Content Curator
Pitting corrosion, also known as pitting, is a very localized type of corrosion that results in the random formation of tiny holes in metal.
- Removal of a tiny region causes it to become anodic (oxidation reaction), causing pitting corrosion, whereas cathodic (reduction reaction) removal of an enormous area causes local galvanic corrosion.
- With only a small amount of ion diffusion, the corrosion penetrates the entire mass of the metal. Only a specific area of the metal surface is impacted.
- The material initially develops a layer of rust, which later produces the holes.
- Because it is challenging to identify and combat, it has been shown to be more destructive than corrosion.
- A calibrated microscope is used to determine the depth of the corrosion.
Key Terms: Corrosion, Rust, Oxidation, Mechanism, Cathode, Homogeneous Corrosion Damage, Ions, Metals, Anode
What is Pitting Corrosion?
[Click Here for Sample Questions]
Pitting corrosion is a specific type of corrosion that causes "holes" or cavities to form in the material. Because pitting is more challenging to detect than homogeneous corrosion damage, it is thought to be more harmful. Pits are frequently hidden by corrosion products.
- Pitting is a type of corrosion that occurs in materials that have protective films.
- It is an attack with localized holes on the metal's surface.
- The attack can penetrate the metal very rapidly, while some parts of the metal surface remain free from corrosion.
Why Does Pitting Corrosion Occur?
[Click Here for Sample Questions]
Atmospheric gases, reactive chemical species in free form, such as chlorides cause Pitting Corrosion. The primary cause of this pitting corrosion is chloride. It penetrates the metal's passive layer and dissociates oxide bonds.
If the metal's surface comes into contact with water. Over the metal surface, a coating of water droplets may form. Corrosion is started by the ions in the water droplets reacting.
Also Check:
Effects of Pitting Corrosion
[Click Here for Sample Questions]
Pits may develop on the metal as a result of pitting corrosion. These pits can range in size from shallow and wide to deep and small.
- Pitting corrosion reduces the thickness of metal.
- Because of this, there are structural flaws, and metal cannot withstand stress.
- Finally, the metal begins to shatter, rendering it completely worthless.
- If pitting corrosion is not stopped, it results in pipe leaks, electrical shorts, and significant mechanical problems.
Pitting Corrosion
Pitting Corrosion Mechanism
[Click Here for Sample Questions]
The oxidation of the metal produces a localized acidity as part of the pitting corrosion mechanism.
- This localized acidity is kept under control by the considerable separation of the cathodic half-reaction and the anodic half-reaction.
- Anions migrate electrostatically toward the created pit as a result of the acidity.
- Localized acidity also creates a possible gradient. The side products produced during the corrosion process frequently fill the pits that develop on the metal's surface.
- The corrosion that takes place when a metal is exposed to an oxygen-rich sodium chloride solution can be used as an example to better understand the pitting corrosion mechanism.
- Now, the pit serves as an anode and the metal's surface as a cathode.
The electrolyte's chloride anions are drawn to the excessively positive charge produced by the local formation of metal cations in the pit.
The metal hydroxide and hydrochloric acid that is produced as a result of the reaction between the metal chloride molecules and the water in the environment accelerate corrosion even further.
Read More: Corrosion Inhibitor: Definition, Types, Application and Mechanisms
Types of Pitting Corrosion
[Click Here for Sample Questions]
Pitting corrosion can take on a number of forms. The material that is impacted and the direction of the grain inside that material has a significant impact on the pit's shape.
The most frequently impacted metals and alloys are those that are passive, such as aluminium and stainless steel. However, corrosion can harm practically any metal or anything that is susceptible to it.
Trough Pitting Corrosion
Trough pit have an uneven, hemispherical, or cup shape. When the passive film (protective layer) is damaged, trough pitting corrosion takes place, creating deep and narrow troughs in the metal wall. Their smooth surfaces reveal the metal's crystal structure. These can easily pierce materials up to their thickness, such as a roof sheet, truss, or gutter part.
Sideway Pitting Corrosion
Sideways pits can take the form of horizontal grain attack, undercutting, or subsurface forms and are covered with a semipermeable membrane of corrosion product (rust). Sideways pitting corrosion has a very quick rate of metal penetration.
Due to the fact that the metal surface will appear to be untouched and corrosion-free, it is exceedingly challenging to identify. It could seem as though there is very little damage if there are only a few little patches of rust.
Pitting Corrosion Test
[Click Here for Sample Questions]
A laboratory test, called the pitting corrosion test, is performed to determine a metal's susceptibility to pitting corrosion. A metal sample is subjected to a corrosive solution as part of the test, and the size of the pits that develop on the metal's surface is then measured.
The metal is more prone to pitting corrosion the smaller the pits are. There are various distinct methods to test a steel grade’s pitting resistance. They are:
- CPT (Critical pitting temperature): This method of determination is frequently used.
- ASTM G48, practice A and E: Since the acidic solution combines with oxidizing ability and chlorides, it is one of the toughest pitting corrosion tests conducted for stainless steel.
- ASMT G48 practice A: It is a performance test at a single temperature.
- ASMT G48 practice E: Determines the CPT.
Pitting Corrosion Treatment
[Click Here for Sample Questions]
To fully expose the pits and eliminate the corrosion products, it is best to use the appropriate cleaning techniques. It is best to use products that don't overly harm the base metal. It is advised to use a pointed tool to occasionally probe the pits during cleaning.
By this, the degree of any corrosion or undercutting can be gauged. The openings can be appropriately enlarged by using a wire brush to clean the pit vigorously. By removing the corrosion byproducts and the damaged metal, we can assess the degree of damage.
Now, a final inspection of the metal substance is done to check whether it is clean and free of debris. Applying a base primer coating and one or two more top layers can be done if you are relieved that the rust was discovered in time. The component will need to be replaced if the damage is significant and it is too weak. Some methods of Treatment of Pitting Corrosion include:
- Zinc Phosphate Priming: One of the most common coating techniques to prevent pitting corrosion is zinc phosphate priming. Corrosion resistance is improved by specially designed primers like zinc phosphate.
- Chemical Coating: Chemical coating applies a precisely prepared powdered substance to a steel surface using electrostatic force or compressed air. After that, it is melted to create a silky protective coating. This type of treatment for steel makes it extremely resistant to peeling, scratching, and cracking in addition to protecting it from corrosion and UV damage.
Read More:
Preventing Pitting Corrosion
[Click Here for Sample Questions]
Wherever feasible, environmental conditions that contribute to pitting corrosion should be minimized. Controlled and reduced amounts of humidity, temperature, chloride, pH acids, and salt should be used.
- Metal that is at risk should be covered with a reactive metal that corrodes more quickly for cathodic protection.
- This may take the shape of a coating made of galvanized zinc.
- By acting as the anode and starting to corrode first, the reactive metal will prevent pitting in the substrate.
- Corrosion-resistant metals, such as alloys, are preferable for chemical reaction defense. In situations with high amounts of chloride ions, titanium, nitrogen, chromium, and molybdenum alloys perform exceptionally well.
Pitting corrosion can be controlled by:
- Using a coating to shield metal surfaces from pitting
- Use of more materials that are resistant to corrosion
- Making sure that the liquids in contact with the material are regularly rinsed away
- Cathodic protection is used
- Avoiding areas of stagnation
- Use of inhibitors and monitoring fluid chemistry
- Keep the material's protective film intact.
Things to Remember
- Pitting Corrosion is a specific type of corrosion responsible to create holes, thus reducing its thickness.
- Use of alloys rather than pure metals, painting of the metal surface, and the use of rust-free materials can help to prevent it.
- Pitting corrosion happens when the anode (exposed metal) is tiny and the cathode (damaged coating) is large.
- The strength and corrosion resistance of a metal surface can be significantly reduced as a result of pitting corrosion.
- Pitting corrosion's primary side effect is a reduction in metal surface area.
Sample Questions
Ques. What is Pitting Corrosion? Explain with an Example. (2 marks)
Ans. Pitting Corrosion is the type of localized corrosion that leaves pits and holes on metal surfaces. Because it is dangerous to detect, it is more harmful than typical corrosion.
Oxides are produced in the passive layer as a result of exposure to moisture and aeration. These begin the process of corrosion. This frequently happens with iron metal. Alloys are therefore preferred today.
Ques. What is the treatment for pitting corrosion? (2 marks)
Ans. There are several ways to deal with pitting corrosion. We ought to employ corrosion-resistant materials. You can utilize cathodic and anodic protection. The substance may be painted or covered with a second coating of metal. The metal can be kept in a location with regulated airflow, temperature, and moisture levels.
Ques. What is a test for pitting corrosion? (1 mark)
Ans. Pitting tests are performed to determine whether a metal is likely to experience localized corrosion that results in surface holes. According to a governing standard, pitting tests are performed using various metal and solution combinations (ASTM G48).
Ques. Which substance stops corrosion from pitting? (1 mark)
Ans. The effectiveness of these alloying components on a material's resistance to pitting corrosion is as follows: Molybdenum (Mo) + Nitrogen (N) + Chromium (Cr)
Ques. What is pitting depth? (1 mark)
Ans. The vertical distance between the level of the lowest landing and the bottom of the pit is referred to as pit depth.
Ques. What is the difference between pitting and crevice corrosion? (2 marks)
Ans. In the start phase, pitting and crevice corrosion can be distinguished.
Pitting corrosion is started (on plane surfaces) by metallurgical variables alone, as opposed to crevice corrosion, which is started by a difference in oxygen or ion concentration in the electrolyte. When a pit first forms, it resembles a crevice greatly in terms of geometry and pitting and crevice corrosion propagation electro-chemistries converge.
Ques. What is pitting? (1 mark)
Ans. Pitting is a technique used to add dents, scratches, and cracks to the hard seed coat. This makes it easier for water and oil to diffuse between the cotyledons and husk during migration.
Ques. Which of the following metals is highly prone to pitting corrosion?
a. 18-8 stainless steel by sulfuric acid with FeCl3
b. Titanium by sulfuric acid with FeCl3
c. Nickel by sulfuric acid with FeCl3
d. Copper by sulfuric acid with FeCl3 (2 marks)
Ans. The correct answer is (a) 18-8 stainless steel by sulfuric acid with FeCl3.
Explanation: Pitting is one of the most destructive and insidious forms of corrosion. 18-8 stainless steel is highly prone to pitting corrosion by sulfuric acid in the presence of FeCl3. Whereas other metals are relatively resistant to pitting. Because it mainly depends on oxide film for its corrosion-resistant.
Ques. Hole-forming pitting is a type of extremely localized attack that affects metal. Explain. (1 mark)
Ans. Pitting and crevice corrosion are the localized corrosion types, that results in corrosion on very confined corrosive sites. Localized attack of pitting results in holes in the metal.
Ques. Consider the following statements regarding corrosion:
a. The formation of a film of atoms or molecules on the surface of an anode so that corrosion is slowed down, is called passivation.
b. Local corrosion attack resulting from the formation of small anodes on a metal surface, is known as intergranular corrosion.
c. Preferential corrosion occurring at grain boundaries or at regions adjacent to the grain boundaries, is called pitting corrosion.
Which of the above statements is/are correct? (3 marks)
Ans. The correct answer is (a). The formation of a film of atoms or molecules on the surface of an anode so that corrosion is slowed down, is called passivation.
Explanation: Pitting is another form of very localized corrosion attack in which small pits or holes form. They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction.
It is an extremely insidious type of corrosion, often going undetected and with very little material loss until failure occurs.
The mechanism for pitting is probably the same as for crevice corrosion in that oxidation occurs within the pit itself, with complementary reduction at the surface.
Ques. Discuss the use of inhibitors in corrosion control. (5 marks)
Ans. A corrosion inhibitor is a chemical agent that is used in a certain environment, such as air or water, to reduce the rate of corrosion of metals exposed to that environment. Corrosion inhibitor, or CI, is the name given to it.
- A corrosion inhibitor is a chemical that can be added to liquids or gasses to slow the pace at which a particular material corrodes (typically a metal). The use of corrosion inhibitors is widespread in business, process, and industrial settings. These applications include some of the following.
- Corrosion Metals are shielded from anodic corrosion and rust by inhibitors. Typically, a chromate coating is applied to the metal surface in order to achieve this.
- In order to help prevent cathodic corrosion, oxygen scavengers can be used as CIs. They work by interacting with dissolved oxygen in the surrounding environment.
- Fuel pipelines must be protected against rust and corrosion at all costs. As a result, CIs are essential for protecting these pipelines and reducing the possibility of mishaps.
- Heating system metal pipes are susceptible to corrosion. Additionally, CIs are crucial to protecting these pipelines.
Also Check Out:
Comments