Corrosion – Basics, Mechanism, Types

In this article, we will know about 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 the material

Why do we need to understand about corrosion?

The annual cost due to corrosion of materials and protection against corrosion will increase all over the world.

Countries	Estimated Money
United States	13 Billion dollars
China	10 Billion dollars
Gulf countries	11 Billion dollars
India	8.7 Billion dollars

For example, one large chemical plant will spend around 2 million dollars a year for painting steel to prevent rusting.

In automobiles industries, the painting cost approximately equals to 1/3 of the vehicle cost. Worldwide the money spends to prevent corrosion increases year to year. If we reduce corrosion or preventing corrosion will drastically change the world economy. Hence understanding the corrosion is very essential.

Corrosion Mechanism (Electrochemical Aspects)

Before we are going see the various types of corrosion, it is essential to now the corrosion mechanics.

 

The electrochemical nature of corrosion illustrated with an example of zinc and hydrochloric acid. When zinc placed in dilute hydrochloric acid a vigorous reaction occurs; hydrogen gas evolved and then zinc dissolves forming a solution of zinc chloride. The reaction is as follows

Zn + 2HCl   ——->  Zncl₂ + H₂   

From the above equation, we understand the chloride ion not involved in the reaction. Hence the above equation is written as

Zn + 2H⁺  ——->     Zn ⁺² + H₂

From this equation, we understand the zinc ion reacts with hydrogen ions in the acid solution to form zinc ions and hydrogen gas. During this reaction, zinc oxidized to zinc ions and hydrogen ion reduced to hydrogen. The reaction divided into two reactions.

• Oxidation
• Reduction

In the above reaction oxidation (Anodic reaction)

Zn    ——>   Zn⁺² + 2e

The reduction reaction (Cathodic reaction)

2H⁺ + 2e  ——>   H₂  

The increase in valence or production of an electron indicates the Oxidation or Anodic reaction. 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 illustrated in the figure below. Here the zinc atom has 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 to zinc, both iron and aluminium 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

Consider an iron immersed in water or seawater and it’s exposed to the atmosphere. Now corrosion will occur due to the anodic reaction is

Fe  —–>    Fe⁺² + 2e

Also, the medium exposed to the atmosphere, it contains dissolved oxygen. Both water and seawater are nearly neutral, thus the cathodic reaction takes place as follows

O₂ + 2H₂O + 4e   —–>   4OH^–

Now, remember that the sodium and chloride in the seawater do not participate in the reaction. The reaction is only between iron and water. The reaction is rewritten as follows

2Fe(OH)₂ + H₂O + ½ O₂   —–>   2Fe(OH)₂

Now the final product called “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 present in acid solution.

Since the anodic and cathodic reactions occur during corrosion are mutually dependent, it’s possible to reduce corrosion by reducing the reaction rate of either reaction.

Polarization:

The concept of polarization is very important to understand the behaviour of corrosion and corrosion reaction. Various physical and chemical factors limit the electrochemical reaction.

Normally polarization classified into two types

• Activation polarization
• Concentration polarization

Activation polarization:

It refers to 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.

Step 1: (Refer above figure the steps marked inbox)

The hydrogen ions absorbed or attached on the surface of the metal (Zinc) before the reaction proceeds.

Step 2:

Following step 1 the electron transfer will occur resulting in a reduction of the metal

Step 3:

The hydrogen ions then combine with the electron to form hydrogen molecules.

Step 4:

These hydrogen molecules then combine to form hydrogen gas.

If Step 1 to 4 occurs at a slower phase, then the speed of reduction of hydrogen ions decreases.

Concentration Polarization:

It refers to an electrochemical reaction controlled by the diffusion of ions in the electrolyte. It’s shown in the below figure. In this case, a number of hydrogen ions in solution (diluted acids) quite small and the reduction rate is controlled by the diffusion of hydrogen ions to the metal surface.

In this case, the reduction rate 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

Galvanic corrosion occurs when a metal or an alloy 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 called galvanic corrosion.

The corrosion rate of the less corrosion-resistant (active) anodic metal increased, while the more corrosion-resistant (noble) cathodic metal or alloy 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. They are three general types of concentration cell corrosion:

1. Metal ion concentration cells
2. Oxygen concentration cells, and
3. 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

The Pitting corrosion is localized corrosion, the corrosion causes 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

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 normally occurs on bolts, gasket area and other points of metal contact.

Fretting Corrosion

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 corrodents in the fluid attack the freshly exposed metal surface. 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.

Intergranular Corrosion

It’s a localized form of corrosion. 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. The intergranular corrosion mainly occurs on the Heat Affected Zone of the weld metal area. 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

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.