Crevice corrosion refers to a localized form of corrosion that occurs within narrow gaps or crevices on the surface of a metal, particularly stainless steel. It is considered insidious because it typically takes place in hidden or inaccessible areas that are not immediately visible.
Crevice corrosion initiates in confined spaces, such as crevices or under deposits, where the access of oxygen and other oxidants is limited. The presence of stagnant conditions, often caused by capillary forces, allows for the accumulation of corrosive agents within the crevice. Over time, the passive layer, which normally protects the metal from corrosion, can be compromised due to the restricted supply of oxygen and the accumulation of aggressive substances.
Factors such as dissolved metal ions, decreased pH levels, and the presence of chloride ions further contribute to the breakdown of the passive layer and the progression of crevice corrosion. These factors create a corrosive environment within the crevice, leading to accelerated corrosion rates and potential failure of the metal component.
Crevice corrosion is particularly problematic because it can occur in critical areas, such as flange joints, threaded connections, or areas where deposits accumulate. Detecting and mitigating crevice corrosion is crucial to maintaining the integrity and longevity of metal structures and components.
How Crevice Corrosion Takes Place?
Crevice corrosion occurs when a metal surface has narrow gaps or crevices that create confined spaces. The process of crevice corrosion typically involves the following steps:
Formation of a crevice: The crevice can be formed by various means, such as gaskets, threads, or surface irregularities. It creates a confined space where stagnant conditions can occur.
Restricted oxygen supply: Within the crevice, the access of oxygen and other oxidants is limited. This can be due to the narrow opening or the presence of deposits that obstruct the flow of oxygen.
Accumulation of corrosive agents: Stagnant conditions promote the accumulation of corrosive substances within the crevice. These can include moisture, chloride ions, and other aggressive chemicals.
Weakening of the passive layer: The passive layer, which typically forms on the metal surface and provides protection against corrosion, can be weakened or disrupted due to the restricted oxygen supply and the presence of corrosive agents.
Electrochemical reactions: Within the crevice, electrochemical reactions take place. The metal in the crevice acts as an anode, while the surrounding area acts as a cathode. This leads to the corrosion of the metal within the crevice.
Accelerated corrosion: As the corrosion progresses, the crevice becomes an aggressive environment. The breakdown of the passive layer and the continuous supply of corrosive agents result in accelerated corrosion rates within the confined space.
Mechanism of Crevice Corrosion
The main cause of initiating crevice corrosion is differential aeration, which occurs due to differences in oxygen availability within the crevice. As shown in Figure below, the dissolved oxygen present in the crevice is consumed over time. When the oxygen supply becomes restricted, a differential aeration cell is established.
Mechanism of Crevice Corrosion
“Crevice corrosion occurs when a narrow gap allows the entry of corrosive electrolyte, creating stagnant conditions. The dimensions of the crevice do not have a definite threshold for corrosion to occur. Crevice attack is similar to pitting corrosion, involving localized attack on the surface.“
In the presence of aerated sodium chloride, the anodic and cathodic reactions for stainless steel in crevice corrosion are as follows:
Anodic reaction: M → M+ + e-
Cathodic reaction: O2 + 2H2O + 4e- → 4OH-
As the anodic reaction progresses, charge balance is maintained through hydrolysis, leading to the following reaction:
M+ Cl- + H2O → MOH + H+ Cl-
At equilibrium, the concentrations of the chemicals involved can be represented by the equation:
[MOH] = [M+] [OH-] / [H+]
Why is crevice corrosion a problem?
Crevice corrosion is a significant problem for several reasons:
Localized damage: Crevice corrosion targets specific areas, such as narrow gaps, crevices, or areas beneath deposits. It leads to localized damage concentrated in these vulnerable locations, making it challenging to detect and address.
Hidden nature: Crevice corrosion often takes place in hidden or inaccessible areas that are not readily visible during routine inspections. This makes it difficult to identify and monitor the corrosion progression, allowing it to continue undetected until significant damage occurs.
Accelerated corrosion rates: Crevice corrosion can lead to accelerated corrosion rates compared to uniform corrosion. The stagnant conditions within the crevice, coupled with the accumulation of corrosive agents, intensify the corrosion process. This can result in rapid metal degradation and potential failure of the component.
Structural integrity risks: Crevice corrosion can compromise the structural integrity of metal components. If left unchecked, it can lead to material loss, pitting, cracking, or even perforation of the metal surface. This poses serious safety risks, especially in critical applications where the failure of the component can have severe consequences.
Cost implications: Repairing or replacing components affected by crevice corrosion can be costly. The localized nature of the damage often requires targeted repairs or replacement, which can be time-consuming and expensive. Additionally, the consequences of component failure, such as production downtime or safety hazards, can result in significant financial losses.
Challenging detection and prevention: Detecting and preventing crevice corrosion can be challenging due to its hidden nature and the complex factors that contribute to its initiation and progression.
Crevice corrosion refers to a localized form of corrosion that occurs when an electrolyte becomes trapped in the crevices, cracks, or joints of metal surfaces. This phenomenon can lead to pitting and deep crevice corrosion, both of which can significantly compromise the structural integrity of the affected metal surfaces.
To prevent structural failure, it is important to be aware of the four types of crevice corrosion:
Ohmic drop of potential
This type of crevice corrosion affects passive alloys or metals that exhibit an active-to-passive current peak in their anodic polarization curve. It occurs when the passive metal undergoes anodic dissolution at cathodic current peaks. Low alloy steels, such as carbon steel and ferritic or martensitic stainless steel, are particularly susceptible to this form of crevice corrosion due to their high vulnerability to pitting corrosion.
Carbon steels, with their higher iron content, are generally more susceptible than alloys. Passive alloys are less prone to this type of corrosion compared to active alloys. Therefore, using passive alloys in environments with high electrolyte concentration can reduce the risk of ohmic drop of potential.
Electrochemical depassivation
Crevice corrosion by electrochemical depassivation occurs as localized corrosion in the presence of specific anions above a critical potential. This type is commonly observed in nickel alloys, which have low resistance to anodic dissolution and high cathodic current density.
To prevent electrochemical depassivation crevice corrosion, it is necessary to increase pH and flow rate or consider using less susceptible alloys. Signs of this type of corrosion include pits surrounded by a white or colorless film.
Atmospheric crevice corrosion
Atmospheric crevice corrosion arises when water accumulates or remains trapped in crevices when the structure is exposed to the elements. Dew or rainwater can easily enter these crevices, and even when the surfaces dry due to sunlight and wind, corrosion can persist within the confined spaces. The continuous wet-dry cycle can accelerate corrosion by concentrating salts in the crevices.
Differential aeration
Crevice corrosion by differential aeration cells occurs due to variations in oxygen concentrations on a metal surface. Metal that has been in contact with higher oxygen concentrations becomes a cathode, slowing down corrosion.
Conversely, metal in contact with lower oxygen concentrations becomes an anode, resulting in an increased corrosion rate. This type of corrosion is commonly observed in carbon and low alloy steels, which have a high degree of porosity that allows oxygen to come into contact with the metal surface.
Factors Influencing Crevice Corrosion
The major factors that influence crevice corrosion:
Crevice Type
Crevice Geometry
Material
Environment
Crevice Type
The type of crevice, whether it is metal-to-metal or metal-to-non-metal, can affect the occurrence and severity of crevice corrosion. Different materials and contact configurations can create varying electrochemical conditions within the crevice, influencing the corrosion process.
Crevice Geometry
The geometry of the crevice plays a significant role in crevice corrosion. Factors such as the size of the gap, depth of the crevice, and surface roughness can impact the accumulation of corrosive agents, the availability of oxygen, and the formation of stagnant conditions. Narrower gaps and deeper crevices tend to enhance crevice corrosion.
The material composition of the metal involved in the crevice is crucial. Alloys with higher levels of corrosion-resistant elements like chromium (Cr) and molybdenum (Mo) tend to exhibit better resistance to crevice corrosion. The microstructure and mechanical properties of the metal, including the presence of sensitization or grain boundaries, can also influence crevice corrosion susceptibility.
Environment
The surrounding environment significantly affects crevice corrosion. Factors such as pH, temperature, halide ions (e.g., chloride, bromide), and oxygen availability play crucial roles. Acidic conditions, high temperatures, and the presence of halide ions can accelerate crevice corrosion processes by promoting metal dissolution and depassivation. In contrast, higher pH levels and reduced halide ion concentrations can reduce the likelihood of crevice corrosion.
Crevice corrosion causes
Crevice corrosion arises from a combination of two factors: the presence of a corrosive environment and the existence of a narrow crevice that restricts the movement of electrolyte. This restricted flow of electrolyte causes an accumulation of ions within the crevice, leading to the formation of localized corrosive conditions.
Several factors contribute to the occurrence of crevice corrosion:
Stagnant or trapped solutions: When a liquid becomes trapped in a crevice, it becomes stagnant, creating an environment conducive to corrosion. For instance, the entrapment of water between two metal surfaces, such as beneath a gasket or seal, can initiate crevice corrosion.
Differential aeration: Differential aeration corrosion contributes to crevice corrosion when different areas of a metal surface experience varying levels of oxygen exposure. This scenario often arises when one part of a metal structure is exposed to the air while another part is submerged in water, causing localized corrosion within the crevice.
Concentration of corrosive agents: Confined spaces can lead to an increased concentration of corrosive agents like chloride ions, intensifying the likelihood of crevice corrosion. This heightened concentration can prompt the breakdown of the protective oxide layer on the metal surface, initiating crevice corrosion.
Material susceptibility: Certain materials are more susceptible to crevice corrosion than others. For example, materials with a lower resistance to pitting, such as specific types of stainless steel, are particularly vulnerable to crevice corrosion.
Temperature and humidity also play significant roles in crevice corrosion. Elevated temperatures and high humidity levels accelerate the rate of crevice corrosion, exacerbating its effects and increasing the likelihood of material degradation within the crevice.
What materials are susceptible to crevice corrosion?
Materials that are susceptible to crevice corrosion include:
Stainless Steels: Certain grades of stainless steels, particularly those with lower chromium and molybdenum content, are more susceptible to crevice corrosion. Examples include austenitic stainless steels like 304 and 316, which, despite their overall corrosion resistance, can experience localized corrosion in crevices or stagnant environments.
Aluminum Alloys: Aluminum and its alloys, especially in marine or chloride-rich environments, are prone to crevice corrosion. Crevice corrosion can occur in aluminum alloys due to the accumulation of corrosive agents within crevices or gaps, leading to localized attack and pitting.
Copper Alloys: Copper and its alloys, such as brass and bronze, can be susceptible to crevice corrosion, particularly in the presence of certain electrolytes. The presence of chloride ions, for example, can initiate crevice corrosion in copper alloys.
Nickel Alloys: While nickel alloys generally exhibit good corrosion resistance, certain compositions can be susceptible to crevice corrosion under specific conditions. Factors such as halide ions, temperature, and pH can influence the susceptibility of nickel alloys to crevice corrosion.
Titanium Alloys: Titanium and its alloys are generally resistant to corrosion, but they can still be susceptible to crevice corrosion in aggressive environments. The susceptibility of titanium alloys depends on factors such as alloy composition, surface condition, and exposure to elevated temperatures.
How to evaluate a material’s resistance to crevice corrosion?
To evaluate a material’s resistance to crevice corrosion:
Conduct laboratory tests using electrochemical techniques like potentiodynamic polarization and electrochemical impedance spectroscopy.
Perform specific crevice corrosion tests, such as ASTM G78 and ASTM G190, which simulate crevice conditions and assess the extent of corrosion.
Consider long-term exposure to real-world environments where crevice corrosion is likely to occur and monitor the material’s corrosion behavior.
Review the alloy composition and corrosion data of materials, prioritizing those with higher levels of corrosion-resistant elements.
Take into account environmental factors like pH, temperature, chloride ion concentration, and oxygen availability.
Utilize computer software, such as CRA-Compass, to assess crevice corrosion resistance for specific temperature and chloride concentration conditions.
When selecting alloys to avoid crevice corrosion, the following factors should be considered:
Chromium Content: Higher chromium content promotes the stability of the passive layer and improves resistance to crevice corrosion. Alloys with increasing chromium content generally exhibit better resistance.
Nickel Content: Nickel supports the stability of the passive layer, but alloys with higher nickel content may not necessarily provide better resistance to crevice corrosion compared to alloys with higher chromium content.
Molybdenum Content: Molybdenum has a significant impact on increasing resistance to crevice corrosion. It helps to slow down the rate of attack once depassivation occurs.
Nitrogen Content: Nitrogen can also contribute to improving crevice corrosion resistance in stainless steels.
Based on these considerations, the following alloys are commonly known for their resistance to crevice corrosion, listed in decreasing order of resistance:
6% molybdenum austenitic stainless steels (e.g., 1.4547 and 1.4529)
Preventing crevice corrosion is crucial to maintaining the integrity and longevity of metal structures. Here are some effective measures to prevent crevice corrosion:
Design considerations: During the design phase, minimize the presence of narrow crevices or gaps where electrolyte can become stagnant. Opt for smooth and continuous surfaces, avoiding the use of overlapping or tightly fitted components that can create crevices. Promote good drainage and avoid designs that trap moisture or fluids.
Material selection: Choose materials with high resistance to crevice corrosion. Stainless steels, particularly those with molybdenum content, are often preferred for their superior resistance to crevice corrosion. Consider the specific environment and corrosive agents the material will be exposed to and select the appropriate alloy or coating.
Protective coatings and barriers: Apply protective coatings, such as paints, epoxy coatings, or specialized corrosion-resistant coatings, to the surfaces vulnerable to crevice corrosion. These coatings act as a barrier, preventing the direct contact of the metal with the corrosive environment.
Proper installation and sealing: Ensure proper installation techniques, paying attention to sealing and gasket materials. Use suitable sealants and gaskets that are resistant to corrosion and can effectively fill crevices, preventing the ingress of moisture or corrosive substances.
Regular inspection and maintenance: Implement a comprehensive inspection and maintenance program to detect and address potential crevice corrosion issues promptly. Regularly monitor crevice-prone areas, including joints, fasteners, and areas with potential crevice formation. Promptly repair any damage, replace worn-out sealants or coatings, and address any signs of corrosion.
Proper cleaning and debris removal: Regularly clean surfaces and remove any debris or deposits that can accumulate in crevices. This helps eliminate potential sources of localized corrosion and ensures proper drainage and fluid flow.
Control environmental factors: Control the environmental conditions that contribute to crevice corrosion. Monitor and manage temperature, humidity, and exposure to corrosive agents like chlorides. Implement effective corrosion control measures, such as maintaining proper ventilation, controlling moisture levels, and removing contaminants from the environment.
Cathodic protection: Consider implementing cathodic protection systems, such as sacrificial anodes or impressed current systems, to provide an additional layer of protection against crevice corrosion. These systems help divert corrosion currents away from vulnerable areas, reducing the risk of crevice corrosion.
Pitting Corrosion vs. Crevice Corrosion
Pitting Corrosion
Crevice Corrosion
Occurs when the protective oxide layer on stainless steel breaks down, leading to the formation of small pits on the metal surface
Similar to pitting corrosion, it starts with the breakdown of the protective oxide film, but occurs in crevices
Pits can grow deep enough to perforate the tube wall
Corrosion takes place within tight crevices, such as between tubing and tube supports or underneath deposits
Environments with higher chloride concentrations are prone to pitting corrosion
Seawater diffusion into a crevice creates a chemically aggressive environment for crevice corrosion
Visual inspection can detect reddish-brown iron oxide deposits and pits
Crevice corrosion can only be observed when a tubing clamp is removed
Pitting corrosion is more likely at high temperatures
Crevice corrosion can occur at lower temperatures compared to pitting corrosion
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