1.What are the consequences of improper substrate surface treatment?
Incomplete degreasing: Grease and dust on the substrate surface (galvanized steel, aluminum, etc.) are not thoroughly cleaned, forming a barrier layer that prevents proper coating adhesion.
Chemical conversion film issues: The conversion film (such as phosphating or chromating films) is uneven, too thin, or defective. This film is crucial for enhancing coating adhesion and corrosion resistance; if it is damaged, adhesion will be significantly reduced.
Surface moisture: If the substrate surface is not completely dry before coating, residual moisture vaporizes during baking, causing blistering of the coating.
2.What impact do coating process defects have?
Incomplete coating curing: Insufficient oven temperature or time results in incomplete cross-linking and curing of the coating, leading to insufficient coating strength and susceptibility to damage.
Excessively high coating viscosity or excessive coating thickness: High coating viscosity or excessively thick coating in a single application causes rapid surface curing while preventing the complete escape of internal solvents or air, resulting in a "half-cure" phenomenon. This can lead to blistering under heat later.
Air introduced during roller coating: During the coating process, the coating roller may introduce air into the coating, forming tiny bubbles. These bubbles may develop into blistering during subsequent use.
3.What are the reasons during transportation, storage, and installation?
Physical Damage
Affects such as bumps and scratches: During hoisting, transportation, or installation, the surface coating may be bumped or scratched by hard objects, damaging the integrity of the coating and providing a pathway for moisture and corrosive media to penetrate.
Improper Storage Environment
Humid Environment and Large Temperature Differences: If the color-coated rolls are stored in a humid, poorly ventilated environment, especially if placed directly on the ground, the temperature difference between day and night will cause condensation on the surface of the sheet. If the color-coated rolls are stored in roll form, moisture will enter the gaps between the rolls, and long-term contact will lead to edge or localized corrosion and bulging.
4.What are the reasons for its use in the environment?
Corrosive Media Intrusion
Starting from Cuts or Damage: Cuts are unavoidable during the installation of pre-coated steel sheets. If these cuts are not treated with anti-corrosion agents (such as sealant), corrosive media such as water, acid rain, and salt spray will infiltrate through them.
Penetration Process: These media first corrode the internal metal substrate (such as the galvanized layer), producing corrosion products such as rust. Rust is much larger than the original metal, generating enormous expansion stress that pushes up the surface coating, forming bulges. This is a process that spreads from the edge or damaged point of the sheet inwards.
Prolonged Exposure to Harsh Environments
High humidity, high salinity, and strong acid/alkaline environments (such as coastal areas or near chemical plants) greatly accelerate the above corrosion process.
5.How to identify and prevent it?
Dense, uniform small bumps, distributed over a large area. Source control: Select manufacturers with stable quality and good reputation. Strengthen factory inspection.
The bulge starts at the edge or cut of the board and gradually spreads inward; rust is visible after peeling it off. 1. Treat cuts and scratches with anti-corrosion agents during installation (e.g., apply special anti-corrosion paint or sealant).
Consider drainage in the design to avoid water accumulation.