in this case the reduction rate is controlled by processes occurring within the bulk solution rather than at the metal surface.
Types of Corrosion:
- Galvanic Corrosion
- Concentration cell corrosion
- Pitting corrosion
- Crevice corrosion
- Fretting corrosion
- Intergranular corrosion (IGC)
- Stress Corrosion Cracking (SCC)
Galvanic corrosion occurs when a metal or an alloy is electrically coupled to another metal, alloy, or conductive non-metal in a common, conductive medium. A potential difference usually exists between two dissimilar metals, which cause a flow of electrons between them. This flow of electrons causes corrosion, this type of corrosion is called as galvanic corrosion.
The corrosion rate of the less corrosion-resistant (active) anodic metal is increased, while that of the more corrosion-resistant (noble) cathodic metal or alloy is decreased.
Concentration cell Corrosion
Concentration cell corrosion is a form of galvanic corrosion. This type of corrosion will occur when two or more areas of a metal surface are in contact with different concentrations of the same solution. There are three general types of concentration cell corrosion:
- Metal ion concentration cells
- Oxygen concentration cells, and
- Active-passive cells.
The lower the metal ion concentration the greater will be the solution potential of the metal and its tendency to dissolve or corrode.
Pitting corrosion is a localized corrosion. The formation of holes in the surface of the material. Pitting can have various shapes and can act as a stress concentration point. Pits are also placed for corrodents to settle and become more concentrated, thus producing a cumulative damage effect to the material.
The corrosion damage due to pitting is more difficult to detect and predict when compare to other types of corrosion.
Crevice corrosion is similar to pitting corrosion. Any crevice acts as a concentration cell in which corrodents can settle and become more concentrated and thereby more quickly attack adjoining material. Crevice corrosion has been attributed to one or more of the following
- Increase in acidity
- Build up of a damaging ion
- Lack of oxygen
- Depletion of an inhibitor
Crevice corrosion is fixed by the design of the system but normally occur on bolts, gasket area and other points of metal contact.
Fretting corrosion occurs in an area where metals may slide over each other with the presence of an electrolyte. This type of corrosion mainly occurs in the heat exchangers. Where the tubes are in contact with intermediate supports (baffles) in the heat exchanger. The metal to metal rubbing causes mechanical damage that removes protective oxide coatings. The freshly exposed surface is then attacked by the corrodents in the fluid. The vibration of long tubes in the heat exchanger may expose to fretting corrosion. Reduction of vibration is thus one way of minimizing this type of corrosion.
Inter-granular corrosion is another localized form of attack. In that, the corrosion will occur on the grain boundaries instead of inside the materials. The intergranular corrosion mainly occurs in the stainless steel materials due to the depletion of chromium in the grain boundaries. In severe cases of intergranular corrosion, the grain boundaries will appear rough to the naked eye, with particles of loose grain evident. It is believed that intergranular corrosion is due to HAZ (Heat Affected Zone) of weld metal area or inadequate heat treatment method. Good heat treatment practices reduce intergranular corrosion and another approach is to use a modified alloy that has tighter grain boundaries.
Stress Corrosion Cracking (SCC)
Stress corrosion cracking is a progressive type of failure that causes cracks at stress levels well below a materials yield point. It is caused by a combination of material properties now recognized as affecting mostly high strength alloys, it occurs in part under high tensile strength and the grain structure of the material. Hence the term corrosion cracking can be improved by loading or they may be residual stresses created in the material during forming or welding. Stress corrosion cracking increases with the temperature of the corroding medium.
Erosion corrosion is the acceleration or increase in the rate of deterioration or attack on the metal surface because of relative movement between a corrosive fluid and the metal surface. Generally, this movement is quite rapid and mechanical wear effects or abrasions are involved. Metal is removed from the surface as dissolved ions or it forms solid corrosion products which are mechanically swept from the metal surface.