Corrosion: Meaning, Types, Prevention, Factors

Corrosion is an irreversible natural process in which pure metals (unstable) other than noble metals are transformed into chemically stable compounds such as oxides, sulphide, hydroxide, carbonates, and other compounds. It is responsible for the degradation of a material. In this article, we will understand the process of conversion, its examples, theories, and how to prevent it.

Key Terms: Corrosion, Rusting, Factors, Wet Theory, Dry Theory, Electroplating, Coating

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What is Corrosion?

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Corrosion is the chemical or electrochemical destruction of metals and alloys in the presence of an environment to form undesirable compounds. Corrosion processes are the reactions of metals with environmental organisms. It is the process of a body gradually deteriorating due to the attack of air gases on the metal's surface. It is known that more stable compounds such as oxides, sulphides, carbonates, and others form as a result of corrosion. Rusting of iron is the most frequent type of corrosion that occurs in our environment. (Fe₂O₃.xH₂O).

Rusting of Metal

Corrosion is a term used to describe the degradation or destruction of solid metallic material and the subsequent loss of that material as a result of an unwanted chemical or electrochemical attack by the environment at its surface. Corrosion is thus a reversal of the metal extraction process.

Water-line Corrosion

When water is kept in a steel tank, corrosion begins along the line just below the water meniscus level. As we know, the concentration of oxygen below the water level is very low, thus metal parts below the water level work as anodes, while metal parts above the water level have a higher concentration of oxygen, therefore this region acts as a cathode.

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Examples of Corrosion

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• Rusting of iron - When iron is exposed to the elements of the environment, it forms a coating of reddish scale and Fe₃O₄ powder.
• When exposed to moist air containing CO₂, a green layer of basic carbonate - [CuCO₃ + Cu(OH)₂] forms on the surface of copper.

Aluminium Corrosion

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Factors Affecting Corrosion

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• When a metal is more reactive, it is more vulnerable to corrosion.
• If there are impurities in the pure metal, it will corrode more quickly.
• Mostly in saline water, metal readily gets corroded.
• Air and moisture content cause rusting of Iron.
• When the temperature is high then the rate of corrosion increases.
• Corrosion mostly takes place in the cuts or bends area of the metal.

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Disadvantages of Corrosion

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Corrosion is a gradual process that happens solely on the surface of metals, but the losses sustained are significant. Machines, equipment, building materials, and all forms of metallic items, structures, and so on are all destroyed. As a result, the costs suffered are enormous, with global corrosion losses estimated to be in the range of 2 to 2.5 billion dollars each year.

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Theories of Corrosion

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There are two theories of corrosion:

• Wet theory of Corrosion

When metals are exposed to oxygen, moisture, soil, and other environmental factors, they get corroded by an electrochemical process. On the metal surface, discrete anode and cathode zones are formed during this process. Corrosion is always caused by an oxidation reaction and the release of an electron at the anodic regions of metal. The freed electrons from the anode travel to the cathode, where they are reduced in the presence of a neutral aqueous electrolyte solution. Iron rusts in the presence of ambient oxygen. Anodic patches emerge on a metal surface when it splits, and the metal components act as cathodes. The overarching theory of corrosion is known as the electrochemical theory. Because corrosion occurs in the presence of moisture, it is also known as the Wet Theory of Corrosion.

• Dry theory of Corrosion

This type of corrosion is caused mostly by the environment's direct chemical action, such as the direct attack of atmospheric gases like O₂, halogens, H₂S, SO₂, N₂, and anhydrous organic liquids on the metal surface with which they come in with contact.

Direct action of oxygen at low or high temperatures, usually in the absence of moisture, causes oxidation corrosion. Except for noble metals like Ag, Au, and Pt, all metals are attacked by oxygen and oxidized at high temperatures. All metals are slightly damaged at normal temperatures. Alkali metals such as Li, Na, K, Rb, and alkaline earth metals such as Be, Ca, and Sr, on the other hand, are attacked quickly and easily oxidized.

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Types of Corrosion

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1. Crevice corrosion

When two connected surfaces (metal-metal or metal-nonmetal crevice) come into contact, crevice corrosion occurs. It is the localized corrosion of the surface of a metal or alloy. The connected surface region of crevice corrosion has a lower oxygen content than the rest of the area. The section with the lowest oxygen concentration becomes the anode, while the portion with the highest oxygen concentration becomes the cathode. 

The absorption of oxygen step of the electrochemical theory of corrosion is related to the corrosion mechanism. A specialized kind of corrosion known as crevice corrosion can occur whenever there is a difference in ionic concentration between any two local locations of a metal. Inside the washers are gaskets, and bolt heads are sites where crevice corrosion might happen. All grades of aluminium alloys and stainless steels are susceptible to crevice corrosion.

Crevice Corrosion

2. Pitting Corrosion

When the protective layer on the metal surface breaks, a pit forms, and a tiny pit (anode) forms on the metal surface. Because of the varying amounts of oxygen in contact with the metal surface once the pit has formed, the corrosion process becomes extremely quick. The section (pit) with the lowest oxygen concentration becomes the anode, while the portion with the highest oxygen concentration becomes the cathode.

Pitting Corrosion

3. Galvanic Corrosion

When an electric contact establishes between two metals that are electrochemically different and are in an electrolytic environment, galvanic corrosion can occur. It refers to when one of these metals breaks down at a joint or junction. This type of corrosion is best shown by the degradation that occurs when copper comes into contact with steel in a saltwater environment. When carbon steel and aluminium are connected and submerged in seawater then the aluminium corrodes faster while the steel is protected.

Galvanic Corrosion

4. Uniform Corrosion

It is corrosion occurring at a consistent rate across a metal surface that is exposed. Oxygen is the primary cause of this type of corrosion of steel and other such metals and alloys in the natural environment. Silver tarnishing, Rusting, Nickel fogging, are all such examples of uniform corrosion

Uniform Corrosion on Metal

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Prevention of Corrosion

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Corrosion can be prevented through:

A. Metal Coatings

1. Anodic Metal Coating (Galvanizing)

• Galvanizing is the technique of applying a thin layer of zinc to iron or steel sheets to protect them from rusting. 
• First, the iron or steel object, such as sheet, pipe, or wire, is cleaned by pickling with a diluted H₂SO₄ solution for 15-20 minutes at 60-90⁰Celsius. 
• Any scale, filth, oil, grease, or rust, as well as any other contaminants, are removed from the metal surface during pickling. 
• The article is thoroughly washed and dried. After that, it's submerged in a bath of molten zinc kept at 425-430^oC. 
• To avoid oxide formation, the surface of the bath is covered with an ammonium chloride – flux. 
• When the piece is removed from the bath, it is coated in a thin layer of zinc. 
• Galvanization is the best example of anodic metal coating. 
• Galvanization is the process of covering a base metal (to be protected) with molten zinc (cathodic in nature) by dipping the base metal (anodic in nature).

2. Cathodic Metal Coating (Tinning)

• The cathodic metal coating is a metal coating that is cathodic to the base metal. 
• Any metal (cathodic in nature) lower in the galvanic series, such as Sn, Cu, Ni, and others, is coated on the surface of a metal (base metal/ anodic in nature) to protect it from corrosion.
• Tinning is the best example of cathodic metal coating. 
• Tinning is the process of covering a base metal (to be protected) with molten Tin by dipping the base metal.

B. Scarification Anodic Prevention

• The metallic structure to be protected (base metal) is connected to a more anodic metal by a wire in this protection method, such that all corrosion happens at that anodic metal and the metallic structure (cathode) is protected. 
• The more active metal used in this way is known as a sacrificial anode. 
• When the corroded sacrificial anode is totally consumed, it is replaced with a new one. 
• Mg, Zn, Al, and their alloys are widely used as sacrificial anodes because they have a low reduction potential (are more reactive) and are at the higher end of the electrochemical series. 
• The sacrificial anode approach also protects underground water pipelines and water tanks.

C. Electroplating

• Electroplating is the process of depositing metal (anode) on the surface of metals and alloys using electrolysis (cathode or base metal). 
• The electrolyte solution containing the metal ions to be deposited on the base metal is dipped into the two electrodes. 
• When a direct current is applied from an external source, the coating metal ions migrate to the cathode and form a thin layer on the base metal's surface. 
• Metals such as Au, Ag, Zn, Sn, Cr, Ni, and others can be electroplated onto the surface of a base metal (Cathode).

Copper Electroplating

D. Impressed Coating Protection

• To neutralize the corrosion current and convert the corroding metal from anode to cathode, an impressed current from an external source is delivered in the opposite direction. 
• As a result, the anodic corroding metal becomes cathodic and corrosion-resistant. 
• The external DC source provides the impressed current. 
• To safeguard the negative terminal of the DC source, it is attached to metal, whereas the positive terminal is connected to the anode. 
• Graphite, stainless steel scrap iron, and other materials can be used as anodes. Water tanks, water and oil pipelines, transmission line towers, and other structures use the impressed current protection system.

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Things to Remember

• Corrosion is the chemical or electrochemical destruction of metals and alloys in the presence of an environment to form undesirable compounds. Example: Rusting of Iron.
• There are two theories of corrosion: wet theory and dry theory.
• Various types of corrosion are crevice corrosion, pitting corrosion, galvanic, and uniform corrosion.
• Corrosion is more likely to affect welded steel joints. When stainless steel connections are welded, regions susceptible to corrosion can form.
• When the cathode is larger than the anode, the cathode desires a high number of electrons while the anode only has a few electrons. Cathode steals electrons from the anode so corrosion occurs.
• Corrosion prevention is intimately connected to efficient waste management.
• Corrosion can be found almost everywhere. It can be found in the pipes that deliver water to our homes, it can also be found in automobiles, railroads, and bridges.

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