What is corrosion?

Corrosion is described as the process in which a certain element is degraded or deteriorated due to the presence of multiple chemicals in the surrounding. Generally, corrosion happens if all the subatomic particles on a metallic object are oxidized, for example, rust on iron structures. In air or water, metals lose electrons to other particles (like oxygen), leading to the formation of an oxide with the metal.

What are different types of corrosions?

Some of the most essential types of corrosion are:

• Uniform corrosion
• Galvanic corrosion
• Pitting corrosion
• Dezincification
• Atmospheric corrosion
• Erosion corrosion
• Crevice corrosion
• Localized corrosion
• Intergranular corrosion(Stress corrosion)

Uniform corrosion

Uniform corrosion is mostly found in ferrous metals like aluminum, lead, cast iron, wrought iron, and alloys. A layer (rust) on the surface of the metal is created if placed in corrosive surroundings like air or water. In order to prevent uniform corrosion, thicker elements for corrosion allowance should be used.

Galvanic corrosion

Galvanic corrosion (dissimilar metals) takes place in a galvanic couple, where active element deteriorates. Galvanic corrosion is usually found in materials like copper, aluminum, iron, and their alloys. It can be detected or predicted with the help of electromotive series as well as galvanic series.

Pitting corrosion

Pitting corrosion occurs in small areas by forming micro-pits on the surface of the material. Passive metals and their composites are most likely to get affected by pitting corrosion. In pitting corrosion, a pitting factor is utilized to determine the harshness, where the pitting factor is the fraction between the most bottomless pit on the surface and average penetration.

The following three diagrams are types of pitting corrosion.

Dezincification

The selective dissolution of active metals from a specific alloy in a corrosive surrounding is described as dezincification. Examples of dezincification are tin bronzes in hot brine, alloys of copper-nickel in condenser and others. The beneficial conditions for dezincification are alkaline water, permeable coatings on the surface of tubes, and various others. Plug type dezincification and uniform layer dezincification are two types of dezincification.

Atmospheric corrosion

Atmospheric corrosion happens on the element's surface due to the reaction of pollutants, chemical compositions or moisture in the atmosphere. Dry atmosphere, surface coating, coating of polymers are some of the essential methods for preventing atmospheric corrosion.

Erosion corrosion

Erosion corrosion is one of the types of corrosion in which deterioration of several metals and their alloys happens because of corresponding motion amongst metal exterior and corrosive liquids. Pump impeller built of stainless alloy, valves, blades of steam turbines, and tunnel walls of the condenser is some examples where erosion-corrosion is most likely to happen.

Crevice corrosion

Crevice corrosion occurs between two metal or non-metal surfaces due to concentration gradients. The factors which affect crevice corrosion are environmental conditions, roughness, alloy composition and metallographic structure. The crevice corrosion can be prevented by using higher alloys, rivets joints at the edge of the metal and by reducing the use of soldering and welding.

Localized corrosion

In localized corrosion, a small part of a component's surface corrodes due to the presence of material of a different nature in a non-homogeneous environment. Some other types of corrosion like crevice corrosion, pitting corrosion, and intragranular corrosion are related to localized corrosion.

Intergranular corrosion

The intergranular corrosion refers to the attack close to grain boundaries in a metal surface. The presence of precipitation or impurities and depletion of an alloying material at grain boundaries leads to intragranular corrosion. This type of corrosion is common in stainless steel welded surfaces.

Prevention methods for corrosion

The following are five mostly used methods for preventing corrosion.

• Barrier coating
• Hot- dip galvanization
• Alloyed steel (stainless steel)
• Cathodic protection
• EonCoat

Barrier coating

The barrier coating is one of the simple and easiest prevention methods as compared to others. In this method, the coating of paint, fibre or plastic, nylon and other things adhere to the surface of the element and form a thin layer. It protects from the electrochemical charges and resists to surrounding factors. Although, the major disadvantage of barrier coating is that it needs to be reapplied because durability is low.

Hot-dip galvanization

The hot-dip galvanization includes molten zinc in which the steel surface is dipped. A strong alloy coating is observed on the surface due to the reaction between iron (in steel surface) and zinc. This tight bond works as a protection from the atmosphere and is extensively used in artistic sculptures and playing equipment. Some drawbacks of the galvanization process is that the exhalation coming from zinc coating is hazardous, and the coating is not permanent. It can peel away from the surface.

Alloyed steel

Alloyed steel provides better protection to the surface than galvanization or barrier coating. In this method, several properties of metals are combined and used on surfaces for protection from resistance. Also, different alloys of metals can be mixed for providing resistance in different conditions and give more flexibility. One of the major drawbacks of alloyed steel is that they are very expensive.

Cathodic protection

Cathodic protection prevent corrosion by transforming active metals into passive metals with the help of electron transfer from different sources like galvanic anodes. Mostly, magnesium, aluminium and zinc are used as anodes for producing electrons. The cathodic process has great effectiveness, but its maintenance cost is high because it needs to be replaced more frequently. In high resistive surroundings, the cathodic protection is not much effective.

Combination coating (EonCoat)

In this type of coating, all of the above methods are included. The metal surface is initially alloyed, and then by providing a thick layer of inhibitors, it is protected from corrosion. For example, in industries, water-resistance barriers are formed to separate water from the metal surface, but this method is not effective, and eventually, rust is formed on the surface. So, the combination coating (Eon-Coat) is applied by chemically bonding with steel and forming an alloyed layer. The layer will protect the surface even if water reaches and doesn't get rusted. This prevention method is inexpensive, low maintenance and long-lasting.

Common Mistakes

• Students have the misconception that electrons flow from the cathode to the anode through the electrolyte solution.
• Students make a mistake while studying stainless steel that rust will develop on it. However, if not properly maintained over time, rust can develop on stainless steel.
• Corrosion and rusting are two different terms, students makes mistakes in between them.

Context and Applications

The topic classification of types of corrosion is very much significant in the several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Civil Engineering
• Master of Technology in Mechanical Engineering
• Doctor of Philosophy in Mechanical Engineering
• Diploma in Mechanical Engineering
• Bachelor of Science in Physics
• Master of Science in Physics

Related Concepts

• Dissimilar metals
• Corrosion rate
• Filiform corrosion
• Chemical reaction
• Electrochemically and microbiologically
• Protective coating and sulfidation
• Electrochemical reaction
• Corrosion-resistant and corrosion-related
• high temperature corrosion
• Metal surface and non metal surface

Practice Problems

Q1. Which of the following reactions are involved in corrosion of metals?

a. Physical reactions

b. Combustion reaction

c. None of these

d. All of these

Correct answer: (c)

Explanation: Corrosion reaction describes two types of reactions, namely oxidation and reduction reactions. There is no physical combustion reaction in the corrosion of metals. Chemical reactions have oxidation and displacement reactions in metal corrosion.

Q2. Name the type of equation related to corrosion rate.

a. Faraday's equation

b. Nernst equation

c. Both options (a) and (b)

d. None of the above

Correct answer: (a)

Explanation: Faraday's equation describes the corrosion rate. It will include the oxidation and reduction reaction analysis and analyses of the concept of equivalent weight. Corrosion rate can be obtained with the help of corrosion current.

Q3. Which of the following types of corrosion is very dangerous?

a. Stress corrosion

b. Atmospheric corrosion

c. Pitting corrosion

d. Erosion corrosion

Correct answer: (c)

Explanation: Pitting corrosion comes under a destructive form of corrosion. Also, the crevice localized types of corrosion can be studied in the dangerous types of corrosion. But pitting is highly dangerous as it destroys the metal structure silently.

Q4. The corrosion prevention method, cathodic protection uses:

a. Cobalt

b. Sulphur

c. Aluminium

d. All of the above

Correct answer: (c)

Explanation: The cathodic protection involves the use of aluminium. Generally, the metals employed for the anodes are namely aluminium and magnesium. This cathodic protection is categorized under a very effective method for small steels types.

Q5. Identify the type of corrosion which is also known as chemical corrosion?

a. Wet corrosion

b. Oxidation corrosion

c. Both (a) and (b)

d. None of these

Correct answer: (d)

Explanation: Chemical corrosion describes the metal's slow depreciation (destruction). It generally involves the metal oxidation process. Wet and oxidation corrosion are not known as the chemical corrosion processes.