1.What are the processes of osmotic pressure bubbling and peeling?
The coating itself contains microscopic defects, mechanical damage, or cuts.
Moisture (humidity) from the environment penetrates between the coating and the galvanized layer/substrate through these defects.
When moisture reaches the zinc-steel interface, it triggers electrochemical corrosion, producing corrosion products (such as zinc oxide and rust).
These corrosion products are several times larger than the original metal, generating enormous internal pressure (osmotic pressure).
This pressure "pushes" the coating off the substrate, first forming blisters, which accumulate water and corrosion products.
Over time, the blisters enlarge and merge, eventually leading to large-scale peeling and detachment of the coating.
Relationship with humidity: High humidity provides a continuous source of moisture for this process, greatly accelerating penetration, corrosion, and blistering.
2.What are the effects of damaging the coating adhesion?
Residual Salts from Pretreatment: If the substrate is not thoroughly cleaned before coating (chemical conversion treatment stage), soluble salts (such as chloride ions and sulfate ions) may remain on the surface. In high humidity environments, these salts will absorb moisture, forming a high-concentration electrolyte under the coating. This not only accelerates corrosion but also directly damages the chemical bonds between the coating and the metal, leading to decreased adhesion.
Water Molecule Penetration: Even without salts, prolonged exposure to high humidity or immersion in water can allow water molecules to gradually penetrate the interface between the coating and the substrate. This weakens the physical adsorption and chemical bonding between the two, reducing adhesion and making the coating easier to peel off by external forces.
3.What are the long-term hazards of condensate?
Process: In areas with large diurnal temperature variations, condensation easily forms on the inner surface of color-coated steel sheets. If building ventilation is poor, this condensation can persist for a long time.
Impact: This is equivalent to the coating being submerged in water for an extended period, representing an extreme case of the two aforementioned destructive mechanisms. It leads to a continuous decrease in adhesion and rapid corrosion, ultimately causing the coating to blister and peel off from the inner surface.
4.What are the synergistic effects with other factors?
Coating Quality/Construction Defects + Humidity:
Poor Pre-treatment: Unclean substrates and poor-quality conversion coatings result in weak adhesion. Humidity quickly penetrates these weak points.
Incomplete Coating Curing: Incompletely cured coatings have a loose structure and poor water resistance. High humidity directly leads to coating softening and loss of adhesion.
Mechanical Damage + Humidity:
Scratches and cuts during installation and transportation provide direct entry points for moisture. Humidity causes these damaged areas to rapidly expand into areas of peeling.
Temperature Difference and UV Aging + Humidity:
UV radiation causes coating aging, chalking, and the formation of microcracks.
Humidity (rain, moisture) penetrates through these microcracks, triggering corrosion and blistering of the underlying layer, leading to the flaking off of the aged coating in large sections.
5.How can I determine if humidity is involved in coating peeling?
If rust products (reddish-brown rust or white zinc salts) are present: it is almost certain that moisture participated in the electrochemical corrosion process.
If the flaking surface is damp or has water stains: this directly proves a relationship with humidity.
Flaking pattern: If the flaking occurred after large-area blistering, and there was liquid inside the blisters, humidity is one of the main causes.