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Home / News / Industry News / Marine Coating Intact After Application — Why Does It Blister After Seawater Immersion?
Industry News
1
A Complete Film Does Not Mean Stable Long-Term Seawater Resistance
When marine coating has just been completed: the surface is continuous and flat, the film coverage is complete, and initial adhesion performance is good. These characteristics only indicate that the coating is in normal condition in the short term — they cannot fully reflect the protective capability under long-term immersion. A complete film does not mean blistering will not occur after prolonged seawater contact.
2
Long-Term Seawater Action Causes Moisture Permeation
In a seawater environment: moisture continuously contacts the film, salt ions continuously migrate into the coating interior, and the coating is subject to sustained wet-heat effects. If the film's barrier capability is insufficient, moisture may gradually enter the coating interior and diffuse toward the substrate interface.
3
Interfacial Corrosion Readily Causes Blistering
When moisture and salt ions reach the interface between the coating and the metal substrate: corrosion reactions may occur at the metal surface, corrosion products gradually accumulate, and interfacial bonding strength continuously declines. As internal pressure increases, the film surface may develop blistering and lifting.
4
Film Density Affects Long-Term Protective Performance
A marine protective coating with stable performance typically requires: good barrier capability, a stable film structure, and low moisture vapour permeability. If micro-pores exist inside the coating or the structure is non-uniform, seawater more easily enters the interior, accelerating coating ageing.
5
Substrate Treatment Also Affects Coating Service Life
If the steel surface of marine structures has: insufficient rust removal, residual salt on the surface, oil contamination, or unsuitable surface roughness — all of these reduce the bonding stability between coating and substrate, making blistering after prolonged immersion more likely to occur.
6
Multi-Layer Protective Systems Require Good Interlayer Matching
Marine protection typically uses a multi-coat system: anti-corrosion primer, intermediate coat, and topcoat. If compatibility between different coatings is insufficient, interlayer bonding is weak, or cure states are mismatched — after long-term seawater action, interlayer failure readily occurs.
7
Why Initial Testing Shows No Problems
Because when the coating has just been completed: the film structure has not yet been damaged, the corrosion process has not yet begun, and the protective barrier is still intact. But as long-term seawater immersion continues: moisture continuously enters the film, salt ions act continuously, and interface defects gradually enlarge — latent problems only slowly manifest as blistering and delamination.
Summary
Marine coating blistering after prolonged seawater immersion is fundamentally the combined result of moisture permeation, salt ion action, interfacial corrosion, and declining coating protective capability.
A complete film does not mean stable seawater resistance
Moisture and salt ions gradually enter the coating interior
Interfacial corrosion causes film blistering
Film density affects protective service life
Substrate treatment determines long-term adhesion stability
Multi-layer systems require good system matching
When analysing marine coating blistering problems, in addition to focusing on application quality, it is necessary to comprehensively assess film performance, substrate treatment, interlayer bonding, and long-term seawater resistance capability.
Frequently Asked Questions
Does the salt spray test predict seawater immersion performance?
Salt spray testing and seawater immersion testing evaluate different aspects of corrosion resistance. Salt spray applies a wet saline mist to the coating surface; continuous immersion subjects the entire coating to sustained hydrostatic pressure and ionic diffusion. A coating that performs well in salt spray may behave differently under immersion conditions, and both test types should be evaluated for marine applications.
Why do weld seams and plate edges blister first in marine structures?
Weld seams typically have higher residual stress from the welding process, more complex surface geometry, and potentially thinner film coverage at the weld profile. Plate edges also tend to have thinner film build. Both zones have reduced barrier mass and higher local stress — making them the first locations where moisture permeation, interfacial attack, and blistering develop ahead of surrounding areas.
