C-P Systems
What Is Crevice Corrosion? | Process Engineering Glossary
What Is Crevice Corrosion?
Crevice corrosion is a localized corrosion mechanism that occurs in narrow gaps or confined spaces where stagnant solution becomes depleted of oxygen and accumulates aggressive species such as chlorides and hydrogen ions, creating a locally acidic, deoxygenated environment that aggressively attacks the metal surface within the crevice even when the surrounding exposed surface remains unaffected. Crevice corrosion is particularly damaging to stainless steels and other alloys that depend on a passive oxide film for corrosion resistance, because the depleted conditions within the crevice break down the passive film locally while the surrounding cathodic area drives accelerated attack within the small crevice volume.
Crevices in process equipment and piping occur at flanged connections under gasket surfaces, at threaded joints, beneath deposits, under insulation that traps moisture, at tube-to-tubesheet joints, and at any geometry where solution can enter but cannot freely circulate and refresh.
Applications
Material Selection for Crevice-Prone Geometries
Corrosion studies identify service environments where crevice corrosion is a credible mechanism and specify materials with adequate crevice corrosion resistance for the specific process chemistry and temperature conditions.
Design to Minimize Crevice Geometries
Equipment and piping design can minimize crevice corrosion risk by avoiding crevice-creating geometries where possible, using full-penetration welds instead of fillet welds, and specifying smooth gasket contact surfaces.
Inspection Targeting at Crevice Locations
Mechanical integrity inspection programs target known crevice locations for focused examination, since crevice corrosion produces highly localized damage that general visual inspection may not detect.
Benefits
Explains localized attack in otherwise resistant materials. Understanding crevice corrosion explains why stainless steel and nickel alloys can fail locally at crevices while the exposed surface shows no corrosion, preventing incorrect conclusions about material adequacy.
Enables design-phase prevention. Recognizing which geometries create crevice corrosion risk enables designers to eliminate unnecessary crevices or specify crevice-resistant materials during design rather than discovering the problem in service.
Supports targeted inspection resource allocation. Knowing which locations and materials are susceptible to crevice corrosion supports focusing inspection resources on the specific locations where this mechanism is most likely to produce damage.
Limitations
Not all crevices can be eliminated by design. Some crevice geometries, such as flanged connections and threaded fittings, are inherent to practical equipment design and cannot be entirely avoided, requiring material selection to address the crevice corrosion risk.
Crevice corrosion testing conditions may not match service. Standard crevice corrosion test methods use specific crevice geometries and solution compositions that may not precisely replicate actual service conditions, requiring engineering judgment in applying test results to wall thinning predictions.
Crevice corrosion can initiate stress corrosion cracking. Localized attack from crevice corrosion can create stress concentrations that initiate stress corrosion cracking in susceptible materials, producing a combined damage mechanism more severe than either alone.
FAQ
How does crevice corrosion relate to pitting corrosion?
Crevice corrosion and pitting corrosion share the same electrochemical mechanism of passive film breakdown, but crevice corrosion initiates at lower temperatures and lower chloride concentrations because the crevice geometry accelerates the local environment degradation.
How does crevice corrosion relate to corrosion under insulation?
Corrosion under insulation creates crevice-like conditions where moisture is trapped between the insulation and the pipe surface, and on stainless steels this environment can initiate crevice-type attack.
How does crevice corrosion relate to fitness for service?
Fitness for service evaluation of crevice corrosion damage assesses the remaining wall thickness at the locally attacked area and determines whether continued operation is acceptable.
Reference
About C-P Systems
SETTING THE STANDARD FOR CHEMICAL ENGINEERING FIRMS EVERYWHERE
Through unmatched professionalism, knowledge and experience, we set the industry bar for chemical engineering firms. With decades of chemical plant engineering and piping design experience, our team of licensed engineers can handle any project scope.