Series: Part A
improving the longetivity of metal expansion jointsCORROSION MECHANISMS AND PREVENTION
Uniform
Corrosion
what is uniform corrosion?
Uniform corrosion, also referred to as general corrosion, involves an even and gradual thinning of the metal surface of the metal expansion bellow when exposed to a corrosive internal or external environment. In metal expansion bellows, this typically occurs when the material is continuously exposed to aggressive media without adequate corrosion allowance or material compatibility.
Unlike localized corrosion, uniform corrosion does not cause immediate cracking or leakage in the pipe expansion joints. Instead, it slowly degrades the mechanical properties of the bellows over time. Because expansion bellows are designed with very thin walls to provide flexibility, even small amounts of uniform metal loss can have a significant effect on performance.
critical impacts on the metal expansion joint
Pressure Loss
Reduction in wall thickness leading to lower pressure capacity than the design rating.
Spring Rate Shift
Change in bellows spring rate, affecting system flexibility and load transfer.
Fatigue
Decrease in fatigue life, resulting in reduced movement cycle capability.
Safety Margin Loss
Gradual loss of safety margin, often unnoticed until failure.
what is pitting corrosion?
Pitting corrosion is a highly localized and aggressive form of corrosion, most commonly associated with pipe expansion joints which are installed in chloride-containing environments. It is frequently observed in the stainless steel expansion joints operating in coastal areas, offshore installations, cooling water systems, or chloride-bearing process media.
Stainless steels depend on a thin, invisible chromium-oxide passive layer for corrosion resistance. Chloride ions can penetrate this layer at weak points such as inclusions, weld heat-affected zones, or microscopic surface defects, initiating deep pits. Because metal expansion bellows have very thin walls, a single pit can rapidly progress into a through-wall failure.
critical impacts on the metal expansion joint
Pressure Loss
Gradual Pin-hole leakage through the bellows wall.
Weak Expansion Bellow Element
Formation of multiple pits leading to crack initiation
Fatigue
Severe reduction in fatigue life due to stress concentration.
Expansion Bellow Squirm
Loss of pressure integrity and increased risk of bellows squirm or rupture.
Pitting
Corrosion
Crevice
Corrosion
what is crevice corrosion?
Crevice corrosion in a metal expansion joint occurs in shielded or stagnant zones where oxygen replenishment is restricted. Common locations within the metal expansion bellow element which are prone to crevice corrosion include:
• Gaps between the inner sleeve and deep convolutions
• Incomplete weld penetration areas
• Overlapping components or trapped gasket interfaces
Within these crevices, stagnant moisture and chemicals create an environment that becomes oxygen-depleted and increasingly acidic, leading to autocatalytic corrosion attack.
critical impacts on the metal expansion joint
Material Loss
Wall loss leading to leakage and reduced service life of the metal expansion joint.
Weak Expansion Bellow Element
Localized thinning that acts as stress concentrators
Fatigue
Development of pits that promote fatigue cracking in the expansion joint.
Sudden breakdown
Hidden corrosion that is difficult to detect externally
what is Stress Corrosion Cracking (SCC)?
Stress Corrosion Cracking (SCC) is a brittle and catastrophic failure mechanism that occurs in normally ductile materials such as 300-series stainless steels. SCC requires the simultaneous presence of :
1. A susceptible material
2. Tensile stress (residual or applied)
3. A specific corrosive environment
Metal expansion bellows are inherently susceptible to SCC due to:
- Thin wall construction of the expansion bellow element
- Deep convolution forming stresses due to continuous movement and pressure.
- Residual stresses from welding and forming of the pipe expansion joint.
- Continuous pressure thrust, thermal movement, and pipe misalignment with regards to the pipeline system.
SCC is one of the most critical failure modes and must be addressed through material selection, stress relief, and environmental control.
critical impacts on the metal expansion joint
No visible warning
Sudden and complete loss of containment resulting in breakdown.
Weak Expansion Bellow Element
Due to SCC, the expansion bellows become weak and the performance reduces, before finally cracking.
Distinct pattern of corrosion
SCC can be observed by its characteristic branched, transgranular cracking pattern.
Stress Corrosion Cracking (SCC)
Galvanic
Corrosion
what is galvanic corrosion?
Galvanic corrosion occurs when dissimilar metals are electrically connected in the presence of an electrolyte. In metal expansion joints, this is common where:
• Stainless steel bellows are welded to carbon steel flanges
• Stainless steel is combined with duplex steel or nickel alloys
• Weld areas experience damaged passivation
In poorly engineered assemblies, the metal expansion bellows material may become the anodic component, accelerating corrosion at the most critical and thin sections.
critical impacts on the metal expansion joint
Pressure Loss
Formation of pinholes and localized pits at the dissimilar metal joints
Convolution root damage
Damage concentrated at convolution roots
Fatigue
Gradual and significant reduction in fatigue and pressure capacity.
Corrosion on weld areas
Rapid wall penetration near weld zones, thinning of material near dissimlar metal areas within the metal expansion joints.
what is erosion corrosion?
Erosion–corrosion is a combined mechanical and chemical attack caused by high-velocity flow of corrosive media within the stainless steel expansion joints. Turbulence, entrained solids, or flashing fluids can impinge directly on the thin bellows wall, especially at the first convolution or flow disturbance zones.
critical impacts on the metal expansion joint
Material loss
Accelerated wall thinning at impact points
Material surface damage
Surface roughening leading to further corrosion acceleration
Fatigue
Pin-hole leaks and premature fatigue failure
Initial convolution failure
Localized failure near the inlet convolutions, mainly the first convolution as it is exposed to higher flow velocity and turbulence
Erosion
Corrosion
External
Corrosion
what is external corrosion?
External corrosion in an metal expansion joint is often more dangerous than internal corrosion, as it may progress unnoticed while internal inspections show no abnormalities.
Common external corrosion sources
• Atmospheric corrosion from humidity and industrial pollutants (SO2, chlorides)
• Marine and coastal environments with salt-laden air
• Corrosion Under Insulation (CUI) due to moisture ingress and chloride leaching
• Chemical splash or vapor exposure from wash-downs or process leaks
• External galvanic corrosion at contact points with carbon steel supports
• External erosion–corrosion from high-velocity wet air or steam impingement
critical impacts on the metal expansion joint
Unnoticed Damage
Loss of pressure containment without internal corrosion evidence
Sudden downtime
Sudden leakage or rupture in the expansion bellow, resulting in downtime.
Fatigue
Reduction in fatigue life and flexibility
Start of SCC
Initiation of stress corrosion cracking, resulting in exponential damage of the expansion joint.
Reducing corrosion-related failures in metal expansion joints is not achieved through a single design choice or material upgrade. It is the result of systematic engineering decisions made across the entire lifecycle of the expansion joint—from initial specification through manufacturing, inspection, and field application. At Ocelflex, corrosion is treated as a multi-variable engineering problem, not merely a material issue.
Every metal expansion joint designed and manufactured by Ocelflex undergoes a comprehensive technical evaluation before release for production. Key parameters considered include:
Correct material selection based on actual service conditions
Optimization of bellows geometry and convolution design
Controlled forming and welding practices
Post-weld cleaning and surface passivation
Use of internal sleeves and flow liners
Prevention of crevice formation through design
Quality control, inspection, and full traceability
Field experience feedback and continuous improvement
Corrosion resistance and lifecycle performance are treated as core design objectives at Ocelflex, ensuring that every metal expansion bellow is engineered for durability, safety, and long-term operational integrity. With a focus on corrosion-aware engineering, Ocelflex delivers stainless steel expansion joints that perform reliably under real operating conditions—not just on paper.