CCT – Critical crevice corrosion temperature of stainless steels and nickel alloys

CCT (Critical Crevice Temperature), or critical crevice corrosion temperature, is a parameter used to assess the resistance of stainless steels, duplex and superduplex steels, and nickel alloys to the initiation of crevice corrosion under specific test conditions. This parameter is particularly important in chloride-containing environments, because even a material with high resistance to pitting corrosion can experience localized damage at crevice locations.
Typical areas at risk of crevice corrosion are under gaskets, washers, deposits and contaminants, in flange and lap joints, under bolt heads and wherever a narrow space is created that restricts the free exchange of solution with the environment.
CCT is not a physical constant of a material nor is it the maximum allowable operating temperature. It is the result of a test conducted according to a specific procedure. Therefore, when comparing CCT values, the test method, environment, surface preparation, and the geometry and pressure of the element creating the gap should always be taken into account.

What does CCT mean?

The critical crevice corrosion temperature (CCT) defines the lowest temperature at which crevice corrosion initiation is observed under the conditions of a given test method, meeting the criteria specified in the test procedure. The higher the CCT value obtained under comparable conditions, the greater the material's resistance to the initiation of this type of corrosion.
However, this does not mean that the material is unconditionally resistant below the CCT value in every installation. Actual operating conditions can differ significantly from laboratory conditions in terms of chloride concentration, pH, electrochemical potential, oxygenation, fracture geometry, flow, sediments, microorganisms, and exposure time.

Why does a gap increase the risk of corrosion?

Stainless steels and many nickel alloys owe their corrosion resistance to a thin passive layer that forms on the surface. In an open environment, oxygen and the appropriate chemical conditions promote the maintenance and reconstruction of this layer. In a narrow crevice, the situation can be completely different.
Limited solution exchange between the interior of the fracture and the surrounding environment creates a locally different environment. As the process progresses, oxygen consumption, chloride ion concentration, and pH reduction may occur. The conditions within the fracture become increasingly aggressive and can lead to the loss of the material's passivity.
Once a stable process begins, corrosion can develop locally under a seal or deposit, while the adjacent, freely accessible surface of the same component remains undamaged. This is why crevice corrosion is particularly dangerous – its development can remain invisible for a long time during a routine inspection of the installation.

CCT vs. CPT – what's the difference?

CPT (Critical Pitting Temperature) describes the critical temperature for the initiation of pitting corrosion in a specific test, while CCT (Critical Crevice Temperature) refers to the initiation of corrosion in an artificially created crevice. Both parameters refer to local corrosion, but do not describe exactly the same phenomenon.
In practice, the CCT is usually lower than the CPT for the same material tested in a comparable system. This is because the crevice creates local conditions that are more aggressive than those found on the freely accessible surface of the material.

How is CCT determined?

One of the primary standards used for laboratory evaluation of pitting and crevice corrosion resistance is ASTM G48 . This standard utilizes a highly oxidizing chloride environment containing iron(III) chloride and includes several separate test methods.

In the current ASTM G48, Methods C and E are used to determine the critical pitting corrosion temperature, while Methods D and F are used to determine the critical crevice corrosion temperature. Method D is intended for determining the CCT of nickel-based alloys and chromium-containing alloys, while Method F is used to determine the CCT of stainless steels.

In CCT testing, elements creating controlled cracks are attached to the sample surface. This is important because the crack geometry and clamping force influence the conditions between the sample surface and the crack-forming element. Therefore, values ​​obtained using different procedures, clamping forces, or crack-forming elements should not be uncritically compared.

Comparison of the CCT of selected stainless steels and nickel alloys

The values ​​below demonstrate the scale of differences between materials, but should be interpreted only in conjunction with the test method. Particularly well-documented comparative data are available for tests in an acidified 6% FeCl₃ solution and ASTM G48 procedures. Differences between sources for the same grade may result from, among other things, different procedures, surface preparation, crevice formation, and specific melt composition.

* These values ​​are intended to illustrate differences between steel groups; they should not be considered guaranteed properties of a specific product. ** For nickel alloys, differences between published values ​​clearly demonstrate the impact of the specific test procedure. For example, Alleima's data for ASTM G48 Method D/Method F yielded different values ​​depending on the method used to create and pressurize the crack.
The comparison reveals a very important pattern. The popular 316L steel has relatively low resistance to crevice corrosion initiation in aggressive chloride tests. With increasing chromium, molybdenum, and nitrogen content, this resistance can increase significantly. 6Mo steels, superduplex steels, and appropriately selected nickel alloys are among the materials with significantly greater resistance to localized corrosion.

Why is CCT usually lower than CPT?

The difference between CPT and CCT stems primarily from the specific chemical conditions created within the crack. On an open surface, the material remains in contact with the surrounding environment, whereas within the crack, solution exchange is limited.
Oxygen concentration may decrease, chloride concentration may increase, and the local environment may become acidic. As a result, the passive layer may lose stability at temperatures lower than those needed to initiate stable pitting on the free surface.
For example, Haynes data obtained in acidified 6% FeCl₃ show for 316L a CPT of about 15°C and a CCT of about 0°C, for 254 SMO about 60°C and 30°C, respectively, for Alloy 625 about 100°C and 40°C, and for Alloy C-276 about 150°C and 55°C. This shows that very high resistance to pitting corrosion does not automatically mean equally high resistance to crevice corrosion.

The influence of gap geometry on corrosion

Not every crack poses the same risk. Its width, depth, the ratio of the crack's internal to external surface area, and the degree to which fluid exchange is restricted are all factors that matter.
In practice, hazardous conditions may include:

• under flange gaskets,
• under washers and screw heads,
• in overlapping connections,
• under mineral deposits and process products,
• under biofilm and biological contaminants,
• in places where metal elements come into contact with plastics or elastomers,
• in insufficiently cleaned construction joints.

For this reason, a structure's resistance to crevice corrosion cannot be assessed solely based on steel grade. The joint design, workmanship, and surface cleanliness are equally important.

The influence of chemical composition on CCT

Chromium is primarily responsible for the formation and stability of the passive layer. Molybdenum significantly improves resistance to local corrosion in chloride environments, while nitrogen is a particularly important alloying addition in duplex, superduplex and high-alloy austenitic steels.
The high content of these elements is one of the reasons for the significantly greater resistance of 6Mo and superduplex steels compared to standard 304L and 316L steels. However, chemical composition alone does not tell the whole story. Actual resistance is also influenced by microstructure, intermetallic phase precipitation, heat treatment, welding, and surface condition.

CCT and PREN

The PREN (Pitting Resistance Equivalent Number) is a useful tool for initially comparing alloy resistance to localized corrosion in chloride environments. For stainless steels, the following relationship is often used:

PREN = %Cr + 3,3 × %Mo + 16 × %N

An increase in PREN is usually associated with an increase in resistance to pitting and crevice corrosion, but CCT cannot be directly calculated from PREN values. PREN is primarily based on chemical composition, while CCT results also depend on microstructure, surface finish, and detailed test conditions.
For high-alloy nickel materials, modified equations are also used to account for other elements, including tungsten. Therefore, comparing the classical PREN of stainless steels with values ​​calculated for nickel alloys requires caution.

Influence of surface and welding on CCT

Surface preparation can significantly impact the results of crevice corrosion resistance testing. Roughness, scale, weld discoloration, iron contamination, and machining residues can create areas conducive to localized corrosion.
ASTM G48 notes that surface preparation can significantly affect test results. This means that values ​​obtained for ground or etched laboratory samples do not always accurately reflect the performance of the actual component.
Welding can also alter the material's microstructure and the local chemical composition of the surface layer. This is particularly important in duplex and superduplex steels, where improper thermal cycling can lead to unfavorable changes in phase equilibrium or the formation of precipitates that reduce corrosion resistance.
After welding, it is also important to properly remove oxides and heat discoloration and, where required, to carry out appropriate pickling and passivation.

CCT and gaskets and flange connections

Flanged joints are one of the classic sites for crevice corrosion. The gap between the metal and the gasket can trap electrolyte and limit its exchange with the environment. Similar conditions can occur under washers, fasteners, and in metal-to-plastic joints.
Therefore, when designing systems operating in seawater, brine, or other media containing chlorides, it is important to consider not only the resistance of the material itself but also the geometry of the structure. Limiting the number of crevices, allowing fluid drainage, eliminating deposits, and selecting appropriate seals can be just as important as selecting a higher grade of steel.

Does CCT determine the maximum operating temperature of a material?

No. CCT is a critical temperature determined in a specific laboratory test and should not be considered a direct limit of safe operating temperature.
ASTM G48 is an accelerated test designed primarily to compare materials' resistance to the initiation of localized corrosion in a highly aggressive chloride environment. The standard emphasizes that these tests cannot directly determine material behavior in non-chloride environments or the rate of further localized corrosion propagation.
For example, a material with a specific CCT in a laboratory FeCl₃ solution may behave completely differently in seawater, dilute NaCl solution, or a real process medium. Its behavior will be determined simultaneously by temperature, chloride concentration, pH, potential, flow, sediments, fracture geometry, and exposure time.

CCT in practical material selection

When designing equipment exposed to chlorides, it's worth analyzing CCT along with other material parameters and information. Of particular importance are:

• the type and actual chemical composition of the material,
• PREN,
• CPT and CCT determined by a comparable method,
• chloride concentration,
• operating temperature,
• pH and electrochemical potential of the environment,
• presence of seals, overlapping joints and deposits,
• surface condition,
• welding technology and post-weld treatment,
• possibility of cleaning and inspecting the element during operation.

In structures containing numerous fractures, CPT analysis alone may not be sufficient. In such cases, CCT may be a more demanding and practically relevant comparative parameter.

How to reduce the risk of crevice corrosion?

The most effective protection doesn't rely solely on the use of increasingly alloyed materials. Proper device design is also crucial. Narrow gaps should be eliminated whenever possible, lap joints should be replaced with properly welded joints, liquid and sediment traps should be avoided, and surfaces should be easily cleaned.
In particularly aggressive environments, superduplex steels, 6Mo austenitic steels, or appropriate nickel alloys may be necessary. However, the final selection should be based on an analysis of actual operating conditions, not solely on a single PREN, CPT, or CCT value.
CCT – Critical Crevice Temperature – is the critical crevice corrosion initiation temperature determined under defined test conditions. This parameter is particularly important in systems operating in chloride environments and containing gaskets, flanges, overlaps, deposits, or other crevice-forming areas.
The CCT is usually lower than the CPT because a much more aggressive local environment can develop within the crevice than at the free surface of the material. Therefore, high pitting corrosion resistance does not guarantee equally high crevice corrosion resistance.
When comparing materials, the method by which the CCT was determined should always be verified. Values ​​obtained from different procedures should not be considered directly equivalent, and CCT alone should not be interpreted as the maximum allowable operating temperature of the material.

Types of corrosion of steel and metals

Uniform corrosion of steel and metals
Pitting corrosion of stainless steels
Crevice corrosion of stainless steels
Intergranular corrosion of stainless steel
Stress corrosion cracking SCC
Galvanic corrosion
Erosive corrosion
Cavitation corrosion
Corrosion fatigue
Fretting corrosion
Microbiological corrosion MIC
High-temperature corrosion

Division of corrosion according to the environment

Marine corrosion – corrosion of steel and metals in seawater
Atmospheric corrosion
Ground corrosion
Corrosion in fresh and industrial water
Corrosion in chloride environments
Corrosion in acidic environments
Corrosion in alkaline environments
Corrosion in gases and exhaust gases
Corrosion in sewage and sewer systems
Corrosion in chemical and petrochemical environments

Related topics

CPT – critical pitting corrosion temperature of stainless steels and nickel alloys
PREN – pitting corrosion resistance index for stainless steel
CCT – Critical crevice corrosion temperature of stainless steels and nickel alloys