1. Zavedení
Nerez is one of the most widely used materials in a vast array of industries, thanks to its unique combination of strength, odolnost proti korozi, a estetická přitažlivost.
From automotive parts to food processing equipment, stainless steel’s versatility and durability make it the material of choice for applications requiring both strength and longevity.
Však, despite its excellent corrosion resistance, stainless steel is not immune to certain types of corrosion.
One such corrosion phenomenon is intergranular corrosion, which can severely impact the material’s structural integrity.
Understanding how this corrosion occurs and how to prevent it is crucial for maintaining the longevity and performance of stainless steel products in various industries.
V tomto blogu, we’ll explore intergranular corrosion in stainless steel, its causes, how it impacts the material, and effective methods to prevent and mitigate it.
2. What is Intergranular Corrosion?
Mezikrystalová koroze (IGC) is a localized form of corrosion that occurs at the grain boundaries of stainless steel.
Unlike general corrosion, which affects the entire surface of the material, intergranular corrosion attacks specific areas, weakening the metal and potentially causing premature failure.
The corrosion process leads to the depletion of chromium at the grain boundaries, reducing the steel’s resistance to further corrosion.
How it Differs from Other Types of Corrosion
Intergranular corrosion is different from other common forms of corrosion, such as general corrosion and pitting.
General corrosion is uniform and affects the entire surface of the material, while intergranular corrosion specifically targets the grain boundaries, leading to a more localized degradation.
Důlková koroze, na druhé straně, forms small, deep holes or pits, usually in areas where chloride ions are present, but does not directly affect the grain boundaries.
How It Occurs
The primary mechanism behind intergranular corrosion is the formation of chromium carbides at the grain boundaries, which occurs at certain temperatures.
When stainless steel is exposed to temperatures between 450°C and 850°C (840°F – 1560°F), carbon from the material combines with chromium, forming chromium carbides.
This process is known as sensitization. The formation of chromium carbides depletes chromium from the grain boundaries, leaving those areas more susceptible to corrosion.
This creates a pathway for aggressive chemicals or moisture to penetrate, exacerbating corrosion.
3. Causes and Factors Contributing to Intergranular Corrosion
Chromium Depletion
Chromium is a crucial element in stainless steel, providing its corrosion-resistant properties.
When chromium is depleted at the grain boundaries due to the formation of chromium carbides, the material loses its ability to resist corrosion in those areas.
This significantly weakens the steel and can cause it to fail over time, particularly when exposed to harsh environments.
High-Temperature Exposure
Heat treatment processes such as welding or annealing can expose stainless steel to the critical temperature range that promotes the sensitization process.
During welding, například, the heat input can cause localized areas of the material to reach these temperatures, triggering the formation of chromium carbides.
It’s important to manage the heat input during these processes to prevent intergranular corrosion.
Carbon Content and Alloying Elements
The carbon content in stainless steel plays a pivotal role in its susceptibility to intergranular corrosion. Higher carbon content accelerates the formation of chromium carbides.
Alloying elements such as titanium, Niobium, or molybdenum can be used to stabilize the chromium in the steel and reduce the likelihood of intergranular corrosion by preventing carbide formation.
4. Types of Stainless Steel Prone to Intergranular Corrosion
Intergranular corrosion can affect various grades of stainless steel, but some types are more susceptible due to their composition and specific characteristics.
Understanding which grades are prone to this issue helps manufacturers and engineers make informed decisions when selecting materials for different applications.
Austenitická nerezová ocel
Austenitic stainless steels are among the most commonly used types in the industry due to their excellent corrosion resistance and versatility.
Však, they are particularly vulnerable to intergranular corrosion,
especially when exposed to temperatures between 450°C and 850°C (840°F – 1560°F) during welding or other heat treatments. The most commonly used austenitic stainless steels include:
- Stupeň 304: This is the most popular austenitic grade and is widely used in food processing, konstrukce, a chemický průmysl.
Však, when exposed to high temperatures, it can experience chromium carbide precipitation at grain boundaries, making it susceptible to intergranular corrosion. - Stupeň 316: Known for its superior corrosion resistance, particularly in chloride environments,
316 stainless steel can also suffer from intergranular corrosion if improperly heat-treated, especially in high-temperature processes like welding.
Why it happens:
In austenitic stainless steel, high carbon content can lead to the formation of chromium carbides at the grain boundaries during the sensitization process.
The depletion of chromium at these boundaries reduces the material’s resistance to corrosion, leaving the stainless steel more vulnerable to degradation.
Ferritická nerezová ocel
Ferritic stainless steels contain higher amounts of chromium and lower amounts of nickel,
which gives them magnetic properties and makes them more resistant to stress corrosion cracking compared to austenitic grades.
Však, ferritic grades are still susceptible to intergranular corrosion, especially if they are exposed to sensitizing temperatures.
- Stupeň 430: Commonly used in automotive exhaust systems and kitchen appliances,
this ferritic grade can suffer from intergranular corrosion if exposed to the critical temperature range during welding. - Stupeň 446: Known for its resistance to high-temperature oxidation,
446 ferritic stainless steel is still vulnerable to intergranular corrosion under certain conditions, particularly after heat treatments.
Why it happens:
Ferritic stainless steels have a lower nickel content than austenitic grades, which means they are less prone to sensitization at higher temperatures.
Však, they can still face chromium depletion at grain boundaries if exposed to prolonged heat, especially in welding processes.
Martensitická nerezová ocel
Martensitic stainless steels, which are high in carbon and offer excellent hardness, are widely used in applications that require strength, jako jsou lopatky turbíny, ventily, and knives.
While they are generally less susceptible to intergranular corrosion than austenitic and ferritic steels, they can still suffer from this type of corrosion, particularly in high-carbon grades.
- Stupeň 410: A common martensitic stainless steel grade used in the aerospace and automotive industries, 410 is prone to intergranular corrosion if not properly heat-treated.
Martensitic stainless steels tend to experience carbide precipitation at the grain boundaries when exposed to high temperatures.
Why it happens:
High carbon content in martensitic stainless steels can lead to carbide formation at the grain boundaries,
similar to the process in austenitic steels, making them vulnerable to intergranular corrosion.
Duplexní nerezová ocel
Duplex stainless steels combine the properties of both austenitic and ferritic stainless steels, nabízí rovnováhu odolnosti vůči síle a korozi.
While duplex stainless steels offer improved resistance to stress corrosion cracking and pitting, they are not immune to intergranular corrosion.
- Stupeň 2205: One of the most widely used duplex stainless steels, 2205 is designed for use in more aggressive environments, such as chemical processing and marine applications.
Však, it is still susceptible to intergranular corrosion if not properly controlled during heat treatment.
Why it happens:
Although duplex stainless steels have a balanced microstructure of austenite and ferrite,
the high chromium content and alloying elements such as molybdenum make them prone to sensitization in certain conditions.
If the alloy is exposed to high temperatures during welding or processing, chromium carbides can form at the grain boundaries, increasing the risk of intergranular corrosion.
5. Effects and Consequences of Intergranular Corrosion
Intergranular corrosion can have significant detrimental effects on stainless steel components, impacting their functionality, bezpečnost, and lifespan.
Reduced Mechanical Properties
- Pevnost: Intergranular corrosion attacks the grain boundaries, which are critical for maintaining the structural integrity of the material.
This can lead to a reduction in tensile strength and load-bearing capacity. - Tažnost a houževnatost: The affected areas become brittle and lose their ability to deform without breaking, reducing the overall ductility and toughness of the component.
- Odolnost proti únavě: Components suffering from IGC may experience premature fatigue failure due to the initiation of cracks along the weakened grain boundaries.
Material Failures
- Critical Applications: In industries such as aerospace, automobilový průmysl, petrochemický, a výroba energie,
where stainless steel is used in high-stress environments, IGC can cause catastrophic failures.
Examples include cracking or rupture in pressure vessels, potrubní systémy, výměníky tepla, and other critical machinery parts. - Real-world Examples: Failures in stainless steel structures like bridges, Offshore platformy,
and chemical processing equipment due to IGC highlight the importance of preventing this type of corrosion.
Například, a small crack initiated by IGC can propagate under cyclic loading conditions, eventually leading to the complete failure of the component.
Aesthetic Damage
- Visible Corrosion Marks: While not always immediately apparent, IGC can lead to visible signs of corrosion that affect the appearance of stainless steel products.
These may include discoloration, Pitting, or roughening of the surface, particularly noticeable in consumer goods, architektonické prvky, and kitchen appliances. - Impact on Surface Finish: Even if the functional performance of the part remains intact,
aesthetic damage can reduce the value and marketability of products, especially in applications where appearance is crucial.
Other Considerations
- Náklady na údržbu: Detecting and repairing components affected by IGC can be costly and time-consuming.
Regular inspections and maintenance schedules must be implemented to monitor and address potential issues before they escalate into more serious problems. - Replacement Costs: In severe cases, components may need to be entirely replaced if the extent of IGC compromises their structural integrity beyond repair.
This leads to increased operational costs and potential downtime in industrial settings.
6. Prevention and Mitigation of Intergranular Corrosion
Intergranular corrosion is a serious issue for stainless steel, particularly in critical applications where the material must withstand harsh environments and maintain structural integrity.
Fortunately, there are several methods to prevent or mitigate the occurrence of intergranular corrosion, from material selection to specific processing techniques.
Below are the most effective strategies for combating this type of corrosion.
Use of Low-Carbon Alloys (L or H Grades)
One of the most effective ways to reduce the risk of intergranular corrosion is to use low-carbon or stabilized stainless steel grades.
Low-carbon alloys contain less carbon content, which minimizes the formation of chromium carbides at grain boundaries.
These alloys are particularly important for applications involving welding or heat treatments that would otherwise cause sensitization.
- 304L and 316L Grades: These low-carbon versions of the commonly used 304 a 316 grades offer improved resistance to intergranular corrosion without compromising their mechanical properties.
They are ideal for high-temperature applications such as food processing equipment, chemical storage tanks, and other industrial machinery that requires welding. - 347 a 321 Stupně: These stabilized grades contain titanium or niobium, which bind with carbon during the welding process to prevent chromium carbide formation.
These alloys are suitable for high-temperature applications, such as in the aerospace industry, where heat exposure is frequent.
Proč to funguje:
By reducing the carbon content, or by stabilizing the carbon through alloying elements like titanium or niobium,
these materials are less likely to undergo sensitization and are thus more resistant to intergranular corrosion.
Proper Welding Techniques
Welding is a common source of intergranular corrosion, as it introduces localized heat that can lead to chromium carbide precipitation at the grain boundaries.
Aby se tomu zabránilo, proper welding techniques must be followed to minimize the risk of sensitization.
- Control Heat Input: When welding stainless steel, it’s critical to control the heat input to prevent excessive temperatures that can lead to sensitization.
This is especially important in the heat-affected zone (Haz), where the material is most likely to undergo the transformation that causes intergranular corrosion. - Po západním tepelném zpracování (PWHT): Po svařování, it is often necessary to perform a solution annealing process.
This involves heating the material to a high temperature, followed by rapid cooling to dissolve any chromium carbides that may have formed during the welding process.
This treatment helps to restore the material’s corrosion resistance. - Use of Stabilized Grades for Welding: As previously mentioned, using stabilized grades such as 321 nebo 347 in welding applications can reduce the risk of chromium carbide formation.
These grades are designed to withstand the elevated temperatures associated with welding and heat treatments.
Proč to funguje:
By controlling the welding parameters and using post-weld treatments, you can effectively reduce the chances of sensitization and mitigate the risk of intergranular corrosion.
Passivation and Surface Treatments
Passivation is a chemical process that enhances the natural oxide layer of stainless steel, Zlepšení jeho odolnosti proti korozi.
Passivating stainless steel helps to reduce the likelihood of surface degradation, including intergranular corrosion.
- Pasivace: This process involves treating the stainless steel with an acid solution (typically nitric acid) to remove free iron and other contaminants from the surface.
This treatment promotes the formation of a dense, passive oxide layer that enhances corrosion resistance and helps to protect against intergranular corrosion. - Pickling and Electropolishing: In addition to passivation, moření (a process that uses an acidic solution to remove impurities) and electropolishing
(which uses an electrolytic process to smooth the surface and improve corrosion resistance) can further enhance the surface quality of stainless steel.
These treatments help to prevent corrosion by removing contaminants that might otherwise contribute to galvanic reactions or localized corrosion.
Proč to funguje:
Passivation and other surface treatments improve the uniformity and durability of the oxide layer on stainless steel, which in turn helps to reduce the risk of intergranular corrosion.
Proper Material Selection and Design
The choice of material and the way parts are designed can also have a significant impact on reducing the likelihood of intergranular corrosion.
Properly selecting stainless steel grades and designing components to minimize the conditions that lead to sensitization can help prevent this form of corrosion.
- Consider the Environment: For applications that involve exposure to high temperatures or aggressive chemicals,
selecting the appropriate stainless steel grade (NAPŘ., low-carbon or stabilized grades) je zásadní.
Například, if the material will be exposed to high heat or welding, using a grade like 304L or 316L would be beneficial. - Design for Stress Relief: Parts should be designed to minimize areas of high stress, as stress can exacerbate the effects of intergranular corrosion.
Incorporating features like rounded corners and avoiding sharp edges can reduce stress concentrations and mitigate the risk of corrosion.
Proč to funguje:
Selecting the appropriate material and designing components to minimize stress and high-temperature
exposure ensures that the material will perform optimally and resist intergranular corrosion.
Regular Inspection and Maintenance
Detecting intergranular corrosion early can help prevent significant damage to components. Regular inspections are essential for identifying signs of corrosion before they lead to failure.
- Vizuální kontrola: The first step in identifying intergranular corrosion is visual inspection.
Common signs of intergranular corrosion include cracks, Pitting, or discoloration along the grain boundaries. - Nedestruktivní testování (Ndt): Techniques such as ultrasonic testing, X-ray analysis, and dye penetrant testing
can help detect internal or surface defects that may indicate intergranular corrosion.
These methods are valuable in industries where maintaining the integrity of critical components is paramount.
Proč to funguje:
Early detection through routine inspections can prevent more severe damage and allow for timely corrective actions,
helping to maintain the longevity and performance of stainless steel components.
7. Detecting Intergranular Corrosion
Vizuální kontrola
Visual inspection can reveal signs of intergranular corrosion, including cracks along the grain boundaries.
These signs often appear as surface discoloration, Pitting, or cracking, especially in areas subject to heat treatment or welding.
Nedestruktivní testování (Ndt)
Techniques such as ultrasonic testing, X-ray diffraction, and metallographic analysis are commonly used to detect intergranular corrosion without damaging the material.
These methods allow for early detection of corrosion and help prevent failure in critical applications.
Electrochemical Tests
Laboratory tests like the Huey test and Strauss test are widely used to evaluate the susceptibility of stainless steel to intergranular corrosion.
These electrochemical tests expose the material to a series of controlled conditions to simulate corrosive environments and assess its resistance.
8. Závěr
Intergranular corrosion is a serious issue that can affect the performance, dlouhověkost,
and safety of stainless steel components, particularly when exposed to high temperatures during manufacturing.
By understanding the causes and mechanisms behind this type of corrosion, industries can adopt preventative measures
such as using low-carbon alloys, controlling heat during welding, and applying surface treatments.
Early detection through proper inspection and testing methods can further reduce the risks and help maintain the integrity of stainless steel in demanding applications.
If you’re looking for high-quality custom Stainless Steel products, výběr TENTO je perfektním rozhodnutím pro vaše výrobní potřeby.
