Introduction
We know that corrosions simply mean rusting. However, the question is what is pitting corrosion. Pitting Corrosion is the form of corrosion that particularly occurs at one spot. We can call it localized corrosion also. It only affects a particular part of the metal surface. Initially, it forms a layer of rust which later forms the holes in the material. It is proven to be more harmful than corrosion because it is hard to detect and work against. The depth of the corrosion is measured using a calibrated microscope. In this article, we will discuss what is pitting corrosion, its mechanism, test, damages caused by pitting corrosion, and how to prevent it.
Causes of Pitting Corrosion
Now, We know that Pitting corrosion is a kind of corrosion that is confined to a small area. However, the question arises what causes this pitting corrosion? The answer is the Environment. Gases present in the atmosphere, reactive chemical species in free form like chlorides. Chloride is the main source of this pitting corrosion. It attacks the passive layer of metal and breaks the bonds of oxides.
If the metal surface is exposed to the water. Water droplets can form a layer over the metal surface. Ions present in the water droplets react and initiate the process of corrosion.
So, there are two factors - Environmental and metallurgy. These two factors determine if this can be stopped or not. If we restrict the aeration (oxygen supply to the surface) and prevent the surface from getting wet, Pitting corrosion can be prevented.
Pitting Corrosion Mechanism
Pitting corrosion is initiated by the oxidation process. Exposure of the passive layer of metal with air and water initiates the oxidation process at the localized part. This leads to the acidification of ions formed by oxidation. These two processes are part of an electrochemical reaction.
Oxidation occurs at the anodic part and reduction occurs at the cathodic part. These half cells constitute the electrochemical cell, which forms at a small site.
The reaction occurring at the anodic site is shown below:
Fe → Fe2+ + 2e-
The reaction occurring at the cathodic site is shown below:
½ O2 +H2O + 2e- → 2(OH-)
Overall, the reaction is shown below:
FeCl2 + 2H2O →Fe(OH)2 + 2HCl
The HCl formed in the pit increases the acidity. A potential gradient is also set in this localized pit region. It attracts the ions from the other nearby sites. The holes which are formed at the surface of the metal, get filled with the side product of the corrosion process.
In the presence of chlorine ions, holes keep on growing via an autocatalytic mechanism.
Pitting Corrosion Test
A number of tests are available to detect pitting corrosion. A few of them are listed below:
ASMT-G48-Practice A
ASMT-G48-Practice E
CPT(Critical Pitting Temperature)
ASMT-G48 Practice A and E are the toughest tests that are being conducted on stainless steel, while CPT is the most commonly used method.
Damages Caused by Pitting Corrosion
As we already know, it affects a localized part of the metal. It creates a rusty layer over the metal surface, eventually forming holes in it. It reduces the thickness of metal. This leads to structural defects and metal cannot handle stress. Finally, cracking starts and metal becomes totally useless. Pitting corrosion causes pipe leakages, electric short circuits and major machinery faults if not prevented.
Corrosion damage causes an economical loss of about ＄300 billion annually in U.S. Industries.
Pitting Corrosion Prevention
Till now, we understood what is pitting corrosion, what are the causes and damages. Now, we will discuss the ways to prevent it.
We should use such materials that are resistant to environmental factors such as aeration and moisture exposure.
We can apply anodic or cathodic protection layers over the metal surfaces.
We can paint the metal surface or apply industrial coatings.
The zinc spray metalizing process is extensively used to cover the metal layer with the layer of zinc because it is prone to environmental factors.
We should keep the metal materials in an environment with less moisture exposure, optimum temperature and aeration should be controlled.
Conclusion
In this article, we discussed the pitting corrosion definition, pitting corrosion examples, causes, mechanism, damages, and prevention methods. It is a localized kind of corrosion that is caused by environmental factors. It forms holes and causes thickness loss. There are a number of tests available to detect it. It could be prevented by using alloys rather than pure metals, painting the metal surface, using rust-prone materials, and maintaining the protective film over the metal surface.
FAQs on Pitting Corrosion
1. What is pitting corrosion and where is it commonly observed?
Pitting corrosion is a very localized form of corrosion that leads to the creation of small holes or 'pits' in a metal. It is considered particularly dangerous because these pits can penetrate the metal's thickness without significant loss of mass on the surface, making them difficult to detect before a failure occurs. A common example is the formation of pits on stainless steel surfaces when exposed to environments containing chlorides, such as saltwater.
2. What are the main causes of pitting corrosion?
The primary cause of pitting corrosion is the localized breakdown of a metal's passive protective layer. This breakdown can be initiated by several factors, including:
- Exposure to aggressive chemical ions, most commonly chloride ions (Cl⁻).
- Non-uniformities in the metal's surface composition.
- Mechanical damage like scratches or dents that disrupt the passive film.
- The presence of stagnant or low-flow solutions, which allows aggressive ions to concentrate on a specific spot.
3. Can you explain the electrochemical process behind pitting corrosion?
The process of pitting corrosion is an autocatalytic electrochemical reaction. It begins when the passive layer breaks down. The small, exposed area becomes an anode, while the surrounding passive surface acts as a large cathode. Inside the pit, metal ions are formed (e.g., Fe → Fe²⁺ + 2e⁻), which then react with water and chloride ions to form an acidic, aggressive solution. This acidic environment prevents the passive layer from reforming and accelerates the corrosion process locally, causing the pit to grow deeper.
4. If stainless steel is designed to be corrosion-resistant, why is it still susceptible to pitting corrosion?
Stainless steel's resistance comes from a thin, passive layer of chromium oxide (Cr₂O₃) on its surface. While this layer is very effective against general corrosion, it can be compromised by specific aggressive ions, especially chlorides. Chlorides locally attack and break down this protective film, creating a tiny anodic site. The large surrounding passive area acts as a cathode, concentrating the corrosive attack on that one small point, leading to the formation of a deep pit. So, its overall resistance paradoxically makes the localized attack more intense when it does occur.
5. How does pitting corrosion differ from uniform corrosion?
The main difference lies in the nature and predictability of the metal loss.
- Uniform Corrosion: This type of corrosion occurs evenly across the entire surface of the metal. While it leads to a loss of material, the rate is often predictable, and failure can be anticipated. An example is the general rusting of an unprotected steel sheet.
- Pitting Corrosion: This is a highly localized attack, forming deep pits in specific spots while the rest of the surface remains largely unaffected. It is far more insidious because a small, barely visible pit can lead to structural failure, making it much harder to detect and predict.
6. Why is preventing pitting corrosion a critical concern for aircraft structures?
Preventing pitting corrosion is critical for aircraft due to safety and structural integrity. Aircraft are often made of high-strength aluminium alloys, which are susceptible to pitting, especially when exposed to moisture and de-icing salts (containing chlorides). A single, deep pit can act as a stress concentrator, initiating fatigue cracks under the cyclic loads experienced during flight. This can lead to catastrophic structural failure without any obvious signs of widespread corrosion on the surface, making regular and thorough inspection essential.
7. What practical methods are used to prevent or control pitting corrosion?
Several methods are used to control pitting corrosion, often in combination:
- Material Selection: Using alloys with higher resistance to pitting, such as those with increased molybdenum content (e.g., 316 stainless steel over 304).
- Protective Coatings: Applying paint, epoxy, or metallic coatings to create a barrier between the metal and the corrosive environment.
- Cathodic Protection: Using a sacrificial anode or an impressed current to make the component the cathode in the electrochemical cell, thus preventing it from corroding.
- Environmental Control: Reducing exposure to aggressive ions like chlorides, controlling pH levels, and eliminating stagnant conditions where ions can concentrate.
