What Is Crevice Corrosion?

Crevice corrosion is a localized attack that occurs within confined spaces (crevices) where stagnant solution conditions develop. The crevice restricts oxygen transport, creating a differential aeration cell: the metal inside the crevice becomes anodic and corrodes, while the surrounding exposed surface remains cathodic and protected. Crevice corrosion initiates at lower temperatures and lower chloride concentrations than pitting corrosion on the same alloy, making it the more aggressive of the two mechanisms.

Crevice Corrosion Mechanism

Stage	Description
1. Initial uniform corrosion	Oxygen is consumed equally inside and outside the crevice
2. Oxygen depletion	Limited access restricts oxygen replenishment inside the crevice; dissolved oxygen is consumed
3. Differential aeration	Oxygen-rich solution outside the crevice drives cathodic reaction; the oxygen-depleted crevice interior becomes anodic
4. Acidification and chloride enrichment	Metal ions hydrolyze inside the crevice, lowering pH (to 1-3); chloride ions migrate in to maintain charge balance
5. Accelerated attack	The acidic, chloride-rich, oxygen-depleted environment inside the crevice prevents passive film repair; corrosion accelerates

The mechanism is similar to pitting corrosion (both involve local acidification and chloride enrichment) but crevice corrosion starts at inherently shielded locations rather than requiring a passive film defect.

Common Crevice Locations in Piping

• Under gaskets: the gasket-to-flange contact surface, especially with spiral wound gaskets on raised face flanges
• Socket weld joints: the gap between the pipe end and socket weld fitting shoulder
• Threaded connections: thread roots trap stagnant fluid
• Under deposits/scale: corrosion products, biological growth, or sand deposits create artificial crevices
• Lap joints: overlap zones in lap joint flanges and reinforcing pads
• Under insulation: moisture trapped between insulation and the pipe surface (corrosion under insulation, CUI)
• Tube-to-tubesheet joints: the expanded or welded interface in heat exchangers

Critical Crevice Temperature (CCT)

The Critical Crevice Temperature (CCT) is the minimum temperature at which crevice corrosion initiates in a standardized test (ASTM G48 Method D). The CCT is always lower than the Critical Pitting Temperature (CPT) for the same alloy; typically 15-20°C lower.

Alloy	CPT (°C)	CCT (°C)	PREN
316L	~25	~5	24.2
Duplex 2205	~50	~30	35.0
Super duplex 2507	~75	~55	41.9
6Mo (254 SMO)	~75	~50	43.3
Alloy 625	>85	>70	51.2
Alloy C-276	>85	>75	69.4

Pro Tip

When selecting stainless steel for seawater or chloride service with flanged connections, size the material selection based on the CCT (not the CPT), because flanged joints inherently contain crevices at the gasket-to-flange interface. A grade that resists pitting on an open surface may fail by crevice corrosion at the first gasket.

Prevention Methods

Strategy	How It Works
Eliminate crevices by design	Use butt welds instead of socket welds and threaded joints; full-penetration welds instead of lap joints
Material upgrade	Select alloys with CCT above the maximum operating temperature
Gasket selection	Use PTFE-coated or elastomeric gaskets that seal tightly and resist fluid ingress
Weld configuration	Seal-weld socket connections and reinforcing pads to prevent fluid entry
Surface preparation	Smooth flange face finish reduces micro-crevices; pickle and passivate per ASTM A380
Cathodic protection	Shifts potential below the crevice corrosion initiation threshold (offshore applications)
Environmental control	Reduce chloride content, lower temperature, or add corrosion inhibitors

For environments containing H2S, chlorides, and elevated temperatures, crevice corrosion risk must be evaluated alongside stress corrosion cracking and pitting corrosion requirements per NACE MR0175/ISO 15156. In such cases, super duplex (2507) or nickel-based alloys are often the only viable solution for flanged connections.

Read the full guide to steel corrosion