Pitting and Crevice Corrosion Resistance of Stainless Steels

Pitting and Crevice Corrosion Resistance of Stainless Steels

Stainless steels are known for their excellent corrosion resistance and are widely used in chemical processing, marine environments, oil and gas, and structural applications. However, under certain aggressive conditions, particularly in environments containing chlorides, stainless steels are prone to localized corrosion, including pitting and crevice corrosion.

What is Pitting Corrosion?

Pitting corrosion is a highly localized form of attack that creates small, deep cavities, or “pits,” on the metal’s surface. This type of corrosion usually happens when the passive oxide layer on stainless steel is damaged by chlorides or other ions.

While the total metal loss may be small, pits can penetrate quickly and deeply, and may cause:

• Leakage in tanks and pipelines
• Structural weakening
• Crack initiation leading to stress corrosion cracking

Pitting often initiates at inclusions, surface defects, or areas with mechanical damage.

What is Crevice Corrosion?

Crevice corrosion occurs in confined spaces where stagnant solution is trapped, such as:

• Under gaskets and washers
• Beneath deposits or biofilms
• At bolted joints
• Under disbanded coatings

In these crevices, oxygen depletion leads to differential aeration cells, which produce localized acidification and increased chloride concentration. This damage to the passive film causes an aggressive localized corrosion.

Crevice corrosion is usually more aggressive than pitting corrosion because its geometry accelerates corrosive attack.

Factors Affecting Resistance

The resistance of stainless steels to pitting and crevice corrosion depends on several factors:

1. Alloy Composition

Elements such as chromium (Cr), molybdenum (Mo), and nitrogen (N) significantly enhance resistance. Molybdenum, in particular, improves resistance to chloride attack.

2. Chloride Concentration

Higher chloride levels increase the risk of localized corrosion, especially in marine and coastal environments.

3. Temperature

Elevated temperatures accelerate passive film breakdown and increase susceptibility.

4. pH and Oxygen Levels

Low pH and stagnant conditions promote localized corrosion.

Pitting Resistance Equivalent Number (PREN)

The resistance of stainless steels to pitting is often estimated using the Pitting Resistance Equivalent Number (PREN):

PREN = %Cr + 3.3(%Mo) + 16(%N)

Higher PREN values indicate greater resistance to pitting and crevice corrosion. For example:

Testing for Pitting and Crevice Corrosion

Several standardized tests are used to evaluate resistance:

• ASTM G48 – Pitting and crevice corrosion testing in ferric chloride solution
• ASTM G150 – Critical Pitting Temperature (CPT) determination
• ASTM G78 – Crevice corrosion testing
• Electrochemical methods for measuring breakdown potential

The Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT) are commonly used parameters to compare materials.

Prevention and Mitigation

Localized corrosion can be minimized through:

• Selecting higher alloyed stainless steels with higher PREN
• Avoiding stagnant design geometries
• Maintaining smooth surface finishes
• Reducing chloride exposure
• Implementing regular cleaning and maintenance
• Applying cathodic protection where appropriate

Proper fabrication practices, including control of welding parameters and post-weld cleaning, also play a critical role.

Applications Where Resistance is Critical

Resistance to pitting and crevice corrosion is especially important in:

• Marine and offshore structures
• Desalination plants
• Heat exchangers
• Chemical processing equipment
• Food and pharmaceutical industries

Failure in these environments can lead to leaks, contamination, and costly downtime.

Conclusion

Pitting and crevice corrosion can be quite aggressive, but they are also localized. The exact composition of a steel, the environment in which it is placed, and the design of the component all affect its susceptibility to corrosion. Selecting the proper material, performing standardized tests, and following sound engineering principles are essential to good performance.

Infinita Lab: Your Material Testing Partner

Contact Infinita lab for Pitting and Crevice Corrosion Resistance of Stainless Steels testing by major benefits like: 

• End-to-end testing management, faster turnaround, reduced administrative burden.
• Confidence in accurate results and reduced stress in vendor coordination.
• Enhanced reputation for product reliability and innovation.
• Engineers and R&D managers focus on core work rather than testing logistics.

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